Raw water nitrate concentration can sometimes vary greatly, and water utilities need a reliable, sustainable and low-maintenance technology to ensure safe drinking water. Real-time monitoring of nitrate concentration utilising spectrophotometric sensors offers security in the treatment of the water and helps to optimise the blending process, while saving energy and reducing costs.
Nitrogen is an important nutrient for plants and is therefore found naturally in the food chain in the form of nitrates and nitrites. About 80 per cent of dietary nitrates come from eating vegetables, fruit, and processed meat, but if intake is excessive, nitrates can be harmful to health.
The European Food Safety Authority suggests we consume no more than 3.7mg per kg of body weight a day of nitrate (equivalent to 260mg per 70kg adult) and 0.06-0.07mg per kg for nitrite.
If drinking water with a high nitrate content is also consumed, this can be harmful. Water authorities publish recommended maximum nitrate concentrations for the drinking water supply.
Australia follows the Australian Drinking Water Guidelines (ADWG) published by the National Health and Medical Research Council (NHMRC) and the Natural Resource Management Ministerial Council. The ADWG sets a limit value in drinking water of 50mg/l for nitrates and 3mg/l for nitrites in line with WHO recommendations.
Most parts of Australia have low to moderate nitrate concentrations in water sources, safely below the guideline value. However, regions of intensive agriculture or areas where nitrogen-based fertilisers are heavily used may experience higher water nitrate concentrations due to agricultural runoff and leaching.
Nitrate monitoring
Intelligent monitoring of nitrate content not only provides important information about water quality, but also for undertaking nitrate treatment measures as effectively as possible. Sampling may be periodic at water sources, but continuous monitoring is necessary during water treatment.
Continuous monitoring of nitrate content not only helps water utilities to comply with legal limit values, but also to save costs. Monitoring directly at the raw water sources makes it possible to optimise downstream processes for the management of feeds with different qualities. Used before water treatment, it also enables the reliable adjustment of the parameters for denitrification and the control of downstream mixing with low-nitrate raw water.
Monitoring is also recommended after the denitrification process. It provides early indications of possible saturation, contamination or exhaustion of the system and can prevent breakthroughs. It can also be used to automate the regeneration, cleaning and replacement of beds, membranes and resins. This minimises maintenance efforts and maintains the efficiency of the system.
Measurement technologies
Depending on the sampling frequency, three common test methods are available for determining nitrate concentration in drinking water.
Colorimetric method
With the colorimetric method, the nitrate concentration is measured indirectly: Cadmium particles in a reagent reduce the nitrate to nitrite, which reacts with a coloured indicator. The colour intensity of the sample treated in this way changes according to the nitrate concentration and can also be evaluated manually. In order to avoid falsification of the results, however, vessels and devices that come into contact with the sample must be kept scrupulously clean and constantly checked.
Ion-selective electrodes
With an ion-selective electrode (ISE), nitrate content is continuously determined potentiometrically. Most commercially available sensors must be calibrated regularly and operated under certain conditions to minimise drift and wear.
Spectrophotometry
The equipment costs for the two methods above are relatively low, however they require the use of chemical solutions and, particularly in the case of online systems, frequent maintenance work, which causes high follow-up costs.
Today, systems that require continuous monitoring rely on spectrophotometric sensors. They determine the nitrate concentration from the direct proportionality of the absorption of their light source at a specific UV wavelength. Due to the low maintenance requirements, this technology is becoming more and more popular despite the higher initial costs.
The Bürkert solution
The Bürkert Type MS09 sensor is a spectrophotometric sensor designed specifically for determining the nitrate content in drinking water using UV absorption measurement. It utilises a xenon flash lamp as a light source and can measure nitrate content with reduced interference through three different detection channels. The nitrate content is determined at 212nm, the organic matter at 254nm and the turbidity at 360nm.
The MS09 sensor is insensitive to cross-influences in the water. Measurement can be carried out in raw water as well as in pure water, and nano-coated glass reduces maintenance.
Bürkert also offers the Type MS08 optical absorption sensor operating in the UV range at 254nm specifically for detecting dissolved organic matter in water. The SAC 254 and turbidity 530 values can be measured, as well as an application-specific correlation of the TOCeq, BODeq and CODeq. SAC 254 sensors are commonly recommended in Europe to exclude final safety risks for the consumer.
Both the MS09 and MS08 sensors are compatible with Bürkert’s Type 8905 and 8906 measurement stations.
Bürkert can provide a complete solution – including continuous sensing, measurement and control valves – for continuous nitrate monitoring in denitrification and water blending systems.
This sponsored editorial is brought to you by Bürkert. For more information, visit www.burkert.com.au.
