No Perfect Chlorination
While there are many forms of disinfection, by far the predominant one is chlorination. When dealing with chlorination, there are five (5) common alternatives: Gas, Sodium Hypochlorite (Bleach), Calcium Hypochlorite (powder or tablets), Trichloro-s-triazinetrione (commonly called TriChlor) (tablets), and On-Site Generation of bleach and/or mixed oxidants. Each of these methods of chlorination has advantages and disadvantages. Unfortunately, this means that there is no perfect form of chlorination. At Chemical Feeding Technologies, Inc., we handle all of the above methods of chlorination. With 36 years of experience, we are able to explain the advantages and disadvantages of each form of chlorination to allow a customer to decide which method of chlorination is the best for them.
When chlorine is mixed with water, it produces hydrochloric (HCl) and hypochlorous (HOCl) acids. The rate of formation of HCl and HOCl is pH dependent and the HOCl is the stronger disinfectant.
When selecting a chlorination method, a key item to consider is if you have hard water (Calcium carbonate in your water). When you raise the pH of hard water, calcium carbonate will precipitate, or fall out of solution. This calcium carbonate is a white floc that starts out as a soft sludge but will eventually harden like concrete.
Gas Chlorination
Chlorine gas is 100% pure chlorine and does not degrade over time. It is supplied to users as a compressed liquefied gas. Since mixing chlorine gas with water produces an acidic solution, there is no effect on calcium carbonate in the water. For this reason, gas chlorination systems tend to be very low maintenance.
Because of the potential for chlorine gas leaks, a gas chlorination system should be supplied with a variety of safety devices, such as chlorine gas leak detectors and automatic chlorine cylinder shut-off systems. Because of this, gas chlorination systems will have a high capital cost. In most situations, the low chemical cost per pound of chlorine gas provides the lowest operating cost.
Bleach
Bleach is produced by bubbling chlorine gas through 50% sodium hydroxide (caustic soda). While chlorine gas does not like to be in solution, more can be dissolved into solutions with higher pH values. Since caustic soda is pH 14, it will hold up to 17% chlorine concentration. However, this bleach solution is very unstable and chlorine gas will come out of solution over time, depending on temperature and agitation. While the bleach is produced at 17% concentration, it is sold as 12.5% concentration. If you get a fresh batch of bleach, you might actually have 15+% chlorine. However, since the bleach manufacturers do not know how long it will be before the bleach will be delivered to you, they sell it as 12.5% concentration. The degradation of the bleach will continue after you have received it and you need to make sure that you do not order too much bleach at one time. The chlorine gas that comes out of solution is disinfection that you paid for that you will not be able to use.
If you are chlorinating a small well, the 12.5% bleach will be too strong to pump reliably and you will need to dilute it to 1% or 2% concentration.
Because of the high pH of bleach, calcium carbonate will precipitate from hard water where the bleach contacts the water. If you are pumping 12.5% bleach, this will be at the point of injection. If you are diluting the bleach with hard water, the calcium carbonate will start forming in the dilution tank and carry through your metering pump and to the point of injection. Bleach systems on small wells tend to be very high maintenance.
The capital cost for a bleach system is low, a pump and a tank. The operating cost of a bleach system will be 2 – 3 times the cost of a gas system.
Calcium Hypochlorite Tablets
Cal Hypo is produced by mixing chlorine gas with binding agents which are then dried into a powder. The powder is then compressed into a tablet. The cal hypo tablet will typically be 65% chlorine and 35% binding agents. When you dissolve the cal hypo tablet, it produces a chlorine solution typically less than 1% in concentration and at a high pH.
With the high pH, calcium carbonate will start precipitating in the tablet dissolving tank. Also, not all of the binding agents will dissolve, so you will also have a sludge being produced from the undissolved binding agents. This creates a very high maintenance system.
Cal hypo systems can have a low capital cost (depending on brand of tablet feeders) but have a high operating cost, 3 – 4 times the cost of a gas system.
TriChlor
TriChlor is produced by mixing chlorine gas with isocyanurates which is dried into a powder and then compressed into tablets. The tablets are 90% chlorine. When you dissolve the TriChlor tablet it produces a chlorine solution less than 1% in concentration with a low pH (acidic). The 10% binders all dissolve and the acidic solution does not cause calcium carbonate to precipitate. This creates a very low maintenance system.
TriChlor systems have a low capital cost with a high operating cost, about 2 – 3 times the cost of a gas system.
On-Site Generators
On-site generators (OSG) produce 0.8% concentration bleach, or 0.4% concentration bleach with mixed oxidants, from salt solution (Salt + electricity -> bleach + caustic soda). The water used in the OSG system is first softened so there is no calcium carbonate in the bleach solution. The production of caustic soda is minimized and produces a solution pH of approximately 9, so there is minimal effect on the calcium carbonate where the solution is injected into the well water.
Early OSG systems required significant operator maintenance. If maintenance was not performed it had negative effects on the operation of the OSG systems. Current OSG systems are much more automated and require very little operator intervention. Because the solutions that are being produced are less than 1% concentration, the solutions are considered non-hazardous.
OSG systems have a high capital cost with an operating cost about the same as chlorine gas.
Summary:
As you evaluate various forms of chlorination, you need to think about the variables and which ones are most important to you: Capital cost, Operating cost, Maintenance, Safety? As mentioned earlier, chlorine gas is perceived as being dangerous. However, throughout the years it has been shown that 60 – 70% of all reported chlorine accidents are actually caused by bleach and cal hypo.
Pro’s Con’s of Chlorination Methods
Summary of Pros and Cons of various chlorination methods
|
Pro |
Con |
Cl2 Gas |
- Low operating cost.
- Produces acidic solution so calcium carbonate does not form.
- Easy maintenance.
|
- High capital cost
- Perceived to be unsafe.
|
Bleach |
- Low capital cost.
- Perceived to be safe.
|
- High operating cost
- Caustic solution causes calcium carbonate to precipitate.
- Difficult maintenance
|
Cal Hypo Tablets |
- Low capital cost.
- Perceived to be safe.
|
- Highest operating cost.
- Produces caustic solution that causes calcium carbonate to precipitate.
- With 35% chemical binders, not all of the binders dissolve and form sludge along with calcium carbonate.
- Difficult maintenance.
|
TriChor Tablets |
- Low capital cost
- Perceived to be safe.
- Produces acidic solution so calciumcarbonate does not precipitate.
- With only 10% chemical binders all of thebinders do dissolve into solution.
- Easy maintenance.
|
- High operating cost.
|
On-Site Generation |
- Low operating cost.
- Solution is only slightly caustic so no noticeable precipitation of calcium carbonate.
- Easy maintenance.
|
- High capital cost.
|
Notes:
- Even though chlorine gas is perceived to be unsafe, 60 – 70% of all reported chlorine accidents involve either bleach or calcium hypochlorite.
- Relative operating costs, low to high: Cl2 gas/OSG, Bleach, TriChlor, Cal Hypo