GB2236689A - Continuous solids/liquid separator - Google Patents

Abstract

A suspension of solids and water is fed at 7 to the top of a granular filter bed 2, the water filters downwards to outlet 5 and air is fed upwards continuously or intermittently through the bed to prevent accumulation of solids therein, and to enable a continuous discharge of concentrated suspension from above the bed by outlet 8. A backwash liquid inlet 6 and overflow 9 are also provided. The separator is used to concentrate the solids fraction of the effluent from an activated sludge process for recycling. <IMAGE>

Description

Continuous Solids/Liquid Separator This invention relates to solids/liquid separators of the granular filter medium type and to a method of preventing solids accumulation on the filter.

In a biological sewage or effluent treatment process, or indeed in some types of water purification process, solids produced by the process are separated from the mixed fluid, allowing a purified product to pass forward and more concentrated suspension of solids from this separation to return to mix with an incoming stream to enable these solids to assist in the purification process. An example is activated sludge treatment, where the solids comprise a biomass which has to be separated from the final effluent and returned to the aeration section. In water purification by coagulation, some systems employ recirculation of solids to enable the concentration in the flocculation section to build up with the resultant acceleration of flocculation and improvement to settlement.

Conventionally a sedimentation tank is employed, with a pump or recirculator to return the solids. At times the settlement may be poor and interference can arise because of wind, gas evolution or deterioration of settleability.

It is possible to abstract the product water with an integral separator within an aeration tank or within an oxidation ditch. In one example a series of triangular beams is installed to define a settlement zone above from a duct beneath, through which the mixed suspended solids pass. Such a system has the advantage that the external separator is eliminated together with the means for returning the solids.

It is furthermore fairly common practice to add to a polishing filter after settlement to reduce the residual solids to an even lower value.

One type of granular medium filter employs air distributed above the bed of media to minimise deposition of solids at the surface. Such a filter has been advocated as a means of improving the quality of effluent going forward to biological treatment thereby reducing the oxygen and hence the power demand. In this device air is supplied from lateral pipes laid above the medium bed.

In another type, an aerated medium bed, with air flowing upwards countercurrently to the downward flow of water, provides a supported growth biological treatment system. Such systems are backwashed regularly like conventional granular media filters. In neither this nor the previous device is there any attempt to feed floc or biological solids onto the filter bed. The only solids arriving are those left after settlement or other treatment of the raw feed. Furthermore in neither case is there continuous abstraction of solids from the supernatant water volume.

This invention comprises a form of granular medium filter which may be used as a continuous separator, in which an incoming stream of suspended solids is divided into a filtrate with a lower concentration of solids and a concentrate with a higher concentration of solids, the concentrate being capable of being returned either to an integral or to an upstream reacting system. It has been found that a continuously aerated bed of granular medium of appropriate size will permit water to percolate downwards concurrently to air flowing upwards and that the air will prevent settlement and the formation of any cake or deposit at the surface of the granular bed whilst at the same time providing a separating action.

It would appear this action occurs at the top surface or near the top surface of the medium bed. Any filtering action within the depth of the bed is an entirely separate process. Such solids penetrating into a bed require backwashing in a conventional manner but the quantity involved is a small proportion of the total arriving.

According to the present invention we provide a solid/liquid separator comprising a granular medium bed with means for supplying a suspension of solids and water to the top of the bed, means for removing concentrated suspension from the top of the bed, and means for removing filtrate from the bottom of the bed with, optionally, means for backwashing and scouring, so that, in use, water filters downwards simultaneously (either continuously or intermittently) with an upward flow of air, whereby the air prevents accumulation of solids at the surface of the bed and permits continuous filtration, and thereby the separation of a concentrate and a filtrate, both of which are continuously discharged from the region of the granular bed.

An aerated granular medium bed used in the above manner may be operated in a batch manner to concentrate solids produced in upstream processes; or it may be used on a continuous flowing basis replacing the sedimentation tank in a conventional activated sludge plant; or indeed as a settlement with the same tank or chamber or channel or as an aeration system in conventional activated sludge plant oxidation ditches or any of the variations in activated sludge systems. Furthermore the aerated granular medium device may be used as an aerating device in conjuction with any superimposed aerated biological system of which it may form part.

The separator is outwardly similar to a conventional granular medium filter with a vessel containing a suitable medium which will tend to be coarser than conventional filter sand in order to permit the passage of air upwards at the same time as the downward flow of water. The vessel is provided with an underdrain which will collect the water passing through and may also distribute backwash water to clean the body of the granular medium bed. Also means are provided for distributing air required for the continuous operation of the device. The same or a separate distributor may be used to supply air for separate or combined air scour to assist cleaning of the body of the medium as distinct from the service air.

The vessel may, as already indicated, be free standing and supplied with feed and discharge pipes or channels or it may be constructed as an integral part of another process unit.

The granular medium may comprise sand, anthracite or indeed any other medium of appropriate Size and density such as expanded clay, granular carbon, pumice, etc. By way of example only, activated sludge may be concentrated to levels in excess 1,000 mg/l, at surface flow rates in excess 1 m/hr with a granular medium bed of particles of approximately 3 mm diameter with an air flow in the region of few mm/s surface rate. The bed depth is not critical for purposes of this invention but to exploit the polishing action of the filter, the bed depth will probably be at least 600 mm.

By way of Example, the separator will now be described with reference to the accompanying drawings, in which: Figures 1 and 2 represent schematic sectional views of two separators.

The separator comprises a filter chamber 1 filled with filler media 2 supported over an underdrain 3 comprising both a drain and an air distributor. The underdrain 3 is provided with an inlet 4 for a source of air and a backwash inlet 6 coupled to a filtrate outlet 5. In Figure 1, a feed inlet 7 is arranged to pass a solid/liquid suspension for treatment across the top of the media 2 to a concentrate outlet 8. The top of the chamber 1 is provided with a backwash channel or outlet 8.

In the modified version of Figure 2, the feed inlet 7 and concentrate outlet 8 are replaced by channel 10 such that the filter chamber 1 is a sump at the bottom of the channel 10.

A number of such separators may be installed, e. g.

in parallel, in the same chamber and may be backwashed independently of each other, the displaced solids passing into the ongoing cross flow.

The service air for surface cleaning may be continuous, intermittent or applied at variable rate.

Excess solids in the system may be bled from the return line or channel and concentrated by settlement in the conventional way or they may be passed to another separator of similar design operating under conditions to maximise sludge concentration rather than a high flux of purified water.

The solids to be separated may comprise any matter that settles less rapidly than the granular medium in the separator.

Claims (4)

1. A solid/liquid separator comprising a granular medium bed with means for supplying a suspension of solids and water to the top of the bed, means for removing concentrated suspension from the top of the bed, and means for removing filtrate from the bottom of the bed with, optionally, means for backwashing and scouring, so that, in use, water filters downwards simultaneously (either continuously or intermittently) with an upward flow of air, whereby the air prevents accumulation of solids at the surface of the bed and permits continuous filtration, and thereby the separation of a concentrate and a filtrate, both of which are continuously discharged from the region of the granular bed.

2. A separator according to claim 1 substantially as desribed herein.

3. A separator substantially as described herein with reference to the accompanying drawings.

4. A method of separating suspended solid from a solid-liquid suspension by passing the suspension over a granular medium filter and removing concentrated solids from the top of the filter and filtrate from the bottom of the filter, in which air or other gas is passed upwards through the filter simultanteously with the downward flow of liquid so as to prevent accumulation of solids at the surface of the medium.