Questions about the abatment performance

The performance of the stages for the abatement of micropollutants is verified using 12 substances defined in the ordinance from the Department of the Environment, Transport, Energy and Communications (DETEC) on the verification of the abatement performance of measures for the abatement of organic micropollutants at WWTPs. The water protection ordinance (WPO) has been in effect since December 2016. The twelve substances are divided into two categories: “very well abatable substances” (group 1) and “well abatable substances” (group 2).

Group 1:

  • Amisulpride (active pharmaceutical ingredient)
  • Carbamazepine (active pharmaceutical ingredient)
  • Citalopram (active pharmaceutical ingredient)
  • Clarithromycin (active pharmaceutical ingredient)
  • Diclofenac (active pharmaceutical ingredient)
  • Hydrochlorothiazide (active pharmaceutical ingredient)
  • Metoprolol (active pharmaceutical ingredient)
  • Venlafaxine (active pharmaceutical ingredient)

Group 2:

  • Benzotriazole (corrosion protection)
  • Candesartan (active pharmaceutical ingredient)
  • Irbesartan (active pharmaceutical ingredient)
  • 4-Methylbenzotriazole and 5-Methylbenzotriazole mixture (corrosion protection)

The criteria for selecting the substances and further explanations can be found in the explanatory report on the DETEC ordinance (in German, French or Italian).

All twelve substances must be measured. Since the concentrations of the substances can vary greatly, the WPO requires the analysis of 48-hour composite samples (WPO “Gewässerschutzverordnung” Annex 3.1 Number 41 Paragraph 1), both in the influent to the WWTP (raw wastewater or the effluent from the primary clarifier) and in the effluent from the WWTP.

At least six of the substances mentioned are to be used to calculate abatement performance, whereby the substances must be represented in a ratio of 2:1 from group 1 to group 2.

If fewer than six substances are available for the calculation, substitute substances may be defined in consultation with the canton and the Swiss Federal Office for the Environment (FOEN).

The required abatement performance is achieved if the mean value of the individual abatements of all substances used for the calculation is at least 80%. This value must be met in each sampling, not as an annual average. Depending on the size of the WWTP, at least 8–24 samples (in the first year) or 4–12 (if the requirements were met in the previous year) must be analyzed. Several non-conformances are permissible per year, depending on the size of the WWTP in accordance with Annex 3.1 No. 42 WPO.

A cantonal office monitors abatement performance. An accredited private or cantonal environmental laboratory usually measures the substances to verify the abatement performance on behalf of the WWTP.

If the abatement performance of 80% is not met, the responsibility to order measures lies with the canton. (The same applies when other requirements on the effluent quality are not met.)

Analyzing a wastewater sample for the 12 indicator substances (according to the DETEC ordinance) costs, as of 2017, around CHF 350–500. The accuracy of micropollutant analysis is comparable to other measurements, such as dissolved organic carbon (DOC) or chemical oxygen demand (COD) analyses, and is 10–20% in relation to the measured concentrations of the individual substances. Since the difference in substance concentrations in the influent and effluent is decisive in calculating the abatement effect, and the degradation capacities of the individual substances are averaged, the resulting accuracy of the calculated abatement performance is 1–3%.

Frequent sampling and analysis are too expensive and time-consuming for daily operation. WWTP operators therefore have auxiliary variables at their disposal for monitoring purposes (cf. the concept for monitoring treatment performance, in German or French). In addition to recording the operating consumables used (O2, O3, powder activated carbon [PAC]) and the DOC, the (online) measurement of the spectral absorption coefficient (SAC) at 254 nm is particularly suitable. The measuring instruments are commercially available and are continuously being adapted to monitor micropollutant abatement in wastewater. The decrease in SAC correlates with the abatement of various micropollutants in both ozonation and activated carbon processes. For monitoring treatment performance, it is therefore recommended to measure the SAC at 254 nm in the inlet and outlet of the micropollutant abatement stage in addition to periodically measuring micropollutants. As an alternative to online measurement of the SAC value, periodic measurements of aggregate samples also provide information on micropollutant abatement.

By constantly optimizing the SAC measurement, WWPT Neugut has developed an innovative regulation strategy for ozonation whereby ozone consumption could be significantly reduced (Schachtler and Hubaux, article A&G, No. 5 2016, in German).

Overall, a combination of micropollutant analysis and online signals can be used to ensure that, with proportionate effort, the WWTPs reliably comply with the requirements for abatement performance.

When selecting the 12 indicator substances, it was considered that all substances are abated similarly well in the common processes for the abatement of organic micropollutants (i.e. ozonation or activated carbon processes). Thus, no process is preferred in the process selection based on the determination of the 12 indicator substances.

If necessary, changes can be made in the DETEC departmental ordinance (e.g. if insufficient concentrations of substances are present in the influent to calculate the abatement effect). The selection of substances is based on their suitability for verifying the abatement performance (i.e. the selected substances are not well biodegradable, measurable with common analytical methods, measured continuously, in sufficient concentrations in the WWTP influent, etc.). However, care is also taken that the selected substances are abated similarly well with both ozonation and an activated carbon process. Thus, WWTPs with already implemented micropollutant projects are not disadvantaged.

In small catchments with short retention times in the sewer network, the concentrations and loads of micropollutants can fluctuate greatly. Due to these dynamics, volume-proportional sampling in the influent of the WWTP has difficulty in capturing the concentration peaks. If necessary, the WWTP influent samples for determining the abatement performance can also be taken in the effluent of primary treatment (see implementation aid “Operation and Control of Wastewater Treatment Plants”, in German or French), where the concentration peaks are smoothed and thus easier to record. Three quarters of Swiss WWTPs already take influent samples in the effluent of primary treatment.

This question was investigated using modelling (the report on these investigations is available here, in French). The results have shown that the greatest sampling errors occur in small WWTPs (for < 8,000 residents; Raff) when sampling in the inflow (raw wastewater) to the WWTP. The error can be greatly reduced when sampling the outflow of the primary clarifier for a more representative sampling because the concentration peaks are smoothed out. In general, it can be said that from a WWTP size of 10,000 Raff upwards, the relative sampling error in the primary treatment effluent is less than 1%. Since the influence of return flows on the measured concentrations in the effluent of the primary clarifier is minimal in normal cases, it is recommended to sample the effluent of primary treatment when possible.

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