Residual municipal solid waste as co-substrate at wastewater treatment plants: An assessment of methane yield, dewatering potential and microbial diversity.

Separately collected organic fraction of municipal solid waste, also known as biowaste, is typically used to fill the available capacity of digesters at wastewater treatment plants. However, this approach might impair the use of the ensuing digestate for fertilizer production due to the presence of sewage sludge, a contaminated substrate. Worldwide, unsorted municipal solid household waste, i.e. residual waste, is still typically disposed of in landfills or incinerated, despite its high content of biodegradables and recyclables. Once efficiently separated from residual waste by mechanical processes, the biodegradables might be appropriate to substitute biowaste at wastewater treatment plants. Thus, the biowaste would be available for fertilizer production and contribute to a reduction in the demand on non-renewable fertilizers. This study aimed at determining the technical feasibility of co-digesting the mechanically separated organic fraction of residual waste with sewage sludge. Further, key parameters for the implementation of co-digestion at wastewater treatment plants were determined, namely, degradation of the solids and organics, specific methane production, flocculant demand, and dewatered sludge production. The microbial community and diversity in both mono- and co-digestion was also investigated. Semi-continuous laboratory scale experiments showed that the co-substrate derived from the residual waste provided a stable anaerobic co-digestion process, producing 206 to 245 L of methane per kg of volatiles solids added to the digester. The dewaterability of the digestate increased by 4.8 percentage points when the co-substrate was added; however, there was also an increase in the flocculant demand. The specific dewatered sludge production was 955 kg per ton of total solids of co-substrate added to the digester. Amplicon sequencing analysis provided a detailed insight into the microbial communities, which were primarily affected by the addition of co-substrate. The microbiota was fully functional and no inhibition or problems in the anaerobic digestion process were observed after co-substrate addition.

The economic efficiency of the co-digestion at WWTPs: A full-scale study.

The methane and digestate production from biowaste (BW, 95% food waste and 5% garden waste based on fresh mass) and grease trap sludge (GTS) co-digestion at the Grossache-Nord WWTP (Austria) as a basis for a cost-benefit analysis was determined using two approaches: The first one was to determine the specific methane yields (SMY) and total solids (TS) removals (%) of the used substrates in biomethane potential (BMP) tests. In the second, the full-scale process data from a supervisory control and data acquisition (SCADA) system were analyzed. From these data, the SMY of the sewage sludge (SS) was calculated for a period without co-digestion and applied to the study period. Thus, it was possible to calculate the methane and digestate production from the co-substrates. Both approaches produced different co-substrate SMYs and TS degradation results. In the approach using the BMP, the SMY was 518 m3/t TSadded and the TS degradation was 77%. For the full-scale method, these values were found to be 620 m3/t TSadded and 66%, respectively. However, the cost-benefit analysis of both approaches indicated that electricity generation from co-digestion can cover the associated costs. The benefit to cost ratio was 1.14 and 1.08 for the BMP and full-scale approach, respectively. The application of the respective approach depends on the availability and quality of full-scale process SCADA data.

Determination of the dewatered digestate amounts and methane yields from the co-digestion of biowaste as a basis for a cost-benefit analysis.

Co-digestion is the simultaneous digestion of two or more substrates and a common practice at wastewater treatment plants (WWTPs). The amounts of methane and digested sludge produced are key parameters for evaluating the economic efficiency of co-digestion. However, the share of dewatered digestate produced from co-substrates is not known. Synergistic effects in co-digestion, i.e. a better biodegradability compared to the mono-digestion of each substrate, might reduce the amounts of digested sludge and increase methane yields. However, these effects might also influence the calculation of methane and digestate quantities from co-substrates. The main objective of this work was to provide a basis for the cost-benefit analysis of biowaste (BW) co-digestion at WWTPs for this data. Therefore, continuous and batch experiments with sewage sludge (SS) and BW co-digestion were conducted and evaluated for methane and digestate production, and possible synergistic effects. BW co-digestion led to an additional production of 0.35 t total solids (TS) of dewatered sludge per ton TSadded in continuous and 0.23 t TS of dewatered sludge per ton of TSadded in batch experiments. The methane yield from BW was 441 L/kg TSadded in continuous experiments and 482 L/kg TSadded batch test. No synergistic effects were observed in both batch and continuous co-digestion experiments. Batch tests were found to be suitable for a rough estimation of the co-digestion economic efficiency key parameters. Continuous experiments are recommended to obtain more robust data. A cost-benefit analysis found that electricity production from co-digestion can generate savings of 88-170 €/t TSadded compared to grid purchase.

Comparison of two mechanical pre-treatment systems for impurities reduction of source-separated biowaste.

The treatment of source-separated biowaste is still a challenge due to its high proportion of impurities. Biowaste bins are intended exclusively for the collection of biodegradable matter, such as food, kitchen and garden waste. However, plastics, metals, glass and textiles are also found in biowaste bins. If not properly removed, these impurities cause problems to the treatment facility and depreciate the quality of the final product, when the biowaste is converted to compost. There is ongoing discussion whether the existing treatment systems are able to remove impurities, especially plastics, from biowaste thoroughly enough to ensure that the produced compost complies with state regulations. In this work, two wet mechanical pre-treatment systems were tested for their efficiency to remove impurities. The first system consisted of a screw mill, a star screen, and a food unpacking machine (process I). The second system consisted of a shredder, followed by a piston press with 12 mm pore size (process II). Both processes produced a dry output, which contained the concentrated impurities, and a wet output, which could be used as substrate for anaerobic digestion. Results showed that, although 99% of the incoming plastics were efficiently removed in process I, the impurities concentration was still too high to meet the legal standards of plastics concentration in the final product, according to the German Federal Compost Quality Association (Bundesgütegemeinschaft Kompost e.V.). The removal efficiency of glass particles was low for both processes: at least 80% of the incoming particles were transferred to the wet output.