Pyrolysis of Dry Toilet Substrate as a Means of Nutrient Recycling in Agricultural Systems: Potential Risks and Benefits

Abstract

Biochar is increasingly being applied as a soil amendment in agriculture. Biochar is typically produced from plant biomass and contains relatively low amounts of plant nutrients (e.g., N, P, and K), thus providing limited fertilizer value. Human excreta contains plant nutrients that could be recycled to create sustainable agricultural nutrient cycles. This study investigated the potential of biochar derived from a dry toilet substrate as soil amendment. The substrate consisted of urine, faeces, and wood chips, and was pyrolyzed at 500–650 °C for 10 min. The biochar was analyzed for plant available P, water leachable P and K, carbon stability, pH, electrical conductivity, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dioxins, and germination tests with barley and lettuce were conducted to estimate the biochar fertilizer value and potential bio-toxicity. The biochar contained 25.0 ± 1.0 g N/kg dry mass (DM), 33.1 ± 2.1 g P/kg DM and 20.7 ± 0.2 g K/kg DM. 65% DM P was extractable by formic acid solution, 31.7% DM P and 60.5% DM K were water leachable in a ten-day column water-leaching experiment. The biochar complied with European regulations for PAHs, PCBs, dioxins and heavy metal concentrations, except for Zn and Ni. Germination of salt-resistant barley was not affected by biochar doses < 50% DM, while salt-sensitive lettuce germination was inhibited at doses ≥ 2% DM, indicating that the dry toilet substrate biochar induced salt stress. Based on these results, it is recommended that urine separation should be considered for biochar of excreta, which could reduce salt stress while maintaining concentrations of “fixed” or bioavailable nitrogen. Graphic Abstract

Qualitative Risk Analysis for Contents of Dry Toilets Used to Produce Novel Recycling Fertilizers

“…The survival of microorganisms in the environment depends on many factors, including temperature, pH, water content, i.e., humidity, solar radiation, antagonists, and nutrient availability. Accordingly, hygienization can be achieved in a number of ways, including (i) heat treatment such as pasteurization [49,50], thermophilic composting [28,50,51], or carbonization [31][32][33], and (ii) alkaline or acidic treatment with appropriate additives, e.g., urea or lactic acid bacteria [30] and (iii) drying [52]. Many treatment processes are discussed in the literature.…”

“…For materials with a comparatively high Cl content, it is still under research if Cl ions are volatilized in the pyrolytic process or not. The fact that Cl is enriched in fecal biochar indicates that Cl ions largely stay in the biochar [31]. Although heavy metals can accumulate during carbonization, their amounts depend strongly on the source material [209], which therefore requires low heavy metal concentrations.…”

“…Following the separation and collection of excreta from DTs, the treatment and recycling process begins [15,16,18]. Fertilizer products that can be produced from human excreta include (i) treated urine (e.g., stored [19,20] or nitrified urine [21,22], struvite [23][24][25]), (ii) fecal compost [26][27][28] or fermentation products [29,30], or (iii) carbonized [31,32] and/or pelletized [33] feces.…”

Resíduos orgânicos e biocarvões para fins de condicionante de solos

“…Toscan et al (2017) evidenciaram um valor de 4,5% de cinzas para o bagaço de cana-de-açúcar. Bleuler et al (2020) encontram um valor de 8,2% para o lodo e 42% no biocarvão em temperaturas de 500 e 650°C. Cao & Harris (2010) estudando as propriedades do biocarvão de estrume observaram um valor de 96% no teor de cinzas e, associaram esse maior valor a presença de minerais como a calcita e quartzo no estrume.…”

Mosan: Combining Circularity and Participatory Design to Address Sanitation in Low-Income Communities

“…Also, through pyrolysis, pathogens are removed, which makes it safe to handle and decreases the contamination risk [29]. At the same time, key nutrients as phosphorus and potassium are retained in the biochar providing fertilizer or soil amendment value [30] (Fig. 5).…”