Water and Effluent Treatment Magazine
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28 WET NEWS SEPTEMBER 2016 INSIGHT Odour control H2S is known for its unpleasant odour T he human nose is particularly sensitive to Hydrogen Sulphide (H2S), a highly offensive odorous gas reminiscent of rotten eggs. With an odour threshold between 0.005ppm and 1.5ppm, relatively low levels of Hydrogen Sulphide can result in complaints if industrial plants generate exposure at these levels. A chemical waste treatment plant in the UK has therefore installed a monitoring and alarm system to generate alerts when H2S concentrations reach levels that might cause concern with staff and the public. Hydrogen Sulphide is o€en produced by the breakdown of organic matter in the absence of THE CONCEPT • The Honeywell SPM Flex has a detection limit of one part per billion • Each cartridge contains a tape that has been impregnated with chemicals • The SPM Flex can also be employed as a portable instrument, running for up to ten hours • Connected to an Envirologger, SPM data can be viewed on the web NEED TO KNOW 1 H2S is often produced by the breakdown of organic matter in the absence of oxygen 2 It is a highly toxic and flammable gas, causing irritation to the eyes, throat and lungs 3 Levels of more than 700ppm cause almost instantaneous death 4 H2S is the main component of stink bombs THE VERDICT "The key to the success of this system is the sensitivity of the monitor; by setting the alarm level at such a low concentration the wastewater treatment plant is able to raise an alarm before staff and neighbours notice the odour" Colin Craggs, Air Monitors A solution: Gas alarm offers protection The Honeywell SPM Flex alarm system is taking the stink out of odour monitoring at treatment works. A ir Monitors was approached by a chemi- cal waste treatment plant looking to install a Hydrogen Sulphide alarm system, the Honeywell SPM Flex was the obvious choice. "The plant regularly treats sewage, so Hydrogen Sulphide is a significant concern, both for plant staff and for the local community," explains Colin Craggs, from Air Monitors. "The SPM Flex operates a novel cartridge system that enables users to select the specific gas or gases of interest – H2S in this case, and we connected the instrument to an Envirologger, so that data could be viewed on the web, and so that alarms can be issued in a variety of ways – email, text etc." With a detection limit of one part per billion (ppb) the SPM Flex is extremely sensitive; each cartridge contains a tape that has been impregnated with chemicals, which perform analysis of the target gas. The chemical reaction that takes place produces a colorimetric response that, whilst not visible to the eye, is measured by the instrument's internal optics. For continuous operation, each cartridge lasts for around one month before a change is The challenge: Keeping odour to minimal levels Odour emissions from treatment works is an ongoing issue of treatment works, with Hydrogen Sulphide being a major cause for concern. necessary (the instrument alerts the operator beforehand). However, 'extended' cartridges are available, lengthening the operational period to two months. The SPM Flex can also be employed as a portable instrument, running for up to ten hours on an internal rechargeable battery. Craggs says: "In combination with the selection of cartridges available, this provides an opportunity to respond to an emergency with an instrument that is able to test for the gases of specific concern." In applications where continuous monitoring is required, the SPM Flex can be connected to mains power, and the battery effectively becomes a back-up in the event if power failure. Describing the performance of the SPM Flex at the wastewater treatment plant, Craggs says: "The system was installed over six months ago, with alarm levels set at 50ppb, and since that time, alarms have only been issued a handful of times, so the client can rest assured that any unusual H2S levels will be detected quickly, and his staff and neighbours are now protected. "The key to the success of this system is the sensitivity of the monitor; by setting the alarm level at such a low concentration the wastewater treatment plant is able to raise an alarm before staff and neighbours notice the odour, and in doing so, they ensure that potentially harmful exposure is effectively prevented." The system was installed more than six months ago, with alarm levels set at 50ppb, and alarms have only been issued a handful of times oxygen, and for this reason it is a major concern for the operators of waste treatment facilities. The gas is well known as an unpleasant odour, forming, as it does, the main component of the stink bombs that children are able to buy. Stink bombs generally contain Ammonium Sulphide, which, when exposed to air, reacts with moisture to form Hydrogen Sulphide and Ammonia, another unpleasantly odorous gas. One might therefore assume that any problems associated with Hydrogen Sulphide are solely associated with odour nuisance. However, Hydrogen Sulphide is a highly toxic and flammable gas, causing irritation to the eyes, throat and lungs, and can cause death in high concentrations The maximum occupational exposure level (eight-hour time- weighted average) in the UK is 5ppm, with a short-term exposure limit of 10ppm. Exposure to levels more than 20ppm can cause dizziness, and exposure to 100ppm-plus can cause death a€er 48 hours. Levels of more than 700ppm cause almost instantaneous death. The challenge for the monitoring instrumentation sector is to be able to monitor Hydrogen Sulphide at sufficiently low levels continuously without interferences from other gases.