On-Site Chloramination Helps Solve Well Water Treatment Challenge

By Terry Arndt

In recent years the Eastern Municipal Water District, headquartered in Perris, CA, faced a challenge common to many water districts throughout the United States: providing high quality water to an expanding service population. Finding a water disinfection system to accommodate this growth, meet concerns with worker, plant, and community safety, and permit blending of local water with purchased water proved to be a challenge.

Working with Severn Trent Services, the district began using a disinfection system that employs sodium hypochlorite generated on site. As a result, it has been able to significantly improve both overall safety and plant operation.

Established in 1950, Eastern Municipal Water District (EMWD) originally served a largely rural area with a population of about 20,000. Today the district is responsible for supplying water to 440,000 residential, industrial, and agricultural users within a 555 square mile area of western Riverside County. This population is expected to expand to more than 800,000 by 2020.

EMWD had been treating water from its wells by injecting gaseous chlorine to provide a free chlorine residual. The chlorine was stored on site in both one-ton containers and in smaller, 150-lb cylinders. Concerns about the safety of nearby residents and increasingly tough regulations governing the use and storage of gaseous chlorine prompted the district to examine alternatives.

Complicating matters even further was that most of the district's source water is purchased from the Metropolitan Water District (MWD) of southern California. Only 25 percent to 30 percent of the district's water originates from local wells.

All water purchased from MWD is chloraminated. This process uses chlorine mixed with ammonia to produce chloramines, which are four times more stable than chlorine and can reduce the formation of potentially dangerous trihalomethanes (THMs) in water.

Blending the two streams together would allow the district to expand its use of local well water throughout the system, and reduce its reliance on the more expensive purchased water. But co-mingling chloraminated water with chlorinated water has two problems: first, it can affect the taste and smell of the finished water; and second, it can reduce the presence of residual chlorine, decreasing overall effectiveness. When blending a free chlorine residual with a chloramine residual, the free chlorine reacts with the ammonia in the chloramine, leaving the water with little or no residual disinfectant. Without changing the treatment method, blending would end up creating more problems than it solved.

Changing to Chloramination

In 1998, EMWD began limited use of chloramination to determine its effectiveness as a treatment option in one area of the district.

Chloramines are produced by mixing a five-to-one ratio of chlorine to ammonia. After examining various alternatives, the district decided to use on-site generation of sodium hypochlorite, and aqueous ammonia at 19 percent, based on its availability, safety, and economics.

The use of aqueous ammonia in a hot, dry environment is not without problems. Outdoor temperatures in western Riverside County can soar to over 115°F in the summer. This can cause pressure to build in unvented storage containers, inviting leakage at the tank connections and creating safety and process hazards.

Working with Severn Trent, EMWD was able install a safer, more acceptable ammonia storage method along with an on-site sodium hypochlorite generation process as an efficient way to control both the chloramination and water blending processes.

The ammonia storage system consists of an insulated storage tank and a chiller, with both a chiller probe and a temperature probe inserted through the tank's roof.

Aqueous ammonia has a very low boiling point and creates fumes and pressure when vaporizing. The chiller keeps the temperature of the aqueous ammonia at a steady 60°F - cool enough to ensure safer handling and eliminating the problem of the ammonia vapors and off-gassing.

To generate sodium hypochlorite on site, the district turned to Severn Trent's ClorTectrademark system, which uses materials that are safe to handle: salt, water, and electrical power.

Operation begins when softened water is fed into a brine dissolver to produce a 30 percent brine solution. This is diluted to 10 parts water to one part brine. The resulting 3 percent salt brine solution is then passed through an electrolytic cell(s) where it is electrolyzed with a low voltage DC current. The result is 0.8 percent sodium hypochlorite, a concentration below the hazardous material threshold. When the hypochlorite reaches a low level set point, the ClorTec system automatically restarts to replenish its supply.

Control System

Severn Trent also installed a programmable logic controller to maintain system efficiency and keep water quality consistent.

In chloramination, the five-to-one ratio of chlorine to ammonia prevents the formation of unwanted di- and trichloramines, which can create serious taste and odor problems in drinking water. The PLC maintains the ratio based on an input signal for the plant water flow rate and chlorine residual. It then uses this information to control the rate at which the aqueous ammonia and 0.8 percent sodium hypochlorite are pumped into the system, thereby maintaining a chloramine level of 2.5 mg/L. The process PLC also communicates with the site PLC and thus affords the use of a SCADA (Supervisory Control and Data Acquisition) system.

An online continuous chlorine residual analyzer is also used to measure total chlorine residual in the treated water. If desired, the PLC controller can also use free chlorine residual as additional input to help control dosing. By monitoring total residual chlorine, however, EMWD is able to monitor mono-chloramines.

A free residual analyzer is located downstream for alarm purposes only. If a failure occurs in the ammonia feed, the analyzer will detect a free residual and cause an alarm to the SCADA system, prompting operator response. All analyzers used in the control system are reagentless, membrane-type, and measure both free and total chlorine.

The properly chloraminated water can now be blended with the purchased water from MWD without any change in water quality, giving the district operational flexibility, and in the process saving money.

Future Use of Chloramination

The initial success with the chloramination system led EMWD to expand its use throughout the District.

Of the district's 16 potable water sites using chlorine throughout the district, only seven are still using a chlorine gas feed system. Staff anticipate completing the conversion by 2005. They are presently bringing on line a desalter plant and a microfiltration plant. These new treatment facilities will use ClorTec on-site generators with ammonia injection to produce chloramine disinfectant. Size of the units range from 100 lbs per day to one thousand lbs per day.

About the Author

Terry Arndt is Operations Area Manager of the Eastern Municipal Water District. He headed the project highlighted in this article.