Nooitgedacht water scheme nearing completion

Aerial view of the Nooitgedagt WTW

Kevin McRae, COO, AfriCoast Consulting Engineers

The Nooitgedagt/ Coega Low Level Supply Scheme (NCLLS) increases the supply of treated water (sourced from the Gariep Dam) from an average of 70 Mℓ/day to 160 Mℓ/day for Nelson Mandela Bay NOOITGEDACHT Municipality (NMBM). Phase 3 – the last phase of the scheme – is WATER SCHEME nearly complete. By Kevin McRae

NEARING COMPLETION

Initially, the NCLLS was to be implemented as a single project under multiple contracts. However, due to funding constraints, the scheme had to be implemented in phases.

Phase 1: On completion (1993), Nooitgedagt Water Treatment Works (WTW) had a capacity of 70 Mℓ/day and a hydraulic peak capacity of 84 Mℓ/day. A fourth pump was added to the pump station, boosting pumping output to 92 Mℓ/day with three pumps operating and one pump on standby. In 2008, two additional pulsator clarifiers were built, increasing the capacity of Nooitgedagt WTW to 100 Mℓ/day. Additional sludge lagoons and a 10 Mℓ balancing reservoir at Olifantskop Farm was constructed. There was also the implementation of bulk electrical supply to the WTW, and the rising (1 200 mm) and gravity (1 400 mm) mains from the WTW to Motherwell and the Coega Industrial Development Zone.

Phase 2: This included the low-lift pump station building, as well as the construction of the western bank with six additional filters, and pumping equipment, electric and control systems for the low-lift scheme.

Phase 3: This comprised a complete standalone 70 Mℓ/day treatment module at the Nooitgedagt WTW, a 45 Mℓ balancing reservoir at Olifantskop Farm, installation of cathodic protection systems on both the original Nooitgedagt to Motherwell highlevel pipeline and the low-level pipeline built under Phase 1. The building of various bulk pipelines and rehabilitation of structures, as well as the replacement of certain valves and fittings on the Motherwell to Chelsea pipeline was also included.

Nooitgedacht WTW – design Sited on the right bank of the Sundays River, the Nooitgedagt WTW is supplied with raw water from the Scheepersvlakte Balancing Dam on the left bank via a 9.1 km long 1 470 mm diameter gravity pipeline. It is now the largest WTW serving NMBM.

The water treatment process at Nooitgedagt is conventional, comprising chemical dosing, flocculation, settling, filtration and disinfection, followed by distribution. Ultraviolet treatment is added between the settling tanks and filters to guard against Cryptosporidium and Giardia.

Designed as a zero-effluent works where dirty backwash water and settled

Once excavations were completed for the new filter block and clear well, groundwater appeared

Pile caps and ground beams below filters

Settling tanks under construction, with floc channels in foreground

Clearwater pipeline to pump stations sludge supernatant is collected and recycled, the Nooitgedacht WTW has water savings between 6.3 Mℓ/day and 10.5 Mℓ/day.

Starting and stopping the final water pumps to meet demand influences the water levels in the clearwells. Level sensors in the clear wells (east and west) are connected to a programmable logic controller (PLC) that in turn automatically operates the raw water inlet control valves. These valves are electrically actuated ring-needle valves that allow for fine adjustment. As the level in the clear well drops, the PLC opens the valves and the valves are throttled as the water level rises. This allows inflow to match outflow at all times and to operate the WTW close to optimum for any given demand.

Using the PLCs, chemical and chlorine dosing are proportional to flow. In this way, chemical dosing is optimised and costs reduced. Two separate dosing systems are provided. These allow for dosing with either ferric chloride or with a polyelectrolyte.

In Phase 3, conventional gravity tanks replaced the pulsator-type settling tanks used for the first two phases, allowing the Phase 3 works to operate during off-peak electrical usage times, providing additional cost savings.

The WTW operates at full capacity during off-peak times, with both the high- and low-level pumps running, while the new eastern Phase 3 module can be stopped and only the low-level pumps run with the original western modules during peak electrical demand.

The new setting tanks were designed to accommodate the new Phase 3 module that would be switched on and off without compromising treatment capacity or efficiency. The raw water has a fairly high suspended solids load that is easily settled and, therefore, bottom-entry, horizontal-upflow settling tanks were designed and fitted with lamella packs. Sludge collection hoppers were provided for the new eastern module.

During the design stage for Phase 2, the decision was taken to fit the filters with dual lateral underdrain systems. This decision was carried through to Phase 3. The original six filters were refurbished with dual lateral underdrain systems under the Phase 3 contract. Dual lateral underdrains allow for concurrent air and water scour during backwashing and have proven to be up to 30% more efficient than the older false-floor and nozzle-type underdrain systems.

The six new filters were constructed on top of a 3 Mℓ clear well. A 1.3 m diameter gravity pipeline was installed to supply treated water to the low-level pump station.

New chemical dosing storage tanks for ferric chloride and liquid polymers, as well as all associated supply and delivery pipelines were set up. A recycling facility for filter backwash water, consisting of a circular collector reinforced concrete tank and recycle pump station, was installed.

The new sludge pond supernatant pump station and rising mains direct the settled effluent from the sludge ponds to the backwash water collection tanks. The combined effluents are then pumped to the head of works at the entrance to the flash mixer.

Groundwater Once excavations were completed for the new filter block and clear well, groundwater appeared. This came as a surprise as no groundwater was found during any of the deep excavations in Phase 1 and Phase 2 of the project, and none of the geotechnical investigations – including on-site drilling – had found any signs of subsurface water.

Tests done on the water showed traces of chlorine, indicating that the water source was most likely the existing WTW. Investigations undertaken on the existing clear well showed no signs of leaks. Further investigations revealed that the dirty backwash water recovery pipe was leaking at the point it exited the structure. This was repaired and a section of the pipeline replaced. The domestic water supply to the administration building was also found to be leaking and was repaired.

However, the water level in the excavation did not decrease once the leaking pipes were repaired. Further tests on the water no longer showed the presence of chlorine and it was concluded that there must be an underground water source.

The saturated soil had zero bearing capacity and dump rock placed in the

PROJECT FACTS

• Original works was built as an emergency drought scheme and commissioned during 1993 • During construction of the original works, a temporary pump station delivered raw water to Grassridge Reservoir, where settling and chlorination took place • The raw water pipeline to the Nooitgedagt WTW is sized for a 280 Mℓ/day average • Nooitgedagt WTW is the largest treatment works supplying NMBM and, on completion, will have the capacity to deliver a peak supply of 210 Mℓ/day – approximately 70% of current demand • The 45 Mℓ reservoir has an internal diameter of 75 m and a full water depth of 10.4 m • The reservoir and associated chambers required 13 097 m2 of formwork, 713.4 tonnes of reinforcing steel, and 4 518 m3 of all grades of concrete • Over 100 km of bulk steel pipelines have had cathodic protection installed under the project • 30% SMME participation has been achieved, with over 100 small enterprises engaged on the project at different times

excavation to provide a stable working surface simply disappeared.

Additional geotechnical drilling was conducted on the perimeter of the excavation. This revealed the presence of a previously undetected pebble layer underlying the eastern portion of the excavation, which proved to be the source of the water.

Three boreholes were drilled below the excavation and fitted with pumps to reduce the water level.

The bottom of the excavation was backfilled with previously excavated material stabilised with cement. The remaining area was backfilled with unstabilised material. This provided a sufficiently stable platform for the pilling rig to operate and 102 piles – ranging in diameter from 350 mm to 900 mm at depths of 7 m to 13 m – were installed.

Funding challenges Funding for Phase 3 is provided by the DWS. Construction spend during 2017 was initially averaging between R15 million and R20 million per month. Due to the financial crisis the DWS experienced, project budgets were slashed dramatically. The 2017/18 budget for Nooitgedagt was reduced from R92 million to R58.7 million in January 2018. By that stage, expenditure already exceed the amount of the reduced budget.

The 2018/19 budget allocation was insufficient to maintain the full pace of construction, resulting in constant late payments to the contractor who was unable to maintain cash flow and forced to resort to the suspension of the works.

In order to prevent ongoing claims and wasted expenditure, all parties agreed to formally suspend work on the pipelines and at the reservoir until the funding crisis was resolved. Limited work continued at the treatment works.

By February 2019, the funding crisis was resolved, and full production resumed across all sites. Unfortunately, the issue of late payments again became a problem late in 2021, leading to the contractor giving notice to terminate the contract and the suspension of the works. As of January 2022, funding issues have been resolved and the contractor has started to re-establish on-site. Final completion of Phase 3 is now scheduled for July 2022.

Conclusion Without the Nooitgedagt supply scheme, NMBM would have run out of water some years ago, with disastrous consequences for all. Nooitgedagt will ensure that NMBM will not run out of water entirely should the western supply fail completely due to the persistent drought but will, instead, be able to provide a large portion of the metro with a continued supply of quality potable water, albeit at lesser quantities than current demand.

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