Borcherds Quarry WWTW, Cape Town

Borcherds Quarry WWTW, Cape Town

Construction of new degritters, road rehabilitation, infrastructure refurbishment and miscellaneous work.

Client: Water and Sanitation, City of Cape Town
Contract value: R26,291,592
Duration: 12 months
Completed: April 2020
Engineer: JG Afrika

Employer’s Objectives

The Borcherds Quarry Wastewater Treatment Works receives raw sewage from several rising main pipelines from various areas in the Cape Town Municipality. The Water and Sanitation Department identified the need to reduce the volume of grit and solids entering the works and to achieve this objective required new infrastructure construction and refurbishment of existing infrastructure and buildings. In addition, a new weighbridge and associated infrastructure was to be constructed at the Bellville WWTW.

Scope of Work

The Borcherds Quarry WWTW received raw sewage inflow 24 hours per a day 7 days per a week through an inlet manhole, chamber and A-inlet Works to the B-inlet Works. To enable the uninterrupted operation of the raw sewage inflow to B-inlet Works and accommodate other contractors responsible for the installation of the mechanical and electrical equipment and odour control systems, the construction sequence was specified by the Engineer. The basic methodology consisted of:
Replacement of an existing 600 mm diameter pipeline between the inlet manhole and A-inlet works with a 900 mm diameter GRP pipeline and refurbishment of the inlet manhole.
This was followed by breaking into and modifying the other existing inlet chamber and diverting the raw sewage inflow from MH2 through the new 900 mm diameter GRP pipeline to the A-inlet works and an existing pump station, where it is pumped to the B-inlet works. However, because the pump station was unable to manage the peak daily inflow, we had to make use of additional portable pumps to provide sufficient capacity to deal with the peak flows.
In addition to the above, work included construction of a new MCC building and blower building, removal of the existing manual bar screen and screening collection channel, refurbishment work to various existing concrete channels and slabs at the old A-inlet works, diversion of an existing 75 mm diameter water pipeline, construction of new reinforced concrete channels and the degritters with a mechanical system, construction of reinforced concrete plinths for the odour control equipment, washer compactor and grit classifier, construction of new lining to the existing concrete channels at the degritters with a calcium aluminate corrosion protection coating and installation of new GRP covers over all existing and new concrete channels.
While the civil works proceeded the existing control building at Borcherds Quarry was stripped and refurbished and a new guardhouse constructed at the main entrance gate. The building work was undertaken to high quality finishes.
As work progressed on the construction and reconfiguring of the hydraulic structures and concrete water retaining structures, a new 22 m x 3.2 m 60 tonne pit-mounted weighbridge and weighbridge office building, and realignment of existing kerbed islands (to accommodate truck turning movements) were constructed at the Bellville WWTW. This work was programmed to fit in with the activities planned at the Borcherds Quarry WWTW.
The complete the two sites, road upgrades were completed towards the end of the contract, once all other construction work was near to completion. Additional work included some additions to the lighting network, carports for the Administration building carparking areas, housing for the skip and skip manoeuvring equipment, a new water main at the plant with connection points to various buildings and firewater connections and external fire hose reels, cleaning of existing incoming raw sewage pipelines.
In all, 1,850m of new pipelines, 615m of kerbs and channels, 320m3 of concrete and just under 1000m2 of block paving were constructed along with the building works and installation of the mechanical and electrical telemetry and control systems.

Challenges & Successes

The existing WWTW could not always handle the peak flows and resulted in flooding the neighbouring properties from time to time. We undertook to control the flow and created the required capacity in the inlet chamber to further prevent inundation of the properties.
In addition, the waste water treatment works had to remain operational for the duration of the contract which meant making provision for controlling the existing inflow. We resolved the issue through design, installation, operation, maintenance and final removal of temporary diversions, pumps and pipework required to divert sewage flow when tying into existing manholes, structures and pipelines.

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