wastewater ass

8/10/2019 Wastewater Ass

1/21

1.0 PROBLEM STATEMENT

The growth in civilization and industrial activities has caused a number of environmental

problems. For decades, large quantities of pollutants have been discharged into the environment

irresponsibly. Hexavalent chromium is of particular environmental concern due to its toxicity

and mobility and is challenging to remove from industrial wastewater. It is a strong oxidizing

agent that is carcinogenic and mutagenic and diffuses quickly through soil and aquatic

environments. It does not form insoluble compounds in aqueous solutions, so separation by

precipitation is not feasible. Moreover, High strength wastewater can be successfully treated by

using membrane bioreactor (MBR) in different conditions according to the types and

characteristics of wastewater and also MBR parameters operational control. High strength

wastewater contains fats, oil and grease or other organic or inorganic compounds in great amount

according to the types of sources that take part. Several factors need to be taken into

consideration during the operation such as hydraulic retention time (HRT), solid retention time

(SRT), mix liquor suspended solid (MLSS), food to microorganism (F/M), transmembrane

pressure (TMP) and flux. Fouling factors need to be taken seriously because they are the major

problems affecting the performance of the MBR and quality of the effluent. To control

eutrophication in receiving water bodies, biological nutrient removal (BNR) of nitrogen and

phosphorus has been widely used in wastewater treatment practice, both for the upgrade of

existing wastewater treatment facilities and the design of new facilities. However,

implementation of BNR activated sludge AS systems presents challenges attributable to the

technical complexity of balancing influent chemical oxygen demand (COD) for both biological

phosphorus (P) and nitrogen (N) removal. Sludge age and aerated/unaerated mass fractions are

identified as key parameters for process optimization. Emerging concerns about process

sustainability and the reduction of carbon footprint are introducing additional challenges in that

influent COD, N, and P are increasingly being seen as resources that should be recovered, not

simply removed. Energy recovery through sludge digestion is one way of recovering energy

from influent wastewater but which presents a specific challenge for BNR: generation of

sidestreams with high nutrient and low COD loads.


2/21

1.2 PROCESS AND TECHNOLOGY

1.2.1 Physical

1.2.1.2 Screening

Wastewater from all different sewers is collected and brought to the treatment plant. The

wastewater is passed through multiple screenings to remove large debris such as glass, rags, and

stones. Screening protects the downstream units of the treatment plant from obstructions caused

by the larger debris and improves the efficiency of the operations in the later stages. The

screening chamber consists of vertical stainless steel screens at an angle (usually 30 degrees) to

the horizontal. Stainless steel is used to prevent the screens from corrosion. They have uniform

openings to retain large solids. The spacing of the screens determines the size of the particles

removed (Metcalf & Eddy, American Sewerage Practice, 1935).

The screening procedure can be manual or automated. However, automation is preferred

if the screens will be exposed to high flow rates or if the water has large amounts of solid debris.

With the help of mechanical rakes, the screens are constantly scrapped off to remove material

deposited into the powers, which is then dumped into a shaft. The solids are separated by a screw

in the shaft. (Al-Layla, Ahmad, & Middlebrooks, 1980). The most common type of screens has

a mechanical device to scrape off the deposited materials. In order to maintain a minimum

velocity of 0.6m/s to prevent clogging, the dimensions of the channels near the screens are

increased (Al-Layla, Ahmad, & Middlebrooks, 1980). A typical mechanically cleaned bar screen

is shown in the figure 1 below:


3/21

Figure 1: Mechanically Scraped Bar Screen


4/21

1.2.2 CHEMICAL

1.2.2.1 Electrochemical Methods

The use of electrochemical methods represents an interesting option as many

electrochemical and chemical reactions occur simultaneously when they are applied.

Electrochemical treatment techniques have attracted a great deal of attention because of their

versatility and environmental compatibility, which makes the treatments of liquids, gases, and

solids possible. In fact, the main reagent is the electron, which is a clean reagent

1.2.2.2 Electrocoagulation

Electrocoagulation is the electrochemical production of destabilization agents that

neutralize the electric charge of the pollutants present in solution. An electrocoagulation reactor

consists of a reservoir in which the solution is contained and two electrodes: a cathode and an

anode (Carlos E et al., 2012).

1.2.2.3 Iron Electrodes

The electrocoagulation process using iron electrodes involves the liberation of Fe (II)

ions into the solution due to the anodic polarization of a plain carbon steel electrode [22]. When

the pH of the solution is between 6 and 8 Fe (II) ions form insoluble species onto which Cr (VI)

ions are adsorbed and removed from the solution. Iron systems show a high efficiency (>90%)

and studies evaluate mainly the interactions of pH, applied electric current, and application time

on the Cr(VI) removal. The removal of hexavalent chromium by electrocoagulation involves two

stages: the reduction of Cr(VI) to Cr(III) at the cathode or by the Fe2+ ions generated from the

oxidation of the iron anode and the subsequent co-precipitation of the Fe3+/Cr3+ hydroxides. At

low pH values, the reduction of Cr(VI) to Cr(III) by Fe2+ ions is favored, but under these pH

conditions there is no precipitation of Fe(III)/Cr(III) hydroxides. The precipitation of Fe3+/Cr3+

hydroxides takes place at pH higher than 3 due the solubility of metal hydroxide species (both

chromic and iron hydroxides) at the low pH (Carlos E et al., 2012).


5/21

1.2.2.4 Aluminium Electrodes

Aluminum anodes are used to produce aluminum cations which form hydroxylated

species. The pollutants present in aqueous solution are destabilized and then adsorbed on the

Al(OH)3(s) produced.

1.2.2.5 Cr (VI) photocatalytic reduction

Photocatalysis on semiconductors involves three main steps:

(i) Absorption of photons with higher energies than the semiconductor band gap, leading to the

generation of electron (e)hole (h+) pairs in the semiconductor;

(ii) Charge separation followed by migration of these photo-generated carriers in the

semiconductor;

(iii) Surface chemical reactions between these carriers with various compounds; electrons and

holes may also recombine with each other without participating in any chemical reactions.

Another class of non-oxide semiconductor photocatalyst is homogeneous sensitizer

molecules, such as organic dyes and metal complexes. A strategy for achieving effective visible

light harvesting is spectral sensitization of wide bandgap semiconductors (e.g., TiO2) using

sensitizer molecules (Carlos E et al., 2012).

1.2.2.6 Organic matter for Cr (VI) reduction

Environmentally ubiquitous, naturally occurring reductants (quinones, organo-sulfur

compounds and amorphous dissolved organic matter (DOM)) exhibit very slow-yet-measurable

Cr(VI) reduction kinetics under predominantly acidic conditions. In contrast to these organic

reductants, zerovalent iron, aqueous Fe (II), Fe (II) hydroxides, adsorbed Fe (II), and Fe (II)-

chelates have been shown to reduce Cr (VI) very rapidly. Coupling humic acid (HA) and iron


6/21

nanoparticles for Cr(VI) reduction has both synergistic and antagonistic effects. HA can act as an

adsorbent competing for reactive sites on the surface of the Fe(0) nanoparticles, leading to a

decreased Cr(VI) reduction rate. However, the quinone compounds in HA act as electron shuttles

promoting electron transfer, which would have a positive enhancement on the reduction of

Cr(VI) by Fe(0) nanoparticles. HA also stabilizes the nanoparticles preventing agglomeration

which enhances the reactivity and counteracts the inhibitory effect. (Carlos E et al., 2012).

1.2.2.7 Fe(III) photocatalytic reduction of Cr(VI) by organic acids

Recent efforts have focused on the photocatalytic impact of Fe(III) on the reduction of

Cr(VI) by organic acids. The rate of Cr(VI) photoreduction in sunlit natural waters is related to

the amount of Fe(III) present and the nature of the dissolved organic matter substrate and the

organic acid type (classified as low Fe(III) photoreductivity acetate and succinate, and high

Fe(III) photoreductivity citrate and tartrate). The fast reaction between Cr (VI) and organic acids

in the presence of Fe(III) is mainly due to the photoreaction products generated when solution is

exposed to sunlight. (Carlos E et al., 2012).

The reduction of Cr (VI) by organic acids in soils is coupled with apparent adsorption of

Cr (III) by the soil, both of which are influenced by the types of soils, their composition, and

their physical characteristics. The presence of soils significantly accelerates photochemical

reduction of Cr (VI) only at low soil loading. Higher soil loading is not beneficial to the

improvement of Cr (VI) reduction due to the decreased light penetration into the dissolved phase

(Carlos E et al., 2012).

1.2.2.8 TiO2 photocatalytic reduction of Cr(VI) by organic acids

The photocatalytic reduction of Cr (VI) by TiO2 at pH 3 in both the absence and presence

of organic compounds has been extensively investigated for its application to the oxidation of

organic compounds and the reduction of metal ions. The mechanism involves the charge

trasnsfer complex (CTC) formed between TiO2 and small molecular weigth organic acids

(SOA). The mechanism of photocatalysis on titanium dioxide particles involves generated


7/21

electron/hole pairs that must be trapped in order to avoid recombination. In this context TiO2

nanoparticles and nanofibers resulted in an enhanced catalytic activity for photocatalytic Cr(VI)

reduction where the hidrotermal postreatment exhibited the highest catalytic activity among

TiO2 nanoparticles. (Carlos E et al., 2012).

1.2.2.9 Cr (VI) reducing bacteria

Microbial chromium (VI) removal from solutions typically involves the following stages:

(a) The binding of chromium to the cell surface,

(b) Translocation of chromium into the cell, and

(c) Reduction of chromium (VI) to chromium (III).

1.2.2.10 Aerobic Cr (VI) reducing bacteria

The process produces a reactive oxygen species (ROS) that easily combines with DNA

protein complexes. Nevertheless, it is presently unclear whether the reduction of Cr (V) to Cr

(IV) and Cr (IV) to Cr (III) is spontaneous or enzyme mediated.

Two processes are responsible for the reduction of Cr (VI) when aerobic heterotrophic

cells, non-growing cells, growing cells with chromate reductase activity, and growing cells that

have lost the chromate reductase activity where used. The first one is the reduction of Cr (VI)

coupled with growth and the second process is coupled with the endogenous decay of the

biomass [74]. Among the different electron donors, glucose provided the highest Cr (VI)

reduction compared to other electron donors. (Carlos E et al., 2012).

1.2.2.11 Anaerobic Cr (VI) reducing bacteria

In the absence of oxygen, Cr (VI) can serve as a terminal electron acceptor in the

respiratory chain for a large array of electron donors, including carbohydrates, proteins, fats,

hydrogen, NAD(P)H and endogenous electron reserves. Compared to free cells, P. phragmitetus


8/21

cells coated with polyethylenimine-functionalized magnetic nanoparticles, not only had the same

Cr(VI)-reduction activity but could also be easily separated from reaction mixtures by magnetic

force. In addition, the magnetically immobilized cells retained high specific Cr(VI)-reduction

activity over six batch cycles. The results suggest that the magnetic cell separation technology

has potential application for Cr(VI) detoxification in alkaline wastewater. (Carlos E et al., 2012).

1.2.3 Biological

1.2.3.1 A typical MBR processes configuration

In CAS treatment, large clarifying basins are needed to make sure the flocs are

completely settled but by using membranes, there are no more settling processes needed and the

area used for clarifier can be eliminated besides acting as a separator . Figure 2 shows the basic

schematic diagram of MBR configuration. Figure. 2 a shows an immersed membrane bioreactor

(iMBR) or submerged membrane bioreactor (sMBR) module. Figure 2b shows a side-stream or

external membrane module while. For sMBR system, the feed wastewater is directly in contact

with biomass. Then, both wastewater and biomass are pumped through the recirculation loop

consisting of membranes. The concentrated sludge is recycled back to the reactor while the water

effluent is discharged. The purpose of separating the membrane and bioreactor is actually to

reduce the power used by the air diffuser. (Noor Sabrina et al., 2012)

Figure 2: Schematic Diagram for MBR configuration


9/21

1.2.3.2 Membrane Characteristics

The ability of the membrane depends on the size of pores, types of materials, types of

wastewater to be treated, solubility and retention time. Membrane structure plays an important

role in transporting mechanism whether the structure is parallel or in series. Diffusion and

solubility of the component are related to the kinetic ability of the mass transport for membrane.

The solution dissolves into the membrane and separates between retentate and permeates. For the

membrane itself, pore-size membrane participates in kinetic mass transport. The types of

membranes used are different depending on the size of contaminants contacting during the

treatment process. Basically, contaminants with the size of a particle from 1001000 nm use

microfiltration (MF) for removing suspended particles; ultrafiltration (UF) for particle size 5

100 nm, for instance bacteria and virus; and nanofitration (NF) for particles with size 15 nm for

dissolved particles. Two types of materials that are commonly used to construct the membranes

are polymeric and ceramic. (Noor Sabrina et al., 2012)

1.2.3.3 Biomass characteristic

Biomass in activated sludge from industries is heterogenous. Basic nutrients of biomass

are glucose, nitrogen and phosphorus with ratio 100:5:1 in weight. The domination of biomass

can occur through the acclimation process and this depends on the major constituent of feed

wastewater. Microbes take a long time to biodegrade the matters and need high concentration of

biomass (MLSS) to make sure all the organics be totally biodegradable. In anaerobic membrane

bioreactor, HRT and SRT are independent and also produce methane as a side product and odor

whereas they do not use any aeration process and energy saving. Besides that, methane can be

collected as energy generation (Noor Sabrina et al., 2012)

1.2.3.4 BNR Processes

Biological nutrient removal activated sludge (BNRAS) system has become an established

technology in wastewater treatment practice to control eutrophication, and this development has


10/21

been facilitated by an improved understanding of nitrification, denitrification and excess

biological phosphorus (P) removal (EBPR) (Ekama and Wentzel 1999).

1.2.3.5 Conventional BNRAS

These technologies are operated with an anaerobic selector upstream in the process in

which influent rbCOD can ferment and fermentation products can be sequestered by

polyphosphateaccumulating organisms (PAOs). The anaerobic selector in these processes

typically represents 10% of overall tankage volume or less, and additional anoxic zones may be

included based on the requirements to remove nitrogen. One important limitation of these

processes is that, because the retention time of anaerobic selector is relatively small, slowly

biodegradable COD from the influent is not considered to be available for PAOs. As such,

biological phosphorus removal in conventional BNRAS is considered to be dependent on the

availability of influent rbCOD. (Zhirong hu et al., 2012)

1.2.3.6 External Nitrification BNRAS Process

The BNRAS system can be intensified by separating nitrification from the main BNRAS

system. In the former, nitrification takes place in the fixed media external to the suspended

activated sludge. In the latter, nitrification takes place in the fixed media that are placed in the

aerobic reactor (see Integrated Fixed-Film Activated Sludge Process).The underflow sludge

from the internal settling tank bypasses the EN system and is discharged to the beginning of the

anoxic zone, and the overflow supernatant from the internal settling tank is passed on to the EN

systems for nitrification. The nitrified EN effluent is then discharged to the anoxic zone for

denitrification. From the anoxic reactor, the mixed liquor passes to the last reactor, which is

aerobic for stripping nitrogen gas, oxidizing the residual COD and completing the P uptake

process. (Zhirong hu et al., 2012)


11/21

Figure 3: Integration of trickling filters with biological nutrient removal activated sludge systems

1.2.3.7 Integrated Fixed-Film Activated Sludge Process

A widely used approach for upgrading existing activated sludge plants for nitrification, or

simply increasing treatment capacity, has been to convert conventional activated sludge tanks to

integrated fixed-film activated sludge (IFAS) tanks. The IFAS process is well suited for the

upgrade of existing wastewater treatment plants (WWTP) where space or financing is not

available to build additional tanks to meet nitrification requirements. (Zhirong hu et al., 2012)


12/21

1.3 COST

1.3.1 Physical

The screening that been used in wastewater treatment process was high cost. This is

because, the cost to maintaining the screening for any substances in water should be always

observed so that the flow of the water always fluent.

1.3.2 Chemical

One of the main problems using this technique is that large amounts of residual sludge

are generated. The sludge presents difficulties in managing, transporting and final disposal issues

as well as the associated cost. Thus, new technologies are being developed to address these

problems. The copper electroreduction of Cr (VI) to Cr (III) was done in a parallel-plate reactor

and the chemical reduction was done with Na2S2O5. It was determined that the cost of the direct

electroreduction process is about 7 times higher than the chemical method, if carried out at

optimum operating conditions at pH 1.5. The costs of the two processes are closer when the

electrochemical method is carried out at pH 2, but operating time is increased threefold, thereby

increasing the cost.

1.3.3 Biological

Bioreactor acts as a biological treatment processor and the membrane is used as a filter in

the filtration process. Despite its reputation of being a reliable treatment process, the interest in

MBR has slowed down for the past three decades mainly due to the high cost of membrane and

membrane maintenance. The cost of the membrane has inclined together with the growth of

membrane technology and the MBR technology is becoming more acceptable for industrial

applications. In 1990s, submerged MBR was commercialized and it was found that this system

had low operational cost than any other types of MBR. Generally, the biological process shows a

greater performance than the filtration process. The membrane plays a role in separating solid

and liquid whereby the biological process by activated sludge converts the particle waste into


13/21

flocs before it is separated by the membrane. The membrane fouling becomes a major factor

since membrane is used and it is still under research to reduce the problem caused. Apart from

that, high cost maintenance and operation are needed to maintain the performance of MBR and

most of the treatment plants avoid using MBR because of this problem. The operational cost of

iMBR system is less because there is no recirculation loop compared with the sMBR system.

Judd (2006) stated that cost is a major constraint to MBR technology in 1990s because of high

cost of membrane which leads to the increase of maintenance and operational costs. Membrane

cost covers on replacing the severe membrane fouling or corrupted membrane and membrane

cleaning process during maintenance (Noor Sabrina et al., 2012).


14/21

1.4 EFFECTIVENESS

1.4.1 Physical

The quantity of screenings depends on the length and slope of the collection system and

the presence of pumping stations. When the collection system is long and steep or when pumping

stations exist, fewer screenings are produced because of disintegration of solids. Other factors

that affect screening quantities are related to flow, as quantities generally increase greatly during

storm flows. Peak daily removals may vary by a 20:1 ratio on an hourly basis from average flow

conditions. Combined collection systems may produce several times the coarse screenings

produced by separate collection systems (Reynolds and Richards, 1996)

1.4.2 Chemical

Electrocoagulation between iron electrodes and aluminium electrodes Iron systems show

a high efficiency (>90%) and studies evaluate mainly the interactions of pH, applied electric

current, and application time on the Cr(VI) removal. The removal of hexavalent chromium by

electrocoagulation involves two stages: the reduction of Cr (VI) to Cr(III) at the cathode or by

the Fe2+ ions generated from the oxidation of the iron anode and the subsequent co-precipitation

of the Fe3+/Cr3+ hydroxides. At low pH values, the reduction of Cr(VI) to Cr(III) by Fe2+ ions

is favored, but under these pH conditions there is no precipitation of Fe(III)/Cr(III) hydroxides.

The precipitation of Fe3+/Cr3+ hydroxides takes place at pH higher than 3 due the solubility of

metal hydroxide species (both chromic and iron hydroxides) at the low pH.

1.4.2.1 Cr(VI) photocatalytic reduction

TiO2 nanoparticles obtained by hydrothermal post-treatment showed the best

sedimentation efficiency, highlighting its prominent potential as a readily separable and

recoverable photocatalyst. Also, TiO2 nanoparticles (Degussa P25) modified with fullerene

derivative C60(CHCOOH)2 display a higher photocatalytic activity with 97% reduction

efficiency on Cr(VI) ions. In this system the enhanced photocatalytic activity of TiO2


15/21

nanoparticles may be ascribed to the enhancement of the photogenerated electron/hole pair

separation because of the modification of the C60 derivative [60]. La2Ti2O7 is a highly active

photocatalyst for reduction

1.4.3 Biological

1.4.3.1 Biomass characteristics

This system does not consider the flocs growth but still maintains the minimum sludge

production with low F/M ratio (less substrate is presented per unit of biomass) and retaining the

biomass in the reactor and sludge age. Besides that, the formation of flocs makes it easier to

filter. However, if F/M is too low, the biomass in the activated sludge could not grow well , or

else if MBR has very high MLSS it will lead to clogging, low efficiency of aeration and it needs

a large bioreactor (increasing the initial capital cost) . HRT with low level will increase organic

loading rate (OLR) which will end up with reactor volume reduction and reduce the performance

of MBR; whereas if the HRT is high, MBR has a good performance (Noor Sabrina et al., 2012)

1.4.3.2 Sludge Treatment Efficiency

To enhance the efficiency of anaerobic digestion for more biogas production, various

sludge pretreatment processes (solids conditioning) have been advanced in recent years to

improve these rate-limiting processes, including, e.g., mechanical means, thermal hydrolysis,

alkaline hydrolysis, and ultrasonic treatment. To reduce operating costs while increasing the

quality of the stabilized sludge, process modeling of anaerobic digestion has become an

important tool. (Noor Sabrina et al., 2012).


16/21

1.5 LAW/LEGAL ISSUES

Effluent guidelines are one component of the nations clean water program, established

by the 1972 Clean Water Act. The CWA requires EPA to promulgate effluent guidelines for new

categories of dischargers under certain circumstances. See CWA section 304(m)(1)(B) and (C).

In addition, the Clean Water Act requires that EPA periodically review existing effluent

guidelines, pretreatment standards, and standards of performance for new sources and to revise

them "if appropriate" or, in the case of new source performance standards, "as technology and

alternatives change". See CWA sections 301(d), 304(b), 304(g)(1), 306(b)(1)(B). In addition,

sections 304(e), 308(a), 402(a), and 501(a) of the CWA authorize the Administrator to prescribe

BMPs as part of effluent limitations guidelines and standards or as part of a permit.

The national clean water industrial regulatory program is authorized under sections 301, 304,

306 and 307 of the CWA and reflects seven core concepts.

1. Effluent guidelines are designed to address specific industrial categories. To date, we

have promulgated effluent guidelines that address 56 categoriesranging from

manufacturing industries such as petroleum refining to service industries such as

centralized waste treatment. These regulations apply to between 35,000 and 45,000

facilities that discharge directly to the Nation's waters, as well as another 12,000

facilities that discharge into wastewater treatment plants or publicly owned treatment

works (POTWs).

2. National effluent guidelines typically specify the maximum allowable levels of

pollutants that may be discharged by facilities within an industrial category or

subcategory. While the limits are based on the performance of specific technologies,

they do not generally require the industry to use these technologies, but rather allow

the industry to use any effective alternatives to meet the numerical pollutant limits.

3. Each facility within an industrial category or subcategory must generally comply with

the applicable discharge limits regardless of its location within the country or on a

particular water body. See CWA section 307(b) and (c); and CWA section 402(a)(1).


17/21

The regulations, therefore, constitute a single, standard, pollution control obligation for

all facilities within an industrial category or subcategory.

4. In establishing national effluent guidelines for pollutants, EPA conducts an assessment

of (1) the performance of the best pollution control technologies or pollution

prevention practices that are available for an industrial category or subcategory as a

whole; and (2) the economic achievability of that technology, which can include

consideration of costs, benefits, and affordability of achieving the reduction in

pollutant discharge.

5. National regulations apply to three types of facilities within an industrial category:

i) Existing facilities that discharge directly to surface waters (i.e., direct

discharges) are governed by best practicable technology (BPT), best available

technology (BAT), or best conventional pollutant control technology (BCT);

ii) Existing facilities that discharge to POTWs (indirect dischargers) are

governed by pretreatment standards for existing sources (PSES); and

iii) Newly constructed facilities (new sources) that discharge to surface waters

either directly or indirectly are governed by new source performance standards

(NSPS) and pretreatment standards for new sources (PSNS).

6. The use of BMPs, either as an alternative to or to reduce the sampling and analysis to

demonstrate compliance with numeric limitations and standards of the final rule, are

directed, among other things, at preventing or otherwise controlling leaks, spills, and

discharges of toxic and hazardous pollutants.

7. Finally, the CWA requires an annual review of existing effluent guidelines and, if

appropriate, revision of these regulations to reflect changes in the industry and/or

changes in available pollution control technologies.


18/21

1.6 ADVANTAGES AND DISADVANTAGES

1.6.1 Physical

1.6.1.1 Advantages of screening process

Manually cleaned screens require little or no equipment maintenance and provide a good

alternative for smaller plants with few screenings. Mechanically cleaned screens tend to have

lower labor costs than manually cleaned screens and offer the advantages of improved flow

conditions and screening capture over manually cleaned screens. (Crites R and G.

tchobanoglous, 1998)

1.6.1.2 Disadvantages of screening process

Manually cleaned screens require frequent raking to avoid clogging and high backwater

levels that cause buildup of a solids mat on the screen. The increased raking frequency increases

labor costs. Removal of this mat during cleaning may also cause flow surges that can reduce the

solids-capture efficiency of downstream units. Mechanically cleaned screens are not subject to

this problem, but they have high equipment maintenance costs. (Qasim S, 1994)

1.6.2 Chemical

1.6.2.1 Disadvantages using Aluminium electrodes

Higher aluminum coagulant dosing leads to higher Cr(VI) removal but it adversely

affects the treatment efficiency as more aluminum coagulant is required per unit of pollutant

removal [29]. A comparison of aluminum electrodes with the popular iron electrodes for Cr(VI)

electrocoagulation, indicate that iron electrodes are better than aluminum ones. aluminum

electrodes were unsatisfactory for Cr(VI) removal and proposed that at nearly neutral pH both

electrochemical reduction (Cr(VI) to Cr(III)) at the cathode surface and adsorption on Al(OH)3

floc mechanisms were responsible for Cr(VI) exhaustion.


19/21

1.6.2.2 Disadvantages of Fe(III) photocatalytic reduction of Cr(VI) by organic acids

Higher soil loading is not beneficial to the improvement of Cr(VI) reduction due to the

decreased light penetration into the dissolved phase [52]. Fe(III) in soil particles reacts with citric

acid and tartaric acid to form a photochemically active complex, which can be transformed to

stronger reductants than the organic acids through a pathway of metalligandelectron transfer.

Consequently, the reduction of Cr(VI) is significantly accelerated.

1.6.3 Biological

1.6.3.1 Advantages using BNR process.

The objectives of side stream treatment are to use biological processes for removing

ammonia from recycle reject water to reduce the nitrogen load that must be handled in the

activated sludge process. A range of benefits have been identified for the different side stream

treatment systems, for example:

Seeding the activated sludge train with ammonia oxidizing bacteria (AOBs) and nitrite

oxidizing bacteria (NOBs) grown in the side-stream stage, allowing shorter SRTs

(bioaugmentation).

Less carbon substrate is required to denitrify nitrite rather than nitrate compared with

denitrification of a fully nitrified stream.

Less aeration and alkalinity are required to convert ammonia to nitrite rather than nitrate.

The significant benefit of the ANAMMOX process over conventional denitrification systems is

that no organic carbon is added for denitrification, and so there is no increased biosolids

production or emission of CO2. (Noor Sabrina et al., 2012)


20/21

1.6.3.2 The disadvantages of conventional BNRAS

One important limitation of these processes is that, because the retention time of

anaerobic selector is relatively small, slowly biodegradable COD from the influent is not

considered to be available for PAOs. As such, biological phosphorus removal in conventional

BNRAS is considered to be dependent on the availability of influent rbCOD. Another important

disadvantage of conventional BNRAS processes is that, overall, conventional BNRAS processes

require greater tank volumes with associated capital costs, particularly in cold climates where

reliability of nitrification is a concern. This is explained in terms of the growth rate of obligate

aerobic nitrifiers . (Zhirong hu et al., 2012)

1.6.3.3 The disadvantages of integrated fixed-film Activated Sludge Process

The primary disadvantage for this process, however, is that nitrification takes place

within a biofilm, which must have a sufficient degree of oxygen penetration to be reliable.

Because of biofilm thickness and the existence of a boundary layer, there can be significant

resistance to dissolved oxygen penetration into biofilms and, as a result, IFAS processes

typically require dissolved oxygen (DO) setpoints in the bulk liquid on the order of 4 to 6

mgO2=L as opposed to the 2 mgO2=L for conventional, suspended growth processes. (Zhirong

hu et al., 2012).


21/21

1.7 REFERENCES

Al-Layla, M. A., Ahmad, S., & Middlebrooks, E. J. (1980). Handbook of Waste Water

Collection and Treatment. (G. l. culp, Ed.) Garland STPM Press.

Metcalf, L., & Eddy, H. P. (1935). American Sewerage Practice. New York: McGraw-Hill Book

Company

Crites, R. and G. Tchobanoglous, 1998. Small and Decentralized Wastewater Management

Systems. The McGraw-Hill Companies. Boston, Massachusetts.

Qasim, S., 1994. Wastewater Treatment Plants: Planning, Design and Operation. Technomic

Publishing Co., Lancaster, Pennsylvania.

Reynolds, T. and P. Richards, 1996. Unit Operations and Processes in Environmental

Engineering. PWS Publishing Company. Boston, Massachusetts.

Ekama, G. A., andWentzel, M. C. (1999). Difficulties and development in biological nutrient

removal technology and modeling. Water Sci. Technol., 39(6), 111.

Carlos E. Barera-Diaz, Violeta Lugo-Lugo & Bryan Bilyeu , 2012. A Review of Chemical,

Electrochemical and Biological Methods for aqueous Cr (VI) Reduction. Journal of Hazordous

Material 223224.

Noor Sabrina Ahmad Mutamin, Zainura Zainon Noor, Mohd Ariffin Abu Hassan & Gustaf

Olsson, 2012. Application of Membrane Bioreactor Technology in Treating High Strength

Industrial Wastewater. Desalination 1-11.

Zhirong Hu, Dwight Houweling & Peter Dold, 2012. Biological Nutrient Removal in Municipal

Wastewater Treatment: New direction in Sustainbility. American Society of Civil Engineers.

wastewater ass

Documents