Destination Digester: Greatest Vacation Ever

In a wonderful success of Alumni Relations, I volunteered a good portion of a vacation to head back to Clarkson University and spent several long and wonderful days with some friends replumbing and fixing their donated digester. Turned out to be a fantastic trip, fraught with engineering peril, and edged with great friends and a trip to one of the coolest bookstores in the world, BirchBark Bookshop. Clarkson's undergraduate program is very refreshing, and as an Master's student there it was nice to work with a good number of talented undergrads, some of whom came back to help wrench on the digester. This digester was used for various educational pursuits after it was brought online.

Clarkson has a long and successful history with innovation in bio fuels, digesters, and renewable research, and it was nice to continue to support their educational goals with some of my copious free time.

Plain, industrial looking portable building with digester inside Here is the digester. This is parked in a parking lot on campus. As to where it came from, and how Clarkson managed to get it, I have no idea.. The build quality of the unit is excellent, and the concept of the design seems pretty solid to me. The double-wide doors at left open to allow access. The stuff to be digested goes in the sliding door visible in the center of the frame. Just behind there is a chute that drops into a pretty serious grinder system, which is connected via lots of pipes to pumps and tanks. The unit can be reconfigured by changing plumbing systems, making it a useful experimental system. At the top of the unit are vents fans and heat exchangers. Inside, to the left, is mechanical space, and to the right are several large, insulated, interconnected tanks.

Looking inside the mechanical space, showing my colleague working on the pipes. At the left is the generator, at the right is the heat exchange and main peristaltic pump. At the back, you can see the control cabinet and behind there are the large tanks. The green box on the right, behind the heater system pump and expansion tank, is the top of the chute that leads into the food grinder.

Peristaltic pump, you say? Peristaltic pumps, sometimes called hose pumps are capable of pumping high viscocity liquids with good flow control. This is the type of pump that is sometimes used to pump concrete. Each rotation of the pump's rotor moves a given volume of fluid through the pump, and the volume pumped varies linearly with motor speed. Because of this, much smaller versions of this are used for medical applications, such as blood transfusions and medication dosing. The pump works by pushing rollers along a length of pipe, so that the fluid in the pipe never touches the pumping mechanism, keeping them sterile (in medical applications) and clean (in digester agitation applications).

A view inside the front doors of the digester. At the right is the generator control panel and synchronizer relay, and directly ahead is the chute and recirculation valves for the food chopper. The valves and plumbing could be easily re-arranged to allow fo various flavor of operation, including series-flow and parallel-flow through the tanks, plus options for recirculating various stages of digestate back through any of the tanks or the feed system.

A close up of the back end of the generator. We had to change oil, replace the battery, fix the heat exchanger system, rewire parts of the control system, and do some other work. This digester operates in collaboration with a solar thermal and pellet fired energy system to provide extra heat to a greenhouse to enable the plants to get through the cold North Country winter. The heater integration work was not performed on this visit, and was the subject of a fair amount of research and teaching work by Clarkson.

The peristaltic pump. Furnco-style fittings are not that great of an idea for a peristolic pump, as the pressure and vacuum forces that they can produce are quite substantial. We made a design choice to include these in here, so that we would blow off the Furnco fitting rather than blast apart one of the other fittings in the system. When we first plumbed this, it was my job to do it right, and I managed to put the out and in backwards on the pump, which we figured out when we turned it on. The result of this mistake was another lovely geyser, althoug this time of relatively clean fluid, as the digester was filled with water to test pumps, fittings, and seals.