Hestia biodigesters are approximately 5 by 7 feet wide by 5 feet deep, providing about 700 gallons of capacity. Slurry occupies about 600 gallons of this biodigester; the remaining space is for the gas that’s produced. The design is straightforward: an insulated concrete vessel is topped with a steel frame that holds an EDPM pond liner, which expands as gas is produced. There’s an inlet for adding feedstock and an outlet for removing composted slurry. A closed loop of PEX tubing in the bottom of the tank is plumbed to an on-demand water heater to add heat when the slurry temperature drops below 50°F—the temperature at which cryophilic methanogenic bacteria go dormant and stop producing gas. If the climate is mild, it may be enough to build a hoop house over the tank to keep the slurry sufficiently warm in winter. Alternatively, the biodigester could be allowed to go dormant during the colder months.
The first step in building the Hestia’s biodigester is to excavate 28 inches below grade, which makes the height of the inlet right for easy addition of feedstock by 5-gallon bucket. Warren likes to make sure the digester is visible from the kitchen, since the inflation of the rubber top indicates if there is sufficient gas available for cooking Alternatively, a simple pressure gauge could be added to the gas line in the kitchen.
After excavation comes building the wall forms for the 2 cubic yards of aggregate-free concrete, which must be poured all at once to avoid leaks through the walls. A 4-inch PVC outlet pipe and any PEX tubing for adding hydronic heat must be set in place. The PEX tubing will rest on the bottom of the floor of the tank, so two short pieces, one for entry and one for exit, must be embedded in the wall so the rest of the radiant heating system can be attached later.
A concrete truck with a pump is best to fill the forms in one pour. A concrete vibrator (also called a “stinger”) will help remove air bubbles’ weak spots from the concrete. The massive weight of the wet concrete and the agitation of the stinger make it important to solidly secure the forms.
After the concrete cures and the forms are removed, three coats of “moose-milk” finish—a mix of Portland cement and acrylic latex—is painted on to help prevent leaks. The first is a bonding coat of watery-thin consistency. The second coat is thicker (like peanut butter), with a higher ratio of Portland cement. The finish coat is another thin application. After sealing, the tank is filled with water for a leak test. If this goes well, the outside of the tank can be insulated with 3 inches of rigid foam board insulation and then backfilled. For aesthetics, Warren wraps the exterior of the biodigester in chicken wire and then stuccos it.
The 40-mil pond liner and steel frame that serve as the tank top are held in place with 18 anchor bolts inserted into the top of the concrete tank at the time of the pour. The steel frame can be fabricated or purchased from Hestia.
The gas line is attached to a regular barbed fitting secured with a hose clamp. The rubber membrane is sandwiched between two washers and nuts on the threaded end of the fitting. The gas line is a 1/2-inch flex hose, which is transitioned to a standard PVC gas pipe when it goes underground. Burying the PVC line protects it from photodegradation and developing cracks that could lead to gas leaks.
The most common problem is water buildup in the gas line, which can interfere with gas flow. When this occurs, the gas line must be picked up and the water drained back into the digester. Ideally, a water separator could be combined with a pressure-relief valve on the bottom to eliminate excess moisture when the valve is tripped.
The outer forms are reinforced to withstand the pressures of the poured concrete. [Warren Weismann]
The interior forms, with floor and wall reinforcements visible. [Warren Weismann]
The finished concrete pour, with rubber bladder in place. [Warren Weismann]
Details of the frame, gasket, and rubber bladder assembly. [Warren Weismann]
Rigid foam board is used to insulate the tank and help maintain the correct temperature for the bacteria. [Warren Weismann]
Flexible gas lines run from the top of the bladder to a valve and buried rigid plumbing. A stucco and brick facade helps the digester integrate with the garden landscape. [Isaac Marquez]
Construction Time & Costs
For this particular biodigester, construction time will vary depending on a person’s construction experience or if a professional concrete contractor is hired to pour the digester tank. Time will vary from several weekends to several days, separated by a seven-day concrete-curing period.
Materials will cost $1,000 to $1,200, depending on the price of ready-mix concrete in your area and if the concrete company will charge you a “short-load” fee for ordering only 2 yards of concrete. The project can be broken down into excavation and concrete; plumbing and gas piping; and external masonry. The concrete, rebar, battens, and anchor bolts run about $500. Warren highly recommends spending the extra $200 to $300 for a concrete pumper truck to avoid having to “bucket brigade” the concrete. The plumbing and gas piping will be an additional $200, and external insulation and masonry, another $300. (To purchase a complete plan set for $89, visit hestiahomebiogas.com.)
Besides building your own biodigester tank, repurposing old food storage or other tanks is a possibility for biogas generation. You’ll just need to size it correctly and make sure it’s leakproof. A general rule is that the tank needs to be 50 times the size of the daily input to allow for some space for gas to collect. If your input is 15 gallons of material per day, you’d need a 750-gallon tank.