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The polymer shall be dosed by means of eccentric screw type pumps controlled by frequency converters. Flow meters shall be installed on polymer dosing line incoming to each mechanical thickener. 

Thickened excess sludge after the mechanical thickener shall be pumped via thickened excess sludge pumps. The thickened excess sludge shall be pumped directly to the anaerobic digesters or to the digested sludge storage tank. Quantity of excess sludge transferred to anaerobic digester or digested sludge storage tank shall be measured. The pumps shall be speed controlled by means of frequency- converters. Adequate space within the unit shall be left for proper operation and maintenance of all equipment. Access to the machines, pumps and components (such as valves and local control panels) shall be secure and unhindered. 

The capacity of the pumps shall be selected based on the below defined daily operation hours. 

Washing water shall be connected to both the suction and discharge lines of the thickened excess sludge pumps and polymer dosing pumps. This washing system will engage automatically when the pumps are not operated. 

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## 3.3.4.25 Gravity Sludge Thickener for Primary Sludge 

For thickening of primary sludge, gravity thickeners shall be used to increase the dry solids content of the primary sludge to minimum 5 % DS. The sludge screen shall be used before the gravity thickener. The gravity thickener and sludge screen shall be in a building. 

Inside building, the top of this tank shall be additionally covered to prevent the spreading of odor. This top cover part shall be GRP and shall have transparent window covers so that the inside can be seen during operation. Sludge screen shall be installed on the top of gravity thickener. The building shall have a lifting system for maintenance and repair purposes. The waste collected from the sludge screen should be conveyed to a container. The foul air from the gravity thickener and from the building shall be collected and sent to the odor removal. The building door dimension and lifting equipment shall be accordance with the requirements for repair and maintenance of all equipment in the building. 

The thickening device shall be the center-feed type and be equipped with perimeter weirs and launders. Weirs shall be EN 1.4404 and launders shall be reinforced concrete. Weired supernatant shall be transferred to the inlet structure of the anaerobic bio-phosphorus tank. 

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Thickened primary sludge after the gravity thickener shall be pumped via thickened primary sludge pumps. The capacity of the pumps shall be selected based on the below defined daily operation hours. 

