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The Duffins Creek sewage plant (illustrated at right, officially known as the York-Durham Water Pollution Control Plant) is located on the shores of Lake Ontario on the Pickering side of the Ajax-Pickering border. It is one of the largest in Canada. It is at the end of the 'Big Pipe' from York Region. It currently dumps about 340 million litres a day (mL/d) of effluent into the lake. 80% of this is from York Region. All waste receives secondary treatment which means that while it may look clean, it still contains a lot of nutrient chemicals. When testing reveals that phosphorus levels are too high, then and only then, does the plant apply tertiary chemical treatment to reduce the phosphorus to 'acceptable' levels.
For an explanation of sewage treatment, click here for an excellent slide show entitled 'Waste Water 101' prepared by York region, which explains the terminology and processes. It is quite a large file, so it may take a minute or so to load.