Enhanced Energy Recovery From Co-digestion of Toilet Waste and Kitchen Waste Using Water-Conserving Toilets

Farideh Jamali-Behnam, Christchurch Science Centre, Institute of Environmental Science and Research, Christchurch 8041, New Zealand

Ricardo Bello-Mendoza, Department of Civil and Natural Resources Engineering, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand

Maria J Gutierrez-Gines, Christchurch Science Centre, Institute of Environmental Science and Research, Christchurch 8041, New Zealand

Kristin Bohm, Kenepuru Science Centre, Institute of Environmental Science and Research, Kenepuru, Porirua

Fatemeh Jamali-Behnam, Department of Environmental Health Engineering, School of health, Mashhad University of Medical Sciences, Mashhad, Iran

Approximately 21 % of Aotearoa-New Zealand’s population is not connected to a reticulated sewer system. They live in rural areas where households must treat their sewage with onsite wastewater treatment systems, which is commonly a septic tank. Unfortunately, this type of onsite wastewater treatment system does not favour the recovery of resources such as energy and nutrients.

In an era of climate change and worldwide water scarcity, efforts to develop a circular economy approach and recover water, energy and nutrients are necessary. One strategy to improve water, energy and nutrient recovery from domestic wastewater is to separate black water from greywater at source and treat each stream with technologies suited to their characteristics. Anaerobic co-digestion can effectively recover clean bioenergy and biofertilisers from high-strength organic waste streams such as toilet waste and food residues. Co-digestion refers to the simultaneous anaerobic digestion of multiple organic wastes in one digester. The benefits include increased cost-efficiency, the synergistic degradation of treated materials, optimal moisture and nutrient concentrations, the dilution of inhibitory compounds such as ammonia, and the degradation of products such as lipids.

This work presents the results of a batch experiment to investigate the biogas production by anaerobically co-digesting source separated toilet wastewater and kitchen waste with different amounts of water to represent: a) water conserving toilet waste (e.g., vacuum toilets using 0.5-1.2 L water per flush), b) dual-flush toilet waste (using 6 L water per flush) and c) conventional toilet waste (using 9 L water per flush). The main objective of this research was to evaluate the impact of water content on the biochemical methane production from co-digestion of three different type of toilet wastes with kitchen waste. 

The results of this study showed that co-digestion of water conserving toilet waste and kitchen waste accelerated the methane production (443 L CH4 Kg VS-1 in TW1+KW) compared to the toilet waste diluted with higher amount of water. Water diluted waste (for example by using less water efficient toilets) impacted the co-digestion reducing the methane production (400 L CH4 Kg VS-1 inTW9+KW).