Water Week

EWN Publishing

Pollution fears: discharges from desalination plants include phospinocarboxylic acid, Sodium metabisulphite 2 dibromo–3–nitrilopropionamide, acidic detergents

Posted by waterweek on 25 September 2007

The paper ‘Making Water – Desalination option or distraction for a thirsty world?’, June 2007, warned ocean or water discharges from desalination plants, would include phospinocarboxylic acid, Sodium metabisulphite 2 dibromo–3–nitrilopropionamide and acidic detergents. These were cleaning chemicals for membranes, reported the author Phil Dickie, of the WWF.

Gross characteristics of the discharge water compared to the intake water included;

• a small increase in temperature;

• increased acidity;

• doubling of suspended solids; and

• increased iron and sulfate content.

All flows to the outflow: Membrane performance is affected by chemical scaling from impurities in water, by biological growth and by simple clogging of the membranes. The widespread use of chemicals to overcome these issues is another potential issue with discharges from desalination plants.

Pretreatment process: As described in assessment documentation for one plant a typical pretreatment process to prevent fouling of the membranes includes;

• the removal of suspended solids, chlorination or disinfection of the water;

• the addition of iron chloride as a coagulant;

• sulphuric acid to adjust pH;

• Several times an hour the filtration system is backwashed with a 12 percent solution of sodium hypochlorite, a biocide;

• on the way to the membranes the feedwater is treated with an antiscalant (phospinocarboxylic acid) at a rate that depends on the quality of intake water – in this case it was forecast at about 4-6 mg/L. The antiscalant is discharged with the brine. The product water is then treated with lime to bring its acidity into line with drinking water standards.

• Sodium metabisulphite is added to the discharge water to neutralise any free chlorine;

• a broad-spectrum biocide (containing 2, 2 dibromo–3–nitrilopropionamide) is added to the filtration and RO systems at approximately weekly intervals to prevent growth of microorganisms;

• two to four times a year depending on the degree of membrane fouling, both filtration and RO membranes undergo “chemically enhanced cleaning” with acidic detergents.

All to the ocean or the water-system: Most if not all of these treatments are discharged with the waste brine stream, although the discharge of the cleaning wastes to sewer was raised as a possibility for this particular plant. Gross characteristics of the discharge water compared to the intake water include a small increase in temperature, increased acidity, a doubling of suspended solids and increased iron and sulfate content. The biocides used are described as breaking down in relatively short periods and most are described as having a low potential for bioaccumulation.

Reference: Phil Dickie, WWF for a living planet, ‘Making Water – Desalination option or distraction for a thirsty world?’, June 2007.

Erisk Net, 23/9/2007

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

 
%d bloggers like this: