WATER & WASTE TREATMENT
EnhancEd PROcESS WITh OPTIMISEd cOnTROL
the selector. The design of this selector and recycle rate allows for the formation of macroflocs in which simultaneous nitrification, denitrification, BOD5 removal and biological phosphorus removal occurs. The reactor aeration blower continues to run with no incoming flow and is controlled either by a dissolved oxygen monitor in the aeration zone which controls to a manually set point, or by oxygen uptake rate (OUR), when the first set- point is automatically calculated by the PLC. The aeration blower is then stopped, allowing the sludge to settle. Finally, the settled, treated wastewater is discharged by a motor driven decant arm, the sludge is removed and the cycle restarts. The system has three Operating Modes:
The project has been very much a collaborative one with regular meetings between Southern Water, Trant Engineering and Te-Tech’s ACT team to ensure compliance with Southern Water standards as well as Water Industry Mechanical and Electrical Specifications (WIMES). This has been enhanced by Te-Tech’s in- house control and instrumentation expertise and knowledge of the industry
Southern Water’s Hawkhurst South Wastewater Treatment Works gets an upgrade using enhanced SBR technology, new control panels and MCCs to optimise the process and reduce energy demand, says Te-Tech’s Stuart Rice...
W
ith stringent new ammonia and phosphorus discharge limits coming into force as part of the Water
Industry National Environment Programme, together with forecast population growth, Southern Water decided to construct a new treatment plant at their Hawkhurst South Wastewater Treatment Works.
in a sequence: fill/aerate, aerate, settle and decant. The process is configured with two or more batch tanks installed in parallel with their sequences out of phase with each other allowing for a continuous flow. During the fill/aerate stage, water enters an
In addition, the
population equivalent was set to rise from 1976 currently to 2285 by 2030 and the U-IMP5 Urban Wastewater Treatment Directive driver dictates that the FFT will increase to 17.1l/s by 2035. To meet the new consents and flow requirements, a new treatment plant is being constructed along with other extensive modifications to the existing works including chemical dosing and sludge handling. Trant Engineering was appointed as Engineering, Procurement & Construction (EPC) contractor for the project using te-cyc advanced cyclic activated sludge technology for the treatment process, and the technology is supplied by Te-Tech Process Solutions.
The treatment process The process is an enhanced sequencing batch reactor (SBR), where secondary biological treatment and associated settlement are combined in a single tank. Treatment takes place in four distinct stages
34 SEPTEMBER 2021 | PROcESS & cOnTROL
anaerobic selector tank and then flows into the aerated zone, aerated by duty/standby variable speed air blowers, and a portion of the sludge is constantly recycled to the inlet of
(DWF) Dry Weather Flow Cycle, (WWF) Wet Weather Flow Cycle and a Maintenance Cycle, each of which has a different operating cycle. In the Maintenance Cycle, one tank is taken offline and the cycle times of the remaining tanks adjust accordingly. The Operating Mode selection can be toggled between manual or automatic. In the latter, the PLC switches between DWF and WWF modes depending on influent flow rate.
Functional Design Specification Clearly, this process needs to be controlled and this aspect of the project was developed by Te-Tech Process Solutions’ Automation, Control and Technology (ACT) team. The team prepares a Functional Design Specification, specifies the instrumentation and PLC I/O and writes the PLC programme. It manufactures MCCs and control panels in house, so develops the project from design through to FAT and on-site commissioning. The OUR control system prevents over-
aeration of the reactor basin, optimising performance and creating energy savings.
It
uses only a measurement of the dissolved oxygen within a reactor basis to determine the oxygen uptake rate of the biomass in real time and optimises the operating cycle times to minimise air blower operation, the principle energy consumer of the system. A survey of several operational te-cyc plants of varying capacities showed that, on average, the energy required for plants using OUR control is 27% less than those without, and around 70% less than traditional activated sludge processes. Trant Engineering also used Te-Tech’s ACT
team to design and build a new top entry Form 4 MCC and a new MCCB distribution board and mains/generator automatic changeover panel for installation in the existing Control Room.
Te-Tech Process Solutions
www.te-tech.co.uk/solutions/automation- control-technology
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