Finding the chase and cutting to it

My 6th podcast was all about resource management over the baking hot summers. While the heat waves were happening in the East of England, our local news channel asked residents what they’d like to see happening. More than a few people asked this question: we’re on an island, surrounded by sea, can’t we just use that?

Desalination.

The title sums it up, high salinity (saltiness) is not useful for many creatures, particularly not land mammals like the naked apes we are.

Desalination is the process for taking salt out of water, but potable (drinking) water has a bit more done to it besides. Our land harvested water is cleaned: soil particles, animal waste (including ours), and harmful chemicals are all removed down to a microscopic level.

While the sea might look lovely and blue, it’s a thriving eco-system with many of these features in it, not least sodium chloride which is harmful in great quantities.

The deep sea reality.

Harvesting sea water close to land multiplies some of these materials, including basic silt. Even further out, the number of living organisms (plant, animal, and other) is high in number. This means biological filters need to be used. Yet the flotsam is carefully looked after so as not to disturb the natural balance: a disaster for the natural world and highly damaging for land health too!

Such pipe work is complex and tough. Deep sea pipes need to withstand the harsh saliferous environment with exposed weather events and tides and currents all pressuring the system. 24/7, 365 days a year, these systems, vital for human life, need to be robust in one of the most extreme environments on the planet.

This is a non-trivial task, that needs to be maintained. For an installation and maintenance point of view, it is not a quick and easy fix.

Polishing the diamond.

Having achieved harvesting the water and coarsely filtering it, this is repeated to a finer and finer grain. We can’t filter out the salt, but much of the other detritus is done mechanically first.

The process of removing the salt is done through ‘exchangers’ like isobaric ones. Isobaric exchangers work by capturing the pressure hydraulic energy from a high-pressure brine reject stream. The water leaves the membranes, transferring it directly to the incoming seawater feed. By using a rotating ceramic rotor or piston chambers to act as a fluid interface, the high-pressure waste stream pushes against the fresh intake water. Systems like these recycle up to 98% of that pressure without needing to convert it back into electrical energy first, drastically reducing the load on the main high-pressure pumps. Often the salt is saved as a by-product and sold as seasoning.

This takes energy. For a highly efficient plant, it costs between 2.5 and 3.5 Wh per litre to do this stage. The total processing of land harvested water is 0.2 to 0.5 Wh per litre.

Ironically, this water is relatively pure, apart from some microbes. Pure H₂O is not actually good to drink, or push through the UK water system. Some necessary minerals are put back, by passing the water through beds of crushed limestone. This gives the purified water calcium and magnesium carbonate to balance the pH and give it a crisp, natural taste. And protect the UK distribution systems.

We then need to kill any bugs remaining, to make it potable, as we do with land harvested water.

So tricky, but not out of reach. Surely better than rationing water with hose pipe bans?

In 2010, Thames Water opened their Thames Gateway Water Treatment Works (TGWT) in Beckton, East London. It uses brackish water, which has salt in it but at a lower level than sea water. Sea water has 35 parts salt per thousand.

As it is expensive to run and has faced operational hurdles, TGWT has largely sat idle. TGWT only comes into operation when water supplies are short and the conditions are favourable. To give it context, TGWT produces 100 to 150 million litres of water a day. That’s pulling out 1,100 tonnes of salt a day.

At 2.5 Wh a litre, TGWT uses 250,000 kWh or 250 MWh as day. A more usual site produces the same amount of water for 20 MWh. If TGWT ran every day, it would use 0.03% of the electricity generated across the UK.

If every coastal county across the UK had a desalination plant, we could have 5,500 Ml a day for a sum of 5.02 TWh per year. Or 2% of the current draw on the electrical generation in the UK.

We haven’t discussed that to keep building costs down, desalination plants need to be sited in towns, close to water systems, and energy sources. Thinking of the discussions going on about pylons bringing in cheap electricity from off-shore wind farms, 55 times, across the UK.

While the recent droughts have brought this idea back to the fore, it is not a quick and easy sticking plaster. It certainly can’t reduce the price of water or reduce the need to be careful what we use.

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If you want to know more about how this technology is used throughout the world, please check out Wikipedia’s Desalination by country page. There’s an interesting technology called Seawater Greenhouses, which desalinates water purely for crops. It requires less processing as the water doesn’t need to be potable – think of your water butt collecting rainwater for your garden use…