Innovative Methods in Wastewater Disinfection
Using alternative methods of disinfection can reduce long-term costs and diminish chemicals introduced into the ecosystem
Wastewater can have several meanings, depending on the industry or application. Blow down from cooling towers is considered wastewater; leachate recovery water in greenhouses contaminated with pathogens can be considered wastewater; and water processed in sewage plants is definitively considered wastewater. Finding alternative methods to disinfect water before it is introduced back into the ecosystem or to disinfect reclaimed water, has always been a challenge. Environmental concerns have driven researchers, operation managers and maintenance employees to look at alternative or innovative methods to achieve bacterial reduction or elimination.
The biggest factor in alternative methods is usually cost. The initial capital required overshadows the long-term benefits. Often, over a long period of time the return on investment (ROI) can actually save thousands of dollars in chemical and maintenance costs.
There are various methods of pretreating incoming water sources to remove contaminants that can help reduce chemical and labor costs. The water used for cooling towers, for example, can be costly when purchased from municipal sources, and the surcharges for blow down exacerbate these expenses. Even when well or bore water is used, the chemicals needed to control iron, hardness and other contaminants can raise costs.
Therefore, the overall analysis of system design needs to be evaluated closely to reflect actual savings from the capital investment expenditure. By using alternative methods of disinfection, long-term costs can be reduced and chemicals introduced into the ecosystem diminished.
Let’s examine some of the alternative methods of disinfection—ozone, UV, copper ionization and filtration—their uses, capital investment and ROI. These methods can be employed as standalone systems or used together to complement their disinfection and contaminant controlling qualities. Capital investment and third-party testing has been an obstacle, which has kept these methods from being widely used.
Ozone. Technological advances, cost reduction, sizing requirements and reliability of ozone systems have helped make ozone a viable alternative for disinfection. Ozone use has been approved and considered safe by the EPA and the USDA in food processing and is accepted as GRAS (generally regarded as safe). Unfortunately, some systems are designed poorly, and the anticipated results cannot be achieved.
One of the biggest problems in sizing ozone systems is the fact that many people don’t understand how ozone reacts with different materials. Ozone-compatible materials must be used in order to keep the system from failing prematurely. Piping, contact chambers, pump seals, and any other part of the system that comes in contact with the ozone must be able to resist breakdown from the highly oxidizing effect and corrosiveness of ozone.
Stainless steel pump heads made up of 304L- and 316L-grade stainless steel must be incorporated in the design. Hypalon or Gortex seals and other materials must be used to keep systems from failing. Off-gas chambers and ozone destruct units must be implemented to direct excess ozone from areas that could be sensitive to the oxidizing effects and health-related issues.
Another cause of ozone failure is incorrect load factor. This can be caused by BODs (biological oxygen demands), CODs (chemical oxygen demands) and other inorganic substances, such as iron, manganese or sulfur contaminants. If the load is not measured properly or a pilot study is not conducted to gather information, the ozone amount would not be sized correctly.
Subsequently, too much or not enough ozone would be injected. Because ozone reacts with many contaminants at different rates, these determinations must be made in order to achieve desirable results and a smooth running system. Costs may seem high, but overall long-term benefits have shown rapid payback in most installations.
UV systems. Flow rates, turbidity, demand and transmission levels are some of the factors that must be considered in order to achieve a successful UV water treatment installation. UV bulbs and protective quartz sleeves require maintenance and replacement at regular intervals. One solution available is coiled fluoropolymer tubes, which are great for bulb protection and help reduce cost and maintenance. The coiled tubes offer easier and safer replacement compared to quartz sleeves because they do not break and are less expensive. There are clear fluoropolymers available that provide high transmission levels, and the turbulence caused by the coiled structure reduces shadowing and increases contact time, thus creating better UV dosage rate.
Copper ionization. Copper ionization reduces bacteria and also helps reduce scaling in pipes and holding tanks. This method is not widely used but should be examined closer because of its positive effects when used in conjunction with ozone, UV and filtration.
Copper ionization technology is very effective for cooling towers for example. By using low levels of copper, chlorine use can be reduced drastically for bacteria or algae control. The EPA has established the MCL for copper at 1.3 ppm. Levels of .5 to .7 ppm are very effective in controlling algae and bacteria, thus reducing expensive chemical use.
Filtration. Filtration is another control method that is being employed in wastewater systems. Advanced membrane technology is becoming more effective in removing pathogens. Cost and footprint reductions are making membrane technology more attractive. These systems can filter millions of gallons of water per day and the efficient membranes keep initial costs lower, and maintenance easier and less expensive.
Defining the Methods
Let’s go back and examine the different ways ozone can be utilized as an alternative method of disinfection.
Ozone disinfection. Ozone can be bubbled into water to reduce bacteria, algae and other pathogens. Porous PTFE tubing is very effective for bubbling ozone into water. The PTFE tubing is porous, flexible and well suited for many applications. PTFE tubing works great with atmospheric tanks because ozone gas rises up through the water and oxidizes contaminants. Longer contact time of the ozone results in better bacterial control and contaminant oxidation. This method can also help reduce “dead spots” within a system. The dead spots occur when ozone is sucked into the water with a venturi, but isn’t circulated properly to all areas, resulting in bioslime build up due to the lack of proper ozone dosage. The tubing can be placed in hard to reach areas throughout the tank, which helps keep the water moving and reduces dead spots.
Ozone is very good for oxidizing iron and manganese in atmospheric tanks. If the ozone dose or contact time is too low, the dose rate will not sufficiently control bacteria. Systems must be designed to overcome the load factor, and all parameters must be considered. Baffles in the tank can help slow the rise of ozone and assure better transfer into the water.
Another method of injecting ozone into the water is by using a venturi along with contact tanks to get maximum ozone transfer into the water. These tanks allow longer contact time and are under pressure, so the ozone can dissolve more efficiently into the water. Sizing a tank to provide the proper contact time for the flow rate needed is one of the main parameters for successful system design.
Using UV-generated ozone or Corona Discharge (CD) is another method. Ozone dosage must be applied properly or results will be marginal. The production of ozone with UV is limited because of various factors, such as inconsistent output from UV bulbs and variable airflow. UV ozone cannot produce the weight percentage that CD units can produce; however, this method has found place in many applications.
On the the other hand, CD units produce very high levels of ozone by weight percentage. Oxygen concentrators and air dryers reduce the nitrous oxide produced from CD units. When used correctly, ozone is one of the best methods of oxidizing contaminants. Calculating the proper dose can be accomplished, but a pilot study proves to be the best method for optimum evaluation.
UV disinfection. UV systems used to reduce and control bacteria have proven to be very reliable. Understanding the water quality and determining the correct dosage rate of the UV needed is very important for proper application. The formula to determine the dosage rate is as follows: Dose (mW-sec/cm2) = Intensity (mW-sec/cm2) x Time (sec). Dose may also be expressed in mJ-sec/cm2, which is becoming the new standard expression. The UV lamp power determines intensity. Time is calculated by the UV exposure duration of the process fluid. The dose rate is affected by the following factors:
- Flow rate;
- Water temperature;
- Water quality;
- Lamp life; and
- Bulb protection materials.
Flow rates are the principle variable in any water treatment application. Typical disinfection systems are rated at the maximum flow that will provide a 30,000 mW-sec/cm2 dose at the end of the lamp’s usable life.
Water temperature is only a factor in low-pressure lamp systems since the operating temperature is lower. Low-pressure UV lamps, which are very similar to standard fluorescent bulbs, operate between 120 and 240 V and obtain power outputs ranging to more than 100 watts with current draws of less than 500 mA. Low-pressure UV output drops drastically if process water rises above or drops below the optimum temperature. Medium-pressure lamps, which are stable under all temperature conditions, have a higher operating temperature; hence the output is not affected.
Turbidity and other water contaminants block and/or absorb UV light, while organic and inorganic materials increase the dose required. Other factors are color, metals and/or suspended and dissolved solids.
UV light technology is not new, but the improved use of UV technology can enhance disinfection systems. Side-streaming UV into wastewater is becoming a good method of controlling bacteria in cooling towers and other wastewater applications. This allows for lower flow rates but still provides very good control of pathogens and bacteria. The coiled tubing has proven to help designers overcome sizing problems because it allows longer contact times and more turbulent flows.
Ionization. Copper and silver ionization have been used for centuries. Egyptians used copper and silver containers to control bacteria in their water. Pioneers used copper and silver coins to keep their water fresh. Today’s technologies use electronic controllers to emit copper and/or silver into water with an electrical charge. Copper or copper/silver electrodes are used to emit positively charged ions into the water for dis- infection purposes. These ions disrupt enzymes in bacteria to stop their proliferation. Copper use works well in bacterial control since it is more widely accepted and not as regulated as silver.
Both need to be monitored in order to keep levels low and not disrupt the ecosystem.
Membrane filtration. Today, membranes are very advanced. They have less effect on flow rate, are cost-effective and easy to maintain. System footprint is also improved. For example, an application that once used 25, 8-in. membranes can now be accomplished with five, 18-in. membranes and still deliver the same quality and quantity of water. This results in tremendous savings in initial startup and maintenance costs without compromising water quality. Many municipal systems are using this technology to conserve water by filtering the backwash and reusing this water in the main system.
No matter what technology or combination of technologies are used as alternatives for disinfection, it must be understood that these technologies usually have higher initial capital investment. It must also be understood that all water treatment equipment has maintenance and other related costs to keep equipment working optimally. The alternative treatment methods discussed in this article provide a rapid payback when compared to chemical costs, maintenance, storage, and labor associated with traditional treatment methods. Over the long run, ozone, UV, ionization and filtration are economical and environmentally feasible. wqp