Anaerobic treatment of domestic sewage at low temperature

The main objective of this thesis was to asses the anaerobic treatment of domestic sewage at low temperature and the possibilities to optimize the performance of high-rate anaerobic systems treating domestic sewage at low temperature.

The anaerobic biodegradability of domestic sewage and its fractions was investigated in batch experiments. The results showed a high potential of anaerobic treatment of domestic sewage due to its high biodegradability (74 %). The results showed a high biodegradability of the colloidal fraction (86 ± 3%) indicating that the low removal of colloidal particles in continuous high-rate anaerobic reactors is due to low physical removal rather than biodegradability.

Both results of batch and continuous experiments demonstrated that the removal of SS prior to anaerobic treatment of domestic sewage not only prevents the deterioration of the methanogenic activity in the anaerobic reactor but also improves the removal of colloidal particles. Therefore, for the pre-treatment (removal of SS) of domestic sewage at a low temperature of 13 °C, a comparison between an anaerobic filter (AF) and an anaerobic hybrid (AH) reactor with flocculant sludge with an HRT of 4 h was carried out. The media in the AF and the top of the AH reactor were vertical sheets of reticulated polyurethane foam (RPF) with knobs. The sludge in the AF reactor was only attached to the media and all accumulated sludge in the bottom of the AF reactor was wasted regularly to avoid reactor clogging. The AF reactor showed a significantly higher suspended COD (CODss) removal (82%) than the AH reactor (53%) and the performance of the AF reactor was more stable especially in rainy periods. The results showed that the wasted sludge from the AH reactor was more stabilized. However, the wasted sludge from the AH still needs post stabilization. Therefore, the AF reactor is recommended for the pre-treatment of domestic sewage at low temperature. The results of batch experiments with RPF media with and without an anaerobic biofilm showed that the removal of CODss in the AF reactor is mainly due to physical filtration and entrapment, while the removal of colloidal COD (CODcol) depends on the presence of a biofilm.

The results of the treatment of pre-settled sewage at 13 °C demonstrated that the AH reactor with granular sludge has a significantly higher total COD (CODt) removal than the UASB reactor due to the significantly higher CODcol removal. Therefore, the AH reactor with granular sludge was selected as a second step for treatment of domestic sewage at low temperature. The performance of the AF+AH system treating domestic sewage at a low temperature of 13 °C at different HRTs was investigated. The results showed that optimization of CODss and dissolved COD (CODdis) in the AF and AH reactor respectively could be achieved with an HRT of 4+4 h for the two-step system without deterioration of the maximum methanogenic activity of the granular sludge in the AH reactor. For optimization of CODcol a long HRT of 8 h is needed in the AH reactor. With an HRT of 4+8 h for the two-step AF+AH system, a CODt removal efficiency of 71% was achieved with 92, 60 and 55% removal efficiency for respectively CODss, CODcol and CODdis. (author's abstract)