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Settling Tanks Effective in Removing Phosphorus

by 5m Editor
15 February 2010, at 8:35am

CANADA - Researchers with the University of Manitoba are hoping to harness anaerobic digestion to increase the amount of phosphorus recovered from liquid swine manure using settling tanks, writes Bruce Cochrane.

Researchers with the University of Manitoba's Faculty of Agricultural and Food Sciences are participating in a project which uses low-tech settling tanks to recover magnesium ammonium phosphate from raw liquid or anaerobically digested hog manure for use as phosphorus fertilizer.

Ph.D. candidate Joe Ackerman explains phosphorus in its soluble form can combine with magnesium and ammonium to form a crystal that can be used as a slow release fertilizer.

Joe Ackerman-University of Manitoba

This crystal dissolves slowly.

It's got ammonia in it as well as phosphate in it and magnesium is not a problem with most soils.

We set up a pilot scale reactor which was basically two 500 litre tanks and that was operated this summer pumping supernatant into it and dosing it and then collecting this precipitate that came out.

That was quite successful.

We were able to remove about 70 per cent of total phosphate from the liquid that we ran through.

That's with the overnight settling.

We'd like to bring that time down.

The product, once it was dried down and analysed, it came out as about six per cent total phosphorus and a little more than six per cent total nitrogen and about four per cent potassium.

It's a reasonable fertilizer.

There's areas we'd like to perfect and develop though and that will be this coming summer.


Mr Ackerman notes a small portion of the phosphorus in liquid swine manure is in the soluble form but during anaerobic digestion the acidity increases causing more phosphorus to become soluble and available.

He says bench scale studies planned for this summer will evaluate the proportion of phosphorus that can be made soluble under anaerobic digestion.

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