Vaalkop WTP focuses on filtration upgrade

The water treatment plant at Vaalkop Dam south of Johannesburg is being upgraded to incorporate a Counter Current Dissolved Air Flotation with Filtration system, as Black & Veatch project manager Paul Urban explains.

An upgrade of the WTP at the Vaalkop Dam in South Africa is the site for the third-largest specified filtration system (Cocodaff) in the world.

Black and Veatch Africa, through its subsidiary Paterson Candy International South Africa (PCI) in joint venture with Turbo Projects was appointed by Magalies Water as the main mechanical contractor for the plant upgrade.

The system upgrade is split into two portions:

  • The Vaalkop Phase 5 extension at the Vaalkop Dam;
  • The construction of new pipelines and reservoir at Boschpoort.

The upgrade includes the construction of an additional WTP. The basic infrastructure of the extensions is designed for a capacity of 120Ml/d of potable water, however, the initial treatment module will produce 90Ml/d.

Vaalkop project

PCI’s scope of supply for the Vaalkop Project, covered the design, manufacture, supply and commissioning of the plant.

It utilises a Dissolved Air Flotation (DAF) system called Cocodaff (counter current dissolved air flotation with filtration) system. It is contained within one process module and is the third-largest Cocodaff unit in the world.

The flotation process relies on the production of micro bubbles, which adhere to the floc particles. This reduces the effective density of the floc particles causing them to rise to the surface to form a scum or sludge blanket.

This scum blanket is removed periodically allowing the clarified water to be filtered thereafter. An automatic control system controls the rotation of the Cocodaff units during operation.

The project embraces new raw water pumps, chemical-dosing plant, primary flocculation and settling tanks, secondary flocculation, flotation, filtration, sterilisation, supply to a new clear water reservoir and high lift pumps for delivery into the supply system.

Certain modifications to the existing flotation units on the Phase 2 and 3 scheme are included with six new flotation units at Phase 2 and 3.

Cocodaff process

The main process stream through the Cocodaff plant is gravity flow. Partially treated water is collected from three horizontal setting tanks and conveyed to a common flume via 800mm, 1200mm, 1400mm and 1600mm diameter piping.

The flume serves six secondary flocculation bays and when the fourth settling tank is added, a further 800mm diameter connection will be added to this piping and the flume will then serve eight secondary flocculating bays. The inlet gates to the secondary bays are of the weir type with free discharge characteristics on the downstream side.

This arrangement fulfils two important functions. It splits the incoming flow equally to each operating unit, and also provides a flash mixing facility at the entry to the flocculating bays for the coagulating chemicals, which are added at this time.

Each flocculating bay has a nominal retention volume of four minutes and is provided with under and over baffles to create the flocculating energy and prevents short-circuiting. At the first flow-under baffle an adjustable restricting plate is provided for adjusting the flocculating energy.

At the outlet of each flocculating bay a penstock is provided to pass the water into an inlet duct to the associated Cocodaff unit. The duct runs down the centre of the unit and connections are taken from it to 18 inlet manifolds serving the process.

Each of these manifolds are equipped with two upward facing conical discharge pipes which introduce the incoming flocculated water to the Cocodaff process at approximately 700mm below the top operating water level. The cones provide even distribution and velocity over the unit area whilst not disturbing the scum blanket.

At a level which is just below the inlet manifold pipes, a further manifold is provided in each half unit to distribute a high-pressure air saturated re-cycle flow, which is equipped with a series of nozzles.

The resulting micro-bubble blanket rises vertically to meet the downward flowing flocculated inlet water flow to provide the counter current flow pattern of the process. The reaction causes the flocculated particles to be conveyed to the surface to form a scum blanket.

The clarified water from this process now continues to flow downward to rapid gravity filter beds at the bottom of each cell. These beds are of the dual media type.

Each half filter bed is equipped with a PCI type ‘D’ underdrain system, which is designed to collect the filtrate and evenly distribute the air scour and washwater flows used to cleanse the bed.

When the depth of the scum blanket at TWL builds up to a point where evacuation is required, the outlet flow from the unit is temporarily stopped. The surface level is allowed to rise by a small amount, until it is at the point to discharge over into the scum channels placed along the side of each half bed.

At this time the inlet flow to the unit is stopped and transferred to a surface flush channel above the inlet duct. The incoming water is now introduced to each half unit via two ski-jump weirs. This pushes the scum blanket into the scum channels, which discharge to a common waste channel and thus to the waste disposal system. The evacuation of the scum blanket only takes a few minutes, thereafter the unit is returned to normal operation.

After filtration, the filtrate passes through an outlet control valve, which is automatically controlled to maintain the TWL in the unit, within very fine limits. Downstream of this valve the filtrate flows into an outlet chamber, which is provided with a retaining wall to always ensure the valve operates under a positive head.

After flowing over this wall, the filtrate from each unit is passed into a common outlet flume, which is connected to the clear water reservoir by a 1200mm diameter pipe.

Duty and standby air blowers are provided to effect an air scour to the filters during a cleansing programme, together with three washwater pumps (two on duty and one standby) to backwash the filter beds after the air scour.

Initiation for a washing programme is automatic and all valves, penstocks and washing machinery are automatically controlled during a cleansing cycle. The backwash sequence can be initiated manually, after which the washing process shall be carried out automatically.

The recycle water for the flotation process is provided by five pumps drawing from the clear water reservoir. When the plant is extended to a capacity of 120Ml/d, a further pump will be added to provide a four on duty, two on standby configuration.

Pumps elevate the pressure of the water and deliver this via a common main to two saturator vessels mounted in the upper filter gallery. These units are sized to cope with the up-rated plant capacity of 120Ml/d.

Three high-pressure compressor units are supplied (two duty and one standby) to provide air for delivery to the saturator vessels. These units are also sized to cope with a plant flow of 120Ml/d.

All the main valves and penstocks for the six Cocodaff units and their associated flocculation units are required to be operated during automatic de-scums or during automatic washing cycles. These valves and penstocks are equipped with pneumatic actuators except for the filter outlet and wash water inlet valves, which are supplied with electrical actuators.

All these actuators are provided with end of travel limit switches to monitor their correct positioning during an automatic wash. Separate duty and standby compressors are provided for operation of the pneumatically operated valves and penstocks.

One solenoid valve is provided per unit, which is connected to a water supply line to provide a spray facility to help scum evacuation during de-scumming of the float.

A centralised control panel in the upper filter gallery contains a PLC programmed to control and monitor all functions of the plant, including the sludge discharge valves for the primary settling tanks.

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