CA1133151A - Treatment of wastewater - Google Patents

Abstract

Abstract Treatment of Wastewater A method and apparatus for the treatment of wastewater in which the wastewater is circulated around a system comprising a downcomer and a riser communicating with each other at their lower ends, the level of the wastewater in the downcomer being maintained above the level of the wastewater in the riser and providing a hydrostatic pressure head which causes circulation of the wastewater around the system at a pre-selected rate, a gas containing free oxygen is supplied to the wastewater as it passes through the downcomer, and the wastewater in pumped from, or from near, the top of the riser back into the downcomer (or vessel com-municating therewith) at a position(s) above the highest level to which the wastewater in the downcomer can fluctuate during normal operation of the system when operating at maximum load.

Description

1 ~ 307~9 Treatment of Wa~tewater ~ his invention relates to the treatment of liquid-borne biologically-degradable waste material, hereinafter referred to as wastewater which term is to be understood to include all types of biologically degradable domestic and industrial waste ma-terials, for example normal domestic sewage, the effluents produced b~Y
farms, food factories and other industries producing such waste.
The methods generally emploged in the treatment of waste-water comprise essentially a primary treatment by physical methods such as screening and sedimentation to remove large suspended solids followed by a secondary treatment by biological methods to remove organi.c materials. The present inve~tion relates to a secondary t~atment stage.
~he present invention relates to a method for the treat-ment o~ wastewater of the kind which co~prises the steps of circ-ulating the wastewater around a system comprising a downcomer and a riser communicating with each other at their lower ends, the level of the wastewater in the downcomer being maintained above the level of the wastewater in the riser to provide a hydrostatic pressure head which causes circulation of the wastewater around the system, supplying a gas contain.in~ free oxygen to the waste-water as it passes through the downcomer, and pumping the waste-water from, or from near, the top of the riser back into the down-comer at or near the top thereof.
The present invention also relates to an apparatus for the treatment of wastewater of` the kind comprising a downcomer and ,. ~
-:

2 B 30799 a riser com~unicati~g with each other at their lower ends, means for suppl~i~g a gas containing free oxygen to the wastewater as it passes through the downcomer, means for pumping the wastewater from, or from near the top of the riser back into the downcomer at or near the top thereof, to maintain the level of the waste-water in the downcomer above the level of the wastewater in the riser.
In the accompanying drawings, ~igures 1, 2 and 3 illus-trate, diagramatically, examples of the kind of apparatus for the 19 treatment of wastewater suitable for use in the present invention, while ~igs. 4 and 5 illustrate, diagramatically, examples of modi-fications made thereto in accordance with the invention.
Ae ill~strated in ~igures 1, 2 and 3, the apparatus com-pri3es a downcomer 1, a riser 2, and a pu~p 3 which recirculates wastewater from, or from near, the top of the riser 2 back into the downccmer 1 at or near the top thereof. Although the waste water must be pumped up from the riser to a higher level in the downcomer, in the interests of power econo~y it is not pumped to a hi~her level than that required to achieve the desired rate of circulation around the system. Wastewater to be treated is intro-duced into the apparatus (preferably into the downcomer 1) at any desired rat0 at a con~enient point 4 and, after being recirculated around the apparatus many times, treated wastewater is removed from the apparatus at sub~tantially the same rate at any convenient point 5.
A gas containing free oxygen (for example air) i8 intro-duced into the downcomer 1 at such a rate and at any convenient point such that it is substa~tially all e~trained by the downwardly flowing wastewater. Preferably the gas is introd~ced into the downcomer 1 at a point 6 between about one-tenth and six-tenths of the length of the downcomer measured from the surface of the waste-water therein. A gas containing free oxygen, for example air, may also be introduced into the riser 2, for example at 7.
Preferably the top of the riser 2 is of enlarged cross-section, forming a basin 8 in which most of the gas bubbles become

3 ~ ~0799 disengaged from the wastewater and escape to the atmosphere beforethe wastewater reaches the pump 3. It is stressed that although it is necessary to disengage gas bubbles from the wastewater before it is p~mped back into the downcomer, it is not essential to pro-vide a basin for this purpose. If the pump is of a suitable type,e.g. an archimedean screw pump, gas disenga~ement will occur if the wastewater is pumped directly from the top of the riser 2 back into the downcomer~
~he pump 3 may be of any type suitable for pumping the wastewater from the basin 8 up and back into the downcomer 1.
i Examples of uitable pumps are centrifugal pumps, axial flow pumps, and positive displacement pumps, for example archimedean screw pumps. The latter have the added advantage of causing further dis-engageme~t of gas bubbles from the wastewater and of not shearing flocs.
~ nder completely stable operating conditions, the level A - A of the wastewater in the downcomer 1 would be at a substant-ially constant height above the level of the wastewater in the basin 8, so that there would be a substantially cons-tant hydro-static pressure difference or pressure head between the two, whichwould cause the wastewater to flow down the downcomer 1 and up the riser 2 at a substantially constant rate. However, we have found that, in general, completely stable operating conditions do not occur and that there are temporary fluctuatio~s in the pressure head, associated with tempora~y fluctuations in the rate of flow of the wastewater around the apparatus; these fluctuations usu~lly rectify themselves and cause no undue disturbance of the flow pat-tern, so that the operating conditions can be fairly described as normal.
Let us suppose that the level of the wastewater in the downcomer 1 is at A - A and the rate of flow of wastewater around the system, starts to decrease due to a random fluctuation of some kind. This increases the voidage in the downcomer 1 (namely the proportion of air by volume in the air/liquid m;~ture) and further slows down the rate of flow of the wastewater around the system.

4 ~ 307~9 While this slowing-do~n process is taking place, however, the level of the wastewater in the top of the downcomer 1 is grad~
ually rising ~rom its original level A - A (since the flow rate around the system is slower and the pump 3 is continuing to pump water into the top of the downcomer 1), with a corresponding in-crease in the hydrostatic pressure head, until it reaches a level~
for example ~ ~ ~, where the increased hydrostatic pressure head is sufficient to speed up the wastewater flow ra-te and restore it to its original value. Once this has been done9 the wastewater level will gradually sink back from 3 - ~ to its original level A -A~
On the o-ther hand, let us suppose that for some reason the wastewater flow rate starts to increase beyond its original rate. In thi~ case, the level A - A will begin to fall, with a corresponding decrease in the hydrostatic pressure head, until it reaches a new level, for example C - C, at which point the hydro-sta-tic pressure head is so reduced that the wastewater flow rate is slowed down again to its original value. Once this has been done, the wastewater level will gradually rise from C - C to its original level A - A. ~rom the foregoing it will be understood that the lines A - A, ~ - ~ and C - C represent respectively the mean, the maximum and the miniumum levels in the downcomer under normal operating conditions - that is to say that between the levels ~ ~ 3 and C - C fluctuations in the flow rate will rectify themselves without outside assistance. It will be appreciated that these levels will be different for any two different waste water treatment systems and will depend upon a large number of different factors including, for example, the rates for which the apparatus is designed, the characteristics of the pum~, the charac-teristics of the wastewater to be treated, and many other factors.
~hese mean, maximum and minimum levels can be determined for each wastewater treatment system.
Since it has hitherto been thought that during normal operation the level A ~ A remains substantially constan-t and it has not been realised that it may, in fact, fluctuate between 3 - 3 and C - C, it might be thought (referring to Figs. 1 to 3) that the most suitable and economical level at which to pump the wastewater back into the downcomer would be a-t or near the mean level A - A. It would not be thought economical to pump it back into the dow~comer at a level substantially higher than the mean level A - A since this would entail additional costs i~ power for driving the pump, which are already a major factor in the total cost of operating the system. ~evertheless, as we have shown above, it has now been realised that the mean level A - A can ri.se to the level ~ - ~, and hence, in order -to maintain normal circul-ation of wastewater and air around the system at the flow rate forwhich the system was designed, the pump must be capable of deliver-ing wastewater back into the downcomer at a level substantially highsr than A - A, and preferably at least as high as the level 3 -~.
~he level ~ - ~ may be defined as the highest level to which the wastewater in the downcomer can fluctuate during normal operation o~ the system.
~ he present invention provides a method for the treatment of wastewater which comprises the steps of circulating the waste-water arou~d a system comprising a downcomer and a riser communic-ating with each other at their lower ends, the level of the waste-water in the downcomer being maintained above the level of the wastewater in the riser and providingr a hydrostatic pressure head which causes circulation of the wastewater around the system at a pre-selected rate9 supplying a gas containing free oxygen to the wastewater as it passes through the downcomer, and pumping the wastewater from, or from near, the top of the riser back into the downcomer (or vessel communicating therewith) at a position or positions above the highest level to which the wastewater in the downcomer can fluctuate during normal operation of the sys-tem ~hen operating at maximum load.
The present invention also provides an apparatus for the treatment of wastewater, comprising a downcomer and a riser com-municating with each other at their lower ends, means for supplying a gas containing free oxygen to the wastewater as it passes through the downcomer, means for pumping up the wastewater from, or from

5 ~

6 ~ 30799 near, the top of the riser and back into the downoomer (or vessel communicating therewith), the said pumping means being capable of pumping the wastewater back into the downcomer at a position or positions located at a level which is above the highest level to which the wastewater in the downcomer can flucuate during normal operation of the apparatus when operating at maximum load.
~ igure 4 shows the upper por-tion of the apparatus of Fig. 1 duly modified in accordance with the present invention. It will be appreciated that Figs. 2 and 3 can be modified ~n similar ~nner. In ~ig. 4, the shaft had a depth of 80 metres and was designed for treating domestic sewage including industrial com-ponents and so that the waste water had an optimum mean velocity around the sha~t of o.6 m/sec., a minimNm head range of 2.6 metres (that is to say, the height of the level C - C above the level of the wastewater in the basin 8), and a maximum head range of 3.1 metres (that is to say, the height of the level ~ ~ above the level of the wastewater in the basin 8). The fluctuations of the level A - A therefore had an amplitude of 0.5 metres.
In Fig. 4, the pump 3 pumps the wastewater from the basin 8, at the top of the riser 2, up and back into the down-comer at 9, if necessary through a suitable vessel such as a conduit or casing (not shown). ~he position (or positions~ 9 a-t which the wastewater re-enters the downcomer from the pump 3 is (or are) located above the level ~ - ~, which is the highest level to which the wastewater in the downco~er oan fluctuate durin~
normal operation. There is therefore a brea~ in the flow of the wastewater between the re-entry position(s) 9 and the level of the wastewater in the downcomer. ~he wastewater may be allowed to fall freely (as a waterfall) from 9. Such free-fall of the wastewater will cause it to entrain air, which is undesirable;
therefore the wastewater i8 preferably caused to flow from 9 to the level of the wastewater in the downcomer through means which inhibit the en-trainment of air, for example down a spiral chute (not shown).
As described above7 the apparatus of ~ig~ 4 was designed 3 ~ ~ ~

7 ~ 3799 for treating certain specific effluents, at specific rates, but it will be understood thatq in general, an apparatus according to the invention will be designed to treat a range of effluents over a range of rates. ~hus at certain times the apparatus may be carrying out its heaviest duty at its ma~imum capacity, in which case the mean level in the downcomer will be at A - A (but fluct~ating between ~ - ~ and C - C) as shown in Fig~ 4. At other times, however, it will be carrying out less than its heaviest duty at less than its maximum c~pzcity, in which case the mean level in the downcomer will be at A - A (but fluct-uating betwee~ Bl - ~ and Cl - Cl) as shown in Fig. 5. In this case it would be wasteful -to pump the wastewater back into the downcomer at the highest point ~ - ~ when it is only necessary to do so at Bl _ Bl.
~his difficulty can be overcome as shown in Fig. 5, wherein the pump 3 is an archimedean screw pump. When the mean level in the downcomer is at A - A, the wastewater is delivered from the top of the pu~p into the do~noomer just above the level ~ he pu~p is, however, provided with valve means or the like 10, which can be opened when the mean level sinks to A - A
so that the pu~p then delivers wastewater through 10 at a level just above ~ _ 31. ~he chan~eover from using the whole of -the pump 3 to using only the bottom end thereof (and vice versa) can be effected ma~ually as and whe~ desired; or it may be effected by a time switch (not shown), if it is known in advance when the change from level A - A to Al - Al (or vice versa) is going to take place - as in the case of installations for treating domestic sewage; or it may be effected by means (not shown) responsive to si~nals derived from the level of the wastewater in the downcomer, such that when it sinks below the level 31 and Bl the valve means 10 is opened a~d when it rises above the level 31 _ ~1 the valve means 10 i3 closed. Since the level of the wastewater in the downcomer is a function of the amount of oxygen-containing air being supplied to the apparatus, the opening and closing of the valve means 10 may be operated by a signal responsive to an ~3~.

8 B 30799 increase or decrease in this parameter. ~here may be a plurality of such valve means located between the top of the pu~p 3 and the illustrated valve means 10 so that the wastewater may be removed from the pump into the downcomer at a plurality of different levels depending upon the type of duty the apparatus is carrying out.
An alternative (not illustrated) would be to have two separate pumps 3, one delivering wastewater into the downcomer just above the level ~ - ~, and the other delivering wastewater into the downcomer just above the level 31 _ ~1, with means for starting and ætopping them alternately. Self-evidently there m~
be additional pumps delivering waste water into the downcomer at levels intermediate ~ - ~ and PA/R~ ~
25 April 1980

Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for treating wastewater utilizing a downcomer-riser system, the downcomer and riser communi-cating directly only at their lower ends and indirectly, through a pump means, only at their upper ends, with the level of wastewater in the downcomer being maintained above the level of wastewater in the riser to provide a hydrostatic pressure head which causes circulation of the wastewater around the system at a pre-selected rate, the level of wastewater in the downcomer fluctuating between a maximum and minimum level for which self-rectification of the flow rate will occur, said method comprising the steps of:
supplying a gas containing free oxygen to the waste-water in the downcomer as it circulates down the downcomer;
effecting disengagement of gas bubbles from the wastewater prior to recirculation from the riser to the downcomer;
pumping the wastewater from an upper portion of the riser back into the downcomer so that the wastewater is discharged into the downcomer at an area above said level of wastewater, and varying the position at which the wastewater is discharged into said downcomer in response to said level of wastewater.

2. A method as recited in Claim 1 wherein said pumping step is practiced so that the wastewater is allowed to fall freely from its area of discharge to the downcomer.

3. A method as recited in Claim 1 wherein said pumping step is accomplished by delivering the wastewater from the riser to the downcomer while inhibiting the entrainment of air therein.

4. Apparatus for treating wastewater, comprising:
a downcomer;
a riser communicating directly with said downcomer only at the bottom ends of said riser and down-comer, and communicating indirectly only at the upper ends thereof;
a level of wastewater being maintained in said down-comer above the uppermost part of said riser so that a hydrostatic head is provided effecting circu-lation of wastewater down said downcomer and up said riser, at a pre-selected rate, and the level of wastewater in said downcomer fluctuating between maximum and minimum levels for which self-rectification of the flow rate around the system will occur;
oxygen-containing gas introducing means disposed in said downcomer for introducing oxygen-containing gas into the wastewater circulating down said downcomer;
pumping means for pumping wastewater from an upper portion of said riser back into said downcomer so that the wastewater is discharged into the downcomer at an area above said level of wastewater, and means for varying the position at which the wastewater is discharged into said downcomer in response to said level of wastewater.

5. Apparatus as recited in Claim 4 wherein said pumping means comprises means for inhibiting the entrain-ment of air in the wastewater during pumping and discharge thereof from said riser into said downcomer.

6. Apparatus as recited in Claim 5 wherein said pump-ing means includes a spiral chute.

7. Apparatus as recited in Claim 4 wherein said pumping means includes a first conduit extending from said riser to said discharge area of said wastewater to said downcomer; a second conduit extending from said first conduit to said downcomer at a lower point of said down-comer than said area where said first conduit discharges wastewater to said downcomer; and valve means disposed in said second conduit.