Composting away wood waste.

New research shows that adding horse manure to wood waste may help to reduce bulk and toxicity of manufacturing residue while turning it into beneficial compost.

Furniture plants produce a large quantity of wood waste during their manufacturing cycles. These wastes are a mixture of hardwood and softwood sawdust, wood flour, wood dust, wood chips, wood flakes, fabric scraps, and small amounts of finishing, polishing and gluing compounds.

One viable disposal method for sawdust residue is burning it for energy. Unfortunately, there are numerous small furniture industries that generate wood waste residues that have no facilities for burning wood.

Sales of sawdust to other firms typically result in a negative cash flow, as the cost of transportation is usually higher than the price received. Incineration on-site is very expensive and the incinerator must meet Environmental Protection Agency (EPA) emission regulations. Disposal in landfills is also becoming prohibitive. One small Mississippi company located in the Mississippi Delta estimates spending $6,000 per month for land-filling its residue waste. Alternative disposal options would greatly benefit many different wood products industries.

Recently, we have been conducting research to evaluate the biotechnology of composting furniture manufacturing residues with added horse manure as a viable disposal option for furniture wood waste residue. Composting is an aerobic biological decomposition process that converts biodegradable solid organic matter into a stable humus material. Composting of wood fiber waste prior to land application has potential advantages of increasing humic components, eliminating toxicity problems and lowering transportation costs by reducing the bulk mass.

Methodology

Manufacturing wood waste residue was obtained from a furniture plant in Mississippi. The wood wastes were cleared of large-sized wood and fabric materials and well-mixed. The moisture content (MC) of the mixed wastes was five percent. Test units consisted of nine treated plywood 2-feet by 3-feet by 1-foot compost boxes. A 2-feet by 3-feet weedproof synthetic fiber mat was laid out on the bottom inside of each box. Two inches of pea gravel was spread over each mat to facilitate water and air flow.

Test units were set in triplicate for each treatment in a completely random design. Treatments were: controls (no additives), five percent added horse manure (wt/wt basis) and 10 percent added horse manure. Sixty pounds of the sawdust were put into three randomly placed boxes for controls. The remaining sawdust was mixed with fresh (one-day old) horse manure. Sixty pounds of these mixtures were placed in the remaining randomly located six boxes.

In order to achieve a rapid stabilization by composting, the material undergoing decomposition should have an MC near 50 percent. Therefore, the MC in each box was adjusted to 50 percent by adding distilled water. Samples were taken from each box and analyzed for pH, MC and toxicity. Boxes were monitored weekly for MC and adjusted if needed. Aeration was accomplished by thoroughly mixing the sawdust every seven days with a compost aerator. Aeration is needed when the temperature exceeds 70 [degrees] C or the MC exceeds the 60 percent level.

The contents of each box were removed after 180 days, weighed and tested for toxicity. The pH levels remained near 6 to 6.3 throughout the study. No isolation from material was conducted to identify the organisms involved in the composting process. However, it is highly likely that a vast group of microorganisms were active in the biodegration process, because compost boxes were located outdoors.

Toxicity analyses were performed with a Model 500 Microtox Toxicity Assay. The Microtox test procedure has been widely used and accepted by regulators to quantify the toxicity of hundreds of pure chemicals, effluents and leachates that could be toxic to humans and the environment.

Results, Application and Discussion

In comparison to day 0, all treatments showed a significant weight reduction after 180 days. The control group was reduced from 60 pounds to 46.5 pounds, a weight reduction of 23 percent. The wood waste with 5 percent manure was reduced by 43 percent, to 34.5 pounds, while the wood waste with 10 percent manure was reduced by 48 percent to 31.5 pounds. While weight reduction was significantly greater for manure-amended treatments, the differences between the 5 and 10 percent horse manure samples were not significant. This could be a very important cost saving for large-scale composting of wood wastes.

To evaluate the results more accurately, original manure weight was subtracted from total weight, and the percent loss was measured. The same pattern was observed. Both manure treatments showed a significantly greater weight loss than controls, but there was no significant difference between the 5 and 10 percent treatments (a 39 and 42 percent weight reduction, respectively).

The composted material was a mixture of completely and partially degraded wood waste and had a much darker color in comparison to the controls. This material could be spread over agricultural land, e.g., used for nursery rooting material.

Toxicity of the wood wastes could be a major problem in disposal of these materials but usually is not. However, composting significantly reduced the toxicity of finishing, gluing and polishing compounds that are usually associated with these wastes. The greatest decrease in toxicity occurred within the first 30 days. A slight increase in toxicity at the end of the study could not be due to anaerobic activities, because the material within the boxes was well-aerated, but may possibly be attributed to the spraying of herbicides for grass control in the vicinity of the compost boxes.

Results obtained from this study indicate that composting could be an economical, simple, safe and viable option for disposal of wood wastes. Because of the small particle size, these types of compost are particularly well-suited as a soil amendment in areas with low organic matter. Several area manufacturers could combine their resources and build a large-scale, agriculturally located composting site and supply local farmers and nurseries with their products. The material could also be taken back to the forest. Research is continuing with other types of manure and agricultural residue.

Hamid Borazjani, Susan Diehl and Harold Stewart are, respectively, associate professor, assistant professor and senior research assistant for Forest Products Lab., Forest and Wildlife Res. Center, at Mississippi State University.