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New seawater plan for KZN

https://www.iol.co.za/news/new-seawater-plan-for-kzn-1668834

Written by Tony Carnie

Durban – Umgeni Water has announced plans to develop two desalination projects on the Durban coastline – one at Tongaat in the north, the other near Illovu in the south.

If both plants are built they would supply enough tap water to meet about a quarter of Durban’s current daily demand.

However, the proposals have to be subjected to comprehensive financial feasibility approvals and an environmental impact assessment (EIA).

There is also a question mark over whether Eskom is able to provide enough electricity to meet the power demands of the plants, which would purify water through reverse osmosis.

The new Umgeni plan also coincides with a more controversial, parallel proposal by the eThekwini municipality to reclaim and purify sewage water to generate similar volumes of drinking water for the city.

While the so-called “toilet to tap” plan is expected to be significantly cheaper than desalination while helping to reduce the rising level of sewage pollution in eThekwini’s rivers, public perceptions around the reuse of sewer effluent could play a significant role in the decision on which option is to be implemented.

Earlier this month, Umgeni Water and the CSIR published information on the desalination projects as part of the EIA and invited the public to identify any concerns.

According to the Umgeni Water proposal, the northern desalination plant would be close to the beach at Tongaat.

In the south, two alternative sites have been proposed near Illovu, both of them about 3km inland from Winklespruit.

Seawater would be sucked through inlet pipes located 650m to 1km from the coast and pumped to plants, where it would pass through ultra-fine membrane filters to remove salt and other minerals.

High salt levels

The brine effluent would then be pumped back into the sea through outlet pipes extending between 350m to 500m offshore.

Umgeni suggests that by using a diffuser system on the outlet pipes, the salinity levels of the brine effluent would be diluted to about three percent above normal salinity levels within 10m of the diffuser points.

The potential impacts of high salt levels on sea life were not discussed in the documents, but were likely to be studied and debated.

Each plant would generate about 150 million litres of drinking water from 390 million litres of seawater each day. The plants each have an energy demand of about 25 megawatts.

Earlier this year, Durban water and waste chief Neil Macleod warned that the city was running out of fresh water, despite the construction of the Spring Grove dam near Mooi River.

He cautioned that the water from the new dam could be depleted in about two years because of rapid growth in demand.

Although a feasibility study was under way to build a dam on the uMkhomazi River, it was unlikely that extra water would become available before 2030, at the earliest.

In the interim, eThekwini had to make a choice soon about re-using and purifying sewer effluent or desalination.

Both options used an almost identical (reverse osmosis) purification technology.

According to Macleod, desalination plants had a high electricity demand “which Eskom would currently be unable to guarantee”.

He also suggested that desalination costs were about double the current price of Umgeni Water supplies.

Another advantage in reusing sewer effluent was that it could reduce the growing volume of sewage pollution in estuaries and the amount pumped into the sea. – The Mercury