40 of 74  Construction of Giresun Wastewater Treatment Plant  Tender Dossier Volume 3 Employer’s Requirements  Section 3: Plant Description  from at least three operating anaerobic sludge digestion process with proposed alternative process like  thermal hydrolysis and ultrasound disintegration).  Efficiency of digestion  : ≥50% reduction of volatile solids  * For the calculation of the volume of the thickened excess sludge used in the digester volume  calculation, excess sludge produced at maximum wastewater temperature of 12  C shall be taken into  account. A storage tank for digested sludge from anaerobic digestion process shall be provided for Stage 2  requirements. The tank shall be constructed as a circular concrete tank. The upper part of this unit shall  be concrete having transparent window covers (hinged covers) so that the inside can be seen during  operation. Epoxy coating shall be applied to digested sludge storage tank that come into contact with  the biogas so that the coating protects concrete against the effects of H$_{2}$S gas and a threat of sulphuric  acid formation. Epoxy product to be used for coating is subject to Engineer’s approval. Overflow  structure shall be provided.  3.3.4.27 Digested Sludge Storage Tank The tank shall be equipped with a submersible mixer to prevent settling of solids.  Minimum retention time (for thickened excess sludge and primary sludge)  :1 day  Sludge after the digested sludge storage tank shall be pumped via digested sludge feeding pumps. The  capacity of the pumps shall be selected based on the table in section 3.3.4.30 defined daily operation  hours. The digested sludge shall be pumped directly to the dewatering facilities. Quantity of sludge  transferred to dewatering facilities shall be measured. There shall be one pump for each dewatering  equipment proposed and one stand by pump shall be provided in total.  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.  A permanent crane or lifting system shall be provided to carry out the maintenance works for  dewatering facilities feeding pumps. 3.3.4.28 Digested Sludge Feeding Pump for Dewatering Facilities Washing water shall be connected to both the suction and discharge lines of the feeding pumps. This  washing system will engage automatically when the pumps are not operated. In addition, the service  water line shall be installed to the appropriate places of this pumping station and sufficient number of  connection points shall be supplied. The treated water should be used for this purpose.  3.3.4.29 Gas holding tank and combined biogas heating and energy plant (CHP) The digester gas that is produced shall be collected and stored in a gas holding tank. The gas holding  tank shall have sufficient capacity for a minimum of 12 hours gas production for Stage 2.  The gas shall fuel a CHP unit for electricity production. The Contractor is solely responsible for the  operational concept of the electric power utilization, i.e. feeding into the local network of the WWTP  or direct delivery to the main public network. The Contractor shall negotiate the particular conditions,  3_Vol3_d4.3_Section 3_Plant Description_GIR_rev08 