Effect of the inoculation level in aerobic biodegradability tests of polymeric materials

The composition and the quantity of the microbial inoculum are well-known factors affecting the reproducibility of results of biodegradability tests. Experiments with polyhydroxybutyrate/valerate (PHBV, a biopolymer), cellulose (natural polymer) and acetate (soluble substance) as substrates were carried out to evaluate the impact of non-controlled inocula on test response. The European Committee of Normalization (ECN) Standard for packaging materials was the method chosen to evaluate the ultimate aerobic biodegradability, in order to establish carbon balances. Inocula were obtained from activated sludges sampled in the aerated tank of a municipal wastewater treatment plant. Activated sludges were collected at different periods, so the inoculum concentration (X0) in the test was dependent on the seasonal variations of the source. As the initial carbon content (S0) was maintained to a well-defined value, various S0/X0 ratios were obtained. At high S0/X0 ratios, cellular growth and soluble residual carbon accumulation were favoured and low mineralization levels were obtained. An increase in the quantity of mineralized carbon due to a low S0/X0 ratio seems to support the notion of standardized biodegradability tests in relation to this parameter, which could enhance the reliability of test results of polymeric materials.