Treating both wastewater and excess sludge with an innovative process

Joumnal of Fromeral Sienes Vol. I.No6,pp.749- -756.2003ISN 10-02CAII - 26291XArticle ID: 1001-0742<(20003 >06-0749-08CLC number: X703 Document code: ATreating both wastewater and excess sludge with an innovativeprocessHE Sheng-bing, W ANG Bao-zhen, WANG Lin, JIANC Yi-feng(Schol of Municipal & Enrionomental Enginering, Harhin Iosiute of Technlogy, Harbin 10090. China. E-mail: heshenping @ sina.com)Abstract: The invaive process consists of biologiral unit for wastewaler treatment and onation unit for res sludge reatment. An serobiemenbrane bioreactor( MBR) was used to remove organics and nitrogen. and an anarobie reactor was added to the biologiral uni for the releaspof phospons cnnind a arobic sludge l0 enhance the removal of phospbons. For the exces sludge produed in the MBR, which wus fed toone rontu:t colunn and reacted with ozoe , then the oronated sludge was retuned to the MBR for further hiolngead tratment. Experimentalresults showed lhat this process could remove organcs, mitrogen and phosphorus eficicnl, and the removals for COD, NH)-N, TN and TPwere 93.17%，97.57% .82.77% and 79.5% . rspectively. Bach test idicaed that the sperifie ririfcation rate and peie drinifcatioinrale of the MBR mere 1.03 mg NH3.N( gMLSS'h) and 0.56 mg NOr-N/( gMISS.h), and deinication sems to be the re-iniig step.Inder the lest conditions，lhe sludge concentration in the MBR was kept at 5000- 6000 mg/L. and the wasird sludge w制1 0zonated at an ozonedose of 0. I0 kgQ)/kgSS. During the experimental period of lwo months, no exces sudger was wasted. and a rero withdrawal of excess sludkpewas implemented. Through eoomie analysis , it was found that an aditiona ozonation operting coet for treatment of both vestewater andexcrss sludge was only 0.045 RMB Yuan( USD 0.0054)1m3 waslewaler.Keywords: wastewaler treatment; excess sludge: aerobice menubrane bioreacte( MBR) ; anerbic reacor; onatin; ro withdrawal of exctssludgeIntroductionUrban wastewater is usually treated by conventional activated sludge process in China, which involvesthe biodegradation of organic pollutants by activated sludge in aeraled bioreactors, and then the activatedsludge could be separated by gravitaional stling in clanifiers. The optimal sludge concentration isgenerally in the range of2→3 g/L, which imposes large size of bioreactor( Defrance, 1998). In addition,a large amount of excess sludge is produced and the separation, dewatering, treatment and disposal of thisexcess sludge represent major investment and operating costs. With the rising costs and restrictions onsludge disposal, the minimization of sludge yield has become of inereasing imporance( Chen, 2000).Up to now，numerous researches have been conducted 10 reduce the production of excess sludge inwastewater lreatment( Wang，1991; Yasui， 1996; Lee, 1996; Sirand, 1999; Ghyool, 2000). However,there are some drawbacks in heir tests, i.e., minimization of excess sludge often results in sludgebulking, meanwhile, nitrogen and phosphonus can nol be removed efectively and efluent often can notmeet the requirement of National efluent standard . Therefore, it becomnes necessary to modify conventionalwastewater treatment process in order to reduce excess sludge production as well as to get high quality o[efluent.The aim of this study was to minimize the production of excess sludge a well as to get an excellentefluent. To achieve both the goals, combined aerobic MBR and anaerobic reactor process was chosen totreat wastewaler, and excess sludge produced in MBR was reacted with ozone，then the ozonated sludgewas retumned to the MBR and mixed with influent for further biological treatment.M laterials and methods1.1 Experimental equipmentA hollow fiber polyethylene ultrfitration( UF) membrane module with a total surface area of2.0 m' ,pore size o[ 0.05 μm and a lengh of0.35 m was used as the中国煤化工! lhe erobiend anaerobic reactors were 16.3 L and 5 L respectively .Freactorsrs wasMYHCNMHG750HE Sheng-bing et al .Vol.15F了1Toveat 1|maintained at 26弋 withthermostat. A water floal cock wasused to control water level of the|Haerobic reactor to keep the balance ofinfluent and efluent ( membrane13 EMuent|1411pemeate). The air was fed into the[2H13reactor with a mie:ro-bubble airdifuser, and air flow was adjusted by11|an air flow-meter. The mixed liquid2flowing into the anaerobie reactor waskept in suspension with a stirrer. TheSupernatant with rich phosphorushydraulicetention limes of6 tobe treated cbemicallywastewater in aerobic and anaerobicreactors were both 6 h. In addition,ozonation of the excess sludge wasFig. | Schenatic of the experimental syslemncarried out in a contact column with5. air difuser; 6. anaerobie reacor; 7. stirer; 8. rehum pump for sludge relesedan inner diameter of 5 cm and aphosphonus; 9. uir coprsorsr, 10. pesurzrd air slorage vesel; 11. air fow-mrter: height of 1.2 m，Ozone was12. air dier; 13. one getato; 14. 0zone contart tractor for sludge; 15. onaled intnoduced into the columnn through asludge returm pump; 16. clleting device for ozone ff-aisdiffuser located at the botton of thecolumn, and the (2z0nated sludge was returmed to the aerobic reaclor through a pump. A schematic diagramof the process configuration is presented in Fig.I.Experimental raw water and analytical methodsTable 1 The chancteristics of the tet wastewaterA synthetic domestic wastewater was used in this testParameterRangtMeanstudy，which was prepared by mixing tap water with certainpH6.72-8.167.44quantities of starch, sugar, NH,GI, Naz HPO, , NaH,P04COD, mng/L157.98- -352. 14232.0828.54- 39.8532.95 .and sodium bicarbonate， and its characteristics areTN. mg/L32.25-42.5636.89summarized in Table I.TP, mg/L2.5-5.03.721.3 Analytical proceduresCOD, NH,-N， TN, TP, MILSS and MLVSS were determined using the standard method issued by theEnvironmental Prolection Agency(EPA) of China(1989). pH was measured using glass eletrodesconnected to a pHS-3C pH meter. D0 was detected with an YSI( MODEL 50B) dissolved oxygen meter.The bilogial treatment unit was run for 4 weeks under contolled conditions lo reach steady state .Then the test continued for a total of two months， and the performance of whole treatment syslem wasanalyzed .2 Results and discussion2.1 Ozonation of excess sludgeOzone is a very reactive oxidizing agent. It can Teact with sludge compounds in two different ways, i.e., the direet and the indirect reaction( Stachelin, 1985) . Both reactions occur simultaneously. While theindirect reaction is based on the short-living hydroxyI radicals which do not react seeifically, the directreaction rate is lower and depends on the structure of the reactants. In this test, ozone was used as a celllysis agent to disintegrate the sludge cells and improve their biod中国煤化工ted sludgewas reltumned to the aeraion lank for fllowing biological treatme:TYHCNMHGNo.6Treating bouth wastewaler and excess sludpe with an invative prxess2.1.1 Characteristies of excess sludge ozonationIn this process configuraion, both sludge disintegration and waslewater lrealment were donesimulaneously in the same treament system. The schematic diagram is shown in Fig.2.Inlucot organicsCo0hCO: ↑H20Organic iternedintes十H20+ H20SolubJeOrganicsSolub!eEasy-biodegndable orgunicsBiodegradation，SludgcOxidutionBiosyoutbesisCell lysis( Sludgo+bydrolysis_Much less cxcess sludgeBiological unitOzonatioo unitFig.2 Schenatic diagran of the treatment procesThe process consists of ozonation and bidegradation stages . The ozonation enhances the biologicaldegradability of the ozonalted sludge ，which is then deconposed into three parts: one is directly oxidizedinto CO2 and H20, and another part is converted into soluble inltemediate organic produrts through celllysis and hydrolysis, and the third part is the remained sludge, which accounts for only a small pontion ofthe oniginal excess sludge. With an assumption that the biomass mineralization at oz0nation stage isnegligible ，then mass balances at biological treatmnent stage are formulated as follows( Yasui, 1994):V. = YQ(L,- 1.)- bVX.- Q.X.- 0AX. + yQe(X, + X.),(1)dX.v.' A =- QrX.- QnX.+ b'Qe(X. + X,).(2)Under steady conditions, the let -hand sides of Equations (1) and (2) are equal to zero, and boththe equations can be combined into the following form:Qw/V= YN, - bX,1X -[1-(y + b')]Qe/V.(3)Where，V is the efective volume of biological reacor(L); X, is the active biomass concentration ibiological reactor( mg SS/I.);名is the inactive bionass concentation in biological reactor( mg SS/L); Y isthe sludge yield( g SS/g COD); y is the yield of ozonated sludge(g SS/g SS); b is the specific decay rate(d-1); b' is (he residual raio of ozonated sludge to original excess sludge(g SS/g SS); l is the infLuentflow rate(Ld); Q# is the flow rate of the wasted excess sludge(Ld); Qp is the flow rate of ozonatedsludge(Ud); N, is the onganic loading rate(kg COD/(kg MLVSS'd)).It can be seen from the Fquation (3) that the sludge wasting rate is related to the parameters of Y,N, b. X/X, QR and {1- (y+ 6')]. Once the biological trealment process and the quality ofwastewater are delermined, Y, N, and b become constant, and the sludge wasting rate is dlependent on X.1x，Qn and[1-(y+ 6')]. x,1X reresents the active component in sludge and[1-