Abstract
A two-stage temperature-phased mesophilic anaerobic digestion assay was carried out to study the interaction between various biological pretreatment conditions and the possible synergistic co-digestion of microalgae and primary sludge. The study of growth kinetics of the biochemical methane potential test revealed that a maximum of 36% increase in methane yield was observed from co-digestion of a substrate pretreated by thermophilic aerobic conditions (55 °C and HRT = 2 days) and an 8.3% increase was obtained from the anaerobic pretreated substrate (55 °C and HRT = 3 days). Moreover, no synergistic effects on methane yields were observed in co-digesting the substrate pretreated with high temperature (85 °C). The study also identified specific conditions in which interaction between biological pretreatment and co-digestion might substantially reduce methane yield. Careful optimization of operating conditions, both aerobic and anaerobic pretreatment at moderate thermophilic conditions, can be used as a biological pretreatment to enhance methane yield from the co-digestion of microalgae and primary sludge.
Highlights
Environmental pollution associated with wastewater treatment plants has become a serious global challenge demanding concerted action [1]
Enhancing methane production through co-digestion from existing anaerobic digestion without increasing the volume of digesters has been observed in previous works [6,7,9]
The yields from mono-digestion of primary sludge in the second and third tests were significantly better than that of co-digestion. This is mainly because both co-digestion and mono-digestion of microalgae entail longer retention times for the digestion of complex biopolymers of algae while the primary sludge retains a higher amount of readily available soluble organic substances for methane production [6]
Summary
Environmental pollution associated with wastewater treatment plants has become a serious global challenge demanding concerted action [1]. Microalgae used for reducing pollutants in wastewater can be consumed during biogas production in anaerobic digestion (AD). Even though its methane yield is quite low due to recalcitrant cell walls and low C/N ratios [5], various carbon-rich sources have been co-digested with the microalgae to enhance biogas production. There have been some successful attempts to produce biogas from the co-digestion of microalgae and primary sludge in the anaerobic digester [6,7,8]. Co-digestion trials have been generally conducted to reduce the time lag of methane yield [9] as well as improve the methane production of mono-digestion by facilitating synergies between substrates via allocating appropriate C/N ratios and reducing the risk of methane inhibitors [7]
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