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 11.9-13.9 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 http://www.habmigern2003.info/PDF/methane-digester.pdf

• Godfree, A., and Farrel, J. 200. Process for managing pathogens. J. Environ. Qual. 34(1), pp. 105-113.

• Krishna, P. (2009) Strategies to enhance sludge processing through anaerobic digestion. Retrieved from http://www.tawwa.org/TW11Paper/803.pdf

• Green Power India Organization. Biogas benefits. Retrieved from http://www.greenpowerindia.org/biogas_benefits.htm