Protecting wildlife from surface water run-off

Turbidity monitors are contributing to high-accuracy flocculant dosing at a Devon landfill site above a haven for aquatic wildlife

A problem that almost all landfill sites face is the treatment of surface water run-off and in particular, removing suspended solids prior to discharge. Where the landfill site is above a beautiful river valley and haven for aquatic wildlife, the problem is all the more acute.

For Devon Waste Management, surface water run-off from its Deep Moor site outside Great Torrington goes back into the water course and eventually back into the River Torridge. But before it is discharged into a local stream, the water is subjected to treatment that, following an upgrade in 2005, incorporates a flocculent dosing system that is reliant on the performance of two Partech Turbi-Tech monitors.

Accepting 85,000 tonnes of domestic and commercial waste a year, Deep Moor is Devon Waste Management's only landfill and covers 35 hectares. About half of it has already been landscaped, a further third is currently being landscaped, and around 20% is currently active. All the roads and partially restored areas generate surface water, and this collects at the lowest point on the site. The Environment Agency's discharge consent level for the site is 60mg/l of suspended solids.

The problem the site operators have to contend with is the varying amounts of surface water run-off from the roads, which pick up suspended solids. The surface water runs to a lagoon where Devon Waste Management operates a flocculent treatment system, which drops out all the suspended solids from the water.

Time for a change
Historically, the flocculent treatment dosing system has been controlled by turbidity meters that feed results back into the computer system. This provided a valuable service. But Devon Waste Management constantly reviews the latest environmental technologies to comply with discharge consents from the site. Working with Greenacre Pumping Systems of Newton Abbott, it identified that an automated plant would ensure the highest accuracy for results and data recording, and contribute to reducing maintenance costs.

"The replacement treatment system uses a Turbi-Tech sensor located in a manhole at the inlet to the dosing station, which provides a 4-20ma signal to the control panel in the dosing station and allocates a dosing rate per unit of flow," says Jim Kent of Greenacre Pumping Systems.

"This is then integrated with a 4-20ma signal from a flow meter. The dosing rate is different for flocculent and coagulant and is set in accordance with amounts decided by Devon Waste Management chemists. The PLC program enables the start/finish points and the slope of any curve of the dosing stages for each chemical - it is a curve that can be adjusted to any shape, but only in five stages. The second Turbi-Tech monitor is located in the outlet point and reads the turbidity level to ensure that it is below the consent level."

The Partech Turbi-Tech 2000 sensor is calibrated in accordance with Reference Method 2540 D Total Suspended Solids (Dried) and is operated in conjunction with the Partech 7200 Monitor. This sensor uses infrared light that is either scattered or absorbed by the particles in suspension, with the amount of received light being proportional to the level of suspended solids.

The geometry of the sensor, either light scatter or light attenuation, is chosen to suit the suspended solids or turbidity range. The amount of received light is converted into suspended solids by the 7200 Monitor using algorithms that have been developed specifically for these applications.

"The Turbi-Tech sensor has been designed to cope with the problems of fouling that are associated with wastewater treatment," says Angus Fosten, sales & marketing director at Partech Instruments. "The Turbi-Tech is ideal for the Deep Moor site where the operating location is some distance from the operating centre because it incorporates an integral cleaning mechanism that ensures accurate measurement without routine operator involvement."

Monitoring is constant
Colin Brown, projects manager at Deep Moor, adds: "All information collected in the dosing unit is transferred via telemetry to the main office control room. In addition to this constant monitoring, the discharge stream is monitored weekly both by ourselves and the Environment Agency. The dosing system is special because we have always wanted to ensure that the site has operated an effective system."

He goes on: "The success of the system is down to Greenacre, which was able to combine its knowledge of pumping systems with an awareness of the capabilities of Partech's turbidity monitors. The challenge that we face here is the very wide turbidity, which results from the different conditions on site. Coupled with this are the varying water run-off flows, which can range from 10l/sec up to 80l/sec. The system had to cope with a huge range on both scales."

The dosing control system reads both the turbidity and the flow and then works out the dosage and multiplies the dosage according to the flow. Operators at Deep Moor say that since the flocculent dosing system was commissioned, it has provided high levels of accuracy and efficiency.

Partech Instruments
www.partech.co.uk

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