Chemical Dosing for Dechlorination

Introduction

Many consider chlorination, through OSEC, chlorine gas dosing or liquid sodium hypochlorite dosing as an essential component of water treatment systems for inlet fouling control as well as tertiary disinfection of final product water. However, in some cases, dechlorination is also a required treatment step within a water treatment process for which the chemistry may not often be considered.

Dechlorination is the process of removing either all or a specified percentage of total chlorine residual within a potable or wastewater stream. Within potable water applications, dechlorination is used to reduce the chlorine concentration to an acceptable level before distribution to the wider network. In some extreme cases where taste and odour are a significant issue, total dechlorination is carried out followed by rechlorination at a lower level. This removes the taste-producing nuisance residuals and stops any formation of trichloramine in the distribution network. Wastewaters (i.e., such as those from power plant cooling towers) are dechlorinated prior to disposal to eliminate chlorine residuals which are toxic to aquatic life in the receiving waters.

Dechlorination Chemistry

There are generally two accepted methods of chemical dechlorination – via the use of either sulphur dioxide gas (SO2) or by the addition of liquid-based sulphite compounds such as sodium bisulphite or sodium metabisulphite, often abbreviated to SMB. Both are considered here in terms of their reaction chemistry, but this article will go on to consider sulphite chemical dosing in more detail.

Sulphur Dioxide

The reactions involved in dechlorination utilising sulphur dioxide are shown below. The first reaction is to create an aqueous stream of sulphurous acid by dissolving sulphur dioxide gas in water:

The produced sulphurous acid then reacts with chlorine residuals as follows:

The above equations illustrate that all chlorine species can be dechlorinated by the addition of sulphur dioxide gas and the produced acids can affect the alkalinity and possibly the pH of the treated water. It is important to note that for each part of chlorine removed, 2.8 mg/l alkalinity as CaCO3 is consumed in the process [1]. However, it is generally accepted that most potable and wastewaters have sufficient alkalinity available and so there is no cause for concern regarding lowering pH with sulphur dioxide addition.

There has been some concern about the fact that excess sulphur dioxide gas may consume a significant volume of dissolved oxygen in receiving waters downstream of a wastewater discharge that been dechlorinated using the above method. The reaction between the produced excess sulphite ion from dechlorination and the dissolved oxygen is:

From the above equation, four parts of SO2 are required to remove one part of dissolved oxygen. Due to kinetic limitations, there is never sufficient time to complete the above reaction at the required concentrations. For example, Chen and Gan discovered that even with an excess sulphite concentration of 2.6 mg/l in plant effluent, this had no effect on effluent pH [3]. To further complicate matters, the dissolved oxygen concentration increased from 4.8 to 6.1 mg/l following dechlorination and was determined to be due to turbulence caused by mechanical mixers in the process. There is no empirical data available to demonstrate that sulphur-based compounds used for dechlorination can affect dissolved oxygen concentrations or pH in receiving waters or indeed in the dechlorinated effluent prior to discharge.

Sulphite Compounds

Sulphite compounds are always used as aqueous solutions, mainly within installations where sulphur dioxide is not practical or where storage of sulphur dioxide gas is considered as an unacceptable hazard. The aqueous sulphite solutions are applied to the process using metering pumps, usually digital stepper motor driven diaphragm pumps. Dosing control and process monitoring of such dosing systems is much more complex and requires more instrumentation than that used for sulphur dioxide dosing systems.

Four sulphur compounds are considered as alternative chemicals to sulphur dioxide for dechlorination: sodium sulphite, sodium bisulphite, sodium metabisulphite and sodium thiosulphate. Sodium sulphite is only available as a white powder or crystals and is extremely difficult to handle as it is strongly hygroscopic and is therefore discounted as a dechlorinating agent. Sodium thiosulphate is slow to react with chlorine, therefore is not amenable to metering applications and is almost always considered for use as a laboratory agent. Thus, only sodium bisulphite and sodium metabisulphite are considered practical for use in chemical dosing systems required for dechlorination.

Sodium bisulphite is a white powder purchased as aqueous solutions up to 44% w/w concentration with a solubility of 515 g/l at 20 degC. It can be handled either in stainless steel, PVC or fibreglass storage tanks and is usually always metered using diaphragm type dosing pumps. Sodium metabisulphite, also known as sodium pyrosulphite, is a cream-coloured powder which is readily soluble in aqueous solution (650 g/l at 20 degC) and is available in various aqueous concentrations from vendors.

The reaction between sodium bisulphite and chlorine is as follows:

Utilising a 38% w/w aqueous solution of sodium bisulphite, each part of chlorine required 1.46 parts of sodium bisulphite for removal [1]. In addition, for each part chlorine removed, 1.38 parts alkalinity as CaCO3 will be consumed in the chemistry.

The reaction between sodium metabisulphite and chlorine is as follows:

From the above equation, each part of chlorine residual required 1.34 parts of sodium metabisulphite and for each part of chlorine removed, 1.38 parts alkalinity as CaCO3 is consumed which is similar to sodium bisulphite [1].

Sodium thiosulphate is generally deployed as a laboratory dechlorinating agent immediately prior to coliform concentration determinations. It is not used on a plant scale for dechlorination because it: a) reacts with chlorine in a step-wise fashion, thereby unacceptably increasing the reaction time, b) its reaction is stoichiometric only at pH2 and c) it’s ability to remove chlorine varies greatly with pH. The reaction is described as follows:

Conclusion

From the above, it is obvious that sodium metabisulphite is the aqueous sulphite chemical of choice for practical dechlorination of clean and/or waste water streams. pH is largely unaffected by this method of dechlorination and is readily achieved using standard chemical dosing skid packages.

Ross-Shire Engineering (RSE) are the UK’s leading specialists in the off-site design, build, installation and commissioning of chemical dosing systems for clean and wastewater applications both for municipal and industrial water treatment processes.

For further information regarding our chemical dosing systems, please contact: [email protected]

References

[1] Black and Veatch Corporation, White’s Handbook of Chlorination, Chapter 11, Dechlorination, pages 572 – 593 (2009)

[2] Brandt, M.J., Johnson, K.M, Andrew, M., Elphinston, J., Ratnayaka, D.D, Twort's Water Supply (Seventh Edition), Chapter 11 - Disinfection of Water, Butterworth-Heinemann, Pages 475-511 (2017)

[3] Chen, C.-L. and Gan, H.B. Wastewater Dechlorination State of the Art Field Survey and Pilot Studies. Cincinnati, OH: Municipal Environmental Research Laboratory, EPA-600/S2-81-169 (1981)