This week, I worked on addressing my concerns about the water content expected in the waste that would prevent composting or solid fuel recovery from being feasible. A low-energy way to maintain a reasonable water content is by draining into a leach (evaporative) field. This pursuit resulted in the setting up of a spreadsheet that takes an estimated volume of wastewater that could be generated, local meteorological and evapotranspirative factors to calculate the required area of the leach field. I’m populating the calculator with regional soil (permeability) , weather (reference evapotranspiration rate) and vegetative (species, landscape and microclimate factors) data so that it can be used as a tool to judge the feasibility of composting/solid fuel per case based on land availability. My intent is to set it up in a user-friendly format that can take the basic inputs of number of female and male occupants and the location of the home, to display the land area requirements.
The field would have layers of gravel and top soil (to allow for the plants to take root), and the filtration occurs through evapotranspiration. The 2 feet deep “field” will be housed in a concrete/brick enclosure with a liner to prevent leaching to the soil. The enclosure will be raised, or a mud mound provided to prevent rainwater run-off from creating any contamination.
This week we took a step back and thought about how to consolidate our findings for the quarter and how to compare different technologies. This led us to the conclusion that the main alternatives to biogas, which deal with waste on a very local, home-centered level, were composting and biochar. Furthermore, we decided to begin considering specific toilet designs as we wrap up our work on the engineering aspect of the project. Both of these alternative technologies have the barrier of excess water (as described in an earlier blogpost), so this week I researched the toilet designs that treat this water. Specifically, I looked at the evapotranspiration trench (and the concept of trenches in general). The leachate water is routed from the toilet, either by being diverted through a third hole or by trickling through the solid waste and draining through a pipe located at the bottom of the tank. This pipe needs to be about half a meter above the leachate trench, in order to properly fill the trench. Then, the trench is filled with layers of sand and gravel; the sand holds water that can evaporate, and water that doesn’t evaporate filters down through the gravel. A layer of topsoil is above these layers and holds plants, which helps the evaporation (often, a geotextile sheet is between the topsoil and the sand/gravel, to keep the soil in place. This sheet also helps draw water up for evaporation.) Typical depths for the trenches are 0.6-0.8 meters; as detailed in Afroz’s post, it is possible to calculate the required area of a trench for a given amount of wastewater.
As mentioned by Afroz and Sonya, this week we focused on methods of dealing with the leachate that has to be removed in order for either compost or biochar production to be viable. This week I also looked into kiln designs for creating biochar. In a previous post I mentioned the design Re:char is using which features a removable container for the feces which is placed in the chimney of the kiln. This design has the benefit of allowing users to create biochar without interacting with the waste. However, it has the drawback of only accommodating one container at a time. A top-lit updraft kiln might be a solution to this problem. For this design a large outer drum is constructed, either out of brick or steel. The container has air inlets at the bottom that will allow a supply of air for the combustion of the fuel. Three smaller covered metal containers containing the biomas to be converted into biochar (in this case, feces) are placed inside the outer drum. Fuel is packed around the containers and a lid is placed on top of the outer drum to limit the supply of oxygen. A chimney is then added to create an upward draft to sustain the burn. This design has the benefit of being easy to use and fairly simple to construct. The most difficult part of construction will likely be obtaining a suitable metal lid and chimney. The ability to fit more than one container into the kiln at once will make it more feasible to have only one kiln for a community or cluster of houses and will make the process more efficient.