Treatment of micropollutants in wastewater: Balancing effectiveness, costs and implications.

In this contribution, we analyse scenarios of advanced wastewater treatment for the removal of micropollutants. By this we refer to current mainstream, broad spectrum processes including ozonation and sorption onto activated carbon. We argue that advanced treatment requires properly implemented tertiary (nutrient removal) treatment in order to be effective. We review the critical aspects of the main advanced treatment options, their advantages and disadvantages. We propose a quantification of the costs of implementing advanced treatment, as well as upgrading plants from secondary to tertiary treatment when needed, and we illustrate what drives the costs of advanced treatment for a set of standard configurations. We propose a cost function to represent the total costs (investment, operation and maintenance) of advanced treatment. We quantify the implications of advanced treatment in terms of greenhouse gas emissions. Based on the indicators of total toxic discharge, toxicity at the discharge points and toxicity across the stream network discussed in Pistocchi et al. (2022), we compare costs and effectiveness of different scenarios of advanced treatment. In principle the total toxic load and toxicity at the points of discharge could be reduced by about 75 % if advanced treatment processes were implemented virtually at all wastewater treatment plants, but this would entail costs of about 4 billion euro/year for the European Union as a whole. We consider a "compromise" scenario where advanced treatment is required at plants of 100 thousand population equivalents (PE) or larger, or at plants between 10 and 100 thousand PE if the dilution ratio at the discharge point is 10 or less. Under this scenario, the length of the stream network exposed to high toxicity would not increase significantly compared to the previous scenario, and the other indicators would not deteriorate significantly, while the costs would remain at about 1.5 billion Euro/year. Arguably, costs could be further reduced, without a worsening of water quality, if we replace a local risk assessment to generic criteria of plant capacity and dilution in order to determine if a WWTP requires advanced treatment.

Influence of operational conditions and wastewater properties on the removal of organic micropollutants through ozonation.

The objective of this study was to evaluate the influence of operational conditions and wastewater properties on the removal of pharmaceuticals, contrast media and antibiotics through ozonation, in order to facilitate the optimization of treatment and its implementation on a full scale. Pilot-scale ozone oxidation trials were performed on treated wastewater, before and after post-precipitation, over a seven-month period, including summer and winter months. Hydraulic retention times as short as 7 min were found to be sufficient for organic micropollutant removal. A short hydraulic retention time reduces both investment costs and land use. Neither the choice of ozone dispersion method, a static mixer or a Venturi injector, nor the wastewater temperature had any significant effect on the removal efficiency of organic micropollutants, however, higher removal was achieved after on-site post-precipitation with aluminum chloride.

Is dissolved COD a suitable design parameter for ozone oxidation of organic micropollutants in wastewater?

Ozone oxidation of organic micropollutants in biologically treated wastewater was investigated in pilot-scale after a high- and a low loaded activated sludge process. Higher ozone doses were required to remove organic micropollutants in the effluent wastewater from the high loaded activated sludge process. Further comparison of the micropollutant removal was based on normalized ozone doses, expressed as g O3/g DOC and g O3/g soluble COD (sCOD). A clear difference was noted for the two effluents when the micropollutant removal was normalized by DOC. This difference disappeared almost completely when the removal was linked to ozone doses normalized by sCOD. The dose-response curves for the organic micropollutants were practically linear in the removal range up to 95%. A linear prediction model was developed and compared with literature values to test the transferability of the obtained results. Results from this comparison indicated that the slope of the dose-response functions could be used to predict the removal efficiency of organic micropollutants at a third plant with an average uncertainty of 10%. The modeled ozone requirements were then set in relation to the COD concentrations in the discharged water from approximately 90 Swedish activated sludge treatment plants with and without nitrogen removal. This comparison highlighted the need for a well-functioning biological treatment for an effective ozone oxidation of organic micropollutants. The results in this study suggest that soluble COD should be further explored for design and modeling of ozone oxidation of organic micropollutants in biologically treated wastewater.

Automatic control of the effluent turbidity from a chemically enhanced primary treatment with microsieving.

For chemically enhanced primary treatment (CEPT) with microsieving, a feedback proportional integral controller combined with a feedforward compensator was used in large pilot scale to control effluent water turbidity to desired set points. The effluent water turbidity from the microsieve was maintained at various set points in the range 12-80 NTU basically independent for a number of studied variations in influent flow rate and influent wastewater compositions. Effluent turbidity was highly correlated with effluent chemical oxygen demand (COD). Thus, for CEPT based on microsieving, controlling the removal of COD was possible. Thereby incoming carbon can be optimally distributed between biological nitrogen removal and anaerobic digestion for biogas production. The presented method is based on common automation and control strategies; therefore fine tuning and optimization for specific requirements are simplified compared to model-based dosing control.

Microsieving in primary treatment: effect of chemical dosing.

Primary and chemically enhanced primary wastewater treatment with microsieving (disc or drum filtration) was studied at the large pilot scale at seven municipal wastewater treatment plants in Europe. Without chemical dosing, the reduction of suspended solids (SS) was (on average) 50% (20-65%). By introducing chemically enhanced primary treatment and dosing with cationic polymer only, SS removal could be controlled and increased to >80%. A maximum SS removal of >90% was achieved with a chemical dosing of >0.007 mg polymer/mg influent SS and 20 mg Al(3+)/L or 30 mg Fe(3+)/L. When comparing sieve pore sizes of 30-40 µm with 100 µm, the effluent SS was comparable, indicating that the larger sieve pore size could be used due to the higher loading capacity for the solids. Phosphorus removal was adjusted with the coagulant dose, and a removal of 95-97% was achieved. Moreover, microsieving offers favourable conditions for automated dosing control due to the low retention time in the filter.