Sulphide monitoring gets a makeover
New technology is making the continuous monitoring of sulphides in wastewater effective and desirable, says Michael Strahand, general manager of Analytical Technology (UK)
In addition, when the temperature of water and its sulphate content are particularly high, sewage quickly becomes septic leading to the production of high levels of hydrogen sulphide (H2S). Concentrations of H2S as high as 500ppm in the air above the sewer are not uncommon and can cause odour problems for both plant operators and people living nearby.
In order to remove sulphides from water it is necessary to add oxidants, which is an expensive and time-consuming process. Therefore, treatment facilities endeavour to prevent their formation through constant measurement and monitoring of dissolved sulphides in water.
Due to the poor quality of water samples used for sulphide measurement, fouling of the analytical system is a concern and accurate measurement has been a challenge. New developments mean it is now possible to continuously monitor sulphide levels in water with minimal maintenance and adjustment.
It is also important to control gaseous emissions from wastewater treatment plants (WwTPs) by introducing scrubber systems for pollution control. Many of these scrubbers employ a wet process using hypochlorite solution to remove H2S from air streams prior to discharge. Consequently, gas detectors have been developed that continuously monitor H2S levels to tell users when their odour control system has failed.
Gas phase H2S in wet wells is often measured to give an indication of whether a dissolved sulphide problem exists. Oxidants are then added if required.
Another approach involves monitoring H2S in order to prevent sulphide formation occurring at all. The measured concentration of dissolved sulphide is then used to decide the dosing rate of oxidants such as hydrogen peroxide and ferric nitrate to eliminate sulphide before it causes problems. Until now, the dosing control has been based on indirect measurements. A traditional method of measuring dissolved sulphide concentration in water uses analysers employing selective ion electrodes (SIE). This method provides adequate results but is expensive and requires regular maintenance, including frequent zeroing and calibration adjustments to maintain accuracy.
Other systems use oxidation reduction potential (ORP) to indicate dissolved sulphides. A semi-direct method is to measure ORP/redox in water and use this to predict H2S levels. As the measurement is non-specific, the values can be affected by many other sample components. As Redox is not sensitive enough to measure low H2S levels, probes have to be immersed in the sample and as a result can foul easily.
Odour control in WwTPs often require the use of scrubbers, which are used to remove pollutants from exhaust streams. Monitoring of H2S in both inlet and discharge air has presented problems for standard gas sensors. Due to humid conditions and condensing levels of water vapour, the water droplets can be a barrier to the diffusion of H2S into the sensor, resulting in inaccurate measurements.
New methods of monitoring sulphides, developed by companies such as Analytical Technology (AT), overcome issues associated with sulphide measurement and odour control. AT's A15/81 Dissolved Sulphur Monitor provides an enhanced process for sulphide measurement in solution, resulting in a system that can operate continuously with minimal maintenance and adjustment.
Instead of using an SIE sensor, the measurement is achieved using a polarographic H2S gas sensor. As the sensor is separated from the sample, fouling does not occur, eliminating one of the major problems of H2S measurement. In practice, a small amount of sample is pumped into the analyser and mixed with acid, leading to the production of H2S as a result of chemical reaction. The sample flows to an air-stripping chamber, where the measurement of gas phase is carried out and the monitor displays an equivalent sulphide concentration in mg/l. This gas stripping technique of monitoring sulphide in solution is extremely sensitive, achieving measurements of low parts per billion. The monitor's design also means that monthly zeroing and calibration intervals are more than adequate.
In addition, AT's Q45S Wet H2S Gas Detector, used in scrubber systems, provides a lower priced alternative to expensive 'tape based' H2S monitors. This gas detector is used to monitor the efficiency of odour control systems and alert the operator of any drop in efficiency.
New sensor technology allows continuous monitoring of H2S where condensing humidity conditions are normal. Measurements may be made either at the inlet to scrubber systems where concentrations can run as high as 200 PPM or at the outlet where concentrations are ideally down below 0.5 PPM.
As these systems often monitor gas streams with condensing levels of water vapour, provision has been made to eliminate water droplets from the sensor as this could present a barrier to the diffusion of H2S into the sensor. An optional air-purge system controlled by the transmitter periodically delivers blast of air across the critical sensor surfaces to remove water droplets. This system ensures a clear gas diffusion path to the sensor and reliable measurements on a continuous basis.
To successfully monitor dissolved sulphides as part of odour control, water and wastewater companies should invest in simple and reliable approaches that measure sulphide in solution. New instruments allow true control of the dosing systems and accurate monitoring of sulphide levels in municipal wastewater collection systems and industrial wastewater processing. The A15/81 Dissolved Sulphur monitor used in conjunction with the Q45S Wet H2S Gas Detector is a complete solution to monitor and control levels of sulphide, combating odour problems.