Selectors in Advanced Biological Wastewater Treatment Systems

8/10/2019 Selectors in Advanced Biological Wastewater Treatment Systems

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SELECTORS IN ADVANCED BIOLOGICAL WASTEWATER

TREATMENT SYSTEMS

S.Eswaramoorthi

Introduction:

One of the main problems associated with every wastewater treatment facility is the maintenance of

MLSS to the required level, and handling issues associated with sludge bulking. Moreover, all

technological advancements in biological wastewater treatment viz., sequential batch reactor S!"#,

moving bed biofilm reactor M!!"#, integrated fi$ed film activated sludge %&'S# process(,

membrane bioreactor M!"# etc., are aimed at handling these issues. %n the case of S!", if sludge

settleabilityis diminished the concentration of MLSS goes down as most part of the bacterial

population escapes the unit during decanting phase. %n M!!" systems, though high organic loading

rate, increased influent flow rate, reduction in hydraulic residence time )"*# and foot print could

be achieved, low sludge volume inde$ S+%-# of as low as - mL/g was reported to occur due to

the formation of small0si1ed flocs 'hmadi et al., 2-((#. *his poses operational problems with the

secondary clarifier due to poor sludge settleability. %n addition, when ammonia content in the

influent is high, enhanced nitrification in the M!!" results in higher amount of nitrate in the

effluent, resulting in bulking sludgein the secondary clarifier 3olic et al., 2-(4#. %n the case of

M!" systems, membrane filtration eliminates sludge settleability issues, butsludge filterabilityis

affected when poorly settled sludge is generated in the reactor. )owever, in compartmentalised

M!" system2,the S+%-values were found to be optimum, favouring good sludge settling

characteristics 5halekhani and 6inati1adeh, 2-(4#. *his compartmentalisation is akin to selector

design, and demonstrated that incorporation of compartments within the e$isiting S!", M!!", and

M!" shall favour lower S+%-values.

Sludge settleability has a direct impact on the maintenance of MLSS by affecting the quantum of

return sludge in a conventional activated sludge process. *he main advantage of proper

maintenance of S+%-is that it improves stability of the treatment system by improving the

concentration of MLSS whereby, the ability to withstand shock loads becomes significant. Since the

( *he difference between M!!" and %&'S is as follows7 %n the M!!" process plastic media with larger surface area

is used for biofilm growth and there is no return sludge supply. 8hereas, %&'S is a suspended growth system that

incorporates attached growth media within the suspended growth compartment and employs recirculation of thesludge.

2 9lugged flow regime is maintained in compartmentalised M!" system. *his compartmentalisation is akin to theselector configuration which retards filamentous bacterial growth.


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concentration of MLSS determines how much time it takes for the reduction of !O: to the required

level, sludge settleability also has an impact on si1ing of the treatment units and )"*. %n addition to

saving on capital investment towards reduction of treatment unit si1e, reduction in operational costs

can also be achieved if we are able to maintain proper S+% -. %n order to solve these problems,

selectorsare used to attain preferential growth offloc-forming bacteria. )ere a discussion is made

on how sludge settleabilty issues can be handled. *he discussions made here are general in nature

and applicable to all biological wastewater treatment systems.

Measuring sludge settleability:

Sludge settleability issues may arise under two scenarios7 i# when &/M ratio is high, and ii# when

&/M ratio is low. 8hen the &/M ratio is high, hydrous sludgeis generated whereas, low &/M values

lead tofilamentous growth. *o address this problem, settleability of the sludge is measured. Several

methods have been developed for measuringsludge settleabilityof which two of them are widely

used7 i# Sludge +olume %nde$ S+%-; mL/g#which is defined as the ratio between


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floc0forming bacteria Stypka, (BBC#. *he choice of employing aerobic, ano$ic and anaerobi

c selector is largely dependent on which configuration most favours the proper maintena

nce of :S+%-in the biological treatment unit. !y controlling sludge settleability and maintaining

:S+%-D (-- mg/L it is possible to increase daily flow and organic loads )ulsman, (BB2#. *he

si1ing of the selector should be based on the level and biological o$idation kinetics of rb3O:

.

&undamentally, there are two configurations for selector design7 i# aerobic selector followed by

anaerobic o$idation; ii# anaerobic selector followed by aerobic o$idation. Fse of aerobic selector

shall convert ammonia present in the effluent into nitrateG, which can be eliminated in the following

anaerobic biological o$idation step. Similarly, use of anaerobic selector shall remove e$cess nitrate

present in the wastewater by denitrification. %n this case, the ammonia present in the effluent may be

converted to nitrate in the aerobic o$idation stage that follows. )owever, by recirculation of the

effluent from the aerobic compartment to the anaerobic selector, it shall be possible to remove thise$cess nitrate. Such a configuration also reduces the costs associated with the requirement for the

addition of e$ternal carbon sources H as the organics present in the influent are utilised as a carbon

source. Sometimes, instead of a single stage selector, cascade of two, three or, four selectors can be

employed 'lbertson and 3oughenour, (BB>#. Such cascaded selectors have higher efficiency than a

single0stage selectors. One of the reason for better functioning of cascaded selectors is that it

prevent the problems arising from short0circuitingCapart from establishing a concentration gradient

that favours the growth of floc0forming bacteria. 8hen the selectors are cascaded, each selector

may or may not have different si1e and hydraulic retention time to get optimised results. Similary,

they can be either aerobic, ano$ic, or anaerobic.

*he main requirement for good selector operation is that we should be able to obtain higher

substrate concentration and high substrate removal efficiency Mangrum, (BBC#. Fnder such a

circumstance, the growth of filamentous bacteria is retarded and that leads to good sludge

settleability. *he ma$imum substrate utilisation rate occurs at high substrate concentration where

bacterial growth rate is also high Metcalf and Eddy, 2--#. *his forms the basis for the design of a

good selector. Since selectors are located at the head of the wastewater treatment system where

wastewater is received, the substrate concentration in the influent is comparably higher than any

other treatment unit down the stream. *his high concentration of the substrate in the selector results

rb3O: H rapidly biodegradable chemical o$ygen demand 0 is due to the fraction of organics present in the

wastewater that degrades at a comparatively faster rate. %t is very much essential to estimate the quantum of rb3O:

and pb3O: 3O: arising from particulate biodegradable organics# and their degradation kinetics to have a bettersystem in place.

G *his is a two0step biological o$idation process whereby ammonia in the wastewater is initially converted into nitrite

and then to nitrate.C Short0circuiting problem arises when the influent leaves the reactor without spending design )"*. Short0circuiting

problems mainly arises from insufficient mi$ing. One of the reasons for such poor mi$ing is improper placement ofinlet and outlet in the reactor.


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in higher growth rate of bacterial population. *he relationship between substrate concentration g

bs3O:/m#Band substrate utilisation rate g/mIday# is given by the following equation eq0(# and

depicted in &ig.((-7

"suJ kKS/s S# ........ eq0(#

where,

"suJ substrate utilisation rate, g/mIday;

kJ ma$imum specific substrate utilisation rate, g substrate#/g microorganismsIday#;

K J biomass concentration, g/m;

s J half0velocity constant, substrate concentration at one0half the ma$imum specific substrate

utilisation rate, g/m;

S J growth limiting substrate concentration in solution, g/m

;

&rom &ig.(, it can be observed that at low biomass concentration for e$ample K J 2- mg/L#, the

substrate utilisation rate increases e$ponentially. )owever, when the biomass concentration

increases K J --- mg/L#, the relationship tends to become linear. *he high concentration of

biomass in the selector could be achieved either by returning the sludge or, providing more time for

biological growth.

B bs3O: arises from biodegradable soluble organics present in the wastewater.

(- *ypical values of k@ma$imum specific substrate utilisation rate g substrate#/g microorganismsIday#, and s half0velocity constant 0 substrate concentration at one0half the ma$imum specific substrate utilisation rate, g/m#A at

2-N3; K is the biomass concentration g/mor, mg/L# and assumed to be 2- g/mor, 2- mg/L# in the influent to the

selector during start0up and --- g/mor, --- mg/L# after sufficient biomass has developed over a period of time.*hough kand s values are typical, actual measurement of biomass concentration K# should be made in the

sample collected from selector for estimating substrate utilisation rate. *he values of kand s should also beadusted for temperature if the wastewater temperature is different from 2-N3.


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&ig.(7 "elationship between substrate concentration and substrate utilisation rate. *ypical values of

kand s Metcalf and Eddy, 2--# were used with assumed values of K J 2- g/mand --- g/m

for the preparation of the chart. ?ote that the substrate utilisation rate is different for different

bacterial population. !y suitably selecting the S"*, it shall be possible to favour the growth of one

type of bacteria over the other.

*o be successful, the selectors must be properly designed and operated. Fsing data obtained from

full0scale wastewater treatment plants, 5abb et al.2--# concluded that ano$ic selectors do not

control filamentous bulking in long mean cell residence time plants((. %n such cases, elimination of

ano$ic 1one is e$pected to reduce sludge bulking problems. Similarly, aerobic selectors do not

control filamentous bulking in short mean cell residence time plants. %t was also found that ano$ic

and anaerobic selectors do not control filamentous bulking in short mean cell residence time plantsif selector volume is large enough and/or selector MLSS concentration is high enough.

Conclusion:

%rrespective of the developments made in the filed of biological wastewater treatment systems such

as S!", M!!", %&'S, M!", etc., sludge settleability remains as one of the most important

parameter that can affect system stability, removal efficiency of organics present in the wastewater,

and ability to withstand shock loads. 9roper selection of a selector aerobic, ano$ic or anaerobic#

and provision of enough number of stages cascaded selectors# shall prove to be useful in system

operation and maintenance. Such measures shall not only help improve the performance of the

wastewater treatment plant, but also reduce capital investments and operational costs. *hough

adoption of selectors is not common in the case of S!", M!!", %&'S and M!" systems, a good

design should consider its incorporation to accrue benefits.

(( Mean cell residence time is the average time that a microorganism spends in the activated sludge process. %t ismeasured in days, and synonymous with solids retention time S"*#. *he mean cell residence time is an important

parameter that e$ert control over other parameters and therefore the performance of any biological wastewatertreatment unit.


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References:

(# 'hmadi, M., %1anloo, )., Mehr alian, '., 'miri, ). and Sepehr, M.?. 2-((# Fpgrading of

ish %sland Marka1i wastewater treatment plant by M!!", Pournal of 8ater "euse and

:esalination, 1(#7 24024B. doi7 (-.2(/wrd.2-((.-C.

2# 'lbertson, O.E. and 3oughenour, P. (BB># 'erated ano$ic o$idation0denitrification process,

Pournal of Environmental Engineering, G2-0G2.

# 3olic, M., !entley, :., 9arra, 3., and and Lechter, '. 2-(4# M!!" )elps Seafood

Manufacturer Meet "egulatory "equirements, 3lean 8ater *echnology %nc., Los 'ngeles,

3alifornia. :ownloaded from

http7//www.cleanwatertech.com/library/weftec2-(seafoodmanuscript.pdfon -( ?ov 2-(4.

4# 5abb, :.M.:., :e Lange, +.9., 3hien, M.)., Esquer, M.'. and Shao, Q.P. 2--#

%nvestigating the fundamental basis for selectors to improve activated sludge settling, 8ater

Environment "esearch &oundation "eport -(03*S04a.

># 5halekhani, 5.". and 6inati1adeh, '.'.L. 2-(4# 9rocess analysis and optimi1ation of

industrial estate wastewater treatment using conventional and compartmentali1ed membrane

bioreactor7 a comparative study, %ranica Pournal of Energy and Environment, 52#7 (-(0((2.

:O%7 (-.>C2B/idosi.iee.2-(4.->.-2.-(.

# )ulsman, '.3 (BB2# *he effect of type, si1e, position and recycle ratio of the ano$ic 1one

on low &/M filament bulking in nitrogen removal activated sludge systems, Master of

Science in Engineering dissertation, Fniversity of 3ape *own, (C pp.

G# Mangrum, 3.".L. (BBC# *he effect of ano$ic selectors on the control of activated sludge

bulking and foaming, Master of Science dissertation, +irginia 9olytechnic %nstitute and State

Fniversity, GB pp.

C# Metcalf and Eddy 2--# 8astewater Engineering7 *reatment and "euse, 4th edition revised

by 5eorge *chobanoglous, &ranklin L. !urton, ).:avid Stensel, *ata Mc5raw0)ill

9ublishing 3ompany Limited, pp.(C(B.

B# Stypka, '. (BBC# &actors influencing sludge settling parameters and solids flu$ in the

activated sludge process, %n7 E. 9a1a, E. Levlin, !. )ultman Eds.#'dvanced 8astewater

*reatment, "eport ?o.4, Point 9olish0Swedish "eports, :ivision of 8ater "esources

Engineering, :epartment of 3ivil and Environmental Engineering, "oyal %nstitute of

*echnology, Stockholm. %S!?7 B(0G(G-002.
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Selectors in Advanced Biological Wastewater Treatment Systems

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