Technology Brief

Composting Produce Waste and Wax Coated Cardboard: Pilot Study Results Using A Low Technology Approach

A Pilot study examining the feasibility of composting produce waste and wax coated cardboard (WCC) using a low technology approach was conducted. The project was conducted to promote the development of programs for composting produce waste and wax coated cardboard generated by grocery stores. The specific reasons for conducting the project are presented as follows:

1. The composting of produce waste at yard debris composting facilities has significant potential for improving the nutrient content, overall value and marketability of yard debris compost products.

2. Compost producers, waste haulers and grocery stores in the Puget Sound Area have begun to implement produce waste and wax coated cardboard composting programs and express interest in the expansion of such programs.

3. The low technology approach presents a very feasible and economical means of recycling produce waste and wax coated cardboard throughout Washington state and the country.

4. Grocery stores achieve significant savings in disposal costs by participating in a composting program.

5. Regulatory and solid waste agencies are requesting more technical information regarding this waste recycling technology.

Methodology

A total of seven experimental piles were constructed at Iddings, Inc. located in Kent, WA. Various mix ratios of produce waste, WCC and yard debris were incorporated into the seven piles. The ratio of produce waste incorporated ranged from 1:2 to 1:5, produce waste:yard debris on a volume basis. Wax coated cardboard was incorporated in a ground form in two piles and as whole boxes in one pile. A hammermill was used to grind the WCC. In all three piles the ratio of cardboard to the other pile constituents ranged from 1:5 to 1:10, WCC:other ingredients. The performance of each pile was carefully monitored during a 12 week composting process.

Overall, the project results indicated the inclusion of produce waste and WCC had no negative effect on the yard debris composting process. The addition of produce waste was noted to increase the pile temperatures. The produce waste piles also had lower oxygen levels than the yard debris only piles, suggesting the addition of produce waste could increase the generation of odors. However, odor measurements indicated there was no difference in the amount of odor generated by the produce waste piles and the yard debris pile. High mercaptan levels observed in low produce piles during the first ten days of composting were attributed to the low porosity of these two piles. Final product analyses indicated produce waste compost had a higher nutrient content that the yard debris compost product. Trace metal analyses of each experimental pile, including the WCC piles were similar, and well below the United States Environmental Protection Agency regulations for the utilization of biosolids (sewage sludge) compost (40CFR part 503).

Wax coated cardboard had no noticeable negative effect on the composting process when used at a rate of 15 percent of the initial mix on a volume basis. The amount of WCC used in the project was based on the generation rate at the supermarket. However, the amount of WCC used in the mix could be increased without negatively affecting the process.

Project Results

The ability of WCC to provide porosity and the contribution of boron from the WCC to the final product are two issues that should be considered when using a higher ratio of WCC. As a bulking material, WCC is dry and absorbent and therefore works well for adjusting the moisture content of an initial mix. However, with respect to providing porosity, WCC is inferior to the woody, brushy fraction of yard debris. WCC loses its structure when it becomes wet, and physically and biologically degrades faster than the woody yard debris fraction. As a result, a mix with a high ratio of WCC will become much less porous as the process continues, than a mix containing woody material as the bulking source.

The boron content of the produce waste compost products and the yard debris product did not differ. The boron levels are also well below the concentrations that have been reported to cause plant toxicity problems. Boron analyses of uncomposted WCC samples also suggest that boron toxicity would not result from the use of WCC even at very high rates. However, other studies have found plant toxic levels of boron in the compost product when WCC is incorporated at a high rate. Compost producers should be aware of this potential problem and test their product regularly until they are assured a problem does not exist.

Samples of different colored WCC (uncomposted) were analyzed for eight trace metals. The analytical results indicate that the trace metal content of colored and uncolored WCC were very similar. The trace metal levels of the WCC samples were noted to be at least two orders of magnitude lower than the federal trace metal limits for the utilization of biosolids compost (40CFR part 503).

The wax used in the production of WCC is produced from the distillation of crude oil. Consequently, questions have been raised as to the presence of organic contaminants in petroleum based waxes. One sample of WCC (uncomposted) was analyzed for volatile organic aromatic compounds (EPA Methods 5030/8310). These compounds are often found in products such as gasoline and motor oil that are produced from the distillation of crude oil. None of the compounds analyzed were detected, suggesting these impurities are not present (at concentrations greater than the detection limit) in WCC. However, only one sample was analyzed. Additional analyses should be performed to confirm these results.

Conclusion

In summary, the project results support the use of a low technology approach for composting produce waste and WCC. Due to the potential for generating odors, facilities in the start-up phase of composting produce waste will need to monitor the process closely and develop a strategy for limiting the persistence of anaerobic conditions. The maintenance of aerobic conditions is the most critical aspect of limiting odor generation. The following activities should be considered for maintaining aerobic conditions when composting produce waste:

· decreasing the cross sectional area of the pile,
· increasing porosity of the initial mix,
· limiting the ratio of produce waste to yard debris,
· increasing the frequency of windrow turning.

The pilot project was conducted with the assistance of Iddings, Inc. a compost processor located in Kent, WA. This technology brief was prepared by the Clean Washington Center. The Clean Washington Center is the Managing Partner of the Recycling Technology Assistance Partnership (ReTAP). ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology. ReTAP is also funded by the U.S. Environmental Protection Agency and the American Plastics Council.

Report Dated: June 1993