Experimental Study on Minimization of Sludge Production by Ozonation Process

Journal of Donghua University (Eng. Ed. ) Vol.20, No.3 (2003) 69Experimental Study on Minimization of Sludge Production byOzonation ProcessHE Sheng-bing(何圣兵)Schood of Municipul & Emionenal Engineing. Hurtin lsiue of Tchrology, Harbin, 15090XUE Gang(薛罡)TU91 AColle of Enironmental Science and Enginering, Donghua Uniersiy, Shanghai, 20051WANG Bao-zhen(王宝贞), WANG Lin(王琳)School of Municipal & Entironmental Enginering, Hurbin Intiute of Technology , Harbin, 150090hypothesis of reducing sludgeproduction under cryptic growth conditions, oz0neMaterials and Methodswas used as cell lysis agent to treat excess sludge,and then the ozonated supernatant was returned toTwo vertical flow submerged bioflm systems were run inthe aeration tank. The results show that COD andparallel; one as a control system with discharging the excessNH: -N removal efficiencies in ozonation process weresludge directly, the other as a test system with recycling of87. 96% and 84. 42%，respectively. Mcanwhile, a lowsupematant that was achieved by ozonaing the excess sludge.excess sludge yield coefficient of 0.113 (g ss/g CODEach of the reactors had an aeration tank volume of 16.3 L andremoved) was obtained. Compared with that of thean inclined tube sedimentation tank volume of 8. 5 L. Thecontrol test, the process configuration decreased thevolume of biomass carrier was 30% of that of the aerationexcess sludge production by 51.3%。tank. Ozonation was achieved in a column reactor with aKeywords: ozonation， sludlge minimitation, cnyptic grouth,diameter of 4 cm and a height of 100 cm. A schematic of theexcess sludge yieldprocess is presented in Fig. 1.IntroductionTo vent↑A large quantiy of biosolids can be produced in aconventional wastewater treatment plant， the handling,treatment, and ultimate disposal of waste biosolids accounts for4,from 40% to 65% of the operating costs of the plant". Sincethe use of sludge in agriculture is debatable and sludgeincineration cannot be a systematic solution, minimization ofSolids towassludge production should be preferred.Solids to waslc◆Every tchnology used for minimization of sludgeproduction should not impact the efficiency of the wastewaterFig. 1 Schematic arangenent of the experimental systemtreatment. Lee and Welander minimized sludge production byI - -control system process configuration;predating protozoa and metuzoa, the apparent sludge yields inI --test system process configuration;the process varied between 0.01 and 0.23 kg TSS/kg COD1- storage tank; 2- -balance tank; 3--submerged biofilm reactor;removedl21. Haner and Mason demonstrated that the net4- -inclined tube sedineotation cank; 5- -air copressor;biomass growth could be reduced under cryptic6-山storage tank; 7- 0zone generator;conditions". Which is to say, when the organic carbon in8- -contact reacior for ozone and sludgemicroorganism is used as substrate of metabolism proceA mixture of starch, sugar， NH,CI， K,HPO," andsludge production will be reduced. Based on these hypotheses ,KH,PO with an equivalenl average COD of 426 mg/L andit was assumed that oz0ne could be used as a cell lysis agent,an average BOD of 244 mg/L was uilized as the food sourcebecause ozone is a strong oxidation agent, it can promote theto represent typical domestic wastewater. The ratio COD :biodegradation of the cell, then the treated supematant wasN: P was about 100: 10: 1.retumed to the acration tank, which will minimize the netThe detention time of wastewater in submerged biofilmexcess sludge production.aeration tank was 6 hours, and all sttled sludge was returnedto the aeration tank from stling chamber in both the controlRecived Jan.2, 2002Corepourdence should le adresed ln HE Sheng bing. Dr. ↓E-muil; heshengbin@ sina. com中国煤化工YHCNMHG70Joumal of Donghua University (Eng. Ed.) Vol. 20, No.3 (2003)and test systems. In aeration tank, DO was about 3 mg/L andCOD of sludge before ozonation became SCOD at the samethe temperature was kept at 26 t with a heater.ozonation time. These results indicated that sludge ozonationled to both solubilization and mineralization of the sludge.Results and DiscussionCOD decreased from 866 mg/L to 768 mg/L and SCODincreased from 115 $0 193 at a ozonation time of 3 hours. .1Influence of ozonation on COD,Ozone could disrupt the cell walls and cause the release of theSCOD, VSS/sS and SV% of sludgeorganic substance from the walls. The rate of carbonFollowing procedures were done in order to determine thesolubilization was high in the first 2 hours and accounted forozone dosage for treaing excess sludge: ozone gas was86% of that in total 3 hours of ozonation, and the sludgesupplied at the bottom of the contact reactor, SS, VSS, COD,mineralization rate decreasedwith the prolongation oSCOD and SV% of the activated sludge were obtained atozonation time. This phenomenon might be due to that the lowdffrent reaction time. Then the plots of COD, SCOD, VSS/concentration ozone couldn't oxidizc sludge cells to a greaterSS versus ozonation time were drawn. Based on all trends, aextent. The influence of ozone concentration on tesuitable ozone dosage might be achieved with which the cellsmineralization and solubilization of sludge should be furthecould be solubilized and mineralized to a certain degree as wellinvestigated. In addition, the ratio of inorganic substanceis the reatment cost for excess sludge should not be tooincreased with the ozonation time, which is presented inexpensive. The esult is shown in Figs.2,3,4.Fig.3.It could be seen that VSS/SS decreased from 0.842 to)00 r500.775 in3 hours of ozonation time. It was an indication that700 t8二28-=-8200part organic substance in biosolids was solubilized or50曾mineralized by ozone. During the first 2 hours, organics00。00decreased quickly and decreasing rate became slow with the300 t-0-COD-0- SCOD0只extended ozonation time.00 lFig.4 shows the relationship between ozone dosage and>.52.5the SV% of the sludge. SV% was found to decrease withOzonation time/hincreasing oz0ne dosagc. It can be seen that SV% decreasedfrom 42 to 21.5 in 3 hours of ozonation time ，which indicatedFig.2 Relationship between ozone time and COD, SCODthat the sludge stting characteristic was improved greatly. Itwas assumed that the filamentous bacteria in activated sludgewere damaged and their structure was changed by ozone also.0.Consequently, these resutls suggested that the czonation has aremarkable effect on maintaining and improving the sludge舅0.7settlig characteristics. Previous study has reported similar” 0.results of ozonation on the sludge stting characterctics'Based on the above experimental findings, 2 hours wasconsidered to be an appropriate ozonaion time. Ozone00.511.22.53concentration multiplies by ozonation time and air flow, anddivides by MLSS of the sludge , then the paratmeter of 0.025Fig3 Efect of ozonation on VSS/sS of sludgekg O,/kg ss was obtained.2 Comparison on treatment effect of theS0two systems40 FThe two reactors were run for 1 month under identical。30conditions to reach steady state. During this period, both曼2(reactors were fed with the sane synthetic fee at an average1(nlow rate of 65.2 Ld and the excess sludge was wasted fromboth of the reactors daily. In the test reactor, the daily excesssludge wastage from the test reactor was ozonated with an00.5.，11.522.ozone dosage of 0. 025 kg O/kg ss. Upon ozonation, thesludge was stted for 1.5 hours and the supermatant was thenFig.4 Effect of ozonation on SV% of sludgerecirculated back into the aeration tank of the test reactor.The data were obtained with an ozone gas concentration ofThus, the test reactor received the solubilized sludge with a1.2mg/L，SCOD concentration increased whilecorCOD of about 630 mg/L in addition to the infuent syntheticconcentration decreased with increasing ozonation time.feed.Especially, more than 10% of COD decreased at an ozoneThe ozonation of the test reactor continued for 76 days.dosage of 25 mg Og/g MLSS. Furthermore, about 9% of" of the reacos for uhe中国煤化工YHCNMHGJournal of Donghua University (Eng. Ed. ) Vol. 20, No.3 (2003) 7|total of 76 days are given in Table 1.The sludge yield coeffcents of the control reactor variedfrom 0. 163 to 0. 306 and averaged 0. 232, whereas theTable 1 Operating parameters of the controland test reactorsozonated test reactor had an average yield coeffcient of0. 113. .Compared with the low yield coefficient of the control reactor，ParumelerControl reactor Test reactorozonation process decreased its value by 51. 3%.3 Economic analysisInfluent flow mte (Lday)65.2An ectric fee of about 10 Yuan (RMB) is needed toRecirculated ozonated sludge (L/day)3.16produce 1 kg of ozone gas. Based on the experimental data,Delention time (hrs)the operation cost of sludge ozonation could be calculated. ASs of the excess sludge