A primary safety concern associated with the use of composting toilet residuals as a soil amendment (or fertilizer) are contaminants of emerging concern (CECs) . Also referred to as micropollutants, CECs are substances that we consume and then excrete (either unmetabolized or as their metabolites); they include hormones, pharmaceuticals, and personal care products . CECs pose significant risks to human and environmental health , . CECs are found in excreta and therefore have the potential to be found in any fertilizing products derived from excreta . The degree to which CECs are discharged to the environment, either as a fertilizing product or effluent, depends on the design of the sanitation and waste treatment system , . In comparison to septic tanks and conventional centralized wastewater treatment plants (WWTPs), composting toilets have the capacity to achieve significant treatment and degradation of pharmaceuticals and micropollutants.
Septic tank systems and conventional WWTPs are major input sources of CECs to the aquatic environment with potentially significant impact on surface water, groundwater, and drinking water supplies , , . Septic tanks are designed to inactivate pathogens, mitigate release of nutrients to aquatic environments, and reduce the biochemical oxygen demand; their design does not target the removal of CECs . Conventional centralized WWTPs only partially remove CECs from the influent wastewater; pharmaceutically active compounds were identified in wastewater effluent and in sludge applied to farmland . Both septic tanks and WWTPs discharge effluent to aquatic environments where CECs degrade slowly with the potential for greater negative health impacts .
It is possible that WWTPs and septic tanks could be redesigned (with significant impacts on capital costs and overall footprint) to offer an improved management of CECs –. However, composting toilets offer several advantages for the management of CECs relating to their compatibility with source separation and effective treatment mechanisms for managing CECs:
- Source separation refers to the separate management of urine and faeces. Urine contains the major fraction of the nutrients, hormones, and pharmaceutical residues excreted by humans; a source separation system can be used to separately treat and manage urine with significant impacts on energy efficiency, greenhouse gas emissions, discharge of CECs to the environment, and final recovery of nutrients for reuse . With composting toilets, there is the potential for improved control: urine and excreta can be collected and treated appropriate to their specific requirements and overall pollution management can be simplified , .
- Storage and treatment of excreta can minimize risks from CECs . Composting is effective for the treatment of CECs: temperature appears to be the most significant factor , . Additionally, the production of struvite (mineralized phosphate) from urine significantly reduced the concentration of CECs , .
In addition to the potential advantages of composting toilets in managing CECs relative to WWTPs and septic tanks, composting toilets have the potential to better conserve water and energy use, steward biochemical cycles (e.g. nitrogen, phosphorous), and minimize greenhouse gas emissions . In other words, our choice of sanitation and waste treatment is intimately connected to the greatest sustainability challenges of our time . So instead of being a barrier to the implementation of composting toilets, the presence of CECs in excreta serves as an opportunity to paradigm shift from the conventional to more ecological and sustainable systems.
The Hornby Island Composting Toilets Study is evaluating the feasibility of installing a composting toilet residuals treatment facility. A key aspect of the study is the technical design of the facility, which will take into consideration how to manage the pathogen content, recover nutrients, and treat the CECs present in the incoming material. There are certain facility design characteristics (e.g. the pH and temperature of operation) that can be optimized to degrade the CECs of the residuals, and there will be ongoing monitoring of the material to guarantee that the compost produced is safe to use as a soil amendment (or fertilizer). Increased usage of composting toilets and the availability of a composting facility to treat and monitor composting toilet residuals would offer greater public and environmental health protection relative to current sanitation coverage on Hornby Island.
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