Water Week 5: Wastewater

This week, our team is trying to narrow down our scope to design the physical biogas plant model.  We thought we were sure about a centralized design, but after speaking to various professionals and receiving differing advice and suggestions, we decided to move forward to lay out both centralized and decentralized systems and determine pros and cons for the style that would work best for Anam.  The main differences would involve scale of plant capacity and point of waste transport, so we can still build our model for the physical plant for either system type.

This week, I determined the key specific questions we needed to ask Sebastien and Jeanette, Ph.D. students at Stanford that do research on anaerobic digestion and biogas plant design for developing countries, in order to help my team choose the most effective design structure.

One primary concern is over the feasible material types available in Nigeria for the biogas sludge containment tank.  We want to identify existing supply chains for our necessary materials and make sure that they are feasible and sustainable over time.  We are currently looking into concrete, plastic, steel, PVC piping and their respective costs and availability.

Another design consideration is how we plan to separate methane from the total biogas captured if that is a necessary step for the end-uses, and how to loop the gas back to the residential systems for cooking or other uses like electricity for cooling.  For a centralized system, the transport costs are greater to pipe the gas back to each residence.  For a decentralized system, the transport costs are minimal.

In terms of tank capacity, we have different questions regarding both systems.  For a large centralized tank, we need to ask whether we can continue to scale up the biogas tank or whether a larger system becomes inefficient after a certain volume, especially with limited technology and materials.  For a decentralized system, which minimizes piping and transport, we are wondering how small a biogas system can be to be effective.  We are concerned one family of 6 individuals might not generate enough waste to create a biogas system that can power at least cooking systems for each household, but perhaps a cluster of 4 families might be sufficient.

We are also concerned about whether there will be any sanitary issues with the proximity of a decentralized system and human waste to the residential area.  Even though the plant would be underground, the operators have to handle final post-treatment biosolids.

We are also curious whether transporting final biosolids treated waste from many decentralized locations underground to landfills/agricultural use is more difficult than piping waste sludge initially to a centralized location and having the final biosolids waste be transported from one final location.

We also want to ask Sebastien and Jeanette whether they have relatively accurate estimates or assumptions for sewage sludge waste density, solid content, etc. that they use for their digestion models, and whether they recommend initial separation of liquid and solid waste through composting-style toilets before treating solid waste sludge.  We are also wondering the rate at which waste typically cycles through the biogas tank so that we know whether our tank should hold a capacity for 5 days, 10 days, etc.  Additionally, for wet digestion, we plan to ask whether there is an optimal volume of water that is required to keep the plant at its maximum efficiency, or whether keeping the sludge as dry as possible is the best design.

We are meeting with Jeanette on Monday, and we will be sending the above questions I wrote up to Sebastien since he is out of town on conferences for the next week.  Once we have these answers, we will be able to detail our instructions and design manual step by step and work on a cost estimate.