Conversion of Municipal Sludge to Biodegradable Plastic  

Project Title: Conversion of Municipal Sludge to Biodegradable Plastic

Applicant: The Hong Kong University of Science and Technology

Total Approved Grant: $103,000

Duration: 01/05/95 to 30/04/96

Project Status/Remarks: Completed

Scope: To develop a biological process for the production of biodegradable plastics from municipal sludge which contains about 70% organic materials by weight so that the landfill capacity could be saved considerably and less contamination would be caused to the local environment.

Summary of the Findings/Outcomes: By using an innovative technology, a biodegradable thermoplastic was made from a local municipal sludge. About 50% of volatile solids of the raw sludge was reduced by thermophilic anaerobic digestion and hence about 30% of sludge mass was reduced for a typical raw sludge containing 60% (w/w) of total solids. Compared with the raw sludge, the treated sludge contained less pathogens (Biosolid A) and less biodegradable components with less formation of biogas. The effects of temperature, pH, hydraulic and solid retention times and the solid content of raw sludge on sludge digestion and formation of volatile fatty acids were obtained in a continuous flow digester. The soluble organic compounds measured as total organic carbon (TOC) from the digested sludge were used by a bacterium Alcaligenes eutrophus to produce polyhydroxyalkanoates (PHAs), a biodegradable thermoplastic. Polymer content in cells reached 33-34% (w/w) and TOC conversion 70-80%. The "sludge" thermoplastic was a copolymer of three monomers with 74% (w/w) of C4 monomer. The melting point of the "sludge" polymer was 167oC. Plastic films (0.2-0.5 mm thick)made from the "sludge' polymer were degraded very fast by aerobic and anaerobic bacteria in a municipal wastewater treatment plant; more than 70% mass in 5 weeks. The films were decomposed by 10-20% mass in sea water and soil in 5 weeks.