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Recycling agricultural wastes to produce hot water (original)

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The original content of this page, "Recycling agricultural wastes to produce hot water (original)", was authored by Ole Ersson, and was written from his point of view. It was ported with permission from Experiments in Sustainable Urban Living.


Two composting systems which use waste biomass (such as kitchen refuse and agricultural wastes) and human wastes for heating water are described. Since they are simple and inexpensive to construct, use widely available materials, and require minimal technical expertise, they may be ideal for developing nations. When properly constructed they require minimal maintenance, are free of odors, and pose no public health hazard. One system has been constructed and has been producing hot water for household use in Oregon for 2 months. A second more simple design is proposed. Inputs are: cold water, waste biomass, and human waste. Outputs are: hot water and compost. Materials required: wire or woven fencing and piping.


Composting is a time-honored process for the conversion of agricultural or gardening wastes into fertilizer. It is a cornerstone of organic gardening. The process is simple and requires little expertise. Most gardeners and farmers understand that by accumulating waste biomass in a central location the natural decomposition process is accelerated. Biomass consists of kitchen waste (inedible portions of fruits and vegetables, such as peels; spoiled food), agricultural or forestry wastes (weeds, prunings, and the remains of crops that have been harvested, such as corn cobs or husks removed during a milling process), and manures. The compost process consumes these waste materials, producing valuable fertilizer and improving garden hygiene. When properly constructed and maintained, it is odorless and free of vermin. Additionally, as anyone knows who has seen composting in action, an important by-product is heat.

The role of nitrogen and carbon in the compost process

The living systems which decompose matter require carbon-rich matter and protein (which contains nitrogen) in their food (substrate). Carbon provides energy for metabolic processes and is found in the structural matrix of plants (the cellulose in wood or plant stalks). Nitrogen provides a source of amino acids to construct the protein enzymes necessary to convert carbohydrate into energy. It is found primarily in other plant parts, such as leaves, and in animal matter or manure. Nitrogen-rich materials tend to be dense, non-porous, and decompose anaerobically and with little release of heat, often producing foul odors and attracting insects. However, when enough carbon is added to nitrogen-rich materials, a porous matrix is formed with a large surface area upon which microbes can multiply. The resulting decomposition process becomes aerobic, releasing much heat, and prevents insect infestation and unpleasant odors. An additional benefit is the thermal destruction of pathogens, such as viruses and parasites. In practice, this means a clean, aesthetically pleasing process.

The chemical decomposition consists of the oxidation of the carbon-hydrogen molecular bond. This releases carbon dioxide, water, and energy in the form of heat. It is the same chemical process which occurs when wood is burned. However, living systems are able to do this at lower temperatures by using enzymes as catalysts.

A composting hot water heater

I would like to describe a composting system I built in Oregon that uses composting principles to supply all the household hot water for my family of 2 adults and 3 children. The primary input in this process is waste biomass from tree pruning businesses. It consists of branches pruned from hardwood or coniferous trees that have been coursely ground by chipping machines. This material is carbon-rich (i.e., contains mostly wood) and nitrogen-poor (i.e., contains few leaves) and by itself decomposes slowly. To this we add nitrogen-rich kitchen and garden wastes, additional garden and yard wastes from the community such as leaves and lawn clippings, and fecal matter and urine (humanure) collected in our toilet. Combining these two types of materials greatly accelerates decomposition and heat production.

Importance of adequate biomass

Proper composting requires adequate mass to create an environment suitable for bacterial growth. Small amounts of vegetation will decompose slowly and produce little heat which is rapidly dissipated. This favours fungal growth which is why fungi are the primary decomposers in nature. However, when adequate mass is brought together, as in a compost pile or bed, sufficient heat is produced to deter fungal growth and favour heat-tolerant bacteria. The mass serves not only as a substrate for heat production, but also as an insulator for the environment in the interior. The outer surface is at ambient temperature; the temperature of the substrate increases until it reaches its maximum in the interior. In our compost bed, we have measured the interior temperature at 130 degrees Fahrenheit.

A waterless toilet provides valuable nitrogen

In our household system we use humanure as a valuable source of nitrogen. We collect it with a simple water-free toilet consisting of two five-gallon buckets and a conventional toilet seat. One bucket is used as the receptacle and is fitted with a removable seat on a flange. A second contains wood chips or sawdust, which, after each use, is added in a thin layer to the first to cover and seal odors. When the first bucket is filled, the seat is transfered to a second bucket which is then empty and becomes the new receptacle. The nitrogen-rich humanure is then added to the compost bed. To maintain aesthetics and for proper hygiene, it is always important to cover any additions to a compost bed with a clean layer of woody or other high-carbon material.

Extracting heat with a heat exchanger

If sufficient mass is present, heat can be drawn off by a simple heat exchanger. We have installed such a heat exchanger in the form of a coil of flexible plastic pipe embedded in the interior of our compost bed. Heat from the decomposition penetrates the pipe, thereby heating water which circulates inside. Cold water enters at one end of the pipe. When a faucet is opened at the other end, hot water will emerge until the incoming cold water replaces the heated water and cools the pipe. Then it is necessary to wait for the heat from the hot mass to again penetrate the pipe and heat the fresh cold water. The amount of hot water that can be drawn off in one "serving" depends on the diameter and length of the pipe. In our system we used a 1.5 inch diameter pipe 100 feet long. This provides a reservoir of hot water of approximately 9 gallons, enough for several quick showers or a single load of laundry.

Using heat to warm a greenhouse

In designing my composting system, I wanted to utilize the heat for both household hot water and to warm a greenhouse in Oregon's cool winter. Therefore, I decided to use a large amount of biomass, which is available free from local companies. I used a three-foot deep layer of the primary substrate (ground tree prunings) as the floor of the greenhouse, whose dimensions are approximately 16' by 28'. This mass is contained within walls constructed of three layers of straw bales, stacked together like bricks, which are also available as a free agricultural waste product in Oregon. I used straw bales because they also perform well as a foundation for the sides and roof of a greenhouse. I plan to construct the roof this Fall by using parallel arches which span the two sides and which will have transparent plastic sheeting draped over them to enclose the interior space, protecting it from the cold outside. The compost bed floor is constantly releasing heat into the space above it. The straw bale sides and plastic roof should help retain this heat in the greenhouse. The heat exchanger is embedded in the deep layer of compost which makes up the floor. Therefore, the heat from the decomposition process provides heat for both water (which is pumped into the house) and to heat the space of the greenhouse itself.


Preliminary measurements (at two months old) showed that the initial 10 gallons of water emerging from the heat exchanger was greater than 130 degrees Fahrenheit. After this, it gradually cooled down to about 100 degrees as it reached 20 gallons. The water entering the heat exchanger was about 45 degrees, demonstrating that the compost beds heated the water more than 80 degrees! We have been using the water it generates for all household needs since its construction. Since installing our system about two months ago we have been able to disconnect our electric hot water heater and still have 24-hour hot water. The space heating ability of the composting process has not yet been tested. No unpleasant odors are present.

A simpler design

I would like to propose the following much simpler design for the generation of hot water alone from waste biomass. A "bare-bones" system would consist of a structure that contains the compost bed in the form of a cylinder with sides constructed of woven wire fencing. No posts or other structural materials would be required because the pressure of the biomass pushing out against the fencing will hold the fencing vertical. Embedded in this is a coil of plastic or other pipe which serves as a heat exchanger. Ideally, this will be pressurized water which can then be delivered to where it will be used. The biomass consists of whatever high-carbon material is available in the locale. The size of the cylinder will depend on the diameter of the coils of the pipe. I would suggest that the sides of the cylinder be one to two feet outside the coiled pipe. This will ensure that sufficient insulation exists to maintain the pipe at a high temperature. The 1.5 inch pipe I used came in coils about 6 feet in diameter. Thus, a cylinder approximately 10 feet in diameter and 4 feet or more high would accommodate this size pipe. The biomass decomposition begins when a source of nitrogen (such as leafy matter or manure) is added.


The major cost of this project consists of the heat exchanger. The plastic pipe described above cost $45 in Oregon. Additional parts to connect to an existing plumbing system (valves, supply pipe) could be expected to cost about $10-20. The only other cost is the fencing or other support to contain the biomass in a cylindrical shape. A 35 foot length of woven wire fence 5 feet high costs about $30 in Oregon. Thus the total budget for this project is less than $100.

The future

I estimate we are actually extracting only a small percentage of the heat produced (several tens of gallons of hot water per day). One Peace Corps volunteer suggested that we could better utilize the heat by constructing a jacuzzi. Indeed, a simple pool could be easily constructed by excavating a depression in the middle of the compost bed and lining it with a heavy plastic or rubber sheet to contain the water. Then, by circulating the water in this pool back into the heat exchanger when it cooled below a desired level, a compost-powered hot tub could be built. I believe the water in a hot tub is typically 105 degrees. Therefore, the temperatures achieved by composting should be adequate. Only experimentation will tell! I invite anyone attempting to harness the power of composting in novel ways contact me with a progress report. The possibilities are manifold!

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