2.5.08 Four methods, one goal – wastewater disinfection in China

A comparison of disinfection procedures in the effluent discharge of Chinese wastewater treatment plants has highlighted the existence of alternatives to conventional chlorination. As part of a joint research project funded by the BMBF, the IWAR institute of the Technische Universität Darmstadt (Darmstadt Technical University) tested four different procedures in conjunction with the Tongji University in Shanghai (period of study: August 2006 – March 2011).

The many hygienically relevant micro-organisms (viruses, bacteria, protozoa, worm eggs) present in the wastewater – even after biological cleaning – necessitate adequate purification of the water before it can be introduced to sensitive surface waters (especially prior to reuse). While wastewater disinfection is a legal requirement in the People’s Republic of China, this process is frequently omitted for cost and operating safety reasons (Xin, 2004).

An alternative to conventional disinfection methods is required for a number of reasons. The most common procedure, chlorination, is generally accompanied by the formation of unwanted disinfection by-products. Other disadvantages of chlorine and its compounds are the ever-present concerns regarding operating safety and the limited effectiveness against chlorine-resistant organisms. Therefore, the aims of the joint research project are to:

• Improve the hygienic water quality in the effluent of municipal treatment plants to protect against waterborne diseases
• Provide a scientifically verified contribution to the issue of cost-effective application of effective, innovative wastewater disinfection processes
• Avoid new risks by minimising the formation of disinfection by-products

Employed methods

Relevant factors influencing the choice of disinfection method include effectiveness, operational safety, investment and operating costs, practicality (transport, storage, production etc.) and the creation of unwanted by-products. Having been selected during a pre-project literature study, the processes tested over the course of the project were as follows:

UV radiation: UV rays with a wavelength of 245 – 265 nanometres alter the nucleic acid in the cell nucleus, thus resulting in the irreversible loss of the cell’s multiplication capacity and subsequent inactivation of the cell when its regenerative ability is exceeded. No addition of disinfectants remaining in the water occurs with this disinfection method. As a result, negative environmental and health impacts can be largely eliminated, though a potential depot effect in the discharge water is also excluded. The UV system features two replaceable UV lamps with outputs of 80 and 120 watt. The desired radiation dose can be set via a control unit (from 80 – 800 J/m).

Electrochlorination: The electrochlorination system produces gaseous chlorine (Cl) and other electrochemical oxidants from table salt, water and electric current on site, thus eliminating the transport and storage of chlorine (gas). Chlorine causes the oxidative destruction of the cell wall of micro-organisms. The maximum dosage is 20 mg Cl₂/l.

Chlorine dioxide: The disinfectant properties of chlorine dioxide can be mainly attributed to its high oxidation potential (approx. 2.5 times higher than that of chlorine gas). When wastewater is disinfected with chlorine dioxide rather than chlorine (gas), the potential for formation of ecologically damaging compounds is lower, since no trihalomethanes (THM), chlorophenols or reaction products with ammonium and amino compounds are produced. The dosage of chlorine dioxide is between 1 and 20 mg ClO₂/l. Chlorine dioxide (ClO₂) is created on site from hydrochloric acid and sodium chlorite by means of the chlorite/acid process.

Ozone: Ozone (O₃) is one of the most effective disinfectants. Ozone attacks the cell membrane directly or permeates the wall to enter the cell interior, where it attacks the DNA, RNA or other cell components, this inactivating the cell. Wastewater ozonation can also be used to remove pharmaceutical residues, endocrine disruptors as well as odourants and colourants (Schuhmacher, 2006). Ozone generators are used to create ozone on site by means of electrical discharges from industrially produced oxygen. Ozone dosage varies between 2 and 20 mg O₃/l.

All of the above procedures (with the exception of ozonation, which was only tested in Germany) were operated and examined over the course of the project using identical semi-industrial test systems at a communal treatment plant in Darmstadt-Eberstadt and in Shanghai.

In addition to the choice of disinfection process, the manner in which the water is pre-treated is also of decisive importance – both with regard to the success of disinfection and the potential for by-product formation. In many cases, the overall performance of a disinfection system and the potential health risks are greatly determined by the shadowing and inclusion of micro-organisms in wastewater particles. In this project, disinfection capacity was assessed by using standard methods for microbiological cultivation to quantify the indicator organisms E. coli, total coliforms, enterococci and somatic coliphages.

Achieved results

Both micro-screening and sand filtration can reduce CODThe chemical oxygen demand (COD) is a measure of the amount of water-based substances that can be oxidised under specific conditions. It indicates the volume of oxygen (in mg/l) required for the oxidation of these substances. concentrations by around 30%, UV absorption (at 254 nm) by approx. 10% and turbidity by some 80%. During test phases I to III (see figure), all four disinfection processes were able to reduce the level of indicator organisms below the detection threshold or by up to four orders of magnitude (Bischoff, 2009). An increased toxicityDegree to which a substance can damage a living or non-living organism. of the wastewater, measured as the effect on the luminescence of Vibrio fischeri organisms, was not significant in this case (order of toxicity rise: Cl₂>O₃>ClO₂; UV radiation: no increase).

In addition to the disinfectant dosage and the organic substances contained in the wastewater, the water temperature was also found to have a significant impact on the success of the disinfection process. Phase IV was terminated after four weeks, as stable operation of the disinfection system was rendered impossible after two weeks by the increasing biofilm growth. As a result, a critical assessment was made of wastewater disinfection procedures in which no biological treatment methods are employed. Depending on the employed dosage, the examined disinfection methods with prior biological wastewater treatmentWith biological wastewater treatment, the organic compounds contained in wastewater are subjected to a biodegradation process. Degradation is performed mainly by micro-organisms in conjunction with dissolved oxygen in the case of ► aerobic processes or in the absence of oxygen in the case of ►anaerobic processes. Inorganic compounds and biomass are created through conversion processes. The most frequently used method of biological wastewater treatments is the ►activated sludge procedure. produced environmentally sound water that could be introduced even to sensitive surface waters and is perfectly suitable for a range of reuse scenarios. A more detailed assessment of the test results can be found in the final project report.

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