Minimum Pipe Flow Velocities and Sediment Behaviour in Wastewater Transport
Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Water transport has in all times been a fundamental part of a society’s infrastructure. Especially the handling of wastewater poses great challenges in today’s growing cities. In modern wastewater transport systems, pressurized pipes are frequently used where the water is moved by pumps. It is often recommended to have a minimum pipe flow velocity such as 0.7 m/s to avoid sediment accumulation in the pipes. By introducing variable frequency drives the wastewater pump speed can be optimized for varying pump station inflow. This means that in many cases pipe flow velocities are lower than recommended, which is sometimes compensated for by introducing high speed pipe flushing. This study aims to contribute to a greater understanding of how a lower pipe flow velocity will impact the sedimentation. The main goal is to establish new guidelines concerning minimum pipe flow velocities.
Characteristics for solid particles in sanitary wastewater, like grit and sand, are determined by collecting sludge samples from Fors’ and Henriksdal’s wastewater treatment plants. The findings are used to reproduce sediment transport behavior in a laboratory environment. Sand transport is studied in a clear PVC pipe. The vertical and horizontal transport velocity as well as the full suspension velocity is found. It is also found that the sand forms dunes of a height and length characteristic for the flow velocity. The sediment transport is related to changes in pipe pressure over a measuring distance of 6.4 m. It is found that when sand is introduced into the system an increase in pressure can be detected using a differential pressure sensor.
Tests are performed in implemented pipes at two pump stations subjected to different types of wastewater. Kalvsvik’s pump station in Haninge transports sanitary wastewater from a residential area. The water contains small amounts of sand from leakage and the station normally operates at pipe flow velocities of 0.45 m/s. By measuring the pressure when flushing the pipes at high speed it is found that there are no changes to indicate any prior sedimentation. It is also found that lowering the velocity to 0.1 m/s for as long as the station operation allows is not sufficient to initiate a sediment accumulation.
Tärnsjö’s pump station transports industrial waste from a nearby tannery. The station has recurring problems with sedimentation due to animal hair, protein and fat in the wastewater. As the maximum achievable velocity of the system of 0.33 m/s is insufficient for pipe flushing, a soft plug is inserted for swabbing. It is found that the cleaning reduces the pipe pressure by 20 %.
From the laboratory studies and the field testing it is concluded that for systems transporting sanitary wastewater a pipe flow velocity of 0.45 m/s is sufficient to avoid sedimentation. For applications where rough wastewater like fat and grease is involved, there are no indications that the velocity can be lowered further than the current recommendations of 0.7 m/s.
Place, publisher, year, edition, pages
2013. , 59 p.
Wastewater transport, sedimentation, minimum pipe flow velocity
IdentifiersURN: urn:nbn:se:kth:diva-124020OAI: oai:DiVA.org:kth-124020DiVA: diva2:632288
Vogt, Damian, Ass. Prof