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6 WASTEWATER

6.1 Overview Umgeni Water currently owns and operates the Darvill and Ixopo Wastewater Works (WWWs). It also operates the Howick WWW for uMgungundlovu District Municipality under a management contract, and the Albert Falls North and South WWWs as part of the management of Albert Falls Dam. All existing operations are based on current technology, namely aeration basins for biological nutrient removal and clarifiers for the separation process. New technology such as membrane separation would allow for the treatment of at least a fifty per cent increase on present volumes in activated sludge (i.e. all these plants). Umgeni Water is currently investigating this at pilot scale. The quality of water produced may make newer technology a necessity as DWA raises standards to compensate for increased water demand and population density.

6.2 Darvill Wastewater Works The Darvill WWW (Figure 6.1) serves the Msunduzi Municipality. This WWW has a current biological treatment capacity of 65 Mℓ/day. Current average daily inflow (Nov 2007 to Nov 2013) is approximately 81 Mℓ/day (Figure 6.2) which is more than the plant’s capacity. The graph also indicates the spike in flow in the summer seasons when there is ingress of stormwater into the sewer system.

Figure 6.1 Darvill Wastewater Works.

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Figure 6.2 Average daily inflow (Mℓ/day) to Darvill WWW.

Umgeni Water is currently investigating options to upgrade Darvill WWW to cope with the increased inflows and predicted wastewater demands (Figure 6.4). The average dry weather flows (ADWF) within the Darvill WWW catchment are expected to grow to about 90 Mℓ/day by 2020 (Figure 6.4). Hence, it is proposed that the plant be initially upgraded by at least 35 Mℓ/day. The upgrade design will make allowance for expansion in the future to a maximum capacity of 120 Mℓ/day, which is forecast to occur in 2024. As a summary the proposed civil and process infrastructure upgrades at Darvill are as follows:-

Biological treatment

o Convert the existing Aerobic Reactor to an Anaerobic / Anoxic reactor

o Construct a new reinforced concrete Aerobic Reactor (40 150 m3)

Air for Biological Treatment

o Blower House

o 4 x 645 kW Blowers each with a rated delivery of 7 m3/sec @ 90 kPa

o Air Header Mains

o 22 680 diffusers in three aeration lanes

Secondary Settling

o 2 x 35 m diameter secondary settling tanks

Advanced Treatment

o Reinforced Concrete Submerged Membrane Tank

o Submerged Ultra-Filtration (UF) Membrane System complete with suction pumps,

backwash blowers and CIP chemical storage and dosing

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Figure 6.3 Darvill WWW Proposed Infrastructure Upgrades

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The UF Membrane System will be designed, but not constructed until feasibility studies on water reuse at Darvill are complete. The UF Membrane System will produce water of sufficient quality that it can be used as the feed to a water reuse plant. If the water reuse plant does not materialise then water of this quality will not be required and Darvill WWWs effluent can be discharged into the Msunduzi River as normal. More information is available on the Darvill Reuse project in Section 4. The present method of disposal of sludge by spray irrigation to land is operating adequately, but has its limitations especially as the capacity of the works increases to cope with growth. Alternative methods of sludge treatment and disposal are thus being investigated in order to determine the optimal future disposal strategy. A feasibility report which looks at the management and disposal of sludge produced by the works has been completed and the following recommendations have been included in the proposed upgrade:-

Refurbish the existing Gravity Thickeners

Construct a new sludge treatment building with mechanical thickening of waste activated sludge and digester sludge dewatering plant.

Construct a new sludge sump for blending thickened primary and waste activated sludge and new digester feed pump station

Construct two new “Egg-Shaped” digesters.

Develop a composting facility.

Figure 6.4 Projected Inflow into Darvill WWW

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6.3 Ixopo Wastewater Works Ixopo WWW (Figure 6.5 and Figure 6.6) serves the town of Ixopo in the Harry Gwala (Sisonke) District Municipality. The WWW has a design capacity of 1.0 Mℓ/day and is designed to be doubled. Sludge is dried on beds and disposed of on a local farm that was owned by Umgeni Water, but has been sold to the Harry Gwala (Sisonke) District Municipality. The average daily inflow to the Ixopo WWW is shown in Figure 6.7

Figure 6.5 Ixopo Wastewater Works.

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Figure 6.6 General layout of the Ixopo WWW.

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Historically, an upgrade to the WWW had not been envisaged, until the inflow to the works climbed to 20% above the design capacity just prior to 2013. Plans are now in place to increase the capacity with the construction of an additional clarifier (3.25 Mℓ/day), now to take place later this year. However, the urgency for this planned upgrade no longer exists as the inflows to the works decreased significantly in 2013. The reduced inflow is because of blockages in the Ixopo sewer network, which overflow into the environment. As a result return flows to the wastewater water works have reduced as is evident in Figure 1.6.

6.4 Howick Wastewater Works and Mpophomeni Wastewater Works

Umgungundlovu District Municipality provides wastewater treatment services for the uMngeni Local Municipality at the Howick WWW. A large pump station at the Mpophomeni WWW site and a set of six of smaller pump stations in Howick transfer wastewater for treatment at Howick WWW. Umgeni Water has a management contract with Umgungundlovu District Municipality to operate this WWW on their behalf. Howick WWW (Figure 6.8) has a design capacity of 6.8 Mℓ/day and is currently treating 6.4 Mℓ/day. Mechanical dewatering equipment installed in 2013 has alleviated operational problems to a degree but the works is currently fully utilised. If additional treatment capacity is required, the WWW will have to be upgraded to a membrane bioreactor (MBR) or similar.

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Figure 6.7 Average daily inflow (Mℓ/day) Ixopo WWW

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Mpophomeni WWW is currently not functioning, and is only used as a transfer station to Howick WWW. This WWW will be reinstated as a treatment facility as soon as the necessary finances have been raised by Umgungundlovu District Municipality. The proposed plant will be designed to treat 6 Mℓ/day with the possibility of upgrading the works to 12 Mℓ/day. The site has space for at least 20 Mℓ/day. The Mpophomeni WWW upgrade is required to free up capacity at Howick WWW and to also cater for planned development in the area. The following developments initiatives by the municipality will be serviced by the Mphophomeni WWW in future:-

Refurbishment of existing sewage reticulation system in Mphophomeni Township will increase

wastewater flows to the works (ADWF 3.6 Mℓ/day)

The development of the Khayelisha social housing development on the banks of Midmar Dam

(ADWF 1.3 Mℓ/day)

Planned light/mixed industrial development park (3 Mℓ/day)

These major infrastructure developments and others have been taken into consideration in the planning for the upgrade of Mphophomeni WWW. Umgeni Water has been requested to manage the implementation of the upgrade to the works and to take over its operation on completion of construction. Construction is envisaged to take place towards the end of 2014.

6.5 Albert Falls North and South Wastewater Works

The two Albert Falls WWWs (Figure 6.9) are transportable activated sludge units installed for the construction of the dam in the early 1970s. They were probably originally designed for approximately 100 m3/day (0.1 Mℓ/day). The bulk of the current feed to these WWWs is from conservancy tank contents delivered by tanker from the Albert Falls and Bon Accord resorts. Umgeni Water refurbished the WWW to a Sequencing Batch Reactor (SBR) configuration with a treatment capacity of 40 m3/day to suit present demands.

Figure 6.8 Howick Wastewater Works.

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Figure 6.9 Albert Falls Wastewater Works.