A Change for the Better
Some New York water districts find that switching from sodium hypochlorite to calcium hypochlorite helps optimize chlorination while eliminating problems
- By Jim Brennan
- Jul 01, 2006
Management at drinking water treatment facilities looking for an alternative to gaseous chlorine (Cl2) or sodium hypochlorite (NaOCl) should seriously consider dry chlorine in the form of calcium hypochlorite briquettes. Drinking water treatment facilities switching from sodium hypochlorite to calcium hypochlorite for disinfection are finding this alternative to be an efficient solution for ensuring consistently accurate chlorination while eliminating many long-standing operations, maintenance, and safety concerns associated with NaOCl use.
Probably the most concentrated geographic area in the United States in which to witness this firsthand is on Long Island, New York. Today, 18 of the 42 water districts in Nassau and Suffolk Counties (the two counties making up the heart of Long Island) use calcium hypochlorite as their primary drinking water disinfectant, and a number of other districts are currently transitioning to cal hypo.
The majority of these Long Island water districts that have switched to cal hypo have replaced their sodium hypochlorite systems with "spray technology" chlorinators that use dry solid calcium hypochlorite (Ca(OCl)2) briquettes containing a minimum of 65 percent available chlorine (AVCl2) by weight. The districts have found these systems provide chlorine solution in the desired range far more consistently than their former NaOCl systems, thereby allowing them to avoid a number of performance problems.
NaOCl Alternative Needed
One of the primary reasons for the earliest adoptions of calcium hypochlorite on Long Island was area safety. Nassau and Suffolk Counties are two of the most densely populated counties in the country. Here, a great concern exists among county and local leaders, and the general community, of tanker trucks filled with hazardous material driving busy roads and through residential neighborhoods.
Compounding this, New York State's requirements for chemical containment, delivery and off-loading of materials, such as commercial NaOCl, are some of the most stringent in the nation. Plus, because 15 percent sodium hypochlorite solution is deemed a hazardous substance under state regulations, Long Island water districts that use the chemical must address issues of secondary containment in bulk storage systems at all their sites.
With cal hypo use, restrictive chemical tank and transfer station requirements do not apply because chlorine solution less than 1 percent is not subject to New York State's stringent chemical bulk storage rule. Secondary containment is not required.
In the late 1990s and early 2000s, management at many Long Island water districts began observing the successful use of calcium hypochlorite for chlorination by several early adopters in the area. These early users were finding that, besides eliminating or significantly reducing stringent delivery, storage, and handling requirements, this alternative disinfection approach provided more effective treatment while eliminating many of the performance and maintenance problems associated with NaOCl use. Roslyn Water District, located in Nassau County, and South Huntington Water District, in Suffolk County, began switching from commercial bleach to calcium hypochlorite in 2001. Both water districts, like many of the other water districts on Long Island, have found it to be a better chlorination approach.
Roslyn Water District
The Roslyn Water District, established in 1910, is one of the oldest public water suppliers in Nassau County. Today, the district has 5,812 service connections and supplies drinking water to a population of approximately 17,000 within a 5.1 square-mile area; it's served by a transmission-distribution system of about 90 miles. The district's source of drinking water is groundwater drawn from seven individual wells and from one well field containing eight wells connected to a common suction pump. Maximum usage in 2004 topped 6.3 million gallons per day (mgd).
The district began chlorinating its water in 1999. According to Richard Passariello, water district superintendent, the use of sodium hypochlorite brought a number of problems and concerns almost immediately.
"First thing, we started getting a lot of taste complaints because the solution strength of the sodium hypochlorite often fluctuated widely," he said.
Commercial NaOCl begins to degrade as soon as it is made and, depending upon temperatures, can lose a significant amount of its strength in 30 days. This can produce undesirable byproducts, inconsistent residuals, and a need for constant dosing corrections.
"When the chlorine residual in our distribution system changed, that's when we would begin receiving a steady stream of customer complaints," Passariello said. "It was almost like clockwork."
The tendency of sodium hypochlorite to off-gas, a symptom of degradation, also created both performance and manpower difficulties for the district, according to Passariello. Off-gassing, a common problem with feeding commercial NaOCl, occurs when a pocket of vapor forms in the fluid line and obstructs the normal flow. For the district, off-gassing was air binding (or, "vapor locking") chemical metering pumps, resulting in unscheduled downtime and further chlorine residual fluctuations in the distribution system.
"Air-bound metering pumps were one of our biggest problems, from a performance- and labor-management standpoint," Passariello said. "When it happened, which was often, someone had to go to the site and bleed the pump. With our one well field and seven individual well sites located throughout the district, the unscheduled man hours required to address air-bound pumps really added up."
He related that operator safety was also an important concern with feeding sodium hypochlorite under the well sites' pressurized feed systems: "Our operators simply didn't like working with it."
South Huntington Water District
Seventeen miles away from Roslyn, the South Huntington Water District experienced similar problems with sodium hypochlorite use when it began full-time chlorination operations in 2002. The district has 21 wells in operation with two more under construction. The District has 17,500 service connections, supplies drinking water to a population of approximately 81,760 within a 22-square-mile area, and is served by a transmission-distribution system of about 255 miles of water main. The district pumps an average of 9 mgd, and on peak days pumps in excess of 25 mgd.
In August 2002, the district began chlorinating its water supply full-time as a precautionary measure after routine monitoring revealed positive samples for coliform bacteria in the distribution system. When the mandate for chlorination came, district well sites were already equipped with sodium hypochlorite storage and feed systems, installed as back-up protection in case of such an event. After chlorination operations had been underway for only a short time, however, district management began searching for an alternative approach due to many of the same problems with NaOCl that the Roslyn Water District had faced.
"Our biggest issue was inconsistent solution concentrations entering our distribution system," said Kevin Carroll, water district superintendent. "Dosage control required constant adjustments and, with 14 individual well sites to deal with, that took a lot of manpower. Once we began full-time chlorination we found we needed to devote two people exclusively, as well as a substantial portion of another person's time, to deal with it."
Switching to Cal Hypo Briquettes
When determining the best alternative to sodium hypochlorite use, both Carroll and Passariello said their districts were heavily influenced by the positive and well-documented experience of nearby Plainview Water District, which fully converted from sodium hypochlorite to a calcium hypochlorite briquette chlorinator system in 2001 after a comprehensive two-year study. In the study, the cal hypo briquette feed system maintained chlorine in the desired core range (0.5 to 0.7 mg/L (milligrams per liter) 91.1 percent of the time, compared to a composite frequency rate of only 70.8 percent using sodium hypochlorite.
South Huntington Water District began switching all their systems to calcium hypochlorite in late 2002, converting all 21 wells at the 14 well sites to the cal hypo briquette feed systems in about a year. The Roslyn Water District installed one unit as a trial in 2001, liked it, and began converting its wells to the system the following year.
Generating Hypochlorite Solution On Site
"By generating hypochlorite solution on site, we make it only as we need it," Passariello said. "This eliminates bulk solution storage and all the requirements and hassles that go along with that." The briquettes have up to a two-year shelf life, as opposed to commercial sodium hypochlorite, which can begin to show signs of degradation in fewer than 30 days. Plus, management at both the Roslyn and South Huntington Water Districts have found that bulk dry chemical delivery and storage are easy to handle and has alleviated numerous safety and liability concerns. As much as 200 pounds of briquettes can be stored at each well site.
Passariello and Carroll both reported that, due to the consistency of the chemical solution concentration, operator dosage adjustment is now minimal. And, because calcium hypochlorite solutions from the feed systems are stable, chemical metering pumps do not become air-bound. This has saved significant manpower for both districts by no longer having to make numerous unscheduled site visits to adjust chemical feed or bleed air-bound chemical metering pumps. In addition, Passariello reports that water taste complaints virtually disappeared after switching to calcium hypochlorite because chlorine residual levels no longer widely fluctuate.
Carroll especially likes how easy it was to install the systems at the South Huntington Water District well sites: "Chief Plant Operator Mike Feeney and his crew installed 10 of the units ourselves (sic). After a feeder was set in place, a water connection was made using flexible tubing and then the unit was plugged into an 110-Volt, 20-Amp circuit. We used our existing automatic chemfeed routers, and once the existing injector was replaced with one for delivering calcium hypochlorite solution, it was ready to go."
The footprint of the feed system is only 9 square feet, and bulk liquid storage or containment is no longer required at the Roslyn and South Huntington Water Districts well sites. Plus, on an equivalent AvCl basis, only 1/6 the space is required to store briquettes on site, compared to that required for commercial bleach. This has provided valuable new storage capacity for both districts.
A Better Alternative
Passariello says that operators at the Roslyn Water District are very pleased with the switch from a safety and handling standpoint.
"The dry chemical is easier to handle, and based on the lower solution strength, it provides for reduced potential hazards to plant personnel, compared to our former sodium hypochlorite systems. It's a much better system -- easier to work with, less aggressive, less offensive, and the accuracy in chlorination is far superior."
Although commercial bleach costs less than calcium hypochlorite briquettes, both Carroll and Passariello say that, from a cost standpoint, the reduction in overtime alone justified the switch to calcium hypochlorite. And, when considering other cost factors, including the penalty for NaOCl degradation, pump system failure, safety, liability, and regulatory compliance considerations and costs, the higher up-front cost of the dry chemical over commercial bleach is more than mitigated.
"For us, the ability of our calcium hypochlorite systems to maintain a steady residual has been the key advantage," Carroll said. Passariello agreed: "The number one reason we switched was to maintain a steady residual in our distribution system, which eliminated customer taste complaints. The next reason was operator safety, and the third was to achieve more efficient utilization of manpower."
System In Action
The feed system, together with specially designed briquettes,
works with the natural properties of calcium hypochlorite to achieve solution
consistency within 0.1- to 0.2-percent of the desired core range at all
times -- significantly more consistent than either of the two water districts'
NaOCl systems. The system operation is straightforward: briquettes are scooped
into the chlorinator's hopper, and the unit's spray manifold utilizes supply
water to dissolve the highly soluble briquettes and prepare a fresh, relatively
diluted chlorinating solution. The solution falls into the unit's discharge
tank and is then pumped on demand to the well's discharge main through a
chemical metering pump.
This article originally appeared in the 07/01/2006 issue of Environmental Protection.
James P. Brennan is a technology manager with Arch Chemicals Inc., Smyrna, Georgia. He has been working extensively with water chlorination chemistry for the past 30 years. He can be contacted at (423) 780-2007.