Questions about process selection

Ozone and activated carbon are suitable for the abatement of micropollutants. Both technologies have advantages and disadvantages, and a variant study tailored to each WWTP must be prepared to find the appropriate process. The catchment area and its future developments, the existing infrastructure and the receiving water play an essential role for the choice.

Important criteria for the choice of procedure:

Catchment area

  • Wastewater composition: Industry in the catchment area, e.g. waste- or hazardous waste incinerators, chemical industry, landfills
  • Developments in the catchment area: planned mergers, industry and commerce, medium-term future of the WWTP, population growth, land use plan, etc.

Existing infrastructure

  • Spatial conditions
  • Current effluent quality of the WWTP (e.g. effluent concentration of nitrite, DOC and total suspended solids [TSS])
  • Current and future discharge conditions (e.g. regarding DOC, TSS)
  • Process and capacity of biological abatement

Receiving water

  • Wastewater share
  • Uses downstream of the WWTP (drinking water catchments, bathing waters, agriculture)

More information is available in the article “With Ozonation and Activated Carbon Against Micropollutants“ (in German) and in the presentation “Influences to the Choice of Process“ (in German).

Ozonation, the Ulmer process, dosing PAC in front of the sand filter or in the activated sludge tank are standard processes today. Granular activated carbon (GAC) filtration, the process of GAC in the fluidized bed, dosing PAC into membrane bioreactors and combinations of ozonation and activated carbon are on their way to becoming standard processes. Here you find a description of the processes dating from 2017 (see Wunderlin et al. 2017 in German and French). The processes evolved in the meantime.

No, neither of the two processes is preferred nor stipulated. The goal is to find and implement the best possible solution for each individual WWTP. For doing so, various aspects must be considered, such as, for example, spatial conditions, existing infrastructures, the preliminary biological treatment process and its capacity. Another relevant boundary condition – especially for a treatment with ozone – is the wastewater composition (see also FAQ on the assessment for ozonation). Therefore, it must be clarified at an early stage (before a process is chosen) whether the wastewater is suitable for ozonation.

The article Aqua & Gas, November 2017 (in German and French) describes the changes in the processes for micropollutant abatement from municipal wastewater, the current level of knowledge and future developments in this field.

The two process variants (ozonation and PAC) are now well established. In principle, it can be assumed that the processes will tend to become more compact and cheaper (e.g. by PAC dosing directly in the biological treatment stage or in front of the sand filter). Other process alternatives are, for example, GAC in the static filter as well as in the fluidized bed. Currently, the development of a completely new process is not in sight.

If ozonation is being considered, it must be clarified at an early stage (e.g. as part of the preliminary study) whether a particular wastewater is suitable for ozone treatment. In principal, treatment with ozone has many positive effects: it removes micropollutants, reduces the negative effects of the wastewater on aquatic life and disinfects and decolours the wastewater. It is known, however, that stable, toxic transformation products (oxidation by-products) can be formed in unsuitable wastewater. This is the case, for example, in WWTPs with problematic industrial wastewater discharges (e.g. high bromide loads; see Soltermann et al. 2016 as an article in German). But this is not the case for WWTPs with a largely municipal catchment area.

The VSA recommendation by Wunderlin et al. 2017/2021 (in German and French) describes how to proceed with these assessments and how to interpret the results. These assessments are part of the chargeable costs and are compensated at 75%.

A list of laboratories offering such assessments can be provided on request.

When selecting the appropriate process, the aspect of proper water protection must be considered alongside other aspects such as economic efficiency, state-of-the-art technology and expedient planning. The effects on other sectors of water management (such as drinking water) must be examined.

A process combination (ozone in combination with activated carbon) is technically feasible and allows greater flexibility. However, a process combination is mainly economical for large WWTPs. It should be noted that, even with a process combination, the suitability of wastewater for ozonation must be clarified (see Wunderlin et al 2017/2021).

Pilot tests on the dosing of PAC into biofilm systems are currently (as of 2017) underway. However, it is assumed that both fixed-bed and fluidized-bed systems are only suitable to a limited extent. However, the dosage of PAC to hybrid fluidized bed systems as well as to systems with granulated biomass can work (see factsheet in German or French).

In a PAC process installed after the secondary clarifier (e.g. the Ulmer process or PAC dosage in front of the sand filter), the PAC is usually returned to the biological treatment stage. (The utilization of a so-called multi-stage makes the process more efficient.) For this reason, it is important that the biology has sufficient capacity so that nitrification is not impaired. In the case of direct PAC dosage into the biology, the existing capacity in the biology should be even greater, as more PAC tends to be dosed.

In all three processes, sufficient capacity must also be provided in the fouling because the PAC is conveyed to the fouling together with the sewage sludge. In the VSA recommendation “Definition and Standardisation of Key Figures for Processes for the Abatement of Organic Micropollutants in Wastewater Treatment Plants”, an additional sludge production of 1.5 times the dosed PAC quantity is assumed for PAC stages; therein the adsorbed DOC as well as the precipitated sludge are included.

To reduce mechanical abrasion of the equipment by activated carbon, abrasion-resistant materials must be used.

A semi-technical pilot by the Competence Centre for Micropollutants in Baden-Württemberg (KomS) has shown that PAC has no negative effects on the digestion process. When PAC is applied to a WWTP, the same amount of gas is produced as before the expansion of the treatment plant, although the methane content in the digester gas is slightly higher (approx. 1.5 percentage points) when PAC is applied (see fact sheet in German or French). The application of GAC has no influence on sludge treatment, as the GAC is not returned into the biology and is thus not fed into the sludge treatment with the sludge. (The GAC is regenerated externally.)

At the Neugut WWTP, the optimization of ozonation is already advanced. An innovative regulation strategy for ozonation (BEAR) using ultraviolet probes allows for a stable and optimized operation. Their control and regulation is described in Hubaux and Schachtler 2016 (in German).

The reduction of ozone consumption while maintaining the abatement of micropollutants (the low ozone dosage (LOD) concept) is described in Hubaux and Schachtler 2016 (in German). The multi-chamber dosage concept reduces ozone consumption by up to 20%.

According to current knowledge, it is estimated that the operating costs of a direct PAC dosage into the biology are about 25 to 50% higher compared to the operating costs of an Ulmer process. These higher operating costs for direct dosage are mainly explained by the higher consumption of activated carbon. At WWTP Wetzikon, this process has been implemented on a large scale.

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