Methane production and hydrolysis kinetics in the anaerobic degradation of wastewater screenings

Anaerobic biodegradability and hydrolysis rates of wastewater screenings were determined using the biochemical methane potential test at 37oC. The extent and rate of screenings conversion to methane of this complex and particulate substrate were investigated and since two phases of hydrolysis were identified, corresponding to the different types of materials in screenings, a linear and non-linear model was used. No accumulation of intermediary products was observed in the first phase of hydrolysis and so it was possible to use the methane production rate and a linear model to estimate the hydrolysis rate of this phase. The measured values of 0.061 to 0.127 d-1 are in the range reported for other comparable organic wastes. It was also observed that the inoculum-to substrate ratio has a large impact on methane production rate of screenings during this phase of the process. The difference in biodegradation rates from the materials in screenings could be represented by the modified Gompertz non-linear model which was able to describe the methane production rate of screenings with a high confidence. Screenings were found to have 52% biodegradability on average and this shows the potential for volatile solids destruction. A two-stage process with an improved hydrolysis rate is proposed to ensure that the full potential of the material is exploited.