Composting of the solid fraction of blackwater from a separation system with vacuum toilets – Effects on the process and quality

Abstract This study presents an example of blackwater (0.8 L flush water, faeces, urine and toilet paper) composting from low-flush vacuum toilets, which is a part of a pilot-scale source separation system at a remote tourist facility. Blackwater (BW) was collected from vacuum toilets into a tank and applied over peat filters for particle retention and dewatering on the gravity principle to obtain a solid fraction (SF). Before composting, SF was mixed with pine bark and wheat bran. With no mixing, the thermophilic phase was achieved only in the middle layer and lasted for three weeks. Sanitation was indicated by the inactivation of enterobacterial pathogens, the absence of parasites, and the cessation of phytotoxicity (cress seed germination index ≥80%). High temperatures and high levels of NH 4 + in the collecting tank, the peat filters and six weeks of SF composting are plausible reasons for the reduction of pathogens. At the end of composting, the NH 4 + –N/NO 3 − –N mass ratio was between 0.5 and 3.0, which indicates mature compost that is ready for use in agriculture. In terms of the percentage of dry matter, the nutrients were nitrogen: phosphorus: potassium = 3.0:3.2:1.6, with a favourable ratio of C:N = 10, indicating the biological stability of the produced compost. The tested pilot-scale model has thus proved that the composting of SF is a viable option as a part of the separation system.