nitrates and the quality of water Water Purification Systems UK

Transcription

1 nitrates and the quality of water Water Purification Systems UK

2 nitrates and the quality of water Nitrogen is an essential element in the periodic table. It makes up 78% of the air that we breathe, is vital to human life and critical as a source of food for plants. There is, however, a downside. Nitrogen has to be converted to nitrate compounds to make it accessible to plants. This naturally occurs through the action of nitrogen-fixing bacteria in the soil, which combine oxygen, hydrogen, and carbon with nitrogen to chemically reactive forms. These include ammonium and ammonia, which are then oxidised by micro-organisms to nitrite and nitrate, in a form that is subsequently taken up by plants during their growth. A similar process is also carried out in the production and application of ammonia based fertilisers, which are used in large quantities around the world, in agriculture and horticulture. The problem is that nitrate is leachable and moves readily in water through the soil into rivers, reservoirs and aquifers. This can be a particular problem after high levels of rainfall or if there is excessive irrigation, especially when one considers that only around half of all fertilisers used are taken up by plants. It s also worth noting that microbes in soil can convert nitrate molecules into gaseous nitrous oxide. When released into the atmosphere, this has around 300 times the global warming potential of carbon dioxide.

3 what problems do nitrates in water cause? High concentrations of nitrates in water can have a number of effects. In rivers, lakes and inshore waters, high levels of nitrates dramatically increase the nutrients that promote the growth of algae or cyanobacteria a process known as Eutrophication. In turn, as the algae die and decay the resulting organic matter depletes the level of dissolved oxygen in the water Anoxia causing the death of fish, invertebrates and shellfish. This phenomenon is being seen with increasing regularity around the world as the number of algae blooms grow. For humans, the presence of nitrites and nitrates in drinking water has been linked to occurrences of thyroid cancer, respiratory tract infections, birth defects and premature births. It is perhaps most commonly associated with Methaemoglobineamia, where nitrates in drinking water are converted in the body to nitrites. These react with haemoglobin in red blood cells to form methaemoglobin, which affects the ability of blood to carry oxygen around the body. Most humans over one year of age have the ability easily to convert methaemoglobin back to oxyhaemoglobin, so the total amount of methaemoglobin in red blood cells remains low even if relatively high levels of nitrates have been ingested. However, in children less than six months old, the enzyme systems for reducing methaemoglobin to oxyhaemoglobin are not fully developed, so methaemoglobineamia can occur. This condition is often known as Blue Baby Syndrome. A similar condition can be found in older individuals who have genetically impaired enzyme systems for metabolising methaemoglobin. In the UK, where nitrate levels are generally low, and where the quality of potable water supplies are carefully controlled, methaemoglobineamia is rare; indeed, the last recorded case occurred in the 1950s and was associated with the use of a shallow private well.

4 potable water The UK, in common with many other countries, has adopted the World Health Organisation s Guideline Value for nitrate in drinking water, of 50 mg/l. Tests of tap water by the Drinking Water Inspectorate over a period of time have shown that over 99% of all samples were below this limit. Nonetheless, the Environment Agency has defined a number of Nitrate Vulnerable Zones (NVZ) in England and Wales. These are areas where water sources have concentrations of nitrates above 50 mg/l, or where concentrations are at risk of breaching this limit, caused by local contamination from excessive use of fertilisers, pollution from animal waste or runoff from rubbish dumps. New NVZs NVZs designated in 2002 In the USA, with a higher percentage of ground water sources and wider use of intensive farming practices, the permissible limit or maximum containment level is set far lower, at 10 mg/l (10 ppm). This has been determined to be as close to the country s health goals as possible, considering cost, benefits and the ability of public water systems to detect and remove contaminants using suitable treatment technologies. Although many countries are fortunate in having established and rigorously controlled drinking water infrastructure, where levels of contaminants are minimised to safe levels, this is not always the case in developing nations, or where geographic conditions prevent the installation of permanent water treatment works capable of supplying a local population or industry.

5 There are a variety of technologies capable of removing nitrates, each of which have advantages in different applications, and depend on correct specification, installation and operation to be fully effective. In these instances, water drawn from ground sources or aquifers via boreholes will need treatment to remove a wide variety of potentially harmful pollutants, including nitrate. Solutions include the use of reverse osmosis, nitrate selective ion-exchange resin or biological denitirification. Reverse osmosis and ion-exchange plant can be skid mounted and so can easily be moved to site, either to upgrade existing systems or as a temporary measure during peaks of high nitrate levels. Typically with these systems it is unnecessary to treat 100% of the flow but instead blend purified water with the source water to achieve an overall level of nitrate below that required by the World Health Organisation (WHO). One consideration to bear in mind is that with any ionexchange system the capacity of the resin is fixed and will require regeneration; nitrate removal plants use a weak saline solution for this purpose. The effluent produced from the regeneration process will be rich in nitrate and chloride could exceed consent levels, especially if the site in question is situated in a nitrate sensitive zone. Additional treatment may be required or the waste tankered away if consent levels dictate.

6 water for food, drinks and pharmaceuticals Although water is used throughout industry, the sectors most likely to be affected by excessive levels of nitrates in raw or source water are pharmaceutical and food and beverage. In each case, manufacturers drawing their source water from a mains supply will normally know the composition of the water in advance; in the UK, for example, nitrate levels will be below 50 mg/l. Many industrial processes also depend on source water being stripped of all its active constituents, using a variety of purification methods, so nitrate removal may already be taking place by means of existing process systems. These typically include reverse osmosis (RO), which is capable of rejecting up to 98% of inorganic ions, which includes the nitrate ion, together with almost all colloids, micro-organisms, endotoxins and macromolecules. An alternative method of removal for small volumes of water is to use distillation, where the source water is boiled, the steam is condensed and the resulting nitrate-free liquid used. This process is, however, time consuming and energy intensive. It should be noted that simply boiling water, using carbon adsorption filters, mechanical filters and standard water softeners will not remove nitrates. The main challenge for food and drinks manufacturers in particular, such as craft breweries and specialised food producers, is where raw water is being drawn from an untreated aquifer or local surface source. Here, nitrate levels may approach or from time to time exceed 50 mg/l, making the incorporation of either reverse osmosis or nitrate selective resins - either individually or in combination - in the pre-treatment a necessity. RO systems are often integrated with electrodeionisation (EDi) or continuous electro-deionisation (CEDi) systems. This configuration is essential in the pharmaceutical sector where the European and British Pharmacopeia specifies that the concentration of nitrates cannot exceed 2 ppm in purified water and water for injection (WFI) grade. Water drawn from ground sources or aquifers via boreholes will need treatment to remove a wide variety of potentially harmful pollutants, including nitrate.

7 water for healthcare The healthcare sector uses large volumes of water for applications that include boiler feed, disinfection, sterile services, and haemodialysis. Mains water is generally the preferred source, in which case nitrate concentrations are not normally of concern, with the exception of haemodialysis. In this instance, nitrate concentration in water must not exceed 2 mg/l far below the permissible UK and USA limits for drinking water. Higher concentrations can result in nitrates passing through the dialysis membrane into the blood, from where they are carried into the small bowel. Here, they are converted to nitrites, which are absorbed back in to the blood, causing oxidation of the red blood cells and leading to methaemoglobineamia. The solutions available include high performance reverse osmosis systems, matched to a pre-treatment package designed to meet the characteristics of the feed-water. Typically, this equipment includes a base-exchange softener to remove hardness that would otherwise scale the membranes. Further protection is provided by passing the water through activated carbon filters, to remove free chlorine, chloramines and organic contaminants, with any remaining particulates being removed by a fine filter before the pre-treated water enters the RO system. With groundwater levels of nitrate increasing in some areas, single pass, reverse osmosis alone cannot always be guaranteed to reduce the levels to meet renal dialysis guidelines; therefore, additional nitrate removal using ion-exchange is becoming more common. It is important to control levels of nitrates in sectors that include food and drinks production, pharmaceuticals and healthcare. In a water system designed to produce purified water for renal dialysis the nitrate removal unit would be installed upstream of the reverse osmosis system.

8 learn more As we ve indicated above the presence of nitrates in potable water does not always represent a problem for human health. Indeed, it s worth remembering that nitrates and nitrites occur naturally in some foods and have also been used as preservatives by the food manufacturing sector for many years. It is, however, important to control levels of nitrates in sectors that include food and drinks production, pharmaceuticals and healthcare. There are a variety of technologies capable of removing nitrates, each of which have advantages in different applications, and depend on correct specification, installation and operation to be fully effective. In turn, this relies on the support of an experienced supplier, capable of understanding the subtleties of each application and developing the best technical and commercial solution. Contact SUEZ Water Purification Systems Ltd Bandet Way, Thame, Oxon OX9 3SJ Tel: +44 (0) Fax: +44 (0) info@sueztreatmentsolutions.co.uk Web: Finally, bear in mind that detecting nitrates and nitrites in in water is not straightforward as they are odourless and colourless. The normal procedure is to test samples using ion-chromatography, or spectroscopy, and may therefore require the services of a specialised laboratory. If you would like to learn more about detecting, controlling and removing nitrates and nitrites, or other active elements from your source water, please contact our customer service team. Errors and Omissions excluded. SUEZ reserves the right to change the specification in accordance with our program of continual improvement.