application of pinch technology in water
DESCRIPTION
Análisis PinchTRANSCRIPT
APPLICATION OF PINCH TECHNOLOGY IN WATERRESOURCE MANAGEMENT TO REDUCE WATER USE
AND WASTEWATER GENERATION FOR AN AREA
KJ Strauss
WRC Report No. 1241/1/06
Water Research Commission
APPLICATION OF PINCH TECHNOLOGY IN WATERRESOURCE MANAGEMENT TO REDUCE WATER USE AND
WASTEWATER GENERATION FOR AN AREA
Report to theWater Research Commission
by
KJ Strauss
on behalf of
CSIR M&Mtek
WRC Report No. 1241/1/06ISBN No. 1-77005-488-X
NOVEMBER 2006
DISCLAIMERThis report has been reviewed by the Water Research Commission (WRC) andapproved for publication. Approval does not signify that the contents necessarilyreflect the views and policies of the WRC, nor does mention of trade names or
commercial products consitute endorsement or recommendation for use.
EXECUTIVE SUMMARY
BACKGROUND AND MOTIVATION
It has been indicated by DWAF that the Grootdraai catchment will soon experience severewater shortages as domestic, industrial and agricultural water use increases. It is, therefore,important to develop a systematic strategy or plan to reduce the amount of water used in thearea, as well as the wastewater generated.
Pinch technology has been successfully applied in improving thermal efficiencies in thechemical and process industries over the last 15 years. This has been through the re-arrangement of heat sources and sinks to optimise the overall thermal efficiency of theprocess. By taking advantage of certain parallels between the principles of heat and masstransfer, the systematic design procedures of pinch technology have been extended toaddress the problems of water use and wastewater generation. The overall goal is to reducethe amounts of water used and wastewater generated, with no detrimental effects to theprocess. The options are the re-use of water in different operations; regeneration and re-use; or regeneration and recycling. Freshwater and wastewater flows are reduced in eachcase, and in the latter two cases the contaminant load of the wastewater is also reduced.
The application of pinch technology in the reduction of freshwater use and wastewaterreduction has already been done on a number of plants and processes internationally aswell as in South Africa. Some of these applications have been for industrial complexeswhere a number of plants were considered and an overall water use optimisation has beenconducted.
Therefore the approach of applying pinch technology over a larger area may not necessarilybe a novel one, the application of pinch for a multi-sectoral and multi-users application is.
OBJECTIVES
Following the submission of a research proposal to the Water Research Commission in2000, the project titled "The Application of Pinch Technology in Water ResourceManagement to reduce water use and wastewater generation for an area" was approved.
The objectives of the project were as follows:• Develop an inventory of water users and wastewater generators in the Highveld
Ridge area• Application of a water pinch technology model that optimises the water use and
wastewater generation in the area.
in
PART I - OVERVIEW
WATER DEMAND MANAGEMENT AND PINCH TECHNOLOGY
Water scarcity has been identified as a driver of implementing new technologies for wateruse as well as re-use, recycling and regeneration options. However, there are other driversfor initiating water and wastewater saving initiatives. Hall (1997) identified the followingwater and wastewater reduction drivers:
• Economics
• Regulation and compliance
• Corporate waste reduction goals
• Regional water shortages/resource limitations
• Site infrastructure barriers
The increased awareness of dangers to the environment due to over extraction of water, theimportance of environmental protection and tougher environmental legislation are furtherdriving forces towards reductions in water consumption and wastewater generation.
The scarcity of good quality industrial water and the stricter discharge regulations haveresulted in higher costs for fresh water and the treatment of wastewater respectively. Thisrequires capital expenditure with little or no productive return and there is now considerableeconomic incentive to reduce both fresh water consumption and wastewater generation.This has impacted all types of industries including chemical processing, paper and pulp,manufacturing, petrochemical and electricity generation industries.
The prime objective of pinch technology is to achieve financial savings in the processindustries by optimising the ways in which process utilities, namely energy and water, areapplied for a wide variety of purposes. Pinch Technology does this by making an inventoryof all producers and consumers of these utilities and then systematically designing anoptimal scheme of utility exchange between these producers and consumers.
Pinch Technology provides a method to solve complex multi-stream energy and waterintegration problems. The technology has provided a rigorous means of analysingprocesses. It is based on sources and sinks, and the approach of reuse, recycle andregeneration, and combinations of these. The pinch approach not only sets targets but alsorecommends appropriate network design changes, which maximize the re-use ofwater/energy.
IV
PART II
WATER PINCH AND CATCHMENT MODELLING
There are numerous activities that affect both the quantity and quality of water in acatchment. The activities meet their water requirements by drawing from the surface bodiesand/or groundwater. The effluents generated through these activities are returned to thesame system, creating a cycle of use with external inputs and outputs. The major inputs intoa catchment are rainfall and inflow from other catchments. Inflow from other catchments canbe in the form of surface bodies (e.g. rivers) and groundwater from an upstream catchment,as well as artificial mediums such as canals and pipelines. The major outputs from acatchment are evaporation, transpiration and outflow to other catchments. Outflows to othercatchments can be in the form of surface bodies {i.e. rivers and streams) and groundwaterto a downstream catchment, as well as artificial mediums such as canals and pipelines.
The increased demand from users and the increased number of users has decreased theavailability of water and also the quality of the available water in the Grootdraai catchment.The water in an area has to be managed in such a way that it is not detrimental to the otherusers, especially downstream users. For a catchment, as mentioned previously, there is alimited supply. The abstraction of water and the release of effluent must be managed in asustainable way.
The following measures are used to manage the limited water resources available in acatchment:
• The variation in the quantity of water that enters and leaves a catchment can becontrolled with the building of reservoirs.
• Water use can be regulated by means of permitting, where a user is given theauthority to draw a limited amount of water per unit time. These permits can also beapplied to the release of effluent, where the volumes and quality of the waterreleased is regulated.
• Close monitoring of the water quality at selected points.
The water pinch model developed by Mr. C. Brouckaert (referred to as the "model") from theUniversity of Natal, Durban was used for this study. The decision was taken to use TDS onlyfor the case study, with the focus on whether water pinch can successfully be applied to acatchment situation. It is important that the modelled situation closely resembles the actualsituation in the catchment, while at the same time falling within the constraints of the modelprogrammed in the MATLAB computer package.
The model follows a plant set-up, which is made up of different processes and operations,which have specific water requirements. The input requirements for the different users are inthe form of a source, processes and sinks.
A comparison between water pinch and a catchment situation highlights the limitations withthe application of pinch to a catchment situation. The limitations listed include the followingfactors:
• Distance and altitude difference between "processes"• Limits and varied supply of the water source• Limits posed by the sensitivities of the surrounding ecological environment• The effects of groundwater and its movement• The effects of evaporation and transpiration
In addition to the limitations listed above, the data available for representation of acatchment situation is limited. Comparing a typical production facility with a catchment underthe listed model data requirements shows this:
• Sources - catchments have numerous sources that are highly variable, dataindicating the fate of water that enters the catchment is scarce.
• Processes - catchments have numerous users with different types of waterrequirements, uses and releases. Data for water losses in non-industrial users ispoorly known.
• Sinks - include evaporation and transpiration that varies from site to site dependingon numerous factors and are poorly known.
To model the catchment, a process of data gathering and identification of gaps in the dataneeds to be undertaken. The gaps can then be filled through water balances across thevarious systems in operation in the catchments as well as the catchment itself. The casestudy on the Grootdraai catchment shows a possible approach.
VI
PART
CASE STUDY- GROOTDRAAI CATCHMENT
The Grootdraai catchment is located in the Industrial Highveld, which forms part ofMpumalanga Province. The catchment has a surface area of 7924 km2 and forms part of theUpper-Vaal reach. One major river, the Upper Vaal, drains the catchment, with no rivers orstreams entering the catchment. All streams within the catchment drain into the Grootdraaidam, which is located at the western boundary.
The major users in the catchment draw their water from the Grootdraai dam. The wateravailable for these uses is therefore dependent on the availability of water in the dam. TheGrootdraai dam has a capacity of 364 million m3. The diagram below gives a graphicrepresentation of the water demand from the dam:
DAM OUTLET(SINK)
DAM INLET(SOURCE)
GROOTDRAAIDAM
A
USERS
TRANSFERS TOEXTERNAL
USERS(SINK)
LOSSESEVAPORATION, TRANSPIRATION,
GROUNDWATER, SEEPAGE(SINK)
VII
Information on the users drawing from the dam, with their current demand from the dam andreleases back into the system, are provided in the table below:
Overview of water demand and return by the usersUSER
Irrigation
DEMAND(m3/year)
321 500Tutuka Power Station 47 420 000Matla Power StationSASOLErmelo MunicipalityBethal MunicipalityThuthukane TownshipTOTAL
53 838 000
RETURN(m3/year)
-
-
-
91 250 000 4 015 000*3 600 0005 420 2501 427 556
203 277 306
1 982 1243 011 250
642 4009 650 774
• Released outside the catchment
The application of the pinch model yielded results that showed that in principle all the wastewater of the different users could be re-used, thereby reducing the demand on the dam bythe total of the currently released waste waster. The inflow to the dam would also bereduced, as part of the waste water is currently released up-fiow from the dam. Due to thelimited availability of input information that was required for the model, it was not possible forthe model to optimise the allocation of the waste water streams to the users. It appearedthat the outcome of the model was effectively a random allocation to the users.
A spread sheet calculation was carried out, which showed that the waste water can indeedbe allocated to the different users without infringing on the requirements of the users interms of maximum allowed inlet TDS. All individual users could take a part or all of the totalwaste water. Without further constraints e.g. the cost to transport the waste water oradditional costs for treatment by the user, there was no preference to allocate the wastewater to a specific user. This result confirmed the outcome of the model.
The following conclusions were reached:
• The available information from the users (inlet and outlet quantities of water andrequirements for inlet and outlet TDS) were not optimal input information for themodel to optimise the allocation of the waste streams to different users and thereforethe model output was closer to a random allocation.
• There are large differences between a catchment and a plant situation for which themodel was designed, and in order to use a water pinch type model for a catchment,considerable changes to the current model would likely be required.
VIII
• The modelling as well as the spreadsheet calculation showed that in terms of TDSinlet requirements all waste water could be re-used by the main water users.
• The study catchment area may not be representative for other catchments for tworeasons. In this particular catchment, only a small percentage of the inlet water isreleased as waste water, due to the presence of industries that evaporate most wateras part of their processes. Also, another aspect of this type of industry is that most ofthe TDS in the inlet water is not returned to the surface water of the catchment, butbecomes part of ash disposal sites.
As good water management is important for South Africa in general and more specifically incatchments such as the Grootdraaidam catchment, where water demand is likely to exceedwater supply in the future, it is recommended to investigate the development of a model thatcan reliably simulate all the important aspects of a catchment and thereby help to reducewater use by optimising the allocation of waste water to different users. This model shouldbe based upon the principles of water pinch, but would probably be substantially differentfrom existing models.
DC
TABLE OF CONTENTS
EXECUTIVE SUMMARY Ill
LIST OF TABLES AND FIGURES XIII
GLOSSARY OF ACRONYMS XIV
1 INTRODUCTION 1
1.1 MOTIVATION 1
1.2 OBJECTIVES 21.3 BACKGROUND 2
2 LITERATURE SURVEY 4
2.1 WATER DEMAND MANAGEMENT 4
2.2 PINCH TECHNOLOGY 5
3 CATCHMENT MANAGEMENT 9
3.1 OVERVIEW 93.2 NATURAL ACTIVITIES 10
3.3 MANAGEMENT OF USE 11
4 WATER PINCH AND CATCHMENT MANAGEMENT 12
4.1 WATER PINCH MODEL 12
4.2 MODEL REQUIREMENTS 144.3 MODEL LIMITATIONS 154.4 CATCHMENT DATA LIMITATIONS 16
5 GROOTDRAAI CATCHMENT 18
6 HYDROLOGY 19
6.1 GROOTDRAAI DAM 19
6.2 MODEL INPUT 20
7 AGRICULTURAL USE 28
7.1 MODEL INPUT 30
8 INDUSTRIAL USE 32
8.1 ESKOM[17] 328.2 SASOL[18] 338.3 MODEL INPUT 34
XI
9 MUNICIPALITIES 36
9.1 MODEL INPUT 37
10 MODEL APPLICATION 39
11 RESULTS AND DISCUSSION 42
12 CONCLUSION 45
13 RECOMMENDATIONS 45
14 ACKNOWLEDGEMENTS 46
REFERENCES 47
APPENDIX: MATHEMATICAL PROGRAMMING APPROACH TO WATER PINCHANALYSIS 50
XII
LIST OF TABLES AND FIGURES
PageFigure 2.1Typical composite curve 5Figure 2.2 Pinch design grid 6Figure 2.3 Concentration/mass flow composite curves 6Figure 3.1 Catchment Graphic 10Figure 4.1 A water using process 12Figure 4.2 The superstructure for a simple 2-process network 12Table 4.1 Pinch vs. Catchment Management 15Table 4.2 Comparison between a production facility and a catchment 16Figure 5.1 Catchment Layout 18Table 6.1 Water quality of Grootdraai dam 19Figure 6.1 Grootdraai System 21Fig 6.2 Grootdraai dam system 22Fig 6.3 Dam inlet and outlet 24Figure 6.4 Inflow into dam at Goedgeluk measuring station 25Table 6.2 Grootdraai dam users 25Figure 6.5: Modelled Grootdraai Dam 27Table 7.1 Extraction volumes of Agriculture 28Table 7.2 Selected SAWQG for Livestock Watering and Irrigation 29Figure 7.1 Water balance for Agriculture 30Figure 7.2 Modelled Agriculture 31Table 8.1 Grootdraai Dam Power Station Water Consumers 32Table 8.2 ESKOM additional requirements 33Figure 8.1 Modelled Industrial Users 34Table 9.1: Municipal wastewater releases [1] 36Table 9.2 Municipal water use 37Figure 9.1 Modelled municipal users 37Table 10.1 Model Input 39Fig 10.1 Intake and outlet volumes 40Table 11.1 Results for fixed concentration 42Table 11.2 Results for fixed mass loads 43Table: 11.3 Spreadsheet allocation of waste water to users 44
XIII
GLOSSARY OF ACRONYMS
CSIR
DWAF
HENS
LP
MEN
MENS
MIP
MILP
MINLP
MSA
NLP
SAWQG
TDS
WDM
WRC
ZLED
ACRONYMS
Centre for Scientific and Industrial Research
Department of Water affairs and Forestry
Heat Exchange Network Synthesis
Linear Programming
Mass Exchange Network
Mass Exchange Network Synthesis
Mixed Integer Programming
Mixed Integer Linear Programming
Mixed Integer Non-linear Programming
Mass Separating Agent
Non-linear Programming
South African Water Quality Guidelines
Total Dissolved Solids
Water Demand Management
Water Research Commission
Zero Liquid Effluent Discharge
XIV
PART I - OVERVIEW
1 INTRODUCTION
The CSIR has been commissioned by the Water Research Commission toconduct a Pinch Technology study in water resource management. The aim of thestudy is to reduce water use and wastewater generation for the Grootdraaicatchment area. There is a large number of multiple sector users e.g.petrochemical industry, electricity generation, mining, farming and increasingdomestic water use in this area.
1.1 MOTIVATION
It has been indicated that the proposed area of study will soon experience severewater shortages as domestic, industrial and agricultural water use increases [1]. Itis therefore important to develop a systematic strategy or plan to reduce theamount of water used in the area, as well as the wastewater generated.
Pinch technology has been successfully applied in improving thermal efficienciesin the chemical and process industries over the last 15 years. This has beenthrough the re-arrangement of heat sources and sinks to optimise the overallthermal efficiency of the process. By taking advantage of certain parallels betweenthe principles of heat and mass transfer, the systematic design procedures ofpinch technology have been extended to address the problems of water use andwastewater generation [2]. The overall goal is to reduce the amounts of waterused and wastewater generated, with no detrimental effects to the process. Theoptions are the re-use of water in different operations; regeneration and re-use; orregeneration and recycling. Freshwater and wastewater flows are reduced in eachcase, and in the latter two cases the contaminant load of the wastewater is alsoreduced.
The application of pinch technology in the reduction of freshwater use andwastewater reduction has already been done on a number of plants andprocesses internationally [3] as well as in South Africa [4]. Some of theseapplications have been for industrial complexes where a number of plants wereconsidered and an overall water use optimisation has been conducted.
Therefore the approach of applying pinch technology over a larger area may notnecessarily be a novel one, the application of pinch for a multi-sectoral and multi-users application is.
1.2 OBJECTIVES
Following the submission of a research proposal to the Water ResearchCommission in 2000, the project titled The Application of Pinch Technology inWater Resource Management to reduce water use and wastewater generation foran area" was approved.
The objectives of the project were as follows:• Develop an inventory of water users and wastewater generators in the
Highveld Ridge area• Application of a water pinch technology model that optimises the water use
and wastewater generation in the area.
1.3 BACKGROUND
One of the greatest challenges to be faced in the 21s t century is developing aninnovative strategy to avert serious local and regional water scarcities, and to meetthe rapidly growing demand for water. This includes addressing the issues ofquality, equity and incentives for both water managers and users. Because wateris a key life-supporting resource, its scarcity can have far reaching implications.
South Africa is characterized by erratic and unevenly distributed rainfall. Asregional populations continue to grow, so does the demand for water by allsectors. A regional (Southern Africa) study on water demand management (WDM)was undertaken by the IUCN [5] in recognition of the importance of water to theregion and the fact that supply-side approaches alone are inadequate to addressthe region's water challenges. One of the critical outcomes of the study was thatWDM is not yet an intrinsic part of water resource planning and management inSouthern Africa.
The motivation for the study was the view that if water resource management is tobe sustainable and socially efficient it needs to move away from a historicalemphasis on simply developing new supplies to meet projected water needs. Theemphasis on supply solutions has led to inefficient use of water resources, over-capitalisation in infrastructure, and environmental damage, and yet has still notprovided water security to the region.
The study identified the following factors driving WDM in the region:
• Growing divergence between demand and supply - demand is increasingdue to increasing population and income growth, but supply is fixed bynatural constraints and limited by financial ones.
• Increasing supply costs as countries are forced to look further a field fornew water sources;
• Increasing fiscal restraint resulting from externally structural adjustmentprogrammes i.e. sharper focus on the financial sustainability of publicutilities, on the removal of subsidies, and on cost recovery from consumers.
• Increased potential for WDM - through access to technology andeconomics of scale in urban areas
• Regional water security and risk aversion - WDM offers an alternative tomoving further a field to source water and avoids exposure to risk andpotential regional conflict.
The study also went on to identify a range of measures that have been used to
modify the demand for water. These are:
• Economic measures (e.g. use of pricing policies - block tariff systems),
• Regulations (e.g. use of permits for water use),
• Education and awareness raising (e.g. public awareness strategies),
• Technology improvements (e.g. efficient water use equipment, improved
irrigation systems),
• Water loss control (i.e. control of unaccounted-for water), and
• Water re-use and recycling (e.g. re-use of mining water, recycling ofmunicipal wastewater).
It is therefore proposed that pinch technology be used as a means of assessingthe potential for water re-use, recycling and regeneration in a water stressed area(catchment). If the application of pinch technology for water use managementproves to be successful, then we have another tool for WDM.
2 LITERATURE SURVEY
2.1 WATER DEMAND MANAGEMENT
Water scarcity has been identified as a driver of implementing new technologiesfor water use as well as re-use, recycling and regeneration options. However thereare other drivers for initiating water and wastewater saving initiatives. Hall [6]identified the following water and wastewater reduction drivers:
• Economics - Hall referred to a study conducted by the US Centre for WasteReduction Technologies where it was found that 75% of respondents citedeconomics as the reason for implementing a water/effluent minimizationprogramme. It was proposed that increasing fresh water and effluentdischarge costs are prompting companies to look for means to save onwater use and effluent discharge thereby saving costs.
• Regulation and compliance - regulations can stipulate the quantity andsometimes quality of wastewater discharged by a company. Companieshave to ensure that their releases comply with regulatory requirements.
• Corporate waste reduction goals - some companies set tough goals forwastewater reduction which go beyond discharge consent limits, as there isstill the possibility of environmental impact from their wastewater.
• Regional water shortages/resource limitations - this is generally manifestedin terms of charges, legislation and compliance. There may not be enoughcapacity in the piping and distribution system, or local treatment works maylimit effluent loading. Regional natural based water shortages have beenpreviously discussed.
• Site infrastructure barrier - Plant water systems are generally alreadycomplex due to years of plant modifications. There may be opportunities tomake better use of the existing equipment and even improve the waterquality.
The increased awareness of dangers to the environment due to over extraction ofwater (P Castro et al) [4], the importance of environmental protection (VR Dhole etal) [7] and tougher environmental legislation (Schaareman et al., P Tripathi, RHamilton, Cripps) [8; 9; 10; 11] are further driving forces towards reductions inwater consumption and wastewater generation.
Another important driver towards water demand management as mentioned aboveare the economic aspects. The scarcity of good quality industrial water and thestricter discharge regulations has resulted in higher costs for fresh water and thetreatment of wastewater respectively (Alva-Argaez et al) [12]. This requires capitalexpenditure with little or no productive return and there is now considerableeconomic incentive to reduce both fresh water consumption and wastewatergeneration (R Smith, E Petela) [13]. This has impacted all types of industries
including chemical processing, paper and pulp, manufacturing, petrochemical andelectricity generation industries.
2.2 PINCH TECHNOLOGY
The prime objective of pinch technology is to achieve financial savings in theprocess industries by optimising the ways in which process utilities, namely:energy and water are applied for a wide variety of purposes. Pinch Technologydoes this by making an inventory of all producers and consumers of these utilitiesand then systematically designing an optimal scheme of utility exchange betweenthese producers and consumers.
Pinch Technology provides a method to solve complex multi-stream energy andwater integration problems. The technology has provided a rigorous means ofanalysing processes. It is based on sources and sinks, and the approach of reuse,recycle and regeneration, and combinations of these. The pinch approach not onlysets targets but also recommends appropriate network design changes, whichmaximize the re-use of water/energy.
Pinch Technology is a systematic method of process analysis and design, whichmaximizes use of inherent thermodynamic potential. Thermal (energy) pinch wasdeveloped 20 years ago. The basic premise that heat flows from high to lowtemperature led to temperature/energy composition curves, the grid designmethod and other key concepts (see Figure 2.1 and 2.2). More recentwater/wastewater pinch and related techniques are based on analogous conceptsof contaminated mass flow vs concentration profiles (see Figure 2.3).
WAT FLOW
Figure 2.1 Typical composite curve
ue.
HOT
Figure 2.2 Pinch design grid
MASS FLOW
Figure 2.3 Concentration/mass flow composite curves
El-Halwagi and Manousiouthakis (1989) [14] first applied pinch technology to massexchange network synthesis (MENS). They introduced the use of a minimumcomposition difference, E, which is analogous to the minimum approachtemperature in heat exchanger network synthesis (HENS). They showed howspecifying the value of E locates the mass transfer pinch, which is athermodynamic bottleneck for mass transfer between streams. This allows a targetfor the minimum flow rate of external mass separating agent (MSA) required by anetwork to be determined. This target is analogous to the energy target in HENS.Avoiding the transfer of mass across the pinch ensures that the MSA target is metin design.
The implementation of pinch principles to mass exchanging processes wasannounced by El-Halwagi and Manousiouthakts (1990) [15] and later extended byWang and Smith (1994) [16]. The analysis uses the water concentration profiles ofindividual water consuming processes (washing, steam stripping, extraction, etc.).Individual concentration profiles are combined in so-called ConcentrationComposition Curves, which are analogous to the traditional thermal PinchComposition Curves. Here the temperature/enthalpy curve is replaced by theconcentration-mass of contaminant composite profile. This composite curve ismatched to a straight line through the origin, which represents a minimum watersupply line. This minimum water supply line touches the composite curve at aminimum of two points i.e. the origin and one another. The points other than theorigin are known as the Pinch points. Wang and Smith then presented twomethods to achieve minimum flow rate design. The first is referred to as themaximum driving force method, which uses concentration differences between thevarious streams to target the minimum flow rate. The second method is referred toas the minimum number of water sources method and uses load intervals. In eachinterval only enough water is used to maintain network feasibility, the remainder isby passed and used later. They considered also a case where more that onecontaminant is present and extended their methodology to cover this situation.They also considered the implications of regeneration of wastewater.
With the application of pinch technology, savings can be achieved in both capitalinvestment and operating cost. Emissions can be minimised and throughputmaximized.
2.2.1 SINGLE CONTAMINANT, GRAPHICAL APPROACHES TOPINCH ANALYSIS
Analogies between heat and mass transfer have been used to extend the conceptof pinch analysis to encompass waste minimisation and pollution prevention.Techniques have been developed in order to design optimal mass exchangernetworks (MEN). These minimum flow rate networks minimise the amount of freshwater consumed and waste water produced.
El-Halwagi et al. (1989-1997) presented several methodologies for the design ofMENS, pioneering the extension of the pinch analysis from thermal to massintegration.
Wang and Smith (1994a) developed an approach, which involves the generationof a single composite curve, which is used to set minimum flow rate targets. Themethods developed allow the designer to identify alternative structures for thesame problem. Wang and Smith also considered the possibility of regeneratingwastewater and presented a conceptually based approach, which distinguishesbetween regeneration re-use and regeneration recycling. They later extended thisidea to situations where flow rates are constrained (Wang and Smith, 1995a).
7
2.2.2 MATHEMATICAL PROGRAMMING APPROACH TO WATERPINCH ANALYSIS
The graphical approaches considered so far provide many valuable insights to thewater optimisation problems, but become increasingly difficult to apply whenmultiple contaminants or special process constraints are involved. They alsocannot deal with optimisation in terms of objective functions, which include factorsother than water use, in particular, economic factors. Previous investigators haveused mathematical programming for mass-transfer networks, (e.g. Takama et al.,1980; Rossiter and Nath, 1995); the formulation, which corresponds to the waterpinch approach, was set out by Doyle and Smith (1997) and extended by Alva-Argaez et al (1998). A detailed description of their approach is provided inAppendix A. The Water Pinch software developed by Dr Chris Brouckaert islargely based on this approach.
PART IIWATER PINCH AND CATCHMENT MODELLING
3 CATCHMENT MANAGEMENT
3.1 OVERVIEW
There are numerous activities that affect both the quantity and quality of water in acatchment. The activities meet their water requirements by drawing from thesurface bodies and/or groundwater. The effluents generated through theseactivities are returned to the same system, creating a cycle of use with externalinputs and outputs. The major inputs into a catchment are rainfall and inflow fromother catchments. Inflow from other catchments can be in the form of surfacebodies (e.g., rivers) and groundwater from an upstream catchment, as well asartificial mediums such as canals and pipelines. The major outputs from acatchment are evaporation, transpiration and outflow to other catchments.Outflows to other catchments can be in the form of surface bodies (i.e. rivers andstreams) and groundwater to a downstream catchment, as well as artificialmediums such as canals and pipelines.
The users within the catchment influence both the volume and quality of waterwithin the system. Processes that lead to evaporation, transpiration and seepage,decrease the volume of water in the catchment. Processes that lead to theaddition of pollutant loads to the water, affect the water quality.
Users can be separated into those that directly affect the quality and quantity ofwater in the system and those that affect the system indirectly. Users that directlyaffect the quality and quantity of water are those that release their effluents in theform of pipelines that discharge into surface bodies (point source pollution). Usersthat indirectly affect the quality and quantity of water are those that release theirpollutants with the aid of runoff and seepage (non-point source pollution).
The diagram below (Figure 3.1) graphically demonstrates the different processesthat take place in a catchment.
RAINEVAPORATION Non-point Source
Pollution
Point SourcePollution
GROUNDWATER
Figure 3.1 Catchment Graphic
3.2 NATURAL ACTIVITIES
As previously mentioned, one of the major sources of water for a catchment israinfall. In catchments where there are no rivers flowing into the catchment, rainfallcan be the only source of water. The quantity and quality of water that ends up inthe surface bodies, from rainfall, is dependent on runoff, evaporation and seepage.Runoff and seepage are interdependent occurrences, and in turn, are dependenton the following factors:• Initial moisture content of the soil during a rainfall event
D Soil with a low initial moisture content will absorb more water beforethe onset of runoff occurs
• Soil cover and/or soil typeo The type of cover will determine how much water the soil is able to
absorb before the onset of runoff occurs. The soil type determinesthe permeability of the soil and therefore the amount of water thatcan seep through. It also determines the speed of release of waterfrom groundwater storage into surface bodies.
• Slope of ground• Determines the speed at which water flows which also affects the
amount of water the soil is able to absorb.
Intensity of the rainfall10
L
D Rainfall with a high intensity allows less time for the soil to absorbwater as compared to rainfall with a low intensity.
Upon reaching the major rivers and streams, the characteristics of the water donot remain constant. Physical, biological and chemical factors such as settling,bacterial activity and chemical reactions respectively, significantly change thecharacteristics of the water. Added to this, users located along these water bodiesmay remove water, as well as, return their effluent, which is a lower quality,affecting both the quality and quantity of water along these streams.
3.3 MANAGEMENT OF USE
The increased demand from users and the increased number of users hasdecreased the availability of water and also the quality of the available water. Thewater in an area has to be managed in such a way that it is not detrimental to theother users, especially downstream users. For a catchment, as mentionedpreviously, there is a limited supply. The abstraction of water and the release ofeffluent must be managed in a sustainable way.
For sustainability, the following requirements have to be met, in this order:1. Ecological requirements2. Human requirements3. Agricultural and Industrial requirements
The following measures are used to manage the limited water resources availablein a catchment:
• The variation of the quantity of water that enters and leaves a catchmentcan be controlled with the building of reservoirs.
• Water use can be regulated by means of permitting, where a user is giventhe authority to draw a limited amount of water per unit time. These permitscan also be applied to the release of effluent, where the volumes andquality of the water released is regulated.
• Close monitoring of the water quality at selected points
11
4 WATER PINCH AND CATCHMENT MANAGEMENT
4.1 WATER PINCH MODEL
The water pinch model developed by Dr. C. Brouckaert (referred to as the "model")from the University of Natal, Durban was used for this study. What follows is abrief description of the pinch model.
The basic model of a water-using operation, similar to Wang and Smith's fixed
load model (Figure 4.1) is used, except for the following:1. An alternative option is considered, where the mass load is allowed to vary
in order to fix the outlet concentration of a contaminant;2. A water gain or loss is allowed, to model operations such as cooling towers
or evaporators.
Contaminant 6'jn Water
Water in Water out•
Cjn
Figure 4.1 A water using process
The basic concepts of limiting flows and concentrations, and the relationshipbetween them via the mass balances, are exactly the same as in Wang and Smith.To automate the procedure for finding the optimal set of connections betweenunits, a superstructure for the network is considered (Figure 4.2). This allows, inprinciple, all possible connections.
Waste
water
Figure 4.2 The superstructure for a simple 2-process network
12
The balance over process ican be expressed as
Flow balance: Z^+I^-<5X+Z /V+^) = 0 (1)
Balance for component n: Y f c + Y F .c,+S = ( Y f +YF,+W)C (2)i k , k
WhereF7y is the flow of reused water from outlet of process i to process j
Fjw is the flow of used water from outlet of process / to sink w
Fjk is the flow of fresh water from source k to inlet of process jQn is the concentration of ion n in outlet stream from process I5in is the mass gain of contaminant n over process IWj'is the water gain over process i
Balances of this form exist for each of the P processes and k contaminants in thesystem, and can be viewed as the basic set of process constraints. Specific limitson flows and concentrations, for a particular system, will form additionalconstraints.
To complete the formulation, an objective function must be defined to provide thebasis for optimisation. A general form for the objective function was proposed,representing fixed and variable costs associated with each stream, to beminimised:
This formulation of the problem has non-linearities in the objective function(Equation 3) and in the component balances {Equation 2). It should usually bepossible to use a linearised objective function as an approximation, but, in thecase of fixed contaminant loads, the component balances are intrinsically non-linear. The terms are products of flow rate and concentration, since both arevariables in the same problem. In the case of fixed outlet concentrations, however,Equation 2 is linear in the flow rates, since the concentrations are then knownconstants. Thus, if all processes in the system are of the fixed-outlet-concentrations type, the problem could be formulated to a linear programming (LP)optimisation. Although this is an unlikely scenario, it is reasonable to suppose that,in an optimised system, the concentrations will approach their limiting values. Thismeans that the LP solution could be taken as a good starting estimate for a non-linear programming (NLP) optimisation. Providing a good starting estimate is themost important means of achieving satisfactory convergence in non-linearprogramming. This rationale forms the basis of the linear/non-linear approach.
13
4.2 MODEL REQUIREMENTS
For the purposes of this study it has been decided to use the constraint of TotalDissolved Solids (TDS) only, with the focus on whether Water Pinch can be usedsuccessfully on a catchment situation. It is important that the modelled situationclosely resembles the actual situation in the catchment, while at the same timefalling within the constraints of the model programmed in the MATLAB computerpackage.
The model can also incorporate cost aspects of the different options, but thisfunction was not used as very little cost information was available. What follows isa brief description of these requirements.
As mentioned previously, the model follows a plant set-up. A plant set-up is madeup of different processes and operations, which have specific water requirements.The input requirements for the different users are in the form of a source,processes and sinks.
SourceThe required input for a source is limited to its cost and quality, excluding thequantity available. The reason the model only places a limitation on the quality andnot quantity is because of the intended use of the model, for a plant situation. Thefocus in a general plant situation is that the only limitation is the cost. The modeldoes allow for more than one source, and includes cost indicators for each.
ProcessThe required input for a process is:
1. The maximum allowed inlet concentration2. The maximum allowed outlet concentration3. The flow through the process1
4. The water gains or losses in the process
SinksThe required input for the sinks are similar to that of a process, where thedistinction occurs in what is referred to as the "connectivity" matrix. This matrix isused to manipulate flows to and from specific processes. It allows the user to allowor prevent the flow from one process to another. In the case of a sink, the userwould set the connectivity matrix so that no flow is allowed from the sink.
1 The model only allows for one flow as an input14
4.3 MODEL LIMITATIONS
A comparison between water pinch and a catchment situation highlights thelimitations with the application of pinch to a catchment situation. The limitationslisted include the following factors:
• Distance and altitude difference between "processes"
• Limits and varied supply of the water source
• Limits posed by the sensitivities of the surrounding ecological environment
• The effects of groundwater and its movement
• The effects of evaporation and transpiration
The limitations of the pinch model presented are now discussed by outlining thedifferences between the offerings of the model as compared to the situation in acatchment (Table 4.1).
Table 4.1 Pinch vs. Catchment Management
PinchWater can be routed in any directionInfinite supplyDispose of all effluent
Load doesn't change with flows
CatchmentsLimited by distance and altitudeLimited and varied supplyLimited by ecological requirements anddownstream usersLoad changes with change in flows
15
4.4 CATCHMENT DATA LIMITATIONS
In addition to the limitations listed above, the data available for representation of acatchment situation is limited. Comparing a typical production facility with acatchment under the listed model data requirements shows this:
Table 4.2 Comparison between a production facility and a catchment
Typical Production Facility
Source:Municipal (or other) viapipeline - largely dictated
by facility demand
Users:Machinery with well known(and controlled) intakevolumes and outletvolumes and quality (e.g.TDS)
Sinks:Single pipeline from
combined outlets
Catchment
Sources:
• Rainfall, dispersed over the catchment - highlyvariable from year to year and season toseason
• Inflow from upstream catchment - highlyvariable, unless controlled (buffered) by a dam
• Water transfersDynamic users:Agriculture
• seasonal
• non-point release of effluent
• pollutant load added through runoff, poorlyknown
• water balance poorly known, site specific
(dependent on soil, climate, water table, crop,etc)
Municipal
• seasonal to a small degree
• water losses through piping
• consumptive use (e.g.) gardening poorlyknown
• outlet quality variableIndustrial(see "Typical Production Facility")
Sinks:
• Evaporation (poorly known)
• Transpiration (poorly known)
• Outflow (well known and sometimes
controlled)
• Seepage (poorly known)
• Transfers (well known and controlled)
16
To model the catchment a process of data gathering and identification of gapsneeds to be undertaken. The gaps can then be filled through water balancesacross the various systems in operation in the catchments as well as thecatchment itself. The case study on the Grootdraai catchment shows a possibleapproach.
17
5 GROOTDRAAI CATCHMENT
The Grootdraai catchment is located in the Industrial Highveld, which forms part ofMpumalanga Province. The catchment has a surface area of 7924 km2 and formspart of the Upper-Vaal reach. One major river, the Upper Vaal, drains thecatchment, with no rivers or streams entering the catchment. All streams within thecatchment drain into the Grootdraai dam, which is located at the westernboundary. [Figure 5.1]
O Matla Power Station
SASOL(Secunda)
O Bethal Municipality
Ermelo Municipality
Tutuka Power Station
Thuthukane MunicipalityDirection of flow
Grootdraai Dam
Figure 5.1 Catchment Layout
18
6 HYDROLOGY
On an annual basis rainfall, by its nature, varies considerably over the catchment.There are no streams or rivers leading into the catchment, thus the water availablein the catchment has its only source as rainfall. With its only source as rainfall, itfollows that the available water varies considerably too (Figure 6.3) with somemoderation.The major users, such as industrial and the municipalities in catchment, require asteady supply of water throughout the year. This has led to the construction of theGrootdraai dam. By controlling the flow of water that leaves the catchment throughstorage, the dam is able to provide a constant, but limited supply, throughout theyear.
6.1 GROOTDRAAI DAM
The historical firm yield for the catchment is estimated by DWAF to be 124 million
cubic meters per year [1], where the historical firm yield is the smallest amount of
water that was available in the recorded history of the catchment. The rivers and
streams within the catchment require a minimum flow to meet the needs of their
aquatic environment. This minimum flow is known as the Ecological Reserve. The
estimated Ecological Reserve for the catchment is 27 million cubic meters per year
[1]-
6.1.1 WATER QUALITY
Water quality measurements are taken for monitoring purposes by DWAF at eightstations throughout the catchment. As mentioned previously, the majority of waterextractions, by the major users, occur at the dam. Given that the majority aresupplied from the dam, the water quality measurements taken at the dam wall willbe taken as representative of the quality received by users (Table 6.1). The detailsof the station are as follows:
Station number: C1R002Q01Station name: Grootdraai dam on Vaal River: Near dam wallData collection: 738 samples collected from November 1982 to September 1999
Table 6.1 Water quality of Grootdraai dam
STATISTIC
Maximum
Mean
Minimum
Standard Deviation
TDS(mg/P)
251
164
8
24.9
19
The data above shows a large variation from minimum to maximum. This can bedue to high rainfall, which will have a dilution effect on the TDS within the riversand streams. For the purposes of the study, the number for the will mean waterquality within the dam will be used for the modelling.
6.2 MODEL INPUT
A schematic diagram is used to describe how the various components of thecatchment and its users have been divided into sources, processes and sinks toallow application of the pinch model. The key below describes the differentsymbols and arrows used in the sections that follow:
• A flow that will not be included in the model, but is required in the balanceof the system is listed as a "Constant Flow".
• A flow that will be included in the model is listed as a "Modelled Flow"KEY to diagrams:
XYZ Process XYZ
Constant flow
Modelled flow
To represent the catchment as a whole, it is necessary to include all activities,grouped into their categories. The only input to the catchment as a whole (rainfall)is listed as a source. The outputs are listed as sinks. As discussed previously, thewater reaching the dam from rainfall is influenced by activities in the catchment,both man-made and natural, ending up in the dam for use by the major users.
20
DAMOUTLET
(SINK)
LOSSES*EVAPORATION,
GROUNDWATERSEEPAGE
(SINK)
GROOTDRAAIDAM
RAINFALL(SOURCE)
CATCHMENT
USERS
TRANSFERS TOEXTERNAL
USERS(SINK)
L O S S E S B
EVAPORATION, TRANSPIRATION,GROUNDWATER, SEEPAGE
(SINK)
Figure 6.1 Grootdraai System
Balance for the system:
Sc - Sc-i = Gj - Go
+ R - E- Od - Uod + Uid
(4)
Where,Sc = Storage in catchment for current yearSc-i = Storage in catchment carried over from previous yearOd = Surface water outflow from catchment for current yearGj = Groundwater inflow into catchment for current yearGo = Groundwater outflow from catchment for current yearR = Rainfall on catchment for current yearE = Evaporation (incl. Transpiration) from catchment for current yearUod = Extraction for useUiri = Return from users
21
The major users in the catchment draw their water from the Grootdraai dam. Thewater available for these users is therefore dependent on the availability of waterin the dam. The Grootdraai dam has a capacity of 364 million cubic meters. Itshould be noted that this does not translate into a water availability of 364 millioncubic meters. Instead, the availability of water for a specific year depends onstorage from previous years including inflow to the dam, minus all losses. Thediagram below (Figure 6.2) gives a graphic description:
DAM OUTLET(SINK)
DAM INLET(SOURCE)
GROOTDRAAIDAM
TRANSFERS TOEXTERNAL
USERS(SINK)
USERS
L O S S E S B
EVAPORATION, TRANSPIRATION,GROUNDWATER, SEEPAGE
(SINK)
Fig 6.2 Grootdraai dam system
The balance across the dam is as follows
Sj - S, - Id - OH + Gid - Gad - Ed - + Uk (5)
Where,Sj = Storage in dam for current year, i.e. available water for current yearSj-i = Storage in dam carried over from previous year
= Inflow for current year= Outflow for current year
Gld = Groundwater inflow to dam for current yearGod = Groundwater outflow from dam for current yearRd = Rainfall on dam for current yearEd = Evaporation from dam for current year
Id
Od
22
The balance can be simplified by grouping terms:Rd- Ed = net loss due to rainfall and evaporation = Lc
Gid - God = net loss due to groundwater seepage = Lg
Equation (5) becomes:Sj-Sj-i = I<,-Od + Lg + Lc-Uod + Uid (6)
6.2.1 STORAGE
The pinch model is a steady state model. To overcome the annual changes thatoccur in the catchment, the available data is averaged over the 20 years that it hasbeen collected. In doing this, storage becomes insignificant in terms of the waterbalance. The general water balance for a system is as follows:
Sx = SJM + gainsx - lossesx (for system x)Bur", Sx-i - Sx-2
+ gainsx-i - losses^Therefore, Sx = Sx-2
+ gainsx.i - lossesx.i + gainsx - lossesx
(St - Si_n) = ^(Gain.sx - Lossesx)
For n-large and/or S,-+t * S,,X I
^ (Gainsx - Losses^) - (Sx — SI_n) => J ] (Gainsx - Lossesx)r—n x—n
Following this, equations 4 and 5, over 20 years, can be simplified to the following:-Od + Gj-Go + R-E-Uod + Uid = 0 (4a)
Rd-Ed'Uod + Ujd^O (5a)
The dam is a system that is located within the catchment system. It is located atthe downstream end of a catchment and receives the surface water that flows fromthe catchment.
23
CatchmentSystem
Dam System
A., G
, G<od
Fig 6.3 Dam inlet and outlet.
The dam outlet is an output for both the catchment and the dam itself. The twosystems are compared by using this commonality and making the dam, Od, thesubject of the formula for both systems:
Od = (Gi - Go) + (R-E) + (U* - Uoa) (4b)Od = (GK, - God) + (Ra- - Ed) + ld + (U* - L U (5b)
The inputs and outputs to the catchment that take place outside the dam can be
found by taking the difference between the two systems:
Equation 4b - Equation 5b:
0 = (G; - Go)+ (R-E)-Id- (G* - Got) - (Rd - Ed)
0 = (R-Rd)-(E- Ed) + (Gi-G*) - (Go - G^) - la-
in words,
0 = Rainfall (excl dam) - Evaporation (excl dam) + Groundwater inflow (excldam) - Groundwater outflow (excl dam) - Inflow to dam
Rearranging this,Inflow to dam = Rainfall (excl dam) - Evaporation (excl dam) + Groundwaterinflow (excl dam) - Groundwater outflow (excl dam)
Therefore, the inflow to the dam equals the activities that take place outside thedam. Since the dam receives its surface water from these activities, it is furtherconcluded that the inflow to the dam is a result of, and accounts for, all losses andgains in the catchment, excluding those that occur in the dam itself.
24
6.2.2 DAM INFLOW
The details of the measuring point upstream of the dam is as follows:
Station No. C1H007Station name: Vaal River at Goedgeluk
Figure 6.4 Inflow into dam at Goedgeluk measuring station
The average dam inflow from the above graph is 299 million cubic meters. Thecurrent demand from the dam and releases back into system is provided in thetable below:
Table 6.2 Grootdraai dam usersUSERIrrigationTutuka Power StationMatla Power StationSASOLErmelo Municipality(water from upstreamdam)Bethal MunicipalityThuthukaneTownshipTOTAL
DEMAND321
47 42053 83891 2503 600
5 4201 427
203 277
500
000
000
000
000
250
556
306
RETURN
40151 982
3011642
9 650
-
-
-
000
124
250
400
774
REFERENCESection 7Section 8Section 8Section 8Section 9
Section 9Section 9
25
Therefore,Uod = Demand = 203 277 306 m3/aU*i = Return = 9 650 774 m3/a
Substituting into equation 5a, using the average inflow (299 million m3) and thepermitted use of water from the dam:
299.0 = Od + Lg + Lc- 203.3 + 9.7105.4 = Od + Lg + Lc (6a)
The numbers are in million m3/year
Therefore the sum of dam outflow, the net loss of evaporation and rainfall, and thenet loss due to ground water seepage is on average 105.4 million m3/year aftertaking the use into account.
6.2.3 WATER QUALITY
The quality measured in the dam is also dependent on point releases fromindustry and municipalities (see Figure 6.2). The assumption is made that themass load contribution, for TDS, from the grouped non-point-sources is muchgreater than the mass load from the point sources. To justify this assumption, anindication of the contribution of the non-point-sources is determined by taking thedifference between the average mass load measured, in TDS, at the inlet to thedam and the total mass load, in TDS, from the point sources to the surface waterbodies. Excluded from the calculation, are the users that release their wastewateroutside the catchment and users that do not release wastewater.
Mass Loads:Average TDS at dam = 164 mg/lAverage annual flow into dam = 299 x 106 m3
Total Mass Load = 164 (x 10 3 kg/m3) x 299 x 106 m3
= 49.04 x10 6 kg
Average TDS and volumes released by users:Ermelo = 2 x 106 m3 at 633 mg/P = 1.27 x 106 kgBethal = 3 x 106 m3 at 547 mg/P = 1.64 x 106 kgThuthukane = 642 x 103 at 390 mg/P = 0.25 x 106 kgTotal mass of point loads = 3.16 x 106 kg
= 6.4% of Total Mass Load
26
Based on the previous discussions and assumptions, the modelled situation forthe dam is as follows:
DAM INLET
LOSSES DAM DAM OUTFLOW
USERS
Figure 6.5: Modelled Grootdraai Dam
Using the above configuration the following data were used for the model1. Dam Inlet to Dam
Volume = Average over 20 years of 299 million m3
TDS = 8-251 mg/P, with an average of 164 mg/P
2. Dam outflow + Losses from DamVolume = Average over 20 years of 105.4 million m3
TDS = 8-251 mg/P, with an average of 164 mg/P
3. Dam to UsersVolume = Modelled, with a maximum intake of 203.3 million m3
TDS = 8-251 mg/P, with an average of 164 mg/P
27
7 AGRICULTURAL USE
The volumes extracted by the agricultural sector are recorded by DWAF in theform of permits. Farmers provide information to DWAF on their water requirementsfor specific farming activities, which are collated for catchment management, andcharging purposes. The data on extractions in the Grootdraai catchment areprovided in the table below.
Table 7.1 Extraction volumes of Agriculture
USER
Irrigation
Livestock Watering
SOURCE
BoreholeDams*Rivers/StreamsOther
BoreholeDams
Rivers/StreamsOther
PERMIT (mJ/a)2 202 6907 598 11533 131 898373 297
990 9205 600596 271113209
'Grootdraai dam = 321 500 mJ/a
The agricultural users have three main sources of water, namely, rainfall, surfacewater (dams, rivers and streams) and groundwater. The water extracted from thesurface and groundwater sources are used to supplement the water obtained fromrainfall. The water extracted is therefore dependent on the amount or lack ofrainfall and varies considerably. The release of water is, as mentioned previously,treated as a non-point source, i.e. evapotranspiration and seepage. The volumereleased annually as evapotranspiration, is dependent on numerous factors, whichvary from site to site:
• Type of crop
• Humidity
• Soil moisture content and the height of the water table
• Wind speed
• Temperature
The quality demanded by the two subcategories, namely livestock watering andirrigation, was derived from the South African Water Quality Guidelines(SAWQG)2. The SAWQG serve as the primary source of information fordetermining the water quality requirements of different water users and for theprotection and maintenance of the health of aquatic ecosystems.
A water quality guideline is a set of information provided for a specific water quality constituent.28
The upper boundaries of the No Effects3 water quality range of the SAWQG wereused for TDS. The limits relevant to this study were as follows:
Table 7.2 Selected SAWQG for Livestock Watering and Irrigation
Agriculture Type
Livestock WateringIrrigation
TDS(mg/P)1000267
The Grootdraai dam has an average TDS of 164mg/P, which is of a higher qualitythan is required by the agricultural sector.
For each water quality constituent there is a No Effects Range. This is the range ofconcentrations or levels at which the presence of that constituent would have no known oranticipated adverse effects on the suitability of water for a particular use. These ranges weredetermined by assuming long-term continuous use and incorporation of a margin of safety.
29
7.1 MODEL INPUT
There are numerous users located throughout the catchment. Extraction methodsutilised include both surface and groundwater sources. The effluents generated byagricultural users are non-point releases through seepage into ground water andrunoff into rivers. The use of water in the agricultural sector can be described asfollows:
GROUNDWATERWATER FLOW OUT OF
CATCHMENTRAINFALL
SEEPAGE
GROUNDWATER
SURFACE WATER
SYSTEMBOUNDARY
AGRICULTURE
RUNOFF
SURFACE FLOWINTO DAM
EVAPORATION,TRANSPIRATION
SUPPLY FROMGROOTDRAAI DAM
Figure 7.1 Water balance for Agriculture
Irrigation serves as a supplement to the shortages from rainfall. For a waterbalance across the system, the focus is on the inputs and outputs:
RA +
UOA =
EA +
GOA + I A
Where,RA = Rainfall onto agricultural landUOA = Water demand from damEA = Evaporation and transpiration from agricultural land
GOA = Portion of groundwater flow out of catchment, originating from agricultureIA = Portion of dam inflow from agriculture
30
But,RA is an element/portion of R (total rainfall,)EA is an element/portion of E (total evaporation,)IA is an element/portion of / (total inflow into dam,)
GOA is an element/portion of G (total groundwater flow from catchment;
Since these elements have been accounted for in the overall balance (Section6.2), they do not have to be considered further. Therefore, the only variable notaccounted for thus far is water demand from the dam for agricultural purposes. Ofall the numerous agricultural users, only one irrigation user actually draws from theGrootdraai dam. Following this argument, the model should only include this userand exclude the rest of the agricultural users that do not draw from the dam.
Based on the above arguments, the modelled situation for agriculture is thereforeas follows:
Figure 7.2 Modelled Agriculture
X to AgricultureVolume = Permitted amount of 321 500 m3
TDS = Maximum of 267 mg/P
31
8 INDUSTRIAL USE
The industrial sector consists of users located inside and outside the catchment.The major users are ESKOM and SASOL. A description of these users follows inthe subsections below.
8.1 ESKOM [17]
8.1.1 WATER REQUIREMENTS
Eskom has two power stations that draw water from the Grootdraai dam, namely,Tutuka and Matla. The required volumes are presented in Table 3.
Table 8.1 Grootdraai Dam Power Station Water Consumers
Power Station
Tutuka
Matla
Permit(m3/year)
47 420 000
53 838 000
The quality requirements of power stations are such that they can be operated onwater of poor quality provided sufficient water treatment is undertaken. With theadded desalination, increased brine disposal is required, where brine is the by-product of water treatment. The disposal of brine has been specifically highlightedby ESKOM as a major problem.
To limit the brine disposal and its associated problems, ESKOM has set thefollowing TDS requirements for the Vaal River system:
• Ideal TDS concentration of less than 120 mg/P
• Tolerable TDS concentration of: 120 - 240 mg/P
• Unacceptable TDS concentration of greater than 240 mg/P
In addition to the TDS requirements listed above, ESKOM also have the followingwater quality objectives for their power stations operating on a raw water supplyfrom the Vaal River system:
32
Table 8.2 ESKOM additional requirements
Parameter
ConductivityTotal Organic CarbonSodiumChlorideSulphate
Permanent hardness(T Hardness - MAlkalinity)M Alkalinity
Total hardness
BariumStrontium
Units
uScm"1
mgkg"1 as Cmgkg"1 as Namgkg"1 as Clmgkg"1 asSO4
mgkg~1asCaCO3
mgkg"1 asCaCO3
mgkg"1 asCaCO3
ugkg1 as Baugkg'1 as Sr
Ideal
<160< 2
< 10<5
< 15
Nil
<60
<60
<30<80
Acceptable
160 to 3202 to 5
10 to 255 to 1515 to 40
<8
60 to 120
60 to 120
30 to 6080 to 120
NotAcceptable
>320>5
>25> 15>40
> 8
> 120
> 120
>60>120
8.1.2 WASTEWATER
Both power stations must conform to the Zero Liquid Effluent Discharge (ZLED)policy. The two power stations therefore release zero effluent into the system. Theeffluent is used instead to transport the coal-ash to the ash disposal site.Thuthukane Township, which forms part of Tutuka power station, has a maximumallowable (permitted) discharge of 1760 m3 per day and TDS of 390 mg/P.
8.2 SASOL[18]
8.2.1 WATER REQUIREMENTS
SASOL extracts 91 250 000 m3 per year of its water requirements from theGrootdraai Dam. This is used for boiler feed water and cooling water. The rawwater, obtained from the dam, is treated by a water treatment plant to meet theirwater quality requirements. The volume of water required increases with anincrease in TDS, e.g. an additional 10 950 000 m3 per year is extracted when TDSincreases from 200 to 300 mg/P. This increases the volume of water required to102 200 000 m3/a for a TDS of 300 mg/P.
8.2.2 WASTEWATER
Most wastewater, that is high in TDS, is used to transport coal ash to the ashdisposal site. Therefore the ash disposal site functions also as a sink for a largepart of the TDS. Approximately 4 015 000 m3 per year at a TDS of 900 mg/P isreleased into the Waterval river, located in the Waterval catchment.
33
8.3 MODEL INPUT
Industrial plants are treated as processes. The pinch program is not equipped tohandle differences between inlet and outlet volumes; therefore to overcome this,the plants with a difference between inlet and outlet volumes are divided into twoprocesses. For the first of the two processes the process volume and the inletconcentration are the characteristics of the water that enters the process. Theoutlet concentration in the model is the actual outlet concentration of the plant, butdoes not have the associated volume that the plant releases. The second of theprocesses is used to account for the change in volume. The inlet and outletconcentration is equalled to the outlet of the first process, while the processvolume is set to the actual outlet volume of the plant.
Based on the above arguments, the modelled situation for industrial users istherefore as follows:
INDUSTRIAL (IN)LOSSES
INDUSTRIAL(OUT)
iFigure 8.1 Modelled Industrial Users
1a) X to Tutuka (in)Volume = Permitted amount of 47 420 000 m3
TDS = Maximum of 240 mg/P1b) Tutuka (in) to Tutuka (out)
Volume = 0TDS = 0
34
2a) X to Matla (in)Volume = Permitted amount of 53 838 000 m3
TDS = Maximum of 240 mg/P2b) Matla (in) to Matla (out)
Volume = 0TDS = 0
3a) X to SASOL (in)Volume = 91 250 000 m3 (TDS = Maximum of 200 mg/P)Volume = 102 200 000 m3 (TDS = Maximum of 300 mg/P)
3b) SASOL (in) to SASOL (out)Volume = 4 015 000 m3
TDS = 900 mg/P
35
9 MUNICIPALITIES
Municipalities either source their input water from the Grootdraaidam or from anupstream dam or tributary. For the modelling it was assumed that all water wasextracted from the dam. No specific information on TDS content for the upstreamdams or tributaries was available and therefore the TDS content of theGrootdraaidam was used for the model.
The municipalities and their wastewater releases are listed in Table 9.1.Amersfoort and Morgenzon municipalities' wastewater is used for irrigation. Giventhe end use and the small volumes, these two municipalities have not beenincluded in the modelling. To prevent municipalities from receiving the wastewaterof other municipalities and users, due to the sensitivity of their requirementsoutside that of TDS, the inlet water quality was limited to that of the dam. The datafrom the DWAF report, the Augmentation of the Eastern Sub-system of the VaalRiver system [17], is described in the table below:
Table 9.1: Municipal wastewater releases [1]
Point Sources
Ermelo Municipality (2 treatmentworks combined)
Bethal Municipality
Armersfoort Municipality
Morgenzon Municipality
Thuthukane Township
Flow(m3/day)
5683
8250
170
400
1760 [17]
TDS(mg/P)
633
547
-
390
Recipient
Klein Kafferspruit
Tributary
(Blesbokspruit)Irrigation
Irrigation / Dilutingmedium (nightsoil)
Leeuspruit
An estimate of the water losses were made by using Ermelo municipality as abase. The inlet volume for Ermelo raw water treatment plant is 3 600 000 m3 peryear [19]. The outlet for Ermelo wastewater treatment plants is 1 982 124 m3 peryear. This is a total loss across the system of 45%. Using a loss of 45% for theremaining 2 municipalities the inlet and outlet volumes for the municipalities are asfollows:
36
Table 9.2 Municipal water use
Ermelo (in)Ermelo (out)BethalBethal (out)Thuthukane (in)- Sourced from Tutuka Power StationThuthukane (out)
3 600 0001 982 1245 420 2503 011 2501 427 556
642 400
9.1 MODEL INPUT
The same situation as far as difference in inlet and outlet volumes occurs inMunicipalities as with Industrial users. The same approach of splitting the user intotwo processes is therefore taken.
Based on the above arguments, the modelled situation for municipalities istherefore as follows:
MUNICI(1
'
PALITY
r
MUNICIPALITY(OUT)
LOSSES
Figure 9.1 Modelled municipal users
1a) Dam to Ermelo (in)Volume = 3 600 000 m3
TDS = 8 - 251 mg/P, with an average of 164 mg/P1b) Ermelo (in) to Ermelo (out)
Volume = 1 982 124 m3
TDS = Average of 633 mg/P
37
2a) Dam to Bethal (in)Volume = 5 420 250 m3
TDS = 8-251 mg/P, with an average of 164 mg/P2b) Bethal (in) to Bethal (out)
Volume = 3 011 250 m3
TDS = Average of 547 mg/P
3a) Dam to Thuthukane (in)Volume = 1 427 556 m3
TDS = 8-251 mg/P, with an average of 164 mg/P3b) Thuthukane (in) to Thuthukane (out)
Volume = 642 400 m3
TDS = Average of 390 mg/P
38
10 MODEL APPLICATION
Based on the data and its manipulation in the previous sections, the water pinchmodel developed by Chris Brouckaert was applied to the following data, used asrepresentation for the Grootdraai catchment. The part of the model that takescosts into consideration was excluded from the modelling as this was not part ofthe scope. The differences between the two parts of the table are explained onpage 52.
Table 10.1 Model Input
Massbalance forindividualwaterusers
ProcessIrrigationTutukaMatlaSasolErmeloBethalThutukaneTotal
Process
IrrigationTutukaMatlaSasolErmeloBethalThutukaneTotal
MaximInlet
0.2670.240.24
0.30.1640.1640.164
BestInlet
0.1640.1640.1640.1640.1640.1640.164
Inlet vol321500
4742000053838000
102200000360000054202501427556
214227316
Inlet vol
3215004742000053838000
102200000360000054202501427556
214227316
MaximumTDS(ton)
85.8411380.8012921.1230660.00
590.40888.92234.12
TDS(ton)
52.737776.888829.43
16760.80590.40888.92234.12
Vol loss321500
47420005383800981850016178762409000785156
Vol loss
32150047420005383800981850016178762409000785156
TDS(Tons)
B5.8411380.812921.127046.5-664.28-758.23-16.42
TDS(Tons)
52.737776888829 4313147.3-664.28-758.23-1642
Outlet vol0
00
401500019821243011250642400
9650774
Outlet vol
000
401500019821243011250642400
9650774
OutletConcn
0.900.630.550.39
OutletConcn
0.900.630.550.39
Outlet(Tons)
0.000.000.00
3613.501254.681647.15250.54
Outlet(Tons)
0.000.000.00
3613.501254.681647.15250.54
In the above table the input information is summarized in the form of a massbalance, taking volume and TDS into account. The table shows that the totalamount of available waste water is 9650774 m3. However the municipalitiesrelease waste water back into the catchment and therefore if this water is re-usedit does not reduce the overall amount of water used in the catchment. In thefollowing diagram the intake volumes and the outlet volumes are representedgraphically showing that the total outlet volumes are 4.5% of the intake volumes.
39
WATER DEMAND WATER USAGE WATER RELEASE
Tutuka Power Station
Matia Power Station
SASOL dischargeto Waterval River
Ermelo Municipality
Beth a I Municipality
Thuthukane Township
Irrigation
Fig 10.1 Intake and outlet volumes
Two scenarios are represented.
The first scenario is based upon the maximum inlet concentrations that the usersaccept as explained in the previous sections. From these maximum inletconcentrations and the actual intake volumes the maximum TDS was calculated.The same procedure was followed for the output streams, therefore theconcentrations in table 10.1 are the maximum outlet concentrations for each user.In the previous sections it was discussed that irrigation and the power stationshave no outlet stream and therefore also no outlet TDS. The loss of both volumeand TDS is 100%. Therefore the modelling parameters are set so that no wastewater from these users is available for others. The other users have an outletstream and therefore also an outlet TDS, but there are losses which can be eitherpositive or negative. For example SASOL has a volume loss of 96% and a TDSloss of 88%. The reason is that most of Sasol's intake water is evaporated ascooling water and brine stream is used to transport coal ash to the ash dump. Themunicipalities reported a water loss of 45%, but a TDS increase {negative loss).
40
The reason for the volume reduction is that part of the intake water is not returnedto the sewage system as it is used for e.g. watering gardens. The reason for theincrease in overall TDS is that the sewage water has a much higher TDS than theintake water.
In the second scenario it is assumed that all users use the best input quality watere.g. water from the dam. It is further assumed that the outlet volumes andconcentrations are the same as in the first scenario. Therefore all volumes in thesecond scenario are the same as in the first scenario, but for the industrial usersthe TDS loss is smaller than in the first scenario. In a real plant situation thevolume of the intake water would have been reduced for the second scenario aswas discussed in the previous section, but no information about this reduction wasavailable and therefore it was not taken into account.
During the modelling a comparison was made between the first scenario and thesecond one during each modelling run, showing how much water could be saved,if all waste water would be re-used.
41
11 RESULTS AND DISCUSSION
The model was applied in two ways, namely fixed concentration and fixed massload.
With fixed concentration, the concentration of TDS was fixed for the outlet waterstream. No matter what is done to the flow rate to/from that plant or process orwhat happens to the inlet concentration, the outlet concentration will remain thesame. The program will adjust the mass load over the process in such a way as tosatisfy the fixed concentration condition. The fixed concentration mode is typicallyused in a plant situation where there is something precipitating or dissolving incontact with a solid phase, so that the outlet concentration is fixed by equilibriumconsiderations. It can also be used in a situation where there is a treatmentprocess with some kind of feedback control to maintain a specified outletconcentration.
The result was as follows:
Table 11.1 Results for fixed concentration
Flows (ML)
From
Cone(kg/m3)
To
IrrigationTutukaMatlaSasol
ErmeloBethal
ThutukaneDam
InletOutlet
Irrigation
0.0
0.0
0.0
30.46.3
7.1
0.0
277.7
0.2513
Tutuka
0.0
0.0
0.0
1992.2984.21501.4321.1
42621.1
0.2184
Matla
0.0
0.0
0.0
1992.2984.21501.4321.1
49038.7
0.2119
Sasol
0.0
0.0
0.0
0.0
7.5
1.3
0.0
102191.2
0.1640.901
Enrnelo
0.0
0.0
0.0
0.0
0.0
0.0
0.0
3600.0
0.1640.633
Bethal
0.0
0.0
0.0
0.0
0.0
0.0
0.0
5420.3
0.1640.547
Thutukane
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1427.6
0.1640.390
Total fromdam
20457.5
Flows if all users are supplied only from dam (ML)
DifferenceDam 321.5 47420 53838 102200 3600 5420.3 1427.6 214227.3
9650.8
With Fixed Mass Load, the mass load change over a plant/process was fixed. Inthis way a constant amount of TDS is added to the stream no matter what is doneto the flow rate to/from the plant or process or what happens to the inletconcentration. The program will adjust the outlet concentration of the stream inorder to satisfy this condition. The fixed mass load approach is the standard modelof a water-using process. The philosophy is that the function of the water is toremove contaminants from the process stream and as the pinch analysis will not
42
affect the operation of the process, the load of TDS will be fixed. The results areshown below;
Table 11.2 Results for fixed mass loads
Flows(ML)
From
Cone(kg/m3)
To
Irrigation
Irrigation
0.0
Tutuka 0.0MatlaSasol
ErmeloBethal
ThutukaneDam
InletOutlet
0.0
0.0
0.0
0.00.0
321.0
0.2513
Tutuka
0.0
0 0
0.0
1.00.0
0.0
0.0
47419.0
0.2184
Matla
0.0
0.0
0.0
1.0
Sasol
0.0
0.0
0.0
4012.0
Ermelo
0.0
0.0
0.0
0.0
0.0 1982.0 0.00.0
0.0
53837 0
0.2119
3011.0642.092552.
00.1640.901
0.0
0.0
3600.0
0.1640.633
Bethal
0.0
0.0
0.0
0.0
0 0
0.0
0.0
5420.3
0.1640 547
Thutukane
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1427.6
0.1640.390
Total fromdam
20457.5
Flows if all users are supplied only from dam (ML)
DifferenceDam 321.5 47420 53838 102200 3600 5420.3 1427.6 214227.3
9650.8
The output of the model for both fixed concentration and fixed mass load showsthat all waste water can be re-used in principle. However the fixed concentrationmodel allocates nearly all waste water to the two power stations and the fixedmass load model allocates nearly all waste water to SASOL. Upon trying tounderstand this difference in allocation it appeared that the number of parametersto be optimised was small and that the model could satisfy its requirements inmany ways depending on chance starting conditions. It is therefore concluded thatthe model cannot indicate the optimum solution, as more than one optimumsolution exist. The differences between the two results above are in fact not nearlyas significant as the output would suggest.
In order to increase the understanding of the possible allocations for the differentwaste water streams to the potential users, a spreadsheet was compiled showingthe different users and how effluent water can in principle be allocated dependingon the maximum TDS levels that the user can tolerate. It must be emphasized thatthis study is considering only TDS as a parameter, and that if allocations would bedone in the real world that all contaminants for which a user has set a maximummust be taken into account.
43
Table: 11.3 Spreadsheet allocation of waste water to users
UserIrrigationTutukaMatlaSasolErmeloBethalThutukane
Vol.321500
474200053838001022000360000054202501427556
SupplierIrrigation
00000
00
Tutuka0000000
Matla00000
00
Sasol44993
401500040150004015000
000
Ermelo71061
198212419821241982124
000
Bethal86461
301125030112503011250
000
Thutuka146524642400642400642400
000
NB Tutuka can take all Sasol's waste water and Thutukane's and Ermelo's or another combinationwithout exceeding its TDS limit.Matla can take aii Sasol's waste water and Thutukane's and Ermelo's or another combinationwithout exceeding its TDS limit.Sasol can take all waste water including its own without exceeding its TDS limit.
Table 11.3 shows options for the users. For example, irrigation as a user can take44993 m3from Sasol or 71061 m3 from Ermelo or86461 m3from Bethal or 146524m3 from Thutukane. A similar situation exists for the other users. From the table itis clear that there is more than enough capacity to re-use all the waste waterstreams without exceeding the inlet requirements on TDS for the individual users.It shows that the larger users can take the total waste water stream of a supplier oreven a number of suppliers. The table also shows that there are many potentialallocations and as long as no additional criteria are set e.g. the cost of transportingthe waste from the generator to the user or criteria for other contaminants, allthese allocations are equivalent confirming the observation that the modelchooses, more or less at random, a solution.
The overall reduction on the demand from the dam would be equal to the totalamount of re-used waste water, which is 9.6 million rrrVyear (4.5% of the currentdemand on the dam). However a large part of the waste water that is currentlyreturned upstream from the dam in the rivers of the catchment is flowing into thedam. If this waste water would be allocated to other users the inflow to the damwould be reduced by this amount which is 5.6 million m3 /year. (2.6% of thecurrent demand on the dam.
44
12 CONCLUSION
The following conclusions were drawn from the study:
• The available information from the users (inlet and outlet quantities of waterand requirements for inlet and outlet TDS) were not optimal inputinformation for the model to optimise the allocation of the waste streams todifferent users and therefore the model output was closer to a randomallocation.
• There are large differences between a catchment and a plant situation forwhich the model was designed and in order to use a water pinch typemodel for a catchment, considerable changes to the current model wouldlikely be required.
• The modelling as well as the spreadsheet calculation showed that in termsof TDS inlet requirements all waste water could be re-used by the mainwater users.
• The study catchment area may not be representative for other catchmentsfor two reasons. In this particular catchment, only a small percentage of theinlet water is released as waste water, due to the presence of industriesthat evaporate most water as part of their processes. Also another aspect ofthis type of industry is that most of the TDS in the inlet water is not returnedto the surface water of the catchment, but becomes part of the ash disposalsites.
13 RECOMMENDATIONS
As good water management is important for South Africa in general and, morespecific, in catchments such as the Grootdraaidam catchment, where waterdemand is likely to exceed water supply in the future, it is recommended toinvestigate the development of a model that can reliably simulate all the importantaspects of a catchment and thereby help to reduce water use by optimising theallocation of waste water to different users. This model should be based upon theprinciples of water pinch, but would be substantially different from existing models.
45
14 ACKNOWLEDGEMENTS
The authors wish to acknowledge the Water Research Commission for financiallysupporting this project.
The authors wish to acknowledge Greg Steenveld for his technical advice duringthe execution of the project, Chris Brouckaert for making the water pinchprogramme available and the valuable contributions that he made in theapplication of water pinch modelling. The authors also wish to acknowledge theother members from the steering committee for making their time available for themeetings and their inputs in the report.
46
REFERENCES
[I] DWAF, BKS, 1998 Augmentation of the Eastern Sub-system of the Vaal
River System: Desktop Study
[2] RossiterAP, 1995 Waste Minimisation Through Process Design, Chapter5, Pinch Analysis in Pollution Protection
[3] Eastwood A R, Tainsh R A, Fien G J, 1998Minimising Wastewater Emissions using Water Pinch TM Analysis: ATechnical White Paper
[4] Brouckaert C J, et al, 1999Optimal location of a membrane treatment plant in a Power Station. Paperpresented at IAWQ International Specialised Conference on MembraneTechnology in Environmental Management, Tokyo
[5] IUCN, Goldblatt, N. et aL, 2000Water demand management: Towards developing effective strategies forSouthern Africa
[6] Hall SG, 1997 Water and effluent minimisation, Institution of ChemicalEngineers, North Western Branch Papers, No. 4
[7] Dhole VR, Ramchandani N, Tainsh RA, 1996Make your process water pay for itself, Chemical Engineering, Vol. 103,Issue 1, pp 100-103
[8] Schaareman M, Verstraeten E, Blaak R, Hooimeijer A, Chester I, 2000Energy and water pinch study at the Parenco Paper Mill, Paper Technology,Vol .41, Part 1, pp 47-52
[9] TripathiP, 1996Pinch technology reduces wastewater, Chemical Engineering, Vol. 103,Issue 11, pp 87-89
[10] Hamilton R, Dowson D, 1994Pinch cleans up, Chemical Engineer, Part 566, pp 42-44
[II] Cripps, H, 2000Pinch technology for waste minimisation, Paper Technology, Vol. 41, Part 1,pp 33-38
47
[12] Alva-Argaez A, Kokossis AC, Smith R, 1998Wastewater minimisation of industrial systems using anintegrated approach, Computers and Chemical Engineering, Vol. 22 -supplement, pp S741-744
[13] Smith R, Petela R, 1994Wastewater minimisation and the design of effluent treatment systems usingpinch analysis, Environmental Protection Bulletin, Part 030, pp 5-10
[14] El-Halwagi, Manousiouthakis V, 1989Synthesis of mass exchange networks, AlChE Journal, Vol. 35, No. 8, pp1233-1244
[15] El-Halwagi, Manousiouthakis V, 1990Simultaneous synthesis of mass-exchange and regeneration networks,AlChE Journal, Vol. 36, No. 8, pp 1209-1219
[16] Wang YP, Smith R, 1994Wastewater minimisation, Chemical Engineering Science, Vol. 49, No. 7, pp981-1006
[17] Discussion with Dirk Hanekom, ESKOM
[18] Discussion with Mario Augoustinos, Roux du Toit, SASOL
[19] Discussion with Ermelo Municipality
48
APPENDIX: MATHEMATICAL PROGRAMMING APPROACH TO WATERPINCH ANALYSIS
DOYLE AND SMITH (1997)
Doyle and Smith considered that a water-using network is not simply a specialcase of a mass-exchange network, because operations such as cooling towers,steam systems and hosing operations cannot be considered as mass-exchangers.They pointed out that non-linear mathematical programming techniques sufferedfrom difficulties in ensuring that they found the global optimum to a problem, ratherthan a local optimum, particular for problems involving many variables. Linearprogramming techniques, on the other hand, can handle very large problems, andglobal convergence is readily obtained. They therefore presented linear and non-linear formulations of the problem, and proposed a combined linear/non-linearapproach to overcome the previously encountered difficulties.
The basic model of a water-using operation (Figure A1) is similar to Wang andSmith's fixed-load model, except that:
i) an alternate option is considered, where the mass load is allowed to varyin order to fix the outlet concentration of contaminant
ii) a water gain or loss is allowed, to model operations such as coolingtowers or evaporators.
Contaminant n 8/» W, Water
•jn
Figure A1 A water using process
The basic concepts of limiting flows and concentrations, and the relationshipbetween them via the mass balances, are exactly the same as in Wang and Smith(1994a).
To automate the procedure for finding the optimal set of connections betweenunits, a superstructure for the network is considered (Figure A2). This allows, inprinciple, all possible connections.
49
/
if 1 2 - • *Wastewater
Fresh^water
Figure A2 The superstructure for a simple 2-process network
In the original paper, several different versions of the balance equations weregiven, which made the treatment difficult to follow. Here a somewhat differentformulation is used, for compactness and clarity.
The balance over process / can be expressed as
Flow balance:
Balance for component n.
ZFji cm + LFjh ciat +Sjr, = I ZFn + z.Fjk +Wi\ci K \ i k
Where
(8)
Fji is the flow of (re-used) water from outlet of process / to inlet of process)FM is tile flow of (used) water from outlet of process / to waste sink wFJk is the flow of (fresh) water from source / to inlet of process jCin is the concentration of ion n in outlet stream from process /5in is the mass gain of contaminant n over process /Wi is the water gain over process /
Balances of this form exist for each of the P processes and k contaminants in thesystem, and can be viewed as the basic set of process constraints. Specific limitson flows and concentrations, for a particular system, will form additionalconstraints.
To complete the formulation, an objective function must be defined to provide thebasis for optimisation. A rather general form for the objective function wasproposed, representing fixed and variable costs associated with each stream inthe system, to be minimised:
(In fact, this form fails to address an important practical issue, namely where thecost associated with a particular stream is dependent on the contaminant load,
50
rather than just the flow rate, however there is no particular problem in includingterms to represent this.)
This formulation of the problem has non-linearities in the objective function(Equation 9) and in the component balances (Equation 8). It should usually bepossible to use a linearised objective function as an approximation, but, in thecase of fixed contaminant loads, the component balances are intrinsically non-linear because the terms which are products of flow rate and concentration, sinceboth are variables in the problem. In the case of fixed outlet concentrations,however, Equation 8 is linear in the flow rates, since the concentrations are thenknown constants. Thus, if all processes in the system are of the fixed-outlet-concentration type, the problem could be formulated to a linear programming (LP)optimisation. Although this is a most unlikely scenario, it is reasonable to supposethat, in an optimised system, the concentrations will approach their limiting values.This means that the LP solution could be taken as a good starting estimate for anon-linear programming (NLP) optimisation. Providing a good starting estimate isthe most important means of achieving satisfactory convergence in non-linearprogramming. This rationale forms the basis of the linear/non-linear approach.
ALVA-ARGAEZ, KOKKOSIS AND SMITH (1998)
The extension to the Doyle and Smith treatment consisted of noting that once aset of flow rates had been obtained from the linear-programming solution, whichassumes that the outlet concentrations are at their limiting values, one cancalculate the corresponding set of concentrations, and determine where theassumptions are in error. If the calculated concentrations are below the limits, theerrors are of no consequence. For concentrations which exceed the limits theerrors can be added into the objective function to be minimised, so that runningthe LP algorithm again will tend to drive the errors to zero. This provides the basisfor a method which uses a series of LP optimisations which converge to the NLPsolution, taking advantage of the particular mathematical structure of water pinchproblems.
A further refinement introduced binary variables corresponding to each possibleconnection in the network. For these, a value of 1 indicates that the connectionexists, and a value of 0 that it does not. This formulation allows automatic controlof features such as the elimination of streams that fall below a specified flow rate,or the maximum number of connections allowed in the network, to avoid excessivecomplexity. These variables move the optimisations into the class of Mixed Integer(Ml) programming - once again MILP is very much more tractable than MINLP.
51
•sau..
Other related WRC reports available:
The application of pinch analysis for the rational management of water and effluent in anindustrial complex
Brouckaert CJ: Buckley CA
The chemical processing industry, in the RSA and internationally, poses environmental challenges as it ischaracterised by a relatively low water use but the effluents produced are often concentrated and can contain toxicor inhibitory contaminants. Compounded by the low availability of water in the RSA, the management trend is toreduce both water use and effluent generation at source during production processing. Process integration is aholistic approach to process design, retrofitting and operation which emphasises the unity of a process orprocesses so that overall eco-efficiency can be sought.
Pinch analysis is a process integration tool first developed for optimising the design of heat recovery systems. Thetechnique was subsequently extended theoretically to water-using systems with the objective of minimising wateruse by maximising water re-use. Modern water pinch analysis is a set of systematic formal mathematicaltechniques for handling the complex problem of hierarchical water allocation to a multi-process system involvingmultiple contaminants, and choosing the "best" strategy according to selected priorities including overall costminimisation.
The aims of this project were to apply and assess water pinch technology as an approach in the RSA forminimising water use and effluent generation in a chemical complex, to refine the technique as necessary, and totransfer expertise in the technology to industry, regulators and academics. The initial case study targeted was theAECI Umbogintwini industrial complex consisting of 13 individual factories with site services coordinated by aseparate company (AECI Operational Services). Due to interest expressed by industry, during the course of thestudy three other investigations were also commenced, at Sanachem (agrochemicals). Eskom Lethabo (powergeneration) and Mondi Merebank (paper milling).
The results obtained may be classified into two categories, namely specific results at the sites studied and generalresults relating to the development and application of water pinch analysis. At Sasol Polymers chlor-alkali plant,one of the three largest water users in the AECI complex, the study identified significant potential savings in wateruse (72%), effluent generation (45%), HCI use (2.9%) and NaOH use (4.2%). At Sanachem, the batch-wiseproduction of agrochemicals such as pesticides and herbicides uses water as a reaction or washing solvent andgenerates effluents which are contaminated with toxic organics with a high hazard rating. The pinch investigationidentified measures to reduce water use by around 40%. proportionate reductions in the generation of toxic andother effluents, and simultaneously a 25% increase in production capacity due to reduced batch times. At LethaboPower Station, the particular objective was to establish the "best" use of an existing reverse osmosis (RO) plant inthe system. The pinch analysis quantified the balance to be struck between the quantities of river water, minewater and regeneration chemicals used and the volume and quality of the net effluent generated, and showed thatin one scenario it was technically feasible to use the RO plant to operate the combined mine and power stationtogether on a zero-liquid effluent basis with identifiable cost considerations.
Regarding the general conclusions about water pinch analysis, the study showed that (a) a clear and systematicpicture was given of the water requirements of a system of processes, (b) areas in which water efficiency couldmost beneficially be improved were highlighted, and (c) a rational tool was provided for negotiation on water usetargets amongst industry, regulators and other role players. Overall it was recommended that water pinch analysisshould be extended in theory (for example to include concomitant energy use) and in practice (by furtherimplementation). Industry has already demonstrated significant interest in applying water pinch analysis as an aidtowards better water and effluent management, and its use is expected to grow.
Report Number: 851/1/03 ISBN No: 1 77005 028 0
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