Biochar addition reduces non-CO2 greenhouse gas emissions during composting of human excreta and cattle manure.

Ecological sanitation combined with thermophilic composting is a viable option to transform human excreta into a stabilized, pathogen-free, and nutrient-rich fertilizer. In combination with suitable bulking materials such as sawdust and straw, and additives such as biochar, this could also be a suitable waste management strategy for reducing greenhouse gas (GHG) emissions. In this study, we conducted a 143-days thermophilic composting of human excreta or cattle manure together with teff straw, organic waste, and biochar to investigate the effect that biochar has on GHG (CO2 , N2 O, and CH4 ) and NH3 emissions. The composting was performed in wooden boxes (1.5 × 1.5 × 1.4 m3 ), GHG were measured by using a portable FTIR gas analyzer and NH3 was sampled as ammonium in an H2 SO4 trap. We found that the addition of biochar significantly reduced CH4 emissions by 91% in the cattle manure compost, and N2 O emissions by 56%-57% in both humanure and cattle manure composts. Overall, non-CO2 GHG emissions were reduced by 51%-71%. In contrast, we did not observe a significant biochar effect on CO2 and NH3 emissions. Previous data already showed that it is possible to sanitize human fecal material when using this composting method. Our results suggest that thermophilic composting with biochar addition is a safe and cost-effective waste management practice for producing a nutrient-rich fertilizer from human excreta, while reducing GHG emissions at the same time.

Nutrient dynamics during composting of human excreta, cattle manure, and organic waste affected by biochar.

Ecological sanitation via thermophilic composting could be a promising solution to the lack of sanitation and limited access to fertilizers, particularly in developing countries. Here, we conducted a 185-d thermophilic composting experiment with human excreta, and separately with cattle manure, mixed with kitchen scraps, teff [Eragrostis tef (Zuccagni) Trotter] straw, sawdust, and biochar (BC) by using an appropriate-technology approach. We followed the dynamics of the most important macronutrients (N, P, K), temperature, moisture, pH, electrical conductivity, cation exchange capacity, as well as content of organic matter, organic C, Ca, Mg, and micronutrients throughout the process. Low N (<47%), P (<9%), K (<11%), Ca (<18%), and Mg (<21%) losses and the temperature profile indicated a well-functioning thermophilic composting process. Compost temperature was >60 °C for 7, 6, 5, and 8 consecutive days for treatments containing human excreta, human excreta amended with BC, cattle manure, and cattle manure amended with BC, respectively, suggesting a final compost product free of pathogens. The compost mixture with cattle manure and BC reached a significantly higher temperature than the same variant without BC, with a maximum value of 65.9 °C on Day 6. For all treatments, final germination index values >100% indicated compost maturity and the absence of phytotoxic substances. Biochar addition reduced losses of organic matter (18-23%), C (33-42%), and N (49-100%) and decreased the amount of extractable NO3 - (32-36%) in the final compost. The tested ecological sanitation concept via thermophilic composting is thus a promising strategy to improve access to cheap fertilizer by safe and sustainable sanitation and waste management.