Biosolids, sometimes called sludge, are generated as a byproduct of wastewater treatment at meat and poultry processing plants as well as at municipal treatment plants. Land application of biosolids is the most common means of disposal providing nutrients for crops, but due to increasing environmental awareness, the practice may not be available to processors in all parts of the country in the future.
Generally, biosolids are accumulated in lagoons and are pumped out for land application once or twice a year, or in some cases every few years. Odor from the storage lagoons can become a problem. Composting wastewater treatment plant biosolids on a more frequent basis with other materials can provide a solution that eliminates sludge storage at the processing plant and presents an alternative to direct land application of the sludge.
Several poultry processing plants in North Carolina and Virginia are sending their wastewater treatment biosolids to McGill Environmental Systems composting facilities in Chatham and Sampson counties in North Carolina. McGill has a third composting facility under construction near Waverly, Va., which will be operational in the spring of 2008. McGill charges a tipping fee for removal of the biosolids from the processing plant.
McGill blends materials that are mostly water like DAF skimmings and liquid from restaurant grease traps with dryer materials like leaves, branches, paper and other carbon sources. Everything from unpainted, untreated wood pallets, clean wood waste, yard waste, furniture manufacturing byproducts, animal manure and bedding, paper and cardboard to unpainted sheetrock is composted. The composting facility provides an alternative to landfills for disposal of these products.
McGill composts at indoor facilities to allow operation 365 days a year regardless of outdoor temperatures or weather conditions. Static pile, forced aeration composting is employed in the compost facilities which each have approximately three acres under roof and represent a $5 to $7 million investment.
McGill compost facilities have three main areas: blending, processing, and curing. The blending area includes storage for wet and dry feed stocks, plus amendments and bulking agents. Raw feed stocks are blended with amendments and bulking agents to provide nutrients, balance the moisture content, create adequate pore space for the movement of air through the composting mass, and produce a homogeneous mix. From the blending area, material is moved to the processing area where a front end loader operator places the blended mixture into a composting bay. Channeled concrete floors in the processing area facilitate forced aeration. During processing, temperature probes placed near the center of the compost pile relay information to a computerized process control station. The computer controls fans according to preset temperature levels, pushing air into the floor channels and up through the composting mass to remove excess heat and moisture.
After two to three weeks, when primary processing is complete, the material is removed from the bay by a front end loader operator, screened, and moved to the curing area for another, three to four weeks. When fully stable, the compost may be rescreened and/or blended with other materials prior to shipment. The composted finished product is sold in bulk form to landscape contractors, erosion control contractors, golf courses and farmers to be utilized as a soil amendment.
Tyson Foods’ Glen Allen, Va., broiler processing plant is testing composting of its DAF skimmings with McGill. Around 5.5 loads of DAF skimmings were being sent to an outside rendering plant on a daily basis from the Glen Allen plant. Because of the high water content of DAF skimmings, having this material rendered is a net cost to the poultry company. Also, around 2 million gallons of waste activated sludge from the wastewater treatment lagoons was pumped out and land applied each year at a net cost of around $150,000. The Glen Allen plant is testing a process where the waste activated sludge is put back into the wastewater stream prior to the DAF. The DAF uses an acid polymer mix and the flocculent is then skimmed off and trucked to a McGill compost facility.
By putting the activated sludge back in the wastewater, DAF skimmings have increased from 5.5 loads per week to between seven and eight loads. These DAF skimmings are trucked to the compost facility on a daily basis. Composting the biosolids eliminates three potential environmental challenges for the poultry processor, on site storage of the biosolids, land application of raw biosolids, and odor problems associated with on site storage and land application of biosolids.