Co-digestion of pig slurry with an algae-rich municipal wastewater sludge

Increasingly, covered anaerobic lagoons are being considered by the Australian pork industry to manage greenhouse gas (GHG) emissions and recover methane (CH₄) for energy production. Algal biomass produced in high-rate algal ponds (HRAP) treating piggery wastewater removes CO₂, contributing to GHG mitigation, and is an additional source of biomass energy that could be released via co-digestion with pig slurry (Buchanan et al. 2013). The objective of this study was to investigate an optimum feed ratio for co-digestion of wastewater grown algal biomass with pig slurry for CH₄ production.

Algae-rich sludge (ALBAZOD; a mixture of algae, bacteria, zooplankton and detritus) was collected from a dissolved air flotation plant and a pig slurry sample was collected from a piggery in South Australia. Experiments were established in 30 L plastic batch anaerobic digester vessels, which were seeded with 20 L of anaerobically digested sludge obtained from the two sites described. The reactors were purged with N₂ gas and digested under room temperature (17−25°C) for 3 months with manual mixing by rotating the vessels once per day. Six experimental groups were studied as follows: 100% pig slurry (PS); 96.5% PS + 3.5% ALBAZOD (A); 92.9% PS + 7.1% A; 85.4% PS + 14.6% A; 67.8% PS + 32.2% A; and 100% A. All experiments were performed with triplicate analysis (n = 3) and the ALBAZOD percentages were calculated based on volatile solids (VS) per g of dry weight (APHA 1995). The results were statistically analysed by independent samples T-Test (95% confidence interval, P ≤ 0.05).

The highest CH₄ production was observed from the 96.5% PS + 3.5% A mixture (Fig. 1), with a production of 0.344 L/g VS removed and a slightly lower production of 0.339 L/g VS removed from 100% PS. However, no significant difference was found on CH₄ production compared to the 100% PS. The CH₄ production decreased as the ratio of ALBAZOD increased in the mixture. When the ALBAZOD ratio was beyond 7.1% A, the CH₄ production decreased to below 0.200 L/g VS removed. The lowest CH₄ (L/g VS removed) was observed from the 100% A control experiment with an average of 0.040 L/g VS removed over the first 73 day period, that then rapidly increased up to 0.174 L/g VS removed at d 91.

Fig. 1.  Cumulative methane (CH₄) production calculated based on per gram of volatile solid (VS) removed from co-digestion of pig slurry (PS) and ALBAZOD (A) over 91-day period. Values are means ± SE (n = 2). ▵: 100% PS; : 96.5% PS + 3.5% A; : 92.9% PS + 7.1% A; : 85.4% PS + 14.6% A; : 67.8% PS + 32.2% A; : 100% A.

The results suggested that although there was a slightly increase in overall CH₄ production with the optimum ALBAZOD mixture, the ratio is crucial in order to achieve optimum CH₄ production between pig slurry and ALBAZOD because it is known as poorly degradable. In conclusion, anaerobic digestion and co-digestion can capture energy in the form of CH₄ which can be converted into electrical energy further enhancing the sustainability of the pork industry (Miao et al. 2014; Astals et al. 2015). Further investigations of pre-treatment with ALBAZOD to increase its biodegradability would seem warranted to optimise this research.