Shredded rubber tires as a bulking agent in sewage sludge composting

This study evaluated shredded rubber tires mixed with raw primary sludge to determine the optimum particle size and mix ratio for efficient composting. Of the three sizes and mix ratios tested, the 1. 27-2. 54 cm size and the rubber chip to sludge mix ratio of 2:1 were found to be optimum. Anaerobically digested and secondary biological sludges were then composted with the optimum size rubber chips. Due to their high moisture content and low C/N ratio, all three sludges required a moisture absorbing amendment and supplemental carbon. Sawdust was tested and found to be a suitable amendment. Heavy metal levels increased due to the concentrating effect as organic matter was decomposed. Rubber chips contributed zinc and iron to the finished compost. Although elevated after five cycles, the increases in zinc and iron were not high enough to limit the use of shredded rubber in composting sewage sludge.