Pumps and Systems Wastewater treatment 4 th cleaning stage with active carbon and ozone Eliminating man-made trace substances Dipl.-Ing. (FH) Michael Birmelin Man-made trace substances in water courses are a continually growing problem. These substances are compounds produced by man, which are very resistant to biological decomposition and which enter our potable water reserves through waste water. Substances, such as active pharmaceutical ingredients, the ingredients of cosmetics or cleaning products, agricultural pesticides and herbicides as well as hormones, contaminate waste water even if they only occur in concentrations that are often hard to detect. However, many of these substances and micro- pollutants, some of which are harmful, do not biodegrade well or cannot be removed with standard waste water treatment methods. Processes beyond those currently used for waste water treatment are needed to reduce the introduction of trace substances from municipal waste water. The article below focuses on this very task. It describes solutions involving two promising procedures: waste water treatment with active carbon and treatment with ozone. Adsorption by active carbon The active carbon can either take the form of powdered activated carbon (PAC) or granular activated carbon (GAC). As well as mechanically, biologically and chemically cleaning the waste water as usual, the active carbon also eliminates trace substances. Powdered activated carbon is added to the waste water in the contact reactor and mixed. The trace substances attach to the surface of the active carbon. Then the loaded active carbon sinks to the bottom of the sedimentation tank and is virtually all removed through filtering. The COD (chemical oxygen de- mand) and P tot (total phosphorous content) parameters are also reduced to below the threshold values required by the AbwAG (German Waste Water Charges Act). If using granular activated carbon, this is not added to the waste water, instead the waste water flows through a tank filled with it, which acts like a filter. The carbon is replaced and some is regenerated for other applications. It is to be expected that electrostatic forces of attraction also result in microplastics attaching to the active carbon along with the trace substances when using PAC and GAC, thereby eliminating them too. A fourth cleaning stage is used in clarification plants in many places. This is prescribed by law in many countries, including Switzerland. It allows the introduction of micro-pollutants into water courses to be significantly reduced. Adverse impacts on ecosystems and the contamination of potable water resources by micro-pollutants from municipal waste water have been proven to be minimised in such cases. Two processes for this have emerged and can be implemented on a large scale: the treatment of waste water with active carbon and with ozone. These methods generally eliminate more than 80 % of trace substances. Fig. 1: Big Bag emptying unit with multi-screw feeder for accurate, dust-free metering of powdered activated carbon (PAC). With the aid of a multi-screw feeder, the system wets, disperses and meters PAC with high precision and without dust. 28 PROCESS TECHNOLOGY & COMPONENTS 2021
Fig. 2: The emptying funnel of the Big-Bag system is matched to the bag size used. A special seal ensures dust-free emptying into the powder storage container. Waste water treatment with PAC A special PAC metering unit meters powdered activated carbon by volume in a reproducible manner and with an accuracy of more than one percent for large-scale elimination with powdered activated carbon (PAC). This level of accuracy is achieved without gravimetric weighing equipment. The Big Bag emptying and metering station with multi-screw feeder, wetting cone and injector wets, disperses and meters the active carbon with great accuracy in a dust-free process. The emptying and metering station is designed for use with Big Bags with an outlet nozzle. The Big Bag is suspended in the frame with the aid of a forklift. The metering unit’s docking station is matched to the Big Bag’s outlet nozzle and ensures dust-free drainage. The manufacturer offers a vibrator as an extra safety measure. The multi-screw feeder’s selfcleaning mechanism ensures that the active carbon is metered smoothly. It is transported to the wetting funnel with a reproducible accuracy of more than 1 %. Thanks to this excellent accuracy, precisely the required amount of active carbon is metered, resulting in optimum adsorption of the trace substances. There is no over- metering and the costs associated with this. A Venturi mixer adds the suspension produced to the waste water. The active carbon generally adsorbs up to 80 % of the trace substances and slowly sinks to the bottom of a sedimentation tank. This process can be accelerated by adding flocculants. The resulting sludge of active carbon and precipitant is then dewatered with the clarifier sludge, dried and sent to an incinerator. Oxidation using ozone Unlike adsorption with active carbon, ozone is one of the most powerful oxidising agents, which breaks down trace substances through oxidation or converts them into smaller molecules. They are then easier for micro-organisms to break down in a post-treatment stage or can be removed by means of adsorption with GAC. The positive side effects of using ozone are that the waste water is partially disinfected, discolouration and the COD is greatly reduced. As with active carbon, the contact time and specific metering are key to technical implementation. Ozone is generated on-site in an ozone generator. The gas used for the large-scale process is liquid oxygen. Problem solver for process engineering and sewage technology www.eggerpumps.com Turo ® Vortex series T and TA Suitable for high solids concentrations and shear sensitive products in the chemical industry and for clogfree pumping of raw sewage with fibres and sludge. Iris ® Diaphragm Control Valve Highly precise and energy saving control of flow rate through concentric Iris ® diaphragms. For aeration airflow control in WWTP’s and for gases or liquids in industry. Ecological disinfection and oxidation Fig. 3: Ozone generators guarantee high operational reliability with minimized operating costs. They are almost maintenance-free and generate ozone from compressed air or oxygen. As a highly reactive gas, ozone is generated on site from oxygen and introduced to the water directly without interim storage. With Ozonfilt ozone systems, the oxygen operating gas is fed to the ozone generator under pressure. The SWISS ENGINEERED PUMPS SINCE 1947 Switzerland Emile Egger & Cie SA Route de Neuchâtel 36 2088 Cressier NE Phone +41 (0)32 758 71 11 Germany Emile Egger & Co. GmbH Wattstrasse 28 68199 Mannheim Phone +49 (0)621 84 213-0
