Wastewater investments focus on control and automation

Paul Boughton

As the latest investments show, everything from water quality to leak detection depends on having the very best in automation and control technology.

One of the major UK water companies, Severn Trent Water currently serves over eight million customers. Each year the company hosts a two-day event, the quality working day, to promote projects and supply teams to promote projects and supply teams which have delivered exceptional standards in their relative disciplines.

Lintott Control Systems is currently working on a framework agreement with Severn Trent Water on the design and delivery of its fluoridation project for the supply of packaged modular designed chemical dosing systems to a number of locations across the region.

During the recent quality event, this approach won two of the three awards it was entered for: a gold award for collaborative working and a bronze award for highest standards.

The fluoride dosing equipment which is located in the region has come to the end of its economic life

Due to the age of the equipment and the changes in operational approach the equipment was not able to be replaced on a like-for-like basis and required an entire re-engineering and re-assessment of an approach to be standardised for future supply.

Specialist resin

Equipment developed by Lintott utilised a 4m wide by 4m high by 10m long glass reinforced plastic (GRP) kiosk with an additional specialist resin coating to prevent the attack of acid onto the glass construction. The equipment was then divided into three distinct areas covering control, dosing and bulk storage.

The control section was fitted with a Lintott's control system and Lintott software in an entirely sealed room to protect the equipment from any potential fume or liquid attack.

Because of the implicit risks to public health in the event of chemical overdosing Lintott designed, supplied and installed control and software system to SIL1 standards, incorporating hardwired safety shutdown systems monitored on several layers.

The control system incorporated and Allen Bradley ControlLogix PLC and a PanelView Plus 1000 HMI. The software was produced using Severn Trent Water function block libraries and graphics, although due to the SIL rating Lintott Control Systems was required to develop a solution to enable the PLC to have an additional protected area of the software code that monitors the PLC internal system safety shutdowns. This protected area of the PLC is isolated from general PLC access by a second layer of security. The HMI also contains full trending, manual control and status monitoring facilities.

The dosing room contains duty/standby dosing and transfer pumps in fully segregated polypropylene dosing skids. Adjacent to the dosing room is the bulk storage facility which houses bulk storage chemical tank, day tank and product overflow waste tank,all built to BS EN 12573 and located in a 110 per cent bund. The whole system is serviced by forced ventilation and extraction to ensure chemical fumes are not able to build up over time. All pipework is PVDF and exotic metals where metallic components are used.

Pre-packed equipment

As the equipment is pre-packed Lintott was able to build it off-site and then have it fully testing and ready for installation in a 16-week period. By the use of innovative production line techniques and 3D modelling, Lintott ensured that the time sensitive programme did not have an adverse affect on quality and also provided system standardisation with its increased commercial and health/safety/environment benefits.

Meanwhile Welsh Water is investing £200 million to upgrade its water treatment works across Wales by 2015. This includes the water treatment plant at Cwellyn, where Rotork intelligent electric valve actuators have been installed to control the flow through a new state-of-the-art extension to the works.

The new plant at Cwellyn has been designed and built by Black & Veatch, one of Welsh Water's asset management alliance partners for water supply capital investment schemes. The new treatment regime is designed to clarify the water before it enters the existing works in order to deal with changing raw water conditions and consistently meet all water quality standards.

Rotork IQT direct drive quarter-turn intelligent electric actuators have been installed to operate butterfly valves controlling the flow of water throughout the new treatment plant (Fig. 1).

Patrice Nadouce, mechanical engineer for Black & Veatch at Cwellyn, explained that Rotork was selected following a competitive bidding process. The selection criteria included value for money, reliability, low cost of ownership and operator familiarity.

All the Rotork IQT actuators are Profibus DP-enabled and linked to an existing distributed control system (DCS) on five two-wire bus networks. The operation of the plant is supervised by a SCADA system designed by Oasis Engineering.

Water from Llyn Cwellyn, some two kilometres distant, gravitates to the new treatment plant at a rate of up to 850 m3/hour. The flow initially passes through inlet static mixers where coagulant is dosed to maximise the performance of the dissolved air flotation (DAF) process. The flow is divided into three identical DAF streams, which consist of one flocculator followed by one DAF cell where particles in suspension are floated to the surface to form a sludge blanket. The clarified water is then filtered through first stage rapid gravity filtration (RGF) before being pumped to the existing treatment works through ultra-yiolet disinfection to deactivate cryptosporidium.

Automatic desludging

The Rotork IQT actuators, including some modulating units, control all valve operations including the inlet works flow control, automatic desludging of the DAF cells, automatic backwashing of the RGF plant and the plant treating the effluent created by the clarification and filtration processes.

The successful completion of the upgrade project at Cwellyn will safeguard the drinking water quality for 70 000 Dwr Cymru Welsh Water customers in the Caernarfon area.

In Germany, Siemens Industry Solutions Division has worked with Siemens Corporate Technologies to develop a method of finding and continuously monitoring leaks in water pipes.

Siwa LeakControl is based on flow measurements and special software for evaluation. It can be used with all sizes and types of pipes, and helps to keep water losses as low as possible.

Precise knowledge of water losses is essential for operating and planning the maintenance of drinking water networks efficiently. The condition of the pipes cannot be continuously monitored, but an indication can be obtained, for example, from the volume of water losses. There is a wide variety of ways of doing this, but they are generally only applied at selected points.

For example, some are based on the evaluation of noise levels emanating from leaks in the networks of water pipes. This investigation is performed either at regular intervals, such as annually, or only when leaks are suspected. However, such a procedure makes it difficult to detect a suspected leak immediately.

This is the strength of the new Siemens Siwa LeakControl location and monitoring system, which not only continuously checks for leaks, but also pinpoints them automatically. This is done by setting up district metering areas, in which the inflows and outflows of water are measured by ultrasonic flow meters.

Existing measuring systems, such as magnetic inductive flow meters, can, of course, also be integrated. The measurements are passed to an evaluation system, which detects leaks and allocates them to the individual zones by statistical methods and a model-based, network-wide mass balance.

The suspected leaks are then found by temporarily installed acoustic sensors, and finally pinpointed down to the exact meter by correlators

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