WATER SUPPLY/Plant upgrade eliminates gaseous chlorine
Hickory, N.C., is eliminating the use of gaseous chlorine in its water treatment process and replacing the chemical with sodium hypochlorite (NaOCl) generated on site. The move will help the city reduce costs of regulatory compliance and chemical storage and handling; and it will alleviate public concerns about potential chlorine leaks.
As an economic and social hub in northwestern North Carolina, Hickory began upgrading its water treatment system in the 1980s to make it a state-of-the-art regional resource. The system treats 12 million gallons of potable water per day, and it has a capacity of 32 million gallons per day. As it expands to meet growing demand, so too does the city’s need for handling, storing and applying disinfecting chemicals.
Currently, Hickory uses chlorine gas for disinfection, consuming approximately 650 pounds per day in summer peak periods and approximately 340 pounds per day the rest of the year. Three-fourths of the gas is injected at a single pre-chlorination point in a flash mixer, with the remainder injected at two post-chlorination points in a filter piping gallery.
Because chlorine is hazardous, municipal systems that use the chemical are subject to a variety of federal handling, storage, monitoring, safety and application requirements. The costs associated with meeting those mandates, as well as safety concerns for city personnel and the public, prompted Hickory to consider alternatives to gaseous chlorine. The Public Services Department recommended on-site NaOCl generation.
The generation process involves running a low-voltage current through a diluted solar salt brine in a cell, with the electrolytic reaction producing NaOCl and hydrogen. The hydrogen is vented safely into the atmosphere, and the mild (0.8 percent) bleach form of chlorine that remains is injected in the water supply in the same manner as the gaseous chlorine. (The typical, approved method of generating NaOCl on site uses 2.5 kilowatt hours of electricity (AC) reacting with 3.5 pounds of solar salt dissolved in 15 gallons of water to produce one pound of equivalent chlorine.)
In 1999, Hickory officials hired Spartanburg, S.C.-based Hayes, Seay, Mattern & Mattern to prepare a preliminary engineering report for the project. Later, the company prepared a design and bid specifications package for replacing the gaseous chlorine system.
The system design — which incorporates the Clor-Tec NaOCl generator from Fort Washington, Pa.-based Severn Trent Services — received state approval last November. Hickory is issuing a request for bids this month, and the city expects to start construction next month. From design to installation, the project will cost an estimated $600,000.
When Hickory’s project goes online in March 2003, the city will become the first in North Carolina to generate NaOCl on site. It also will join the more than 1,500 U.S. municipal treatment facilities that have converted to on-site systems. (Most of the others are in western states, where chlorine is regulated more heavily than it is elsewhere, but the technology is spreading to the East. For example, in Florida alone, more than 60 plants have converted to on-site NaOCl generation since 1995.)
In addition to being safer than gaseous chlorine, the bleach solution produced by the on-site generation process is not subject to the many stringent regulations of USEPA and OSHA. Over a 20-year lifecycle, costs associated with on-site generation — i.e., electric and startup expenses — average a little more than the chemical and operating costs for a gaseous chlorine system. However, chemical costs are lowered with on-site generation, as are maintenance costs; the NaOCl maintains a long residual in the distribution system, reducing the frequency of system “flushes” to purge latent bacteria.
Ñ Gary McGee, city manager, Hickory, N.C.