## 3.3.4.20 Return and Excess Sludge Pumping Station 

The excess and return sludge pumping station shall be designed, constructed and equipped regarding the requirements set in Table 2-1. 

The pumping station and all equipment shall be located in a building. The requirements for the drainage, foul air collection and clean water supply system stated in Section 3.3.4.34 and 3.3.4.35 should be met for the pumping station inner area. 

The Contractor may also propose transfer of excess sludge by installing an electrical actuator valve control system on return sludge pipeline without using excess sludge pumps. In this methodology, connection lines from return sludge line to mechanical thickener and digested storage tanks shall be provided. An electrical actuator valve control system and flow measurement instrumentation shall be installed on these connection lines, separately. This methodology shall also include an electrical actuator valve control system and return sludge flow measurement instrumentation installed on return sludge line prior to discharge to anaerobic bio-phosphorus tanks. 

Excess and return sludge solid content shall be measured in accordance with “Measurement and Instrumentation” section of this document. 

Return sludge flow measurement shall be a standard in-line flow meter (electromagnetic) for continuous flow monitoring and recording. Easy access to flow meter for installation, dismantling and maintenance shall be secured and unhindered. For this purposes suitable lifting system shall be provided. 

The accuracy of flow measurement device shall be ±0.5 % of the measured flow. The flow measurement device shall have a minimum capacity of 30% higher than maximum (Q$_{WW,h}$) flow of second stage. The measured flow and status of the pumps should be monitored by the SCADA system. 

Return sludge pumping station shall consist of separate and independent pump sumps for each secondary clarifier. In addition, one spare pump sump shall be provided. There shall be one pump in each pump sumps including spare pump sump. An ultrasonic level measurement sensor shall be provided for each pump sum including spare pump sump. 

Each secondary clarifier tank’s sludge suction pipeline shall be equipped with a suitable electrical actuator valve or penstock at the inlet point of the return sludge pump sump to maintain the separate and simultaneous sludge suction from each tank. Each pump sump shall be connected separately to spare pump sump with a suitable electrical actuator valve or penstock. 

Each return sludge pump shall be designed and installed for serving one secondary clarifier. Bottom level of pump sum shall be minimum 1.5 m lower than the bottom level of sludge hopper of secondary clarifier to ensure drainage of the secondary clarifier via return sludge pump. Return sludge line between the secondary clarifiers and return sludge pump sums shall be inclined towards pump sums with an appropriate slope. 

The internal volume of the return and excess sludge pumping station shall be dimensioned to allow access to all instruments and maintenance and repair of the instruments. A lifting system with overhead travelling crane shall be provided to carry out the maintenance works for return and excess sludge pumps. 

