Mixed culture (activated sludge) polyhydroxyalkanoates (PHAs) produced in conjunction with wastewater treatment and compounded into bioplastics have applications in agriculture. Rates of biodegradation and any influence of degradation products need to be characterised in-situ in soil environments for applications involving PHA-based product degradation (either as a controlled intent in product service or a criterion for product end-of-life management). We performed such characterisation as part of the initial stages of product development of native copolymers of poly-3-hydroxybutyrate-
co-3-hydroxyvalerate (PHBV). This PHBV was accumulated in and recovered from activated sludge produced in pilot and full-scale wastewater treatment processes. The rates of PHBV biodegradation in soil were quantified using a broad range of HV content material (12?mol % -72?mol % ) and referenced to a cellulose control. For mixed culture PHBVs, 90 % biodegradation was expected to occur between 10.7 and 22.2 months. Germination studies using a standard cress seed assay indicated that PHA biodegradation products had no influence on germination activity in soil. Soil extracts examined during polymer degradation were non-toxic (Microtox).
Mechanisms of biodegradation were examined by means of physico-chemical characterisation and Atomic Force Microscopy (AFM). Biodegradation occurred at the polymer surface and the rate was governed by both biodeterioration and depolymerisation, which were shown to be controlled by a combination of copolymer composition, crystallinity, microstructure and surface morphology.
