Reducing P-discharge from Riga WWTP, Latvia

Solution to which problem

Eutrophication caused by intensive P-discharge and sludge washouts.

Technical conditions

WWTP of 1 000 000 PE, using mechanical and biological treatment, and generating about 7 000 tonnes of sewage sludge produced annually. The treatment process of mechanical filtration through screens, sand sedimentation, mechanical treatment in radial sedimentation pools that have a volume of 16,000 m3. Biological treatment based on an anaerobic-anoxic-aerobic treatment chain, afterwards directed into secondary sedimentation ponds.


Purchasing and installing the equipment. Storage and dosing tanks for phosphorus precipitation chemical to improve the phosphorus removal efficiency; additional precipitation tank was purchased and installed by funding from John Nurminen Foundation in 2012-2013 to allow both pre- and simultaneous precipitation of P; Magnetic flow meters to provide exact waste water flow measurements needed in steering of the process; An additional sludge thickening centrifuge to achieve the necessary capacity for excess sludge removal and to prevent wash-outs of activated sludge.


Full implementation of the HELCOM recommendation on phosphorus removal, significant phosphorus load reduction in comparison with the situation without chemical precipitation - over 100 tons less phosphorus is discharged annually from the Daugavgriva WWTP. In addition, sludge wash-outs that have been estimated to contain up to 10 tons of phosphorus each, will be avoided.

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