Interaction between digestion conditions and sludge physical characteristics and behaviour for anaerobically digested primary sludge

The interaction between digestion conditions and the sludge physical characteristics and behaviour was investigated for anaerobically digested primary sludge in completely stirred tank reactors (CSTRs). The CSTRs were operated to maintain sludge retention times (SRTs) of 10, 15, 20 and 30 days and temperatures of 25 and 35 °C. The change of the floc size as a result of digestion was examined using wet sieve analysis (0.100, 0.125, 0.200, 0.500 and 1.000 mm). The results reveal a substantial reduction in all floc sizes with improving digestion conditions. Digestion leads to the transfer of bigger flocs into smaller ones, which has a remarkable effect on the sludge physical behaviour. The majority of the raw and digested flocs are smaller than 0.100 mm. The dewatering results showed the existence of an optimal SRT for dewaterability at 20 and 15 days for the reactors operated at 25 and 35 °C, respectively. The dewaterability of sludge digested at less favourable conditions, viz. 10 days at 25 °C deteriorates due to increase of small flocs generated from destruction of larger flocs. The digested sludge settling results showed a slight worsening but insignificant trend with increasing the SRT.