Mapping of emissions from reporting point sources and Estimation of emission factors from reporting waste water treatment plants

Sweden has reported data to the E-PRTR (European Pollutant Release and Transfer Register) since the year 2007. However, there has been no general survey of the reporting point sources with respect to which substances that are reported by the companies in a particular industry, neither which method that has been used to report the emissions. Earlier work has shown that only a few organic compounds were reported from very few waste water treatment plants (WWTP) in SMP, e.g. DEHP, alkylphenols, nonylphenol and PAHs. Earlier studies had also shown that there seems to be a huge under-reporting of the emissions of organic substances.

The project had two sub-projects:

  • The purpose of the first sub-project was to map the data reported for 2011 to the E-PRTR (releases to water and air and as off-site transfers to WWTP), for all reported E-PRTR pollutants and all industry sectors in Sweden. With mapping is here meant to sort, organize and group the emissions data and methodology for the development of emission data for facilities in the same industry sector.
  • The purpose of the second sub-project was to develop emission factors for estimating emissions to water from municipal WWTP, for 18 substances / substance groups. These emission factors could be used by WWTP in the reporting of emissions data in the emission declarations in the annual environmental report which in turn are used to the E-PRTR reporting.

The result for the first subproject showed that generally, for all the receiving media and pollutants, there was higher percentage of reported releases below the thresholds values to E-PRTR compared to releases above the thresholds. The results from this project indicated that the thresholds to E-PRTR, in many cases, may be too high. Emissions of all pollutants are not expected to occur from all sectors. There was however some examples of sectors missing expected release data, for instance, releases of CH4 and N2O to air from sector 7 (Intensive livestock production and aquaculture). Organic substances as a group was rarely reported except for release of dioxins and PAH to air, AOX to water and phenols and TOC/CODCr to water and as off-site transfer to waste water. If reported, the emissions generally were below the thresholds to E-PRTR.

The use of determination methods (C, M, E) for reporting releases to air, water and as off-site transfer to waste water varied with different pollutants and receiving media. Generally, measurement methods were more frequently used and the estimation methods were used less often.

In the second subproject ratios based on measurements could only be developed for 4-tert-octylphenol and 4-nonylphenol, due to a large number of values below the detection limit.The ratios vary between the different WWTP, which may depend on both the normal variance due to different load, size and technical properties of the WWTPs. The results based on a STP (Sewage treatment Plant) model showed that the ratios for individual substances differ between the included WWTPs, by up to a factor of 30 (for trichloroethylene. The water/sludge ratio differs not only between chemicals and different WWTPs, but may also vary between years. TSS (Total Suspended Solids) in the effluent water and the volume of incoming water and its properties were parameters that had a high influence on the factor for the included substances. These two factors are prabably also an explanation for the large variations in factors calculated by measurements.

Both methods (measurements and modelling) illustrate a large variation between different WWTPs, and an inappropriateness of using one common substance specific ratio for emission calculations. This leads to the recommendation that WWTP specific ratios should be derived that can be used to estimate emissions with outgoing water from sewage sludge data. There are relatively few WWTPs that have to report to E-PRTR and for those WWTP’s modelling is the preferred method, since it proved to be difficult to measure most substances in the low concentration that is needed.

The results from the STP model also indicated that there were only a few substances that are close to or above the emission threshold for reporting; DEHP, nonylphenols, octylphenols, PBDE, PAH and chloroalkanes. This is important information for the WWTPs. In a previous study, 17 substances were identified to be of potential concern, largely based on concentrations below the detection limits. This study limited the number of substances or groups to six.