Desalination 249 (2009) 571–576

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Slow sand filtration of UASB reactor effluent: A promising post treatment technique

Vinay Kumar Tyagi a, Abid Ali Khan a, A.A. Kazmi a, Indu Mehrotra a, A.K. Chopra b,⁎a Department of Civil Engineering, Indian Institute of Technology, Roorkee, Indiab Department of Zoology & Environmental Science, Gurukul Kangri University, Haridwar, India

⁎ Corresponding author.E-mail addresses: vinayiitr@rediffmail.com (V.K. Tya

(A.A. Khan), kazmifce@iitr.ernet.in (A.A. Kazmi), indumprofakchopra@yahoo.co.in (A.K. Chopra).

0011-9164/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.desal.2008.12.049

a b s t r a c t

a r t i c l e i n f o

Article history:Accepted 11 December 2008Available online 6 October 2009

Keywords:BODColiformsPost treatmentSlow sand filtrationSuspended solidsUASB

The study was carried out to evaluate the feasibility of slow sand filtration as a promising post treatmentmethod for the up-flow anaerobic sludge blanket (UASB) reactor effluent. Laboratory scale filter column of10 cm diameter and 0.54 m sand media depth was used to study the process performance. It was found outthat slow sand filtration with 0.43 mm effective sand size is the most effective at a filtration rate of 0.14 m/h.It is capable of removing 91.6% of turbidity, 89.1% of suspended solids (SS), 77% of chemical oxygen demand(COD) and 85% of bio-chemical oxygen demand (BOD), 99.95% of total and fecal coliforms (TC and FC) and99.99% of fecal streptococci (FS). Slow sand filters efficiently reduce the mass of suspended material andextend the filter run for more time (7 days) at a hydraulic load of 0.14 m/h as compared to the hydraulic loadof 0.19 m/h and 0.26 m/h. Therefore, due to excellent effluent quality, it can be said that slow sand filtrationwould be a promising technology for the post treatment of small-scale UASB reactor effluent in developingcountries, where treated effluent can be reused for various recreational purposes i.e. gardening andirrigation, as well as for safe discharge.

gi), abidkdce@iitr.ernet.infce@iitr.ernet.in (I. Mehrotra),

l rights reserved.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The great increase in water demand due to the continuous growthof human population has aroused a strong interest in wastewaterreuse as a way of industrial and municipal water supply. Whenappropriately treated and recycled, wastewater can become a feasibleoption as a water source. Furthermore, this alternative reduces theenvironmental impact [1].

A major dilemma in this context is regarding the choice of anappropriate treatment technology to achieve optimal reuse of waterand nutrients at a minimal energy expense. Anaerobic treatmentdoesn't require oxygen and therefore no energy input is required andin turn will yield energy in the form of biogas (Methane: CH4).Anaerobic treatment plants have limited space requirements andtherefore can be planned at locations within or just outside the city[2]. In addition, due to low sludge production and better stability ofthe sludge under anaerobic treatment, the cost involved in furthertreatment of sludge can be considerably reduced [3].

However, the residual concentration of organic (BOD and COD)and microbiological (quantified by fecal coliforms) pollutants in theanaerobic reactor effluent usually exceeds the maximum permissiblelevel prescribed by the effluent discharge standards of mostdeveloping countries including India [4–6]. From this standpoint,

post treatment of anaerobic effluent is necessary to reduce theseparameters to the required level.

At present, UASB facilities in India are integrated with pond systemfor post treatment. However, due to low detention times, theseintegrated ponds are found to be ineffective for the removal ofpathogens, BOD and TSS. Nevertheless, several studies show thatpathogenic microorganisms and BOD5 and TSS can be removed byvarious systems such as aerated lagoons, downflow hanging sponge(DHS), rotating biological contractor, trickling filters, biological aeratedfilters [1,2,7,8]. Most of these processes require high energy and a hugecapital cost for operation and maintenance and in addition even theeffluent quality is not in compliancewith the standards for the disposal.

Regarding the effluent quality, cost efficacy and operationalsimplicity, slow sand filtration can be considered as one of the mostpromising post treatment options. Various researchers investigatedthe effectiveness of slow sand filters for tertiary treatment ofwastewater at laboratory and pilot scale using different hydraulicloading and sand size, and suggest that slow sand filters are capable ofremoving BOD and SS, turbidity and total coliforms up to 86%, 68%,88% and over 99%, respectively [9–13]. Slow sand filters have beenused for the treatment of high quality surface waters [14–16], as wellas for the treatment of secondary effluents [9,17–22]. The sand filtershave multiple variables and have been conventionally designed onhydraulic loadings as well as on organic loading rates. It is aneconomical techniquewhich requires less skilledmanpower due to itssimplicity [23–27].

Although a lot of work has been carried out on slow sand filters as atertiary treatment process of biologically treated municipal sewage,

572 V.K. Tyagi et al. / Desalination 249 (2009) 571–576

not sufficient studies have been conducted regarding the posttreatment of UASB reactor effluent. Therefore, the objective of thisstudy was to investigate the performance of slow sand filter as posttreatment option for the UASB reactor effluent and its feasibility insatisfying water reuse standards. The experiments have been carriedout at the bench scale filter column. The performance has beendetermined with respect to the removal of turbidity, SS, BOD, COD,and coliforms bacteria (total and fecal) efficiency.

2. Material and methods

2.1. Set up of the experimental column

The principal apparatus employed in this investigation consists of100 mm (10 cm) internal diameter vertical Perspex tube of 120 cm(1.2 m) in height, packed with mixed filter media to a height of 54 cm(Fig. 1).

The column was filled with locally available natural sand. The2500 g of sand was sieved mechanically on a set of sieves and sievefractions were weighed. Ninety-three per cent of the sand was in fullrange of 0.212–0.425 mm grain size; the main fraction (with 74.4% oftotal weight) had a grain size of 0.425 mm.

Before filling it into the column, the sand was thoroughly washedwith distilled water to remove clay and other mineral contaminantsfrom the sand particles and dried in an oven at 120 °C overnight.

The lower section of the column contains 11 cm depth of coarsegravel (4.75 mm) above 2 cm underdrain of spherical glass balls. Thecoarse gravel was overlaid by 11 cm layer of 2 mm fine gravel which isfollowed by 22 cm of 1.18 mm coarse sand along with a 10 cm of0.15 mm fine sand at the top. The effective size (d10) and uniformitycoefficient (d60/d10) of the combined sand media were 0.43 mm and2.35, respectively.

The depth of the sand bed was so arranged that its top surface wasat a level with the flanged joint in the filter tube. A perforated PVC discwas mounted on top for the inflow tube to the column in order tosupply homogeneously wastewater to sand bed in column. Anoverflow opening port was attached at the top along the wall of filtercolumn. A T joint connection was inserted at the bottom of the filtercolumn to withdraw the filtered water and through which water isadded for cleaning the filter media. The Perspex tube was covered

Fig. 1. Schematic diagram of laboratory scale slow sand filter column.

with black ribbon to prevent the growth of algae on the media. Aconstant inflow was maintained by a peristaltic pump. The filter wasfed from the UASB reactor effluent, collected in a 100 L capacity plastictank. In order to maintain all the solids in suspension, UASB reactoreffluent i.e. influent for slow sand filter column, was slowly andcontinuously stirred at the rate of 70 rpm.

The criterion applied for terminating the filter run was theattainment of head loss i.e. due to clogging of filter. The desiredflow rate could not be maintained over this head loss. Cleaning of thefilter was done at the time of termination of the operation bybackwashing the filter. Slow sand filters are usually not backwashedbut in this case it was necessary as the influent turbidity and SSconcentration were higher than the recommended values.

The filter column was operated at different hydraulic loadings of0.14 m/h, 0.19 m/h and 0.26 m/h.

2.2. Sample collection

A UASB based sewage treatment plant of 38 MLD capacity at thecity of Saharanpur was selected for regular monitoring over a periodof 4 months (March 2007–June 2007). Grab samples of wastewaterwere obtained at the outlet of UASB reactor.

2.3. Sample analysis

Samples were collected from the outlet of filter column at theintervals of 6 h. As a result, four samples were obtained within 24 h.All the samples were assayed formicrobiological (total coliforms, fecalcoliforms, fecal streptococci) and physico-chemical parameters (COD,BOD, TSS and turbidity) as per Standard Methods [28].

3. Results and discussion

The range and average values of the physico-chemical andmicrobiological characteristics of the UASB reactor effluent used asthe influent for slow sand filter are reported in Table 1.

3.1. Determination of the optimal flow rate/hydraulic load

Filtration rate control is the key element in operation of filters. Fortreatment of surface water, generally a filtration rate of 0.1 to 0.32 m/h is recommended [29] but up to 0.6 m/h is reported in literature [30].This study was conducted in two phases. The first phase employed theoptimization of flow rate. The filter was operated at three differentflow rates of 1.0 L/h, 1.5 L/h and 2.0 L/h, i.e. hydraulic loading of0.14 m/h, 0.19 m/h and 0.26 m/h, respectively. The flow rate of slowsand filter was optimized with respect to the head loss measurement(as H2O) as well as residual total suspended solids (TSS) and BOD ofthe treated effluent. Fig. 2 displays the head loss profile of 7 daysoperational period at three selected flow rates. The shortest duration(4 days) of filter operationwas observed at a hydraulic load of 0.26 m/h. The head loss development at a hydraulic load of 0.26 L/h and0.19 m/h were observed faster than that of 0.14 m/h. The head loss ata hydraulic load of 0.26 m/h and 0.19 m/h reached at 44 cm and 46 cm

Table 1Characteristics of UASB reactor effluent.

Parameters Values (average)

pH 7.1–7.8 (7.73)Turbidity (NTU) 35–60 (56.5)TSS (mg/L) 110–180 (168)COD (mg/L) 109–256 (120)BOD (mg/L) 38–55(50)Total coliforms (MPN/100 mL) 2.3×105–2.3×107 (4.3×106)Fecal coliforms (MPN/100 mL) 4.3×104–9.3×106 (4.3×106)Fecal streptococci (MPN/100 mL) 2.3×104–2.3×106 (2.3×106)

Fig. 2. Head loss development in slow sand filter column at different flow rates(hydraulic loadings).

Fig. 4. SS removal in slow sand filter column at different flow rates (hydraulic loadings).

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H2O in a filter run of 90 h (app. 4 days) and 114 h (app. 5 days),respectively. However, the longest duration (7 days) of filteroperation was observed at a hydraulic load of 0.14 m/h. At thisparticular hydraulic load, the head loss development was observedslowest (40 cm H2O in 7 days).

The observations revealed that slow sand filter column extend itsoperation up to 40% more at a filtration rate (hydraulic load) of 1.0 L/h (0.14 m/h) in contrast to the flow rate of 1.5 L/h (0.19 m/h) and2.0 L/h (0.26 m/h). The filter run was terminated when the filtercolumn got fully clogged although the filtrate quality was stillacceptable (in terms of SS and BOD) for discharge in surface watersas well as for unrestricted irrigation.

In all three cases of different flow rates (Figs. 3 and 4), the averageSS and BOD of treated effluent was b30 mg/L at the time of filtertermination and in order to meet the water reclamation and effluentdischarge limits (SS: 100 mg/L and BOD: 30 mg/L) [32].

The BOD and SS removal efficiency of filter was observed to vary inthe range of 34–95% and 37–94% (flow rate: 1 L/h); 29–88% and 26–90% (flow rate: 1.5 L/h) and 27–82% and 22–85% (2.0 L/h), respec-tively. After 2 days of filter run, filter gave N80% BOD and SS removalat a hydraulic load of 0.14 m/h. Almost 80% of the SS and BOD wereremoved at a hydraulic load of 0.14 m/h. This percentage removal ofSS and BOD dropped only to 70% and 60% when hydraulic load wasincreased to 0.19 m/h and 0.26 m/h, respectively. Therefore, slowsand filters efficiently reduce the mass of suspended material andextend the filter run for more time at a hydraulic load of 0.14 m/h ascompared to the hydraulic load of 0.19 m/h and 0.26 m/h. Thus,lowering the flow rate increased the duration of the filter run withoutdeteriorating the filter effluent quality.

Fig. 3. BOD removal in slow sand filter column at different flow rates (hydraulicloadings).

3.2. Effect of slow sand filtration

Once the optimum flow rate has been determined, filter columnwas operated at a hydraulic load of 0.14 m/h in order to ascertain theefficiency of slow sand filtration with respect to the removal ofturbidity, SS, BOD, COD, total coliforms, fecal coliforms and fecalstreptococci.

3.2.1. Turbidity removalTurbidity is one of the most important parameters for monitoring

the performance of a filter. It is believed that turbidity serves as acarrier for nutrients and pathogens which can result in biologicalactivity.

The turbidity removal was observed to be significantly high andstable since the start of the experiment. The turbidity level in the filterinfluent ranged from 35 to 60 NTU (Avg. 56.5 NTU), whereas at aparticular operational period, the effluent turbidity levels lie in therange of 1.6 to 6.2 NTU (Avg. 2.9 NTU) during the entire study. Thismay be due to the maturation of biological layer at the top of the sandmedia. Following the development of this layer, the purifying bacteriabecome well established and play an important part in the treatmentprocess. The percentage turbidity removal ranged from 77% to 96%(Avg. 91.60%) during the entire study period. The average percentageremoval of turbidity i.e. 91.6 % in this study at the sand depth of 54 cmand effective size of 0.43 mm was better than the values reported byAl-Adham [12]. He reported a percentage turbidity removal of 88% forthe smaller size of sand with an effective size of 0.23 mm and a sanddepth of 84 cm. Our results are concomitant with those reported byCleasby et al. [31] (97.5% turbidity removal), for surface water wherethe influent turbidity concentration ranged from 0.35 to 18.1 NTU.

The observations suggest that using a combination of coarse andfine sand in a deep bed, rather than fine sand in a shallow bed leads toimprove the efficiency.

3.2.2. Suspended solidsThe concentration of suspended solids in the effluent of UASB

reactor was significantly high i.e. 110–180 mg/L (Avg. 168 mg/L) anda complementary polishing was observed in sand column with anaverage SS removal of 89.05% (range: 82–94%). However, the effluentSS concentration was varying from 11 to 30 mg/L (Avg. 17 mg/L) after36 h of filter run to entire study period. The observed SS removal of89% in our study is significantly higher to those reported by Al-Adham[12] i.e. only 68% at a hydraulic loading of 0.16 m/h and an effectivesand size of 0.23 mm. Our observations are in good agreement withEllis [9], as he observed almost similar (90%) removal of suspendedsolids.

The effluent concentration of SS is in compliance with the Indianstandards of effluent discharge in surface water bodies [32].

Fig. 5. SEM images: attached growth of (A) cocci and (B) rod shaped bacterial (C) diatoms (D) protozoa and (E) dense algal mat onto the sand particles.

Table 3Results of paired t-test: two sample for means between turbidity, SS, COD, BOD and TC,FC and FS.

Variables TC FC FS

Turbidity Data pairedr2 0.97 0.91 0.93

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3.2.3. BOD and COD removalAs expected, the average removal of both BOD and COD during the

first few hours (36 h) was modest i.e. only 43% and 34%, respectively.The average filtered BOD was 9.7 mg/L (Min.: 3.6 mg/L; Max.:11.5 mg/L), i.e. almost three times lower than the permissible limitof 30 mg/L [32].Whereas, the average BOD removal was 85% (Min.77%Max.: 93%) during a study period of 7 days. The observed removalefficiency of BOD is remarkably superior to the 35–45% removal ofBOD suggested in published results [23–25] of slow sand filteroperation, whereas, similar to those (86%) reported by Al-Adham [12].The overall COD concentration in filter effluent was measured as23.44 mg/L (Min.:15 mg/L;Max.: 27 mg/L). Slow sand filter was foundefficient in COD removal i.e. ranging from 71 to 83% (Avg. 79%) for themajority of the operational period. At the end of the filter run, the BODand COD removal efficiencies were 78 and 71%, whereas, the residualeffluent concentration of BOD (11.5 mg/L) and COD (27 mg/L) wasobserved under the permissible limit of effluent discharge [32].

3.2.4. Bacterial removalBacterial dynamics in slow sand filters consist of growth, decay

and protozoa grazing [33]. Filtration was effective in reduction of fecalcoliforms bacteria and in no case did the effluent of either filter mediaexceed the required geometric mean of 25 fecal coliforms/100 mL[34]. Samples for microbiological analysis were routinely collected at6 h interval from the outlet of filter column. The average influent

Table 2Correlation matrix for physico-chemical and microbiological variables measured fromfilter effluent.

Parameters Correlation coefficient, r

Turbidity SS COD BOD TC FC FS

TurbiditySS 0.97106COD 0.94738 0.96977BOD 0.95123 0.94946 0.98451TC 0.96539 0.96606 0.94082 0.94132FC 0.91326 0.97435 0.95739 0.92035 0.94954FS 0.92568 0.90996 0.81513 0.79754 0.90924 0.87322

concentrations of coliforms (TC and FC) and FS throughout the filterrun were 4.3×106 and 2.3×106 MPN/100 mL, respectively. Whereas,the lower effluent concentration of TC (Avg. 2.1×103 MPN/100 mL),FC (Avg. 1.3×103 MPN/100 mL) and FS (Avg. 3.8×102 MPN/100 mL)was observed after 36 h of operation. The average percentage removalof coliforms (TC and FC) and FS were observed 99.95% and 99.99%,respectively. That is significantly higher than 96% reported by Al-Adham [12] at a sand depth of 1.05 mwhich further indicates that theeffective sand size of 0.43 mm and a greater height of sand bedimproved the removal rate of total coliforms. Cleasby et al. [31]reported that the average percentage coliforms removal was over 99%using an effective sand size of 0.32 mm and sand depth of 94 cm. Theresults of the present study are better when compared to the findingsof Ellis [9] who reported the average percent removal of 97% for totalcoliforms bacteria using the effective sand size of 0.30 mm and thefindings of Bellamy et al. [35] who reported that the average coliformsremoval of 97% using the effective sand size of 0.29 mm and sand bed

t-statistic 2.5 2.7 3.6t-critical 2.1 2.1 2.1p b0.05 b0.05 b0.05

SS r2 0.97 0.97 0.91t-statistic 4.2 4.2 4.4t-critical 2.1 2.1 2.1p b0.05 b0.05 b0.05

COD r2 0.94 0.96 0.82t-statistic 6.9 6.9 7.3t-critical 2.1 2.1 2.1p b0.05 b0.05 b0.05

BOD r2 0.94 0.92 0.80t-statistic 8.0 5.2 5.9t-critical 2.1 2.1 2.1p b0.05 b0.05 b0.05

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depth of 97 cm. Our findings were also found superior pertaining tothe 90% removal of coliforms reported by Al-Sawaf [11].

After 3 days of continuous filter run, the effluent FC concentrationwas observed below the permissible limit of 1000 MPN/100 mLprescribed by WHO [36] for unrestricted irrigation.

The high coliforms removal efficiency achieved by slow sand filter ispartly explained by the slow filtration rate and fine effective size of thesand, but is also attributed to biological processes in the layer of slimematerial that accumulates above the sand surface (schmutzdecke) andwithin the upper layers of the sand bed. Most of the purification occurredat or about the surface sand layer in the mixture of humus, sand, algae,protozoa and metazoan referred as the “filter-skin schmutzdecke”. Thisbiologically active schmutzdecke layer onto the top of sand media wasconsidered themajor reasonof suchahighcoliforms removal in slowsandfiltration. The thickness of the schmutzdecke was approximately 0.5 cmafter 2 days of filter operation and increased to 2.0 cm after 7 days on thefilter bed. The SEM(Scan ElectroneMicroscopy) of the schmutzdecke layerwas performed and overall some 10–15 micrographs at variousmagnifications were taken of sand grains. Although it was not possibleto draw any quantitative conclusions from these photographs theimpression was evident of a very dense biomass of attached solids onthe sand grains viewed. The bacteria of cocci and rod shapes, diatoms,protozoans (Paramecium sp.) are frequently present and dense growth ofalgal mat are apparent the general impression is of an abundance of lifeinto the schmutzdecke layer (Fig. 5).

Due to the heavy growth of biomass the rate of removal is highestin the upper part of filters i.e. caused by better oxygen conditions,higher numbers of active protozoa, high bacterial biomass and smallerpore sizes due to biological clogging. Adsorption onto the sandparticle surface and predation by protozoa are two major bacterialimmobilization processes. According to Ellis [9] the removal ofcoliforms organisms was achieved principally in the surface layerbut this removal also continued, sometimes substantially through thewhole sand bed. The schmutzdecke removes natural organic matter,transforms synthetic organic compounds and retains pathogens,producing microbiologically safe water.

3.3. Interrelationships of physico-chemical and microbiological variables

The relationships between physico-chemical (Turbidity, SS, CODand BOD) and microbiological variables (TC, FC and FS) wereinvestigated by regression analysis of the data obtained for filtereffluent during the filter run at a hydraulic load of 0.15 m/h. Table 2depicted that the removal of turbidity, suspended solids and organicpollutant (COD & BOD) appeared to be related to the reduction innumbers of all types of indicator organisms (TC, FC & FS) with acorrelation coefficient r varying from 0.91 to 0.97 for turbidity and SS;0.82–0.96 for COD and 0.80–0.94 for BOD.

Our observations support the hypothesis that all the physico-chemical and microbial parameters exhibit a similar decreasing trendin slow sand filter as well as the decrease is driven by a common force.The removal of physico-chemical parameters in slow sand filters takesplace by straining and attachment by/to medium and previouslyremoved particles. Almost similar pattern of removal is followed bybacterial indicators i.e. by straining and attachment to biofilm growthand capturing by predators (suspension feeders and grazers) [37,38].

In order to test the significance of r values, t-distribution wasemployed in all cases. At 5% probability level it was observed that thevalues of t-stat are higher than the t-critical values (obtained from t-distribution table; Table 3) in all cases. It shows that the chances oferror in drawing out conclusions are less than 5%.

3.4. Paired two sample mean t-test

In order to determine whether the observed differences between twosets of experimental data were significant or not, data were subjected to

paired two sample mean t-test. Results obtained from the statisticalanalysis of data are summarized in Table 3. The results obtained frompaired t-test between turbidity and TC indicate that the critical values of t(2.1) obtained from the t-distribution table at 5% probability level is lowerthan the observed values of t-stat (2.5–3.6). Almost similar results werefound between turbidity and FC, FS and hence null hypothesis is rejected.Thus, it can be concluded that the real and significant differences existbetween the obtained experimental data for SS and TC, FC, FS in filtereffluent.

Similarly when paired t-tests were conducted between SS, CODand BOD in relation to TC, FC and FS, it was observed that the critical tvalues obtained from t-distribution table at 5% probability level arequite lower than the observed t statistical values in all cases. Thus,null hypothesis is rejected in these cases too. Hence, it is concludedthat the differences that exist among the observed sets of experi-mental data are quite obviously significant and not obtained bychance. These interrelationships can be helpful in routine monitoringas well as up gradation of slow sand filter efficiency in time.

4. Conclusions

Slow sand filtration could be a promising process for the posttreatment of small-scale UASB effluent. The quality of filter effluent interms of turbidity, BOD, SS and coliforms could be achieved for reusepurposes. Filter runs time and head loss is found to be mainlydependent on hydraulic loading rate. The maximum filter run of7 days was obtained at 0.15 m/h hydraulic loading. Development ofhead loss was extremely small during the initial period of filteroperation, which later increased exponentially and reaches itsmaximum value within 7 days. Furthermore, various pollutionindicators have been correlated in terms of turbidity, SS, COD, BOD,TC, FC and FS. All variables showed strong positive correlations withremoval of each other in the sand filter which proved to be vital forquick evaluation of microbial removals. The filter run of 7 days isconsidered to be short as the average influent turbidity (56.5 NTU)was high for a slow sand filter operation. Thus, to overcome thisproblem, roughing filters can be used to remove excessive turbidityprior to SSF treatment. As, the gravel rock roughing filters have beenused for decades as a pre-treatment method for source waters subjectto high fluctuations in turbidity.

List of symbols

The following symbols are used in this paper:

BOD = bio-chemical oxygen demandCPCB = Central Pollution Control BoardFC = Fecal coliformsFS = Fecal streptococciMPN = Most probable numbersSS = Suspended solidsNTU = Nephlometric turbidity unitPVC = Poly vinyl chlorideUASB= Up-flow anaerobic sludge blanketWHO= World Health Organization

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