Thursday 15 April 2010

Metals monitor improves influent understanding

Sometimes you see a name which you know many people outside of the area might find strange and you yourself smugly know where it is. This is a story of such a place - Fazakerley.The United Utilities wastewater treatment plant at Fazakerley (It's near Liverpool in North West England, said he smugly!) has installed a continuous metals analyser from Cogent Environmental to help investigate variable heavy metal content arising in the plant's catchment.

United Utilities staff are working with the British Environment Agency to gain a better understanding of the heavy metal sources in the catchment so that an appropriate solution can be identified. The monitor, an OVA5000, is providing continuous monitoring data at the plant's outfall for Copper, Nickel, Cadmium and Lead.

The continuous, real-time heavy metal monitoring instrument was first installed on a 3 month trial in April 2009. Since that time the OVA5000 has performed very well with no failures or significant downtime and was therefore purchased so that continuous monitoring could be extended.

The Fazakerley plant receives effluent from domestic sewage and industrial premises. The average flow is 54,000 m3/day with a maximum flow of 140,000 m3/day. This volume represents a high proportion of the flow in the receiving watercourse.

Alison Summersfield, Lead Operational Scientist for Wastewater, has been involved with the trial since its inception. Commenting on the value of real-time continuous data, she says, "We realised from the beginning that sampling for laboratory analysis would not be feasible because the concentration of heavy metals in the influent is extremely variable and consequently spot measurements are unlikely to provide meaningful data. In addition, results from laboratory analysis can incur long delays by which time the situation will have either changed or disappeared. The suggestion to employ an OVA5000 came as a result of successful trials on another site at which continuous chromium monitoring was conducted.
"Real-time data enables us to identify the frequency and timing of high concentrations and qualitative data may help us to identify the source of the metals content."

Data from the OVA5000 is currently collected by laptop, but will shortly be connected to the plant's monitoring and control system. In addition, an auto sampler will be triggered by the monitor so that samples can be taken immediately if effluent concentrations exceed pre-determined levels at any time of day or night. This will enable a more detailed laboratory analysis of high sample results to facilitate a closer identification of the pollutants' source.

The OVA5000 is able to measure using both Anode Stripping Voltammetry (ASV) and Cathode Stripping Voltammetry (CSV) techniques delivering limits of detection down to as low as 0.5ppb, depending upon the metal being measured and the sample matrix.

The maintenance requirements of the OVA5000 are minimal, requiring approximately 30 minutes per week by United Utilities staff (pump calibration verification, cleaning sample lines and analysis cells). A small quantity of reagents is consumed in the measurement process and these require top-up or replacement every 2 to 4 weeks. In addition a service contract has been put in place whereby Cogent Environmental staff make quarterly visits to carry out more extensive maintenance.

The OVA5000 is factory configured to meet application requirements and is able to measure up to six sampling streams.
Sample pre-treatment options include acid/ultraviolet light digestion for the removal of potential interferences.
Typical metal applications include: Arsenic (As), Cadmium (Cd), Chromium (Cr), Copper (Cu), Lead (Pb), Iron (Fe) Mercury (Hg), Molybdenum (Mo), Nickel (Ni), Zinc (Zn) (Cogent Environmental continuously works to expand this list)

In addition to applications involving the monitoring of wastewater discharges, the OVA5000 is also employed in many countries including China, Korea and Taiwan to monitor river water for the identification of sources of heavy metal pollution and for intake protection for drinking water. It has also been deployed in process control applications.

Summarising her experience with the OVA5000, Alison Summersfield says, "The overall objective is to ensure that effluent metal concentrations remain within the consent for the site and this would be more difficult if we did not have access to continuous data. The OVA5000 has run constantly since April 2009 and since that time it has proved to be very reliable. Data from the monitor compares favourably with lab analysed samples. "As we establish a more detailed picture of the heavy metal patterns at the plant we look forward to being able to work with the Environment Agency to ensure that the environmental performance of the plant is optimised."

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