Measurement of dissolved H2S, (hydrogen sulphide), in difficult applications such as tanneries, circuit board waste and waste water treatment
12 May 2005, News release from ATi UK Limited
ATi has developed a new dissolved hydrogen sulphide monitor that for the first time enables you to measure levels of H2S in difficult applications.
Hydrogen sulphide is often present in the waste water from tanning processes. Oxidation of sulphide to sulphate must be carried out prior to standard waste treatment processes.
Sulphide is usually present at level between 1000 and 200ppm. Oxygen is used as the oxidant. Due to the expense, it is not desirable to overdose. Sulphide levels are usually reduced to between 0.5 and 10ppm.
Until now, measuring dissolved H2S at this level in tannery waste has been difficult and prohibitively expensive.
Redox/ORP monitors are sometimes used but they are non linear and not sensitive enough at the low levels of H2S.
Measurement and control of H2S levels is vital because the presence of even low levels of H2S will have a detrimental impact on subsequent biological treatment processes.
Hydrogen Sulphide gas is used to treat exhaust etching solutions from PCB industries, containing high level of Copper under Ammoniacal complex form. The gas is used to precipitate Copper Sulphide. The sulphide level must be always in a small excess (few ppm, usually 5 - 10) in order to be sure that all Copper is precipitated. Measurement of dissolved sulphide would allow control of the process.
Waste Water Treatment
Sulphide can also build up in wastewater collection systems due to the anaerobic conditions that frequently occur. When sulphides occur they can cause damage to concrete structures, they can lead to septic condition and create "black primary tanks" and are also the primary cause of odour problems.
To remove sulphides it is necessary to add oxidants. Many companies measure gas phase H2S in wet wells to give an indication of whether there is dissolved sulphide problem. Measurement of sulphides in water has also sometimes been carried out using selective ion electrodes, while this method is sensitive, such system require frequent zeroing and calibration adjustment.
The measured or calculated concentration of dissolved sulphide is then used to determine the dosing rate of oxidants such as hydrogen peroxide and ferric nitrate that are used to eliminate sulphide before they can cause problems.
Measuring H2S in these applications has been made easier ATi's new improved method for monitoring sulphide in solution. The measurement is done using a polarographic H2S sensor that is never in contact with the sample.
In operation a small amount of pre filters sample is pumped into the analyser and mixed with acid. In acid solutions chemical reactions lead to the production of H2S. The sample then flows to an air-stripping chamber. Measurement of gas phase H2S is carried out and the monitor displays an equivalent sulphide concentration in mg/l
This approach eliminates the big problem associated with H2S measurement. Because the sensor never comes into contact with the sample fouling does not occur.
The A15/81 consists of two parts. A chemistry module where the sample is conditioned and an electronic display unit that shows the sulphide concentration. The electronics have 4-20mA outputs and alarm relays as standard.
The gas stripping technique of monitoring sulphide in solution provides an extremely sensitive on line monitor, measurement down to low parts per billion can be achieved. The design also means that monthly zeroing and calibration intervals are more than adequate.
The key benefits of the ATi dissolved sulphide monitor are:
· Continuous, accurate measurement of true dissolved sulphide levels
· Low maintenance due to non-contact gas phase measurement
· High accuracy and sensitivity measurement for fine control
Try one now
ATi are inviting all operators who are serious about improving their control of sulphide levels to judge the A15/81 Dissolved Sulphide monitor for themselves.
For further information please email ATi UK Limited