By Lydia Peraki, Sustainabity Specialist

In addition to the indirect emissions that are derived mostly from the energy use, direct GHGs, methane (CH4), nitrous oxide (N2O) and biogenic carbon dioxide (CO2) are emitted during the wastewater treatment processes (WWTP). Moving towards climate neutrality by improving the energy balance of the WWTP is an issue of great attention at this moment. It is noticeable that great improvement has been reached to maximize energy efficiency and to recover renewable energy from the wastewater by using traditional wastewater treatment technologies such as activated sludge and anaerobic digestion. However, not much has been done regarding reducing the emission in these sites. This is directed by several European and national policies and initiatives, in conjunction with the European Union’s 2030 Climate and Energy Policy Framework which requires a 40% reduction in GHG emissions by 2030 related to 1990 baseline. To achieve this goal, there is growing research into the maximization of the energy recovery and increased methane production by using alternative technological approaches.

However, the approaches mentioned above can be considered as adaptive measures for global warming. These adaptive measures can only reduce fossil fuel consumption and its associated carbon emissions (Indirect GHG emissions), whereas few have looked at the additional possibility of using wastewater and the organics included as a valuable source for products and fuels. Regarding the quantity of wastewater produced annually and its positive relation with population, there is great potential for contribution from the wastewater treatment to meet the Paris agreement, national and organizational goals.

Water authorities cannot consider wastewater as a by-product which has to be treated and processed, but as a source for energy, raw materials, and clean water. Therefore, many research institutions and water authorities have already joined forces to reach a more sustainable growth of the water industry and the national goals, considering that the focus has been changed due to the high energy demand but also the high amounts of released direct, indirect and biogenic emissions during the treatment processes on WWTPs.

Definition climate neutrality

Under the Kyoto Protocol, Paris Agreement, COP21 and other definitions (Levin et al., 2015), it seems that Climate Neutrality can be reached if CO2 and other greenhouse gases are decreased to minimum and the remaining are offset with carbon sequestration. However, this definition is not precise since this minimum level is not quantitatively defined. Under the Clean Development Mechanism (CDM) of the Kyoto Protocol, an organization must participate in projects of saleable certified emission reduction credits (CER) equivalent to tones of CO2, which can be contributing to the Kyoto targets. Of COP21, climate neutrality means that every ton of anthropogenic GHG released is compensated with an equivalent amount of CO2 removed. However, this term does not represent the need for mitigation concerning global warming. Therefore, ‘climate neutrality’ should be defined as the annual zero net anthropogenic GHG emissions without including the possibility of compensation by offsetting. This definition means that although energy neutrality cannot be in a different line of climate neutrality strategies and implementations, it should not be incorporated into the actual carbon footprint but to be analyzed separately.

Classification of GHG emissions

Water resource recovery facilities release gases such as nitrous oxide (NO2), carbon dioxide (CO2) and methane (CH4) which essentially contribute to global warming. The GHG released in the WWTP are categorized into three Scopes. The direct anthropogenic (Scope 1), indirect internal (Scope 2) and indirect external (Scope 3) emissions of an industrial plant as characterized in the General Reporting Protocol for Climate Registry by the United Nations (2013).

Scope 1 includes the direct greenhouse gas emissions which occur from sources that are owned or controlled by the company, while the CO2 emissions from combustion of biomass, which is produced on-site, are excluded from this scope. Specifically, direct emissions from WWTPs derive from biological carbon, nitrogen and phosphate removal processes and sludge management. The CO2 is emitted from organic matter degradation, N2O from nitrification and denitrification and CH4 from anaerobic digestion. Scope 2 consists of the GHG emissions that occur from the use of electrical and thermal energy, as defined by the Greenhouse Gas Protocol Initiative, 2004. Scope 3 constitutes other indirect GHGs which are accountable to emissions from sources that are not owned or controlled by the company.

Moreover, the Intergovernmental Panel on Climate Change in the Guidelines for National Greenhouse Gas Inventories (2019) also clarified the need for tracking and reporting biogenic CO2 emissions separately from the other emissions due to the origin of the organic matter. Elements of carbon in biomass were contained in living organic matter. This means that the carbon is not derived from fossil fuels, which creates the need for a different method for calculating and classifying carbon. This method (CO2 emissions from biomass combustion) applies only to CO2 and not to CH4 and N2O, although these are also emitted during biomass combustion. This happens due to the non-biogenic origin of CH4 and N2O, unlike CO2, and thus these emissions are categorized into Scope 1.

Definition and boundaries of a WWTP Carbon Footprint

There are several different methodologies available to determine a carbon footprint such as IPCC-2006, WSAA-2006, LGO-2008, Brindle-2008, NGER-2009, GWRC-2011 (Pagilla et al., 2009). However, after evaluating these methods, it was verified that the total carbon footprint of a WWTP cannot be calculated by using only one of these methods, since these require on-site measurements and calculation of actual GHG emissions based on an annual time frame. Therefore, different approaches had to be combined to define and calculate the actual carbon footprint of a WWTP. The informative value of carbon footprint examination is based on a well-selected use of emission factors for the calculations. Various databases and sources are presently accessible, mostly for indirect anthropogenic emissions (INCOPA, 2014; SimaPro, 2007, STOWA, 2014). For direct emissions of CH4, few research and measurements have been identified while the examination of N2O emissions from activated sludge processes has been intensively analyzed throughout the last decade. Nevertheless, the decision on a representative emission factor of N2O is still debatable due to the broad variety of the results.

It is noted that CO2 emissions from biological wastewater treatment are generally not considered in the carbon footprint of WWTP because it has not a fossil origin. However, some studies have pointed out that around 20% of the carbon present in wastewaters can be of fossil origin and fossil CO2 emissions from wastewater treatment are underestimated.

Is there any solution?

AR5 announced the necessity of CO2 extraction up to 90% in the second half of the century, to reach a reduction of the global temperature target. In this regard, some studies conclude at the importance of alternative approaches such as the balancing emissions by capturing the carbon and implementation of negative emission techniques. Negative emission technologies (NETs), as mitigating actions for global warming, are currently available only on a theoretical level. Although these technologies, such as microbial electrolytic carbon capture (MECC), microalgae cultivation, bioenergy combined with carbon capture and storage, biochar, direct carbon capturing and others have been researched over the last few years, they have gained the public concern after the recently AR5 announcement.

Therefore, it is confirmed the need for mitigation measures and utilization of biogenic emissions associated with the wastewater as announced by AR5. This also clarifies the need for drastic actions where CO2 could be extracted and reused.

To sum up, climate neutrality can be reached in a WWTP if alternative mitigating actions such as NETs and other physical, biological options could be implemented. Many water boards have already explored and enlarged the potential of carbon capture and its utilization for further use as energy, raw materials, and fertilizers. Some WWTPs are already researching some circular economy initiatives such as recovery of cellulose, producing bioplastics and struvite which also act in the CO2 footprint reduction direction. However, it is recommended that more attention must be brought on avoiding biogenic CO2 emissions and/or on capturing and reutilizing this fraction. Finally, based on research in a Dutch WWTP, it was found that not only climate neutrality can be reached but also energy neutrality or even negativity is possible. Although NETs are still in theoretical level, it is estimated to have negative emissions since endogenous and exogenous CO2 can be captured.