Indian Journal of Engineering & Materials SciencesVol. 5, August 1998, pp. 217-222

Bacterial removal from secondary sewage effluent by a combineddownflow floating medium flocculator/prefilter and sand filter

P Peiris", J J Bailey', H H Ngob & S Vigneswaran'"

'Faculty of Science and Technology, University of Western Sydney, Hawkesbury, Richmond, NSW 2753, Australia

bFaculty of Engineering, University of Technology, Sydney, PO Box 123, Broadway, NSW 2007, Australia

Received 23 February 1997

A semi pilot-scale study conducted with the combined downflow floating medium flocculator/prefilterand sand filter indicated that, it is an effective filter systemfor removing bacteria from the secondary sewageeffluent. Since the filter system can remove more than 99% faecal coliform and faecal streptococci from thesecondary sewage effluent, the chlorine dose required to achieve the effluent discharge standard of 200cfu/lOOmL will be significantly reduced. It was found out that a suitable alum dose of 35 mgIL obtainedfromjar test was found to be an optimum dose for the filter system. The results also indicated that there wasno significant effect of filtration velocity on bacterial removal.

Deep bed filtration has a number of applications inwater and waste water treatment and in industrialwater reuse. The type and the size of filter mediaare the most important parameters in affecting theretention of particles within the filter bed, toachieve superior effluent quality and longer filterrun times. In conventional deep bed filters, thefilter media generally used are sand (single medium),anthracite and sand (dual media) or anthracite,sand and garnet (multi-layer). To fluidise thesehighly dense media during backwashing, a largequantity of water at a high backwash rate isrequired. In a water treatment plant, the water usedfor backwashing could be up to 5% of the totaldaily water production. Backwash requirementscan become very high in the case of directfiltration (more specifically in contactflocculation/filtration) where flocculants are addedto achieve flocculation, and the entire solid-liquidseparation is within the filter bed itself. With theobjective of reducing the backwash requirement,the use of synthetic buoyant filter materials (lessdense than water) has been proposed forflocculation and filtration in water and tertiarywaste water treatment!", The backwashing offilters of this type can be achieved with a smallquantity of water at a much smaller backwash

*For correspondence

velocity. Apart from this advantage, the floatingfilter media are also found to have a high solidretention capacity and low head loss development.Hence the use of floating filter media in directfilters (where a contact flocculation/filtrationarrangement is adopted) is a promising solution. Itovercomes the disadvantages of direct filtration,such as limited retention capacity, shorter filterruns (lengths of filtration cycle) and high energyrequirement for backwashing. .'

A laboratory-scale experimental study with akaolin clay suspension, natural surface water, andwaste water in a saturated down flow poly-propylene medium flocculatorlprefilter showedthat this system has a good pollutant removalcapacity. Further, it led to very low head lossdevelopment and produced uniform, microflocs(26-40 urn) suitable for direct filtration". Sinceboth flocculation and a significant amount of solid-liquid separation take place within the filter beditself, this filter can be used as a static flocculatorand a prefilter in place of conventional processesof flocculation and sedimentation. In this system,flocculation occurs during the contact of raw waterand flocculant within the pores of the medium (themixing is created by the flow of raw water andflocculant through the filter pores). This isfollowed by the separation of particles and floes inthe filter medium. A subsequent polishing filter

218 INDIAN J. ENG. MATER. sci., AUGUST 1998

(such as a sand filter) can then remove theremaining solids. The Super Waki-Shimizu system(using 1.1 mm diameter polystyrene beads in a 0.6m deep filter bed with upflow filtration) reducedthe suspended solids (SS) in the secondary sewageeffluent from 10-30 mg/L to 3-5 mgIL (ref. 5). AJohnson's upflow filtration system (using 4-6 mmdiameter, irregularly shaped, polyethylene beads)used as a prefilter at Bargo Water Treatment Plantin Australia successfully increased the existingcontact filter run time. It led to turbidity and colourreductions from 8 NTU and 25-30 HU to 0.7-3NTU and 7-25 HU respectively". A two-stagebuoyant coarse media flocculator proposed bySchultz et al.7

• provided an effective treatmenthigher loading rates (30 m/h) and shorter residencetimes (2-3 min) than the mechanical flocculatorstypically employed in treatment plants.

The combined system of floating medium-sandfilter has been developed successfully with theconcept of using floating medium filter as aprefilter whilst the sand filter as a subsequentlypolishing filter". This system is a direct filter thatoperates in a down flow mode, under constant headpressure, and incorporates in-line flocculation.Coagulated waste water enters the combinedsystem at the top where it comes into contact withthe floating medium. Flocculation then occurs in

the floating medium due to the promotion ofinterparticle contacts by the water flow aroundindividual grain of media. This is followed by theseparation of particles and floes by floating filtermedium. Thus, it has a dual function offlocculation and solid-liquid separation. A sandfilter wiII then help to remove the remaining solids.The performance of this system was remarkable interms of head loss development and filtratequality9.~o. However, the combined system has notpreviously been assessed. in terms of' bacterialremoval, and therefore a main objective of thisstudy is to investigate the effectiveness of thissystem in bacterial removal from secondarysewage effluent.

MethodsExperimental set-up

A series of filter experiments using thecombined system of downflow floating medium -sand filter were carried out with in-lineflocculation arrangement (Fig. 1). The combinedsystem used consisted of a perspex filter column(120 mm inside diameter (i.d.), 2 m height).Polypropylene beads (diameter = 3.8 mm; density= 0.87 g/crrr') were packed in the column to adepth of 480 mm and restrained by a grid (stainlesssteel coarse mesh) at the top. A coarse sand (ES =

SandFilter

To Foul Water TankRapidMixing

DosingPump(Alum)

Sampling( inlet )

BuoyantMedia

FlowMeterTo

-===manometer

_----Backwash Water

. Effluent I Sampling

Fig. I-Experimental set-up

PEIRIS o: al: BACTERIAL REMOVAL FROM SECONDARY SEWAGE EFFLUENT 219

1.7 nun; UC = 1.3) was placed at the bottom of thefilter column to a depth of 600 mm. A rapid mixingdevice was fabricated by tightly winding a 3-6 mmdiameter tube around a column to produce a rapidmixing time of approximately one second for allexperiments. Commercial alum (AI2(S04)3.16H20was used as tlocculant. The effluent from thesecondary clarified of a STP was used. The qualityof this waste water is sumrnarised in Table 1.

During the experiments, the filtration velocity inthe filter was maintained constant at a known value(7.5, 10 and 12.5 m'/m2 h). Samples were taken atthe inlet (prior to the chemical dosing point) and atthe outlet hourly. The filter column wasbackwashed after each filter run using acombination of air (60-80 psi) and water (mainspressure). Typical cleaning of the column involvedair scouring (5-10 s) followed by water backwash

Table I-Specific characteristics of waste water used over theexperimental period

Parameter Range Average

pI!Faecal coli forms (cfu/IOO ml.)Faecal streptococci

(cfu/lOO mL)

5.87-7.60 6.861.1x I06 -8.0x I03 1.0 x 105

2.7xI04-8.0xI02 3.3 X 103

10M

4000'"E=gi'0 ~ooou_- ~~'ti.-.!2011

10M

(30 s). This procedure was repeated three timesand followed by a final backwash with water for 5min. Enumeration of faecal coliforms and faecalstreptococci were carried out using the membranefiltration method outlined in the standard methods(A WW A, APHA, WPCF, 1996). Headloss throughthe bed was directly recorded from the manometerreading.

Results and DiscussionEffect of alum dosing on bacteria removal

To obtain an estimate of the alum concentrationrequired for the maximum removal of bacteria, aninitial jar test was performed. It is quite clear fromthe results presented in Fig. 2, that an alum dose of35 mgiL was the optimum for this secondaryeffluent. One can see from the figure that anyfurther increase in alum dose beyond 35 mgiL hasvery little effect on the bacterial densities. Thebacterial density actually starts to increase againwhen an alum dose of 50 mgiL is used, particularin the case of faecal streptococci.

With the jar test 'optimum' was determined andthe effect of alum dose was then tested using thecombined system by varying the doses fromo mg/L to a dose 28.6% under dose (25 mg/L) to

210

10

10 20O+---~--~--~--~---+--~--~----~--~--+---~Oo

Alum concentration(mgIL)

Fig. 2-Results of the Jar test (initial faecal coliform andfaecal streptococci were 9.0x103 and 9.0x102 cfullOO rnl.,respectively).

220 1NlJlAN 1. LNli. MATER. SCL, AUli UST 1<)<)8

an optimum dose (35 mg/L) to a 28.6% over dose(45 mg/L). From the results of the analysis ofvariance (ANOY A) for faecal colifonns, faecalstreptococci, it is seen that significant differences(I %) occur between the various alum doses forboth faecal coliforms and faecal streptococciremoval. This shows that alum dosing is necessaryif the system is to achieve maximum bacterialremoval from secondary sewage effluent. Figs 3and 4 and Table 2 indicated that although an alumdose of 45 mg/L led to highest removal efficiency,a jar test optimum dose was sufficient to achievean excellent result both in bacterial removal andhead loss development (up to 99% bacterialremoval with the very low head loss developmentof smaller than 20 ern after 240 min of filtrationtime at a filtration velocity of 10m3 1m2 h ).

Effect of filtration velocityThe performance of the filter system was

studied at three different filtration velocities (7.5,10 and 12.5 m3/m2 h) using alum as a flocculant. Itis seen from the analysis of variance that for the

100

99

98

97

96-;>-O~ 95E 0al-a::: 94

93

92

91

90

filtration velocities, there was no significant effecton the faecal coliform and faecal streptococciremoval achieved by the filter system (Table 3).This means that the filter system is just as effectiveat removing bacteria when operated at a filtrationvelocity of 12.5 m3/m2 h, as it is when operated at7.5 m3/m2 h. This phenomenon can be explained asthe bacteria might be associated with the 'large'suspended organic matter in the effluent, whichwould flocculate quite easily. From Fig. 3, amaximum faecal coliform removal of 99.69% wasobtained for a filtration velocity of 10 m3/m2 h,

Table 2-Mean per eent bacteria removal for various alumdoses

Alum doserng/l.

Mean % removal(faecal coliforms)

Mean % removal(faecal streptococci)

o 72.94" 77.22b

25 95.39" 91.89b

35 98.68 96.6145 99.60 98.23

Note: a. b denotes significant differences at the I% level,determined by Least Significant Difference (LSD)

99.69

Filtration velocity

m3J m2• h

Alumdose

25mgIL

hg 3-laecal coliform removal efficiency

PEIRIS et al.: BACTERIAL REMOVAL FROM SECONDARY SEWAGE EFFLUENT 221

c; 95>-O~ 94EO4»-0:: 93

92

99.46

12.11Filtration velocity

mll m2 h

Alumdose

Fig. 4--Faecal streptococci removal efficiency

Table 3-Mean percent bacteria removal for various filtrationvelocities

Mean % removal(faecal eoliforms)

Mean % removal(faecalstreptoccoci)

Filtrationvelocity(m3/m2 h)

7.510.012.5

92.1790.6892.10

91.5290.8390.62

using an alum dose of 45 mg/L. For all filtrationvelocities tested, more than 99% faecal coliformremoval was achieved except one filter run whichwas operated at velocity of 10m3 Im2.h with analum dose of 35 mg/L «99% faecal coliformremoval). It is believed that compositional changesof the influent between various days, is onepossible explanation for the exceptional case beingless than 99% efficient, as bacterial density didhave a relatively large range over the experimentalperiod. As shown in Fig. 4, greater than 99% faecalstreptococci removal was only achieved fortwo of the nine conditions tested, com parted tofive for faecal coliforms (Fig. 3). Average faecalstreptococci removals ~reater than 99% were

achieved at a filtration velocity of 7.5 m3/m2.husing alum doses of 35 and 45 mg/L. The lattergiving rise to a maximum faecal streptococciremoval of 99.46%.

ConclusionsThe experimental results lead to the following

conclusions:- More than 99% bacterial removal was

achieved by the filter system;Standard Jar Test procedures enable designersto select the optimum alum dose for the filtersystem. This study showed that suitableconditions for the filter system to obtain themaximum bacterial removal were: (a) alumdose of 35 mg/L and (b) filtration velocities- 3 2(up to 10.0 m 1m .h.

The combined system of down flow floatingmedium - sand filter is therefore very effective atremoving bacteria from secondary sewagetreatment.

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New Filtration Media and their Use in Water Treatment,

222 INDIAN J. ENG. MATER. scr., AUGUST 1998

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