Crawley STW Extensions

This contract for the complete mechanical and electrical works associated with the extensions and improvements of Crawley Sewage Treatment Works (adjacent to Gatwick Airport) was awarded by Thames Water utilities Limited in March 1990. The ?2.2 million contract for the mechanical and electrical works was completed on schedule in May 1991.

The provision of sophisticated monitoring equipment and networked PLC control throughout the plant together with a full SCADA system was a key feature of the this project.

On completion, a rigorous programme of oxygen transfer testing was conducted in the new Stage 3 aeration tanks to verify the guaranteed performance and to ensure compliance with discharge consent standards.

The mechanical and electrical works at this site feature:

- Fine bubble diffused aeration
- Stormwater pumping
- Return and surplus activated sludge pumping
- Anoxic tank mixing
- Sludge consolidation
- Standby power generation
- Primary tank auto-desludging
- Final Settlement

Plant Instrumentaton and Monitoring
Sophisticated equipment installed to monitor existing and Stage 3 flows, levels, pressures, rotation, suspended solids and dissolved oxygen, providing both local control and overall plant monitoring signals.

Electrical Works
Extensive modifications and additions to existing control panels and installation of new panels together with all necessary cabling were carried out during the course of this contract with minimum disruption to the existing plant operations.

Final Settlement
Half-bridge scraper mechanisms installed in the three new 23 metre diameter final settlement tanks serving the Stage 3 treatment stream. Refurbishment of existing final settlement tanks including the replacement of one half bridge scraper mechanism and installation of scum removal equipment and "V" notch weirplates to the remaining tanks.

Plant Control and SCADA
Overall plant control based on five networked PLC's together with a Dexterity SCADA package running on a VAX VMS computer system. Facilities are also provided for separate data interchange between the PLC network and the existing Datacall System to enable designated alarms to be transmitted via the PSTN whilst the works are unmanned.

Fine Bubble Diffused Aeration
Six variable speed Roots air blowers, each rated 110kW, installed in the existing blower house serving the existing and Stage 3 treatment streams.

Flexible membrane fine bubble diffusers installed in the three new aeration tanks capable of transferring in excess of 500kg of oxygen per hour into the Stage 3 treatment stream. The diffuser installation provides tapered aeration, automatically regulated in response to both primary and secondary dissolved oxygen monitoring.

Extensive refurbishment work was carried out on existing aeration tanks including the installation of new flexible membrane diffusers and replacement of weirstocks and penstocks.

Stormwater Pumping
Submersible pump unit rated 132kW installed in the existing main works pumping station forebay, providing additional standby stormwater pumping.

Return and Surplus Activated Sludge Pumping
Three submersible RAS pump units and two submersible SAS pump units installed in the new pumping station, serving the Stage 3 treatment stream.

Anoxic Tank Mixing
Two submersible mixer units installed in the new anoxic tank, serving the Stage 3 treatment stream.

Sludge Consolidation
Automation of the existing sludge consolidation facilities.

Standby Power Generation
Diesel driven generating set rated 750 KVA, installed in the main works pumping station and replacing a 375 KVA unit that was prepared, removed and mothballed for long term storage.

Primary Tank Auto-Desludging
Nine rotary lobe pump units installed adjacent to existing primary tanks to provide automatic desludging.