Transesterification of waste activated sludge for the reduction of excess sludge and biodiesel production

Date
2016
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Publisher
University of Delaware
Abstract
Managing large quantities of sludge generated from biological wastewater treatment poses both technical and economical challenges to the treatment of domestic and industrial wastewater. One approach that has been receiving considerable attention for decreasing sludge production is the cell lysis in the wasted sludge and returning the lysates back to aeration tank to promote the cryptic growth. Various cell disruption techniques, which include chemical, physical, mechanical and biological methods, have been successfully applied to achieve greater than 50% reduction in excess sludge production. However, these techniques have not been successfully adopted in fullscale applications primarily due to additional costs associated with these energy intensive or high chemical-demanding processes. In this study, disintegration of waste sludge by in-situ transesterification was studied as a new lysis technique to promote the cryptic growth. It is hypothesized that disintegration of waste activated sludge (WAS) by in-situ transestrification may be a novel and cost-effective technique to overcome the economic limitations of current lysis methods in the lysis-cryptic growth strategy for sludge reduction. Transestrification disintegrates cell membranes by disrupting phospholipid bilayer structure of bacteria cell membrane. In addition to extracting fatty acids, transestrification simultaneously transforms them to methyl esters, thus allowing the recovery of high-value product (biodiesel) from sludge, which may off-set the additional operation and chemical costs typically associated with lysis-cryptic growth strategy. The objectives of this study were 1) to investigate the recovery of biodiesel and the bioavailability of residual by-products from transesterification of waste activated sludge, 2) to demonstrate and validate the feasibility of transesterification as a viable cell disintegration technique for lysis-cryptic growth strategy using a benchscale sequencing batch reactor (SBR), and 3) to develop the mathematical model for SBR system coupled with the transesterification process. In-situ transesterification was explored as a cost-effective method to solubilize waste activated sludge (WAS) and produce biodiesel. The most critical parameters were heating temperature for protein and H2SO4 concentration for carbohydrate. All reaction parameters except for H2SO4 concentration and water content gave the positive effect on the lipid conversion to biodiesel. Xylene-facilitated transesterification showed the higher production of biodiesel than hexane-facilitated reaction in all transesterification tests. Under the optimum transesterification condition, the soluble material contained 93 mg glucose/g TSS, 302 mg BSA/g TSS, 80 mg biodiesel/g TSS, 60 mg P/g TSS (489.1 mg P/L) and the degree of COD solubilization was 47.5%. The composition of fatty acid methyl esters (FAME) in xylene extract was similar with palm oil biodiesel. The physical properties (51.1 cetane number and 5.3°C cold filter plugging point) of WAS biodiesel was comparable with vegetable oils. This study suggested that in-situ transesterification can efficiently solubilize waste sludge and simultaneously produce biodiesel having a satisfactory quality. Comparative study of WAS disintegration showed that solubilization of COD (49.5%) by transesterification and its product biodegradability (70%) was comparable to thermal NaOH hydrolysis (35.8% COD solubilization and 40% biodegradability). A laboratory-scale sequencing batch reactor (SBR) was operated with recycling transesterification-treated WAS back to the aeration basin, and 70% recycling of solubilized biomass resulted in 48% reduction of excess sludge generation. Quality of effluent from the test SBR was comparable (52.1 mg/L of TSS and 43.1 mg /L of SCOD) to that from the control SBR (40.5 mg/L and 42.5 mg/L) during 95 days of operation. This study demonstrated that transesterification can be a feasible technique for reducing excess sludge from activated sludge process while simultaneously producing valuable by-products (i.e. biodiesel). A mathematical model was developed for the activated sludge process coupled with the transesterification process. Biological and chemical reactions occurring in the model system were described by mass balance equations. The model was calibrated with the operation data of the laboratory-scale SBR. The key biokinetic model parameters (Y0, YLys, YMeOH, b, and XA) for the model were determined from the biological oxygen demand (BOD) tests of solubilized lysate from batch transesterification. The developed model simulated the excess waste activated sludge (WAS) production, treated water quality, and biodiesel production under various operational scenarios. The production of both excess WAS and biodiesel significantly increased at the following operation conditions: < 12 hours hydraulic retention time (HRT), < 10 days sludge retention time (SRT), and > 50% of return activated sludge (RAS) recycling. Increasing the transesterification efficiency had no significant effect on the yield of biodiesel in the model system. The simulation results of treated water quality was a satisfactory level between 7.8 and 12.4 mg COD/L under all scenarios except for poor operating case (19.2 mg COD/L under 80% methanol recovery and 70% RAS recycling). This model may be useful for the design of activated sludge system coupled with transesterification process and the prediction of its system performance.
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