A series of impoundments occur on the final 46 km stretch of the Ruhr river where phosphorus, nitrogen, and organic carbon are present abundantly, causing several heavy algal blooms during the year. This is detrimental for the treatment capacity of the water works. Also, an oxygen shortage is always recorded in the river after algal decay at low flows, thus requiring artificial aeration. By balancing all oxygen supply and consumption it is shown that about 2/3 of the oxygen demand is due to algal decomposition. When evaluating the water quality data of the past 30 years and by conducting special laboratory tests, results show that phosphates initiate algal growth. Therefore, a phosphate model has been developed for the river which predicts the phosphate content for 1988 and 1998. It is expected that by reduction of this phosphate content maximum algal growth can be cut by about 50 %.

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