One for the buoys…

Measurement of nutrients within wastewater is becoming increasingly important both for end-of-pipe monitoring of discharges and for process control. Current control strategies for optimising the nitrification and denitrification processes of biological nutrient removal (BNR) plants and other wastewater treatment designs are often limited by long lag times in the availability of laboratory data. Being able to measure directly at any point throughout the treatment system would often be a significant advantage for the process operator.

Online ammonia and nitrate analysers that have been widely used for clean water applications have been adapted with various designs of filtration and sample supply systems to enable operation within the wastewater control environment. However, their use is often restricted to sample streams with the minimum of solids, i.e. on the final effluent. Even here, fouling from bacterial and algal growth, leading to high maintenance and reliability issues, often limits the real benefits of such online measurement. An example is Southern Water, who spent three years evaluating a wide range of ammonia measurement systems and concluded that all suffered from similar maintenance issues. Most of the key issues were centered not so much on the analysers themselves, but rather the sample handling systems required.

A new design concept of in situ analyser is now available and has, for the first time, proven to provide reliable measurement of ammonia and nitrate, even directly within activated sludge. Since the sample supply system is often the weak link, the Isco-STIP ammonia process buoy has been designed so that there is no requirement for a sample pump, pipework, filtration systems or even analyser cabin.

The novel design incorporates an internal sample settlement chamber, the reagent and calibration solutions, with the measurement sensors housed in a submersible casing. The buoy is filled by the hydrostatic pressure of the water and emptied by air pressure. Typically, reagents will need to be re-filled after 5-8 weeks – a simple and easy procedure.

Both the ammonia and nitrate process buoys are equipped with a purgeable cell in which the wastewater is separated from the sludge and solids before being fed into the measurement cell. During the measurement phase caustic addition occurs using an ammonia gas-sensing electrode, controlled by the pH sensor to help ensure high measurement accuracy and low reagent consumption. In a similar procedure for nitrate measurement, the ionic strength is controlled via a conductivity probe. Nitrate is measured with an ion-selective probe. Both process buoys incorporate automatic calibration using the standard addition method which compensates for the variability in the wastewater.

Controlling costs

Typically, 30-40% of the cost of a conventional analyser can be directly related to the sample supply system. Such a system is not required with the process buoy and can be installed simply within a few hours which makes it particularly suited to existing treatment plants and industrial applications.

The process buoys are ideally suited as tools to help understand and potentially control the nitrification and denitrification process within a wastewater treatment plant. This sort of data provides immediate indication of the effectiveness of the nitrogen removal processes and provides opportunity to implement control strategies. For example, inadequate aeration can result in poor conversion of ammonia to nitrates and produce ammonia surges at the effluent discharge.

A typical application for continuous nitrate monitoring is to control chemical addition such as methanol in the denitrification process. The nitrate buoy can provide a control signal linked to a metering pump for chemical injection. This can yield savings by avoiding wasteful overdosing.

Final effluent Monitoring

Southern Water has recently awarded Envitech a framework agreement to supply its ammonia monitors for final effluent consent requirements. The Isco-STIP ammonia process buoy proved itself as being the most reliable and cost effective solution for this application. Results from trials undertaken at Fullerton WWTP also demonstrated the accuracy of the data when compared to the laboratory method.

Treatment plant and process performance will improve and costs of operation will decrease when online analysers such as process buoys are used to both minimise short term load variability and adjust the plant to actual conditions.

Understanding of the wastewater treatment process by treatment operators will increase as online data is made available. The Isco-STIP process buoys demonstrate that such data is now available at lower cost with the benefits of direct in situ measurement with simple and quick installation and improved reliability of measurement.