Abstract
Human excreta are a sustainable, economical source of nutrients, and can be used to produce recycling fertilizer for horticulture by collecting and processing the contents of dry toilets. Herein, we discuss the key categories of risk associated with the main groups of materials commonly found in dry toilets. The study was part of the development of a German product standard for marketable and quality-assured recycling fertilizers from human excreta for use in horticulture. Particular attention is paid to ensuring that the fertilizer is epidemiologically and environmentally harmless and that the quality of the recycling fertilizer is adequate in terms of low pollution and nutrient availability. In sum, the risk of transmissible human pathogens lies within the human excreta, particularly feces; plant materials added during composting are of particular phytosanitary relevance; pharmaceutical residues in excrements and chemical additives are potential sources of pollutants; non-biodegradable contaminants can cause pollution and injury; and the horticultural risks involve mainly the ammonia emission potential and in some cases the salinity effects of urine. These risks can be reduced significantly (i) with education of users around proper operation of dry toilets and the consequences of adding inappropriate waste, (ii) with facilitation of proper use with general waste bins and clear instructions, and importantly (iii) by using modern sanitization and cleaning processes and testing for harmful substances under the guidance of local laws and regulations, ensuring safe and high-quality fertilizers. In conclusion, the benefits of using dry toilet contents to produce fertilizers for use in horticulture are unquestionable. Our analysis highlights the need to support recycling optimization and awareness for the purpose of a sustainable circular economy and to minimize the risk of harm to humans and the environment overall.
Highlights
In Europe, human excreta are almost exclusively discharged into wastewater, and nutrient recycling from human excreta is not yet part of a circular economy (CE) [1,2,3]
According to Schönning and Stenström [16], most pathogens in human excreta are excreted with feces, as part of the large microbial content of feces: 25–54% of dry matter (DM) of feces consist of bacteria [44]
The eggs of helminths are excreted with feces, with the exception of bilharzia, which is the only known worm whose eggs are known to be expelled via urine [16]
Summary
In Europe, human excreta are almost exclusively discharged into wastewater, and nutrient recycling from human excreta is not yet part of a circular economy (CE) [1,2,3]. These nutrients are often contaminated with heavy metals and micro-plastic from sources other than toilets. Due to this contamination, field fertilization with sewage sludge has been restricted or banned by many national governments, including Germany [9]. By means of decentralized collection, treatment and use of human urine and feces material flows [11, 12], regional, CE-orientated nutrient recycling can be achieved (Fig. 1)
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