Reporting river analyses
Paul Fisher of AET examines the developments of a biosensor for onsite monitoring of ammonia in rivers and effluents
Ammonia is one of the most widespread toxic pollutants in our rivers. It is a breakdown product of human and animal waste, and European and UK legislation imposes strict limits on its concentration in environmental waters. The legislative pressure to control ammonia emissions to water is not limited to the UK - it applies to the whole European Union (EU) and throughout the developed world. Control strategies need analytical data. As an illustration the Environment Agency (EA) carries out some 110,000 ammonia analyses each year and sets limits on ammonia concentration in effluents for about 10,000 industrial and business sites that discharge to controlled waters in England and Wales. These dischargers also make many ammonia measurements to ensure their effluents do not exceed their permit levels for ammonia concentration. The uncertainty of total ammoniacal nitrogen analysis has to be suitable for its environmental and regulatory purposes, normally 10% random error, 10% systematic error and 0.03mg/l limit of detection1. Monitoring is currently carried out by taking samples to an analytical laboratory, where they are analysed by methods that can be demonstrated to have suitable performance and traceability.
It would be preferable to make the measurements directly on-site to reduce costs and to facilitate a rapid response to problems but there are no commercially available field instruments that meet the performance requirements. Some permanently-sited instruments can achieve the required performance but their size and cost makes widespread use impractical. Test kits for ammonia are useful to provide guidance on-site but they are not easy to operate in the field. Ammonia ion-selective electrodes are unable to meet the performance specification because they also respond to sodium and potassium ions. Moreover, the EA's experience with conventional field instruments is that they have a high technical support cost of essential maintenance and calibration to ensure their continuing accuracy.
The healthcare industry has led the way in moving accurate medical monitoring from laboratories to direct site measurement by developing 'one-shot disposable' instruments that are pre-calibrated and require minimal operator skill or technical support. They are based on solid state biosensors mass produced by a screen printing process at sufficiently low cost to enable once-only use to be feasible. Applied Enzyme Technology (AET), Gwent Electronic Materials (GEM) and the EA are collaborating in a DTI Bio-Wise-funded project to produce one-shot disposable biosensors. The ammonia biosensor is a one-shot disposable test. It combines the materials expertise of GEM, which has developed a screen printable ink that contains an electron acceptor, and the protein stabilisation technology of AET. The electrochemical test was initially developed by Professor John Hart from the University of West England as the result of a LINK grant with the other partners - the EA, GEM and Jenway Instruments. The test utilises the enzyme glutamate dehydrogenase. In the presence of ammonium ions, the cofactor NADH is converted to NAD by the GLDH enzyme, resulting in the generation of electrons at the sensor surface.
These electrons are detected by the presence of an electron mediator that is impregnated within the working electrode material. The instrument then detects the change in current at the sensor surface, which is directly proportional to the ammonium ion concentration. In close collaboration with the EA, AET and GEM have developed a hand-sized portable field testing unit.
In conjunction with a low-skill sampling system, which sits on top of the sensor, they have delivered a low-skill accurate, fast, cheap method for testing ammonia levels at the site of testing. GEM and AET are currently scaling up the manufacture of the complete biosensors to at least 500,000/annum in the first instance. The instrument has been rebadged as the Handilab. The complete biosensors plus the manual sample dispensing system was independently evaluated in the EA's chemical laboratory at Starcross, near Exeter, that holds UKAS accreditation for conformance with international analytical quality assurance requirements. Comparison with reference laboratory analysis for a range of river water and effluent samples between 1-10mg/l ammonia showed the biosensor achieved the 20% uncertainty target and underpinned the fundamental viability of the system design. A series of iterative system modifications, followed by evaluations at the Starcross laboratory, are now in progress to fine-tune the commercial product. Latest results from more than 2,000 sensors now tested at the Starcross laboratory show good agreement with the lab-based tested on a wide range of sewage effluent, river and marine based samples.
This data demonstrates how close the development is to its target performance. AET believes that within a short time it will complete its objective of producing an accurate, low-cost, pocket-sized field instrument for total ammoniacal nitrogen analysis that requires minimal operator skill and no technical support 1As defined in the WRc publication NS 30, A Manual on Analytical Quality Control for the Water Industry, MJ Gardner, 1989.