Production of biodegradable thermoplastics from municipal sludge by a two-stage bioprocess

Biodegradable thermoplastics, polyhydroxyalkanoares (PHAs), were produced from municipal sludge in a two-stage bioprocess; anaerobic digestion of sludge by thermophilic bacteria in the first stage and production of PHAs from soluble organic compounds in the supernatant of digested sludge by Alcaligenes eutrophus in the second stage. Local municipal sludge of 0.3-5 wt \% dry solids containing 50-70\% volatile solids was digested under thermophilic (50-65 degrees C) and anaerobic conditions producing a mixture of fatty acids and other minor fermentation products. The residual sludge contains about 50\% of the original volatile solids. Volatile solids reduction and formation of soluble organic compounds from sludge were investigated under different temperature, pH, hydraulic (solid) retention times. More soluble organic compounds were produced at high temperature (65 degrees C) and pH (> 5.5) with little change in volatile solids reduction. In the second stage, harvested from a nutrient-rich culture, A. eutrophus produced PHAs from the soluble organic compounds in the supernatant of digested sludge under aerobic and nitrogen-limited conditions. The amount of PHAs accumulated in A. eutrophus accounted for 34\% of cell mass, comparable to the amount of PHAs produced from pure volatile fatty acids (33\% of cell mass). About 78\% of the total organic carbon in the supernatant was consumed by A. eutrophus. The conversions of four major acids in the supernatant by A. eutrophus were found to be 87.6\% (acetic acid), 62.6\% (propionic acid), 56.8\% (butyric acids) and 32.0\% (valeric acids), respectively. The sludge PHA is a co-polymer containing mainly C-4 monomers (74 wt \%) and has a melting point of 167 degrees C which is 9 degrees C lower than that of poly-3-hydroxybutyrate (PHB). The sludge thermoplastic and PHB had a similar biodegradability in soil, 22-27\% in 5 weeks. In sludge suspensions, 70\% of sludge plastic was decomposed and utilized by anaerobic and aerobic bacteria in 6 weeks. Copyright (C) 1997 Elsevier Science B.V