University survey compiles data on composting operations, volumes and types of feedstocks processed, product quality and ways to expand markets.
IN THE FALL of 2001, the University of Georgia's Engineering Outreach Program (EOP) completed a statewide composting survey designed to help expand the industry and organic materials recycling in Georgia. Some goals of the survey were to visit each composting operation in the state to collect data on the types of feedstocks being composted, origin of feedstocks, annual throughput volumes, maximum throughput capacity, quality of finished product, equipment owned or contracted, price of marketed compost, market sectors for finished product, type of composting method employed, permit requirements, and the quantity of finished material stockpiled. Global Positioning System (GPS) coordinates were obtained to create a digital map of the facilities for the state, a compost sample was taken for lab characterization at the university, and technical and educational assistance was provided if necessary.
The survey found Georgia has 38 facilities actively composting that handle over 550,000 tons/year (tpy) of organic materials in addition to five new facilities coming on line later this year. While half of these facilities are private operations, they handle more than 80 percent of the recycled organics. More than 35 percent of the composted materials are handled through operations that manage a variety of feedstocks, while yard waste composting operations handle the second most at nearly 20 percent of the total materials composted.
The EOP analyzed finished compost samples from 33 of the 38 operations in the state. Lab analysis included: moisture content, bulk density, pH, organic matter, heavy metals, soluble salts (conductivity), nitrate-- nitrogen, ammonium-nitrogen, total nitrogen, total carbon, C:N ratio, phosphorous, potassium, percent inerts, particle size, biological stability (respirometry), and maturity (germination rate).
PHYSICAL CHARACTERISTICS
Moisture contents ranged quite a bit depending on whether the material was stored under shelter or left outside. The driest material was seven percent moisture while the wettest was 51 percent; average moisture content was approximately 34 percent. Bulk density of the characterized samples was partly dependent of the original feedstocks.
Yard trimmings composts, some food residuals composts, municipal solid waste (MSW) compost and agricultural residuals composts tended to be lighter than manure, industrial by-products, and biosolids composts. Bulk materials were as light as 385 lbs per cubic yard and as heavy as 1,400 lbs per cubic yard. The state average was approximately 900 lbs per cubic yard. All products met U.S. Composting Council (USCC) standards for inert materials present in the finished compost, less than one percent on a dry weight basis. Of the 38 operations, 15 screen their finished products and all screen to 3/8 inches. Organic matter content was quite variable as well ranging from 13 percent to 63 percent depending on the amount of carbon material or bulking agent used in the initial recipe as well as the length of time the material was composted. The state average was approximately 30 percent.
CHEMICAL CHARACTERISTICS
The pH levels of Georgia's composts signify that most of the products are good quality. While the range was from 4.9 to 8.6, the majority of the products were near neutral and averaged 6.5. The pH levels seemed to be affected most by type of feedstocks used and length of composting time. Composted biosolids seem to consistently have lower pH levels and materials that have short residence times seem to have higher pH levels.
Soluble salt contents are most often a result of the initial feedstocks and curing time. Some animal manures and food waste composts often have higher salt contents; however, curing time can help to reduce these levels. Soluble salt content (conductivity) ranged from 0.09 to 25.20 mmhos/ cm. The average was 5.5 mmhos/cm, which exceeds the Georgia Department of Agriculture's (GDA) standards for horticultural grade compost set at 4 mmhos/cm. Of the 38 products tested, 12 exceeded GDA standards; they were mostly derived from food waste composts and poultry litter composts with a couple of biosolids and yard trimmings compost products testing high as well. It should be noted that soluble salt levels exceeding 4 mmhos/cm may still represent a high quality material that is well suited for other applications such as landscaping and erosion control.
The carbon to nitrogen ratio (C:N) of the sampled products ranged from a low of 7.6 to a high of 147.2, with an average of 16 if the highest sample is removed. This is a positive indicator for the state's products, and is often influenced by the initial recipe and thoroughness of composting. Materials that seemed to produce high C:N ratios were wood residuals and some yard trimmings composts, MSW compost, and compost made from pulp and paper mill solids.
Ammonium nitrogen levels ranged from 1 ppm or 0.0001 percent to 4876 ppm or 0.49 percent. The average was approximately 500 ppm or 0.05 percent. The composts with the highest levels were poultry litter composts and some of the food residuals and biosolids composts. Nitrate nitrogen ranged from 1 ppm or 0.0001 percent to 1817 ppm or 0.18 percent with an average of 330 ppm or 0.03 percent. Again the poultry litter composts were consistently higher followed by some food residuals composts and biosolids composts. TKN followed the same trend ranging from 0.18 percent to 3.57 percent with an average of 0.75 percent. The wood residuals and yard trimmings composts generally were lowest in nitrogen followed by those products that had feedstock recipes high in wood residuals or yard trimmings.
Total phosphorous ranged from 54 to 18,930 ppm or 0.0054 percent to 1.9 percent with an average of 5500 ppm or 0.55 percent. Generally the poultry litter and biosolids composts had the highest levels of phosphorous followed by food residuals, MSW, yard trimmings and wood residuals composts respectively. Potassium ranged from 151 ppm or 0.015 percent to 34,570 ppm or 3.46 percent. The state average was 7300 ppm or 0.73 percent. Again the poultry litter composts were the highest followed by food residuals composts, biosolids, MSW, wood residues, and yard trimmings respectively.
Finally, all compost samples were analyzed for metals and micronutrients, including: aluminum, boron, calcium, cadmium, chromium, copper, iron, magnesium, manganese, molybdenum, sodium, nickel, lead, sulfur, silicon, and zinc. Metals and micronutrient levels are generally a product of the initial feedstocks. While none of the composts exceeded EPA 503 standards and should pose no concern for any application, some trends were evident.
Calcium and manganese levels were highest in the poultry litter composts, and pulp and paper compost. Chromium levels were higher in biosolids composts, boron levels were higher in poultry litter materials, and iron levels higher in yard trimmings composts. Sodium was generally higher in the food waste composts, poultry litter composts, pulp and paper, and MSW composts respectively. Nickel was generally higher in the pulp and paper, MSW, and biosolids composts. Lead was highest in the MSW compost and some of the poultry litter composts. Silicon seemed to be higher in some of the food waste composts and biosolids composts. Zinc appeared to be higher in some of the poultry litter composts, biosolids and MSW composts. There were no apparent trends for cadmium, copper, molybdenum or sulfur. Levels of metals and nutrients proved to be and are highly variable between composts and no conclusions should be made on composts and/or feedstocks in general based on data presented here.
MARKETING QUALITY COMPOST
The physical and chemical characteristics of Georgia's composts ranged quite a bit depending on feedstocks, management style, and intended application for the end product. In addition, time of sampling and replications may produce different or more consistent results that were beyond the scope of this study. While some operations compost specifically as a waste management tool, less attention is given to the quality of the product and its consistency, especially if the material is used for internal purposes or given away. Those materials that were composted the longest and given ample curing time seemed to produce higher quality products, especially if the end use was for market sales.
While standardizing finished compost products may help the composting industry in Georgia, it may only serve a handful of the larger private composting operations, and of course, the consumer. The Georgia Composting Association (GCA) and the EOP at University of Georgia are interested in creating standards for compost products in the state as well as specifications for various applications. The Georgia Composting Association is considering endorsing the USCC's Seal of Testing Assurance (STA) program and may provide incentives for GCA composters to become STA certified. In addition, the GCA may also create their own stamp of approval for high quality compost produced within the state. The stamp can be used as a marketing tool for GCA composters and can serve to ensure consumers that the material they are buying is a certified high quality material that was made in Georgia with materials recycled from Georgia industries and municipalities.
The University of Georgia has already completed a set of recommended specifications for using compost in erosion and sediment control applications. These recommendations are evolving as research is completed on the environmental impacts of using compost and mulch in erosion and sediment control applications. They can be found on the web at www.ces.uga.edu/pubcd/B1200.htm.
[Author Affiliation]
Britt Faucette is with the University of Georgia's Department of Biological and Agricultural Engineering.
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