The market for the use of PAA in wastewater is fast emerging as an alternative for disinfection.
Compared with other oxidants, PAA: is a stronger disinfecting agent; has a shorter residual retention time; retains its storage strength for longer. The dosing and control methods are the same as for any other disinfection system - flow control, residual control or monitoring, or the combination of both (known as compound loop control). The only element that was missing from the control requirements for PAA was an on-line analyser that could accurately measure below 1ppm (mg/l) and down to 0.1 ppm (mg/l).
To overcome this, CSB developed an analyser for the PAA market that measures accurately between 0 and 1 ppm (mg/l). There are now no restrictions to the control and monitoring of PAA in wastewater and the only remaining challenge appears to be the cost.
Chlorine is generally considered to be the cheaper option. However, when taking into account all related costs, we can demonstrate that this is not the case. Where chlorine is used the residual must be eliminated by using bisulphite or an equivalent - we can assume 10% of chlorine dose remains to be eliminated i.e. 10% additional cost added to delivery costs and two lots of chemical storage systems.
PAA dosage on average is 2.5 ppm against chlorine 5-8 ppm. We can use a low figure of 6 ppm as an average, therefore 2.4 times the dose rate of PAA. As chlorine degrades 10% faster than PAA in higher temperatures we can take 10% of 2.4 = 0.24 giving a total of 2.64 times the apparent cost of the chlorine to compare with PAA. Therefore the two chemicals are at least comparable. In fact PAA is cheaper in real terms and the price will probably reduce as it becomes more widely used.
It should be noted that PAA cannot be used in systems using chlorine and or vice versa as they cancel each other out, it also requires relief valves at potential pressure build up areas.