Modelling the dynamic response of a secondary clarifier to a sea water incursion at the Radford STW
Burt, D.1, Ganeshalingam, J.1 and Hammond, C.2, 1MMI Engineering Ltd, UK, 2Hyder Consulting, UK
(free)
Abstract As part of a recent study of a secondary clarifier optimisation for the Radford coastal Sewage Treatment Works (STW) in Plymouth, a Computational Fluid Dynamic (CFD) model has been used to consider the dynamic performance of the secondary clarifiers during storm flows with sea water incursions. The addition of seawater into the system increases the density of the liquid phase at the influent producing an enhanced gravity current. There is significant site evidence to suggest that a sea water incursion degrades clarifier performance but the mechanisms for this are not clearly understood. This behavior has not previously been investigated with a modelling approach
The paper describes the development of a CFD model of the Radford STW final clarifier No 4 to determine the performance under dynamic changes in influent flow including storm influx and a 10% seawater incursion for a duration of up to 1 HRT (Hydraulic Retention Time). Two inlet modifications are investigated; one with an upturned bellmouth and the other with an Energy Dissipating Inlet to test whether dispersing and mixing the load at the influent could exert some control over the enhanced gravity current.
Keywords Activated Sludge, Mass Flux Theory (MFT), EDI (Energy Dissipating Inlet), Computational Fluid Dynamics (CFD)
Introduction
Radford Sewage Treatment Works (STW) is managed and operated by South West Water Ltd and serves a population of approximately 22,000 from the Plymstock area to the east of Plymouth. Over the next 20 years, it is anticipated that the catchment population will grow by around 8,000 0. This will result in higher average flow and loads for treatment. South West Water Ltd and Hyder Consulting Ltd have carried out a review to identify the asset upgrades required to deal with future growth and historical performance issues, particularly sea water incursion.
The source of the sea water is a result of seawater ingress in to the sewer system. This sea water is consequently conveyed to the STW.
A number of options for extending Radford STW were considered including increasing the volume of the activated sludge aeration tanks, increasing the area of final settlement tanks and improving the characteristics of the activated sludge. All options involve increasing the capacity of the oxygen transfer system, remedial works to reduce saline infiltration and measures to reduce the degree of filamentous foaming. There is significant site evidence to suggest that a sea water incursion degrades clarifier performance but the mechanisms for this are not clearly understood. This behaviour has not previously been investigated with a modelling approach.
In the initial phase of Computational Fluids Dynamics modelling, the performance of the existing secondary clarifiers is assessed under steady state conditions to identify potential improvements for current and future flow conditions.
The secondary clarifier design was then tested for influent flow profiles approximating a diurnal flow cycle for a storm flow event as well as for a seawater incursion event where an influent flow was contaminated with 10% by volume of seawater. The performance of the secondary clarifier, measured by the effluent concentration and bed depth, were monitored over the period of the storm and seawater incursion event and the following hours up to 6 hydraulic detention time (HRT).
This paper mainly discusses the secondary clarifier response to a storm and sea water incursion.
