Valuable Routes for Sewage Sludge Utilization: Effect of Temperature and Hydraulic Retention Time in the Acidogenic Fermentation Process

The disposal of sewage sludge potentially reaches the 50-60% of WWTP’s total operation cost. Such stream can be considered a renewable carbon source to produce added-value products. Different pre-treatment methods have been applied on thickened sewage sludge (SS) coming from the domestic wastewater treatment plant (WWTP) of Treviso (northeast Italy) to favour its acidogenic fermentability. Alkaline (pH 9-11) and thermal (50-70°C) hydrolysis were applied separately and in combination The following fermentation process was addressed to the recovery of volatile fatty acids (VFA) as valuable building blocks substances. Batch fermentation tests were conducted at lab-scale under controlled temperature (T): 20, 37, 55 and 70°C by using an available mixed fermentative consortium as inoculum. Thermophilic T (55°C) was chosen in the following semi-continuous fermentation process (fill and draw), carried out with three different hydraulic retention time (HRT; 4-5-6 days). In terms of organic matter solubilisation, the thermal hydrolysis (70°C) allowed to obtain a soluble chemical oxygen demand (CODSOL) concentration around 10.0 g/L, with no additional benefits from the combined alkaline treatment. The batch acidogenic fermentation tests highlighted the T effect on acidification performances; thermophilic trials (55°C) showed the highest CODVFA/CODSOL ratio (0.81). The three semicontinuous tests (HRT 4-5-6 days) were followed for 45 days (roughly), under the chosen thermophilic condition (55°C). The highest fermentation rate was obtained at 4.0 d as HRT (22 mgCODVFA/gVS d); on the contrary, the highest fermentation yield (0.30 gCODVFA/gVS) and CODVFA/CODSOL ratio (0.73) were obtained at 6.0 d as HRT. In practice, low HRT selected for a mixed consortium with high fermentation rate, but less efficient in the conversion of the organic matter into VFA. No HRT’s effect was instead observed in the VFA composition, always rich in acetic (29-31% COD basis) and butyric acid (31-32%), and poorer in propionic (14-15%), valeric (12-13%) and caproic acid (11-12%).