An evaluation of the treatment performance of small wastewater treatment plants with the capacity of ≤50 PE (population equivalent) was carried out using data collected in external monitoring reports, taking Upper Austria as an example. External monitoring data for 2009–2018 were available for this analysis. About 2′500 small WWTPs are in operation in Upper Austria. The main technologies implemented include about 870 Sequencing Batch Reactor (SBR) plants, 630 conventional activated sludge (CAS) plants and 535 vertical flow (VF) wetlands. The data set for the present evaluation comprises a total of approx. 14′000 external monitoring reports from approx. 2′000 small WWTPs. This paper presents the results of this evaluation. The evaluation showed that all technologies meet the legal requirements regarding BOD5, COD and NH4-N effluent concentrations. However, for small WWTPs with a polishing stage consisting of a VF wetland, the median values of the effluent concentrations and the limit value exceedances are significantly lower. There is no evidence of a significant deterioration in treatment performance with the age of the plant for any technology. However, for all technical WWTPs such as SBR and CAS plants, a greater fluctuation of the measured values (i.e. the median values of the effluent concentrations) can be observed when the plant is longer in operation. Compared to technical WWTPs, this fluctuation is significantly smaller in VF wetlands.
The European Union (EU)’s legislation on urban wastewater requires all agglomerations with a population equivalent (PE) above 2000 people to undergo a secondary (mechanical/physical and biological) wastewater treatment. Agglomerations below 2000 PE, though, fall outside the scope of the current EU’s legislation. As such, their regulation is heterogeneous across the various EU member states, and there is no systematic collection nor reporting of data enabling an estimation of their actual significance as a source of pollution for the receiving water bodies. Here we present a spatial model to delineate agglomerations in a GIS, based on population distribution and land cover. From the model results, in the EU, we identify 364,650 agglomerations with 2000 PE or less, housing a cumulative population of about 75 million inhabitants. We then calculate the organic matter and nutrient loads these agglomerations can discharge, assuming they presently undergo primary wastewater treatment, and the reduction of loads that can be expected under different treatment scenarios, together with the corresponding treatment costs based on a simple cost model. Using a conventional shadow price for the organic matter and nutrients removed, we show that all treatment scenarios show a benefit-to-cost ratio (B/C) above (or close to) 1. However, only a scenario of secondary treatment applied to all agglomerations above 1000 PE provides sufficient safety margins on the B/C. This suggests the opportunity to expand the scope of the current legislation down to agglomerations of this size, while addressing smaller agglomerations depending on their actual impacts on the receiving water bodies, through “appropriate treatments” defined by the local authorities.
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