Testing the water
New methods and sensors for measuring nutrient levels in water systems can help prevent harmful eutrophication, says Rosa Richards
Nutrients are essential for biological systems, but excessive amounts can create problems of algal blooms (and eutrophication) in rivers, lakes and bathing waters. In the UK, phosphorus has been the major reason for failure of Good Ecological Status for the European Water Framework Directive (WFD). Although levels of 0.25 milligrams per litre (mg/l) or even 0.1mg/l are now technically achievable, is this always desirable from the perspective of energy use and the resulting carbon emissions? And how are nutrients monitored in water?
In the UK, as elsewhere, point source discharges of industrial wastewater into surface water are regulated. A company wishing to discharge wastewater must apply for a permit, which sets the limits for the constituents and volume of discharges in order to protect the receiving water body. In response to the UK government’s Red Tape Challenge in 2011, the Environment Agency (EA) committed to using flexibility within water quality permitting. The EA in England has begun trials to assess innovative ways of regulating discharges using flexible catchment permitting and nutrient balancing.
At full stretch
Flexible permitting uses a range of options that can be used individually, together or interchangeably, but the EA is open to new ideas and proposals. Traditionally, industry will design wastewater treatment systems to overperform against a permit limit, in order to reduce the risk of non-compliance. A water company with an ammoniacal nitrogen limit of 3mg/l may design wastewater treatment processes to achieve 1.5mg/l. This buffer drives extra cost, energy use and carbon emissions.
In contrast to permitting point source discharges individually, catchment permitting allows discharge permits to be set up collectively to the whole catchment scale, using this overperformance as a stretch target (Figure 1). This is achieved through an operating techniques agreement (OTA) to link multiple permits. If one site does not achieve its stretch target but another over-achieves, the target load reduction in the catchment can still be met overall (Table 1). The OTA is linked to the permit by a specific condition, and enforcement action can be taken if the agreement is not complied with. “This can be a more efficient and effective way of achieving water quality objectives in a catchment,” says Barrie Howe, national senior regulatory advisor at the EA.
Indeed, a cost benefit analysis of a trial by Wessex Water (2017-2020) in the Bristol Avon catchment has identified savings of £23m, as well as carbon emission savings (Table 1). So far, says Howe, the results are “very promising”.
Traditionally, end-of-pipe techniques were used to achieve water quality objectives in water bodies. Catchment nutrient balancing (CNB) uses alternative methods to reduce nutrients, such as land use changes; these types of solutions are encouraged by the WFD. Buffer strips can reduce nutrients entering rivers from agricultural run-off, while a carefully sited settlement pond on a tributary can prevent sediment pollution of the receiving river. Industry can use these kinds of measures to augment treatment at wastewater treatment works (WwTW), with OTAs used to link measures to the permits. This can allow water quality objectives for the catchment as a whole to be met while reducing cost and carbon emissions – as well as providing other ecosystem services benefits, such as reduced siltation or habitat creation. These innovative approaches are proving to be successful in saving money and carbon emissions while
still achieving sufficient protection of water bodies.
“The EA is expecting to receive many more proposals for flexible catchment permitting in the coming years,” explains Howe. “We are investigating the application of flexible permitting to chemicals, metals and organics in future.”
In order to evaluate the effectiveness of measures to control pollution in catchments, monitoring is vital. The EA mainly uses handheld meters for spot tests and telemetered sondes (water sensors connected to telemetry for continuous monitoring and transmission of results) to measure the common water quality parameters of temperature, conductivity, pH, dissolved oxygen, turbidity, ammonium and chlorophyll. They also use probes to measure oxidation or reduction potential, blue-green algae and dissolved organic matter. Phosphorus levels are harder to assess and require expensive equipment; an online soluble reactive phosphorus analyser is currently being trialled.
The optical probes for dissolved oxygen, turbidity and chlorophyll work well and are low maintenance, but nutrient monitoring is more difficult, says Jerome Scullin, senior field scientist at the EA. “Measuring phosphorus is difficult as we are still using large semi-portable instruments that are labour intensive,” he explains. “The phosphorus sensor involves wet chemistry, so we still need site visits every couple of weeks to top up reactants, and the fluidics in the sensor are quite small, which has its advantages but is pretty useless in winter where there’s a danger of freezing.”
There are some advanced top-end sensors on the market to measure phosphorus in real time, such as the Hydrocycle-PO4, with built-in intelligence that flags up when data quality starts to reduce and maintenance is needed. This sensor can be adjusted to measure water quality in freshwater or seawater. Scullin says that monitoring ammonium also has its challenges: “Although the ion-selective electrode is small, cheap and very portable, it suffers from interference and drift that are characteristic of those sensors.”
There are developments on the horizon for measuring nitrate and nitrite. In response to a US Environmental Protection Agency (EPA) Septic System Sensor challenge, TE Labs in Ireland has developed an online nitrate and ammonium sensor called Ecosens Aquamonitrix, which combines microfluidics (tiny quantities of fluids used in small tubes) with a LED detector to measure the absorption of wavelengths of light by water samples mixed with reactant. Accuracy so far is proving very good. Aquamonitrix is currently undergoing performance testing, and ISO 4034 Environmental Technology Verification verification reports and statements are due to be completed in February 2020. Multiple online units could be connected to cloud storage systems so that they can be accessed by customers to monitor different sites.
“Measuring phosphorus is difficult as we are still using large semi-portable instruments”
The University of Southampton is also working on a microfluidic sensor system, this time using oil droplets. The system miniaturises wet chemistry reactions combined with detection of absorption of light using a UV LED detector. It provides low-cost, on-site, real-time measurement of nutrients at high frequency, and with low reagent use and a high limit of detection. A combined nitrate/nitrite sensor has already been developed and field tested. At the time of writing, a phosphate sensor is due to be commercialised by the end of 2019, and an ammonium sensor will follow in 2020; the system will then be adapted to detect other nutrients, pollutants or biochemical molecules. A data management and sharing protocol has already been developed for data transmission purposes.
The EA is leading the way with innovative ways to regulate wastewater discharges, including flexible catchment permitting and CNB. Monitoring is vital and although nutrient monitoring has its challenges, developments will soon mean on-site, real-time monitoring is possible with transmission of data to a central database via telemetry.
Rosa Richards is an independent environmental consultant specialising in water policy and monitoring. She is a freelance science writer and programme manager of the Sensors for Water Interest Group (SWIG).
This article is based on a SWIG workshop on developments in nutrient monitoring, held in July 2019.
Rosa Richards is an independent environmental consultant specialising in water policy. She is a freelance science writer, programme manager of the Sensors for Water Interest Group (SWIG), and distance learning tutor of integrated environmental management at the University of Bath.