Lipid Composition And Biodiesel Production By Enzyme Catalysis From Sludge Of Municipal Wastewater Treatment Plant

With the exacerbation of global energy shortage and environmental deterioration,biodiesel had attracted more attention because it is clean and renewable. However thehigh cost of raw materials, severely limited the industrialization of biodiesel producing.Therefore, it is necessary to seek a cheap and sustainable raw material. Municipalsewage sludge containing lipid, could be used as a raw material for the production ofbiodiesel. In this paper, the sludge or scum as the research object were used to studythe effect of drying methods on the extractable lipid content, main components andfatty acid composition of extracted of lipid from sludge. The scum was collected fromthe grit chamber and the primary sedimentation tank and the sludge was collected fromthe primary sedimentation tank and the secondary sedimentation tank from the No.4wastewater treatment plant(WWTP) in Xi'an. Then the scum from primarysedimentation tank was used as raw material to study the catalytic conditions of lipaseNovozym435and the yield of biodiesel. The main conclusions were as the following:(1) The extracted contents of lipid were differed in different sludges. For example,the lipid content of the scum from the grit chamber had the maximum value (37.40%of dry sludge), while the sludge from the secondary sedimentation tank had theminimum value, only7.67%of dry sludge.(2) The saponification value and acid value of these four sludges were different.Compared with the saponification value, the difference of acid value is smaller. Thesludge lipid from the primary sedimentation tank and the secondary sedimentation tankhad a high saponification value. The extracted lipid of the sludge or scum from the grit chamber and the primary sedimentation tank, were mainly free fatty acids,saponification and acylglycerol (AGs), which could be converted into fatty acid methylester (FAME), and the percentage accounted for more than50%of the sludge lipid.However, only about25%of the sludge lipid from the secondary sedimentation tankcould be converted into biodiesel.(3) The fatty acids in sludge lipid mainly included myristic acid (C14:0), palmiticacid (C16:0), palmitoleic acid (C16:1), stearic acid (C18:0), oleic acid (C18:1) andlinoleic acid(C18:2). The relative contents of fatty acid composition in different sludgelipid were different, which led to the content of the saturated and unsaturated fatty acidester different in the biodiesel. Even the same sludge drid by different methods alsohad different content of the fatty acids. In addition, freeze-drying sludge had a highercontent of saturated fatty acid compared with the sludge dried by baking.(4) When the scum from the primary sedimentation tank was used as raw material,the optimum conditions to produce biodiesel through enzyme catalysis was as thefollowing: the ratio of methanol and sludge, tert-butyl alcohol and sludge, n-hexaneand sludge (v/w) being10,12, and8respectively; the ratio of Lipase Novozym435and sludge being0.6:1(w/w); reaction temperature and time being24h and45?.Under the above conditions, a maximum FAME yield of9.95%was obtained.(5) When the enzyme of Lipase Novozym435and Lipozyme TLIM wascombined at the ratio of3:1(w/w), the FAME yield was8.85%, which was89%of theFAME yield gained at the optimum condition. From the view of yield and cost, thereasonable ratio of lipase Novozym435and Lipozyme TLIM was3:1.(6) In order to reduce the cost of enzyme catalysis, Lipase Novozym435wasused repeatly in this experiment. The results showed that organic solvent could recoverthe lipase. The efficiency of the lipase would remain at least75%of the originalefficiency when it was used for5times. Thus recycling the enzyme would be a goodway to reduce the cost of catalysis.