Enhanced Process with Optimised Control

Southern Water’s Hawkhurst South Wastewater Treatment Works gets an upgrade using enhanced SBR technology, new control panels and MCCs to provide plant optimisation and reduced energy demand, says Stuart Rice.

With 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 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 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 through the system.

During the fill/aerate stage water enters an anaerobic selector tank and then flows into the aerated zone which is aerated at a controlled rate by duty/standby variable speed air blowers, and a portion of the sludge is constantly recycled to the inlet of 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: (DWF) Dry Weather Flow Cycle, (WWF) Wet Weather Flow Cycle and a Maintenance Cycle. In DWF mode the operating cycle is set at four hours although, over the course of a day the cycle times are shifted by several minutes to avoid the cycle starting at the same time every day. In the WWF mode, the overall cycle time is reduced to 3 hours and the individual phase durations are reduced accordingly. 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.

Published in Process & Control, September Issue Read full article: https://content.yudu.com/web/69r/0A170s6/PCsept21/html/index.html?page=40&origin=reader

Automation, control & technology: https://www.te-tech.co.uk/solutions/automation-control-technology

System integration: https://www.te-tech.co.uk/solutions/automation-control-technology/system-integration-and-operational-technology

te-cyc™ technology: https://www.te-tech.co.uk/products/wastewater-treatment-products/te-cyc

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