Effects of post anaerobic digestion thermal hydrolysis on dewaterability and moisture distribution in digestates.

Ester Rus,Kjell R Jonassen,Bjørge Westereng,Greeley Beck,Svein J Horn,Matthew J Higgins,Pål J Nilsen,Geir H Sørland,Oda K Svennevik,Odd Egil Solheim

doi:10.2166/wst.2019.379

Abstract

Organic waste fractions such as sewage sludge, food waste and manure can be stabilized by anaerobic digestion (AD) to produce renewable energy in the form of biogas. Following AD, the digested solid fraction (digestate) is usually dewatered to reduce the volume before transportation. Post-AD treatments such as the Post-AD thermal hydrolysis process (Post-AD THP) have been developed to improve the dewatering, but the mode of action is not well understood. In this study, samples from 32 commercial full-scale plants were used to assess the impact of Post-AD THP on a broad range of raw materials. Maximum dewatered cake solids after Post-AD THP was predicted by thermogravimetric analysis (TGA). Post-AD THP changed the moisture distribution of the samples by increasing the free water fraction. A consistent improvement in predicted dewatered cake solids was achieved across the 32 samples tested, on average increasing the dry solids concentration by 87%. A full-scale trial showed that dewatering Post-AD THP digestate at 80 °C improved dewatered cake solids above the predictions by TGA at 35 °C. In conclusion, dewatered cake solids were significantly improved by Post-AD THP, reducing the volume of dewatered cake for disposal.

Highlights

There was no significant difference in predicted cake solids by thermogravimetric analysis (TGA) for the 2015 and 2016 samples demonstrating good reproducibility of sampling procedure, treatment and analysis
Predicted dewatered cake solids by TGA of Post-anaerobic digestion (AD) thermal hydrolysis process (THP) digestates were compared to the cake solids of original dewatered cake with open symbols representing digestates from source separated food waste (SSFW) (Figure 1)
Regardless of the original cake dryness or any use of pre-AD treatment, the predicted cake solids were on average increased by 87% by application of Post-AD THP at 165 C for 45 min

Summary

INTRODUCTION

Anaerobic digestion (AD) is a common method to reduce and stabilize sewage sludge and organic waste More than 60% reduction in dewatered cake for disposal has been reported for a full-scale Post-AD THP plant (Kolovos et al ) and similar results were found in the laboratory for sewage sludge digestate (Svensson et al ). Reported improvements in dry solids concentration of dewatered cake due to industrial application of Post-AD THP on sewage sludge surpassed the effect of Pre-AD THP (Gerstner et al ). This improvement makes PostAD THP a promising technology for plants with high cake disposal costs, as cake disposal savings are becoming an increasingly important economic advantage of installing THP in novel wastewater treatment plants (TaboadaSantos et al ). The aims were to: (1) study the effect of Post-AD THP on the free water and predicted dry solids concentration of dewatered cake, and (2) study the effect of high temperature dewatering on the dry solids concentration of dewatered cake

MATERIALS AND METHODS

RESULTS AND DISCUSSION

CONCLUSIONS