M. Macias-Corral, Z. Samani, A. Hanson, R. DelaVega and Kent Hall
BioCycle March 2004, Vol. 45, No. 3, p. 55
New Mexico ranks seventh in the nation in milk production, with the dairy industry in the state having grown from 25,000 dairy cows in 1975 to 320,000 in 2002. While a major source of income and employment, the industry remains as a major source of environmental concerns. The New Mexico dairy industry is currently generating about 1.3 million tons of manure each year with potential adverse environmental impact due to uncontrolled methane emission and contamination of air, surface water, groundwater and soil. In addition, the generated waste represents a financial liability for the dairy with disposal cost of $1-$2/ton.
Traditionally, manure has been used in agriculture as soil amendment, either through direct application or after composting. However, land application in New Mexico is restricted due to high salt content of the manure, limited water supply, limited agricultural land and low rainfall. Farmers are generally reluctant to use manure in their fields due to high cost of manure handling and salinity problems associated with manure application.
On the other hand, the recent USDA/ EPA Unified National Strategy for Concentrated Animal Feeding Operations (CAFO) would require animal feeding operations (those with 750 dairy cows or 1,000 cattle) to develop a comprehensive nutrient management plan for field application of waste. The new CAFO strategy, which is designed to minimize contamination and to protect surface and groundwater, will further increase the cost of feedlot waste management, and tend to further decrease the interest in land application of animal waste.
An alternative management approach would be to convert manure into methane and a compost by-product which can be used in agriculture as well as in the greenhouse industry.
ANAEROBIC DIGESTION SYSTEM
Anaerobic digestion (AD) is a biochemical process where complex organic compounds can be broken down by a microbial consortium in an oxygen free environment. The by-product, biogas, is a mixture composed of methane, carbon dioxide and other trace gases. Most commercial technologies have been developed to handle low solids content (less than 12 percent). The application of current AD technologies in New Mexico is limited due to the high solid content of the manure which is normally higher than 18 percent. This is due to open corral management practice in the dairy industry and dry climates of New Mexico which result in manure with high solid content. A pilot plant two-phase AD system was constructed at the campus of New Mexico State University (NMSU) to evaluate the feasibility of using an alternative AD system in New Mexico.
TWO-PHASE ANAEROBIC SYSTEM
The two-phase system installed at NMSU consists of two different reactors: a solid phase reactor and two up-flow anaerobic filter (UAF) reactors, as shown in Figure 1. The solid phase reactor consists of a metal container with capacity of 8 m3. The UAF reactors have two PVC columns with capacity of 0.4 m3 each. The UAF reactors are filled with plastic packing material and seeded with methane bacteria.
The process starts when the solid phase is loaded and sealed using a plastic polyethylene liner. Water is added from the top using a sprinkler irrigation system and the leachate is collected at the bottom through a subdrain. At the early stage of the operation, the leachate has a low pH, which is transferred to the UAF and converted to biogas by methane producing bacteria. The residual leachate is then returned to solid phase carrying with it the methane bacteria which in turn seed the solid phase.
After a short period of typically 7 to 10 days, the solid phase becomes a biogas producing machine without the need for mixing. Gas production from the solid phase continues until the organics are exhausted. In the end, about 40 percent of manure is converted into biogas resulting in about 100 m3 of biogas of which about 72 percent is methane. The 60 percent remains are ready compost. The entire process takes about four to six weeks.
The compost produced by the two-phase AD system has more nitrogen than aerobically composted manure. In the aerobically composted manure, nitrogen is lost in the form of ammonia due to the comparatively higher temperature of the compost and continuous turning (See Table 1).
When compared to the traditional composting process, AD has two other important advantages, especially in dry arid climates: lower water consumption and less processing time.
The aerobic composting of manure in New Mexico takes about eight to nine months and requires about 1.14 m3 of water per cubic meter of produced compost. The AD system takes four to six weeks , and requires less than 0.1 m3 per cubic meter of compost.
GREENHOUSE EXPERIMENT
In order to test the AD composted manure and its impact on plants, a greenhouse experiment was conducted at the Sierra-Vista Growers’ greenhouse – using AD composted manure and comparing it with aerobically composted manure under the same conditions. The photographs show the comparatively higher plant growth with AD compost than the aerobic compost. The higher plant growth with AD compost is attributed to its higher nitrogen content.