Excess sludge thickening with a decanter centrifuge at Riga WWTP, Latvia
Thickening centrifuge is designed to reduce water in an efficient way with lower energy input and lower chemical consumption. It can reach a DS content of about 5–7 %. The required chemical dosing with centrifuge thickening is low (1.0−1.5 g/kg DS); however, the energy consumption might be higher compared to the other mechanical thickening methods. It can also operate without any flocculant aid, but in such case the separation effect is much lower than with a flocculant.
Components installed in the solution
Continuously operating centrifuge with horizontal solid-wall bowl developed specifically for the requirements of thickening municipal sewage sludge. The frame is of open design with gravity discharge of the clarified phase. Capacity 10 - 300 m³/h.
in 2012 a thickening decanter centrifuge was installed at Daugavgrīva WWTP in Riga, which has a total capacity of 1 000 000 PE and at that time produced about 6 857 tonnes of DS to be disposed of annually.
In the case of this particular WWTP, primary sludge is obtained with the primary sedimentation, having a retention time of 2.5 hours and achieving a DS content of 4 to 5 %. Then the excess sludge is mechanically thickened with the centrifuge (polymer consumption 2–4 g/kg DS), which increases dry solid content up to 5–7 % resulting in a total DS content of 3 % after the digester. The biological digestion takes place at a temperature of 37 °C for 14 to 20 days. The biogas stored in a 2 500 m3 gas storage tank up to 5 hours, is used for operating the combined heat and power plant (CHP), operation of which is outsourced and run by a separate company.
Digested sludge has been conditioned with polymers after digestion (8 g/kg DS) and dewatered with centrifuges, achieving a DS content of 22 %. All the sludge is destined to be disposed of for use in agriculture, the most common way for sludge disposal in Latvia.