CA1042568A - Sewage treatment - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Pure oxygen or a gas containing more oxygen than air is injected under pressure into sewage held in or flowing through a sewer. This injection can be used to prevent the concentration of dissolved oxygen in the sewer falling to a level at which there occurs bacterial reduction to hydrogen sulphi?? or sulphate present in the sewage. The injection can also be used to oxidise to sulphur any sulphide dissolved in the sewage. The pure oxygen or the gas contain-ing more oxygen than air may be injected into sewage flowing through a sewage pipe forming part of a rising sewer, a pumping sewer or a gravity sewer, into a pump used to transfer the sewage through the sewer, or into a part of the sewer where sewage is collected before being transferred through the sewer.

Description

;8 Thi8 i~venti-n rolates to the treat~ent of sewage while it is being held in or is 10wing through a ~ewer.
Problems can ari~e in conveying sewage through ~oth ri~ing ~ewers and gravity sewers as a result of the action of facultative bacteria present in the sewage. The6e bacteria normally respire oxygen dissolved in the sewageJ but if this supply of oxygen i~
exhausted, they obtain their oxygen for respiration by reducing compounds containing oxygen, particularly sulphates.
This bacterial reduction of sulphates yield~ hydrogen sulphide, a malodorous~ poisonous gas. Sometimes lethal concen-trations of hydrogen sulphide are created at locations that are accessible to maintenance engine-rs, and there i8 frequently e~cape of hydrogen sulphide from the sewer to above ground. In particular, hydrogen sulphide can be formed in rising main sewer~ in view of the exclusion of air from between the pumphouse and the discharge end of the ~ewer. In conseguence the pre~ence of hydrogen sulphide i~ often detected ~u~t beyond the outlet of a rising sewer.
Formation of hydrogen sulphide is particularly marked under hot climatic conditions, since the~e conditions promote bacterial activity.
Another problem at~ributable to hydrogen sulphide i~ the corro~ion of gravity sewers. Thi~ i8 caused by aerobic bacteria oxidising hydrogen ~ulphide to ~ulphuric acid on the damp walls of a gravity sewar in the air ~pace abovo tho level of the sewage.
Various methods have boen tried i~ order to pr~vent the bacterial activity which form~ hydrogen sulphide. For example, bacterial activity can be inhibited b~ the addition of gaseous chlorine, but thi~ can result in the ~terili~ation of the ~ewage.
~oreover, chlorine it~elf is both highly corrosive and toxic.

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5~;8 It is pos~ible to freshen sewage by adding air. With air, however, once a concentration of dis~olved oxygen of 10 ppm has been attained, no more oxygen will dissolve. Furthenmore, the sewage i8 usually saturated with nitrogen before the air i~
introducod. Accordingly, none of the nitrogen contained in the air is dissolved. This undis~olved nitrogen can cause ga~ locks and other hydraulic proble~s in the sewer.
The present invention aims at providing a treatment which combats bacterial fonmation of hydrogen ~ulphide in the s wer.
Accordingly the present invention provides a process ~ -of treating sewage while it i8 being held in or i8 flowing through a sewer, in which oxygen-rich gas (as herein defined) is injected under pressur6 into the ~ewage.
By this proce~s there can be maintained a concentration of dissolved oxygen sufficient to prevent bacterial present in the --~
sewage reducing compound~ containing oxygen. Treatment with ~ -oxygen-rich gas can also oxidise any sulphide dissolved in the sewage .
The invention also provido~ apparatus for treating ewage while it is being held in or i8 flowing through a sewer, including a source of oxygen-rich gas (as herein defined) J a conduit connected between the source and at least one in~ector loeated in the ~ewor for the introduction of the oxygen-rich ga~
into the sewage.
~ he term ~-owor~ a~ used in this specification, includes within it~ ~cope a conduit which is connected between a source of untreated sewage and a treatment plant (e.g. an activated sludge or biological filtration plant~ or, if the sewage is not to be treated in such a plant, a place of dispo~al. The term ~sewer~, .. . . .
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11~4ZS68 as used in this specification, al80 include~ within it~ scope any pumping equipment used to transfer ~ewage through the re~t of a suwex.
The proce6~ according to the present invention can ~e performed in a gravity sewer or a rising 6ewer. A gravity 6ewer 610pe~ downwards in the direction of the flow of sewage there-through 60 that the pas6age of the 6ewage is effected by gravity.
On the otherhand, in a rising sewer transfer of sewage i6 effected by a pump. A ri6ing sewes may comprise a conduit which 10 follows the undulation~ of the ground in which it ha6 been lain. ~ ~;
Thi~ type of ri6ing sewer i8 60metimes referred to as a pumping 6ewer or main, and possibly length6 of it will have a downward inclination in the direction of the flow of the sewage. Alter-natively, a rising sewer can be a cyli~drical pipe which i8 .
inclined 80 that it ri~es continuously or for mo~t of it6 length in the direction of flow of the sewage therealong.
In a rising 6ewer, the treat~ent i8 preferably performed in a length of ~onduit ~orming a part of the sewer, but can al~o be perform~d in any ~ump in which ~ewage i6 collected bofore being pagsed through the re~t of the sewor, or in the volute of any pu~p u6ed to pas~ the ~ewage through the re~t of the sewer.
In a ~erie~ of ~ewers, it i~ generally aesirable to in~ect the oxygen-rich ga~ into a ~ew-r ~ufflciently noar to the 60urce of th~ sewage to en~ure that bacterial reduction of compoun~s containing oxygen and ~ulphur does not start before the sowage r~aches the region whor- oxygen-rich gas i8 injected. If d~6ired, oxygen-rich ga~ can be injected into ~ore than one sewer in the series.
By the term ~oxygon-rich ga~, a6 u~ed herein, is meant .. .

1~25t;8 pure oxygen itself, or an oxygen-containing ga~ mixture having a proportion of oxygen greator than that of air.
The oxygen-rich ga~ can, if desired, include a small proportion of ozone or other gas which has a beneficial effect.
If desired, the oxygen-rich gas can be formed by enriching air in oxygen.
In a rising sewer, in particular, it is generally necessary to dissolve in the sewage substantially all the oxygen and any other gas contained in the oxygen-rich gas. Volumes of 10 undissolved gas might hin~er pumping of the ~ewage through the -~
ri~ing main. Hence care should desirably be taken to ensure that sub~tantially no insoluble gas be introduced into any volume of sewage contained within a ri~ing sewer. It i8 therefore, in general, desirable to use an oxygen-rich gas which contains at least --90~ by volume of oxygen, and which preferably contain~ at lea~t 9g~
by volume of oxygen. Thi8 keeps to manageable proportions the amount o~ any unwant~d gas, such as nitrogen, that need6 to be diE~olved in the ~ewage.
In performing the process according to the pre~ent invention it i~ u~ually not de~irable to in~ect 80 much oxygen-rich gas a~ any one region as to ex~eed itR solubility limit in the sewage. Conseguently injection at discrete interval~ along the length of the ~ewer or a ~ewage system is sometimes de~irable.
A variety of source~ of oxygen-rich gas are available.
- For oxample, pure oxygen could be supplied from an in~ulated ves~el in which liquid oxygen i~ stored an~ pas~ed through an evaporator prior to its introdu~tion into the sewage. Alternatively pure oxygen could be ~upplied ~rom one or ~ore gas storage cylinders.
Oxygen-rich air could be ~upplied rom an air separation plant ,: ,. . . .

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- ~4'~S~8 operating a pressure swing ad~orption-de~orption cycle. If required, another gas or gases could be admixed with the oxygen or oxygen-enriched air.
The oxygen-rich gas may ~e conveniently injected into the sewer at a pres~ure in the range 1.5 to 5.0 bars. A pres~ure from 5 to 10 bars i~ also suitable.
The oxygen-rich gas i~ preferably in~ected into sewage in a rising sewer in fine bubbles, typically having a dia~eter of 0.05 to 0.15 mm, 80 as to facilitate its dissolution. In some instance~, though, difficulties could ari~e in achieving ~ubstantially complete di~olution of the oxygen-rich gas. Such difficulties can, for example, occur in a gravity sewer where it i5 uaually desirable, but often not pos~ible, to introduce the oxygen-rich gas well below the level of the ~owage. On the other hand, difficulty in di~solving the oxygen-rich ga~ i8 unlikely to ~;
arise if the gas i8 injected downstream of the pumphouse of a rising sewer. ~ -In a gravity ~ewer or the 8ump of a rising ~ewer failure to di~olve sub~tantially all the oxygen in the oxyqen-rich gas could be dangerou~. ~his i8 because inflammable vapour i8 some-ti~es pre~ent in the air ~pace above the level of the sewage.
Tbu8 undis~olved oxygen entoring the air space could cause an explosion.
In order to avoid any dificulty in offecting it~
dis~olution, the oxygen-rich ga~ may fir~t be introducod into a pre~urised ~tream of ~ewage or water, which stream i8~ then injected into the ~ewage in the sewer. The st~eam o ~ewage i8 preferably taken from sewage ~lowing through the sewer. This may be performed in accordane with the proce~s de~cribed in our ~, ', ' ' .

copending Canadian Pa~ent Application No. 189,687. Such a system i8 particularly suitable for use with a gravity main or a sump in the pumphouse of a rising sewer.
A proportion of any hydrogen sulphide fonmed in a gravity sewer will enter the air ~pace above the level of the ~ewage and thus cannot be oxidised in the gravity sewer by the injection of oxygen-rich qas. merefore, if the incoming sewage contains di~solved oxygen, it is generally desirable in a gravity sewer to inject the oxygen-rich gas sufficiently near to the inlet of the ~ewer to ensure that bacterial reduction of compounds containing oxygen and sulphur does not start before the sewage reaches the region or region~ of injection. ~his is often a de6irable practice in a rising ~ewer, too. Consequently it is often preferable to inject the oxygen-rich ga~ near to the inlot of a gravity or a rising sewer.
In a rising sewer, however, any hydrogen sulphide formed will be disJolved in the sewage and will not be relea~ed until the sewage i~ discharged from the 8 wer. Accordingly, sulphide pre~ent in tho sewage entering a ri~ing ~ewer or fonmed during it~
rosidence therein can be oxidised by the process according to-tho present invention. Preferably the oxygen-rich gas is inje~ted for this purpose at a ~ufficient di~tance from the outlet of the -~
~ewer as to ensure that all the sulphide is oxidi~ed before the ~ewage i~ di~charged from the sower.
In a rising sewer the oxygen-rich ga~ i:3 proforably injected at one or more regionE between the outlet of the sewer and a non-return valve which i8 located in the sewer near to the pump 80 as to prevent back-flow of the se~age. If the oxygen-rich gas is introduced at ~uch a region or regions, it is preferably .
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11! ~Z5~8 injected for at least the groater part of a period during which sewage i8 pumped through the sewer.
If desired, the oxygen-rich ga~ can be in~ected into the sewage collected in a sump prior to its being pumped through the rest of a rising sewer, or it can be injected into the volute of the pump of a rising sewer whilst the sewage is being passed through the rising sewer.
In many sewers there might be no aceess open~ng vertically above a region where it i8 desired to introduce the oxygen-rich gas. In such circumstances there can often be provided a rigid or flexible pipe which leads from the ~ource of the oxygen-rich gas through an a~cess opening in the sewer, extends along the sewer and terminates in the in~ector.
The outlot or outlets of the injector are preferably pointed in a downstream direction with reference to the sewage flow 80 a~ to reduce the tendency for solid matter to block the outlet or outlets.
The minimum quantity of oxygen-rich gag required to prevent the concentration of dis~ol~ed oxygen in the sewage from falling to or below a level at which there starts the bacterial reduction of ~ompounds containing oxygen and sulphur depend~ on a nu~er of factors. Important factors are the temperature of the sewage, the time ~or which the sewage i~ re~ident in the ~ewer, and the rate at whi~h the sewage consume~ di~solved oxygen. A1JO
important ars the internal dimen~ions of the sewer since populations of facultative basteria tend to be present on the inner walls of the ~ewer. A~ all these parameters can be determined empiri~ally the minimum quantity of oxygen-rich gas that needs to be injected into a given volume of sewage can readily be calculated -:: ' . . .
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163~'~568 given a knowledge a the dissolved oxygen concentration of the incoming ~ewage. A dissolved oxygen moter can be u~ed to mea~ure the dissolved oxygen concentration.
It i8 ~esirable to monitor one or both of the dis~olved oxygen concentration or BOD (Biochemical Oxygen De~and) of the sewage downstream of where the oxygen-rich gas is introduced and to adjust the rate at which the oxygen i8 in~ected in accordance with changes in the monitored value. A dissolved oxygen meter ~-can be used to measure the dissolved oxygen concentration and an on-site BOD detector can be u~ed to measure the B09. ThiR
procedure i8 able to take account of unexpected diurnal and climatic ~ariations in the oxygen demand exerted by the sewage.
If desired, a control system can effect the adjustment auto-matically, the rate of in~ection being increased, preferably in steps, if the dissolved oxygen content of the sewage falls by a chosen amount and being decreased preferably in steps, if the di~olv~d oxygen content of the sewage rises by a chosen amount.
For example, electrical signals generated by the dissolved oxygen meter can be u~ed to actuate one or more solenoids in the control circuit of a solenoid valve controlliny the pas~age of the oxygen~
rich g~8 to the in~ector.
In a ri~ing ~ewer whose length i8 ~ufficiently short to require introduction of the oxygen-rich gas in one region only, ~ -it is convenient to monitor the di~solved oxygen content at or near to th- outfall of the ~e~er. If t~o len~th of the rising sewer is ~uch that introduction of oxygen-rich gas at more than one region i8 desirable, the di~solved oxygen concentration i8 ~ :
preferably monitored betwoen regions and downstream (with reference to the direction o~ flow of the sewage) of the region nearest the _ g _ , .. : ' ' , ' " " ' . ' : ' ' '~' ' ', ~. ,' : .

l~Z5~8 outlet.
If the treatment if required to oxidise di~olved sulphide to sulphur, the quantity of oxygen-rich gas to be injected can be that re~uired both to oxidise all the sulphide dissolved in the ~ewage and to prevent further bacterial reduction of sulphate.
Moreover, the concentration of the dissolved sulphide can be monitored up~tream or downstream (with reference to the direction of flcw of the sewage) of where the oxygen-rich gas is injected, and the rate of injection adjusted in accordance with increase~ or decrea~es in the measured dissolved sulphide concentration. The degree to which lead acetate is darkened by ~ulphide is one way of estimating the di~solved sulphide concentration.
There is frequently con~iderable diurnal fluctuation in the rate at which sewage flows into the sump or sump~ of the pumphouse of a rising sewer. In consequence, the period for which each given volyme of sewage is held in a rising sewer will vary.
Con~equently it will be desirable to make correspond~ng variations in the rate of injection of oxygen each time a volume of sewage -i8 pu~ped. ~oreover, diurnal and ~easonal fluctuation6 in the t~mperature and the BaD of the sewage can al~o require appropriate adjustment~ to be made in the rate at which oxygen i8 in~ected each time a volume of sewage is puMped. As c milar patterns of variations in the rate of flow of sewage into the ~ump or JUmp8 of a rising ~ewer, in temporature and in th- BoD of the incom-ng ~-wage tend to occur oach day, it is po~sible to predetermine how the rate of oxygen injection need be varied each time sewage is pu~ped. For thi~ purpose a digital or analogue computer can be u~ed. A small digital computer i~ particularly suitable. If desired, the computer ~an be used to control the injection of the oxygen-rich - . . . , :
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5~8 gas aut~matically.
The process according to the pre~ent invention can readily be performed in an establi~hed sewage ~ystem. Moreover, the in~ection of oxygen-rich gas can help to reduce the BOD of the sewage.
The apparatus according to the present invention will now be described, by way of exampleJ ~ith reference to the accompanying drawings, of which~
Figure 1 is a schematic representation of a rising sewer 10 with apparatus for injecting oxygen into sewage flowing there- -through, and Figure 2 i~ a schematic representation of a gravity sewer with apparatus for injecting oxygen into ~ewage flowing thorothrough. ~-With reference to Figure 1, a ri~ing ~ewor 2 has an inlet 4 serving an und rground puaping ~tation 6. In the pumping station i~ a sump 3 which receives sewage from the inlet 4, and a pump S whose inlet i8 connected to tho outlet of the sump 3 a~d whoso outlet is conne~ted to the inlet of a conduit 7 along ~20 which sewag- i~ pumped. The conduit 7 rises in the direction of the flow of ~ewage therethrough a~d ends in an outfall 8 through whi~h ~owage falls into a gravity sew~r (not shown).
Located near to the inlet of the conduit 7 is a non-roturn (or ro~lux) valv- 9 to prevent backflow of ewage into the pu~p 5. An in~ector pipe 16 i~ locat-d in tho conduit 7 near to the non-return valvo 9 and between the valve g and the outfall 8.
axygen to be in~ected into the sewer ~rom the pipe 16 i~ conducted thereto through a conduit 18 pa~sing ~rom an oxygen converter 20 (located aboYe ground) downwards through an acces~ ~haft 22. The .
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l~ S68 oxygen converter 20 compri~es a vacuwm-insulated container 24 and a vaporiser 26.
Passage of oxygen into the injector-pipe 16 i8 controlled by a flow control ~alve 28. Thi8 valve i9 de~irably a solenoid valve which i8 operated automatically to adjust the rate of flow of oxygen to the pipe 16 in accordance with variations in the value of the concentration of dissolved oxygen measured at the outfall 8 of the rising main 2 by a dissolved oxygen meter 30.
With reference to Figure 2, a conduit 40 in communication with a volume of sewage 42 flowing downwards through a gravity sewer 44 leads upwards to the inlet of a high Fressure pump 46 which ia located above ground. The outlet of the pump 46 is connected to a conduit 48 which terminates in an in~ector pipe S0 ~ -located below the level of the inlet of the conduit 40. A pipe 56 -is connected between a cylinder 5~ containing compre~sed, pure oxygen and a chamber 54 located in the conduit 48 downstream of ~he pump 46.
In operation, 0.5 to 10~ of the flGw of ~ewage i8 withdrawn from the gravity sewer 44 through the conduit 40 by the pump 46 operating at a pre~ure of 40 p8ig. The ~o-formod pressuri~ed stream of sewage pas~os into the conduit 4a, and oxygen i~ injected under pre~ure into it in the chamber 54. This for~s an oxygenatod ~tream of ~ewago which i~ thon introduced back into th~ ~owago flowing through tho gravity ~ewer by tho ~n~ector pipa 50.
The proces~ according to the present invention is now illu~trated by the following example.
~xamPle 26 800 litres of dome~tic ~owage per hour were passed .

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into the sump of a rising sewer 1140 m in length and 200 mm in diameter. The concentration of dissolved oxygen in the incoming sewage was 6 ppm (part~ per million). The temperature of the incoming sewage wa~ 15C.
The 8ump of the rising sewer had a total capacity of 5000 litres, and each time it was filled, pumping was started and continued for a period 2 ainutes 50 seconds only. During each period of pumping 0.108 m of pure oxygen gas was injected into the sewage at a pressure of 3 bars. In~ection of the oxygen was discontinued when pumping was stopped.
No trace of hydrogen ~ulphide was detected at the outfall of the rising sewer. -~

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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process of treating sewage in a sewer to maintain aerobic metabolism prior to being subjected to primary treatment and to ensure that substantially all oxygen is dissolved or consumed in the sewage, which process comprises the step of dissolving gas containing at least 90% by volume of oxygen in the sewage so as to maintain aerobic metabolism in the sewage.

2. A process as claimed in claim 1, in which the oxygen-rich gas contains at least 98% by volume of oxygen.

3. A process as claimed in claim 1 or 2 in which the concentration of dissolved oxygen in the sewage or the BOD of the sewage is monitored downstream (in respect of the direction of flow of the sewage) of where the oxygen-rich gas is injected, and the rate at which the oxygen-rich gas is injected is adjusted in accordance with changes in the monitored value.

4. A process as claimed in claim 1 or 2 in which oxygen-rich gas is introduced into a pressurised stream of sewage or water, which stream is then injected into the sewage in the sewer.

5. A process as claimed in claim 1 or 2 in which the oxygen-rich gas is injected at two or more separate regions in the sewer.

6. A process as claimed in claim 1 or 2 in which the sewage entering the sewer contains dissolved oxygen, and the oxygen-rich gas is injected sufficiently near to the inlet of the sewer to ensure that bacterial reduction of compounds con-taining oxygen and sulphur does not start before the sewage reaches the region or regions of injection.

7. A process as claimed in claim 1 in which the sewage entering the sewer contains dissolved sulphide, and the oxygen-rich gas is injected at a sufficient distance from the outlet of the sewer to ensure that the sewage leaving the sewer contains no dissolved sulphide.

8. A process as claimed in claim 7 in which the concentration of the dissolved sulphide is monitored downstream or upstream of where the oxygen-rich gas is introduced, and the rate of injection of the oxygen-rich gas adjusted in accordance with increases or decreases in the monitored value.