Recuperative thickening: impact of dewatering on anaerobic activity

The impact of dewatering on methanogenic sludge viability and activity influences both the feasibility of recuperative thickening to increase solids residence time within a digester, and the utilisation of dewatered sludge cake as digester inoculum. Thickening can reduce activity through either air contact (rotary drum, DAF, or belt filter press), or by lysing cells through shear (centrifuge), particularly at high solids. To assess this, two plants with recuperative thickening (rotary drum), and three without, had specific methanogenic activity tested in all related streams, including dewatering feed, cake, and centrate. All plants had high speed centrifuges for final dewatering. The microbial community was also assessed through 16s Pyrotag sequencing. The activity of all samples was in the expected range of 0.2-0.4 gCOD gVS-1d-1. Plants with recuperative thickening did not have lower digestate activity. Centrifuge based dewatering had a significant and variable impact on viability of methanogens from all samples, ranging between 20% -90% decrease Recuperative thickening had a far smaller impact on activity, with a drop of 0%- 20% in activity. Principal components analysis of the microbial populations indicated that most populations were comparable, except that plants with recuperative thickening had a shift towards hydrogen utilising methanogens from aceticlastic methanogens. Further analysis indicated that the drop in activity was correlated with both niche microbial community (represented by principal component 3), and with final cake solids. However, a sample with very wet cake was insensitive despite a presumably sensitive community (high PC3 score), while a sample with very dry cake was insensitive due to an apparently “hardened” community (low PC3 score)