By Robert Nobile
Increasingly stringent US EPA drinking water rules have led to the development of a new water circulation technology aimed at preventing the deterioration of water quality in potable water storage reservoirs for both chloramine and chlorine systems.
In an effort to further improve and protect drinking water, the US EPA periodically announces new disinfection byproduct (DBP) rules. In the past, the emphasis was on the water treatment facility; more recently, attention is being placed on the distribution system. Because about 30 percent of the nation’s drinking water resides in potable storage tanks for some time period, rules currently in effect require water managers to take new steps to maintain water quality in potable water storage tanks.
Unfortunately, either due to a lack of awareness of the problems that occur in potable water storage reservoirs or a lack of good technologies to deal with the problems, many systems are now in a ‘catch up’ mode in regard to preserving water quality in such reservoirs. For instance, since a growing number of potable water suppliers have switched from chlorine to chloramines to disinfect water, they need to perform more sampling in the tanks to closely monitor nitrites caused by ammonia-oxidizing bacteria (AOB). But many chloramines system operators are not yet addressing this problem.
Also, in a recent paper on the Final Stage 2 Disinfectants and Disinfection Byproducts Rule, the US EPA refers to the need for effective mixing of stored water to prevent the formation of DBPs, particularly as it applies to chloramines introduced as chlorine and ammonia.
“When chlorine and ammonia are added simultaneously, good mixing can reduce the time free chlorine has to react with NOM [natural organic matter]… This eliminates the free chlorine almost immediately and reduces the potential for DBP formation… At lower temperatures, the reaction can take longer and mixing becomes more important.”
So, quick and thorough dispersion of both chlorine and ammonia determines the extent of free chlorine exposure and thereby substantially impacts the formation of DBPs.
As DBPs contribute to the deterioration of water quality, the question for many system operators is how to address this deterioration in potable water storage tanks when they can’t even gauge (much less control) how long the water stays in the tank? The answer is to mix the tank continuously so that water age is minimized and actual detention time will be as low as possible. If problems do occur despite thorough mixing, the mixing capability gives operators a good tool for solving any arising problems—by injecting additional disinfectant or by taking the tank through breakpoint chlorination.
A new and proven water-mixing technology is a solar-powered circulation system that is also in wide use for improving water quality in lakes and reservoirs. These circulators are floating units that draw power from photovoltaic (PV) modules mounted on top of the tank. They self-adjust for all water levels in the tank and, depending on size, can pump up to 10,000 gallons (37,854 liters) of water per minute.
The long-distance flow pattern emanating from these solar-powered circulators has been shown to thoroughly mix tanks ranging from 200,000 gallons (757,082 liters) to 100 million gallons (378,541,178 liters) and to work for both chlorinated and chloraminated water. They facilitate thorough breakpoint chlorination, even in very large tanks, whenever necessary and can also be used for ‘boosting’ by injection of chlorine or chloramine (chlorine and ammonia), or chlorite ions. These machines operate 24/7 and can be monitored through the facility’s SCADA (Supervisory Control and Data Acquisition) system. Installations are accomplished through the hatch, without draining the tank or taking it off line.
Joel Bleth, President of PSI, explains that the problems in treating potable water come from the need to constantly and thoroughly replace the disinfectant that is being used up at all the boundary layers of the tank (floor, walls and support members) to keep bacterial growth under control. This is especially critical in warm weather, when bacterial growth rates are the highest and bacteria can quickly deplete the disinfectant at the boundary layers.
Diffusion alone cannot be counted on to keep fresh disinfectant at these surfaces; it is too slow and is hindered by the stratification caused by temperature gradients as low as 0.01°C (32.018°F). Mixing techniques used in the past could not accomplish this job because they involved turbulent mixing, which reaches out just a short distance and then turns around and goes right back into the mixer. So, instead of the whole tank being mixed all the way to the boundary layers, only the water around the turbulent mixer was being mixed. But the new style of mixers, which have near-laminar flow, will reach out up to 800 feet (20.32 meters) in all directions so that all boundary surfaces are getting a constant flow of replacement disinfectant.
One of the most problematic areas for bacterial growth is at the floor of the tank, where there is usually a layer of sediment. The sediment in a chloraminated system is especially troublesome, because free ammonia is always present in chloramina-ted water. AOBs (nitrifying bacteria) thrive when attached to sediment and are more resistant to the chlorine component of chloramines than most other bacteria. The result of no circulation across the sediment is often the formation of nitrite, especially in warm summer months, that exceeds the maximum contaminant level (MCL) of 1.0 mg/L.
The new long-distance mixers have a patented design that allows for constant replacement of the disinfectant at the sediment-water interface (without disturbing or re-suspending the sediment) to keep AOBs in the sediment under control.
Prevention of bacterial growth by continuous mixing is the most effective approach to avoiding deteriorated water quality in storage tanks. Yet, even with thorough mixing there can be problems, particularly in warm weather, when monitoring will sometimes show that the residual disinfectant level is dropping dangerously low as the chlorine is getting used up while combating high bacterial growth. When this occurs, the chlorine or chloramines should be boosted. Or, if nitrite is being formed, chlorite ions can be mixed into the water; they have been demonstrated to be very effective in controlling AOBs.
If the mixer is effective at the boundary layers, boosting of the disinfectant will probably solve the problem. But, if the water quality has deteriorated beyond the point where boosting disinfectant is effective, the tank must be taken offline and taken through breakpoint chlorination and/or cleaned. If breakpoint chlorination is to be performed, the mixing system must be capable of quickly mixing large volumes of chlorine throughout the entire water column so that each point in the tank is at the same place on the breakpoint curve.
To summarize, an effective mixing system can be thought of as having two different roles, one for prevention and one for major corrections. To prevent problems and to head off small problems by boosting, the mixer needs to have good circulation of the disinfectant across all boundary curves. But if the tank needs to be taken through breakpoint, then the mixer needs to have the ability to quickly and thoroughly mix the entire water column.
Unlike nozzle devices and check-valve inflow-outflow piping, solar-powered, long-distance circulation causes no adverse effect on system flow-rate capability, no loss of energy at the nozzle, no losses in pump efficiency and no changes to other distribution system characteristics. It has also proved to be reliable, cost-effective and low-maintenance. In fact, compared to turbulent mixers, the solar-powered circulators mix the entire reservoir right up to the boundary layers with far lower operational and maintenance expenses. The circulators can be equipped with SCADA output, a chlorine injection system and with various solar and 24-hour power kits, depending on reservoir characteristics. The system’s flotation system, together with the variable length intake hose, self-adjusts at all times for peak performance, regardless of water depth in the reservoir.
About the author
Robert Nobile is currently employed as Marketing Manager for SolarBee Inc. and holds a BS in electrical engineering from Bridgeport Engineering Institute, Conn. He has made numerous presentations to consulting engineers, public agencies and SolarBee’s national sales force on the advantages of using submersible pumps. Nobile also presented his paper, “Upgrading Hydro Plants with Submersible Hydroturbines” at Water Power 89 and provided training on how to evaluate the design characteristics of electric motors that are responsible for long life and high efficiency.
About the company
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