Dorothee Lensch

Examination of food waste co-digestion to manage the peak in energy demand at wastewater treatment plants.

Many digesters in Germany are not operated at full capacity; this offers the opportunity for co-digestion. Within this research the potentials and limits of a flexible and adapted sludge treatment are examined with a focus on the digestion process with added food waste as co-substrate. In parallel, energy data from a municipal wastewater treatment plant (WWTP) are analysed and lab-scale semi-continuous and batch digestion tests are conducted. Within the digestion tests, the ratio of sewage sludge to co-substrate was varied. The final methane yields show the high potential of food waste: the higher the amount of food waste the higher the final yield. However, the conversion rates directly after charging demonstrate better results by charging 10% food waste instead of 20%. Finally, these results are merged with the energy data from the WWTP. As an illustration, the load required to cover base loads as well as peak loads for typical daily variations of the plants energy demand are calculated. It was found that 735 m³ raw sludge and 73 m³ of a mixture of raw sludge and food waste is required to cover 100% of the base load and 95% of the peak load.

Evaluation of the energetic potential of sewage sludge by characterization of its organic composition

The composition of sewage sludge and, thus, its energetic potential is influenced by wastewater and wastewater treatment processes. Higher or lower heating values (HHV or LHV) are decisive factors for the incineration/gasification/pyrolysis of sewage sludge. The HHV is analyzed with a bomb calorimeter and converted to the LHV. It is also possible to calculate the heating value via chemical oxygen demand (COD), total volatile solids (TVS), and elemental composition. Calculating the LHV via the COD provides a suitable method. In contrast, the correlation of the HHV or LHV with the TVS is limited. One prerequisite here is a constant specific energy density; this was given with the types of sewage sludge (primary, surplus/excess, and digested sludge) investigated. If the energy density is not comparable with sewage sludge, for instance with the co-substrate (bio-waste, grease, etc.), the estimation of the heating value using TVS will fail. When calculating the HHV or LHV via the elemental composition, one has to consider the validity of the coefficients of the calculation equation. Depending on the organic composition, it might be necessary to adjust the coefficients, e.g. when adding co-substrates.