WO2013088097A1

WO2013088097A1 - Waste water treatment

Info

Publication number: WO2013088097A1
Application number: PCT/GB2011/001727
Authority: WO
Inventors: Ahmad al SHUHA
Original assignee: Eflo International Limited
Priority date: 2011-12-16
Filing date: 2011-12-16
Publication date: 2013-06-20

Abstract

A membrane bioreactor waste water treatment system (20) comprises an inlet screen (2), an aeration process tank (4) to receive waste water from the inlet screen, a membrane process tank (9), a membrane filter (10) submerged in said membrane process tank and through which water permeates to provide a flow (11) of cleaned water, and a flow path (6,8) for passing the waste water from the aeration tank to the membrane tank, said flow path comprising an intermediate screen (7) to arrest the flow of agglomerations of hair and fibre present or formed in the aeration tank (4) and membrane tank (9).

Description

Waste Water Treatment

This invention relates to a method of and apparatus for waste water treatment. It relates in particular, though not exclusively, to a method of and apparatus for the capture and removal of agglomerations of hair, fibre and other such debris material from a waste water treatment installation of the membrane bioreactor type.

Conventional waste water treatment plants employ screens as part of a pre- treatment process to remove debris. These screens typically have openings no smaller than 3mm so as not to unduly impede the through-flow. This screen size allows hair, fibres and other debris material to pass through the screen, these materials later being captured in settlement tanks, sludge wasting tanks and sand filters such that they have no significant detrimental effect on the quality of the treated water discharged from the process.

However, the presence of hair, fibres and other such debris material presents a significant problem in the case of the more advanced membrane bioreactor (MBR) waste water treatment processes which employ membranes instead of settlement tanks and sand filters to separate the biologically cleaned water from the biomass. These membranes typically are submerged in the biological treatment process tank. The hair, fibres and other small debris mix with the biomass to form agglomerations which physically foul the

membranes.

The membranes have a very small pore size, typically less than 0.2 micron. Their filtration rate (referred to also as "flux") is determined by the pressure differential across the membrane, the available membrane surface area and, significantly, the extent to which fouling effects have clogged or otherwise restricted the membrane pores such that the flow rate through the membrane is diminished. In order to maintain a desired filtration rate, or flux, it is most important that the membranes remain substantially free from fouling such as may be caused by scaling or physical effects. Scaling can be considered to be a reversible form of fouling because the membrane can be subsequently restored substantially to the designed flux rating by periodic chemical cleaning whilst the membrane is in situ, submerged in a process tank.

Although scaling may be treated by periodic chemical cleaning, attempts to minimise physical fouling require the provision of one or more inlet screens to screen the waste water that is entering the process, upstream of the bioreactor. Typically the upstream screens have openings of 3 mm or less, and smaller sizes down to in the order of 0.5 mm are employed in some cases in an attempt to enhance the capture of hair and fibres. However the smaller the screen size the greater the difficulty of operation and the greater the requirement for pre-treatment in the form of course screens, grease removal and grit removal upstream of the screen, as well as need for high pressure screen washing systems and washing processes to remove the collected organic material from the screens.

Furthermore, even with a small inlet screen opening size such as down to 0.5 mm, it is found that small hairs and fibres still pass through the screen and into the treatment process, thereby becoming present in both the aeration and membrane tanks. Within these tanks the physical action of the aerators and the high concentration of the sticky, aerobic biomass interact with the hair and fibres suspended in the tank contents and form agglomerations of hair and fibre.

These agglomerations are found to form as irregular shaped, roughly woven balls and plaited ropes and are of a size sufficient to cause significant and detrimental physical fouling of the membranes. In particular it is found that the agglomerations tend to attach themselves to the membranes and cannot be dislodged readily by a conventional aerator driven scouring system. This type of fouling can be considered as irreversible in so far as it requires the physical, manual removal of the agglomerations from the membranes in order to recover an acceptable flow rate through the membrane. It is necessary to remove the membranes from their submerged positions within a process tank either for cleaning, or for substitution of a pre-cleaned

membrane. This results in an undesirable operational downtime and loss of treatment capacity.

Furthermore the removal of the membrane from the process tank and the action of manual cleaning present a risk of damage to the membranes.

Attempts have been made to provide a more efficient means for cleaning the membranes, or for preventing the agglomerations of hair and fibre tending to attach to the membranes but up to the time of the present invention such attempts have not been successful.

In a proposal described in patent publication US 2003/0006200 A1 a portion of the re-circulation flow from the membrane tank to the aeration tank is directed through a screen to remove hair, trash or fibrous materials. However the major portion of the re-circulated flow does not pass through the screen and in consequence the membrane remains exposed to the effects of significant fouling.

It is an object of the present invention to provide a waste water treatment installation of the membrane bioreactor type, and method of operation thereof, which addresses the problem of irreversible fouling and minimises or eliminates the need for removal and physical, manual cleaning of the membrane filter.

In accordance with one aspect of the present invention a membrane bioreactor waste water treatment system comprises an inlet screen, an aeration process tank to receive waste water from the inlet screen, a membrane process tank, a membrane filter submerged in said membrane process tank and through which water permeates to provide a flow of cleaned water, and an intermediate screen positioned between the aeration tank and membrane tank to arrest the flow of agglomerations of hair and fibre present or formed in the aeration tank and membrane tank.

In accordance with another aspect of the present invention a method of treating waste water by a process of the membrane bioreactor type comprises an inlet screen, passing waste water through the inlet screen to an aeration tank, and then passing the waste water from the aeration tank to a membrane tank via an intermediate screen which arrests the flow of agglomerations of hair and fibre present in the flow from the aeration tank.

The method and system installation of the present invention may comprise a re-circulatory flow of retentate from the membrane tank to the aeration tank in a manner known per se, the re-circulation flow rate typically being in the order of four times the average in-flow rate through the inlet screen. Thus both the raw water in-flow through the inlet screen and the re-circulation flow pass through the intermediate screen, hereinafter referred to as the biomass screen.

The function of the biomass screen is to remove the larger, damaging agglomerations of hair and fibre that are formed as the consequence of the physical action in the aeration process within the aeration tank and, typically, also aeration in the membrane tank such that the agglomerations flowing from the aeration tank to the membrane tank are prevented from coming into contact with the membranes. Although some hairs and fibres may pass through the biomass screen due to their individual sizes or their small early form of agglomeration, they cause no harm or only minimum harm to the membrane because a scouring system, as typically employed to clean a membrane, is sufficient to prevent fouling.

In order to avoid unduly minimising the flow rate through the biomass screen the openings of the biomass screen may be larger than the smallest size of the openings of the inlet screen, which typically comprises openings of a 3 mm pore size or less, that size being sufficient for removing larger debris that may damage the membranes. However it is found that the hairs and fibres which are not arrested by the inlet screen develop in the aeration and membrane tanks to form hair and fibre agglomerations which are larger than 3 mm in all three dimensions. Thus a biomass screen having openings of a 3 mm or larger size is effective for minimising the risk of fouling to the

membranes. Preferably the biomass screen has openings of a size in the range 2 mm to 12 mm, more preferably in the range 3 mm to 8 mm.

The biomass screen may be of a vertical, parallel bar type screen and may have a flooded hydraulic profile to eliminate a hydraulic fall through the screen. The screen may be periodically, manually raked clean by an operator or mechanical means such as a mechanical raking system may be provided for continuous or intermittent automatic cleaning. Other screen types such as wire mesh, wedge wire or perforated plate type screens may be employed. Because the biomass screen is positioned downstream of the aeration tank, and upstream of the membrane tank, the screened and collected

agglomerations on the biomass screen are biologically scoured clean of offensive organic material and therefore may be disposed of without further treatment.

Although reference is made herein to a method and apparatus comprising inlet and biomass screens, an aeration tank and a membrane tank, it is to be understood that the method and apparatus may comprise also provision of additional features well known per se, such as anoxic tanks and biomass de- oxygenation tanks.

The present invention is applicable to membrane bioreactor treatment systems which employ membranes of different types. Thus the membrane may be in the form of hollow fibres or flat sheets bonded to an internal carrier structure such as a ceramic structure. Membranes of these types are potentially prone to fouling by hair and fibre agglomerations, and to damage, pariicuiariy damage arising during physical, manual removal of the

agglomerations. The membrane may be a filtration device with a pore size less than 0.3 micron, more typically less than 0.2 micron. Preferably the biomass screen is capable of passing a flow of seven times the average flow rate of the system. That is, with a re-circulation flow rate which is four times the average flow rate of waste water to be treated, and with the peak in flow rate being three times the average flow rate the biomass screen should be capable of accepting a flow of seven time the average flow rate of the process or proportionally more or less if the peak flow rate is predicted to be more or less than three times the average and if the re-circulation rate is more or less than four times the average flow rate.

One embodiment of the present invention will now be described, by way of example only, with reference to the accompanying schematic diagrams.

A membrane bioreactor type waste water treatment installation 20 comprises an aeration tank 4 containing an aeration system 5, and a membrane tank 9 comprising an air scouring system 12 and a membrane filter 10.

Raw waste water flows from a feed 1 to an inlet screen 2 and then into the aeration tank. The aeration tank also receives the re-circulation flow from the membrane tank via re-circulation flow path 14. Clean water permeating through the membrane filter 10 exits from the membrane tank via flow path 11. Additionally a flow path 13 leads from the retentate side of the membrane filter for discharge of excess biomass waste.

The aforementioned features of the installation shown in Figure 1 may each be of an established and known construction and mode of operation.

However, in contrast, and in accordance with the particular feature of the present invention, flow 6 from the aeration tank to the inlet path 8 leading to the membrane tank is via a biomass screen 7. For the purpose of clarity other conventionally known devices such as screen cleaners, pumps and liquid stirrers are not referred to or illustrated.

In this embodiment of the invention the inlet screen comprises openings which have a size of 3 mm or less such that it is able to remove from the incoming flow 1 any debris of a size that is potentially harmful to the membrane filter 10. Although the openings of the inlet screen may be smaller, by virtue of the provision of the biomass screen 7 there is no need for the inlet screen to comprise much smaller openings in any attempt to minimise the flow therethrough of hair and fibres. The flow from the inlet screen 2 through the flow path 3 enters the aeration tank in which the aeration system 5 introduces bubbles of air into the waste water to maintain the contents in an aerobic state. Thus the aeration system 5 delivers oxygen to the micro organisms which oxidise the organic contents of the incoming flow 3.

In operation it is found that within the aeration tank the hair and fibre (and similar debris material) that has passed through the inlet screen combine in the high concentration of sticky biomass to create agglomerations of hair and fibre. Passage of those agglomerations to the membrane tank is inhibited by the biomass screen which, in this embodiment, has openings of 8 mm size.

The biomass screen 7 is relatively robust compared with the membrane and therefore accumulations of the hair and fibre agglomerations arrested by the biomass screen may be removed relatively readily either manually with a suitable rake or, for example, with an automatic rotating rake.

Optionally the biomass screen 7 may be equipped with a water spray cleaning system but that is not essential. The biomass screen operates in a flooded state and relies on the hydraulic flow through the process to drive the flow of fluid from the aeration tank to the membrane tank.

The membrane 10 may be of a type known per se and may, for example, comprise vertical hanging hollow fibres or flat sheets bonded to an internal carrier structure.

Because the biomass screen 7 captures hair and fibre agglomerations from the biomass flow 6 from the aeration tank it delivers to the membrane tank a biomass flow 8 which is substantially free of hair and fibre agglomerations. The re-circulation flow 14 facilitates the return to the aeration tank 4 of any individual, non-agglomerated hairs and fibres in the biomass flow 8 into the membrane tank.

From the foregoing it will be understood that most advantageously the provision of the biomass screen between the aeration tank and the membrane tank arrests the flow of hair and fibre agglomerations such that they do not accumulate on the surface of the membrane filter. Accordingly the need to shut down the installation and interrupt the processing of waste to renew and clean or replace a membrane filter, and the risk of damage to the relatively delicate membrane filter, is thereby avoided or at least substantially minimised. Instead it is necessary merely to effect cleaning of the relatively robust biomass screen, and that may be undertaken automatically without (or with only a minimum) need to interrupt operation of the installation.

A further advantageous feature of the present invention is that it does not call for expensive additional filters or redesign of the membrane filters and may readily be retro fitted to existing membrane bioreactor treatment systems.

A further advantageous feature of the present invention is that it avoids the need to provide an inlet screen having very small openings of 1.0 mm or less and in some cases of 0.5 mm requested by membrane manufacturers.

Screens with these very small openings are burdensome to operate since they arrest very large amounts of material, far greater than the 3 mm inlet screen described in respect of the present invention. This large mass of screenings needs to be washed free of all foul, odorous matter and dried before disposal. Furthermore, the automatic mechanisms required for maintaining screens with small openings in efficient condition without fouling or blocking is extensive and requires significant upkeep. The present invention allows the MBR process to employ 3 mm opening screens and thus advantageously avoids the difficulties described in relation to screens having smaii openings.

EFLODE2

Claims

1. A membrane bioreactor waste water treatment system comprising an inlet screen, an aeration process tank to receive waste water from the inlet screen, a membrane process tank, a membrane filter submerged in said membrane process tank and through which water permeates to provide a flow of cleaned water, and a flow path for passing the waste water from the aeration tank to the membrane tank, said flow path comprising an

intermediate screen to arrest the flow of agglomerations of hair and fibre present or formed in the aeration tank and membrane tank.

2. A system according to claim 1 wherein said intermediate screen comprises openings of a size greater than 3 mm.

3. A system according to claim 1 or claim 2 wherein the intermediate screen comprises openings of less than 8 mm.

4. A system according any one of the preceding claims wherein the openings of the intermediate screen are larger than the openings in the or each screen in the flow path to the aeration tank.

5. A system according to any one of the preceding claims wherein a screen opening size of 3 mm is the minimum opening size of the or each screen in the flow path to the aeration tank.

6. A system according to any one of the preceding claims wherein the intermediate screen is adapted for cleaning by a mechanical device.

7. A system according to any one of the preceding claims wherein the intermediate screen has associated therewith means for automatic cleaning of the upstream side thereof.

8. A system according to claim 7 wherein said automatic cleaning device is operable continuously.

9. A system according to any one of the preceding claims wherein the membrane comprises pores of a diameter less than 0.2 micron.

10. A system according to any one of the preceding claims and comprising a re-circulation flow path for flow of water from the membrane tank to the aeration tank.

11. A system according to claim 1 and substantially as hereinbefore described.

12. A method for treating waste water by a process of the membrane bioreactor type comprising providing an inlet screen, passing waste water through the inlet screen to an aeration tank, and then passing the waste water from the aeration tank to the membrane tank via an intermediate screen which arrests the flow of agglomerates of hair and fibre present in the flow from the aeration tank.

13. A method according to claim 12 and substantially as hereinbefore described.

EFLOCL2