Increasing biogas production by thermal (70◦C) sludge pre- treatment prior to thermophilic anaerobic digestion Presented by Reem Satti

Background Wastewater treatment process overview Facilities are designed in stages, each stage either removes particles or changes dissolved material into a form that can be removed. Modern wastewater treatment plant stages: – Influent – Primary treatment – Secondary treatment – Tertiary treatment – Effluent discharge

Biogas Production The creation of biofuel through anaerobic decomposition of organic materials Benefits – Production of energy – Transforms organic wastes into high quality fertilizer – Improves hygienic conditions – Environmental advantages

Objectives Investigate the effect of a low temperature pre-treatment on the efficiency of digestion of primary and secondary waste sludge

Methods Sludge sampling and characterization – Primary and secondary sludge was obtained from a wastewater treatment plant (MWWTP) near Barcelona. – Samples were collected weekly and stored at 4 degrees Celsius until use – Primary sludge (PS) and secondary waste activated sludge (WAS) are thickened and mixed – Mesophilic anaerobic digestion (38 ◦C)

Methods Low temperature pre-treatment – Beakers with 0.5 L of sludge were submersed in a thermostatic bath at 70 ◦C during 9, 24, 48, and 72 h. – Samples of raw and pre-treated sludge were analyzed for total solids, volatile solids, total dissolved solids, volatile dissolved solids (VDS), volatile fatty acids (VFA), and pH. – Effect of pre-treatment assessed via increase in VDS and VFA

Methods Anaerobic batch tests – Used to determine biogas production of raw and pretreated sludge samples – Conducted at 55 ◦C – Inoculum: thermophilic sludge from the effluent of a 5L continuous stirred tank reactor – Substrate: Pre-treated or raw sludge – Blank treatment with inoculum only (determines biogas production due to endogenous respiration)

Methods Reactor constituents – 100g of inoculum, 50 g substrate (blank treatment: 150 g of inoculum) – Purged with N2 Bottles were incubated at 55 ◦C Biogas production – Pressure increase in the headspace

Methods Analytical methods – Solid content of sludge determined using different procedures including centrifugation. Supernatant underwent filtration and suspended particles were deduced – VFA and biogas composition were determined by gas chromatography

Results and Discussion Total dissolved solids and volatile dissolved solids increased after thermal pre-treatment, as expected – 1.5 g increase VDS in raw sludge compared to g VDS after 9, 24, and 48 h pre-treatment. This means that the proportion of soluble to total organic matter increased by almost 10 times, from 5% to almost 50% after pre-treatment.

Results and Discussion VFA concentration – Acetic and propionic acids were the main VFA generated after 24h. – Butyric and valeric acids were mostly detected after 48 h.

Results and Discussion At day 10, accumulated biogas production was nearly 300 mL for 9, 24, and 48 h pre-treated samples. The control was 200 mL, representing an almost 50% volume increase

Results and Discussion Pre-treated sludge results show that the process was more efficient in terms of biogas production and yield in all cases (30% higher). Methane content was always higher after sludge pre-treatment. Results suggest that a short period (9h) low temp. pre-treatment should be enough to enhance methane production.

Limitations Effluent hygienisation was briefly discussed – Reduced E. Coli, and Salmonella was absent – Should have tested for other bacteria Duration for the control treatment and experimental treatment were different (1 year vs. 6 months) Statistical analysis Shorter periods should be tested (3h, 6h?)

References Directory of Industrial Supply and Agriculture. (2003). Biogas Production. Retrieved from Godfree, A., and Farrel, J Process for managing pathogens. J. Environ. Qual. 34(1), pp Krishna, P. (2009) Strategies to enhance sludge processing through anaerobic digestion. Retrieved from Green Power India Organization. Biogas benefits. Retrieved from