Can Water Conserving Toilet be a Solution to Achieve Higher Energy Recovery from Co-digestion of Toilet Waste and Kitchen Waste?

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. However, septic tank does not favour the recovery of resources such as energy and nutrients.

In an era of climate change, a circular economy is vital. Separating black water from greywater at the source and treating them with specialized technologies enhances water, energy, and nutrient recovery from domestic wastewater. Anaerobic co-digestion efficiently extracts bioenergy and biofertilizers from organic waste like toilet waste and food residues.

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 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.