Influence of anaerobic co-digestion of sewage and brewery sludges on biogas production and sludge quality

Abstract

This research investigated operating parameters and treatment efficiency for the digestion of sewage and brewery sludge. The prime objective of this study was to enhance the quality of treated sludge for use as agriculture fertilizer and to enhance biogas production, a by-product that can be used as an energy source. Three bench-scale completely stirred tank reactor (CSTR) anaerobic digesters were operated at mesophilic condition (36 ± 0.2°C). A mixture of sewage and brewery sludge were used as substrates at ratios of 100:0, 75:25, 50:50, 25:75 and 0:100, based on wet weight basis (w/w). For each digester, the solids retention times (SRT) were 20 days. The organic loading and volatile solids loading were between 1.3–2.2 kg chemical oxygen demand (COD)/m³/day and 0.9–1.5 kg/m³/day, respectively. The digester fed with brewery sludge as co-substrate yielded higher treatment efficiency than sewage sludge alone. The removal efficiencies measured in terms of soluble chemical oxygen demand (SCOD) and total chemical oxygen demands (TCOD) ranged from 40% to 75% and 22% to 35%, respectively. Higher SCOD and TCOD removal efficiencies were obtained when higher fractions of brewery sludge was added to the substrate mixture. Removal efficiency was lowest for sewage sludge alone. Measured volatile solid (VS) reduction ranged from 15% to 20%. Adding a higher fraction of brewery sludge to the mixture increased the VS reduction percentage. The biogas production and methane yield also increased with increase in brewery sludge addition to the digester mixture. The methane content present in biogas of each digester exceeded 70% indicating the system was functioning as an anaerobic process. Likewise the ratio of brewery sewage influenced not only the treatment efficiency but also improved quality of treated sludge by lowering number of pathogen (less than 2 MPN/g of dried sludge) and maintaining a high nutrient concentration of nitrogen (N) 3.2–4.2%, phosphorus (P) 1.9–3.2% and potassium (K) 0.95–0.96%. The heavy metals, chromium (Cr) and copper (Cu) remaining in digested sludge were present at relatively high levels (Cr 1,849–4,230 and Cu 930–2,526 mg/kg dried sludge). The metals were present as organic matter-bound and sulfide-bound fractions that are not soluble and available. The digested sludge could be safely applied to soil as a plant nutrient source, without fecal coliforms or heavy metals risk. A sludge mixture ratio of 25:75 (sewage:brewery), which generated the higher nutrient concentrations (N = 4.22%, P = 3.20% and K = 0.95%), biogas production and treatment efficiency meet the Bangkok Metropolitan Administration (BMA) safety guidelines required for agricultural application. Biogas production and methane at the 25:75 ratio (sewage:brewery) yielded highest amount of VS_removed (0.65 m³/kg) and COD_removed (220 L/kg), respectively.

Keywords:

• Anaerobic co-digestion
• biogas
• organic fertilizer
• toxic heavy metals

Acknowledgments

The authors would like to thank Dr. Katerachada Klankrong for technical assistance and the Drainage and Sewerage Department, Bangkok Metropolitan Administration (BMA) for the provision of sewage sludge.

Notes

^a Moisture content.

^b Total solids.

^c Total volatile solids.

^d Total chemical oxygen demands.

^e Soluble chemical oxygen demand.

^f Total volatile fatty acids.

^a Moisture content.

^b Total solids.

^c Total volatile solids.

^d Total chemical oxygen demands.

^e Soluble chemical oxygen demand.

^f Total volatile fatty acids.