August 14, 2010

The Selinda system was sized to accommodate the kitchen waste from a maximum of 16 guests eating four meals a day. Culhane spent two days during a period of full occupancy grinding the waste generated through meal preparation and plate scrapings through the Insinkerator LC50 generously donated by Emerson Electronics. He determined that each meal (breakfast, lunch, tea and dinner) generated approximately two 25-liter buckets of slurry (food waste ground up with water). Calculating that every 25 liters requires approximately 1000 liters of digester space (based on estimates by ARTI India whose telescoping systems we have been replicating), we decided that we needed about 8000 liters.

Because of cost considerations, we decided that we would try wedding a 5000-liter sealed tank as primary digester to a 2500-liter telescoping system with a nested 2000-liter gas collector acting as secondary digester, giving us approximately 7500 liters of digester space. We also placed at least four vertical 110 mm sewer pipes inside each of the digesters to increase surface area for biofilm formation. This also allows the bacteria to inhabit different thermocline zones. The digester system is made so that the primary digester spills its overflow into the secondary digester, which has the telescoping gas collector in it. We observed that some of the food slurry sinks and some floats. Our theory is that some of the food entering the primary digester will spill over into the secondary digester. Gas created in the primary digester flows from a 1/2-inch plastic tube into the gas collector and gas created in the secondary digester directly raises the gas collector. Hence both tanks were filled with septic tank sewage sludge as starter culture. Note there is no smell or presence of flies or vermin associated with the open telescoping digester and it is safe enough for small children to play around.

Here we display photos of our latest design for larger capacity biodigestion, utilizing a larger sealed tank as the primary digester (mimicking a Chinese fixed dome system, but out of plastic) connected to a secondary digester/gas collector setup in the ARTI India style. The challenge in Botswana was creating the telescoping digester out of two tanks with a similar diameter. To our chagrin, when the 2500-liter and 2000-liter tanks we ordered arrived, we discovered they both had the same diameter! At $330 and $300 respectively, with no alternatives and time running short, we figured that God was challenging us to come up with a solution for people who have similar problems (we’ve never had this situation before). So we are running a very expensive experiment. We cut the 2000-liter tank into four sections, taking eight centimeters out of each side. Next, we reassembled the tank by heat welding the pieces back together. It took about two days with three people working, one using a heated trowel to patch gaps with the trim strips we removed, one using a heated angle iron to melt the strips and bind them, and another working inside the pieces with a large soldering iron, filling gaps. But it seems to be working!