Chemical pretreatment changes the composition of waste water by reducing the paniculate organic matter.

A lower biological sludge production from the sewage facilitates nitrification.

The denitrification capacity of the sewage is decreased, but the fraction of low molecular substances is not influenced by the chemical treatment.

To increase the denitrification capacity, the organic matter removed can be hydrolised into a readily degradable form and used as an electron donor. In Tampere waste water plant the sludge is hydrolysed at 150° C in a low pH environment.

The carbon source was increasing the capacity of the anoxic zone.

The sludge dewatering characteristics were improved.

The sludge is divided into two fractions, one with paniculate organic matter, the other containing a solution of organic and inorganic substances. The heavy metals can be separated and the precipitant recovered and recycled.

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