Efficient elimination of trace substances using a modular ozone system

Positive results from actual use in clarification plants

Efficient elimination of trace substances using a modular ozone system

As a result of the new EU Urban Waste Water Treatment Directive, many clarification plants will need to add an additional treatment stage to remove trace substances. According to current research and practical experience, ozonisation followed by filtration is one of the most effective fourth treatment options to meet the new requirements. With the modular system solution proposed here, it is possible to respond flexibly to rapidly changing waste water volumes and compositions. This means clarification plants can achieve the same or better treatment results with less effort and lower running costs than traditional ozone systems.

New Urban Waste Water Treatment Directive

The updated version of the directive is due to come into force for all EU member states in autumn 2024. This article examines the resulting need for action by clarification plant operators in Germany with regard to the fourth waste water treatment stage. Article 8 of the new directive sets out the requirements for the elimination of trace substances by 2045 for clarification plants of size class 5 (more than 150,000 inhabitants; about 240 plants nationwide) and clarification plants in smaller municipalities discharging into sensitive water bodies.

Trace substances in waste water

Anthropogenic trace substances, also known as micropollutants, include pharmaceuticals, pesticides and detergents, cosmetics and industrial chemicals. In most clarification plants, these trace substances are not yet, or not fully, degraded and retained. Although the human and ecotoxicological effects of individual substances have been studied, little is known about the potential effects of a "cocktail" of several substances in water. The new directive now sets a recommended level for 12 key substances. This means that at least six of these substances must be reduced by at least 80% during waste water treatment. Examples include the painkiller diclofenac and the corrosion inhibitor benzotriazole.

Methods for removing trace substances

Germany has been using activated carbon adsorption, ozonisation and subsequent filtration for several years. Other technologies, mainly oxidative, are being researched but are not yet economically viable for use in clarification plants.

Ozonisation combined with granular activated carbon (GAC) has several advantages: A consistent and high removal rate of more than 80% is achieved despite variable contaminant composition. Oxidisable and partially adsorbable substances are removed prior to activated carbon treatment. As a result, the trace substances are degraded more effectively and activated carbon life is greatly increased.

Challenges in system selection

The most cost effective solution is primarily determined by the composition of the waste water and the concentration of trace substances present. Large variations in water composition and flow rate will result in significant variations in the required ozone capacity.

Each ozone generator has an optimum operating point at which operating costs are minimal. This optimum operating point is determined by the ozone output and concentration and the local electricity, oxygen and cooling water costs. The best efficiency at rated operation should be less than 8 kWh/kg ozone. Dynamic adjustment of oxygen and power consumption based on local costs therefore contributes significantly to reducing the overall operating costs.

Comparison of modular and conventional ozone systems

The determining parameters for the design of the required ozone quantity are dissolved organic carbon (DOC) levels after sedimentation of 5-15 mg/l and an ozone metering rate of 0.3-0.8 g ozone/g DOC. Typical contact times are between 20 and 30 minutes. Particular attention should be paid to the formation of bromate, which will be discussed later in this article.

Ozone systems are often designed for the maximum waste water volume flow and the highest expected DOC content to ensure that the reduction performance is achieved even during peak periods. However, this peak load is rarely needed. Studies of several pilot projects by ProMinent GmbH of Heidelberg, Germany, show that for more than 75% of the operating time of conventional systems, the required ozone capacity is less than 50% of the maximum ozone output. The maximum capacity is only required for 12% of the total operating time. These findings have been confirmed by discussions with clarification plant operators. As the efficiency of an ozone generator is determined by the required ozone output, such a system is effectively oversized for the remainder of the operating year.

In comparison, a modular system is much more efficient: In the pilot project under consideration, up to 16 self-sufficient ozone-generating modules can be operated independently, depending on the ozone demand. The modular design significantly reduces energy consumption, especially in the medium to low capacity range.

Significantly reduced contact times

Until now, contact times of 20 to 30 minutes have been used to effectively remove trace substances at high flow rates. This requires the use of huge concrete and steel contact tanks, which are expensive to build and take up a lot of space. In contrast, the modular system's pilot project at a clarification plant in Luxembourg is delivering positive results despite much shorter contact times. The composition of the waste water is a major problem due to the many sources (domestic and industrial), and it is organically and chemically contaminated with high concentrations of bromide.

The combination of effective mixing and on-demand ozone generation is critical to the elimination performance. This allows the contact time to be significantly reduced to as little as five minutes without any loss of reduction performance.

Positive results with the modular system

Results from the ongoing pilot projects demonstrate the effectiveness of modular ozone systems as a fourth treatment method. Systems that combine ozonisation and activated carbon can remove more than 80% of trace substances. The ProMinent GmbH ozone system used in this practical example, together with effective mixing, allows a significant reduction in contact time. This results in a huge reduction in construction costs and, ultimately, CO2 emissions.

Depending on the waste water matrix and purification performance requirements, the system can be easily adapted to fluctuations at any time, thanks to modules that can be switched on and off. Existing systems can be easily expanded, replaced or modified due to the modular design. The extremely compact design has been shown to save up to 70% of space. The above advantages result in a significant reduction of investment and operating costs.

The pilot system in Luxembourg also has a particularly positive side effect: the waste water discharged from the industrial area contains bromide. Ozone treatment can produce potentially harmful bromate. However, the results so far suggest that due to the good control of the ozone system, the helical mixer for ozone injection and the existing water matrix, bromate formation can be disregarded as a concern despite the high bromide concentrations.

Bibliography & references

Publications from the following institutions and associations were used as sources of information for this article: German Environment Agency (UBA), Federal Ministry for the Environment (BMU), Kompetenzzentrum Spurenstoffe Baden-Württemberg (KomS BW), German Association of Local Utilities (VKU), German Association for Water, Wastewater and Waste (DWA), Federal Statistics Office (Statistisches Bundesamt). This article is also based on ProMinent GmbH's own surveys from the pilot projects mentioned above.

Authors:Egbert Hocke, Water Treatment Process Developer and Wolfgang Matheis, Ozone Product Manager, both from ProMinent GmbH, Heidelberg.