Dewatering of digested biomass

The world’s population is increasing and as cities grow in population and area, efficient and safe handling of sewage is becoming increasingly important. Wastewater treatment generates two products; purified water and sewage sludge. More sustainable options than landfilling of untreated sewage sludge and other organic wastes are desired. Technologies such as the anaerobic digestion (AD) process have been employed to convert a portion of the organic material to renewable energy in the form of biogas. The remaining solid residue after AD (digestate) has been reduced in volume and is more attractive for other disposal options such as land application in the agricultural sector. To minimize the amount of water in the digestate before transportation, solids are separated from the water in a dewatering process. Technologies such as the thermal hydrolysis process (THP) have been developed to increase biogas production and improve dewatering. The amount of water removed from the digestate has a substantial impact on the volume of dewatered cake and thus the transportation costs. It is therefore important to understand the mechanisms underlying the dewatering process. This thesis aimed at improving the understanding of why digestates dewater differently, how this can be predicted and what the effect of the THP is, either in front of AD (Pre-AD THP) or after AD (Post-AD THP). A wide range of digestates from 35 commercial full-scale plants were collected as a part of this study. This study is based on five research papers, which gave the following main results: Overall, the AD substrate composition was shown to highly influence the dewatered cake solids from digestates with conventional AD, Pre-AD THP and Post-AD THP. Thermogravimetric analysis (TGA) was used to predict dewatered cake solids by measuring the free water content. Different dewatering devices produced different cake solids despite having similar free water content, however, the impact was minimal compared to the impact of differences in digestate physicochemical parameters. A universal factor describing the water retention capacity of the digestate was identified and termed C/N•ash. This factor was found to correlate linearly with the predicted cake solids by TGA when plants with conventional AD were separated from Pre-AD THP plants. Moreover, when applied to full-scale data, the C/N•ash was found to correlate linearly with both dewatered cake solids and the polymer dose used in the dewatering process. Both Pre-AD THP and Post-AD THP were found to improve predicted cake solids of dewatered digestate by increasing the amount of free water. Post-AD THP gave bigger improvement in cake solids than Pre-AD THP. Centrifuging the Post-AD THP digestate at 80 °C increased the dewatered cake solids compared to predictions by TGA at 35 °C. The substrate composition also influenced the amount of melanoidin-associated compounds in Pre-AD THP digestate quantified as the concentrations of soluble colloidal chemical oxygen demand, color and dissolved organic nitrogen. Post-AD THP results in a sterilized biosolid fraction where pathogens can grow without microbial competition if recontaminated during storage and handling. This scenario was efficiently mitigated by the addition of a mixed microbial community from compost to the dewatered biosolids. The compost added a robust and diverse microbial community capable of outcompeting the added Escherichia coli and suppressing its’ growth in Post-AD THP biosolids. The results provide a simple solution for control of pathogen recontamination. Overall, this study has identified a parameter that can easily be measured in a commercial laboratory providing information on the water holding capacity of digestates. Predictive tools have been developed and the effect of sludge composition on both dewaterability and the concentration of melanoidin-associated compounds have been quantified. A solution to control pathogen growth in Post-AD THP biosolids has been thoroughly verified showing increased robustness of compost-inoculated biosolids