In recent years regulations for environmental control, health and safety issues have become much more stringent for existing and new landfill sites. The regulations encompass many considerations and some involve the measurement and control of water level in many positions around a waste disposal site.
One application is ensuring contaminated water is contained within a tanked zone and does not escape into the surrounding environment. Water level is also monitored at syphoning points where the contaminated water (Leachate) is removed for processing.
In locations where the leachate is pooled there will also be relatively high concentrations of volatile hydrocarbon gases expelled due to the decomposition process within the landfill. Due to the presence of flammable gases, areas within landfill sites are classed as hazardous zones. It is requirement that any measurement instrumentation that is to be located inside a hazardous zone, should be approved and installed to the appropriate ATEX classification for that area.
A problematic issue for specifying liquid level transducers on a landfill site is ensuring media compatibility with Wastewater of an uncertain chemistry. Leachate is a highly corrosive cocktail which is produced from the slow break down of waste materials which have been heaped onto a landfill site.
Since the presence and proportion of a particular material cannot be accurately predicted or controlled, the type and level of corrosion is difficult to anticipate without some initial experimentation with different instrumentation constructions to identify the most reliable and cost effective solution. A low cost device that lasts 6 months may end up costing more in the long term than a higher spec transducer which lasts 2 years.
A pressure transmitter that is going to be exposed to leachate will need to be constructed from materials that will withstand long term exposure to this highly corrosive media. For water level monitoring, a pressure transmitter is submerged down into a well or borehole to measure the hydrostatic level, which means that the whole device must be compatible with the leachate. The main media considerations for an immersed pressure transducer are ensuring that the body, diaphragm, seals and cable material will be compatible with the type of leachate present at a particular landfill site.
Hydrocarbon gases are a by-product of the decomposition and chemical reactions within a landfill waste. In some wells this gas is recovered in a controlled way and used as a fuel to power machinery on site or to be distributed elsewhere. In order to contain the gas the well is capped and sealed from outside air to prevent the gas escaping into the atmosphere.
This poses a problem for accurate measurement of liquid level since a typical submersible pressure transducer is vented to outside air to compensate for ambient air pressure. However if the air above the water surface is controlled independently to outside air the transducer will be using the wrong pressure reference. Since the gas pressure can rise above and drop below depending on the generation and removal of gas it is necessary to use an additional compound range pressure transmitter and subtract it’s signal from the level reading signal in order to determine a precise measurement of level within the well.
Alternatively it is possible to split out the reference tube and run it back into the top of the capped well. This can introduce installation complications and moisture ingress issues, but it does eliminate the expense and uncertainty error of an additional sensor.