Hybrid Biogas Digester Design

Hybrid batch/continuous design a la UC Davis Anaerobic Phased Solids (APS) system. Also described in David House’s Biogas Handbook (©1978).  The idea is to have multiple batch fed hydrolysis tanks and a single larger biogasification reactor that is “semi”-continuously fed liquid from the hydrolysis tanks. Tanks can be anything from 55-gallon drums on up, depending on volume and type of available waste material and gas production goals.

Hybrid APS Digester

aerated compost floor

Aerated Composter Floor

First steps occur in 'acid' drums, methane production only in the biodigester

Separate Biological Pathways. Methane production only in final stage.

digester diagram

Construction:

  1. Hydrolysis reactors are 55 gallon plastic drums with removable lids, bulkhead fittings connected to piping with quick disconnects. (See also Aquatic Ecosystems for bulkheads)
  2. Piping, valves and pumps. Assume 1″ PVC.  1″ PVC True Union Ball Valve. Each drum can be easily connected and disconnected to a single methane digester via hose with quick disconnect fittings.
  3. Example shown assumes (1) 600 liter methane digester and four each ‘acid’ drums. For a 20 day HRT, fill/empty each drum every 5 days.  Actual operation, number and size of tanks, etc. will depend on testing, and determination of type & quantity of feedstock available. See data to be collected for MEP)

Startup:

  1. Load first ‘acid’ drum with fresh waste material (e.g. kitchen waste, garden/farm waste, etc.)  Top off with liquid pumped from composter underdrain, replace the lid and seal gas-tight.
  2. Simultaneously, fill methane digester with water and bring it up to operating temperature (~35ºC).  Heating system and sources to be determined — ideally, use a combination of SDHW and/or diesel engine coolant.  Add inoculant (e.g. sewage sludge, cow rumen, mud from bottom of swamp, fresh manure, etc.  depending on available sources.)
  3. Continue to monitor methane digester pH, & add buffer as required to maintain minimum pH 6.5.
  4. After 4-5 days, or when first acid drum starts producing gas, slowly pump some of the liquid from 1st drum to methane digester, returning overflow from the digester to the acid module.  The idea is to gradually mix VFA’s generated in the acid module with methanogens in the methane digester.

Operation: (after successful startup…)

  1. Repeat Steps 1 and 3 for the next drum, but using digestate returned from methane digester as inoculant. As soon as drum 2 starts producing gas, repeat for 3rd drum. And so on…  Continue until all drums are full and hydrolyzing waste, producing volatile fatty acids (VFA) — the food that the methanogens eat.

Construction Design Notes:

  1. Integrate with a garbage collection service for the whole community — houses, villas, apartments, hotel, restaurant.  Provide receptacles for homeowners to facilitate keeping organic wastes (kitchen, garden and yard wastes, some kinds of paper) separate from landfill wastes (plastic, glass, metal, some clay-based papers w/ colored inks, etc.)  Also coordinate with permaculture people to estimate type and quantity of agricultural waste.
  2. Design acidifcation tanks to:
    1. Facilitate filling and emptying (non-pumpable) solids — i.e. removable top, drum handling equipment, staging area, water supply, drum washing area. A way to dump the end product on the compost pile. Top must be removable AND be able to be sealed gas-tight. (Consider a floating cover — more comments below…).
    2. Provide quick disconnects on gas and liquid piping connections. And flexible hoses. Make easy to move, fill and dump solids.
    3. Consider a floating cover design. It would provide a way to monitor gas production of each module individually. (As with gas collector, limit switches cycle an air compressor or solenoid valve, depending on gas composition). And solve problems with frequent opening and re-sealing drums.
    4. Maintain ‘acid’ tanks at low pH – optimum for producing VFA’s but too low for methanogens.  Consider filling with microbially rich leachate from composter instead of from methane digester — or in addition to…
    5. Not necessary to to maintain temperature control of acid tanks.
  3. Either insulate methane digester or locate inside an insulated, temperature-controlled space.  Try to think of a simple way to use heat from compost. And/or heat with HW solar collector and/or diesel engine coolant.
  4. Links to info on VFA’s:
    1. http://www.codigestion.com/index.php?id=411&L=1
    2. http://www.ramiran.net/ramiran2010/docs/Ramiran2010_0026_final.pdf
    3. http://www.iiasa.ac.at/Research/FOR/downloads/Kraxner/CD%20Venice%20Symposion%202010_Proceedings/data/data/files/424.pdf
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