Friday, September 28, 2007

Beware the Deadly Microsystems

Could water recycling help prevent blue-green algal growth in our reservoirs?

Sydney’s Lake Burragorang (Warragamba Dam), currently has a considerable growth of blue-green alga on the surface, -see the article and image below.

It shouldn’t come as a surprise to most people that if you dam a river and cause the water to sit in a huge puddle, algal growth is always a possibility. Add to that the nutrients run-off into the reservoir from last month’s rain and the current warm weather, and algal growth becomes almost inevitable.

As the article below suggests, it’s probably not a major problem. Blue-green algae tend to float towards the surface, whereas we have the capability to draw water supplies from much deeper. Nonetheless, these types of algae do produce a number of chemicals which are released into the water. Some of these may simply cause the water to taste or smell bad, but some are highly toxic.

The article quotes NSW Water Minister Phil Koperberg stating that recent testing has “detected some Microsystems” in the water. This could possibly mean that there are lots of computers that have been dumped in the reservoir. However, it is more likely that Mr Koperberg was misquoted and was actually referring to microcystins, -a type of blue-green algal toxin.

The current NSW Government does not plan to construct a planned indirect potable water recycling scheme that involves recharging Warragamba Dam. They have clearly stated that they are opposed to such a scheme because they don’t believe that the public would support it and because it would be expensive.

Nonetheless, it would be interesting to consider the impact that such an IPR scheme may have on the propensity for algal growth. We could look at the relative loads of the key nutrients nitrogen and phosphorus in the reservoir and at how effectively we may be able to dilute these with lower concentrations of nitrogen and phosphorous from advanced-treated recycled water.

Furthermore, recharging the reservoir and extracting the majority of our water supplies from it, would keep the water moving through the system, -preventing nutrient build-up and stagnation. Compare this with the current plan to replace much of the eastern suburbs’ water use with desalinated seawater pumped directly into the distribution system. This would reduce demands on Warragamba supplies (as it is intended to do), causing the water there to travel more slowly through the system.

Just another angle to consider for water supply planning…

Damn that algae - it's a blooming nuisance

Alexandra Smith
Sydney Morning Herald
September 28, 2007

A BLUE-GREEN algal bloom has taken over most of Warragamba Dam, with small levels of toxins found just below the surface.

Recent testing of the algae had shown three positive samples of microcystins, toxins that can cause skin irritation and stomach upsets if consumed in large doses. But NSW Health has stressed that the quality of Sydney's drinking water is not under threat.

Kerry Chant, the acting NSW chief health officer, said the levels of toxins were very low and had been found about three metres below the surface, not from where drinking water was being sourced.

The Water Minister, Phil Koperberg, said the bloom now stretched across 75 per cent of the dam - more than 58 kilometres - and more than twice the area it occupied at the beginning of the month.

He said it was possible drinking water could be mildly affected, but this would be limited, with water being safely drawn from 48 metres below the algae.

"There could be a discernible odour or taste with the water in the coming months," he said.

Warmer weather had provided the perfect conditions for the bloom to grow and could be present until at least Christmas.

"It is a bloom which, due to the warm weather, is likely to persist," Mr Koperberg said.

"It is very unlikely, unless there is some unforeseen meteorological event, that this bloom will either dissipate or disappear during the summer. It's, more likely than not, going to be around at Christmas."

Mr Koperberg said he would be provided with updates on water quality and testing.

"Testing is being conducted regularly by the Sydney Catchment Authority and Sydney Water of both the raw and treated water before it is supplied to Sydneysiders to drink," Mr Koperberg said.

"It is not a health risk whatsoever, even though part of the rigorous testing that Sydney Water and Health undertake has detected some microsystems - which are a group of molecules which contain some toxicity.

"Those levels are well below the Australian standards for drinking water safety.

"The main issue with this bloom is its visibility."


mark said...

The current NSW Government does not plan to construct a planned indirect potable water recycling scheme that involves recharging Warragamba Dam.

Good news.

Anonymous said...

The algal bloom is the result of decades of poor catchment management by the NSW Government.

There is NO way that I will be drinking the water in Sydney this summer. I think it might be time to buy those bottled water shares!

Anonymous said...

You know what bothers me the most about this Blue Green algi in Warragamba ???
Its the fact that ,What happens when we start our Bushire season and they start to helicopter water from Warragamba and then other fires break out say up in the blue Mountains ,,and they put those same buckets that have been in Warragamba into other reseviors ???? They dont wash the Buckets out after each fire, So basically they'll transfer it to other Water supplies that dont have the Bacteria cleaning stystems ????
This is scary stuff folks ,,I think we'll see this Blue/green Algi spread throughout N.S.W and maybe beyond by christmas or early January...

Stuart Khan said...

Hello Anonymous,

Thanks for this comment. I’m certain that you are correct in that transfers of small volumes of water between reservoirs are a good way to spread blue-green algal cells. However, the impact may not be as significant as you might presume.

While they are not perfectly predictable, it seems that algal blooms will occur practically wherever the conditions are right. The ‘right’ conditions can usually be defined simply as any relatively still water source with an elevated concentration of nutrients (nitrogen and phosphorous) and moderately warm weather. This means that we can normally take the introduction (or pre-existing presence) of sufficient algal cells required to initiate a bloom for granted. Likely means of transfer between water bodies include ducks, which may visit numerous water systems.

But still, I agree that bushfire buckets have the potential to exacerbate cross-contamination.

Anonymous said...

"How effectively we may be able to dilute these with lower concentrations of nitrogen and phosphorous from advanced-treated recycled water."

Considering it would involve recyling sewage, which is higher in N and P than the inflow to the dam, the N and P removal efficiency of the treatment is critical.

Can you give typical nutrient concentrations pre- and post- "advanced treatment"?
Also for comparison, what are typical wet and dry streamflow nutrient concentrations?

Stuart Khan said...

Hello Anonymous, great question!

To provide some context, Australian guidelines for recreational water quality include the following information:

Mass developments of cyanobacteria [blue-green algae] are associated with high nutrient concentrations. Phosphorus is usually the key nutrient controlling proliferation, although the availability of nitrogen may be an important variable to assess because it can influence whether or not nitrogen-fixing species dominate. However, total phosphorus concentration in the water body can be a simple guide to the influence of nutrients on cyanobacterial growth. In general, a total phosphorus level of [0.01 – 0.025 mg/L] presents a moderate risk of the growth of cyanobacteria. The phosphorus concentration reflects the potential for increased algal and cyanobacterial biomass. For levels less than [0.01 mg/L], there is a low risk of cyanobacterial growth; a level greater than [0.025 mg/L] provides good growth potential.

Sydney Catchment Authority aims to keep nutrient levels below phosphorus 0.01 mg/L and nitrogen 0.35 mg/L (see Page 23). However, streams leading to Lake Burrogorang (“Warragamba Dam”) routinely exceed these levels (see Page 46) and the reservoir itself rarely complies (see Page 51).

I don’t know the current nutrient concentrations in Warragamba (I’d be grateful if anyone can point me to some recent data), but given the state of the cyanobacterial growth, I expect that phosphorus is probably in excess of 0.05 mg/L.

As a rough guide, I would estimate that raw sewage typically contains something like a Total Nitrogen concentration of 30 – 60 mg/L as N, and Total Phosphorous concentration of 5 – 15 mg/L as P.

How much of this N and P that is removed during sewage treatment is highly dependant on the precise treatment scheme. Various methods are available for biological or chemical removal of N and P, but the sewage treatment plant needs to be designed appropriately in order to be able to use them. A decent tertiary sewage treatment plant (such as those in western Sydney) can routinely achieve effluents of around 1 - 10 mg/L Total Nitrogen and 0.01 - 0.1 mg/L Total Phosphorous.

If membrane treatment is to be subsequently used, it is not uncommon to reduce phosphorous concentrations even further prior to membrane contact. This is because phosphate salts can be troublesome to reverse osmosis (RO) membranes. They tend to precipitate on the membrane, leading to membrane fouling and associated decrease in membrane porosity. Phosphorus pre-treatment is typically undertaken by precipitating with iron (Fe) and removal by settling or at the microfiltration stage.

RO membranes themselves vary in their ability to remove remaining dissolved P and N, but for the predominant ionic species (phosphate and nitrate ions), removals of 1-2 Logs (ie 90 – 99%) can normally be expected.

So while it is very difficult to generalise, I think it is a fair statement that advanced treated recycled water can be prepared to contain less than the 0.01 mg/L phosphorus and 0.35 mg/L nitrogen aimed for (in reservoirs) by Sydney Catchment Authority. By doing so, they will provide a nutrient diluting effect to many reservoirs.

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