Tank water release no simple issue

The scene on Monday as water was released from the Merimbula water tank.
The scene on Monday as water was released from the Merimbula water tank.

A population that doubles in summer, water security and safety and the need to ensure stability of a large water tank are all contributing factors in the release of about 1 million litres of water from the Merimbula tank, every year, according to Bega Valley Shire Council.

There are three tanks that store Merimbula’s water although only two are used, one which holds 3.37 million litres and the other which holds 4 million litres.

Council’s acting water and sewer manager Chris Best said that during the height of the summer period Merimbula uses close to 2 million litres of water a day and having two tanks provides water security for a couple of days in the event of a break in the system and fire fighting resources.

Why can’t we just use all the water in the tank? 

However by April there is no longer the need for both tanks and so the tank next to the Merimbula Rural Fire Service holding 3.37 million litres, is taken offline until October.

Mr Best explained that all the water in either tank cannot be completely removed.

“We can’t empty it because the soil underneath is used to having a heavy weight on it. If we suddenly take that weight off the soil will rise.

“We try to keep it at a normal level. If it’s normally full then that’s what we do to avoid the risk of damage to the tank,” Mr Best explained.

It costs about $750,000 to build a 1 million litre tank, Mr Best said, and so all risk of damage to the tank has to be avoided. 

The water in the Merimbula tanks is sourced from Tantawangelo Weir, stored at Yellow Pinch Dam and transferred through a chlorinator to local distribution tanks.

“Chlorine is a volatile gas, it gases off and there is no chlorine left in a tank that has been left idle over winter,” Mr Best said.

This occurs in the offline tank and by October the water needs re-chlorination to ensure it is safe to drink but there is no facility for secondary chlorination in the Merimbula tanks. Council deals with the problem by emptying some of the water out – about a third which is 1 million litres – and adding water which has been chlorinated.

Chlorine is added to the to the refreshed tank, as well as new water.

“We add around 30 litres of liquid chlorine and use the new water to mix this with the older water. Normally chlorine is added as a gas in a controlled facility near the dam,” Mr Best said.

He said that on Monday, October 8, about 7 per cent of the tank’s total capacity had been drained and that over the week another 23 per cent would be drained.

Can the water that has to be drained, be used in any way?

Mr Best said there were a couple of issues surrounding any potential use of the 1 million litres of water that had to be drained out of the tank.

“If people are buying water from council we must ensure it is safe and meets a certain standard,” Mr Best said.

However this water does not meet those standards as the chlorine has gassed off.

“It is not suitable for drinking and so we have to find a non-portable (not for human consumption) use.

“We need somewhere for a truck to park and to turn around and there is the question of fittings – there are no fittings on the tank to connect to a truck,” Mr Best said.

“Typically trucks hold 15,000 litres and we’re getting rid of 1 million litres in a week. That’s high cost in terms of fuel. It becomes very hard and very expensive,” Mr Best added.

“That’s the reason we refresh rather than replace.

“In a normal year that tank supplies round 510 million litres just for that tank’s supply area. That means that the 1 million litres lost is about 0.2 per cent of the annual supply, in exchange for a more secure water supply for the summer,” Mr Best said.