Full-scale activated sludge transplantation reveals a highly resilient community structure.

Well-functioning and stable microbial communities are critical for the operation of activated sludge (AS) wastewater treatment plants (WWTPs). Bioaugmentation represents a potentially useful approach to recover deteriorated systems or to support specific AS processes, but its application in full-scale WWTPs is generally problematic. We conducted a massive transplantation (in one day) exchanging AS from a donor to a recipient full-scale WWTP with similar process type (biological removal of nitrogen and phosphorus) and performance, but with differences in microbial community structure. The treatment performance in the recipient plant was not compromised and the effluent quality remained stable. The AS community structure of the recipient plant was initially very similar to the donor AS, but it almost completely restored the pre-transplantation structure approximately 40 days after transplantation, corresponding to 3 times the solid retention time. Most of the unique species of donor AS added to recipient AS disappeared quickly, although some disappeared more slowly the following months, indicating some survival and potentially a time limited function in the recipient plant. Moreover, the addition in higher abundance of most species already present in the recipient AS (e.g., the polyphosphate accumulating organisms) or the reduction of the abundance of unwanted bacteria (e.g., filamentous bacteria) in the recipient plant was not successful. Moreover, we observed similar abundance patterns after transplantation for species belonging to different functional guilds, so we did not observe an increase of the functional redundancy. Investigations of the microbial community structure in influent wastewater revealed that for some species the abundance trends in the recipient plant were closely correlated to their abundance in the influent. We showed that a very resilient microbial community was responsible for the outcome of the transplantation of AS at full-scale WWTP, potentially as a consequence of mass-immigration from influent wastewater. The overall results imply that massive transplantation of AS across different WWTPs is not a promising strategy to permanently solve operational problems. However, by choosing a compatible AS donor, short term mitigation of serious operational problems may be possible.

Challenges in carbon footprint evaluations of state-of-the-art municipal wastewater resource recovery facilities.

Wastewater treatment is an important source of direct and indirect greenhouse gas (GHG) emissions, which some wastewater operators report and account for CO2-eq impacts through carbon footprint evaluations. We investigated the challenges with GHG emissions' accounting of three state-of-the-art energy-efficient wastewater resource recovery facilities (WRRFs) and reviewed their CO2 accounting reports. Our study aimed to highlight the major contributors and factors to estimate emissions, including direct N2O and CH4 emissions and propose recommendations for public reporting of CO2 accounting of WRRFs. We categorised emissions as direct (scope 1), background (scope 2), downstream and avoided emissions (scope 3A and 3B) and evaluated how a change in emission factor may affect how close the WRRFs are to reaching CO2 neutrality. The results show that electricity consumption and direct emissions constitute between 20 and 70% of actual CO2-eq emissions and therefore need careful consideration. All three plants have increasingly offset scope 2 emissions over 2014-2019, resulting in a total reduction of approximately 3211 tons CO2-eq, corresponding to 72% of their needed cuts by 2030 set by the Danish government. No standard factors are used across the plants to estimate emissions. We propose some general recommendations that wastewater operators can apply to correctly report and account for CO2-eq emissions. We also recommend that operators move their long-term focus from CO2 neutrality to CO2-eq reduction and make an effort to measure and quantify scope 1 direct emissions properly. A tax on N2O emissions should be introduced in future policies.

Evaluation of greenhouse gas emissions from the European urban wastewater sector, and options for their reduction.

We present an assessment of greenhouse gas emissions from urban wastewater treatment plants in Europe. We propose a quantification in terms of emission factors (kilograms of carbon dioxide equivalents (CO2e) per population equivalent (PE) and year) taking into account all phases of wastewater treatment, from the construction of the infrastructure to the discharge of treated effluents. The assessment includes (1) life-cycle emissions of infrastructure; (2) emissions of dissolved methane in the sewer networks; (3) direct emissions of nitrous oxide and methane from the treatment processes; (4) emissions due to COD and nitrogen in the effluents; (5) indirect emissions due to the generation of electricity and the production of reagents; and (6) emission credits due to energy recovery or biomethane export associated with the anaerobic digestion of sludge. Our estimated emissions range between approximately 50 and 125 kg CO2e/PE/y depending on the type of treatment plant, of which about 20 to 40 are embedded in the infrastructure. We estimate that direct nitrous oxide emissions and indirect electricity emissions are the main contributors in the operation phase, followed by direct methane emissions. By extrapolating these emissions to the ensemble of the European Union's wastewater treatment plants, we estimate a cumulative emission of about 35 million tonnes CO2e/year, of which ca. 14 are due to the infrastructure. We analyse various scenarios to reduce emissions, showing that the efficient use of electricity at the plant and the decarbonisation of electricity would significantly help to improve the CO2e footprint of the WWTPs. In particular, the recovery of methane from biogas and the decarbonisation of electricity may reduce emissions below 27 million tonnes CO2e/year. Extending N removal to the whole territory for all plants above 10,000 PE may contribute to decrease direct nitrous oxide emissions.