Nutrient dynamics and recovery efficiencies in a decentralised faecal sludge and food waste treatment system

Decentralised faecal sludge (FS) and food waste (FW) treatment systems like co-composting system offer more practical solutions for waste treatment and nutrient recovery in low- and middle-income countries, yet nutrient recovery and losses across this system remain poorly quantified.This study aimed to assess the flows, losses, and recovery efficiencies of nitrogen (N), phosphorus (P) & potassium (K) with the goal of recommending measures to minimize pollution to water bodies. Raw FS, FW, compost, and effluent samples were collected at each treatment stage over three treatment cycles from August 2021 – 2022 in Somanya, Ghana. A total of 108 composite samples were collected and analysed for N,P & K using standard procedures. The N, P & K losses at each stage of the treatment system were calculated using the mass balance principle and the nutrient flow diagrams were created using the Sankey diagram generator. Results show that, 59-86% N, 8-40% P and 49-81% K were lost at the dewatering stages for all cycles. Losses were lumped together as either gaseous losses, adsorption to media surfaces or percolate. The overall nutrient recovery efficiency of the system was 6–17% N, 20–37% P & 17-24% K in co-compost and treated effluent. Despite high removal efficiencies in the facultative ponds, the final effluent did not meet EU standards. Effluent may become a resource in geographies that have scarce water and less stringent regulations. This study recommends strategies and approaches such as biochar use, percolate/leachate recirculation and covering of compost piles to reduce nutrient losses. • First end-to-end assessment of NPK flows in a decentralised FS & FW treatment system in SSA • Up to 86% N, 40% P & 81% K losses occurred during FS dewatering stages. • Co-compost recovered only 6–15% N, 21–36% P, and 14–25% K from the treatment system • Treated effluents exceed EU discharge standards for N & P showing environmental risks • Findings provide a framework for designing low-cost, nutrient-efficient sanitation solutions