Virginia Law

A. Dechlorination is a process which effectively reduces free and combined chlorine residuals. Sulfur compounds applied to chlorinated effluents have been established as effective dechlorination agents as follows:

1. Sulfur dioxide (SO₂) is a nonflammable, colorless gas with a suffocating, pungent odor and a density greater than that of air. It rapidly dissolves in water to form a weak solution of sulfurous acid (H₂SO₃) which dissociates to produce sulfite ions (SO₃)^-2, which are the active dechlorinating agents.

2. Sulfite salts used for dechlorination include sodium sulfite (NaHSO₃), sodium disulfite (NaHSO3), and sodium metabisulfite (Na₂S₂O₅). Sodium metabisulfite is the most commonly used. Sulfite salts are available in dry form and are more safely handled than sulfur dioxide. On dissolution in water they produce the same active sulfite (SO₃)^-2 ion.

B. Usage. Both sulfur dioxide gas and sulfite compounds may be considered for use for dechlorination purposes. However, the use of sulfur dioxide gas or sodium metabisulfite in accordance with this chapter will be considered as conventional technology for dechlorination of flows equal to one mgd or more.

1. Sulfur dioxide shall be fed as a gas similar to chlorine gas. Since sulfur dioxide is more prone to reliquification, consideration should be given to heating the sulfur dioxide header. Sulfonator capacity shall be adequate to dechlorinate the maximum chlorine residual anticipated on at least a one-to-one basis at maximum daily flow rates to meet the effluent requirements contained in the issued permit or certificate. Requirements for equipment type, standby capability, spare parts, water supply, measurement equipment, control equipment, and evaporators are the same as for chlorination although the materials of construction may differ.

2. Sulfite salts may be fed in dry form with dry chemical feeders or they can be made up as a solution and fed with a diaphragm pump. With either method, proper feed controls shall be provided. Equipment capacity shall be adequate to dechlorinate the maximum chlorine residual anticipated on the basis of 1-1/2 parts or more sulfite salt to one part chlorine.

C. Features. Gas and dry feed equipment requirements shall be similar to those used for chlorination.

1. The dose mixing shall occur following the design chlorine contact period. Normally, this will require the use of a separate basin designed to thoroughly mix the dechlorinating agent with the contact tank effluent within a period of approximately one minute.

2. As the dechlorination reaction is essentially instantaneous, no further contact time is needed other than that required for mixing.

3. Piping materials shall be suitable for use with the sulfur chemical utilized.

4. Housing for feed equipment required shall be the same as for chlorination. However, sulfur dioxide feed equipment and storage containers shall be physically separated by sufficient distance, or by partition barriers, from the chlorination equipment and storage containers in order to prevent cross contamination of feed lines and to satisfy fire codes. Sulfite salts should be stored in unopened shipping containers until ready for use.

D. Safety. Handling requirements shall be the same as for chlorination, except for sulfite salts, which are nonhazardous.

E. Monitoring. Monitoring provisions shall be the same as for chlorination, except that facilities shall also be provided for securing a sample after dechlorination.

F. Other methods. Other means of dechlorination will be evaluated based on submission of adequate performance data.

1. Granular activated carbon may be used for dechlorination of high quality effluents. The dechlorination reaction is dependent on the chemical state of the chlorine, chlorine concentration, flow rate, physical characteristics of the carbon, and wastewater characteristics. Design considerations are similar to those utilized for other wastewater processing unit operations.

2. For small facilities with a design flow less than one mgd, dechlorination may be accomplished through the use of a holding pond such as effluent polishing pond or a constructed wetlands.