By Gerald F. Connell, Consultant, Capital Controls Co., Inc., Colmar,
PA, and
Ahron Nahmias, Metcon Sales & Engineering, Concord, ON
Chlorination has been said by many to be the most important public health accomplishment in the last century for its role in preventing major water-borne diseases in North America. Chlorine gas has had a long history of successful use in water treatment. For small water systems, chlorine gas has long been the primary disinfectant source because of its low cost, simple operation and stability. Since the majority of small water systems use a groundwater source from wells where the water supply is relatively clean, only disinfection with chlorine is required to provide a potable water supply to the distribution system.
The recent occurrence of bacterial outbreaks in groundwater systems can be eliminated with the use of chlorine for well water treatment at the well head. The operation of chlorine gas systems, properly controlled, safely installed and continuously monitored, offer the user an effective, simple and economical method of disinfection and is effective against bacteria such as E. coli.
Chlorine Requirement
The chlorine requirement must be established to meet the disinfection needs of each well or underground water source. Most well sources will have a chlorine demand consisting of dissolved metals such as iron and manganese, perhaps some organic demand from colour or taste, perhaps an odour from sulphides and perhaps a bacteria demand. The total demand from any or all of these sources may be established in the utility's laboratory or in an independent laboratory. Once the demand is determined and the residual requirements, governed by local regulations, are known, the dosage is established by adding the demand and the residual.
The quantity of chlorine required is determined quickly and easily with a simple calculation. The formula used is:
M3/h x g/M3 = g/h,
where
M3/h = water flow in cubic metres per hour,
g/M3 = chlorine dose in grams per cubic metre (numerically the same as milligrams per litre (mg/L)),
g/h = chlorine gas feed requirements in grams per hour.
So, assuming a well has a pumping rate of 400 M3/h and requires a dosage of 1 g/M3 or 1 mg/L, the chlorine feed rate is 400 grams per hour.
A 400 gram per hour continuous feed rate will be available from a cylinder of chlorine containing 68 Kg. This cylinder will last seven days before requiring replacement, assuming 24 hour operation of the well pump. In most cases, a well pump operates only when the water demand in the distribution system calls for more water. Well pumps normally operate intermittently, controlled by a level in a head tank or system pressure. A chlorine cylinder used under these conditions will be in use for well over seven days at a 400 gram per hour intermittent feed rate.
Most well systems are provided with 100% standby through the use of an automatic switchover system. This provides for continuous availability of chlorine by switching to a standby cylinder when the in-use cylinder is empty. Operating personnel need not be on-site 24 hours per day nor on weekends, and they can replace the empty cylinder during normal working hours.
This article was abridged from Environmental Science & Engineering magazine, which also contains many more articles not posted on our Web Site. See our home page on how to order your subscription. We regret we can only accept orders from Canada and the United States.