Comparison of pre- and inter-stage aerobic treatment of wastewater sludge: Effects on biogas production and COD removal

Biogas Production from Coffee Pulp and Chicken Feathers Using Liquid- and Solid-State Anaerobic Digestions

Agricultural waste, particularly lignocellulose, has been used in the second generation of biogas. Coffee pulp and chicken feathers can be developed as biogas raw materials because of their suitability as a biogas substrate. This study investigates the effect of the percentage of total solids (TS), carbon to nitrogen ratio (C/N, g/g), and delignification pretreatment on biogas production from coffee pulp and chicken feathers, and aims to compose kinetics using the modified Gompertz model. The results show that adjusting the percentage of TS at low-level speeds up the degradation process, which increases chemical oxygen demand (COD) reduction and biogas production. COD reduction and biogas production increase optimally at the 25 (g/g) C/N ratio. Pretreatment delignification aids microorganisms in substrate decomposition, resulting in faster COD reduction and biogas conversion. The 25% TS and 25 (g/g) C/N ratio with the delignification process achieved the best biogas production, with biogas production of 10,438.04 mL. The Gompertz method shows that the difference in TS percentage can influence biogas production. Moreover, the method shows that biogas production is higher with the delignification process than without it.

What factors affect the selection of industrial wastewater treatment configuration?

Industrial wastewater treatment is gaining significance in literature due to stricter environmental policies and increased environmental awareness. The selection of the wastewater configuration encompasses both the treatment as well as several decisions around wastewater collection and disposal pertaining industrial decision-making sphere. However, so far in the wastewater literature, research has mostly discussed either technical features of wastewater technologies, or wastewater policy issues at broader level, without focusing on the industrial decision-making issues and driving factors leading to the selection of a specific configuration. Starting from a literature review, the present study provides an innovative framework of the possible options for wastewater system configuration, as well as major adoption factors by industrial decision-makers. The factors have been classified according to 7 categories, namely: influent-related, technological, economic/financial, internal socio-cultural, external socio-cultural, regulation, site characteristics. The framework, validated with acknowledgeable experts, policy makers and firms, has been preliminarily applied to Italian and Australian food firms. Our investigation reveals that the framework was able to include all relevant problems faced by industries in the selection of a treatment system configuration; besides, the relative importance of factors has been assessed: legal requirements emerge as the most critical factors, followed by volume and discharge fee, the latter particularly interesting for policy makers purposes, since it may guide the decision-making process. Further, the wastewater volume seems to play a key role in our exploratory investigation, with smaller firms preferring a complete off-site treatment to reduce the complexity, whilst larger firms preferring instead more partial or complete on-site treatment configurations for compliance costs reduction. In conclusion, we have provided policy and managerial implications stemming from the study as well as sketched interesting future research avenues.

Inter-stage thermophilic aerobic digestion may increase organic matter removal from wastewater sludge without decreasing biogas production

Combining aerobic and anaerobic digestion in a two-stage system can improve the degradation of wastewater sludge over the use of either technology alone. But use of aerobic digestion as a pre-treatment before anaerobic digestion generally reduces methane production due to loss of substrate through oxidation. An inter-stage configuration may avoid this reduction in methane production. Here, we evaluated the use of thermophilic aerobic digestion (TAD) as an inter-stage treatment for wastewater sludge using laboratory-scale semi-continuous reactors. A single anaerobic digester was compared to an inter-stage system, where a thermophilic aerobic digester (55 °C) was used between two mesophilic anaerobic digesters (37 °C). Both systems had retention times of approximately 30 days, and the comparison was based on measurements made over 97 days. Results showed that the inter-stage system provided better sludge destruction (52% volatile solids (VS) removal vs. 40% for the single-stage system, 44% chemical oxygen demand (COD) removal vs. 34%) without a decrease in total biogas production (methane yield per g VS added was 0.22-0.24 L g^-1 for both systems).