A 50 tonne, 15 metre, driver operated weighbridge supplied by Calibra Weighing Systems is playing a pivotal role at the Severn Trent sewage treatment works on the outskirts of Kidderminster.

A 50 tonne, 15 metre, driver operated weighbridge supplied by Calibra Weighing Systems is playing a pivotal role at the Severn Trent sewage treatment works on the outskirts of Kidderminster. The surface mounted steel weighbridge has been installed as part of a GBP1.4 million upgrade at the site, carried out by main contractors Edmund Nuttall Limited. Calibra Weighing have a long-standing working partnership with Severn Trent and have already installed similar weighbridge systems at nine other sites across their operating region.

The Kidderminster site processes raw sewage coming directly into the treatment works and also that brought in by tanker from surrounding pumping stations and storage tanks.

The sewage goes through a comprehensive treatment process before ending up as digested sludge cake and clean water.

The cake, which has the look and feel of garden compost, contains substantial quantities of nutrients, important trace elements and organic matter.

Severn Trent are investing significant time and money improving the treatment process at their sites in order to increase the amount of treated sewage that can go to agricultural use.

Strict controls on its processing and distribution ensure that it can be safely used by farmers to meet crop nutrient requirements and maintain soil fertility.

Severn Trent offer a comprehensive free support package for the farming community including soil analysis and advice on how much, and where, the sludge cake can used on their land.

This takes into account critical environmental issues and as a result strict records have to be maintained relating to the quantities of sludge cake delivered.

The weighing process at Kidderminster is fully automatic and this allows extended operating periods for the weighbridge.

The system, which is fitted with a master weight terminal at one end and slave key reader at the other, can be used to weigh product entering and leaving the site.

The main terminal enclosure has been designed in close co-operation with Severn Trent and incorporates the Calibra versatile 788 weight module, a thermal ticket printer, GSM modem and integral heater.

The 788 has a wealth of standard features and interfaces, and unlike most competitive products, its functionality can be readily adapted through the standard software.

The optically encoded keys are encoded with a unique hex number which accesses the data base of the 788.

When a vehicle enters the weighbridge, it triggers the induction loop in the entry ramp, automatically setting the system to zero, ready for weighing.

For a vehicle entering the site, the driver stops opposite the slave unit and places his key in the reader.

Each key, which can be easily carried on the drivers key ring, can be programmed to contain specific details of the vehicle, including registration number, contractor name and tare weight.

If the key is accepted, the first weighing is carried out and temporarily stored.

At the same time the green light on the reader is activated allowing the vehicle off the bridge.

If the vehicle is empty, the system checks the weight reading with the stored tare value and flags up any discrepancies outside a preset tolerance.

When the empty vehicle has been loaded the weighing process is repeated.

This time the vehicle enters the bridge from the other end and stops opposite the main 788 terminal.

The driver places his key in the reader and this initiates the second weighing.

Depending on the vehicle designation, the driver may be prompted to press the appropriate button on the 788 display to acknowledge whether the product being transported is raw or digested sewage.

If the weighing is accepted the driver is issued with a printed ticket.

The weighing process is broadly similar for loaded vehicles bringing raw sewage into the site.

Details of each and every weighing are stored within the main terminal and at the end of every day the information is emailed directly from the weighbridge to designated recipients in Excel spreadsheet format.

In the event of power failure, the system can store up to a weeks data in its flash memory.

As the Severn Trent Area Manager for Sludge Recycling Keith Pryce concludes: 'The Calibra weighbridge system is playing an important role in our process at a growing number of treatment sites, providing comprehensive weight and vehicle information.

We currently send around 350,000 tonnes of recycled sludge cake to agriculture annually and this is growing steadily.

There are very strict controls on the application of treated sewage on agricultural land and we have to keep detailed records on the amount delivered to specific farms.

Demand varies significantly throughout the year and the driver operated system gives us optimum flexibility.