COMPOSTING OF PHENOLIC-BONDED SOFTWOOD PLYWOOD WASTE.
SUSAN V. DIEHL [+]
HAROLD A. STEWART [+]
In a 6-month composting study, sawdust from machined softwood plywood bonded with phenol-formaldehyde resin was amended with chicken manure, cow manure, horse manure, cotton gin trash, and inorganic fertilizer. The sawdust and amendments were placed in eighteen 75-liter plastic cans. Moisture was provided through atmospheric precipitation, and the substrate was aerated once or twice per week, depending on the amount of rainfall. Samples were collected at 30-day intervals and analyzed for toxicity and pH. On day 180, the compost cans were weighed and sampled to determine the decrease in dry weight. All treatments showed a significant decrease in toxicity by day 180, and maintained a neutral pH throughout the study with the exception of the horse manure treatment. All treatments showed a reduction in weight. Amended treatments had almost double the percentage of weight loss compared to the unamended control. The composted sawdust was also evaluated in a greenhouse study to determine its effect on the growth of row crop plants. Twenty-five percent of the composted sawdust by weight was mixed with potting soil, in which corn, soybean, and cotton seeds were planted. The pots were watered daily and allowed to grow for 55 days. The chicken manure treatment showed no significant difference in mean dry weight of the plants compared to potting soil. Other treatments were comparable to chicken manure, except gin trash, which showed a significantly lower dry weight for plants compared to the chicken manure and control.
Total North American production of plywood and oriented strandboard (OSB) is expected to grow from 37.5 billion [ft..sup.2] in 1998 to 38.8 billion [ft..sup.2] in 2002 . This increase in demand will result in the generation of more woodwaste for disposal. Most of these woodwastes are currently energy sources for large OSB or plywood manufacturers. Some of these woodwastes are reground for nearby particleboard mills. However, other plywood companies that do not have the capacity to burn the waste must transport it to a location equipped to incinerate the waste or to a landfill. Because of high transportation costs, landfill costs, environmental concerns, and governmental regulations, alternative methods of disposal are being sought.
Phenolic-bonded softwood plywood is manufactured from southern yellow pine and Douglas-fir, and bonded with phenol-formaldehyde (PF) resin with an approximate 2 to 1 molar ratio of formaldehyde to phenol in the presence of alkali (resol) . Phenol-formaldehyde resin is a thermosetting resin mixed with proteinaceous extenders (such as wheat flour, corn-cob, and pecan shells), lignocellulosic fillers, earthen minerals, and alkali to increase the viscosity, prevent overpenetration of the resin, and reduce the resin solids to about 28 to 31 percent in the adhesive mix. This adhesive has reactive methylol groups that condense into larger molecules when heated without the addition of a hardening cross-linking agent. When heated, this resin is three dimensional and mechanically, and/or chemically, attaches to wood surfaces. The resin is water insoluble except for the presence of about 10 to 12 percent alkalinity on a solids basis. The application adhesive ranges in viscosity from 3,000 to 10,000 mPa.s . Cross- linked PF resin has proven to be durable and waterproof in accelerated-aging tests. From a soil exposure standpoint, thermoset PF adhesive is comparable to thermoplastics in that it does not bind up any soil nutrients and it is not detrimental to plant growth when mixed with the soil .
Composting is a proven method that can reduce waste volume and toxicity, and transform woodwaste into a product that can be used as a soil amendment to increase organic matter content and water-holding capacity, and improve the texture of the soil [2-4]. A composting operation can be implemented at a plant site and requires limited knowledge, equipment, and space. The objective of this study was to evaluate several amendments for composting sawdust (from machine-sawn softwood plywood that was bonded with PF resin) and to determine the effect of composted material on three agricultural crops.
MATERIALS AND METHODS
Sawdust (machine-sawn softwood plywood bonded with PF resin) was collected from a furniture company in Mississippi. The moisture content (MC) of the sawdust was 3 percent. Eighteen 75-liter plastic cans held the sawdust and amendments. Five holes were drilled into the bottom of each can to prevent them from retaining water and creating anaerobic conditions. The cans were numbered and weighed, then treatments were randomly selected for each can. Sixteen kilograms of sawdust were placed into each can. The manures in this study were collected from local pastures and a poultry farm. The gin trash (residual from cleaning and ginning of cotton containing a mixture of cotton fiber, seeds, leaves, limbs and twigs) was collected from a local cotton gin and the inorganic fertilizer was Miracle-Gro brand fertilizer. The fertilizer was composed mainly of 36 percent nitrogen, 6 percent phosphoric acid, 6 percent potash, 0.325 percent iron and micronutrients.
Manures were allowed to dry under a hood and were added on a dry-weight basis. One hundred grams of inorganic fertilizer were mixed with one liter of deionized water and stirred. Each inorganic fertilizer treatment received one liter of this solution. After amendments were added, the substrate was thoroughly hand mixed. This study consisted of six treatments that were replicated three times. The treatments were as follows:
1. Control contained sawdust only;
2. Sawdust plus 5 percent chicken manure (dry-weight basis);
3. Sawdust plus 5 percent cow manure (dry-weight basis);
4. Sawdust plus 5 percent horse manure (dry-weight basis);
5. Sawdust plus 3 percent gin trash;
6. Sawdust plus one liter of inorganic fertilizer solution.
The compost cans were randomly placed in two rows outdoors in Mississippi for 6 months, from September through February. The substrate was turned with a compost aerator once or twice per week, depending on rainfall. The MC of each can was adjusted to 50 percent by adding deionized water. Atmospheric precipitation kept the MC around 50 percent throughout this study. Composite samples were collected at day zero and at 30-day intervals for 180 days and analyzed for toxicity using a Microbics Model 500 Toxicity Analyzer and for pH with a pH probe. Toxicity was measured by adding 2 g of substrate to 18 mL of water in a vial and sonicated for 10 minutes. The solution was placed in a refrigerator overnight. The solution was then centrifuged for 10 minutes at 50,000 revolutions per minute and adjusted to a neutral pH if necessary. The toxicity analyzer measures the reduction in luminescence of a marine bacteria challenged with the sample solution to determine relative toxicity levels . Toxicity results are shown a s the percent change of five replications of the sample compared to pure water controls. The pH was analyzed by a Markson Microcomputer pH-Vision 6072. On day 180, MCs were measured, and compost cans were weighed to determine the decrease in dry weight.
At the end of the composting study, about 1200 g of the composted sawdust was collected from each replicate for a soil amendment to test its effect on cotton, soybeans, and corn. Twenty-five percent by weight of the composted sawdust was added to Bactomix potting soil and placed in a 15-cm plastic pot. The seeds used for this study included: 1) soybeans - Hutchinson (80% germination rate); 2) cotton - Delta Pine 50 treated with Del-Max (80% germination rate); and 3) corn - GARST 8514-IT (80% germination rate). This study consisted of seven treatments including one control containing potting soil only. The remaining six treatments consisted of potting soil amended with 25 percent composted sawdust from the composting study. Each treatment was replicated three times for each seed type. After mixing and planting, each pot was placed in a greenhouse and allowed to grow for 55 days. Each pot was equally watered daily, but fertilizer was not added during this study so the effect of the composted sawdust would not b e masked. On day 14, excess plants were pulled by hand in order to leave two plants per pot. At day 55, all plants were pulled and cleaned of all soil. The plants were placed in an oven for 24 hours and dry weights were measured and recorded.
Complete randomized design with three replications for each treatment was used for statistical analysis of composted sawdusts and row crop dry weights. Mean comparisons were made using a least significant difference at the p = 0.05 probability level by the Statistical Analysis System (SAS) program (SAS Institute, Gary, N.C.).
RESULTS, DISCUSSION, AND APPLICATION
All treatments showed a decrease in toxicity and weight when compared to day zero. Treatments amended with chicken manure showed the best results in reduction of toxicity, weight loss, color change, and greenhouse study. Inorganic fertilizer and chicken manure treatments turned dark in color by day 75, while other treatments and controls showed only a slightly visible change in color at the end of the study. All treatments showed a decrease in toxicity at 30 days and continued to decrease until day 180 (Fig. 1). By day 180, the unamended control, chicken manure, gin trash, and inorganic fertilizer treatments showed the lowest toxicity. The cow manure treatment toxicity level was greater than the unamended control and chicken manure treatment. The horse manure treatment showed the greatest toxicity level at day 180. With the exception of horse manure, most treatments began at near neutral pH at day zero, decreased at day 30, and increased back to neutral by day 180 (Table 1). The reason for low pH and relativ ely high toxicity of horse manure is not fully understood. However, one possible explanation could be due to the presence of microorganisms, mainly fungi, associated with horse manure and their biological activities.
Amended treatments showed a significantly higher percentage of weight loss compared to the controls at day 180 (Fig. 2). The same results were found when amendment weight was excluded from the total weight, further confirming biodegradation of plywood sawdust (Fig. 3). Inorganic fertilizer treatment showed the most reduction in weight, but was not significantly different from the other amended treatments.
Dry weights of row crop plants from the greenhouse study showed no significant difference between potting soil and potting soil mixed with composted sawdust amended with chicken manure (Fig. 4). All other treatments were comparable with the chicken manure but not with the potting soil only treatment, except for the gin trash, which had a significantly lower dry weight than both the potting soil only and chicken manure treatments.
This composting study of PF-bonded plywood sawdust found that composting can decrease the toxicity, reduce the weight, and produce a final product that can be used as a soil conditioner. The addition of chicken manure or another high-in-nitrogen amendment can increase the rate of substrate degradation. The final results show that if an inexpensive nitrogen source is not available and time is not a factor, moisture and mixing alone can decrease the toxicity and produce a product that is comparable to potting soil with no adverse impact as a soil amendment after 180 days. For quick composting, an organic or inorganic fertilizer should be added to the substrate. In the greenhouse study, the benefits of the composted sawdust were masked, due to the texture and organic matter contained in the potting soil. Further research is needed to test composted woodwaste in a field study of regular soil that receives normal applications of fertilizer. Also, plants should be allowed to grow for an entire growing season to ana lyze yields of row crop plants. Overall, this study showed that forest products companies that produce PF-bonded woodwaste could implement composting in their remedial plans to reduce landfilling and transportation costs, and perhaps assist other agricultural industries in disposing of their waste.
The authors are, respectively, Former Graduate Student, Professor, Associate Professor, and Research Scientist, Forest Prod. Lab., Mississippi State Univ., Box 9820, Mississippi State, MS 39762. Approved for publication as J. Article No. FP157 of the Forest & Wildlife Res. Center, Mississippi State Univ. This paper was received for publication in October 1999. Reprint No. 9046.
(+.) Forest Products Society Member.
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pH of composted sawdust during the period of 180 days. Treatment Day 0 Day 30 Day 60 Day 90 Day 120 Day 150 Day 180 Sawdust only 7.01 4.02 6.97 6.21 6.73 6.47 6.52 Chicken manure 7.05 6.57 6.66 6.37 6.55 6.58 7.02 Cow manure 6.79 6.70 7.01 6.39 6.77 6.90 6.88 Horse manure 6.70 7.17 6.87 6.43 6.69 5.66 5.22 Gin trash 5.20 4.02 7.10 6.50 6.73 6.52 6.23 Inorganic fertilizer 6.77 4.05 6.57 6.21 6.39 6.51 7.34
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|Author:||WILTCHER, DON; BORAZJANI, HAMID; DIEHL, SUSAN V.; STEWART, HAROLD A.|
|Publication:||Forest Products Journal|
|Date:||Oct 1, 2000|
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