1) Stream Characteristics

The by-product streams for chlorine scrubbing systems vary with the intent for their use. The strength and content depends on the solution chosen for scrubbing. In addition, the final use final use of the by-product will be a factor in the stream characteristics, especially strength and purity.

2) Product or waste

Will the by-product stream be a product or waste?

As a product, usually a hypochlorite, the material is best handled in lined piping and tanks to keep the purity as high as possible. The product is kept cool, minimum exposure to light, and with an alkaline pH to minimize decomposition. The most common by-product of a chlorine scrubbing system is sodium hypochlorite which is used as chlorine bleach. If this product is to be sold, careful analysis of potential contaminants must be made.

As a waste, the by-product stream must be decomposed to get rid of the chlorine value prior to disposal for environmental compliance. In some cases the by-product streams are acidified for the recovery of the chlorine gas and the resulting stream becomes a waste.

3) Environmental

Before disposal of the chlorine scrubber streams the environmental permits with the respective agencies must be checked. The choice of decomposition methods will be dependent upon the regulations. Free chlorine is almost always limited, but in addition certain metals (usually from metal catalysts) cannot be discharged. The total dissolved solids (which are the salts of decomposition) usually have a limit in discharge permits. It may be necessary to adjust pH prior to discharge.

4) Decomposition

There are several factors that accelerate decomposition of scrubber by-product streams:

• Increasing concentration of hypochlorite
• Increasing temperature
• Decreasing alkalinity (or pH)
• Presence of catalysts
• Exposure to light

The effect of variations and combinations of these factors have been used to build decomposition processes.

(1) Thermal

Thermal decomposition is based upon the fact that hypochlorine solution decomposition rate increases with higher temperature. Also higher concentration and low pH accelerate the decomposition. A common method of decomposition is to heat a tank containing a maximum strength hypochlorite solution with live steam. This heating combined with the heat of decomposition results in rapid decomposition of sodium hypochlorite to oxygen and salt.

(2) Catalytic

The presence and/or addition of dissolved cobalt, nickel, copper, and iron in hypochlorite solutions catalytically accelerates decomposition. The presence of salts of these elements combined with the thermal process results in almost complete decomposition of sodium hypochlorite.

(3) Time and Light

All hypochlorite solutions will decompose with time. The presence of sunlight accelerates the process. In some cases shallow ponds with exposure to light have been used to decompose weak solutions of hypochlorite.

(4) Chemical

Certain chemicals react with hypochlorite solutions. Some of these are SO₂, sulfites (Na₂SO₃, NaHSO₃), thiosulfate, and hydrogen peroxide. In most cases the use of these chemicals are too expensive to use as the method for strong hypochlorite decomposition. They are used most of the time in polishing and small batch reactions to remove traces of chlorine. Certain chemicals result in high chemical oxygen demand and therefore discharge permits must be checked.

The combination of waste acids are sometimes used to recover chlorine and return chlorine back to the process. The reaction of acids such as HCl and H₂SO₄ liberate chlorine gas in their chemical decomposition and thus must be used with care.

(5) Effluent pH Adjustment

After decomposition of hypochlorite solutions by any method, acids may be used for pH adjustment prior to effluent discharge.

CAUTION - Acids should not be added before complete decomposition because chlorine will be evolved.