Constructing the largest dewatering facility in the Western Cape

The Zandvliet Wastewater Treatment Works (WWTW) is undergoing a substantial upgrade with the construction of a new primary treatment section, equipped to accommodate up to 90 Mℓ/day average dry weather flow (ADWF), and a new large-scale sludge dewatering treatment facility.

Six waste-activated sludge gravity belt thickener and belt press cascades

Water and wastewater treatment specialist PCI AFRICA was awarded the design and build contract for associated mechanical, electrical, instrumentation, control and automation by the City of Cape Town, with the consulting engineers being Aurecon (now Zutari). This included new inlet works, primary sedimentation tanks, pump stations, as well as sludge dewatering and treatment facilities.

“The Zandvliet WWTW deals with effluent from the eastern parts of Cape Town, including Delft, Blue Downs and Khayelitsha. The new upgrade will assist significantly with hydraulic load relief on the existing works, together with the ability to remove waste-activated sludge (WAS) from the existing bioreactors. Urgent rehabilitation and new additions were required to meet the needs in this fast-growing catchment of Cape Town,” explains Cobus Mellet, project manager, PCI AFRICA. New plant The new sections of the contract comprise: • Part A: dewatering plant located on the eastern side of Zandvliet WWTW • Part B: inlet and primary treatment plant located on the northern side of the

Zandvliet WWTW. Currently, the dewatering plant is in the final preparation phase for commissioning, while the inlet works and primary treatment are in the installation phase.

Flow enters the Zandvliet WWTW via three gravity sewers (Khayelitsha, Blue Downs and Delft). The Blue Downs sewer is not being utilised at present. Since the entrance level of the raw sewage is far below ground level, part of the new inlet

Cobus Mellet, project manager at PCI AFRICA

PCI AFRICA

• Candy Filter Company and The

Paterson Engineering Company – from which the name Paterson

Candy International SA is derived – were late Victorian pioneers in water treatment, dating back to the 1870s. • In South Africa, the first records of

Candy Filters being sold was in 1897. • Paterson Candy, now trading as PCI

AFRICA, has been registered and operating in South Africa since 1953. • As experts in the water treatment field, PCI AFRICA is particularly well known for dissolved air flotation and sand filtration designs across Africa.

The sludge feed to the belt presses prior to the polymer injection

works will lift the water above ground level by means of Archimedean screw pumps at the lower screw pump station. Downstream of the inlet works, there is a second set of Archimedean screws where the upper screw pump station raises the flow for distribution to the primary settling tanks.

After the lower screw pump station, there is a three-phase screening process where different sizes of inorganic material are removed through three different sized screens. Wastewater then passes through the degritting system and reaches the four new primary settlement tanks via the upper screw pump station. These primary settlement tanks work on the principles of gravity where primary sludge settles, with the settled underflow sent for dewatering. The overflow is sent to three different locations on the plant – the existing activated sludge plant, the existing membrane bioreactor plant, as well as a new membrane bioreactor plant are currently being constructed under another contract.

Sludge handling facilities Work on the sludge handling facilities included all the dewatering machinery and peripheral pump stations. “One of the most impressive components of the project is the sheer size of the dewatering facilities due to the enormous amount of sludge that has to be handled. The dewatering facilities will treat both primary sludge (from the four new primary settlement tanks) and WAS (from the biological reactor on the existing works). The WAS is sent to the new dewatering plant in order to maintain the sludge age and mixed liquor suspended solids levels in the bioreactors,” adds Mellet.

The sludge handling facilities can receive: • primary sludge: 240 m3/h • primary cake product: 146 t/day • WAS: 520 m3/h • WAS cake product: 142 t/day. While the WAS cake will be used for agricultural purposes, the primary sludge will be sent to landfill via trucks.

“We used Bellmer sludge dewatering equipment, as it is a well-known company within the water industry and was compliant with all specifications stipulated by the City of Cape Town at tender stage. Preliminary testing on the sludge characteristics has been completed and we are confident that the final product characteristics will be within the commissioning parameters going forward,” says Mellet.

Layout of the plant PCI AFRICA paid close attention to the design and layout of the different components and equipment, as well as the arrangement of piping on the new plant. Mellet states that this was done to optimise efficiency from an operational and maintenance perspective. “Platforms and stairways have provided easy access to all of the components of the plant. The layout of the equipment relative to each other has been designed and placed to improve workflow, where operators can walk easily from one unit to the next. Lifting structures were sited in strategic places where all pieces of heavy equipment could be removed and placed back with ease during future maintenance outages. PCI AFRICA, together with the consulting engineers, also designed the plant to give trucks easy access to key areas where they may have to collect heavy plant equipment during maintenance.”

Health and safety was also a central factor when designing the plant.

PCI AFRICA’s contract also included the full electrical scope, as well as the control and instrumentation of the new Zandvliet upgrade. This included the design and supply of the transformers, minisubs, motor control centres, programmable logic controllers, supervisory control and data acquisition, and associated power and control cabling.