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Old 04-24-11, 10:49 AM   #12
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I spent a few hours reading the site: High-Performance HeatGreen Home Heating System Version 3a and am quite impressed by the thought behind this system. Unfortunately there is no synopsis that explains the system in a few paragraphs. I will try to do so here:

Composting Bioreactor

The HG 3a composting system is a bioreactor designed to maintain composting at temperatures of 125F to 150F, which is the range of thermophiiic bacteria. The bioreactor must be held within that temperature range, requiring cooling to remove heat. A small amount of air must be provided to supply oxygen to the bacteria. One of the outputs of the bioreactor is heat, which can be removed by air cooling and water cooling. The reactor also produces carbon dioxide and water vapor.


The purpose for building a reactor is typically for home heating or greenhouse heating using waste materials. The HG 3a bioreactor has a PVC coil water loop, which can be used for hot water hydronic heating, or potable hot tap water use. There is also an air cooling blower which outputs hot air. Apparently there is little to no odor in the exhaust air, although there is CO2 and water vapor as part of this exhaust air. Other uses for the bioreactor could include water distillation and cooking.

Bioreactor Vessel

The HG 3a system is designed to fit through a doorway, the main part is 2 feet wide by 5 feet high. It is a cylindrical tank made of plywood and designed to be rotated about a 3/4 turn (back and forth) approximately every hour. The tank is insulated with construction foam. Its function is somewhat similar to a front loading washing machine that you would typically see at a laundromat. The vessel is made watertight by lining it with a polytarp. There is an access door and ports for air intake and exhaust, as well as water connections for the loop of PVC tubing inside the tank.


The unit is fed with organic matter, which can be leaves, lawn clippings, chipped wood debris, agricultural waste (corncobs, straw) or agricultural products (hay, corn). The material must be kept dry until added to the bioreactor. Water is added to moisten the material. The amount of heat produced depends on the amount of material added, the fineness of the material, the condition of the bacteria, and the type of material. Once the bacteria are active and the bioreactor is at operating temperature, material is added daily or every few days to maintain the bacteria life cycle. There does not appear to be much waste material produced by the bioreactor, removal of waste material is not mentioned, although it is likely that the vessel must be cleared of waste at least once a season. It is unclear whether the bioreactor can be located outdoors. Examples shown appear to be in a garage or workshop location.

Bacteria Requirements

In order to promote bacterial digestion of the material, the bacteria require the proper temperature, oxygen, moisture and the food source material.

Temperature Control

Maintaining the proper temperature appears to be the most critical issue, because the thermophilic bacteria die when outside of their temperature range. Overcooling of the system by blowing cooling air through the system too long is one way to kill the bacteria. Another way to kill the thermophilic bacteria is to allow the system to overheat above about 170F. Water cooling using the PVC water loop would be another way to cool the vessel, which is not discussed in the article.


Generally the air cooling system provides enough oxygen for the bacteria. Tumbling the digested material by turning the tank hourly assist in getting oxygen to the bacteria.


Water is added initially to the vessel to bring the material to "damp sponge" consistency. It is unclear whether additional water is required with each addition of material, or if water produced as a byproduct of digestion maintains the appropriate moisture level.

Food Source

It appears that the food source can be any compostable material. This would indicate a need for a balance of carbon and nitrogen, often discussed as a mixture of "green and brown". A discussion on composting principles is available here:

Fundamentals of Composting: Why Compost

From the discussion, it would appear that a home could be heated using autumn leaves and grass clippings. The author states that 1 acre could provide enough material to heat a home. The material would need to be dried to preserve it through the winter, which could be done using methods similar to agricultural practices used for hay and straw. For example, cut grass when the weather is predicted to be dry, let it lay on the ground in the sun for a day or two, collect it when dry. It may be convenient to press the dry material into blocks to aid in filling the bioreactor and to preserve the material. I do not have any data to back this up, but the amount of feed material required for one winter might be an uncompressed amount of leaves and grass clippings the volume of one or two mini-vans(?).


A composting bioreactor should be able to heat a home with a homeowner effort similar to a woodstove. Only minor additional energy inputs (blower) are required to operate the bioreactor. In wooded areas, high volumes of autumn leaves as a feed material are available at no cost and are currently collected as waste. Automation of the bioreactor should be fairly simple with an Arduino type controller. The vessel could be constructed for about $200 as shown in the article, or $400 with automation.
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