UNIVERSITY OF NAIROBIDEPARTMENT OF ENVIRONMENTAL AND BIOSYSTEMS

ENGINEERING

FEB 540: ENGINEERING PROJECT- REPORT

2017/2018

DESIGN OF A MEMBRANE BIOREACTOR FOR WASTE WATER TREATMENT AT NORTH GATE SCHOOL, KOMA HILL.

NAME: MAINA MARY WACERA

REG. NO: F21/1977/2013

SUPERVISOR: Dr. CHRISTIAN OMUTO

DATE OF SUBMISSION: 29th JUNE 2018

Submitted in partial fulfillment of the requirements of the Degree of Bachelor of Science in Environmental and Biosystems Engineering in University of Nairobi.

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DECLARATION

I declare that this project is my original work and has not been submitted for a degree in any other University.

Sign………………………..........................Date…………………………………………….....

(MARY WACERA – F21/1977/2013)

This project report has been submitted for examination with my approval as University supervisor.

Sign…………………………………………..Date……………………………………………….

(Dr. CHRISTIAN OMUTO)

Supervisor

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DEDICATION

To North Gate School fraternity for allowing me to do a case study of their waste water treatment plant and believing in me to provide a solution.

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ACKNOWLEDGMENT

To Almighty God for giving me an opportunity to pursue my studies and giving me strength and perseverance for completion of this phase.

To my parents and siblings for always supporting me and being there to encourage me during the tough seasons.

To Dr. Christian Omuto, my project supervisor, for his insight and guidance in this process of writing my project. I cannot thank you enough.

To Eng. Prof Ayub Gitau for encouraging me and ensuring I do not settle for less with the abilities I have.

To my lecturers who have dedicated their time to impart knowledge and engineering skills that were useful in this project and for the future use.

To my colleagues who have been on this journey with me for moral support and academic help they have offered me.

To all my friends who have been a source of strength and support. Thank you.

GOD BLESS YOU ALL.

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ABSTRACT

North Gate School experiences a limited supply of fresh water. The school is located in an arid and semi-arid area that receives low unreliable rainfall. The primary source of water at the school is underground water pumped through a borehole. The boarding school with a population of 255has a daily water demand of 20,000 Lto cater for cooking, laundry, toilet flushing etc. With the school being located far from any sewer line, an onsite waste water management is necessary. Currently, the school is generating 16,000 L of waste water daily. The institutionhas been experiencing an overflow of their biological waste water treatment system. This can be attributed to the lack of maintenance of the conventional activated sludge system and high hydraulic retention time. Given a growing population at the school and the already experienced overflows, this system is not capable of handling the waste water treatment currently and in the future.

The overall objective of this project was to design a membrane bioreactor system for waste water management at the school. The system enables direct reuse of waste water for nonpotable uses after treatment and has low hydraulic retention time. The daily quantity of waste water produced at the school was established through calculation after establishing the daily demand of water. The quantity and pollutant content of the waste water was established from secondary data as labtesting was a challenge. The membrane bioreactor components; anoxic tank, aeration tank and membrane tank were sized through calculations taking into consideration the current and future design flow of waste water. The treatment plant performance was evaluated.

This report contains the introduction, literature review, theoretical considerations, methodology, results and analysis, cost benefit analysis of the implementation of the project, AutoCAD design drawings, recommendations and conclusions. The project was a success as the objective of the project which was to design a membrane bioreactor for North Gate School. Implementation of this project will reduce pressure on the water supply at the school and become save costs in regards to water resources.

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Contents1 INTRODUCTION.............................................................................................................................11

1.1 Background................................................................................................................................11

1.2 Problem statement.....................................................................................................................12

1.3 Problem justification..................................................................................................................12

1.4 Site analysis...............................................................................................................................12

1.4.1 Climate and vegetation......................................................................................................14

1.4.2 Geology and soils...............................................................................................................15

1.5 Objective....................................................................................................................................15

1.5.1 Specific objectives.............................................................................................................15

1.6 Scope.........................................................................................................................................16

2 LITERATURE REVIEW..................................................................................................................17

2.1 Sources of waste water..............................................................................................................17

2.2 Classification of waste water.....................................................................................................18

2.2.1 Black water........................................................................................................................18

2.2.2 Grey water.........................................................................................................................18

2.2.3 Brown water.......................................................................................................................18

2.3 Waste water quality indicators...................................................................................................18

2.3.1 Biological quality indicators..............................................................................................18

2.3.2 Physical..............................................................................................................................18

2.3.3 Chemical............................................................................................................................18

2.4 Waste water treatment systems..................................................................................................18

2.5 Overview of the biological waste water treatment.....................................................................19

2.5.1 Biological Aerobic Treatment............................................................................................19

2.5.2 Biological Anaerobic Treatment........................................................................................21

2.5.3 Process of biological waste water treatment.......................................................................21

2.6 Membrane Bioreactor (MBR)....................................................................................................24

2.6.1 Membrane bioreactor treatment process............................................................................24

2.7 Membrane bioreactor versus the conventional methods of wastewater treatment......................28

3 THEORETICAL FRAMEWORK.....................................................................................................30

3.1 The components of a biological waste water treatment system..................................................30

3.1.1 Waste water source............................................................................................................30

3.1.2 Waste water collection.......................................................................................................30

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3.1.3 Waste water conveyance system........................................................................................30

3.1.4 Waste water treatment........................................................................................................30

3.1.5 Storage system...................................................................................................................30

3.2 Design considerations when designing MBR............................................................................30

4 METHODOLOGY............................................................................................................................32

4.1 Generation of the design concept...............................................................................................32

4.2 Design parameters of membrane bioreactor components...........................................................33

4.2.1 Equalization tank...............................................................................................................33

4.2.2 Anoxic Tank volume..........................................................................................................33

4.2.3 Aeration Tank volume.......................................................................................................34

4.2.4 Membrane tank volume.....................................................................................................34

4.2.5 Hydraulic Retention Time..................................................................................................35

4.2.6 Sludge retention time.........................................................................................................35

4.2.7 Calculating Required Oxygen............................................................................................36

5 RESULTS AND DISCUSSION........................................................................................................38

5.1 Site analysis...............................................................................................................................38

5.1.1 Land cover.........................................................................................................................38

5.1.2 Geology and soils...............................................................................................................39

5.1.3 Population..........................................................................................................................40

5.2 Quantity of water supplied daily................................................................................................41

5.2.1 Average volume of water used per person per day.............................................................41

5.2.2 Quantification of waste water produced.............................................................................41

5.3 Quality determination of waste water........................................................................................42

5.4 Design parameters of MBR.......................................................................................................43

5.4.1 Equalization tank volume...................................................................................................43

5.4.2 Anoxic tank volume...........................................................................................................44

5.4.3 Aeration tank volume.........................................................................................................44

5.4.4 Membrane tank..................................................................................................................46

5.4.5 Calculating required oxygen..............................................................................................47

5.4.6 Calculating pipe sizes........................................................................................................48

5.4.7 Hydraulic Retention Time..................................................................................................48

5.4.8 Sludge retention time.........................................................................................................49

5.5 Effluent quality..........................................................................................................................49

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5.6 Calculating MBR Efficiency......................................................................................................49

5.6.1 BOD removal.....................................................................................................................49

5.6.2 Nitrogen removal...............................................................................................................50

5.6.3 TSS removal......................................................................................................................50

5.7 Operation and maintenance........................................................................................................50

5.8 Summary of results....................................................................................................................50

5.9 Cost benefit analysis..................................................................................................................53

5.9.1 Capital cost........................................................................................................................53

5.9.2 Operational costs................................................................................................................53

5.9.3 Energy demand and power requirement.............................................................................54

5.10 Fixed cost of the MBR...............................................................................................................54

5.11 Operational cost.........................................................................................................................55

5.12 Discussion..................................................................................................................................57

6 CONCLUSION AND RECOMMENDATION.................................................................................58

6.1 Conclusion.................................................................................................................................58

6.2 Challenges encountered.............................................................................................................58

6.3 Recommendations......................................................................................................................58

7 REFERENCES..................................................................................................................................59

8 APPENDICES...................................................................................................................................60

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LIST OF FIGURES

Figure 1:Map showing North Gate School. Source: Google Earth...............................................12Figure 2: Map showing waste water treatment plant at NGS School. Source: Google Earth.......13Figure 3: Map showing NGS School in relation to transport network. Source: Google Earth.....14Figure 4: A graph of average temperature and rainfall at Machakos............................................14Figure 5: A pie chart showing domestic water use........................................................................17Figure 6: A flow diagram showing activated sludge process........................................................19Figure 7:Schematic diagram of a Membrane Bioreactor...............................................................25Figure 8:Submerged and side stream membranes.........................................................................26Figure 9:Flow diagram of Membrane Bioreactor and conventional biological waste water treatment........................................................................................................................................28Figure 10:Design methodology flow diagram...............................................................................31Figure 11:Photo showing North Gate School................................................................................37Figure 12:Photo showing vegetation in the school environs.........................................................38Figure 13:Photo showing soils in the school.................................................................................38Figure 14:Overflowing manhole....................................................................................................39Figure 15:Photo showing waste water treatment at the school......................................................40Figure 16:Operational energy demand..........................................................................................53

LIST OF TABLES

Table 1:Increasing levels of treatment and acceptable levels of human exposure......................................23Table 2:Membrane types and uses.............................................................................................................46Table 3: Membrane type and aeration supply............................................................................................47Table 4:Oxygen requirement based on depth.............................................................................................48Table 5:Pipe selection by membrane type..................................................................................................48Table 6:Effluent characteristics of waste water in an MBR.......................................................................49Table 7:Operation and maintenance of MBR.............................................................................................50Table 8: Table of results summary............................................................................................................50

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ABREVIATIONS AND ACRONYMS

BOD: Biological Oxygen Demand

COD: Carbon Oxygen Demand

DO: Dissolved Oxygen

FS: Flat Sheet Membrane

HRT: Hydraulic Retention Time

MBR: Membrane Bioreactor

MLSS: Mixed Liquor Suspended Solids

SRT: Sludge Retention Time

TSS: Total Suspended Solids

WWTP: Waste Water Treatment Plant

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1 INTRODUCTION

1.1 BackgroundIncreased focus is being put on water reuse and recycling due to the increasing demand on freshwater water supply that is caused by increasing water usage, changes in climate, uneven distribution of water resources, water pollution and unsustainable management.

The increase in population growth is continually leading to strain on the available water resources. One of the ways used to measure water scarcity is per capita availability of fresh water within a nation’s boundary. The total number of people living in a country helps establish the per capita availability of freshwater resources in that country. A country is defined to be water scarce if annual water supply falls below 1000m3 per capita per year (World Water Assessment Programme, 2012). Kenya is classified as a water scarce country with less than 643 m3 per capita of renewable fresh water supplies (Momanyi, Quyen Le, 2005). Projection of population growth shows that water demand is increasing at twice the rate of the growth in population.

There is increasing requirement to use the natural resources sustainably in the present time so that they can also be reserved for the future use. The millennium development goals have been developed as a set target to achieve this. Millennium Development Goal 7 deals with environmental sustainability. One of the ways that has been outlined to realize this is through adequate treatment of wastewater that contributes to less pressure on freshwater resources, helping to protect human and environmental health.

Water recycling is the reusing of treated waste water. The term water recycling can commonly be used synonymously with the expressions water reclamation and water reuse. Through the hydrological cycle, the earth has the natural ability to recycle and reuse water, a process that has occurred for millions of years. Generally, water recycling refers to projects that utilize technology to speed up the natural processes.

Recycled water has the ability to meet most water demands but this is only so if it is adequately treated to ensure water quality appropriate for use. In cases where there is a high chance of human exposure to the recycled water, advanced level of treatment is mandatory. Recycled water is mostly used for non-potable i.e. for non-drinking purposes.

These uses include irrigation in agriculture, landscaping and toilet flushing, construction activities e.g. concrete mixing. In industries recycled waste water can be used on site for cooling machinery. A common type of recycled water is water that has been reclaimed from municipal waste water otherwise known as sewage.

Although most of the water recycling projects have been established to cater for the non-potable water requirements, some of the projects use recycled water in an indirect way for potable purposes. This indirect method involves the recharging of ground water aquifers and expanding surface water reservoirs with the recycled water. In this ground water recharge projects, the recycled water can be spread or inserted into ground water aquifers to enlarge ground water supplies and also to prevent salt water invasion in the coastal areas.

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1.2 Problem statementThere is need to reduce pressure on fresh water resources as water scarcity is a challenge especially in arid and semi-arid areas. North Gate School is located in Kagundo Constituency in Machakos County, Kenya. The area is classified as an arid and semi-arid area. The students board at the school therefore there is a high water demand and this demand will continue increasing with the planned expansion of the school. The design of the membrane bioreactor system for waste water treatment at the school is an effort to counter this problem by reducing the pressure on the fresh water sources. This will be achieved through adequate treatment of the wastewater in the system for reuse in flushing toilets, washing and cleaning activities.

1.3 Problem justificationKagundo Constituency receives an average annual rainfall of 958 mm which is unevenly distributed and unreliable (Republic of Kenya, 2015). North Gate School mainly relies on a borehole as the water source. The school also relies on rainfall water harvested in the bimodal rainfall season. With the limited supply of fresh water sources and the continued expansion of the school, there is need to recycle the waste water generated in the institution. Adequate treatment of the waste water is needed for reuse in flushing toilets and cleaning purposes.

The membrane bioreactor which combines the biological unit and membrane unit for filtration is ideal compared to the conventional activated sludge system for waste water treatment at the school as it will take up less space, easily accommodate room for expansion and water can be directly reused after leaving the system.

1.4 Site analysisNorth Gate School is located in Kagundo town, Machakos County in Kenya. It’s located 299555.00 m E - 299857 m E and 9856304.00 m S – 9856163.0 m S and 1.8 km off Kangundo road.

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Figure 1:Map showing North Gate School. Source: Google Earth

Figure 2: Map showing waste water treatment plant at NGS School. Source: Google Earth

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Figure3: Map showing NGS School in relation to transport network. Source: Google Earth

1.4.1 Climate and vegetationRainfall in this area is low and unreliable. The annual rainfall ranges between 400 mm and 800mm. The precipitation can be termed as bimodal with long rains falling in the March-May period and short rains between October and December.(Moore 1979; porter1965).

The area experiences semi-arid conditions characterized by high temperatures during the day and relatively low temperatures at night. average monthly maximum temperature varies between 22.20 C and 27.30C and the minimum temperature varies between 11.10C and 15.20. Humid conditions and seasonal rainfall are also characteristic of this climate.

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Figure 4: A graph of average temperature and rainfall at Machakos

The vegetation that characterizes this area comprises of short acacia trees on the plains with short grass together with other short and sturdy shrubs which are all drought resistant.

1.4.2 Geology and soilsThe dominant rock type is metamorphic. These rocks are comprised of

mica schist, gneisses granitoids

Igneous rocks are also found in this area and are comprised of pholonites and tuff minerals.

1.5 ObjectiveTo design a Membrane Bioreactor for waste water treatment at North Gate School.

1.5.1 Specific objectivesi. To establish the quantity of waste water generated in the school.

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ii. To establish the quantity and pollutant content of the wastewater discharged. iii. To design membrane bioreactor for design wastewater.

1.6 ScopeThe scope of this project is to design a membrane bioreactor system for waste water treatment at North Gate School at Koma Hill.

The purpose of this system is the reuse of the waste water generated in the school for cleaning, washing and flashing toilets. The design will cover modification of the components of the currently existing conventional activated sludge system for high effluent quality water.

The following areas will not be covered in the design;

1. Biogas production for energy requirements of the system.2. Pretreatment of the black waste water before being injected in the system.3. Piping and storage system of the treated water.

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2 LITERATURE REVIEW2.1 Sources of waste waterWaste water is generated from various uses of water in agricultural, domestic and industrial applications. The waste water is generated through;

a) Agricultural

Slaughter houses

Animal waste

Nutrient runoff

Sediment runoff etc.

b) Domestic

Residential homes

Schools

Hotels/ restaurants

Offices etc.

The percentage use of water for domestic purposes is illustrated by the pie chart below.

Figure 5: A pie chart showing domestic water use

c) Industrial

Manufacturing and processing industries e.g sugar, breweries, tanneries, pulp and paper etc.

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2.2 Classification of waste water2.2.1 Black waterIt consists of faecal matter.

2.2.2 Grey waterIt consists of waste water from laundry, kitchen, cleaning and showers.

2.2.3 Brown waterIt consists of deposits in water e.g. soil.

2.3 Waste water quality indicatorsWaste water consists of organic impurities (plant, animal, human waste) and inorganic impurities such as metals. There are biological, physical and chemical quality indicators as listed;

2.3.1 Biological quality indicators COD BOD5 DO TVS VSS

2.3.2 Physical TSS TDS Temperature DO

2.3.3 Chemical Nitrogen concentration Phosphate concentration Arsenic Sulphates

2.4 Waste water treatment systemsWastewater treatment is done using different methods and they are;

Physical method:  pollutants and contaminants are removed by use of physical forces. Chemical method: in this method impurities and toxic matter are removed through

chemical reactions. Biological method: Pollutants in the waste water are removed through biological

activities.

Biological method is the most common method used for waste water treatment but also incorporates the physical and chemical methods.

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2.5 Overview of the biological waste water treatmentThe biological treatment of wastewater can be dated back to the late 19th Century. By the 1930s, it became established as a standard method of wastewater treatment (Rittmann,1987). Aerobic (presence of oxygen) and anaerobic (lack of oxygen) biological treatment methods are used for the treatment.

2.5.1 Biological Aerobic TreatmentThis is a wastewater treatment where the biological process is carried out in the presence of oxygen. Compared to the anaerobic treatment, it is more efficient and rapid treatment and there is up to 98% removal of organic contaminants. There is effective breakdown of the organic pollutants present in the wastewater and a cleaner water effluent is obtained in comparison to that in anaerobic treatment. The aerobic biological treatment processes comprise;

a. activated sludge processb. trickling filterc. aerated lagoonsd. oxidation ponds

The most extensively used method among the above listed processes is the activated sludge process for both the domestic and industrial wastewater treatment.

a. Activated Sludge Process

It comprises of a multi-chamber reactor unit which uses a high concentration of microorganisms. These microorganisms breakdown organics in the wastewater and remove the nutrients which produces a high-quality effluent at the end of the process. In this method, the wastewater that contains the organic matter together with the microorganisms is aerated by use of a mechanical aerator. This is done in the aeration tank. This process is important in that it speeds up the waste decomposition. Aeration is done by pumping air into a tank. This stimulates the growth of microbial in the wastewater. The discharge from the aeration tank contains the flocculent microbial mass which is known as sludge. The sludge is separated in another tank known as the settling tank that is also sometimes referred to as a secondary settler or a clarifier.

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Figure 6: A flow diagram showing activated sludge process

b. Trickling filters: 

This is the second most commonly used method of biological aerobic treatment. The method is also referred to as percolating/bio filter or sprinkling filters. The filters are the ones used for the removal of compounds like ammonia from the wastewater after it has undergone the primary treatment. Trickling filters are more easily operated and have lower energyrequirements than the activated sludge processes. However, they have a lower removal efficiency of the solids and the organicmatter. They are also more sensitive to low air temperatures, and they can become infested with flies and mosquitoes (UNEP et al. 2004).

c. Aerated Lagoons

This is another method under the aerobic biological wastewater treatment. In this method, an aerated lagoon which is a treatment pond, is provided with motorized aeration whereby oxygen is introduced in the pond so as to stimulate the biological oxidation of the wastewater. The increased mixing and aeration obtained from the mechanical units makes the ponds more tolerant to higher organic loading than the maturation/a facultativepond. The mechanical aeration increases the efficiency of the treatment as there is reduction of the required hydraulicretentiontime(HRT) for aerobic degradation of organics (ROSE 1999). There is also an increase in the removal of pathogens (CURTIS et al. 1992). The discharge from the aerated ponds can be reused or also used for recharge. The settled sludge however requires a further treatment.

d. Oxidation Pond

In this process, there is interaction in the ponds between bacteria, algae and other organisms that feed on the organic matter which has been obtained from primary effluent. The ponds also

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generate effluent which can be used in other applications. Generally, the process is slow and it needs large areas of land. The oxidation ponds are mainly used in places that have small populations and where land is available easily.

2.5.2 Biological Anaerobic TreatmentThis process is effectively used for treatment of high strength waste water.

Organisms that operate in the absence of oxygen are used and the waste water is treated to a level that will allow the discharge to a municipal sewer system. The amount of sludge that is produced in this process is very small when it’s compared to the aerobic treatment. This treatment is a slow process which occurs in numerous different stages. The system is used in the treatment plants for the degradation and stabilization of the sludge. After the process has been completed, the wastewater undergoes many other additional treatments. This process is acknowledged because of its ability to stabilize the water together with little production of biomass. Biogas is produced in the process of bacteria feeding on the biodegradable material during the anaerobic process. In general, this process converts about 40% - 60% of the dissolved organic solids to methane and carbon (iv) oxide.

In the biological process (both aerobic and anaerobic), the biomass formed has to be separated from the liquid stream to obtain the required effluent quality. A secondary settling tank is therefore used for the solid/liquid separation and this clarification is often the restrictive factor in effluent quality of biological units (Benefield and Randall, 1980). Further clarification is needed i.e. tertiary treatment of the wastewater.

2.5.3 Process of biological waste water treatment

1. Preliminary Treatment of Wastewater 

This is the first step in treatment of wastewater and the aim is to remove the large debris, the coarse solids and heavy inorganic material that is contained in the wastewater. This step comprises of physical operations that include:

Screening- in this stage there is removal of heavy solids logged in wastewater. Such materials include paper, plastics, metals etc. and screening helps to avoid damage and clogging of treatment plant equipment at the later stages.

Floatation- It is used for removal of suspended solid particles found in the waste water. Grit removal- Grit chambers are utilized for slowing down of the flow of waste water.

The purpose for this is to settle out solids like sand, ash, eggshells etc. from the water and then the solids are removed manually or mechanically.

2. Primary Treatment of Wastewater

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After the preliminary treatment, the waste water is channeled to primary treatment operations. At this stage the objective is to remove the settleable organic and inorganic solids through sedimentation and also through the removal of materials that can float (scum) by skimming. It incorporates the physical and /or chemical procedures for treatment of wastewater. A sedimentation tank/ primary clarifier is used for removal of many of the suspended solids that can float or settle. Some of the chemicals used in sedimentation include flocculants and coagulants. Sludge that settles at this stage to the bottom of the sedimentation tank is called the primary sludge. It is then collected to undergo further treatment (sludge treatment). In the primary treatment, about 50-70% of the suspended solids and 35% of BOD will be reduced. Very few toxic chemicals are removed at this stage.

Secondary Treatment of Wastewater

The effluent from the primary treatment is channeled for secondary treatment. At this stage the treatment consists of a biological process. The further treatment of the discharge from primary treatment is to remove the remaining organics and suspended solids in the wastewater(Nazaroff & Alvarez-Cohen, 2011). The biological treatment process in the secondary treatment is aerobic and anaerobic as discussed in the section above. Secondary treatment involves biological treatment processes namely activated sludge process, oxidation pond, bio filters, aerated lagoon as also discussed. The primary discharge enters the aeration tank and air is mixed with the sludge. Many microorganisms aid in removal of the biodegradable organic matter. A secondary sedimentation tank lets the microorganisms and the solid wastes to form clusters and settle. In this treatment, 80-90% of all the impurities are removed and also a large percentage of toxic chemicals are removed(Namiiro, 2012).

Tertiary/Advanced Treatment of Wastewater: 

This is the final treatment stage for the wastewater. The main purpose is the removal of particular wastewater constituents that cannot be removed in the previous steps and therefore increasing the quality of the discharge to higher level. Filtration is utilized to remove higher degree of the suspended solids which was not possible through primary and secondary screening and sedimentation. Some of the contaminants removed in this process are heavy metals, particular toxic chemicals and other pollutants. It has the capability to remove more than 99% of all the contaminants from sewage thus producing an effluent equivalent to an almost drinking water quality. It involves disinfection which can be achieved by means of physical sterilizers like UV light and chemical disinfectants such as chlorine(Form, Effluent, & Licence, 2006).

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Table 1:Increasing levels of treatment and acceptable levels of human exposure

Increasing Levels of Treatment; Increasing Acceptable Levels of Human Exposure

PrimaryTreatment: Sedimentation

Secondary Treatment: Biological Oxidation, Disinfection

Tertiary / Advanced Treatment: Chemical Coagulation, Filtration, Disinfection

 

No uses Recommended at this level

Surface irrigation of orchards and vineyards

Non-food crop irrigation

Restricted landscapeimpoundments

Groundwater recharge of non-potable aquifer**

Wetlands, wildlife habitat, stream augmentation** 

Industrial cooling processes**

Landscape and golf course irrigation

Toilet flushing

Vehicle washing

Food crop irrigation

Unrestricted recreational impoundment

Indirect potable reuse: Groundwater recharge of potable aquifer and surface water reservoir augmentation**

* Suggested uses are based on Guidelines for Water Reuse, developed by U.S. EPA.** Recommended level of treatment is site-specific.

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2.6 Membrane Bioreactor (MBR)The idea for combination of the activated sludge process and membrane separation was first reported by research conducted at Rensselaer Polytechnic Institute, Troy, New York, and Dorr-Oliver, Inc. Milford, Connecticut, US (Jyoti et al., 2013). Before the 1990s, majority of the installed MBRs were utilized for industrial water treatment. Submerged membranes development that was firstly introduced by Yamamoto et al., an increase in the number of MBRs for the treatment of municipal wastewater has been experienced. The MBR market is currently undergoing accelerated growth. Membrane bioreactor technology has been recognized in recent years for the treatment of various types of wastewaters whereas the conventional activated sludge (CAS) process cannot manage the high quality effluents because of poor sludge settleability unless tertiary treatment is added(Scott, 2012).

2.6.1 Membrane bioreactor treatment processPreliminary treatment

Just like in the conventional waste water treatment units, the pretreatment of waste water in an MBR system is necessary to remove large particles, grease etc. through grit removal and screening.

A 2mm automated perforated plate spiral screen is fitted to get rid of all the inorganic material from the wastewater(BusseGT, 2011). The material that has been screened is disposed in an offsite disposal unit. The screened influent gravity flows into a screened influent lift station through the perforated screen. This 2mm aperture screen is sized to meet the peak instantaneous flow requirements and provide the required fine screening necessary to protect the downstream membranes.

Equalization tank

The equalization tank provides a shield to manage peak inflows into the other tanks of the wastewater treatment process. It is designed to absorb the peak flows during the peak periods. The tank is fitted with an external mixer to prevent the settlement of solids and to minimize the chances of the tank contents from becoming septic(Dumbrell, 1998).

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Anoxic tank

The anoxic tank receives screened influent at a steady flow rate. Denitrification occurs at this stage. In the absence of oxygen gas, organic materials in the waste water causes the nitrate reduction of nitrate ions to molecular nitrogen which is released to the atmosphere as gas. The anoxic tank provides a region of low dissolved oxygen (0-0.5mg/L) for denitrification (i.e. nitrogen removal) of the wastewater. The tank gets the nitrate abundant recycled mixed liquor from the aerobic or the membrane tank for this process. The nitrate rich mixed liquor return mixes with the organic substrate (BOD) which provides the electron donor for the oxidation reduction reaction. The reduction reaction steps involve nitrate to nitrite, nitric to nitrous oxide and to nitrogen gas which is released to the atmosphere. The anoxic tank is fitted with a submersible propeller mixer to keep the tank contents completely mixed and the solids in suspension. The mixed liquor in the anoxic tank gravity overflows into the Aerobic/MBR tank for biological oxidation(Milano & Zaerpour, 2014).

C10H1903N + 10NO3- 5N2 + 10CO2+ 3H20 + NH3 + 10OH-

A mixer is fitted to efficiently mix the waste water with the mixed liquor returning from the membrane filtration tank. High levels of nitrogen in the water poses major environmental concerns like algae growth and health conditions in humans hence the need for the denitrification of wastewater.

Aeration tank

The aerobic tank receives a steady flow through gravity of mixed liquor from the anoxic tank. Oxygen is delivered in the tank by use of air blowers in a diffused aeration system. The aerobic tank provides for the complete biological nitrification and also the BOD removal from the wastewater. The nitrification process is a two-step biological process. In the first part of the process, ammonia (NH4-N) is oxidised to nitrite (NO2-N). The nitrite formed is then oxidised to nitrate (NO3-N). Fine bubble air diffusers are fitted in this tank and they supply oxygen to the wastewater. The anaerobic and membrane tank contain MLSS concentration that operates at high concentrations of 10,000 mg/L and the mixed liquor flows through to the MBR stage for filtration. Mixed water is then pumped in the membrane tank.

Membrane filtration tank

The Membrane Bio-Reactor (MBR) stage obtains mixed liquor from the aerobic stage. This stage comprises of the required number membranes (submerged flat sheets/hollow fibers) for ultrafiltration of activated sludge. The design mixed liquor suspended solids in this tank is 10,000mg/L as mentioned above. The tanks are fitted with coarse bubble diffusers that provide air scrubbing of the membrane surface to avoid fouling. On the permeate (water that passes through the membrane) side, a vacuum suction pressure is fitted to draw the clear water past the membrane. This process provides for separation of the solids that remain in the tank. The trans membrane pressure (TMP) and permeate flow has to be monitored in order to establish the permeability of the membrane.

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Permeate collection tank

The permeate water is conveyed to the permeate tank where it is then distributed to the storage tank for reuse.

Sludge Tank

Sludge is occasionally wasted from the aerobic and membrane tanks to conserve the design MLSS(Ching, 2010). The sludge produced is then transferred to a sludge tank for anaerobic digestion after which it can be converted to biogas(optional).

Figure 7:Schematic diagram of a Membrane Bioreactor.

Types of membranes

Membranes are made from polymer/ inorganic materials. They are composed of small pores on their surface that can only be viewed using a microscope. Due to their tiny size, the pores only permit water on the other side of the membrane. This water is called the permeate. The size of the pores enables the classification of the membranes(Yang, 2013).

Filtration Class Particle Capture

Contaminants removed Operating pressure ranges

Microfiltration (MF)

0.1-10 Suspended solids, bacteria, protozoa 0.1-2 bar

Ultrafiltration (UF)

0.003-0.1 Colloids, proteins, polysaccharides, most bacteria, viruses (partially)

1-5 bar(cross flow)0.2-0.3 bar (dead-end and submerged)

Nanofiltration (NF)

0.001 Viruses, natural organic matter, multivalent ions (including hardness in water)

5-20 bar

26

Reverse Osmosis (RO)

0.0001 Almost all impurities including monovalent ions

10-100 bar

The membrane configurations that are utilized in MBR’s are hollow fiber, flat sheet and tubular membrane. Hollow fiber and flat sheet are submerged in the mixed liquor and the permeate drawn through the use of vacuum pumps. (Radjenivic et al. 2008). Tubular membranes are usually placed outside the bioreactor.

Submerged and side stream membranes

There are two main designs for MBR plants, the submerged and side stream. In the submerged design, the membrane is submerged directly in the bioreactor e.g. the hollow and flat membranes. In the side stream, the membrane is submerged in a side tank and wastewater constantly recirculated(Visvanathan et al., 2010). The advantage of the submerged membrane design is smaller footprint of the plant and lower capital cost due to lower energy requirements as compared to the side stream membrane bioreactor design.

Figure 8:Submerged and side stream membranes

The first is a submerged configuration with the membrane module immersed in the activated sludge (Fig. 1a). A suction force is applied to draw the water through the membrane, while the sludge is retained on the membrane surface.

27

The second is a recirculated configuration with an external membrane unit (Fig. 1b). Mixed liquor is circulated outside of the reactor to the membrane module, where pressure drives the separation of water from the sludge. The concentrated sludge is then recycled back into the reactor. diffuses compressed air within the reactor, providing oxygen to maintain aerobic conditions. The air bubbles also function to scour the membrane surface and clean the exterior of the membrane as they rise in the reactor.

The submerged configuration is more commonly used than the recirculated configuration because it is less energy-intensive and provides a cleaning mechanism to reduce membrane fouling(Sharrer, Tal, Ferrier, Hankins, & Summerfelt, 2007).

2.7 Membrane bioreactor versus the conventional methods of wastewater treatment

The complete solids removal, a high disinfection capability, a high rate and high efficiency of organic removal and small footprint are collective characteristics regardless the type of wastewater to be treated or the type of commercial process used. (Stephenson et al., 2001). MBR technology is also used in situations where the need on the quality of effluent surpasses the capability of CAS(Elokdi, 2007). Lack of space prevents the addition of new treatment units. The improvement from CAS to MBR can be essential when it comes advance treatment performances. Along with better understanding of emerging contaminants in wastewater, their biodegradability, and with their inclusion in new regulations, MBR may become a necessary upgrade of existing technology in order to fulfill the legal requirements in wastewater treatment plants(Rousseau & Hooijmans, 2007).

28

Figure 9:Flow diagram of Membrane Bioreactor and conventional biological waste water treatment.

29

3 THEORETICAL FRAMEWORKThe quantity of domestic waste water produced is 70-80% of design water flow rate. The design water flow rate in Kenya ranges from 50 – 250 liters/person/day.

Black water (water containing faecal matter) is approximately 30% of domestic waste water produced(Soomaree, Ragen, & Mudhoo, 2015)

3.1 The components of a biological waste water treatment systemItcomprises of;

3.1.1 Waste water sourceThe waste water source is an important parameter so as to connect the waste water generated to the treatment unit. It is also important to know the biological parameters of the waste water depending on its waste water source. This aids in choosing the most effective system of treatment depending on the effluent qualities.

For example, industrial waste has high organic loading and requires biological anaerobic treatment system.

3.1.2 Waste water collectionWaste water needs to be collected to a central location for mass treatment. Collection systems include onsite, offsite or partially onsite. Onsite (DEWATS) include toilets for black water treatment and septic tanks. Offsite treatment involves municipal waste water treatment. Partially onsite system involves the use of exhaust trucks to collect waste water at an onsite system.

3.1.3 Waste water conveyance systemOnce the source and the collection point have been established, a system to transport the waste water from the source to the collection point is necessitated. It includes use of pipes for conveyance. Sewer pipes carry contaminated water which is corrosive if it settles in the pipes. The velocity in the pipes should therefore be self-cleansing in order to avoid the deposition of solids. Transport of waste water under gravity is recommended other than flow through pressure. Gravity flow also minimizes leakage. The flow in the pipes is designed to run partially full at the maximum discharge to ensure no-pressure flow.

3.1.4 Waste water treatmentThis encompasses the primary, secondary and tertiary treatment of the waste water in various tanks as discussed in detail in section 2.1.3.

3.1.5 Storage systemTreated water to be used for recycling is stored temporarily in a storage tank for uses such as irrigation, landscaping etc.

3.2 Design considerations when designing MBR1. Pre-clarification through screening of solids should be used to reduce the aeration rate

requirement hence lower operational costs.2. The level of DO is 0.5 mg in anoxic tank for complete denitrification and 1.0- 2.5 mg/L

in the aerobic tank for complete nitrification to occur.

30

3. The MLSS concentration in the MBR should range from 4,000 mg/L – 10,000 mg/L.4. The design HRT should be between 2-16 hours.5. The design SRT should be between 10-25 days.6. The tank should have a minimum of 0.5 m freeboard at the maximum liquid level7. The minimum oxygen requirement should be 0.6 kg / kg of influent BOD5 and 2 kg/ kg

of influent NH3-N to the MBR.8. The MBR should be designed for an average daily net flux of not more than 57 L/day/m2

of membrane area.9. Air scouring should be at least 0.2 – 1.1 L/min of air / 0.09 m2 of membrane area.

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4 METHODOLOGY4.1 Generation of the design concept

32

Problem identification

Assessment of alternatives

Data collection

Design of system components

Site analysis

Quantification of waste water used

Quality determination of grey water through lab testing

Design of equalization tank, anoxic tank, aeration tank, membrane tank

Figure 10:Design methodology flow diagram.

4.2 Design parameters of membrane bioreactor components4.2.1 Equalization tankWater usage is higher during certain times and it is important to absorb the peak flows during the peak periods.

Peak Factor

PF = 1 + 14

4+√P

Where P is design contributing population in thousands

Peak hourly flow = PF∗Q

24

Peak daily flow = Peak hourly flow * 24

Volume of tank = L*W*D

3W2 = Q

Where Q = Peak hourly flow

4.2.2 Anoxic Tank volumeUsing De-Nitrification Rate:

V = TNc∗Q

[ MLSS∗RR+1

∗DNR]

Where;

V = Required volume of aeration tank.

TNc. = Influent T-N concentration

Q = Influent flowrate

MLSS: Mixed Liquor Suspended Solids concentration in aeration tank

33

AutoCAD drawings

DNR = De-nitrification Rate

R = Recirculation rate

4.2.3 Aeration Tank volumeUsing BOD Loading:

V = BODc∗Q

BODL

Where;

V = Required volume of aeration tank

BODc = Influent BOD concentration

Q = Influent flowrate

BODL = BOD loading

Using Nitrification Rate:

V = TNc∗Q

[ MLSS∗RR+1

∗NR ]

Where;

V = Required volume of aeration tank.

TNc = Influent T-N concentration

Q = Influent flowrate.

MLSS = Mixed Liquor Suspended Solids concentration in aeration tank

NR = Nitrification Rate.

4.2.4 Membrane tank volumeV = Aeration tank volume – Pre-aeration tank volume

Recycle ratio

34

RR =QR Membrane−Q NET

QNET

= QRQNET - 1

Where,

RR = Recycle Ratio

QNET = Net Permeate Rate

4.2.5 Hydraulic Retention TimeThe hydraulic retention time (HRT) is calculated using the formula:

HRT = V

QNET∗60

Where;

HRT = Hydraulic retention time (hours)

V = MBR volume (L)

Qnet = Ton−Toff

Qp

Where Ton = Time MBR is in production

Toff = Time MBR is in relaxation

Qp = Permeate flow rate

4.2.6 Sludge retention time

SRT = V∗XR

QW∗XW

Assuming that XR is equal to XW:

35

SRT = V

QW

Where;

SRT = sludge retention time (days)

XR = volatile suspended solids in the reactor (mg/L)

XW = volatile suspended solids in the waste stream (mg/L)

QW = waste stream flow rate (m3/day)

Or;

Design SRT

= FS * TSRT

Where; FS = TN peak

TN average

= 1.5

TRST = Theoretical SRT

= 1µ

Where µ is specific nitrification rate

Membrane Surface Area

A = Q Inlet

J

Where;

A = Membrane area (m2)

J = Membrane flux (L/day/m2)

Where;

J = Membrane flux (L/day/m2)

A = Total membrane surface area (m2)

4.2.7 Calculating Required Oxygen(Qoxy)= (Qair (l/min)) x (ξ) x (a) x 0.227 Kg O2/m3

36

Where;

ξ is oxygen dissolving efficiency

a is alpha factor

Specific air demand

SAD = QaAm

Where;

Qa = Membrane aeration rate (m3/hr)

Am = Total membrane surface area (m2)

37

5 RESULTS AND DISCUSSION5.1 Site analysisNorth Gate School is located in Kagundo town, Machakos County in Kenya. It is located approximately 70 km from Nairobi, the capital city of Kenya. It is located 1.8 Km from Kangundo road at Nimrod bus station.

Figure 11:Photo showing North Gate School.

5.1.1 Land cover

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The vegetation is comprised of grass and short shrubs indicates the area is an arid and semiarid area.

Figure 12:Photo showing vegetation in the school environs.

5.1.2 Geology and soilsThe soils found in the school and surrounding area is black cotton soil which is classified as an igneous rock.

Figure 13:Photo showing soils in the school.

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5.1.3 PopulationThe school has a capacity of 255. The students are 225 in number while the staff who reside in the school compound are 30.

State of waste water treatment at the school

The waste water treatment system has been experiencing overflows in recent times since the expansion of the school from a population of 145 to 255 in 2017.

Figure 14:Overflowing manhole

The activated sludge system occupies a surface area of 96 km2. Currently, water is chemically treated and pumped after a few days to the farm due to the overflows but this is a temporal solution to the overflows. The water was designed to undergo primary and secondary treatment but due to no maintenance, the water in all compartments has mixed as shown in the diagram below. The water contains ordour which causes air pollution. Black water is first passed through a septic tank before being pumped into the system after 14 days.

40

Figure 15:Photo showing waste water treatment at the school.

5.2 Quantity of water supplied dailyOn average, 20,000 liters of water is pumped from the school’s borehole every day.

5.2.1 Average volume of water used per person per day

= Totalvolume supplied

Population

= 20,000

255

= 78.43 L/person/day

5.2.2 Quantification of waste water produced.Black water

Black water (water containing faecal matter) is approximately 30% of domestic waste water produced(Soomaree, Ragen, & Mudhoo, 2015)

41

=30

100∗20,000

= 6,000 L/day

Grey water

Grey water is approximately 50% of domestic waste water produced ( Mayer et al. 1999).

= 50100 * 20,000

= 10,000 L/day

Total waste water produced

= Black water + grey water

= 6,000 L/day + 10,000 L/day

= 16,000 L/day

= 16 m3/day

5.3 Quality determination of waste waterSecondary data was used to determine the waste water constituents and their concentrations.

Weak Medium Strong

TS 350 700 1200

TDS 250 500 850

TSS 100 200 350

N 20 40 85

P 6 10 20

Cl 30 50 100

Alkalinity 50 100 200

Grease 50 100 150

BOD 100 200 300

COD 250 430 800

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5.4 Design parameters of MBR5.4.1 Equalization tank volume

PF = 1 + 14

4+√P

P = Design contributing population in thousands

Population of the school = 255

P = 255

1000

= 0.255

Peak Factor

PF = 1 + 14

4+√P

= 1 + 1 + 14

4+√0.255

= 4.5

~ 4

Peak hourly flow = PF∗Q

24

Where Q = greywater produced per day

= 16 m3

Using a design flow twice of the current Q to cater for increased waste water generated,

Peak hourly flow = 4∗32

24

= 5.33

4W2 = Q (Peak hourly flow)

= 5.33

Width (W) = 1.2 m

Length to width ratio, L: W = 3:1

Length (L) = 3.5 m

Selected height = 2.0 m

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5.4.2 Anoxic tank volume

V = TNc∗Q

[ MLSS∗RR+1

∗DNR]

TN Concentration in the waste water from table 4. = 20 mg/L

Using a design Q twice the current waste water produced = 16 m3 * 3

= 48 m3

V = 20∗32

[ 5000∗55+1

∗0.02]

= 8 m3

Taking depth as 2 m + freeboard 0.5 m

Depth = 2.5 m

L: W = 3

Length (L) = 3 m

Width (W) = 1.3 m

New computed volume with the freeboard,

V = (3 * 1.3 * 2.5) m3

= 9.75 m3

5.4.3 Aeration tank volume

V = TNc∗Q

[ MLSS∗RR+1

∗NR ]

TN = 20 mg/L

Q = 32 m3

MLSS = 5,000 mg/L

R = 5

NR = 0.02

44

V = 20∗32

[ 5000∗55+1

∗0.02]

= 8 m3

Volume of aeration tank = Volume of anoxic tank

New computed volume with the freeboard,

V = (3 * 1.3 * 2.5) m3

= 9.75 m3

Checking the volume for future design using various parameters;

Case 1

With Q = 32 m3/day

MLSS = 5,000 mg/L

R= 3

NR = 0.02

V = TNc∗Q

[ MLSS∗RR+1

∗NR ]

Volume of aeration/anoxic tank will be; V = 20∗32

[ 5,000∗33+1

∗0.02]

= 8.5 m3

Case 2

Using the design volume 8m3 and finding Q when

MLSS = 10,000 mg/L

R = 5

NR = 0.02

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V = TNc∗Q

[ MLSS∗RR+1

∗NR ]

8 = 20∗Q

[ 10,000∗55+1

∗0.02]

Q = 66.7 m3/ day

These calculations show that the design volume can handle up to 4 times its current design waste water flow rate.

5.4.4 Membrane tankThe size is dependent on the membrane type that is going to be usd

Selection of membrane

Table 2:Membrane types and uses

Type Uses Example of applicationA Designed to be placed in

deep waterNight soil treatment facilities Combined type

E Standard type Combined typeWastewater reuse facilityIndustrial wastewater

F Designed to be placed in shallow water

Waste water reuse facility

Number of cartridges (panels)

Number of cartridges required = Q/S/F

Where;

Q= Average daily flowrate (m3/day)

S= Effective area of membrane cartridge (m2) (0.8 m2)

F = Design flux rate for average daily flowrate (m3/m2/d)

Taking F = 0.5

Number of cartridges = 32/0.8/0.5

= 70

46

Selection of units

Table 3: Membrane type and aeration supply

Type

Height (mm)

Application

Aeration supply (L/min/cartridge)

Membrane case + Diffuser case

Min. Max. Remarks

EK 2500+1000 Deep water (>=4.2m)

7 10 Double deck unit

AS 1000+1500 Deep water 10 15 Single deck unitS 1000+1000 As standard 10 15 Single deck unit FS 1000+500 Shallow

water12.5 20 Single deck unit

FS 75 is selected (75 cartridges and is fitted in shallow water)

The height from the table = 1.5 m

The width = 1.5 m

The surface area = 1.5 m * 1.5 m

= 2.25 m2

If Q doubles to 64 m3

The number of cartridges = 64/0.8/0.5

= 160

FS 200 would be selected

The height = 1.5m

The width = 3 m

Designing the membrane tank for an upgrade in the type of membrane in the event of tripling of the waste water from the current waste demand, then the length is selected as 3 m.

The width = 1.3 m

The depth = 2.5 m

47

5.4.5 Calculating required oxygen(Qoxy)= (Qair (l/min)) x (ξ) x (a) x 0.227 Kg O2/m3

Table 4:Oxygen requirement based on depth

Water depth 1 2 3 4 5

Oxygen dissolving efficiency (%) ξ 2 3.5 5 6 7

MLSS (%) 0 0.5 1 1.5 2

Alpha factor (a) 1.0 0.95 0.85 0.75 0.6

With a 1 FS 75 membrane at a depth of approximately 2 m,

Q(oxy) = (1*75*10L/min) * 3.5 % * 0.95 *0.227 kg O2/m3

= 5.66 kg/day of 02

5.4.6 Calculating pipe sizesThis depends on the membrane unit chosen for the permeate and diffuser port as shown in the table below.

Table 5:Pipe selection by membrane type

Membrane Unit Permeate Port Diffuser port

FS50 ND40 ND40

ES75,FS75,AS100,ES100 ND50 ND50

AS125,ES125,AS150 ND50 ND75

ES200 ND65 ND75

EK300 ND50*2 ND75

EK400 ND65*2 ND75

For FS 75, the pipes selected are;

ND50- Permeate port

ND40- Diffuser port

48

5.4.7 Hydraulic Retention TimeHRT = 3 hours + 8 hours (equalization tank)

5.4.8 Sludge retention timeNitrification rate is dependent on sludge retention time

Theoretical SRT = 1µ

= 10.32

Where µ is specific nitrification rate

Design SRT

= FS * TSRT

FS = TN peak

TN average

= 1.5

Therefore; SRT = 1.5* 10.3

= 15 days

5.5 Effluent qualityThe MBR system produces high quality effluent. Efficiently treated water has the following characteristics.

Table 6:Effluent characteristics of waste water in an MBR

Parameter Unit Value

BOD mg/L <2

TSS mg/L <1

Ammonical Nitrogen as NH3-N

mg/L <0.5

Nitrogen as TKN mg/L <1

Faecal Coliform Count MPN/100ml <2

PH 6.8-7.8

49

5.6 Calculating MBR Efficiency5.6.1 BOD removal

Efficiency = BOD inffluent−BOD effluent

BOD influent∗100 %

=100−2

100∗100 %

= 98 %

5.6.2 Nitrogen removal

Efficiency = TKN inffluent−TKN effluent

TKN influent∗100 %

= 20−2

20∗100 %

= 90%

5.6.3 TSS removal

Efficiency = TSS inffluent−TSS effluent

TSS inffluent∗100 %

=100−1

100∗100 %

= 99 %

5.7 Operation and maintenanceAfter installation of the MBR, it is important to properly maintain the system so as to obtain the required effluent and also to prolong the life of the Bioreactor and its components.

Table 7:Operation and maintenance of MBR

Frequency Maintenance required

Daily None except where there is need for sludge removal

Weekly Sampling of MLSS

Fortnightly/monthly Visual checking of the final effluentChecking on the collection at the screeningChecking of the production of MLSS and sludge

Six-monthly Chemical cleaning of the membrane unit

Annually Draining the membrane tank to visually check on the membrane unit.

50

Every 5 years Inspecting membrane unit panels for wear and tear due to fouling.Cleaning and replacing the membrane unit if necessary.

5.8 Summary of resultsTable 8: Table of results summary

Table of results summary

Quantity of grey water (m3/day)

Design water supply Water pumped from borehole 20,000 L/dayBlack water 30% of design water supply

=30

100∗20,000 6,000 L

Grey water 50 % of design water supply

=50

100∗20,000

10,000 L

Fine screen < 10mm

Equalization tank

Peak Factor (PF) Harmon’s equation PF = 1 + 14

4+√P

P=225

4

Peak Hourly flow = PF∗Q

24 5.33 m3/day

Volume L* W*D = 11.4 m3

Depth, D 2.5 m

Width, W 3W2 = 5.33 1.3 m

Length, L L:W = 3:1 3.5 m

Detention time 8 hrs

Anoxic Tank

51

Volume V =

TNc∗Q

[ MLSS∗RR+1

∗DNR]

8 m3

Freeboard 0.5m

Depth, d 2.0 m

Length, L 3.0 m

Width, W L:W = 3:1 1.3 m

Aerobic Tank

VolumeV =

TNc∗Q

[ MLSS∗RR+1

∗NR ]

8 m3

Freeboard 0.5 m

Depth, d 2.0 m

Length, L 3.0 m

Width, W L:W = 3:1 1.3 m

Membrane tank

Membrane selected FS

Number of catridges = Q/S/F 70

Membrane Height FS 75 1.5 m

Membrane Width 1.5 m

Membrane tank volume 8 m3

52

Freeboard 0.5 m

Depth, d 2.0 m

Length, L 3.0 m

Width, W L:W = 3:1 1.3 m

Total MBR Volume Anoxic tank volume + Membrane Tank Volume

= 29.25 m3

Freeboard 0.5 m

Depth, d 2.0 m

Length, L 12.5 m

Width L:W = 3:1 1.3 m

MBR efficiency

BOD Removal = BOD inffluent−BOD effluent

BOD influent∗100 %

98%

Nitrogen Removal ¿TKN inffluent−TKN effluentTKN influent

∗100%90 %

TSS Removal = TSS inffluent−TSS effluent

TSS inffluent∗100 %

99%

5.9 Cost benefit analysis5.9.1 Capital costThe cost of MBR systems has been high as compared to the conventional systems but this is slowly changing with the increased application of MBR for waste water treatment and recycling. The costs of membranes have significantly reduced with time and also there are more competitors in the production field. In some cases, the capital cost of MBR systems can be lower as MBR’s have a lower land requirement compared to conventional methods.

53

5.9.2 Operational costsThe operational costs are a bit higher compared to conventional systems because of higher energy costs required to counter membrane fouling. The air needed is twice the amount needed to maintain aeration in the conventional activated sludge system. However, the sludge retention time is longer than in the conventional system hence the operation costs can be partially offset.

Figure 16:Operational energy demand.

The figure above shows the operational energy demand of an MBR.

The primary energy demand is energy for aeration. Pumping is the secondary energy demand. For one to get the most cost effective and energy sufficient system, one has to keenly look into the opportunities that are related to design, operations and equipment.

5.9.3 Energy demand and power requirementProcesses in the MBR that contribute to energy costs are aeration, sludge transfer and permeate production. For aeration, 0.025 kWh/ m3 of air is consumed. For pumping, 0.016 kWh/m3 is consumed for internal recirculation, membrane recirculation and sludge pumping. Power requirement for mixer is 8 W/m3(Verrecht et. al. 2008)

On average specific energy requirement in an MBR is 0.6 – 1.2 kWh/m3 of the MBR.

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5.10 Fixed cost of the MBR

No. DESCRIPTION QUANTITIES PERIOD UNIT (Ksh.)

TOTAL COST (Ksh)

1. FS 75 membrane 1 1,167,700 1,167,700

2. Air blower 3 5,000 15,000

3. Labour 10 600 6,000

4. Pump 2 4,000 8,000

5. Equipment installation

200,000

6. Fine Screen 1 35,000

6 Miscellaneous 200,000

1,631,200

5.11 Operational cost

Power requirement

Specific energy consumption = 0.6-1.2 kWh/m3 MBR Volume

MBR Volume (minus equalization tank) = L*W*D

= 9 m * 1.3 m *2.5 m

= 29.25 m3

If;

0.6 kWh = 1m3

P = 0.6 * 29.25

= 12.75 kWh

On average, 1 kWh is 20 Ksh

Cost of 12.75 kWh = 12.95 kWh * 20

= Ksh 255

Hours the MBR is in operation = 9 hours

55

Cost of power/day = 9 * Ksh 225

= Ksh 2,025

It is estimated that operational costs of an MBR is $ 1.77 ~ Ksh 177 per 3,785 L of waste water treated per day(Water Environment Research Institute).

For Q = 16,000 L/day the

Operational cost = 16,0003785

∗Ksh177

= Ksh 748 / day

Per annum (minus the days the school is on holidays),

Operational cost = Ksh 748 * 240

= Ksh 179,520

Assuming power requirement is not included in the operational cost then,

Total cost / day = Ksh 748 + Ksh 2,025

= Ksh 2,773

Benefit analysis

Assuming the amount of recycled water is 65% (10,000L) on the lower limit:

When there is shortage of water, the school purchases salty water at Ksh 6500 / 10,000 L.

Therefore,

1 L = Ksh 0.65

From the 240 school days on average, assuming for 150 days they buy water when borehole supply isn’t sufficient,

Total amount used to buy 10,000 L of water = 0.65 * 10,000 * 150

= Ksh. 975,000

Total operating cost of the MBR in 150 days = Ksh 2,773 * 150

= Ksh 415,950 for 10,000L recycled/ day

After the initial investment, the amount saved annually from recycling water is

= Ksh. 975,000 – Ksh. 415,950

= Ksh. 559,050

56

With an increased system capacity, the amount saved will be more.

5.12 DiscussionFrom the data obtained on quantity of waste water produced at the school and its components, the MBR volume was calculated to be 29.25 m3. The design flow rate used was 32 m3 per day, double the current flow rate. This was done in order to ensure the system had sufficient capacity to handle an increase of waste water produced as the school expands in the future. From calculations, varying various parameters such as doubling the MLSS concentration to 10,000 mg/L, the system was found to have a capacity of handling up to 67 m3 daily waste water flow rate. The membrane type selected was Fibre Sheet membrane with 75 catridges to handle the current flow rate. The HRT was obtained to be 3 hours and SRT 10 days. Daily power requirements were obtained as 12.75 Kw for aeration, pumping and mixing. The efficiency of the MBR was calculated to be 98% for BOD removal, 90% for Nitrogen and 99% for Total Suspended Solids.

57

6 CONCLUSION AND RECOMMENDATION6.1 ConclusionFrom the analysis of the results, the project was a success as the objectives were met. The overall objective was to design an MBR for North Gate school for waste water treatment. The volume of the MBR was obtained from calculations. The design volume will cater for increased waste water generated at the school without having to increase the size of the bioreactor. From the benefit cost analysis, the MBR system proved from calculations that recycling the water for purposes such as toilet flushing, irrigation of school farm and landscape irrigation will cut the costs of pumping water and also from purchasing water when the borehole cannot meet the demand. This will go a long way in helping in management of the waste water at the school for a long time.

6.2 Challenges encounteredIn carrying out this project, some of the challenges encountered were;

1. Inadequate funds to carry out the laboratory testing of the waste water.2. Inadequate information on design specifications of different components of the MBR

which is highly specialized.3. Lack of design drawings of the bioreactor that would help in detailed design.

6.3 Recommendations1. The upgrade of the conventional activated sludge process to the MBR system of waste

treatment at the school as the MBR is cost effective, provides recycled water for use and uses a smaller footprint.

2. The MBR is a highly specified system therefore an experts input would be required to add onto the analysis done before the implementation.

3. A comprehensive bill of quantities before implementation in order to avoid underestimation of the investment cost that would hamper the implementation of the project.

4. Use of solar for pumping and blower requirements to cut down on cost of electricity.5. Use of the sludge produced to provide energy for cooking at the school.

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7 REFERENCES

1.BusseGT. (2011). Small scale sewage treatment system with membrane bioreactor technology.

2. Ching, K. F. (2010). Design and Operation of MBR Type Sewage Treatment Plant at Lo Wu Correctional Institution , Hong Kong, 1–9.

3. Dumbrell, K. (1998). Design and Control of Equalization Tanks, (November).

4.Elokdi, H. (2007). Membrane Biological Reactor Design, Operations and Maintenance, 971(0).

5.Form, A., Effluent, F. O. R., & Licence, D. (2006). WATER QUALITY REGULATIONS APPLICATION FORM FOR EFFLUENT DISCHARGE LICENCE WATER QUALITY LICENSING GUIDANCE PACK a ) Guidelines to Filling in Application Form for Effluent Discharge Licence b ) Fourth Schedule Monitoring Guide for Discharge into the Enviro, 1–25.

6.Milano, P., & Zaerpour, M. (2014). Design , Cost & Benefit Analysis of a Membrane By :

7.Namiiro, A. (2012). Assessment of Membrane Bioreactor and Pre-precipitation Processes for Wastewater Re-use in Agriculture.

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8 APPENDICESAppendice A: Membrane Bioreactor Cross-section

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Appendix B

Appendix C

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Appendix E

Appendice F

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