Hg in sodium hypochlorite

[PEorl]

I was doing some research and found that sodium hypochlorite has mercury in it. The manufacturers are saying there is no mercury in their product, it is undetectable when sampled for. However, newer test methods are out that are more sensitive and mercury has been found.

Should we be worried about the mercury levels in sodium hypochlorite, since many plants are converting from chlorine gas? Are we poisioning the water we are drinking, since mercury bioaccumulates, or is this trace amount not a big deal? Thanks for any opinions! (more information below)......

According to the manufacturer they have no mercury in their product, however, that was not based on the newer EPA Method 1631 which allows for a method detection limit of 0.0002 micrograms/L (required for sampling discharges into Class III marine waterbodies) Samples of the sodium hypochlorite utilized at the WWTP plant ranged from undetectable to 1,400 ng/L. (it was also determined that alum had levels of mercury in it as well, previously undetectable. Other possible sources such as atmospheric, etc were sampled and negligable. The problem was through the mercury in chemical addition).

My question then is posed. Is sodium hypochlorite instead of chlorine gas such a good idea?

 

[bimr]

If the chlor-alkali production process that uses the older mercury cell technology is still being used, perhaps that may be a source of mercury.

In any case, the amount of mercury is minute. You are feeding maybe 2 parts per million (2 mg/1000000 mg) of water of a hypochlorite solution that contains only 2.0e-7 milligram/liter. Multiply those two values together and that’s a lot of zeros. It is not possible to measure such a low concentration.

The supplier listed below has no mercury:

http://www.odysseymanufacturing.com/images/odyssey_bleach_plant_process.jpg

Perhaps some suppliers may be using the threat of mercury as a scare tactic to obtain new business.

Suggest that you specify that the hypochlorite should be mercury free.

Suggest that you specify that the hypochlorite should be mercury free.

 

[PEorl]

Then you would be interested to hear that the sampling was done from this manufacturer's product.

I talked to the manufacturer and obtained there MSDS sheet, and a list of the testing methods and results.....no mercury. However, it was determined by accident, that there was mercury. Because an NPDES permit requires a test method for discharge into a class III marine water body that is more sensitive (EPA Method 1631). This method was used for all samples taken at the WWTP, including the sodium hypochlorite at the WWTP. (The discharge limit of mercury into class III marine water body is 25 ng/L). Not a large number of samples were taken. The concentrations ranged from undetectable to 1,400 ng/L. The product was not always "mercury free".

 

[bimr]

It seems that you should start a quality control check on the manufacturer’s product. Such a quality check is more common in the chemical industry than the wastewater industry. Start out sampling each truckload or batch. Later you may just do random sampling.

If you actually have a concentration of 1,400 ng/l mercury in a sample of hypochlorite, than something is really wrong.

 

[PEorl]

The facility we are discussing is under Administrative Order and recommendations are being made. Timed sampling was completed at various locations within the WRF - influent, process treatment units and effluent, for seven separate sampling events. The timing of sample collection for each sample event was designed to approximate the movement of an incremental volume of ww through the WRF based on calculated hydraulic detention time of each unit process. I have attached a graphic from the report and would include more of the report (it has been submitted to the regulatory agency) but am having trouble with uploading files.

My concern is that the engineering community is supporting the switch from chlorine gas, which is as "mercury free" as one could possibly get on a consistent basis, and is cheaper. Compare that to sodium hypochlorite, which based on limited sampling with the more sensitive test method had low levels of mercury in it. I am wondering if this is what the engineering community should be supporting. Is it safer to drink water with the possibility of mercury at these low levels, or the threat of chlorine gas being dispersed?

 

[bimr]

Mercury sources include crop runoff, natural deposits, batteries, and electrical switches. In addition, mercury detection equipment has been developed so that extremely low quantities can be detected.

Locating the source of mercury contamination is usually quite difficult because you are trying to find an extremely small quantity and the mercury typically might lay in pipe sediment until disturbed by a flow change.

You are also dealing with a situation where the facilities to produce and transport your raw materials are not dedicated to your use. You may be picking up mercury contamination in the transport truck; the contamination being sourced from the previously transported commodity.

Your answer would have to come in the preparation of a study by an epidemiologist. However, I would hazard to guess that chlorine would be more of a problem. What is worse, the exposure to a catastrophic leakage from a concentrated chemical or exposure to the trace of a chemical?

By the way, have you considered the use of UV oxidation in place of chlorine? It seems like it would be more appropriate for your application.

 

[PEorl]

The price tag for the UV system was quoted at around $2 million in late 2007. That is not accounting for operating costs. Alot of money for a smaller city.