Dewaterability of thermophilic digestate: Characteristics and underlying processes

Abstract

In a WWTP operating with full scale mesophilic digestion, a thermophilic digestion pilot and a digestate ammonia stripping pilot were installed. Benefits of the aforementioned sludge treatment processes, such as energy and nutrient recovery, are extensively recognized. However, drawbacks, such as deficient sludge dewatering, are amply disregarded despite their substantial influence on the treatment scheme and treatment cost. Consequently, the installations research resolved to determining the causality of the unsatisfactory dewaterability. The digester feed, mesophilic digestate, thermophilic digestate and ammonia stripped thermophilic digestate were compared. Harsher sludge treatments led to a severe deterioration in dewaterability. An increase in presence of colloids resulted in a worsening of dewaterability, to an extent originating from size and surface charge of colloids; unlike variations in presence and size of supracolloids, which were inconsequential. Moreover, inferior dewaterability was strongly associated with the increased concentrations of carbohydrates, proteins and humics in the bulk and loosely bound phases, and with the decreased presence of proteins in the tightly bound phase. Furthermore, the decreased calcium and increased iron presences in the bulk and tightly bound phases were correlated to a degradation in dewaterability; whilst sodium, potassium, ammonium and magnesium in all phases were found of no or insignificant influence. Recognizing the dewatering determinant factors enables the search for digestate specific conditioning and dewatering methods.