Sulphite Monitor for Removal of Ozone
Removal of ozone or chlorine from water is commonly undertaken in the treatment of potable water and process water. Typical applications include the removal of chlorine from town's/mains water prior to using Reverse Osmosis (RO) membranes. RO membranes have a low tolerance to chlorine. If chlorinated water reaches the membranes it will significantly reduce the membrane lifetime and may even destroy the membrane. Another common application occurs in the treatment of water with ozone. Many plants follow ozone treatment with a Granular Activated Carbon (GAC) plant. It is important that ozonated water does not go onto the GACS filter.
Removal of ozone or chlorine is commonly achieved using SO2 gas, sodium sulphite or sodium bisulphite solutions.
Control of this process of Cl2/O3 removal is often carried out by measuring the lack of Cl2/O3. A plant, may for example use a chlorine monitor to measure the chlorine concentration after the addition of bisulphite. The control set point would be 0.00ppm chlorine.
Ozone removal application
Many water companies use ozone to treat water. Following ozonisation it is very common to use GAC to filter the water.
It is important that ozone does not reach the GAC filters. When using bisulphite it is also important, however, that not too much bisulphite reaches the filters. High concentration of sulphite can be reduced on the GAC to produce H2S which can lead to severe taste and odour problems. Control of excess sulphite is as important as removing the ozone.
A leading K water company asked ATi to supply an on line residual sulphite monitor to improve process control at one of its sites.
Operation of the A15/66
The A15/66 Monitor takes a unique approach to the measurement of sulphite ion concentration. In operation, a small amount of sample is pumped into the system and mixed with acid. In acidic solution, the sulphite ion is converted to sulphur dioxide according to the following reaction:
SO32- +2H+ SO2 + H20
The mixed sample flows into a special chamber where the sulphur dioxide is stripped from the sample. A sensor located in the gas stream measures the released SO2 concentration and displays the results in terms of equivalent sulphite ion concentration.
Sulphite monitors consist of three separate components: a chemistry module where the sample is pH adjusted for measurement, an inlet overflow assembly where raw sample is delivered to the system, and an electronic readout containing the sulphite concentration display, analogue output, and alarm contacts. The A15/66 monitor requires a sample of around 0.5lmin-1 and a power supply. Installation is straightforward.
The first system ran successfully for around 6 months and allowed the customer to reduce the sulphite dosing and control at around 0.010ppm excess sulphite. This resulted in some costs savings but, more importantly, ensured that sulphite levels were enough to ensure that ozone removal had taken place whilst not creating H2S problems on the GAC filters.
A second system has been installed as a duty standby. The customer has started to install the A15/66 on other treatment plants with an ozone removal process.
1. Positive control of excess sulphite ensures that ozone removal is complete. An ozone monitor could fail to zero and give the misleading impression that there was no ozone in the system
2. By allowing true control of bisulphite the costs are reduced.
3. The risk of formation of H2S on the GAC filters is reduced
Other applications for sulphite monitoring
1. Protection of RO Membranes by removal of Cl2
2. Measuring sulphite levels following chlorination and de-chlorination of waste water