Sedimentation of Raw Sewage: Investigations For a Pumping Station in Northern Germany under Energy-Efficient Pump Control

Martin Rinas,Thilo Koegst,Jens Tränckner

doi:10.3390/w11010040

Abstract

Flow control in wastewater pressure pipes can reduce energy consumption but increases the risk of sediment formation due to reduced flow velocity. In this work, the sedimentation behavior of dry and wet weather samples at the inflow of a wastewater pumping station is determined by settling column experiments. Based on the derived characteristic settling velocity (vs) distribution, the impact of energy-efficient flow control on sediment formation in pressure pipes (600 mm diameter) was quantified in comparison to a simple on/off operation. In parallel, the sediment formation for 2 years of pumping operation was monitored indirectly via the friction losses. For the investigated case, settling is strongly influenced by the inflow condition (dry, combined from road runoff). Under combined inflow, the proportion of solids with vs from 0.007 to 1.43 mm/s significantly increases. In energy-efficient mode with smoother operation and shorter switch-off sequences, the sediment formation is significant lower. The mean deposit’s height in energy-efficient control was calculated to 0.137 m, while in on/off operation the mean deposit’s height was 0.174 m. No disadvantages arise over a long period by installing the energy-efficient control. The decreased flow lead under the investigated conditions even to a reduced sediment formation.

Highlights

Sewage transport in urban drainage systems involves in many cases the use of pumping stations (PS)
The key of a minimized energy demand is the reduction of friction losses, resulting from low flow velocities and engendered lower shear stress in connected pressure pipes
Results for for sediment sediment formation formation calculation calculation after after 22 years years monitoring, monitoring, average average diurnal diurnal courses courses s of the settling duration, settling velocity threshold (c and d) and settled mass (m on pipes duration (a,b), settling velocity threshold (c,d) and settled mass on pipes bottom bottom and f), for(a,c,e) rule-based(a, c and control e) and two-point (e,f), for(e rule-basedand two-point (b,d,f). control (b, d and f)

Summary

Introduction

Sewage transport in urban drainage systems involves in many cases the use of pumping stations (PS). Pumps are usually designed to handle maximum design inflow, which rarely occurs under normal operating conditions. While the pump sump is filled with sewage the pump switches on at a defined level before overflow occurs and switches off before it runs dry (two-point control). Pump flow exceeds by far the actual demand, resulting in unnecessary friction losses and in a high energy consumption. The key of a minimized energy demand is the reduction of friction losses, resulting from low flow velocities and engendered lower shear stress in connected pressure pipes. To increase efficiency and minimize energy demand, sewage PS can be equipped with frequency control. Identified side effects are the reduction of operation and maintenance costs and an expected increased durability of pumping aggregates, armatures, and electric engines due to smoother operation conditions

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Conclusion