Water is vital for all living things. In order for our cities and communities to survive long term, water, the essential material for life, must be recycled! With growing population demands for clear, clean, pure water, a growing emphasis has been placed on higher standards for wastewater treatment plants. Potchefstroom is no exception and with this objective in mind, Streamworks was contracted to implement an automated control system for Potchefstroom Waste Water Treatment Plant.
The operation and control of the many steps in the wastewater treatment process is watched from a central control room and the facility is under constant monitoring and control by a team of system operators. The principal objective of the system is to control all sections of the sewage plant, old and new, with particular emphasis on the efficient running of aerators that in turn control the amount of energy consumed by the plant.
System overview
The Potchefstroom Waste Water Control System consists of a distributed Citect scada server situated in the control room. This server workstation communicates with a Citect scada display client situated in the lab, which is a few hundred metres away, via a fibre-optic network. The Citect Server also communicates with the control PLC, a Klockner Moeller PS416 CPU 400, via RS232. This PLC gathers all pertinent plant information regarding status, alarming, trending and plant control. A Microsoft Access Database is used to report inflow and outflow to and from the sewage works via an ODBC connection. The I/O count is in the order of 500 digitals and 100 analogs.
The PLC controls various sections of the waste water treatment plant which are listed here:
* Mixers in the balancing tank.
* Settled sewage pumps in the balancing tank.
* Acid fermentation valve outlet flow.
* Inlet works.
* Primary sludge pump station.
* Old works feed pumps.
* Weir control at the biological reactors.
* Aerators in the biological reactors.
* Waste mixed liquor pump station control.
Plant process
Raw sewage arrives at the Inlet Works from the Potchefstroom catchment area. From the Inlet Works the raw sewage moves to a divisional box where the raw sewage is distributed between two primary sedimentation tanks. The primary sedimentation tanks separate the primary sludge from the raw sewage and allow the sludge to settle on the floor of the tank.
The rotating half bridge scrapers move the settled primary sludge towards a central hopper. Scum baffles on the primary sedimentation tanks retain scum on the surface of the tanks. Scum from the removal beaches on the sedimentation tanks gravitates to a collection sump where two submersible scum pumps propel the scum to the digesters or balancing tanks. The settled sewage is gravity-fed to the balancing tank, which is designed to provide the new works with a constant flow of settled sewage that is kept mixed in the balancing tanks by a series of mixers.
Three submersible sewage feed pumps propel a preset flow of settled sewage to division box 2. Division box 2 controls the flow to the old works and biological reactors A and B. The biological reactors are zoned and have aerators and weirs for controlling the amount of dissolved oxygen.
From reactors A and B, mixed liquid flows to division box 3, which distributes this liquid between the four secondary sedimentation tanks. Phase separation of the mixed liquid is carried out in the secondary sedimentation tank, which is in continuous operation. Each tank has a sludge outlet valve, which allows the sludge to gravitate to the return activated sludge pump station where it is pumped back to the reactors. The waste-mixed liquid then gravitates from division box 3 to the waste-mixed liquid pump station where it is pumped to the sludge lagoons.
The primary sludge gravitates to the primary pump station where it is pumped to the acid fermentation tank or directly to the sludge lagoon. The acid fermentation tank allows the formation of volatile fatty acids from the primary sludge that is mixed in the mixer. An overflow system in the acid fermentation tank carries the overflow back through the main process. Fermented sludge is removed from the fermentation tank by gravity back through the main process.
The entire project is to be completed in a series of phases: Jocks Santos of Streamworks carried out engineering of this particular section of the project as well as onsite training for all system operators.
For more information contact Shannon Solyom, Streamworks, 011 803 4376, [email protected], www.streamworks.co.za
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