Particle size distribution and removal in the chemical-biological flocculation process

Available online at www.sciencedirect.comJOURNAL OFScienceDirectENVIRONMENTALSCIENCES,JESISSN 1001-0742CN11-2629/XJoumal ofEnvironmetal Sciences 19(2007) 559 -563www.jesc.ac.cnParticle size distribution and removal by a chemical-biologicalfocculation processZHANG hi-binl:2", ZHAO Jian-fu', XIA Si-qing', LIU Chang qing', KANG Xing sheng31. State Key Laboratory of Plluion Conrol and Resource Reuse Research, Colle of Emironmental Science and Enginering,Tongji Univrsity, Shanghai 200092, China2. College of Muricipal and Emironmental Engineering, Shandong Jianzhuu Universit, Jinan 250101, China. E-mail: zhazhb@163.com3. Shandong Academy of Envirorunental Sciences, Jinan 250100, ChinaReceived 15 May 2006; revised 7 August 2006; accepled 22 August 2006AbstractThe particle charaterization from the infuent and efluent of a chemical biological foculation (CBF) processlaser difraction device. Water samples from a chemically enhanced primary treatment (CEPT) process and a primary sediment tankprocess were also analyzed for comparison. The results showed that CBF process was not only efctive for both the big sie particlesand small size particles removal, but also the best particle removal process in the three processes of CBF process CEPT process, andPST process (primary sediment tanks). The resuts also indicated that CBF process was superior to CEPT process in the heavy metalsremoval. The high and non-selecive removal for heavy metals might be closely related to its strong ability to eliminate small particles.Samples from dfferent locations in CBF reactors showed that small particles were casier to aggregate into big ones and those disuptedflocs could properly focculate again along CBF reactor because of the biological focculation.Key words: particle removal; chermical-biological focculation (CBF); chemically enhanced primary treatment (CEPT)Introduction(CBF) can improve small particle removal greatly (Blancaet al, 2000; Hallvard, 1998; Poon and Chu, 1999). TheIt was estimated that particle organic matter and phos-basic meaning of CEPT is to use chemical coagulants tophorous accounted for more than 70% of the total organicinduce coagulation or flocculation and allow the finely-matter and phosphorous in municipal wastewater (Levinedivided particles to form large aggregates so that they canet al, 1985; Tiehm et al, 1999). Particles (<10 μm)be easily separated from water by gravity. Through thehave the largest specific surface among all particles andchemical addition, CEPT process achieves good treatmentplay a key role in the dynamics and equilibrium of theefficiency not only in removal of particles, but also inphysico-chemical surface reactions. Trace elements, suchremoval of organics and phosphorous. Blanca and hisas heavy metals, can be adsorbed on the surface of thesecolleagues found that the particle removal efficiency wassmall particles (Daniel and Markus, 1997; Kavanaugh et 82% for the particles in the range of 20 -80 um at theal, 1978). According to a study carried out by Landadosage of 72 mg/L FeSO4 and 1.2 mg/L P-17, whereas theand Jimenez (1997), the number of helminth eggs wasprocess was less effective in eliminating particles smallerin close correlation with the suspended solids number. than 10 um (Blanca et al, 2000). CEPT process couldTherefore, the particle removal, especially small particles,reduce more than 80% suspended solid (SS) and 70% totalis essential and great interest for the municipal wastewaterphosphorous after the addition of 30 mg/L FeCl3 and 0.5treatment plants. Particle removal in primary sediment mg/L polymer (Poon and Chu, 1999). In Europe, CEPTtanks (PST) is achieved primarily through gravity in tradi-process is used in some wastewater treatment plants as ational secondary wastewater treatment plants. PST usually useful way to futher decrease the level of heavy metalshas good removal eficiency for the particles (>50 pum),discharged into water body (Charles and Bernard, 2005).whereas more than 85% of the particles in effluent wereCBF process is a new advanced primary treatment processsmaller than 32 pum (Neis and Tichm, 1997). Compared in wh中国煤化irough chemical coag-with traditional PST, the chemically enbanced primary ulatiocompared with CEPT,treatment (CEPT) and the chemical-biological flocculation CBFYHCNMHGnall particles becauseProject supported by the Hi-Tech Research and Development Programof biological flocculation. Previous studies indicated that(863) of China (No. 202AA601320); the Shandong Environment Pro-CBF was superior to CEPT in the removal of SS (Wu ettection Bureau Program (No.2006032, 2006043) and the Ph.D Fund of al, 2003). Since ss only represents the particles >10 μmShandong Jianzhu University (No. 624006, 2006043). *Correspondingand smaller particles are a substantial part of solids, it isauthor. E-mail: zhazhb@ 163.com.56ZHANG Zhi-bin et al.Vol. 19important to study the removal eficiency of particles <10out to determine which flocculant was the most suitable inμm in CBF process.this study. PAFC was added at the CBF mixing tank and theThis paper aimed to study the characters of particlesCEPT mixing pipe respectively and the dosage dependedbetween 0.4 -2000 um, especially with respect to smallon the influent TP concentration. PAM (dosage 0.5 mg/L)particles in the influent and efluent of the CBF process.was added at the third channels of both CBF and CEPTSamples from a CEPT process and PST process of Antingreactors. The influent temperature was 21.6 -25.5°C, andWastewater Treatment Plant were also studied for compar-pH was 6.79-7.73 during this experiment.ison.1.2.2 Flow chart of Anting Wastewater Treatment Plant1 Materials and methodsThe flow chart of Anting Wastewater Treatment Plant isshown in Fig.2.1.1 Municipal wastewaterInfluent Grit chamber公Raw municipal wastewater used in this experimentSedimentAerated! SecondaryEffluent- >0>口→was collected continuously from the Anting WastewatertanksedimentTreatment Plant, Shanghai, China. After the screens andSludge return↓Waste sludgegrit chamber, the wastewater was pumped to the adjustingWaste sludgetank of the pilot-scale treatment process.Fig. 2 Flow chart of Anting Wastewater Treatment Plant.1.2 Experimental set-upThe primary sediment tank was of typical radial shape1.2.1 Pilot-scale experimental set-upand had an internal diameter of 30 m, and an effectiveThe pilot- scale experimental set- up is shown in Fig.1.height of 6.5 m. Anting Wastewater Treatment Plant hadan average flow of 50000 m'/d, and the hydraulic retentiontime of its PST was 2.0- 2.5 h. PST effuent was sampledMixingCBF reactorBmuentat its overflow weir.Influent1.3 Removal of CODa, ss, NH4*-N, TP, and heavySludge return. Waste sludgemetalsThe influent, CBF effuent, CEPT effuent and PSTAdjusting! Sediment Efluent40→由由山effuent were sampled daily for 30 d, and CODcr, SS,NH4*-N, and TP were measured with their respectivepipeCEPT reactorstandard methods (Wei et al, 2002).Infuent, CBF efuent and CEPT effluent were sampledFig. 1 Pilot-scale experimental set-up.at exactly the same point of time (April 15, 2005), of whichThe CBF process consisted of fast air strring mixingheavy metals were measured with an IRIS Advantage 1000tank, CBF reactor, sediment tank, and sludge return device.Inductively Coupled Plasma Spectrometer (Thermo Jarrellhe mixing tank had an effective volume of 30 m', anAsh Company).effective height of 0.55 m, and its hydraulic retention1.4 Particle size distributiontime (HRT) was 60 s. The CBF reactor consisted of threeplug-flow channels with declining aeration rate, effectiveParticle size distribution of the infuent, CBF effluent,volume of 1.2 m', effective height of 0.55 m, and HRTCEPT efluent and PST effluent was analyzed once a weekof 35 min. Aeration was supplied to the channels throughfor a month and the results were comparable to each other.microhole aeration pipes and the aeration rate could beIn this article, only one result was presented. Particle sizeadjusted separately in each channel. The sediment tank hadditribution was determined by a Coulter Ls230 whichan effective height of 1.2 m, and HRT of 1.5 h. Sludgemeasures particle sizes from 40 nm to 2000 μm by laserwas returned to the CBF reactor with pump (NEMO).diffraction.The CEPT process consisted of mixing pipe, CEPT re-Three samples (1 L) were taken from each chamberactor, and sediment tank. CEPT reactor included threealong the CBF reactor and CEPT reactor, and then theseserially- connected chambers and had the same volumes assamples were allowed to precipitate for 30 min in beakersCBF. Wastewater in the chambers was completely mixedof 1 L. Top 300 ml supermatant was taken from eachthrough mechanical force.sample to analyze its particle size distribution to studyThe capacity of the pilot-scale plants was 100 m3/d.the evolution of flocs along the CBF and CEPT process,The sludge retum ratio of CBF was 33% and dissolved中国煤化工oxygen (DO) in three chambers was contolled at 1.9 -3.22RCNMHGmg/L, 1.3- -2.5 mg/L, and 0.3 -1.5 mg/L, respectively. TheTYHliquid polymeric flocculant PAFC (Al2O3: 10.8%, Fe2O3:2.1 Removal of CODcr, SS, NH4+ -N, and TP1.8%, Guoxin, Jiashan, China) and the aiding flocculantPAM (Shanghai Chemical Reagent Corporation, China)The monthly average concentrations of CODcr, sS,were selected after a series of jar tests which were carried NHs*-N, and TP in diferent samples are presented inNo. 5Particle size distribution and removal by a chemical-biological flocculation process61Table 1.Particles were distributed in 0.4 -3.0 um with the mean sizeIt is shown in Table 1 that the effluent of advanced pri-at 0.819 um in CBF efluent, showing the highest removalmary treatment processes (CBF and CEPT) were superiorefficiency for both large particles and small particles dueto that of PST, and the best removal eficiencies for CODc,to the combination of chemical flocculation and biologicalSS, NH4*-N, and TP was obtained in CBF process.flocculation. Stan and Despa (2000) also found that the2.2 Particle size distribution in infuent, efuent ofCBE,combination of single particle was the rate-limiting stepof flocculation, and the presence of surface proteins overCEPT and PST processesmicroorganisms promoted the aggregation of small parti-The typical size distibutions of particles are shown incles.Fig.3.2.3 Removal of heavy metals in CBF and CEPT0FThe removal results for heavy metals in CEPT and CBFare shown in Table 2.30 F... CEPTMore than 90% of Cr was eliminated whereas the re-moval rate of Cu, Sn, Ag, Pb, Zn, and Ba was only 29.9%, .0f43.6%，78.4%, 30.2%, 57.0%, and 45.5%, respectivelyin CEPT, suggesting that its removal capacity of heavymetals was selective. CBF appeared to be more effectiveand non-selective in heavy metals removal compared withCEPT, and the removal rates for Cr, Cu, Cd, Ag, Pb,50 100500 1000Zn and Al were all above 75%. The diferences in theParticle dianeter (um)removal of heavy metals between CBF and CEPT were .Fig. 3 Particle size distributions in infuent, CBF effuent, CEPT efuentprobably due to their varying capacities in removing smalland PST eluent. The meanings of CBF, CEPT and PST are the same asparticles. The total surface of particles <10 μm in munic-in Table 1.ipal wastewater had a significant effect on the speed andThe particles had the total volume of 689.4x109 μm3equilibrium of physico-chemical surface reactions. Tracein the influent and were distributed in 0.4 -340 um withelements, such as heavy metals, may be adsorbed on thethe mean size at 29.96 um. A portion of large particlessurface of these small particles. CEPT was less effectivewere removed in PST, which resulted in the decreasein small particle removal and its ability to remove heavyof the volume of the particles to 478.0x109 pum3 inmetals depended on the chemical coprecipitation. SpecialPST efuent. PST was less effective in eliminating smallcoprecipitant was usually selective in nature; thereforeparticles, particles in its effluent were mainly distributedCEPT exhibited different removal rates for different heavyin 0.4 260 μum with the mean size at 25.14 um, makingmetals. CBF achieved high removal rate in small parti-no much difference to the influent. The particle volumecles, thus heavy metals adsorbed on these particles werein CEPT efluent decreased to 129.2x109 μum3 due toeliminated significantly from wastewater, resulting in highthe chemical focculation, and particles >110 μm wereheavy metal removal rates with non-selective character.completely removed through CEPT. While, particles >3This was confirmed by the research of Kavanaugh et al.um were almost removed completely in CBF efluent, and(1978), which found that the removal of heavy metals wasmost of particles <3 um were also removed, resulting in theclosely correlated with the removal of small particles intotal particle volume of 5.157x109 um3 in CBF effuent.wastewater treatment processes.Table 1 Remnoval of CODCr, ss, NH4*-N, and TPCODcr (mg/L)NH4+*-N (mg/L)TP (mg/L)ss (mg/L)Infuent171.4+102.213.71+5.183.40+1.79107+12155.6+47.510.48+4.460.96+0.4310+15CEPT85.2+65.211.72+4.931.32+0.8327+20PST120.2+80.612.35+4.993.03+1.2559+66CBF: Chemical-biological focculation process; CEPT: chemically enhanced primary treatment process; PST: primary sediment tanks.Table 2 Removal of heavy metals in CBF and CEPTCr (mg/L)Cu (mg/L)Sn (mg/L)Ag(中国煤化工(mg/L)Ba (mg/L)0.9490.12050.02870.022MHC NMH G2110.23570.0760.08440.01620.0040.1285NCEPT92.0%29.9%43.6%78.4%30.2%57.0%45.5%CBF0.00620.00700.00460.00050.07500.0720.058699.4%94.2%84.0%97.8%88.5%95.0% .75.1%n: Removal rate; CBF: chemical-biological flocculation process; CEPT: chemically enhanced primary treatment process.562ZHANG Zhi-bin et al.Vol. 192.4 Particle size distribution and evolution of flocsThe concentration of particles in CBF reactor was highalong the CEPT processdue to sludge recycle, so these particles collided moreThe particle size distribution of samples taken along thefrequently with each other than in CEPT. On the otherhand, biological flocculation promoted small particles toCEPT reactor are shown in Fig.4.be adsorbed on active large flocs. The volume of particlesin Samples 1,2, 3, 4, and 5 was 48.67x109 um3, 57.38x109um3, 37.86x109 um3, 53.46x109 μm3, and 52.19x109t Sample 1---中Sample 2μm', and particles were distributed in 0.4 146.2 μm, 0.4-+rr Sample 3131.8 pum, 0.4-114.1 pum, 0.4 -123.4um, and0.489.66umrespectively.It was inferred from Fig.5 that along the CBF reac-tor, although flocs were disrupted for many times, they3 2-could properly reaggraded. These phenomena could notbe explained appropriately by traditional chemical floc-culation theory which holds that disrupted flocs hardly050 1001000agglomerate again, so typical chemical flocculation plantsParticle diameter (um)always consisted of serially connected tanks mixed byFig. 4 Particle size distibution of supematant sampled along CEPTmechanical force with deckling rate. Reaggration tookreactor. Samples 1, 2 and 3 are taken respectively from each chamberplace in CBF because biological flocculation helped toalong the CEPT reactor.bridge disrupted flocs together. It is in agreement with theresearch by Wilen and Kristian (2004), which found thatThe volume of particles in the supernatant of Sample 1aerobic biological activity was of the greatest significancewas 104x109 μm', and these particles were distributed infor the observed reflocculation of broken flocs. Previous0.4 -110.6 um with the mean size at 25.89 um, indicatingresearch on microbial community in CBF also showed thatthat focs formed in the first chamber were mainly of smallthere was stable and special biological community after 30size. In the second chamber, some big focs which were notd of running duration (Xia et al, 2005).present in supernatant were observed. The particle volumeAlthough at the end of the CBF reactor, the volume ofof Sample 2 decreased to 53.46x109 um3 and particlesparticles in Sample 5 increased to 52.19x109 μum3 due towere distributed in 0.4 -69.61 pum with the average size atflocs disruption, there was certain height of sludge bed in21.13 pum. After the PAM addition, more small particlessediment tank which could catch small particles due togrew into big flocs in the third chamber. The volume ofthe biological adsorption. Therefore CBF achieved goodSample 3 decreased to 37.84x109 um3 and particles wereparticle removal rate, and the volume of particles in CBFdistributed in 0.4 47.94 um with the mean size at 19.57effuent was only 5.157x109 μm3 in Fig.3.μm. Along the CEPT reactor, the particle size ditributionscurve gradually shifted towards small particles, suggesting3 Conclusionsthat the evolution of flocs was slow and gradual.2.5 Particle size distribution and evolution of flocsParticles in the infuent, CBF effluent, CEPT effluent,and PST efuent were distributed in 0.4 -340 um, 0.4 3.0along CBF processum, 0.4-112.6 pum, and 0.4 -260 um, respectiely. The bestThe particle size distributions of samples taken along theparticle removal efficiency was achieved in CBF and lttleCBF reactor are shown in Fig.5.small particles was left in CBF effluent compared withthose in CEPT effluent and PST efluent, which indicated.0 Fthat CBF were effective in both big size and small size时Sample 2particles removals.什Sample3The removal efficiencies in CEPT process for Cr, Cu,. Sample 5Sn, Ag, Pb, Zn, and Ba were 92.0%， 29.9%， 43.6%，3.0 F78.4%，30.2%, 57.0%， and 45.5% respectively. While,those in CBF for Cr, Cu, Sn, Ag, Pb, Zn, and Ba were.099.4%, 94.2%, 84.0%, 97.8%, 88.5%, 95.0%, and 75.1%.0 trespectively. CBF appeared to be superior to CEPT in theremoval of heavy metals, and the capability of high andnon-selective removal of heavy metals might be closely5 1010010000elat |中国煤化工in flocculation reactorsFig. 5 Particle size distribution of supermatant sampled along CBF reactor.:howYHCN M H Gasier to agaded intoSamples 1 and 4 were taken respectively from the begining of the firstbigger ones and those disrupted flocs could properly floc-and the third channel; Samples 2. 3 and 5 were from the end of eachchannel.culate again in CBF because of the biological flocculation.No.5Particle size distibution and renoval by a chemicalbiological focculation process63ReferencesNeis U, Tichm A, 1997. Particle size analysis in primary andsecondary wastewater effluents[J]. Wat Sci Tech, 36(4):Blanca J, Alma C, Alberto L et al, 2000. Sand and synthetic151-158.medium filtration of advanced primary treatment efuentPoonC s, Chu C W, 1999. The use of frric chloride and anionicfrom Mexico City[J]. Wat Res, 34(2): 473- -480.polymer in the chemically assisted primary sedimentationCharles G, Bernard R, 2005. Contribution of municipal efuentsprocess[J]. Chemosphere, 39(10): 1573- -1582.to metal fuxes in the ST. Lawrence River[]. Environ SciStan S, Despa F, 2000. On the doublet formation in the floccula-Technol, 39: 456- 464.tion process of the yeast cll[J]. Biosystems, 57: 139-145.Daniel K, Markus B, 1997. Particle removal in diferent filterationTiehm A, Herwig V, Neis U, 1999. Particle size analysis forsystems for tertiary wastewater treatment- - a comparison[J].improved sedimentation and filtration in waste water treat-Wat Sci Tech, 36(4): 259 -267.ment[J]. Wat Sci Tech, 39(8): 99-106.Hallvard D, 1998. Optimized particle separation in the primaryWeiF s, XuX B, Yan J C et al, 2002. Water and wastewaterstep of wastewater treatment[]. Wat Sci Tech, 37(10): 43-analysis manual[M]. 3th ed. Beijing; China Environment5Science Publisher.Kavanaugh M, Toregas G, Moon C, 1978. Particulates and traceWilen B M, Kistian K, 2004. Flocculation of activated sludgepollutant removal by depth fitratio[J]. Prog Wat Tech,flocs by stimulation of the aerobic biological activity[J].10(5/6): 197- -215. .Wat Res, 38: 3909- 3919.Landa H, Jimenez B, 1997. Particle size distribution in an ffuentWuZP, XiaSQ, Yang D H et al.,. 2003. Comparative research offrom an advanced primary treatment and removal duringchemical and biological flocculation process to municipalfitraion[]I. Wat Sci Tech, 36(4): 159-165.wastewater[J]. Chongqing Environmental Sciences, 25(7):Levine A D, Tchobanoglous G, Asano T, 1985. Characterization12-14.of the size distribution of contaminants in wastewaterXiaS Q, Wang F, Fu Y G et al, 2005. Biodiversity analysis oftreatment and reuse implications[J]. J Wat Pollut Controlmicrobial community in the chem-bioflocculation treatmentFed, 57: 805- 816.process[J]. Biotechnology Bioengineering, 89(6): 656- 659.中国煤化工MYHCNMHG