Sodium Hypochlorite vs calcium hypochlorite for water dist.

[Fisher31]

Besides the obvious physical differences with sodium hypochlorite and calcium hypochlorite what are the benefits and what is typically recommended for a small water distribution system of about 2 MGD. Is cost a major offset to one or the other?

 

[semo]

Either one works. My clients seem to have better luck with the sodium hypochlorite as it is delivered in a liquid form. I've had some start with calcium hypochlorite; but change over because of the maintenance involved with preparing the solution and clogging the pump suction with undissolved particles.

Because the pumping equipment is the same, it really doesn't matter which one you use. The only cost difference will be the chemical. I've not seen a large cost difference; but, get some prices and see what it is in your area. The sodium hypochlorite can be delivered in different percentage solutions and the costs will be different; but, remember most of the solution will be water and the weaker the soln, the more water you are paying for.

You need to make sure you are looking at equivalent doses of chlorine to make an equal cost comparison.

 

[ch81pc]

Calcium hypochlorite will add hardness to the water which may be undesirable. As mentioned it can be difficult to prepare and dose as it is supplied in solid form. We use sodium hypochlorite at all of our 100+ smaller sites (<10 MGD) without any significant problem. The only problems we get are dosing lances becoming "furred" up with calcium carbonate and the fact that hypo looses its strength on exposure to higher temperatures and sun light. We have over come these by changing the lances regularly and storing a maximum of two weeks supply of hypo at any one site. Hope this is usefull

 

[sbush]

There are distinct advantages for using calcium hypochlorite over sodium hypochlorite, when you are able to use a precise tablet erosion system such as PPG's Accu-Tab system.

The PPG erosion technology uses a unique chlorinator (vat) with a see-through cover to store 3" diameter tablets of 65% calcium hypochlorite. The tablets sit on a perforated grid suspended above a cylindercal wier. Incoming water from a side stream contacts only the tablets at the bottom of the chlorinator feeder vat, so tablets at the top stay dry. No premature dissolving. The tablets erode at a predictable rate according to the amount of water that enters the chlorinator. By controlling the flow rate, highly accurate chlorine dosage can be acheived. Chlorinator effluent is returned to the unchlorinated main stream via a pump (or by gravity, if the chlorinator can be mounted above an atmospheric discharge point such as an open storage tank). PPG's Accu-Tab tablets have a "built-in" scale inhibitor to prevent calcium scale from forming on any of the wetted surfaces of the chlorinator.

The advantages are:

Dry tablets are easy to handle, no spills, no spashing.

The amount of calcium added to water is not significant and it tends to be neutral to pH as opposed to bleach which tends to raise pH.

Tablets come is convenient 55-lb pails where as bleach is stored in drums or bulk tanks.

Tablets maintain there chlorine strength (65%) for years where as bleach can see significant loss in a week.

55-lb pails stack three high, same space as 150-lb gas cylinder, where as bleach drums or bulk tanks require space and containment pad.

The PPG Accu-Tab system has no moving parts in the chlorinator itself where as the bleach uses troublesome metering pumps that require frequent maintenance.

The cost for a PPG Accu-Tab system with manual dosage controls for a 2 MGD plant trying to maintain a 1 ppm free chlorine residual is $6,000 and has a foot print of 24" X 41" X 50" high with 75-lb of tablet storage capacity. For a gravity system, the cost is $2,500 and the foot print is 13" X 20" X 26" high.

 

[countryham]

The subject has been pretty well covered here. I have been mulling over these ideas for many years and I have never seen anyone address the improper use of available sodium hydrosulfite sources to chlorinate potable (drinking water). I don't think that anyone has addressed the fact that sodium hydrosulfite is common commerically sourced and consumed both in supermarket containers as well as bulk for potable uses.No one has ever questioned where each shipment of chlorine originates or where the caustic soda originates....(No Sampling?-No purity standards?)The cheapest source would come from by-products and I doubt that all by-products could be assumed CP (chemically pure). So,-if not CP , How CP are they??????

 

[hydrae]

Countryham

Some states require the chemicals used to treat drinking water have a NSF 60 certification, the certification process is conserative and is not easy to get. While the chemical is not required to be chemically pure, the imputities are monitored to ensure to product is safe to use at registered levels.
An example, I use NaOCl 12.5% from 15 gallon drums (in a small surface water plant I operate). The max dose for treatment is 64 PPM and is listed on the label (the max dose looks high but treatment for hydrogen sulfite consumes at an 8 to 1 ratio) part of shipment receipt is verify the NSF 60 label. Non NSF product is available from the same distributer at a lower price.

In another application, my client is having bulk Aqua Ammonia shipped from the nearest source of certified product 750 miles away in a truck that is used only for NSF 60 Aqua Ammonia. Non NSF product is available locally, but use of the local product would require a sample plan to check for purity and justification to the State.

In Calcium Hypochlorite, the NSF 60 product is more expensive than the Non NSF 60 product as seen in water product catalogs such as USAbluebook

Hydrae

 

[mec01]

I've used both sodium and calcium hypochlorite for small systems. For places that are remote, I prefer calcium since the shipping cost is lower and the product is more concentrated. However, the clogging issues are a pain and storage of the product must be considered.

Sodium hypochlorite can be shipped in large vats. It is simple and care free to use. I prefer it for places that the shipping costs are low.

Lastly, I've also used onsite generation. There are numerous manufactures that made sodium hypochlorite generators. They aren't cheap, but they are nice to use. The main requirements for onsite generation is soft, warm water (over 50 deg f), and a clean abundant supply of softener salt.

 

[sbush]

Regarding mec01's comment about clogging, this is a common problem among most brands of calcium hypochlorite tablets. However, the Accu-Tab tablet manufactured by PPG Industries is the only one on the market today that has a built-in scale inhibitor. The patented 3-1/2" diameter Accu-Tab tablet is also compressed so tightly in the manufacturing process that it doesn't break apart like the Arch and other cheaper brands. With calcium hypochlorite you don't want to cut corners on quality. You'll only get what you paid for.

PPG uses nationally recognized distributors such as Vopak, Brenntag, and many others to stock and distribute the Accu-Tab tablets. So, you don't have to stock large quantities of tablets. Instead, you can simply rely on just-in-time deliveries from a local distributor.

S. Bush

http://www.water-eg.com

 

[CindyL]

Hi,
I am replying to an old thread here. I found this thread very interesting...for us we have to use Calcium because the need to add the chemical is so seldom that the sodium would always be innefective.

How does one figure out concentration of calcium hypochlorite addition to a water treatment system? Is there a book/method/site/formula?

Thanks in advance.
C

 

[BobPE]

CindyL: It would seem to me that if you were requiring large amounts of chlorine, sodium hypo would be the cheaper and by far the more effective chemical.

One of the best ways I use to determine dosage is to jar test you actual water at various times of the year. Books only get you the math, they cannot get you past your system requirements.

Give us some more info on what your system is like and I am sure you will get some more opinions....

BobPE

 

[CindyL]

Thanks,
The paper mill will not use sodium no matter how much cheaper because it is a large amount but not often. I will have the information on my desk by Friday or Monday morning, so you'll hear from me then. I only know the description of my task to be.
C

 

[sbush]

CindyL,

Chlorine is available in different forms. The most common includes gas, liquid chlorine (sodium hypochlorite or bleach) and solid calcium hypochlorite. All three generate hypochlorous acid--the germ killing form of chlorine--when dissolved in water.

If you're already using some type of chlorine, then you probably know what your chlorine demand is. Anything that uses up chlorine such as, iron, manganese, sulfide, bacteria, BOD, constitutes a demand component. Chlorine residual is the amount of free chlorine available for protection of the water after treatment. Therefore, total dose = chlorine demand + chlorine residual.

Once you know how many pounds per day of chlorine is required or the "total dose" from the formula above, then sizing the chlorinator device is easy since 1 pound of chlorine is the same regardless of whether it started out as a gas, liquid or solid. Gas chlorine contains 100 percent of available chlorine, so a 150 pound cylinder of gas will deliver up to 150 pounds of chlorine before it is empty. Bleach contains 12 percent available chlorine (when it is fresh), therefore 100 pounds (or approx. 12 gallons) of bleach contains 12 pounds of available chlorine. Calcium hypochlorite tablets contain 65 percent available chlorine, so 100 pounds of tablets will deliver 65 pounds of chlorine.

The Accu-Tab system by PPG Industries comes in a wide variety of sizes from less than 1 pound per day to more than 1,000 pounds per day. They offer two installation options: (a) a gravity return system is recommended when the chlorine effluent is to be returned to the main water stream in an open tank, channel or filter, and when the chlorinator can be physically located at an elevation higher than the re-injection point; (b) a pressure return system is recommended when the chlorinator effluent must be directed to multiple locations, or any time the chlorinate stream is to re-injected to a pressurized tank or line.

The chlorinator is designed to dissolve the dry tablets using a side-stream from the main water supply which comes into contact with only the bottom layer of tablets which are sitting on a perforated support grid within a cylindrical storage hopper. When water comes in contact with the tablets, they dissolve and the concentrated liquid is re-injected back into the main water line. As tablet dissolve, the level of dry tablets in the hopper will drop and more tablets are added through the removable/see-through cover over the hopper. A throttling valve and rotameter are used to adjust the flow rate through the chlorinator which, in turn, controls the pounds per day of chlorine being fed.

The tablets contain an integral scale inhibitor suitable for water with up to 500 ppm calcium. The tablets are 3" diameter and come in 55-pound plastic pails as well as 400-pound bulk bags. Accu-Tab tablets are NSF-61 listed for drinking water.

The Accu-Tab tablets are formulated and compressed in a patented process. This unique product dissolves in water at a very precise and predictable manner. This is crucial for accuracy and maintaining a relatively constant chlorine feed rate. Beware: not all calcium hypochlorite tablets dissolve with the same accuracy and predictablity as the PPG Accu-Tab product.

I hope this information is helpful.

S. Bush

http://www.water-eg.com

 

[CindyL]

Thanks! Your answers really helped me grasp the scope. I am going to include more detail below.

Activated Sludge Treatment System
Calcium Hypochlorite needed for Intermittent feed to suction of RAS pump to control bulking.


Sodium Hypochlorite Solution Parameters that need to be converted to Calcium
1.5 g CL2/100 g
1.9% (g/100 ml)
1.26 grams/L

Dosing 7 kg Cl2/t-d
2-10 kg Cl2/t-d

Average Consumption 980 kg CL2/d
5.16 m3 sol/d

2 Metering Pumps running 1-10 l/min (already purchased for sodium, would like to use same for Calcium)

Existing Equipment the mill would like to use and therefore order calcium in granular form, not tabular (this is not drinking water). The following equipment was used for Urea, DAP, Caustic.(old equipment, the old way).
Big Bag Unloader
1.5hp/1800 rpm vibrator
Feeder (with indicator)
This is standard “Acrison” Equipment
Screw Conveyor. (0.75 hp)
6” NPS Sch 10 gravity piping to a mixing tank (6’-6”H X 4dia)
They would buy a new storage tank (size?)
And dosing pumps that were bought for Sodium they would like to re-use


If anyone’s interested in commenting on my task at hand, it would much appreciated.

C

 

[sbush]

CindyL,

Granular calcium hypochlorite contains 65 percent of available chlorine, just like tablets. Therefore, if you are currently using sodium hypochlorite (12%), then you can make an equivalent stock solution by mixing 1.538 pounds of granular calcium hypochlorite to each gallon of water in the stock solution tank.

Since you mentioned earlier that you seldom add chemical, be aware that, once dry calcium hypochorite is mixed in water, it is subject to the same degradation due to time and temperature as sodium hypochlorite.

Unfortunately, you're saddled with converting an existing liquid system. The true advantage of using calcium hypochlorite is that it doesn't degrade with a dry storage feeder.

S. Bush

http://www.water-eg.com

 

[CindyL]

Has anyone used Cal Hypo instead of Sodium Hypo for Waste Water Treatment application.
We need to add a 15% solution of Chlorine at a worst case of 150 tons per day but this will only happen once a year or so. They do not want sodium hypo losing stregnth in their storeroom.
The solution will be sent to the suction of the pumps (recirculating sludge) and then to the plant when they have 'floaters' that they have to get rid. They are treating 7000 gal/min. So as little as they need to use it, they need a lot.
I'm thinking that it cannot be done!
Any comments?
Cindy