Wessex Water awarded this G90 contract in March 2000. The original scope was increased to accommodate major electrical modifications found to be required during the design works to give a final contract value of ?650k. Naston was Principal Contractor under CDM for the second stage of this project.
Project Scope The existing SBR process at Portbury Wharf was found to be failing consent conditions so this project would provide improved treatment from increasing retention by conversion to a conventional activated sludge process. Increased aeration facilities were provided to the four process tanks as well as two new final settlement tanks and RAS, SAS and scum pumping stations. A new control system was also installed to improve aeration control.
Aeration Blowers The oxygen requirement for the new process was significantly increased hence two new twin-speed blowers were installed complete with acoustic enclosures to limit noise levels to 75dBA. The five existing blowers were also upgraded to suit their new duties. The blowers are controlled by a new pressure transducer located in the air main.
Aeration Pipework Although the existing diffusers were retained, much of the existing stainless steel pipework was too small for the increased airflow so this was replaced. The new pipework was required to connect to the existing blowers discharge pipework and diffuser dropper pipework so required accurate surveying and fabrication. The changeover required close liaison with Wessex Water to maintain the process operation during the installation works. Process downtime was limited to 2 hours to prevent sludge settlement and microbial distress so all connections to existing pipework had to be made within this period. New ultrasonic flowmeters were installed in the air mains to each of the four aeration tanks to allow improved process monitoring. Each tank is split into three process zones, new modulating control valves and dissolved oxygen monitoring systems were installed to provide accurate process control. Severe foaming was experienced during the installation period so Naston installed temporary dosing equipment to add foam suppressant to the influent.
Final Settlement Tanks Two new final settlement tanks were constructed and provided with half bridge scrapers. These included integral tipping bucket type scum removal systems.
Each tank was equipped with sludge blanket monitoring and trending equipment with radio transmission of signals to the local control panel.
FST Area Motor Control Centre A new motor control centre located within a purpose built kiosk was installed to control the scrapers and RAS, SAS and scum pumping stations. This equipment was PLC controlled from this MCC and is fully operator adjustable using the MMI provided.
RAS/SAS pumping stations A new pumping station was provided with three variable speed submersible RAS pumps and two submersible SAS pumps. MLSS monitoring of the aerated effluent and magnet flowmeters on the RAS and SAS discharge lines were provided to allow process optimisation. The pump station structure also incorporated automated FST desludging bellmouths. Full lifting facilities were provided to allow pump and valve removal for maintenance.
A separate scum removal pump station with two submersible pumps was also provided. Additionally the existing aeration MCCs were uprated and an increased power supply installed to suit the new requirements.
Automation and Control Systems The existing SBR PLC control system was effectively made redundant under the new process, hence a new PLC with MMI was provided to control the new equipment. The process control programme was developed in close conjunction with Wessex Water software engineers to ensure a smooth changeover between the two systems.
Portbury Wharf STW Extensions
