Investigation and Optimization of Small-Scale Fecal Management : As a product from dry toilet solutions in off-grid Swedish holiday homes

Sammanfattning: In off-grid holiday homes, alternative toilet solutions are needed. There is a wide range of dry toilet systems, where urine-diverting systems and incineration toilets are common solutions. Urine-diverting dry toilets require that users need to manage generated fecal fractions. This is often done by private composting or through municipal latrine bucket pick-ups. In this project, fecal management for holiday homes in Sweden is examined from three perspectives by (1) studying the biological phenomenon with composting and how a compost should be managed to generate rich humus, whilst minimizing greenhouse gas emissions associated with the act of composting, (2) examine current user experiences associated with latrine compost management, and (3) map out current latrine management systems, including laws and regulations. The goal was to develop a user-friendly concept for fecal management for urine-diverting toilets, based on this research. The project was carried out in collaboration with Harvest Moon, a company focused on the development of innovative and refined dry toilet systems. The project was initiated with a literature review, the examination of current fecal management systems, and interviews with composting experts and researchers. The background research showed that there is no such thing as perfect compost management since it depends on what end goals the users have. Research also showed that frequently turning the pile, increases ammonia (NH3) emissions, but reduces methane (CH4) formation. Furthermore, biochar can be added as a bulking agent to aerate the compost mass, and aid the hygienization process of such a mass, since it binds e.g., hormones. Regarding composting methods, a static passively aerated compost is not the fastest process but has the least compost mass reduction, which is desired when using it as a soil enhancer. In addition, it requires the least management. This method was therefore chosen for further development. Furthermore, since temperature and moisture are easily measured with sensors, it was deemed interesting to implement such sensors in a final concept, to alleviate management for the user. To assure that the final concept would reflect user needs and wishes, a phase of user studies was then initiated. The studies showed that users generally see latrine compost more as something to take care of, rather than as a resource, and therefore have no interest in using composted humus. The research also showed that because many municipalities require 2-year storage of the material in the composter, users experienced scheduling issues, which ultimately led to an inefficien composting system. The third perspective that was investigated, was the management systems of today and how they are regulated by laws. Each municipality has its own requirements on how latrine composts should be managed. These requirements are based on the Environmental Code, as well as Naturvårdsverket’s recommendation for the implementation of the law. Apart from the compilation of these regulations, this investigation showed that pyrolysis, as well as the centralization of hygienized feces, could be future alternatives to latrine composting and latrine pickup. But due to the short Time-to-Market, and the project's limited time scope, product development towards system innovation was deemed unrealistic within this project.  Insights from these three perspectives created a framework for the concept development phase, which was finalized with building a full-scale functional prototype. During detailed design, the concept was further developed in CAD. The final concept presented in this project is a modular, user-friendly latrine compost that can be adapted to follow different municipal regulations. It has an inner mesh that aerates the compost mass to reduce methane gas formation. The mesh is constructed with hexagonal perforated acid-proof steel. The composter has a push latch mechanism on the lid together with two gas struts, which makes it easy to open since the user only needs to push the lid once, for it to open. Temperature and moisture sensors make it easier for the user to manage their compost correctly, and a front door allows for ergonomic emptying of the finished compost humus. Future development to reduce production costs, simplify the construction, continue the CAD model development, find suitable sensors, develop product instructions as well as perform user tests with the physical prototype should be further investigated.