Instrumental to accurate data
Peta Glenister of Solarton Mowbrey explains why the company has been chosen to supply a range of measurement instrumentation at Anglian Water's showcase WwTW in Lowestoft
The plant, commissioned in December 2001, provides a unique opportunity to demonstrate innovative technologies developed by companies within the Anglian Water Group.
Solartron Mobrey was chosen as the preferred supplier for level and pressure measurement systems as a result of Purac’s previous experience with the products, which include MSP Series ultrasonic liquid level transmitters, 9000 Series hydrostatic level sensors and MSL600 sludge blanket level monitors.
“The Lowestoft plant is equipped with state of the art monitoring and control systems,” said Phil Windle, electrical instrumentation engineer with Purac. “Solartron Mobrey’s level and pressure transmitters proved simple to install and provide an accurate and reliable input to the plant’s control systems.”
MSP100 ultrasonic level transmitters are used in a variety of applications throughout the plant. The two-wire, loop-powered 4-20mA devices feature integral intelligent processing and built-in diagnostics, with Zone 0 approval for hazardous areas. In practice, the extensive gas detection and odour control systems throughout the plant ensure there are no Zone 0 areas, but certain areas are classified as Zones 1 or 2.
Preliminary treatment of raw sewage begins with mechanical filtering using adjustable screens. Periodically the screens become clogged and need to be cleared. Two MSP100s positioned above the inlet stream monitor the level on either side of the screens, when the difference in level exceeds a preset limit, indicating the screens are becoming clogged, a mechanical cleaning operation is initiated.
The inlet flow feeds into four Delcor spiral separators. These rotating devices use centrifugal force to remove heavier material from the main flow. For maximum efficiency the rotation speed must be adjusted continuously in proportion to the input flow. Four MSP100 transmitters are configured to measure the flow over a V-shaped flume at the input to each separator and this data is used to control rotational speed. MSP100s are also used on a number of process chemical storage tanks to provide a continuous and accurate measurement of the level within the tank.
On the foam
Some ultrasonic level technologies can be unreliable in difficult ullage conditions, for example, where foam forms on the surface of a liquid, a frequent occurrence in WwTWs. Hydrostatic level transmitters on the other hand, although unaffected by surface conditions, may be liable to inaccuracy due to sludge build-up on the sensor surface, another common feature of this environment. Solartron Mobrey’s fully submersible 9700 series effectively overcomes these problems, and over fifty are installed in a variety of applications throughout the Lowestoft WwTW, including effluent tanks and sludge digesters.
At the heart of the 9700 series is a ceramic capacitive sensor (CCS). CCS devices offer major benefits over traditional metal diaphragm sensors, especially in wastewater treatment applications, where their reduced susceptibility to the effects of sludge yields significant benefits. The low deflection (0.025mm) of the CCS reduces performance degradation and enables the sensor face to be made flush with its housing, eliminating crevices which encourage sludge build-up. Accuracy is ±0.1% of upper range limit, with excellent long-term stability. The 9700 series also offers 10:1 rangeability with no loss of environmental integrity. Transmitters can be adjusted to give full-scale output for any water level down to just one-tenth of the specified upper range, reducing the number of different transmitters required to meet any need across a large plant. The transmitters can be supplied with Chemraz seals, an important factor in their selection for the industrial effluent stream.
Additional transmitters are used to measure water and gas pressure throughout the plant, for example on water boosters and hotwater systems used to reduce the viscosity of sludge, and on the biogas generators, which are used to provide heating and power. Pressure sensors are also used in Purac’s proprietary Kaldnes system, a moving bed bio-film reactor. Specially designed polyethylene carrier elements support a high density of the bacterial growth which is used to remove BOD, ammonia and nitrogen from the effluent. The process is aerobic and air is bubbled into the effluent through fine nozzles at high pressure. Solartron Mobrey’s 9000 Series pressure transmitters are used to monitor this flow for maximum efficiency.
MSL600 sludge blanket level monitors are installed on each of the three settlement tanks. The control unit is mounted on a rotating bridge with remote sensors immersed in the liquid. Ultrasonic pulses from the MSL600 pass through the supernatant and are reflected by the settling sludge blanket, the timing of the echo is processed to give an accurate estimate of blanket thickness. Integral relays within the MSL600 trigger when the thickness reaches a pre-determined value, to control duty/standby pumps for removing the sludge. Blanket thickness is also relayed to the control room for monitoring by plant operators. A common problem in this application is that the submersed sensors become coated or fouled by sludge and cease to work accurately. The MSL600 has an integral air-purge system for self-cleaning at user-defined intervals, enabling the transmitter to function for longer periods between maintenance and reducing lifetime costs. Although not required at Lowestoft, another key feature of the MSL600 is an optional telemetry link which can significantly simplify installations on continuously moving booms.
Purac has also purchased an AC615 dual mode flow meter from Solartron Mobrey, following extensive tests at the Lowestoft plant. This clamp-on portable device uses sophisticated ultrasonic techniques to measure liquid flow rates in closed pipes. No connections to the pipe are needed, eliminating the possibility of leakage, and liquid flow is unaffected. It can tackle pipes with diameters ranging from 10mm-6.5m, with flow rates in the range 0.01-25m/s and is intended for use by Purac’s commissioning engineers at Lowestoft and elsewhere