Vermicomposting biosolids and organic wastes

Most biosolids produced at wastewater treatment plants in New Zealand, are currently put into landfills, were they can generate methane and produce leachate. Recent Government policy means that this disposal method cannot continue to be used and biosolids must be used beneficially. However, biosolids need to be stabilised before beneficial use. Using earthworms to break down organic wastes has gained momentum in the past decade. Earthworms and microorganisms can be used in the vermicomposting process to stabilise many organic materials including biosolids. This dissertation swnmarises information on vermicomposting obtained from a literature review and from visiting two large-scale commercial vermicomposting operations in Australia. Investigations assessed the potential of vermicomposting biosolids in New Zealand. Initially, the physical, chemical, and microbiological properties of three biosolids and four sewage sludges were measured. Worm acceptance of these materials were then determined. The four most promising materials from North Shore, Hamilton, Te Awamutu and Taupo were vermicomposted for 30 days using Eisenia fetida. The vermicasts produced were then evaluated in a glasshouse pot trial using ryegrass. The biosolids characterised had widely different properties compared to sewage sludges from waste stabilisation basins. Biosolids had more plant nutrients, greater pathogens numbers (as indicated by E. coli) and lower heavy metal concentrations than sewage sludges. Biosolids were initially toxic to worms due to ammoniwn concentrations and required stabilisation for 14 days before acceptance whereas sewage sludges were acceptable within two days if cellulose (as paper or cardboard) had been added. Vermicomposting these materials for 30 days produced vermicasts with lower volatile solids, higher C/N ratio, and significantly reduced indicator pathogen concentrations than the starting material. Ryegrass trials showed that mixing vermicasts with soil significantly increased plant growth, mostly because of the soluble N content. Trial results have shown that vermicomposting is an acceptable method for stabilising biosolids and produces a quality end product highly beneficial for land use.