Civil Engineering August 2022 | Vol 30 No 7

22 August 2022 Civil Engineering treatment, the points of odour generation, namely the head of works, sludge thick- ener and sludge storage tank, are serviced by an odour control system based on a combined bio trickling filter and fixed media biofilter. Figure 3 illustrates the works at 85%. CONSTRUCTION OVERVIEW ANd CHALLENGES The NWWTP site is underlain by mate- rial of high variability, both in terms of the description and constructability. The sites of the treatment plant and the outfall sewer exhibit a copious amount of groundwater at depth ranges from 1.5 m above natural ground level which, when combined with the area’s clay soils and soils with a high plasticity index, results in very poor in-situ founding conditions which had to be improved. This was par- ticularly the case for the sludge thickener, sludge pump station, backwash water tank platform and the platform carrying the intermediate sedimentation tanks and intermediate pump station. Furthermore, because most of the structures are water retaining and thus sensitive to differential settlement – which may induce cracks – carefully thought-out joints had to be used to accommodate movement along with subsoil drains. The need for durable and resilient concrete structures required the use of concrete with a cement content of over 325 kg/m 3 and a strict cement to water ratio of not more than 0.55, thus resulting in concrete of low permeability. The inlet works and sludge thickener, being points of odour generation, are covered. The entrapment of the odor- iferous hydrogen sulphide, if incidental to moisture, would form sulphuric acid which would in turn corrode the concrete structures. Thus, in order to protect the structures, in particular the inlet works (screening channels, primary sedimentation tanks and incidental collection and diversion chambers) and sludge thickener these were coated with a proprietary coating material. Careful design, workmanship, and material selections where necessary to avoid early thermal cracking of the concrete, in particular for the sludge digester where walls of thicknesses up to 500 mm where constructed. Adequate amounts of reinforcement were provi- sioned to ensure crack control for the sludge digesters, while spaced joints were used for structures where joints could be tolerated. The high phosphate levels in the influent wastewater required phosphate precipitation. This will be achieved by the addition of alum both before the final sedimentation tanks and after final sedimentation tanks but before the filtration stage. The two-step dosing of alum will assist in achieving high phosphate removal. PROGRESS TO DATE The project is currently 90% complete and is expected to be completed in August 2022. The complexity and numerous competing technical and operational objectives stemming from the revised effluent disposal requirements have provided the project participants with numerous learning opportunities as they sought a solution to meet the updated effluent regulations while the project was being implemented.  Figure 4 Open excavation at the intermediate pump station and intermediate sedimentation tanks Figure 5 Open excavation at the sludge thickener, sludge pump house and backwash water tank