GB2504927A

GB2504927A - A sewer overflow structure for the removal of floating materials

Info

Publication number: GB2504927A
Application number: GB201213931A
Authority: GB
Inventors: Frank Moy
Original assignee: WRC PLC
Current assignee: WRC PLC
Priority date: 2012-08-04
Filing date: 2012-08-04
Publication date: 2014-02-19
Anticipated expiration: 2032-08-04
Also published as: GB2504927B; GB201213931D0

Abstract

The invention relaters to a sewer overflow structure for providing efficient removal of floating material from a wastewater stream under storm conditions and its routing to the continuation flow of the structure. The structure comprises two chambers, the first 2 has a weir 3 which captures the floating material in its spill flow. This spill flow is then routed past the second chamber 7 to join the-continuation flow 6 in a downstream manhole chamber 5. The second chamber acts as a conventional combined sewer overflow, providing for a continuation flow in which the sinking solids are concentrated and a weir 8 and spill flow (9 figure 2) for discharge to the environment.

Description

I

DOUBLE CHAMBER COMBINED SEWER OVERFLOW

This invention relates to combined foul and surface water sewer systems.

Combined Sewer Overflows (CSOs) are an essential part of sewerage systems where mixed foul and surface water sewers are presert. During periods of heavy rainfall the flow into the sewerage system may exceed the capacity of the network and some of the flow is discharged to a receiving water such as a river or for coastal systems, the sea. The CSO structure has several functions: it provides some storage of wastewatersuch that for smaller rainfall events no discharge occurs; it provides some settlement of solids in the wastewater so that the wastewater suspended solids are concentrated in the continuation flow to the treatment works; it includes scum boards and/or screens to prevent floating sollds such as rags and hygiene products being discharged.

There are a large number of different CSO configurations in use. Typically they consist of a circularor rectangular tank into which the wastewater flow enters, a continuation flow drawing from the invert of the tank, and.a weir at the top of the tank over which the discharge occurs. If no screen is installed, a scum board is used at the weir. Screens where fitted may be arranged at the top of the tank, or may be mounted downstream of the weir to intercept solids which would otherwise be discharged to the environment. . Following an from the Urban Wastewater Directive, most EU countries have programmes to upgrade their existing CSOs arid to build new ones, designed to reduce the polluting loads discharged to surface waters. Where CSOs discharge to surface waters which are amenities with public access, it is especially important to prevent the discharge of visible material, and this is typically plastic, paper and hygiene and contraceptive products which tend to float. Theseare particular eyesores when deposited on the banks of rivers or in vegetation after storms if CSOs have failed to trap the floating debris. This is described as aesthetic pollution due to their unsightliness. If CSO screens are blinded the discharged flow will bypassthe screen and discharge its load of floating material while it is spilling. Maintaining CSO screens is an onerous task for sewerage operators; the loading on screens is irregular and difficult to predict. Operators are faced with making frequent and costly visits to ensure screens are clear or risking screen failure if maintenance intervals are extended. Some CSOs have electrically powered screens which clear themselves, and whilst this improves the screen performance, it is a further maintenance task for operators, and the electrical installation is an additional capital and operating cost. CSOs, although normally hidden, are very numerous; the UK alone has over 13,000 CSOs. A CSO structure which deals effectively with floating and settling solids without the on-going maintenance and energy costs of current designs would have a ready market.

We havenow invented a new CSO structure which makes specific provision to retain floating material in the sewer system and route it into the continuation flow to the wastewater treatment A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings which show a side elevation of the structure in Figure land a plan of the structure in Figure 2.

Referring to Figure 1, the invention consists of two chambers (2 and 7), the size of which depends on the hydraulic characteristics of the sewer system to which it is being installed.

The first chamber has an inlet (1), an outlet orifice (11) and a high level weir (3). The second chamber (7) has an inlet (11), an outlet (10), and a hFgh level weir (8) fitted with a fixed screen (12).

The two chambers are interconnected by the outlet orifice (11) through which flow of effluent is controlled by a fixed orifice plate, adjustable penstock or other flow control device. This orifice allows forward passage of bed load (sediment and heavy sewer debris) and allows a balance of water levels between the two chambers.

The outlet (10) of the second chamber (7) is also throttled by a specified pipe size or a flow control device..

The weir (3) in the first chamber (2) is connected to a bypass pipe (4) that is connected to the continuation pipe (6) at a chamber(S) downstream of the CSO chamber. The size of the bypass pipe (4) will be specified or fitted with a flow control device such as as fixed orifice plate or an adjustable penstock.

The weir (8)in the second chamber (7) is connected to an overflow pipe (9) that discharges to the receiving water.

The relative height settings of the weirs (3 and 8) in both chambers and the relative size of the throttles atli, 10 and 4 depends on the flow balance required at the CSO, the inflow, continuation and spill, and will be subject to specific calculation for each installation.

The operation of the new CSo structure will now be described, referring to Figure 1.

Referring to Figure 1, during dry weather flow conditions, with no storm flows, flow enters the -2 structure at land passes into the first chamber (2). Flow continues through the first chamber outlet orifice (11), through the second chamber (7), the second chamber outlet (10), the continuation pipe (6) and forward to treatment.

Under conditions of high flow, the flow is throttled at the second chamber outlet orifice (10) and the two chambers fill simultaneously due to the U tube' effect through orifice (11). When the depth of flow increases above the soffit of orifice 11, the flow pattern in the body of water in each of the two chambers becomes independent, The stilling of the body of water in the first chamber (2) allows heavy debris to sink to the invert and floatable debris to rise to the water surface. This separation results in a significant reduction of floating debris passing through to the second chamber.

If the water level continues to rise, the water in the first chamber (2) spills over the weir (3) carrying the floating debris into the bypass pipe (4) which flows forwards to the treatment works-via manhole chamber S which joins the continuation flow 6. The second chamber (7) also has a wéir (8) which facilitates the CSO discharge to the environment via the spill pipe (9) in Figure 2.

The weir (8) in the second chamber (7) is set higher than the weir (3) in the first chamber promoting a strongerflow and removal of floating debris via weir 3 due to the height differential. The flow downstream of the second chamber (10) and in the pipe (4) downstream of weir 3 is throttled to endure that the total continuation flow does not exceed the flow capacity of the downstream sewer network.

The flow discharged to the environment from the second chamber (7) is screened (12) to intercept any floating debris. However this will be minimaldue to the intercepting of the debris in the first chamber. The bed load entering the second chamber from the first chamber remains at the invert due to the stilling effect in the second body of water and passes through the outlet (10) and on to treatment via the continuation pipe (6).

When compared with existing CSO designs, the new double chamber CSO will reduce debris discharged to the environment and reduce debris on the screens. There are no moving parts, no electrical power requirement and no need for periodic renewal of the electrical and mechanical systems. The new CSO design can be effectively retro-fitted by partitioning an existing CSO structure or adding an additional chamber upstream.

The floating debris passes to the continuation pipe and to treatment unobstructed by a physical separation device therefore the separation of floating solids will continue under all circumstances.

Discharge. to the environment is from the second chamber where there is little or no floating debris.

This will reduce the requirement for screen cleaning and will avoid the need for litter picking by the system operator which can be required after a CSO with blinded screens has spilled.

The adoption of the new CSO design is expected to produce a valuable improvement in sewage litter retention performance compared with existing CSOs and therefore to contribute to cleaner rivers and beaches, reducing capital and operating costs and significantly reducing the number of complaints to the water company. This will assist UK water companies in achieving a good Service Incentive Mechanism (SIM) score from the regulator. .