GB2350608A

GB2350608A - Sewage treatment

Info

Publication number: GB2350608A
Application number: GB9900962A
Authority: GB
Inventors: David Burt
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-01-15
Filing date: 1999-01-15
Publication date: 2000-12-06
Anticipated expiration: 2019-01-15
Also published as: GB2350608B; GB9900962D0

Abstract

A sewage plant 10 comprises an anaerobic chamber 12 and an aerobic chamber 13. Raw effluent 16 is fed into the anaerobic chamber 12, which preferably has a hemispherically-shaped bottom 24, and settles under the action of gravity into an activated sludge 21 in the bottom of chamber 12. Anoxic liquor produced by fermentation of the activated sludge is passed through a one-way valve 15 located above the sludge level 17 into the associated aerobic chamber 13, the latter preferably forming an insulating jacket about the anaerobic chamber. Uniform fermentation and increased temperature is acquired by the system by virtue of the hemispherical bottom. The aerobic chamber 13 preferably surrounds the anaerobic chamber 12, forming two channels leading to a treated liquor outlet (see Fig. 2). The chambers may have a weir arrangement.

Description

2350608 A SEWAGE TREATMENT SYSTEM AND A METHOD OF PROCESSING SEWAGE This

invention relates, in general, to a sewage treatment system and a method of processing sewage and is particularly, but not exclusively, applicable to a system for use in a domestic environment as an alternative to a septic tank system.

The treatment of effluent is necessary in order to separate sewage sludge and large particles from a suspension in water. Purified water released by both the removal of inorganic material (i.e. so-called At suspended solids") and the reduction of organic matter (that otherwise removes oxygen from the water through a biological process) can then be discharged into a watercourse, such as a river or the sea, for subsequent re-use.

Present-day sewage works separate sludge by filtration, with the filtrate bifurcated and treated separately. Typically, sludge is anaerobically digested by micro-organisms (namely filamentous bacteria), whereas the filtrate is treated biologically by an aeration unit (and sometimes through ozone impregnation) before being allowed to settle in open lagoons.

Degradation of sludge occurs by the anaerobic bacteria which breakdown ammonia and phosphates (for example) to produce nitrification and denitrification in the liquor.

Unfortunately, present systems seldom acquire stipulated Royal Commission Standards (or the like) for biological oxygen demand (BOD) and level of suspended solids, and so it is necessary to discharge treated water not directly to a watercourse but rather to a foul sewer or soak away. Royal Commission Standards presently stipulate a BOD level of twenty parts per million (20ppm) and a suspended solids level of 30ppm, although moves are afoot to improve the quality of discharge by halving these levels.

According to a first aspect of the present invention there is provided a sewage treatment system comprising an anaerobic chamber and an associated aerobic chamber, the anaerobic chamber having a raw sewage inlet and a sewage liquor outlet, the aerobic chamber having a sewage liquor inlet communicating with the sewage liquor outlet of the 10 anaerobic chamber and a treated liquor outlet. Advantageously, the energy stored in activated sludge can therefore be more effectively utilised in the treatment process. In a preferred embodiment, the anaerobic chamber and the aerobic 15 chamber are insulated.

0; Preferably, the anaerobic chamber contains a substantially hernispherically-shaped bottom, which ensures a substantially uniform sludge layer having a dynamic environment. Consequently, anaerobic fermentation is improved.

In another embodiment, the aerobic chamber circumferential ly surrounds at least part of the anaerobic chamber.

In yet another embodiment, the sewage liquor inlet and the sewage liquor outlet are connected via at least two channels, each channel having a series arrangement of an over-weir and an under-weir located between the sewage liquor inlet and the treated liquor outlet. Preferably, a flow of liquor in the at least two channels is in opposite directions. 30 I V In another embodiment, the raw sewage inlet is coupled to a sewage pipe having an exhaust stack, wherein the exhaust stack regulates a gas pressure within the sewage treatment system and the anaerobic chamber is otherwise air-tight.

In a second aspect of the present invention there is provided an underweir coupled, in use, in an effluent flow of a sewage treatment system, the under-weir having an aperture formed in a bottom edge thereof, the aperture sized to guarantee that a self-cleansing velocity of a flow 10 through the aperture is achieved. In a further aspect of the present invention there is provided a method of treating raw sewage comprising the steps of: providing a first tank having a layer of activated sludge and settling raw sewage into the 15 activated sludge to cause anaerobic fermentation to generate a substantially anoxic liquor; extracting the substantially anoxic liquor from the first tank into a second tank; and aerating the substantially anoxic liquor in the second tank to produce an aerated liquor and to cause aerobic digestion therein. 20 The method may further comprise the step of inducing settlement in the aerated liquor by causing the aerated liquor to drop and pass beneath an under-weir. 25 In another embodiment, the method includes the step of forming an evenly compacted sludge having a substantially flat activated sludge surface in a hemispherical shaped first tank. Additionally and/or alternatively, the method provides communication 30 through a common wall between the first tank and the second tank.

Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 is a sectional view of a sewage treatment system according to a preferred embodiment of the present invention; Fig. 2 is a sectional plan view of the sewage treatment system of FIG. 1; FIG. 3 is a diagrammatic section illustrating aerobic flow lines through an aerobic chamber of FIG. 1; FIG. 4 is an illustrBtion of an improved underweir baffle according to a preferred embodiment of the present invention, Referring to FIGs. 1 and 2, a sewage treatment system 10 according to a preferred embodiment of the present invention in illustrated. The sewage treatment system 10 comprises an anaerobic chamber 12 and an associated aerobic chamber 13 that forms, in a preferred embodiment, an insulating jacket around at least an upper portion of the anaerobic chamber 12. The sewage treatment system 10 is shown located within the ground (which therefore provides additional insulation to the anaerobic chamber 12 and the aerobic chamber 13), although the system 10 could equally well be employed in an above-ground situation.

An important aspect to the effective operation of the sewage treatment system 10 of the preferred embodiment is the retention of heat energy within both the anaerobic chamber 12 and the aerobic chamber 13, with the preferred embodiment of FIG. 1 having a common support wall 14 between the anaerobic chamber 12 and the aerobic chamber 13. The support wall therefore acts in an analogous fashion to a heat exchanger.

Of course, another embodiment of the present invention has physically separate anaerobic and the aerobic chambers, with these connected by a one-way (non-return) valve 15 and appropriately insulated piping (not shown).

It is preferable that the anaerobic chamber 12 is totally surrounded by insulation (which could be realised by a combination of earth and the aerobic chamber 13). Alternatively, the aerobic chamber 13 could be constructed to encapsulate substantially the anaerobic chamber 12 and such that substantially all heat generated within the anaerobic chamber 12 is optimised and further transferred to the aerobic chamber for full utilised within the system.

Raw sewage (effluent) 16 is delivered into the anaerobic chamber 12 for anaerobic treatment prior to discharge to the aerobic chamber 13 through the one-way valve 15 located above a settled sludge level 17.

An inlet pipe 18 that projects into the anaerobic chamber 12 contains an exhaust stack 19 for (noxious) gases, such as hydrogen sulphide (1-12S), methane (CH4) and carbon dioxide (C02). The inlet pipe 18 preferably terminates in the middle of the anaerobic chamber 12 to effect even distribution of the solid matter under the action of gravity. The inlet pipe 18, at its termination in the anaerobic chamber 12, has a generally open ended T-shaped confi puration 20 so that solid matter can drop evenly to the bottom of the anaerobic chamber 12, whereas gases can rise upward within the piping. With the exception of the inlet pipe 18 (and the one-way valve 15), the anaerobic chamber 12 is otherwise sealed in order to retain heat; gases can, of course, still exit the anaerobic chamber 12 via a reverse route within the inlet pipe and then out of the exhaust stack 19, but this is generally only controls the gas pressure and is not an effective route for heat dissipation. Of course, the anaerobic chamber 1 twill typically include an air-tight lid.

In contrast with prior art systems that operate an aerobic process before anaerobic fermentation, the preferred embodiment of the present invention reverses this process; the benefits of the reversal will be described subsequently.

The anaerobic chamber 12 contains activated sludge 21 (or the like) onto which pours the raw sewage (effluent) 16 via the T-shaped configuration 20. To avoid surface disturbance of liquor 22 (comprised from raw sewage 16 and activated sludge 21), the down-pipe of the T10 shaped configuration 20 at the end of the inlet 18 is, in operation, always submerged below a surface level 23 of the liquor 22. A crust of (principally) dead micro-organisms will form on the surface of the liquor over a period of time. 15 The activated sludge 21 at the bottom of the anaerobic chamber 12 has a lack of oxygen and digests by fermentation. The organic matter in the sludge 21 is converted into methane and carbon dioxide gas. The anaerobic bacteria are generally of two types; first type converts the complex organic compounds into very simple compounds which create a 20 food source for the second type of bacteria. The second type of bacteria are the most important feature of anaerobic digestion. The second type of bacteria develops very slowly which therefore means that large volumes are required to ensure sufficient numbers; this generally dictates the size the anaerobic chamber 12 (when also taking into 25 account the amount of effluent that is to be treated). The second type of bacteria are very sensitive to temperature; at higher temperatures reproduction occurs at a faster rate which increases the rate of sludge digestion.

The nitrogenous compounds, such as ammonia, proteins and decomposition products, when acted upon by both the first and second types of bacteria undergo a nitrogen transformation cycle. Nitrification is the process when the hydro-carbons and nitrogen are changed by oxidation by bacteria to nitrite, which is then changed to nitrate by another organism; this occurs in the activated sludge 21. It is preferable that the temperature within the anaerobic chamber 12, generally, is raised as high as possible towards blood temperature (about thirty-seven degree Centigrade). At higher temperatures, sludge conditioning is faster such that the slowest (in terms of reproduction capability) bacteria will have sufficient time to reproduce before they die or before partially treated liquor is passed to the aerobic chamber 13.

Phosphorous is an essential nutrient for the bacteria and is the basis for steady temperature. The phosphorous that is concentrated in the activated sludge 21 is degraded by the bacteria and slow combustion of the phosphorous occurs so releasing the essential heat required for improved operation.

The liquor directly above the sludge will be lacking in oxygen (i.e.

anoxic) and denitrification takes place because the micro-organism in the activated sludge will have utilised the oxygen in changing nitrite to nitrate, which is then changed in this anoxic zone to nitric oxide gas, nitrous oxide and nitrogen. Denitrification, in the anoxic zone above the activated sludge 21, therefore eliminates most of the BOD by re-using the oxygen in the nitrates.

The anaerobic chamber 12 is generally cylindrical having a hemispherical bottom 24. Although the anaerobic chamber 12 could take different bottom profiles, it is both preferably and advantageous that the bottom be hemispherical which causes solid material in the raw sewage 16 to slip downwardly forming an evenly compacted sludge and creating a generally flat top activated sludge surface. Consequently, a substantially uniform (void) pressure is established within the activated sludge 21, and there is a more uniform fermentation process and hence an increase in the amount of phosphorous released which gives greater combustion and therefore higher temperature. The void pressure is the pressure within the generally microscopic gas voids created by the fermentation within the activated sludge 21.

The preferred hemispherical shape of the bottom of the anaerobic chamber 12 is also beneficial because it ensures that the activated sludge 21 does not become septic. In other words, there is always movement within the sludge arising from the shape of the hemispherical bottom and gaseous movement and slight thermal eddies set up within the activated sludge 21.

Suspended solids within the anaerobic chamber 12 will be broken down (in some extent) by processing readily appreciated to the skilled addressee.

Since anaerobic digesters, namely micro-orga n isms, take longer to develop (in terms of cultivation and activation) and the activated sludge has a very large stored energy, reversal of sewage processing by performance of anaerobic fermentation makes use of this stored energy to the fullest extent.

As more raw sewage 16 enters the anaerobic chamber 12 a similar volume of (substantially) anoxic liquor 22 is discharged through the non return valve 15 to the aerobic chamber 13. It should be noted that the non-return valve 15 is protected in the anaerobic chamber 12 by a downward sloping cowl or shield 26 designed to prevent raw (untreated) sewage 16, failing from T-shaped configuration of the inlet pipe 18, passing directly into the aerobic chamber 13.

Once anoxic liquor has entered the aerobic chamber 13, an aeration unit 28 (comprised from a ceramic aerator) introduces very small-sized air bubbles 30 (typically 1.5 to 2.5 micrometres in diameter) in the anoxic liquor to replenish oxygen which has been removed in the anaerobic 10 chamber 12. The replenished oxygen is required for a final aerobic bacteria action on the decomposition of suspended solids and organic waste products. The size of the bubbles 30 is such that a majority fall within a range that is neither classified as being dissolved nor diffused. 15 A small compressor is timed to provide actuation of the aeration unit through a twenty-four hour period. Once aerated, bacterial action takes place to remove organic particles remaining within the liquor (which is now almost pure water). 20 It has been found that, at the point of entry into the aerobic chamber 13, the BOD is about 100ppm and the suspended solids about 150. As previously indicated, the aerobic chamber 13 may form a jacket about the anaerobic chamber 12. The structure of the aerobic chamber 25 will now be described in greater detail and with particular reference to FIGs. 2 and 3. In a preferred embodiment, a tortuous path is provided between the non-return valve 15 and an outlet 32 of the sewage treatment system 10. In a preferred embodiment, the aerobic chamber 13 forms an annular jacket about the anaerobic camber 12 above the 30 hemispherical bottom 24, thereby providing two flow paths 36- 38. Each flow path 36-38 contains one over-weir 40-42 and one under-weir 44 46 (sometimes collectively known as "baffles"). A liquor flow 48 therefore rises from the non-return valve 15 and passes over the over weir 40-42 before descending to pass under the under-weir 44-46. It has been found that the BOD level at the under-weir (on the descending side) is about 25ppm, whereas suspended solids are at a level of about 40ppm. The majority of the suspended solid settlement occurs following the over-weir 40-42.

Each under-weir 44-48 contains a self cleansing aperture 50 in its bottom edge, which aperture is typically semi-circular and having a radius of about 6mm-10mm. The self-cleaning aperture 50 maintains a flow path velocity sufficient to prevent accumulation of sediment (namely, suspended solids including humus) adjacent the under-weir 44 46 that could otherwise block a flow path under the under-weir 44-46.

Liquor then rises up the final tank towards the outlet 32 that has a lower lip 52 at the level of the liquor in the aerobic chamber 13. The BOD at the outlet of the system of the preferred embodiment has been found to V be less than 10ppm, with the suspended solids at a level below 15ppm.

Clean water (i.e. liquor overflowing into the outlet 32) is forced over the lip 52 be virtue of the incoming head of water from the anoxic influent.

By dividing the flow, the flow is generally slower and so suspended solids (including humus) are encouraged to settle out. Furthermore, by virtue of the slower flow, there is an increased time for aerobic digestion. Thirdly, by providing two paths about the system 10, uneven settlement of the system 10 in the horizontal plan (or incorrect installation) can be mitigated. Specifically, the over-weirs have a predetermined clearance below the surface of the liquor, which over- weirs have hydraulic characteristics that enable equal flows to pass in either direction towards the outlet 32. Typically, the over-weirs are sited about 20mm-30mm below the liquid surface, and most preferably about 25mm (millimetres). Basically, tilt of the system results in a constant and equal volume of flow across the over-weirs, i.e. a balanced flow is achieved provided that tilt is not excessive.

The volume of the anaerobic chamber 12 is generally at least about twice the volume of the aerobic chamber 13.

It will, of course, be appreciated that the above description has been given by way of example only and that modifications in detail may be made within the scope of the present invention. For example, while the present invention is generally described in relation to a septic tank on a domestic scale, the underlying principles of firstly providing anaerobic fermentation, the jacketing of the anaerobic chamber by the aerobic chamber and the use of a hemispherical (bell-shaped) bottom to the anaerobic chamber can be applied to commercial sewage farms in both an underground and above ground implementation.

Claims

1 A sewage treatment system comprising an anaerobic chamber and an associated aerobic chamber, the anaerobic chamber having a raw sewage inlet and a sewage liquor outlet, the aerobic chamber having a sewage liquor inlet communicating with the sewage liquor outlet of the anaerobic chamber and a treated liquor outlet.

2. The sewage treatment system of claim 1, wherein the anaerobic chamber and the aerobic chamber are insulated.

3. The sewage treatment system of claim 1 or 2, wherein the anaerobic chamber contains a substantially hemispherically-shaped bottom.

4. The sewage treatment system of claim 1, 2 or 3, wherein the aerobic chamber circumferentially surrounds at least part of the anaerobic chamber.

5. The sewage treatment system of any preceding claim, wherein the aerobic chamber includes at a series arrangement of an over-weir and an under-weir located between the sewage liquor inlet and the treated liquor outlet. 25

6. The sewage treatment system of claim 5, wherein the under- weir includes an aperture formed in a bottom edge thereof, the aperture sized to guarantee that a self-cleansing velocity of a flow through the aperture is achieved.

7. The sewage treatment system of any preceding claim, wherein the sewage liquor inlet and the sewage liquor outlet are connected via at least two channels, each channel having a series arrangement of an over-weir and an under-weir located between the sewage liquor inlet and the treated liquor outlet.

8. The sewage treatment system of claim 7, wherein a flow of liquor in the at least two channels is in opposite directions.

8. The sewage treatment system of any preceding claim, wherein the sewage liquor outlet has a lower lip substantially aligned to a level of the liquor in the aerobic chamber.

9. The sewage treatment system of any preceding claim, wherein the sewage liquor outlet is connected to the sewage liquor inlet via a non-return valve.

10. The sewage treatment system of claim 9, wherein the sewage liquor outlet in the anaerobic chamber is located above a settled sludge level, the non-return valve further positioned below a shield.

11. The sewage treatment system of any preceding claim, wherein the raw sewage inlet is coupled to a sewage pipe having an exhaust stack, wherein the exhaust stack regulates a gas pressure within the sewage treatment system and the anaerobic chamber is otherwise air tight.

12. An under-weir coupled, in use, in an effluent flow of a sewage treatment system, the under-weir having an aperture formed in a bottom i.L edge thereof, the aperture sized to guarantee that a self-cleansing velocity of a flow through the aperture is achieved.

13. The under-weir of claim 12, wherein the aperture is centrally located.

14. A method of treating raw sewage comprising the steps of:

providing a first tank having a layer of activated sludge and settling raw sewage into the activated sludge to cause anaerobic fermentation to generate a substantially anoxic liquor; extracting the substantially anoxic liquor from the first tank into a second tank; and aerating the substantially anoxic liquor in the second tank to produce an aerated liquor and to cause aerobic digestion therein.

15. The method of treating sewage according to claim 14, further comprising the step of inducing settlement in the aerated liquor by causing the aerated liquor to drop and pass beneath an under-weir.

16. The method of treating sewage according to claim 14 or 15, further comprising the step of forming an evenly compacted sludge having a substantially flat activated sludge surface in a hemispherical shaped first tank.

17. The method of treating sewage according to claim 14 or 15, further comprising the step of providing a heat exchanger between the first tank and the second tank.