Microbial oxygen uptake in sludge as influenced by compost physical parameters

The wide range of optimal values reported for the physical parameters of compost mixtures suggest that their interactive relationships should be investigated. The objective of this study was to examine the microbial O2 uptake rate (OUR) in 16 sludge waste recipes, offering a range of moisture content (MC), waste/bulking agent (W/BA) ratio and BA particle size levels determined using a central composite experimental design. The 3 kg samples were maintained at a constant temperature and aeration rate for 28 days, during which a respirometer recorded O2 uptake to provide a measure of microbial activity and biodegradability. The cumulative O2 consumption after 14 and 28 days was found to be significantly influenced by MC, W/BA ratio, BA particle size and the interaction between MC and W/BA ratio (p < 0.05). Using multivariate regression analysis, the experimental data was used to generate a model with good predictive ability for cumulative O2 consumption after 28 days as a function of the significant physical variables (R2 = 0.84). The prediction of O2 uptake by the model depended highly on the interaction between MC and W/BA ratio. A MC outside of the traditional 50–60% (wet basis) range still resulted in a high level of microbial O2 uptake as long as the W/BA ratio was adjusted to maintain a suitable O2 exchange in the sample. The evolution of OUR in the samples was also investigated, uncovering strong associations between short and long-term respirometric indices, such as peak OUR and cumulative O2 consumption (p < 0.005). Combining peak OUR data with cumulative O2 consumption after 14 days allowed for accurate predictions of cumulative O2 after 28 days of aeration (R2 = 0.96), implying that future studies need only run trials up to 14 days to evaluate the overall O2 consumption or biodegradability of similar sludge mixtures.