Co-composting of alkaline tissue digester effluent with yard trimmings

Alkaline digestion of animal carcasses is gaining popularity as a method of disposing of animals because of its very effective pathogen control and general ease of operation. Once completed, the resulting high-strength effluent can be released into the municipal sewer systems. In some cases where the municipal system is unable to handle this high-strength wastewater, alternate methods of treatment are required. Co-composting with a low-moisture substrate such as yard trimmings can be an effective option. This paper reports the results of absorption tests to determine the amount of digester effluent (from the Tissue Digestortrade mark process) that can be added to yard waste before leachate production begins. In addition, a low dosage of liquid effluent was added to yard trimmings and composted in laboratory bioreactors. Results show that leachate production begins when 0.6L-effluent is added per kg-unamended yard waste at an original moisture content of 55.6%. The amount of leachate produced increases exponentially following the empirical equation: leachate in mL/kg=0.145 e(6.007Effluent dosage in L/kg) (valid in the effluent addition range of 0-1.2L/kg). Composting of yard waste with effluent showed that the initial pH did not inhibit microbial activity up to 9.39 pH. Variability was high and there was no statistically significant difference in dry matter degradation between treatments (measured range was 1.3-6.0% of initial dry matter). Final compost had nitrogen and phosphorus concentrations of approximately 1% and 0.1%, respectively. The potassium concentration increased with increasing effluent addition and was 1.84% in the 0.2-L/kg treatment. All regulated heavy metals were several-fold below US EPA limits.