Worming the way to finished compost


Courtesy of BioCycle Magazine


When materials from CR&R and other generators are unloaded, a preliminary screening separates out nonorganic contaminants.
 Even though there are source separated programs in places like Temecula and Lake Elsinore, there is the occasional appearance of plastic and glass that needs to be sorted out. Next, Bodlak’s crews put materials through a Vermeer tub grinder that can process up to 200 tons (or 720 cubic yards) a day.

After the initial grinding, water is sprayed onto the piles, and two days later, the grinding process is repeated this time through one-inch screens to produce a much finer material that can be spread onto windrows as “worm food.” As the Eisenia fetida process the green waste, they continually move up in the windrow. “That’s what distinguishes these redworms from the common earthworms that tend to move down into the soil,” Bodlak points out. “As long as the redworms have moist feed on top of them, they keep eating, breaking down material and reproducing.”

When the windrows reach the three feet level, the worms in the top six inches are “scooped up” with a loader and deposited in a newly-formed windrow. The vermicompost in the finished windrow is allowed to dry, then screened to a half to 7/8th inch. The product is sold in bulk to soil blenders, who in turn do the bagging for varied uses.
 During the past decade, livestock facilities have become increasingly aware of the critical issues associated with production, storage, treatment and utilization of animal wastes. Particularly in regions where animals have been concentrated, excessive manure loading rates pose real problems such as the accumulation of nitrates in groundwater and of phosphorus and nitrates in surface waters. These issues create major constraints to profitability as well as growth.

In the swine industry, the current practice for handling manure involves a slotted floor with either liquid storage under the slats or a flush system with lagoon storage. Problems with limited transportability and pollution associated with liquids have the industry seeking new approaches. One concept now being researched in Ohio uses slotted floors and an in situ system for drying swine manure. Termed the High-Rise Hog Building (HRHB), manure in this system is handled as a solid, so there’s no need for lagooning. The concept described in this article is similar to the high rise, deep pit poultry building, where fresh poultry manure is converted from 70 to 75 percent moisture to a dry product below 25 percent that can be transported economically.


The first HRHB unit was set up in 1998 in Darke County, Ohio by Ft. Recovery Equipment, Inc. for 4M Farms. This building is a fairly straight forward adaptation of a conventional 960-head, automated, commercial finishing unit. The facility has now been operated very successfully for three cycles of hog production by 4M farms, and it has been studied over this yearlong production period by the authors. Other HRHBs are being planned or are under construction.

The basic HRHB concept is a patented modification of existing technology such that manure can be partially dried in place and later handled in solid form. Each building is two stories high, built up from ground level. The pigs are housed in pens on the upper level. These pens have slotted floors, and manure and urine fall through to bedding material on the lower level that serves to absorb excess moisture and allows for drying. There are two basic streams of air flow in the building. First, outside air is pulled down through ceiling inlets into the pig space, on down through the slots in the floor, and then exhausted by fans out the sides of the lower level of the building. Second, air is blown through pipes embedded in the lower floor, up through holes in this floor, up through the absorbent bedding and manure mass, and then out of the building through the sidewall fans. These air streams keep the air around the hogs reasonably fresh while serving to draw off excess moisture from the collected manure/bedding.

For a 960-head finishing operation producing 250-lb pigs, final daily manure production contains about 1500 lbs of solids and 13,700 lbs of water. To remove 75 percent of this moisture with air at reasonable winter conditions would require 6,500 cfm. The present floor system was designed with about 3,200 three eighth inch holes, and required just over 4 hp to move this much air at a three-inch water pressure drop. In practice, the floor was built with four aeration zones. Each zone is supplied by a 2.0 hp fan, and at the start of each production cycle design air flow was achieved. By the end of each cycle, however, the pressure drop increased to above five inches water due to manure buildup, and only 75 percent of design air flow was achieved.

Various bedding materials can be used. Sawdust, chopped corn stover and cobs have been tried with reasonable success. Chopped straw, various paper wastes, etc. may be tried as well, and other materials that may be cheaply available could be considered. Basically, the materials need to be reasonably absorbent, and capable of maintaining a porous bed while resisting compacting or crusting over. The bed should be 18 to 24 inches deep initially, and some mixing should be done to maintain consistent moisture and compensate for uneven manure buildup due to the hogs’ well-defined dunging patterns.

Only very minimal leachate from the manure bed was observed no more than a few gallons a day. Thus the manure mix could be safely land applied with little risk of ground or surface water contamination. Auxiliary studies also showed that this mix could be satisfactorily composted if immediate application was not suitable at time of removal from the HRHB.

Air quality was monitored in the building. Ammonia concentrations in the upper level of the HRHB averaged 4.3 ppm, with the highest values, occurring at low winter ventilation rates, always being less than the eight-hour exposure limit of 20 PPM Hydrogen sulfide was never detected in the upper level air, and carbon dioxide values were typically up to about 1,000 PPM as would be expected from the respiration of the pigs.

Gas concentrations in the lower level and just external to the exhaust fans were higher than those within the pig space. Ammonia values averaged 23.3 and 18.3 PPM, respectively, with one winter value being 136 PPM Hydrogen sulfide was detected, but only up to 0.3 PPM On the other hand, carbon dioxide values did show modest dilution as greater mixing occurred.

Final moisture levels of the bedding/manure mix were around 63 percent (wet basis). This was less drying than had been designed for, but was sufficient and the material produced was such that it could be handled as a solid with conventional loading and spreading equipment. Some loss of drying potential was due to airflow reduction as mentioned earlier. In addition, temperature measurements in the bed gave little indication that in-place composting was taking place to generate heat and drive evaporation (although observations of steamy conditions during mixing operations did indicate that there was some heat generation).


Construction costs are 15 to 30 percent higher for HRHBs relative to conventional structures, and could amount to as much as a $45,000 increase in initial outlay. Additional operating costs due to greater ventilation power usage also add about $2,000/year in expenses. On the other hand, the dry manure from an operation of the size examined here could be land applied for about $400/year as opposed to $1,600 to nearly $6,000 for conventional systems with manure moisture levels of 95 to 99 percent. In addition, the more concentrated nutrients in the dry manure mixture would have a value in excess of $15/ton and might, therefore, be worth about $3,000. Thus, while these benefits do not fully compensate for the greater expenses, there is sufficient similarity in costs that the production and environmental benefits previously noted make the HRHB system economically and socially attractive.

While more experience with high-rise hog buildings is certainly needed, initial evaluation indicates that: The modified air handling system of an HRHB provides a stable thermal environment for pigs and maintains low noxious gas levels in the pig space; There are significant production gains that can favorably influence annual cash flow, but additional expenses are also applicable; and The drying bed functions fairly well, allowing only minimal leachate, evaporating more than half of the moisture from the manure deposited on it, and yielding a manure/bedding mix that can be handled as a solid with conventional equipment and either composted or directly land applied.

Both odor and water pollution problems in swine production are being reduced with the HRHB systems due to drying and to aerobic handling conditions, but much more work is needed to verify the full extent of this.

The authors are in the Department of Food, Agricultural, and Biological Engineering, The Ohio State University/OARDC, Wooster, Ohio.

Information in this article was adapted from ASAE papers 994107-4109 (ASAE, St. Joseph, MI) by the present authors and by Terry Mescher, Tom Menke, Mike Veenhuizen and Brian Strobel. The cooperation of 4M Farms and Ft. Recovery Equipment Co., Ft. Recovery, OH, is greatly appreciated.

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