Effect of Dechlorinated Effluents on Dissolved Oxygen of Receiving Streams

Although precise control may be achieved in dosing the sulfur dechlorinating agents, some residual will exist. This residual should be minimized not only because of waste of chemicals but also because of its effect on the dissolved oxygen of receiving streams. The reactions of these residuals on oxygen are as follows:

for sulfur dioxide:

for sodium sulfite:

2Na2S°3 + 2H2O ^ 2S°2- + 4Na+ + 4H+ + 4e- (17.63)

for sodium metabisulfite:

Na2S2°5 + 3H2° ^ 2S°2- + 2Na+ + 6H+ + 4e- (17.66)

The effects of the above reactions on the receiving streams are a reduction of dissolved oxygen and an increase in BOD and COD. In addition, the right-hand sides of the reactions, except those for sodium sulfite, indicate production of the hydrogen ion. Thus, there is the possibility of a decrease in pH of the receiving streams.

Example 17.19 Calculate the residual Na2S2O5 in a dechlorination process that will result in a decrease of pH to 5.0 in a receiving stream assuming the original pH is 7.5.

Solution:

Increase in hydrogen concentration = (10 - 10 ) = 9.97( 10 ) gmols/L Therefore, from Equation (17.68), residual Na2S2O5 = 9.97(,10 - = 4.99( 10-6) gmols/L = 4.99( 10-6)(Na2S2O5) = 4.99 (10-6)[2 (23) + 2(32.1) + 5 (16)] = 9.49( 10-4) gm/L = 0.95 mg/L Ans 17.5.4 Unit Operations in Dechlorination

The unit operations equipment for dechlorination are similar to that for chlorination. In fact, dechlorination equipment is interchangeable with chlorination equipment. The contact tank, however, is not used in dechlorination using the sulfur dechlori-nating agents. The reactions are so very fast that no contact tanks are necessary. As in chlorination, the key process control parameters are precise dosage and monitoring of residual chlorine and adequate mixing at the point of application in the process stream. The unit operation of dechlorination using activated carbon is similar to the one used in the unit operation of carbon adsorption, with the difference, as mentioned before, of the loss of large amounts of activated carbon in the case of dechlorination.

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