Purified water at your fingertips
Veolia Water reveals some of its water purifying techniques that are enabling government to boost use of brownfield sites and clean up, re-use and redevelop contaminated land
Past industrial use has left a legacy of contaminated land that must be cleaned up before it can be re-used. The UK government is extremely keen for this land to be redeveloped for housing and light industrial units, in preference to eating into the countryside and green belt.
Under its brownfields initiative, the aim is that 60% of all new housing will be built on previously developed land, and while obviously not all such land suffers from industrial contamination, much does. A recent high-profile example, the Millennium Dome, was built on the site of an old gasworks.
The land was contaminated with heavy metals and it was essential this was removed before the dome and nearby new housing were built. Typical contaminants include landfill gas, heavy metals and organic chemicals. Detrimental effects to human health, groundwater, surface water and the ecosystem are all likely to result. Remediation is not only necessary if the land is to be redeveloped - industrial contamination can also pose a threat to the cleanliness of the water supply. Trace metals can also originate from natural sources - it is a recognised problem that, for example, many rivers in the south-west of England and Wales are contaminated with arsenic, cadmium, copper and zinc from abandoned metal mines. The first step to remediation is to establish just how much of a problem there is - and this means testing samples from the soil and water courses for contamination.
The low levels of trace metals means careful analysis is required to ensure background interferences do not cause false readings. Many trace elements are toxic, including arsenic, cadmium, chromium, lead, mercury, nickel, selenium and tin. Copper and zinc levels also need to be controlled, and then there are other elements, such as thallium, which are toxic but levels of which are not regulated in the UK. Some of these can be toxic at very low levels, so extremely sensitive tests are essential. The key analytical technique is atomic absorption spectroscopy (AAS).
Flame AAS machines can detect metals at ppm levels which, in this context, is still fairly high. A graphite furnace AAS is much more sensitive and can routinely pinpoint metals at levels of ppb or ppt. Similar sensitivities can be achieved using inductively coupled plasma mass spectrometry (ICP-MS), which can measure more than 30 different elements at once, often at levels as low as ppq.
At such low concentrations, it is vital water used to dilute the samples and standards used for comparison is as pure as possible because any contamination, however small, would be likely to lead to inaccurate results. Accurate calibration of the instrument is also vital, so again the water used to make up calibration standards has to be high-purity. As a result, 18.2M?-cm water must be used for all samples and standards.
PerkinElmer Life and Analytical Sciences makes analytical instruments that are used to carry out trace analyses and provides a demonstration facility at its UK site in Seer Green near Beaconsfield in Buckinghamshire. The instruments there include a graphite furnace AAnalyst 800 atomic absorption spectrometer, for measuring single elements down to ppq levels, and an Elan DRCII ICP-MS, a multi-element analyser that can measure 30 or more different elements in up to 200 samples a day. PerkinElmer relies on Elga LabWater equipment to provide pure water to run these instruments.
The tap water in Seer Green, as is the case in much of south-east England, is very hard, with typically 100ppm of calcium, which must be removed. Water from a holding tank will also typically have high levels of sodium, potassium and magnesium, and it can pick up other metals such as copper, chromium, lead, nickel and zinc from the tank. These must all be eliminated from the water used to make samples and standards.
At PerkinElmer, the water is fed into an Elga Purelab water polisher, via a 40-litre reservoir. The polisher produces water with the lowest levels of inorganic impurities, a resistivity of 18.2 M?-cm, total organic carbon of less than 3ppb and particulate filtration at 0.05µm. The flow rate is up to 1.8 l/m. The inorganics are removed by the ion exchange resins and a wide range of organic contaminants are scavenged by adsorption media. The polisher also contains a short wavelength UV photochemical reactor cell, to provide continuous disinfection and photo-oxidation, ensuring low levels of bacteria and total organic carbon.
A hygienic hollow fibre 0.05µm ultra-micro-filter is also included to eliminate particulates and bacteria and a TOC monitor provides a continuous read-out of organic water quality. The two cartridge pack system used by Elga purifiers is an essential guarantee of the purity of the product water. The resistivity of the water after the first purification cartridge is monitored. When this intermediate resistivity drops, the first cartridge can be replaced before the second cartridge is significantly used. In this way the product water is always protected by the second cartridge and effectively all impurity ions are removed.
With such pure water, it is possible to measure the levels of trace heavy metals down to sub-ppb levels or lower. This level of accuracy is essential if those cleaning up brownfield sites are to be sure all traces of toxic heavy metals have been removed to avoid them leaching into the environment, watercourses and the potable water supply.