Pyrolysis of Dry Toilet Substrate as a Means of Nutrient Recycling in Agricultural Systems: Potential Risks and Benefits
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ORIGINAL PAPER
Pyrolysis of Dry Toilet Substrate as a Means of Nutrient Recycling in Agricultural Systems: Potential Risks and Benefits M. Bleuler1 · M. Gold2 · L. Strande2 · A. Schönborn1 Received: 8 October 2018 / Accepted: 13 August 2020 © The Author(s) 2020
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 10 inhabitants of a shared house. The collected waste had been stored in an airtight vessel for several weeks before sampling. Several random samples were collected from the whole vessel volume (400L). From visual observation, about 25 vol% of the sample consisted of wood chips. Following sample collection, the feedstock was dried for two days at 60 °C in a laboratory oven, and several small samples were taken from the dried feedstock to form a composite sample for the feedstock analyses.
Slow‑Pyrolysis Experiments Nine batches of the dried DTS were pyrolysed with an externally heated pyrolysis reactor (Fig. 1). The temperature was measured with a temperature sensor (Mantelthermoelement Typ K Otom group, Germany) in the center of the pyrolysis steel cylinder. Temperature was recorded manually each minute.
Waste and Biomass Valorization
Feedstock and Biochar Analyses
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Fig. 1 Schematic diagram of pyrolysis batch reactor. (1) Steel cylinder (Ø:76.1 mm, h:300 mm) containing feedstock (2) screw cap (3) ring burner run with propane gas initiating pyrolysis process (4) perforations for the escape of pyrolysis gases (5) aluminium cover (Ø:353 mm, h:500 mm) (6) insulation wool 50 mm (7) outlet for exhaust gases (8) temperature sensor
The reactor was heated with a ring burner powered by propane gas. After 10 to 15 min, the temperature had reached 500 °C. Regulating the ring burner by hand, the temperature was held c
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