Explanation of Technology
Biogas or Anaerobic Digesters are relatively low tech approaches. Reactors are well suited in a variety of applications including the conversion of compost and/or manures. They rely on the action of several types of Anaerobic bacteria (bacteria that live without the presence of Oxygen) as part of a multistage composting process. As the bacteria break down the waste into compounds more assimilable by plant root structures, they emit biogas which can be collected in the biogas reactor and then piped to a storage tank.
Many low tech grassroots have experimented with the use of biogas as fuel for heating water, cooking and heating buildings. In some cases the successful scrubbing of the biogas has led to applications where the gas was used to produce electricity.
Potential Benefits
The benefits to the biogass approach are several and include the rapid breaking down of biowaste using relatively high temps and thus killing most pathogens, seeds and diseases that may be present in plant, human and animal waste while producing rich compost and converting some of the biomass into biogas a potential energy source:
Production of Sludge effluent that can rapidly broken down by worms into a rich compost
Production of Liquid effluent that can be added to plants as a compost tea/irrigation application and possibly configured into a organic hydroponics system.
Production of biogas which is mix of methane, CO2 and hydrogen sulfide.
Reduction of pathogens and disease by creating high temperature beneficial bacteria driven chemical reactions
Design & Materials Used for Construction
Reactors can be made of a variety of materials in fact just about any material which can make an air tight seal that is impervious to corrosion, deterioration and leakage is suitable. There are also a series of reactor designs for different applications. At the most basic level even a lagoon by covering it can be made into system for anaerobic digestion.
Considerations for Potential Application at Arcosanti
For a potential application of biogas technologies at a medium to large scale, various things need to be considered.
What is the feedstock to power the system.
How large will the system be scaled?
What type of technologies/designs will be used?
How will it relation to the Energy Apron Greenhouses?
In the Holcim Contest that Star Cag is working we considered the application of a biogas digester into the Energy Apron Greenhouse design. While it can be used for human sewage apparently there are more pathogens in human waste and this complicates any effort to incorporate this technology for active processing of sewage effluent at Arcosanti. Also the scaling and integration with other growing systems needs to be well thought out as mentioned above with the listing of potential benefits.
Therefore it is suggested that the biogas system can be started relatively easily and should be considered as a possible experimentation and research component of the next phase of the Energy Apron Greenhouse. This will give us plenty of leeway to consider the options without committing to anything concrete at this stage.
Scale-Out Strategy - What is recommended is a multistage process in which one small bioreactor is started to process the food waste that is not recommended for the worms such as meats, dairies, citrus, oils, etc. This can be scaled as appropriate in relation to the successful development of the project.
Site Selection - A nice pad and operational center should be located a reasonable distance from living facilities. The composting area of Camp might be a good location for the system as we test it out.
Premixing - A system for grinding the materials should include a grinding or process machine before the materials are batched in the reactor. This is important to assure a consistent mixture that can easily and rapidly be broken down by the bacteria
Biogas Collection & Utilization - will be collected in bags and then burned to sanitize high pathogen waste as they are added in the experimental process - for example human sewage is much safer in this system if it is sanitized first. In the future the biogas from system can be used to heat water or buildings such as an heating system for the camp greenhouse apartment and an backup heating system for the camp greenhouse. If deployed in Phase II it could be used for making hot water and or a back heating system for that greenhouse.
Sludge for Compost - will be collected and further refined into a nutrient rich compost by adding to a vermiculture system to see if worms can quickly process the waste as stipulated in George Chan's Integrated Farming system (IFS). In the Integrated Farming System developed by Chan in addition to the role of the worms in breaking down the biowaste/sludge, fungi also plays a key role by spawning mushrooms and using the sludge as a substrate for growing this crop.
Liquid Effluents - also have nutrients. They can be applied directly to the soil as a compost tea. More ideally would the addition of a fish farming system. In George Chan's IFS the fish are use to prep the effluent for use as irrigation nutrient medium in a modified aquaponic growing system.
Human Sewage - Various options could be considered before using and experimenting with human sewage in a reactor as mentioned above. With the right prep in using the digestor and the design of the system to ensure meeting of regulatory requirements experimental greenhouse could be set up as part of the Energy Apron to study and manage the human sewage with a plan put in place to eventually replace the sewage lagoon.