Technical realization and implementation of an ozonation stage for municipal wastewater treated according to the state of the art - approaching the state of the art.
Dr. Norbert Kreuzinger firstname.lastname@example.org
After the realization of an area-wide carbon and nutrient removal at wastewater treatment plants, more and more ecological and societal issues and problems are raised, which raise the question of the necessity to further increase the efficiency of wastewater treatment plants. These problem areas include, for example, hygienic aspects, ecotoxicological effects (hormonal potential, mutagenicity; ...) caused by wastewater treated even according to the state of the art, foam development, intrinsic coloration of the wastewater or the input of xenobiotics into the water bodies.
In numerous laboratory tests published in the scientific literature, the basic suitability of wastewater oxidation by means of ozone (ozonation) as a further treatment step to improve the performance of wastewater treatment plants with regard to the treatment of these problem areas could be proven. In the process, basic procedures, framework conditions and also limits of the performance of ozonation were presented. To date, however, there have been no studies on the process engineering and control implementation of ozonation in municipal wastewater treatment plants. Central aspects such as type of ozone input, reaction conditions as well as control of the ozone input or protzes stability and costs have not yet been investigated as a basis for a large-scale application. The aim of this project is to develop basic principles for the practical application of ozonation in the field of municipal wastewater treatment, irrespective of a company-specific implementation, and to make these generally available as decision-making and implementation aids
The following effects of ozonation of wastewater treatment plant effluent were investigated:
Degradation potential of ozonation for anthropogenic trace substances.
Ecotoxicological effects due to the formation of by-products
Effects on endocrinology
Effects on the BOD
Effects on the microbial content and the disinfection effect of ozone
Decolorization of the wastewater treatment plant effluent
The removal of anthropogenic trace substances from wastewater treatment plant effluents is currently a central point of discussion in water quality management.
In the research project KomOzone, ozonation was investigated as a technology for advanced wastewater treatment with regard to the issues of trace substance removal, mutagenicity, ecotoxicity, reduction of bacterial counts and operational implementation, such as applied ozone dose, control and regulation, residence time.
Many of the investigated anthropogenic trace substances, which are not or only to a small extent degraded in conventional wastewater treatment, such as carbamazepine, could be increasingly removed by ozonation. The concentrations in the effluent of the two test reactors connected in series (ozone input tank and reaction tank) did not differ significantly from each other at a total of 16-20 min (2 x 8-10 min). Nevertheless, the 2nd reactor is considered necessary for reasons of process and operational safety (matrix fluctuations, potential ozone overdosing and thus for avoiding ozone bleed-through into the effluent).
The formation of by-products represents a central argument against ozonation. Three mutagenicity tests for different trophic levels were performed during the pilot tests and it can be summarized that treatment of WWTP effluent with ozone did not lead to an increase in genotoxic potential.
Using conventional short-term ecotoxicity tests, no negative effects of ozonation on test organisms were detected. The endocrine activity of WWTP effluent was reduced by treatment with ozone (the estrogenic effect and, to a lesser extent, the androgenic effect). During the 21-day fish test to study the endocrine effect, no negative effects of the tested samples on Medaka fish were detected. Histologically and immunohistochemically, neither estrogen nor androgen induced changes of liver and gonads of the exposed fish could be detected.
Regarding the studied hygienic parameters, it can be stated that viruses were the most sensitive to ozone, followed by naturally present Escherichia coli and enterococci. Ozone treatment resulted in a reduction for those parameters regulated in the EU Bathing Water Directive to below the values specified there for good status. The inactivation of aerobic spores (Bacillus subtilis), which are models for permanent forms of bacteria (cysts) and oocysts, was negligible.
At a specific ozone depletion of 0.6 to 0.7 g O3 g-1 DOC, i.e., in the range considered reasonable for practical application, there was an average 15% increase in BOD5 in the pilot plant effluent. Such an increase of BOD5 in the effluent of a wastewater treatment plant is only a problem if the effluent value is close to the limit value, which is generally not the case with the treatment performance of wastewater treatment plants operated according to the state of the art.
The yellowish inherent coloration of wastewater treatment plant effluent caused by humic substances could be significantly reduced by treatment with ozone.
With regard to the control and regulation of the ozone plant, suitable matrix-specific PLS correlation models for the control parameter TOC in the inlet of the ozonation, as well as the control parameter O3 in the outlet of the plant, could be developed with the help of UV/Vis online spectrometry.
The operating costs of ozonation (0.7 g O3 g-1 DOC) were calculated for a model WWTP with 150,000 p.e. and different effluent DOC concentrations (6, 8 and 10 mg L-1 DOC). This resulted in 0.3 to 0.5 € EW-1 a-1 (0.4-0.7 cent m-3), which is less than 5% of the operating costs of a WWTP > 100,000 p.e. with operating costs of 13.7 € EW-1 a-1 (median from Austrian wastewater benchmarking).
Ozonation of WWTP effluent proved to be a promising technology for the treatment of state of the art treated WWTP effluent in the pilot tests conducted.