Peracetic acid (CH₃CO₃H) is a liquid that kills by oxidation disruption via the Hydroxyl radical HO. It is dosed at lower levels than hypochlorite and chlorine gas. As it breaks down to water and carbon dioxide, there are no requirements to remove the residual as the retention time is much less than for chlorine / hypochlorite, and the discharge requirements are typically below 1 ppm (mg/l).
As there are two points of measurement that can be used - contact chamber or discharge - the CSB analyser has been developed to measure at both positions, using the built-in verification system and cell cleaning methods in both cases.
- Contact chamber: Widely used and shows changes in the PAA dosing levels due to incoming demand variations or flow change. The accuracy of the analyser can be verified by the injection of a predetermined sample value. This can either be taken from the front end of the contact chamber at the injection point of the PAA, or prepared in the laboratory to a standard solution.
- Discharge: Generally used in smaller plants where the residual is retained due to the size of the contact chamber. The analyser is used to monitor at the discharge point and reads the residual at very low levels, even down to zero. This is where the self-verification system of the CSB analyser is both unique and essential. Because the zero reading, which is continuously recorded, could be either correct, or due to a failure of sample to the analyser or of course the analyser. Here it is essential that there is proof of the analyser operational reading. As for point 1. This will register on the analyser trace and gives proof of the operational value of the analyser output.
This system is used where fecal values vary due to the quality of the incoming wastewater. This requires a manual increase of PAA to reduce the fecal value without exceeding the value above 1ppm (mg/l) at the monitored point of discharge.
In wastewater treatment chemical monitoring, one of the biggest problems is the fouling of sensors and monitoring equipment. The use of PAA is no different and keeping the sample as clean as possible (and therefore the cell) increases the reliability of the unit and reduces the day-to-day maintenance of the system.
The CSB analyser is supplied with a self-cleaning filter pump, which delivers the sample to the analyser unit and is used in the operation of a patented manual or automatic vacuum operated self-cleaning cell system. The sample water can also be used to feed the clean sample to a pH monitor or other monitoring units.