By Jan Korzeniowski
Water and wastewater infrastructure is a substantial item in municipal budgets and has major health, safety and environmental impacts. Also, municipal, industrial and recreational development depends on adequate water and wastewater services.
Municipalities have to continue implementation and operation of needed services, even on reduced budgets. Therefore, they need to take more progressive steps when selecting engineering solutions, making sure they offer good service and result in cost savings during implementation and operation. Conventional solutions are often preferred and selected because they are proven and safe. However, they can be more expensive than new, innovative solutions that are proven but less known.
Water and wastewater treatment plants are major infrastructure components. They involve complex structural, electrical, mechanical and process components, which creates opportunity for innovative and cost saving solutions.
Water treatment systems are used in many water supply applications, including surface and ground waters, which may contain naturally occurring or man-made pollutants, such as high turbidity, colour, algae, taste, odour, chlorinated organic compounds, cyanide, hydrogen sulfide, nitrite, phenol and dissolved metals.
Easy to treat surface or ground waters only require simple solutions, predominantly filtration with post-disinfection. But, ground or surface waters with difficult to treat natural or man-made pollutants require more complex treatment. It is here where competitive and innovative technology may be considered and has advantages over conventional technology.
One of the solutions which has wide applications is water oxidation with ozone or ozone and hydrogen peroxide. This is an advanced and complete treatment system which can be used for different conditions. Options include pre-oxidation with ozone and direct filtration, or ozone followed by settling and filtration. Also, coagulation and flocculation can be added to either process, depending on the type and concentration of the pollutant to be removed.
Ozone and hydrogen peroxide oxidation (super oxidation) is used for treatment of waters which contain high levels of dissolved organic carbon, odour caused by geosmin, MIB, or algae and chlorinated organic compounds. High levels of dissolved organic carbon and odour are often present in surface waters from lakes and man-made ponds.
Ozonation, used as a primary oxidation and disinfection treatment component, followed by direct filtration, provides distinct advantages which include bacteriological control (CT requirements) for inactivation of giardia, cryptosporidium and viruses, and enhanced removal of dissolved organic carbon and odour.
Treated water needs only a low dosage of chlorine to maintain the minimum residual in the water distribution system. This prevents formation of high levels of trihalomethanes.
The system shown in Figure 1 is one that we designed. It uses two-stage ozonation with vacuum type ozonators and innovative water recirculation, which is accomplished by a recirculation pump and air aspirator-mixer. Off gas from the first stage ozonation is recycled to the second stage ozonation, which ensures efficient use of ozone and a thorough oxidation process.
Oxidation is achieved by ozone alone, or by ozone and hydrogen peroxide, depending on the type of pollutant to be removed. This system is safer, more efficient, less expensive and easier to operate then many conventional pressure ozonation systems.
Municipal wastewater treatment primarily involves biological treatment with activated sludge to a secondary level or to a tertiary treatment level, using a biological nutrient removal (BNR) system.
Although, there are a number of different BNR system configurations, all use similar mechanical equipment which includes air blowers and diffusers for aeration in bioreactors, vertical mechanical mixers in anaerobic and anoxic reactors, mechanical scrapers in primary and secondary clarifiers, and wastewater pumps to transfer sludge or wastewater between the treatment units. Effluent disinfection is usually achieved in UV light reactors.
We have designed treatment systems using different aeration, recirculation and mixing equipment. Our aeration system uses an aeration pump, usually a high efficiency wastewater effluent or industrial pump, an air aspirator-mixer, additional static mixers and engineered perforated PVC distribution pipes in the bioreactor and secondary clarifier. Similar perforated PVC piping system is provided in the primary clarifier and anaerobic and anoxic tanks to recirculate wastewater and activated sludge.
The aeration pump aerates wastewater and sludge in the bioreactor, recirculates sludge from the secondary clarifier to the bioreactor, diverts unaerated wastewater to the anoxic tank and wastes activated sludge to the waste sludge tank.
The air aspirator is a modified venturi nozzle and the mixers are static spiral type. The aeration assembly is non-plug type with wastewater screened at headworks.
Our aeration system can be used continuously or intermittently as required by the treatment process, with the aeration pump working continuously for mixing the wastewater or sludge. However, if the aeration pump operates intermittently, the sludge which settles at the aeration tank bottom will be re-suspended when the aeration pump is turned on. Mixing in the aeration tank is by air and by wastewater circulation.
Activated sludge from the clarifier and wastewater from the bioreactor flow by gravity to the aeration pump, mix and re-aerate at the air aspirator-mixer. They are discharged back to the bioreactor through the perforated distribution pipes. A portion of the wastewater and sludge mixture is wasted, before aeration, to the sludge stabilization tank and to the anoxic tank as required.
Aeration of wastewater and activated sludge and oxygen uptake by bacteria begin at the air aspirator-mixer and continue in the downstream piping and in the bioreactor. This results in high oxygen transfer rates of over 50%.
This system can be used with concrete process tanks or earth ponds/lagoons, new or existing, to secondary or tertiary treatment level. It can be used economically in existing anaerobic lagoons.
Advantages include a less complex and costly mechanical design, and greater energy efficiency. Using ozone is reliable, cost-effective and does not require the same level of maintenance, cleaning and replacement of equipment parts as other disinfection methods.
Jan Korzeniowski is with J.K. Engineering Ltd. This article appears in ES&E Magazine’s December 2019 issue.