Wastewater Disinfection -- Alternative Methods

By Jeff Roseman

Wastewater can have several meanings, depending on the industry or application. Blowdown from cooling towers is considered wastewater, leachate recovery water in greenhouses contaminated with pathogens can be considered wastewater, food processing water, and water processed in sewage plants is very definitively considered wastewater. Finding new methods to disinfect these waters before they're introduced back into the ecosystem or by disinfecting the water for reclamation purposes (whether for reuse in process systems or irrigation) has always been a challenge. Reducing costs of disinfection remains the biggest concern for decades. But environmental concerns drive researchers, operation managers or maintenance employees to look at alternative, innovative methods to achieve microorganism reduction or elimination (see Table 1 below).

A Matter of Cost
The biggest factor in alternative methods always seems to revolve around cost. Initial capital required overshadows the long-term benefits. Initial capital expense and long-term return on investment (ROI) need to be analyzed closely. Often, the ROI viewed over a longer period of time can actually save thousands of dollars in chemicals and maintenance costs. There are methods of pre-treating incoming water sources to remove contaminants that can help reduce chemicals and labor costs, reflecting major savings. The water used for cooling towers can be costly when purchased from municipal sources and the surcharges for blowdown -- not to mention new federal regulatory requirements -- exacerbate those expenses. Even when well water is used, the chemicals used to control iron, hardness and other contaminants can cause elevated chemical use and labor costs. This in turn creates more frequent blowdown and if discharged to municipal sewer systems, the surcharges again ultimately increase the cost of operation. As one can see, the overall analysis of system design needs to be evaluated closely to reflect actual savings from the capital investment expenditure over time. By using alternative methods of disinfection, long-term costs can be reduced and chemicals introduced into the ecosystem diminished.

Table 1. Industrial wastewater general parametersThe BOD, COD, and TOC measurements are essential parameters to be considered when designing water treatment for industrial wastewater. The following examples of industries and contaminants reflect only a small fraction of industrial water treatment possibilities:INDUSTRY.............................................TYPICAL TREATMENTSSemiconductor..........................................TOC reduction in reclaimTFT-LCD..................................................TOC, acetic acidElectroplating............................................Destruction cyanides, EDTAMining.....................................................Free and combined cyanidesWood treatment........................................PhenolsPetrochemical...........................................BTEX and MTBENuclear power plants..................................TOCOil refineries............................................Vinyl chlorideExplosive manufacturing..............................TNTPharmaceutical manufacturing.......................TOC, endotoxinsLooking at Options
Let's check some alternative methods of disinfection. Ozone, ultraviolet (UV) light, copper ionization can be employed as standalone systems or used together in a synergistic fashion to compliment their disinfection and contaminant controlling qualities. Reduction of chemicals and labor associated with their use has proven to be feasible. Capital investment and third party testing has been an obstacle, which has kept these methods from being widely used. Tried and true chemical use is the norm and alternatives have been long pressed for acceptance because of the initial investment, a lack of understanding as to how the technology functions and/or an unwillingness to try something new.Ozone
Advances in ozone production equipment are helping make ozone a very viable alternative for disinfection. Cost reduction, sizing requirements and reliability have helped make this method more attractive. Many experts now know how to use ozone safely and effectively in a variety of applications. Ozone use also has been approved and considered safe by the U.S. Environmental Protection Agency and U.S. Department of Agriculture in food processing and is accepted as GRAS -- generally regarded as safe. Too many times, though, systems are designed poorly and anticipated results weren't achieved because a crucial parameter was omitted or not factored thoroughly.A big problem in sizing ozone systems is that many engineers don't understand how ozone reacts with different materials. Ozone compatible materials must be used to keep the system from failing prematurely. Piping, contact chambers, pump seals and any other part of the system in contact with the ozone must be able to resist breakdown from the highly oxidizing effect and corrosiveness of O₃. Stainless steel pump heads made up of 304L and 316L grade stainless must be incorporated in the design. Hypalon or Gortex seals and other ozone resistant materials must be used. Off gas chambers and ozone destruct units must be implemented to direct excess ozone from areas that could be sensitive to oxidizing effects and health related issues for humans (see Figures 1&2 at bottom).Another big miscue of ozone failure is load factor, or how different parameters affect ozone demand. The load factor can be caused by BOD, COD and other inorganic substances, such as iron, manganese or sulfur contaminants. If the load isn't measured properly, or a pilot study conducted to gather information, the ozone amount is generally not sized correctly and subsequently too much ozone can be injected or in most cases not enough. Ozone reacts with many contaminants at different rates and these determinations must be made in order to achieve desirable results and a smooth running system. Capital costs may seem pricey, but overall long-term benefits have shown rapid payback in most installations. Savings in chemicals, labor and equipment replacement costs all factor into the investment return.Ultraviolet
UV light systems are affected the same way that ozone systems are in determining successful installation. Flow rates, turbidity, demand and transmission levels, etc., are some of the factors that must be considered in UV disinfection. Total organic carbon, or TOC, removal can be very successful when applied correctly. Maintenance issues must be considered in this type of treatment and conducted on a regular basis to keep the system clean and running efficiently. UV bulbs and protective quartz sleeves require maintenance and replacement at regular intervals. Newer coiled fluoropolymer tubes (see Figure 3) are a great alternative method of bulb protection that helps reduce cost and maintenance. The coiled tubes offer easier and safer replacement compared to quartz sleeves, since they don't break, are less expensive and, therefore, are more cost effective. There are very clear fluoropolymers that provide good transmission levels and the turbulence caused by the coiled structure reduces shadowing and increases contact time, thus creating a good UV dosage rate.Copper ionization
Copper ionization (see Figure 4) is a method of reducing bacteria and also helps reduce scaling in pipes and holding tanks. This method is not very widely used and should be looked at more closely because of the synergistic effects when used in conjunction with ozone, UV and filtration. Cooling towers and agricultural produce washes are very good applications for use with this type of technology. Chlorine use can be reduced drastically and other biocides to control bacteria or algae can be used in moderation. Copper must be controlled when the effluent is released back in the ecosystem, but this method is still a very viable disinfection alternative. Low levels of copper are all that's needed to control algae, fungus, and bacteria. A copper level below the action level established by the EPA is 1.3 ppm. Levels of 0.5 to 0.7 ppm are very effective in controlling algae and bacteria, thus reducing expensive chemical use.Conclusion
Without going into a lot of explanation of how to calculate the dose rate, let it be said that an ozone manufacturer or technician can help with procuring the correct information for any application that may be addressed. Many books have been written on ozone. One good reference book that touches many different applications is the Water Quality Association's Ozone -- A Reference Manual.UV light systems seem to be more widely used within the wastewater industry. Wastewater treatment plants worldwide have proven UV a viable alternative for disinfection if monitored and maintained properly. The coiled fluoropolymer tubing is a product on the horizon that brings newfound ease for installation, maintenance and safety issues. The coils replace quartz sleeves and help designers reduce size requirements while improving safety issues and overhead costs.Ionization is coming into its own, because more installations and third party testing proves its usefulness as an integral part in system designs. When monitored and used at low levels, this technology enhances synergistic effects in scale reduction along with good bacterial control. When ionization is used with ozone, UV and filtration, these technologies enhance each other and provide system operators with a good alternative for disinfection, ultimately reducing costs and environmental impact of chemical use.About the author: Jeff Roseman is the owner of Aqua Ion Plus+ Technologies in La Porte, Ind. He's a Certified Water Specialist, Level IV, with the Water Quality Association and has a background in chemistry and physics from studies in electrical engineering at Purdue University. His specialty certifications are in ozone, disinfection, and membrane (RO/UF) filtration technologies. He has been instrumental in developing systems using copper ionization as an alternative to using chlorine. Contact: 219-362-7279 or [email protected]###