Wat. Res. Vol. 35, No. 13, pp. 3127–3136, 2001 # 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$-see front matter PII: S0043-1354(01)00019-7 INTEGRATED BIOLOGICAL AND PHYSIOCHEMICAL TREATMENT PROCESS FOR NITRATE AND FLUORIDE REMOVAL ALEMAYEHU MEKONEN, PRADEEP KUMAR* and ARVIND KUMAR Department of Civil Engineering, University of Roorkee, Roorkee, 247 667, India (First received 1 February 2000; accepted in revised form 5 January 2001) Abstract}The feasibility of an integrated biological and physiochemical water treatment process for nitrateandfluorideremovalhasbeenevaluated.Itconsistedoftwosequencingbatchreactors(SBRs)in series. Performance of the process in the treatment of 24 synthetic water samples having nitrate concentrationsof40,80,120,160,200,and250mg/l(asN)andfluorideconcentrationsof6,10,15,and 20mg/latdifferentcombinationswasstudied.Denitrificationfollowedbydefluoridationprovedtobethe bestsequenceoftreatment.Inallcasesnitratecouldbereducedtoanacceptableleveloflessthan10mg/l (as N) at 3, 5, and 7h hydraulic retention times (HRTs) depending on its initial concentration. Fluoride concentrationsupto15mg/lassociatedwithnitrateconcentrationsupto80mg/l(asN)couldbereduced to an acceptable level of 1.5mg/l by alum-PAC slurry using alum doses up to 850mg/l [as Al 2 (SO 4 ) 3 16H 2 O] along with 100mg/l of powdered activated carbon (PAC). Additional alkalinity produced during denitrification was used up during defluoridation for maintenance of pH avoiding the need for lime addition. On the other hand, residual organics, turbidity, and sulfide present in the denitrified water were removed by alum and PAC at the defluoridation stage along with fluoride, eliminatingtheneedforanadditionalpost-treatmentstep.Athighernitrateconcentrations(120mg/las N),thealkalinityproducedatthedenitrificationstagewasintherangeof715–1175mg/lasCaCO 3 .This excessivealkalinityinhibitedreductionoffluoridetothelevelof1.5mg/latthedefluoridationstage,using alumdosesupto900mg/lalongwith100mg/lofPAC.Inallcases,afluorideconcentrationof20mg/lin water could not be reduced to the acceptable level of 1.5mg/l. # 2001 Elsevier Science Ltd. All rights reserved Key words}biological treatment, defluoridation, denitrification, integrated process, physiochemical treatment, sequencing batch reactor INTRODUCTION Nitrate and fluoride concentrations in groundwaters exceed desirable levels at various places throughout the world. According to the WHO guidelines and recommendations, the allowable concentrations of nitrate and fluoride in drinking water are 10mg/l (as N) and 1.5mg/l, respectively (WHO, 1984). Excessive intake of nitrate and fluoride can cause methemoglobinemia and fluorosis, respectively. The problem is more acute in rural and small urban communitiesparticularlyinthirdworldcountries.In India alone, 27845 habitations with approximately 25 million people are supposed to have water supply contaminated with fluoride (Paramasivam and Na- noti, 1997; Iyengar and Venkobachar, 1997). Simul- taneous occurrence of both fluoride and nitrate in high concentrations in natural water samples has been noticed at several places. Three such examples areRajasthanstate,andAgraregioninIndiaandthe Rift Valley region in Ethiopia where nitrate and fluorideconcentrationsashighas271mg/l(asN)and 3.2,178mg/l(asN)and21mg/l,and224mg/l(asN) and 26mg/l, respectively have been reported (Gupta, 1992; Pal, 1983; Ashley and Burley, 1994). Chemical precipitation by alum and lime and adsorption by activated alumina or bone char are extensively used for fluoride removal (Iyengar and Venkobachar, 1997), whereas ion exchange and biological denitrification are widely used for nitrate removal (Hoek and Klapwijk, 1987; Gayle et al., 1989). In India, the ‘‘Nalgonda Technique’’ for defluoridation based on fluoride precipitation using alum and lime has been successfully developed. This simple and economical method is widely used for defluoridation of domestic and community water supplies (Nawlakhe et al.,1975;NawlakheandRao, 1990; Bulusu et al., 1979, 1983). However, its use is limited to water samples having fluoride concentra- tions only up to 10mg/l. Water containing high fluoride concentrations (10mg/l) require a high dose of alum which results in an increase of sulfate *Author to whom all correspondence should be addressed. Tel.: +91-1332-72349; fax: +91-1332-75568; e-mail: indumfce@mail.rurkiu.ernet.in 3127