Utilization of digested sewage sludge in lactic acid fermentation

The management of sewage sludge is mostly limited to anaerobic digestion, incineration of digestate and recovery of phosphorous. In terms of resource efficiency, it is recommended to make use of the potential of all organic compounds. Nitrogen compounds, for instance, can find application as nutrients in biotechnological processes. To follow this approach, sewage sludge collected after anaerobic digestion, which had carbon and nitrogen contents of 35.9% (w/w) and 5.6% (w/w), respectively, was first hydrolyzed using 0-1% (w/w) sulphuric acid for 15 minutes at 121°C and the hydrolysate used as nitrogen source in lactic acid fermentation. Even though the focus was on a recovery of nitrogen compounds, the hydrolytic treatment with 1% (v/v) sulphuric acid resulted in a release of 28 mg g^-1 glucose. Because of the complex composition of the obtained hydrolysate it was not possible to quantify the released organic nitrogen compounds. Lactic acid fermentations, however, revealed that the concentration of organic nitrogen compounds was sufficient to efficiently convert 10 g L^-1 of added glucose into 9 g L^-1 lactic acid, and thus it is expected that digested sewage sludge may be an alternative nitrogen source in lactic acid fermentation, possibly combined with the utilization of a carbon-rich feedstock. Such a utilization approach goes beyond the conventional management strategies of digestated sewage sludge and allows a material utilization even after anaerobic digestion.