An Innovative Process to Improve Turbidity and Organics Removal by BAC Filters

Journal of Ocean University of China (Oceanic and Coastal Sea Research)ISSN 1672-5182, October 30, 2006, Vol.5, No.4, pp.387-392http: / www. ouc. edu. cn/ xbywb/E-mail: xbywb@ oul. edu. cnAn Innovative Process to Improve Turbidity and OrganicsRemoval by BAC FiltersMIAO Jia', ZHAO Qingliang'，WANG Baozhen'), LI ji?)， and ZHANG Jinsong)1) School of Municipal and Environmental Engineering， Harbin Institute of Technology，Harbin 150090, P. R. China2) Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055，P. R. China3) Shenzhen Water (Group) Co, Ltd.,, Shenzhen 518031，P. R. China(Received March 22, 2006; accepted August 22, 2006)Abstract The turbidity criterion for the product water of a WTP according to the State Project ‘ 863' on the safeguardtechnology of drinking water in the southern areas of China is 0.1 NTU. The turbidity removal in the activated carbon filterwas analyzed in a pilot-scale test and an innovative technology to improve the turbidity removal in a biologically activatedcarbon (BAC) filter was put forward in order to meet the criterion. Experimental results showed that the enhanced filtrationby adding polymerized aluminium chloride (PAC) into the BAC filter was quite effective in turbidity control. The effluentturbidity was kept at a stable level (mean) of 0.033 NTU with a high removal of about 80% for influent turbidity of 0.110 -0240 NTU with an addition of PAC at 0.05 mgL~ ', meeting the requirement for filtrate turbidity equal to or less than0.1NTUC totally. In addition, the larger the PAC dosage was, the lower the effluent turbidity was. However, further im-provement of turbidity removal was not obvious for PAC dosages beyond 0.10 mg L~，and an optimal PAC dosage in therange of 0.05 - 0.10 mgL~' was proposed.Key words BAC filter; C enhanced filtration; PAC; micro-flocculationratio of more than 90%，However, it has been foundthat there exist in the water nanometric pollutants1 Introductionwhose particle diameters are less than 1 μm，such asThe demand for advanced drinking water treatment fine clay minerals, synthetic organics, humic sub-processes will be on the rapid increase due to the in- stance, oil and algae, which pose direct harm to hu-creasing possible pollution of drinking water sources man health, pollute the water quality severely andand the more stringent quality standards of drinking complicate the treatment process ( Edzwald, 1993;water in China. The major drinking water source in O' Melia et al, 1987). In the conventional treatmentShenzhen City is Shenzhen Reservoir which receives process, it is very common that the flocs are floated tofresh water with light pollution from the Dongjiang the surface and the sand filter can be penetrated easilyRiver, a main tributary of the Pearl River. The source by this kind of pollutants, thus resulting in worsenedwater in Shenzhen City is characterized by low- turbi- filtrate water quality and increased ss loading rate ofdity and high-algae content. Although the source wa-BAC filter with filtrate turbidities possibly unable toter quality has been improved greatly by the biological meet the strict criterion. Therefore, more attentionpre-treatment project and the modification of water should be paid to the influence of BAC filter perfor-supply works between the Dongjiang River and Shen- mance on the filtrate turbidity, for the filter acts aszhen Reservoir, it is necessary to employ an advanced the last treatment unit before the clear water reser-drinking water treatment process to meet the require- voir.ment stipulated in‘ Shenzhen Drinking Water QualityThe influence of BAC filter performance was ana-Upgrading Program in 2010' (Zhang et al, 2005).lyzed in this study and an innovative technology - theTherefore, an advanced treatment process will be im- micro flocculation and enhanced filtration technologyplemented in a demonstration project of Bijiashan was put forward to improve the turbidity removal inWTP.the BAC filter.中国煤化工ic or organicThe turbidity criterion of the demonstration project filtration accelHCNMH G° BAC fiterfor the filtrate of the biologically activated carbon to destabilize the ouustoutto pivulig urbidity, such(BAC) fiter is very strict: 0.1NTU with a guarantee as particles, colloidal matter and algae remaining inthe water and to alter their hydrophilic quality, mak-* Corresponding author. E mail: zhaoql1962@ 163. coming it easier for them to contact the filter media surface388Journal of Ocean University of ChinaVol.5, No.4, 2006and to bremoved more efficiently due to the higher et al. (1999) and Dick (1992), and TOC (Total Or-adsorption and trapping ability of the BAC filtering ganic Carbon) concentration was determined by themedium (Luan et al, 1997; Amirtharajah， 1988).analysis meter of Phoenix type 800. The analysis ofThe mechanism was similar to that of micro- floccula-Permanganate Index was carried out according to thetion and direct filtration (Letterman, 1987), and the acid potassium permanganate method, and UV2s4 wasadsorption ability of filter media surface was fully uti- determined with the ulraviolet absorbance at 254 nmlized to purify the source water by the method of con-(without adjustingpH) by tltact and flocculation.Type 752, which represented the removal efficiency ofthe double _bond NOM ( Natural Organic Material).2 Materials and MethodsThe hydraulic head loss was read directly by a piezo-metric tube on the BAC filter.2. I MaterialsThe flow chart is shown in Fig.1. The sand filter3 Results and Discussioneffluent from water treatment plant was pumped intothe pilot-scale ozone contactor by the submerged pump 3.1 Turbidity Removal by BAC Filterand the oxidized effluent (i.e. influent to the BAC fil-The analysis results of influent and efluent turbidi-ter) flowed into the BAC filter through the static mix-ties of the BAC filter in the normal operation and 24 her and pipes. The filtration accelerator was dosed byafter backwashing are all presented in Figs.2 and 3,the metering pump and then mixed with influent b:the static mixer into the micro -flocs before enteringrespectively .the BAC filter. The sand filter effluent was also used0.y= 0.7968x+0.0015to backwash the BAC filter.， 0.R2-0.8693The pilot-scale BAC filter was used with the follow-ing parameters: carbon diameter = 360 mm, carbonlayer thickness= 2 m，empty bed contact time ( E-! 0.BCT)= 12min, filtering velocity= 10mh - 1 and treat-ment capacity= 1m'h '. The BAC filter had been instable operation for more than two years.Onlineurbidimeter0.40.60.8 .Influent turbidity (NTU)Static mixeFig.2 Regression analysis of BAC influent and effluent tur-idities.BACComputerMeteringfily_=0.8093x+0.004330 tFlow meterR2= 0.9738lus。的0.2520 tBackwashingwaterEltluent0.15 tFlow meter valveValve flow meter.10 tFig.1 Flow chart of the pilot- scale test..05 tThe polymerized aluminium chloride (PAC) is very0.00.1.4effective in removing the organics in low- turbidity wa-Infuent turbidity (NTU)ter due to its strong electro neutralizing and destabiliz-ing capacities, rapid flocculation reaction kinetics andFig.3 Regression analysis of BAC influent and effluent tur-reaction characteristics of combination flocculationbidities 24 h after backwashing.(Luan el al., 1997; Bjrar, 1999). PAC with an Al2O3content of 9. 8% was utilized as the flocculant in theIt can be seen in Figs.2 and 3 that the turbidity wastest .removed by the, BAC filter to some extent and the in-fluent turbidity中国煤化工rable correla-2.2 Analytical Methodstion with the eY片CNMHGr. the BACTurbidity was measured with an online turbidimeter filter played a negative rore In a certain period of timeof 2100SP type (HACH Company) with the accuracywhen the influent turbidity was low due to the waterof 0.001NTU. The measurement of AOC ( Assimil- quality and operation situation of the BAC filterable Organic Carbon) followed the description of Zhou (Fig.4).MIAO J. et al: An Innovative Process to Improve Turbidity Removal by BAC Filter3890.140.123.2 Optimal Chemicals Dosage0.In previous studies, the filtration accelerator dosagehad an important influence on micro- flocculation and0.08enhanced filtration (Luan el al.，, 1997)， and the des-0.06y=0.8706x+0.029tabilizing effect of colloidal matter in water and theR2=0.73150.04 .size and quality of the flocs were all directly affected.08by the chemicals dosage. The flocs were too small ifInfuent turbidity (NTU)the dosage was insufficient, then they could penetrateFig4 Research analysis of BAC effuent turbidity with the :the filtering bed easily, affecting the efluent qualitylow- turbidity influent.severely. On the other hand, oversized flocs caused byan excessive dosage would lead to over trapping in theJudged from long: term operation data, the BAC fil- filter surface layer and the trapping capacity of the fil-ter is capable of turbidity removal to a certain extent,tering bed could not be fully utilized, leading to abut the effluent turbidity can be over 0. 1NTU whenrapid increase of head loss, decrease of the filter periodthe influent water quality is worsened and the removaland reduction of water yield.by the BAC filer is unstable with the effluent underThe turbidity removal efficiencies with differentthe criterion when the influent turbidity is low.chemical dosages are shown in Table 1. It took 20 -Therefore, a novel technology for drinking water safe- 30 min for the sand filter efluent to flow through thety under the condition of high efluent turbidity should BAC filter completely after the addition of PAC andbe provided to make effluent turbidity stable and less then the effluent turbidity remained at a stable level asthan 0.1NTU, and an innovative technology - the mi- shown in the table.cro-flocculation and enhanced filtration technology wasThe turbidity removal by the BAC filter was 27.1%brought forward .without PAC addition. But it was found in the experi-Table 1 Turbidity removal efficiencies with different chemical dosagesPAC dosageMean removal (%)(mgL~I)variation range (NTU)(0. 140 (0.131-0.178)0.102 (0.093 -0.113)27.10.050. 178 (0. 154 - 0.228)0. 068 (0.051 - 0.087)62.40.100.144 (0. 121 - 0.245)0.041 (0.039 - 0.045)71.50.200.186 (0. 127- 0.296)0.038 (0.031 - 0.044)80.30.300. 158 (0.107 - 0.261)0.036 (0.032 - 0.042)77.2ment that the effluent turbidity was less than ogy, the reaction mechanism was not identical to that0.1 NTU, close to the criterion even with the trend of of coagulation-sedimentation completely ( Yu, 2005).increase in the earlier stage, later the effluent turbidi- It takes 30 - 60 min to mix，react and settle in the co-ty increased to over 0.1 NTU and the effluent quality agulation- sedimentation process, while the mixing owas rather unstable even under the condition of con- influent and flocculant used in micro-flocculation andstant influent turbidity. The above results showed enhanced filtration was accomplished in a few min-that, firstly， the effluent turbidity could be reduced utes, in which it was difficult for the particles to beeffctively by the BAC filter with the addition of a integrated into bigger ones in a short time; besides,small amount of PAC when the influent turbidity was the conditions for the formation of larger flocs werein the range 0.150 - 0.200NTU, and the effluent tur- not adequate due to the fact that the influent waterbidity could be kept at a quite stable level and did not quality to the BAC filter was much better than thevary with the fluctuation of influent water quality raw water as there was lttle impurity in the influentwhen the PAC dosage was more than 0.10 mgL- '.to the filter. We therefore come to the conclusion thatSecondly, the turbidity removal by the BAC filter was better filtration could be attained by micro flocculationimproved greatly with increased PAC dosage. Yet fur- and enhanced filtration because the surface property ofther improvement of turbidity removal was not obvi- the particles in中国煤化工at they wereous by increasing PAC dosage beyond 0. 10 mg L~ 1easy to be adsoI YHCN M H Gwhile theef-This implies that when micro- flocculation is applied to fect of size- increascu pas uuc Ull uitc cmancement ofimprove the BAC filter filtration， a small dosage of the filtration process could be neglected.PAC will be able to satisfy the demand.Polymerized aluminum chloride (PAC) was appliedThough the flocculant was used in the new technol- in the experiment, and the optimal dosage should be390Journal of Ocean University of ChinaVol. 5, No. 4, 2006not only suitable for the influent water quality, but al- variation of effluent turbidity of the BAC filter withso beneficial to the formation of micro-flocs ( Lei the fluctuation of influent turbidity in certain ranges.et al, 1999). Therefore, the PAC dosage should beThe test had continued for about 80h and was di-as small as possible while the effluent water quality of vided into 7 stages as shown in Fig.7. The PAC wasBAC filter was to be ensured and a dosage range of not dosed during the first stage of about 15h and the0.05 - 0.10mgL~ 1 was regarded as suitable.mean value of the effluent turbidity was 0.111 NTU,The organics removals by the novel technology withwhich was worse than the effluent criterion requiredlow PAC dosages are shown in Figs.5 and 6.by the demonstration project with the influent turbidi-ty kept at a stable level of about 0.150 NTU. There-8fore, a dosage of 0.05 mg L-' of PAC began to beadded. During the initial half an hour, the effluent望8turbidity variation was not obvious， but then it de-scended continuously, being kept stable in the range of880.038 - 0.050 NTU eventually despite fluctuations in7influent quality, indicating the obvious effect of PACBAC influent PAC=0PAC-0.05PAC-0.10on filtration enhancement. Approximately 9 h later,PAC/ (mgL')the PAC dosing was stopped and the effluent turbidityFig.5 AOC removal by the BAC filter with low PACincreased from 0.046 NTU to 0. 116 NTU graduallydosages.with the influent turbidity in the range of 0. 167-1.0.198 NTU. However, the effluent turbidity dropped。1.to 0.048 NTU immediately with the PAC being addedagain and remained stable with a mean of 0.055 NTU。0.9in spite of the great fluctuation and high level of influ-g 0.6ent turbidity (0.200 - 0.450 NTU) in the following14.5h. Subsequently the PAC was added intermit-tently and the effluent turbidity underwent similar~BAC influent PAC=0PAC=0.05PAC=0.10variations. It was therefore considered that it took 20- 30min for the reaction PAC to set in when it wasFig.6 TOC removal by the BAC filter with low PACadded for the first time and then the effluent turbiditydescended quickly. But the turbidity dropped right. It was found that the AOC removals were 4.65 %，away when PAC was added again. In addition, the6.98% and 6.98% respectively at the PAC dosages ofeffluent turbidity increased gradually instead of imme-diately when the dosing of PAC was stopped, which0, 0.05 and 0.10mgL~I and the 7; removals werecould be caused by the residual PAC in the BAC filter.21. 13%，27.46% and 22. 54 % respectively, whichAccordingly， the intermitent dosing of PAC can beindicated that a part of AOC and TOC could be re-performed as follows: PAC is added when the effluentmoved by the BAC filter, while the novel process didturbidity is higher than the upper limit，for instancenot play a remarkable role in AOC and TOC removals.0.1NTU in this study， and the effluent turbiditywould drop promptly in about half an hour. The PAC3.3 Removal Efciency by Different Operation Modesdosage could be stopped when the effluent turbidity is3.3.1 Intermittent PAC dosagedecreased to the lower limit and would not be startedFirstly，the influence of intermittent PAC dosageuntil the effluent turbidity rises to the upper limit a-on the turbidity removal by the BAC filter was studiedgain. Moreover， automatic drug- feeding equipmentwith a PAC dosage of 0.05 mgL" '. Fig.7 shows thewith variable frequency and speed can be employed inpractical operations (Wu et al, 2003).0.Influent Effluent。0.43.3.2 Continuous PAC dosage后0.3-PAC Dosing「DositStop dosingIn this stage， the influence of continuous PAC自0.2dosage on the removal by BAC filter was studied at the莒0.1same dosage of中国煤化工ater tempera-ture and ozoneMHCNMH G.mgL-'re-17:002:30 12:00 21:00 7:00 16:45 2:15 11:45 21:15spectively.' The' lun alu ellluell turbidity varia-Day IDay 2Day3Day4tions of the BAC filter are shown in Fig.8.Fig.7 Turbidity removal by the BAC filter by intermittentThe effluent turbidity of BAC filter was steady allPAC dosage.along with a mean value and removal of 0.033 NTUMIAOJ. et al: An Innovative Process to Improve Turbidity Removal by BAC Filter391and 80.4 %，respectively, while the influent turbidityvaried in the range of 0.110 - 0.235 NTU.As mentioned above, the PAC dosage in the BAC4 Conclusionsfiltration process was definitely beneficial to the efflu-The effect of BAC filter on turbidity removal effi-ent water quality. However， the increase of water ciency was examined through pilot-scale tests and anhead loss caused by the addition of chemicals should innovative technology to improve the turbidity removalnot be ignored (Lei et al, 1999). The water headin BAC filter was put forward. The conclusions areloss variation of the BAC filter with the filtration time drawn as follows:can be seen in Fig.9.Turbidity could be reduced to a certain extent by0.25 rthe BAC filter and the result was affected by the influ-ent water quality greatly. The effluent turbidity could).2 tInfluentnot satisfy the criterion of 0.1 NTU as required by thedemonstration project of Bijiashan WTP when the in-fluent turbidity was more than 0. 1 NTU. It was0.1 ttherefore necessary to take effective measures to im-Effluentprove the turbidity removal.05上---..........It was shown from the results of enhanced filtrationfor the BAC filter that 1) The enhanced filtration wasJan.5Jan.10Jan.I5Jan.20Jan.25quite effective in turbidity control. The effluent tur-Fig.8 Turbidity removal by the BAC filter by continuousbidity was kept at a stable (mean) level of 0.033 NTUPAC dosage.with the removal of about 80% for influent turbidity0.110 -0.240NTU with a PAC addition of 0.05Backwashing Backwashing Backwashing BackwashingmgL-'; 2) The larger the PAC dosage was, the low-er the effluent turbidity was. However， further im-provement of turbidity removal was not obvious by in-creasing PAC dosage beyond 0.10mgL-'; 3) The re-息2:moval of organics, such as AOC, TOC, etc. by theg2enhanced filtration process was not remarkable;4) Automatic control was proposed to use intermittentPAC dosage in practical operations.Jan.3Jan.8Jan.13 Jan.18Jan.23Jan.28Fig.9 Head loss variation of the BAC filter.AcknowledgementsThe effluent head loss of the BAC filter decreasedThis study was supported by the National Founda-constantly with the filtration time. However, the tion Research Program of China (No.2002AA601 120).formed flocs were not large enough to clog the filter me-dia seriously when the PAC dosage was 0.05 mgL-',Referenceswhich resulted in a slow increase of water head loss.The water head could be recovered to its original valueAmirtharajah, A.. 1988. Some theoretical and conceptualafter each backwashing with the intensity, durationviews of filtration. J. AWWA，80(12): 36-46.and expansion rateof 40m3 m-2h -', 10minand 15%，Bjrar, E., 1999. Coagulation-direct filtration of soft, lowrespectively，which indicated that a PAC dosage ofalkalinity humic water. Wal. Sci. Tech, 40(9): 55 -62.0.05mgL" 1 was suitable for micro- flocculation andDick, V. D. K, 1992. Assimilable organic carbon as an in-dicator of bacterial regrowth. J. AWWA, 57- -65.enhanced filtration.Edzwald, J. K., 1993. Coagulation in drinking water treat-Part of TOC could still be removed by the BAC fil-ment: particles, organics and coagulants. Wat. Sci. 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