CA1049667A - Disposal of biological sludge from pulp mills - Google Patents
Disposal of biological sludge from pulp millsInfo
- Publication number
- CA1049667A CA1049667A CA242,540A CA242540A CA1049667A CA 1049667 A CA1049667 A CA 1049667A CA 242540 A CA242540 A CA 242540A CA 1049667 A CA1049667 A CA 1049667A
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- Prior art keywords
- sludge
- effluent
- stage
- treated
- aerating
- Prior art date
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Abstract
ABSTRACT OF THE DISCLOSURE
The present invention discloses a process for the treatment of surplus sludge in conjunction with a two stage activated sludge process. The sludge is digested during the digestion phase of a pulping operation without detri-mental effects on the quality of the pulp.
The present invention discloses a process for the treatment of surplus sludge in conjunction with a two stage activated sludge process. The sludge is digested during the digestion phase of a pulping operation without detri-mental effects on the quality of the pulp.
Description
:~)49~i~7 The present inYention relates to a process for the purification of effluents and more particularly, relates to a process for the treatment of pulp and paper mill effluents and for the disposal o~ surplus sludge.
Many systems for t~e biological purification of domestic sewage have been proposed. In a very few cases, these systems have been adapted to the treatment of pulp and paper mill effluents; however, the prior art proposed systems do not provide adequately for the disposal of the surplus sludge. This, to a great extent, defeats the - original purpose of attempting to dispose of the contaminatling organic material in the effluent to be treated.
In the prior art of treating domestic and other effluent water, there has been proposed the two stage activated sludge process which is a continuous operation.
In such a system, each stage comprises an aeration tank and a secondary settling tank. The return sludge circulates only within its own stage i.e. part of the sludge from each settling tank is recirculated through its prior aeration tank.
The effluent from the settling tank of the first stage serves as feed for the aeration tank of the second stage and the effluent from the settling tank of the second stage constitutes -the puri~ied effluent. In the aerating tank, the effluent water is aerated in the presence of micro organisms - i.e.
activated sludge. Due to the bacterial activity, the organic ;~
; impurities present in the effluent water are assimilated ; and decomposed. By controlling the oxygen content of each ;~ stage the nature/type of micro organism can be controlled so that the organisms of the second stage can feed on those of the first stage. The sludge in the settling tanks that is not recirculated, builds up and must be withdrawn as surplus sludge.
, .
96~7 While th~ above process has been used for the treatment of domestic effluent water, even in this field pro-blems have been encountered. In particular, the wrong type of micro organism or an improper balance in the various types of oFganism may cause difficulties in the sedimentation or settling tanks. In other words, to work at full efficiency, optimum conditions must be maintained in order to maintain the proper type of organisms at their optimum level. As will be understood by those skilled in this art, the life conditions of the various groups of organisms are markedly dif~erent and the proper conditions for each application have to be worked out.
It is an object of the present invention to provide -~
- a method to purify effluent waters and in particular, pulp and -paper mill effluent waters.
It is a further object of the present invention to ~ , :
provide a method to purify pulp and paper mill effluents which disposes of the surplus sludge in an economic and efficient ` manner.
In one aspect of the present invention, there is pro-vided a process for the disposal of surplus sludge which com-prises injecting said sludge into a digestion step of a pulping -operation whereby, after said sludge is digested, a first portion thereof is retained as a part of the pulp yield, a second portion is digested and discharged as part of a spent liquor from said pulping operation. The surplus sludge for such a disposal process could come from a variety of sources, e.g. chemical or mechanical processes steps; microbiological purification processes including lagooning and other similar processes involving sedimentation :;
- techniques, etc.
In a further aspect of the present invention, there is provided a process for the disposal of surplus sludge ~rom a system which microbiologically purifies the spent liquor effluent from a pulping process by means of an activated sludge, :. . .
.
comprising the step of injecting said surplus sludge into a digestion step of said pulping process whereby, after said sludge is digested, a first portion of said digested sludge .is retained as part of the pulp yield, a second portion is discharged as a ventable gas, and a third portion is rec'ycled through said system to serve as a nutrient for microbiological purification by said activated sludge.
In a further aspect of the present invention, there is provided a process for the microbiological purification of : 10 spent pulping liquor effluent and other raw effluents from a pulp and paper mill which employs a wood digestion process, -comprising the steps of continuously feeding raw effluent into a first aerating tank at a desired loading rate, aerating and decomposing in a first stage the organic material in said ~`
effluent using an activated concentrated sludge to thereby form a first treated effluent, sedimenting said first treated ; effluent to form a treated first effluent, recycling a first portion of the sedimentary part of the biological sludge from said first sedimenting step to said first aerating stage, taking a second portion of the sedimented sludge to form a first sur- . :
plus sludge portion, aerating and decomposing the organic material in said treated first effluent to form a second treated effluent, : sedimenting said second treated effluent, to form a biologically purified effluent, recycling a first portion of the sedimentary : part of the biological sludge from sedimenting step to said sec-ond aerating stage, removing a second portion of the sedimentary sludge to form a second surplus sludge portion, collecting said first and second surplus sludge portions and subjecting them to a digestion step of a wood digestion process.
In greater detail, the treatment of the raw effluent -from pulp and paper mills desirably includes the preparation, ~if required, of the raw effluent prior to feeding the same :
to a first aerating/decomposition tank. Thus, prior to ~, 4g~
feeding the raw effluent to the first aerating tank and depending on the nature of the effluent to be treated, it has been found that particular conditions should be established in this preliminary treatment step. In such a preliminary treatment step, the removal or neutralization of coarse sus-pended solids and other substances such as free sulphur dioxide which could seriously affect proper subsequent microbiological ~;
growth of the bacteria is accomplished. This pre-purification ! may be accomplished by conventional means well known to those -skilled in the related arts. As will be readily understood by those skilled in the art, in many situations, the coarse suspended solids will automatically be removed from the raw effluent due to the nature of the pulping operation - i.e. the blowing of digested pulp into blow pits where the raw effluent is drained off and the pulp mat so formed will itself filter most of the suspended solids.
Also, the pre-purification step may include a pH
adjustment. Preferably, the pH may be adjusted by treating the effluent with a suitable alkali to achieve a pH range of between 7 to 9. This will automatically neutralize any free sulphur dioxide and as has been found, this range will discourage the formation of undesirable filamentous bacteria.
In the case of acid digestion processes, substances such as free sulphur dioxide which are present in the cooking liquor -will be removed to a large extent during the blowing operation. -Many suitable alkalis are known for treating the effluent -e.g. lime, sodium carbonate, sodium hydroxide, ammonia, magnesium dioxide, etc. Use of ammonia is advantageous in that ~ ;
it will pro~ide the nitrogen necessary for proper microbiolo-gical growth as will be discussed in greater detail hereinafter.
In the case of an alkaline digestion process, such as the Kraft and Soda processes, the spent liquor effluent will already be . .
_ 4 _ `. .' ' , :, ~ ' ' .. . . .
alkaline and if it is necessary to adjust the effluent to a lower p~ the use of inexpensive acidic phosphorous substances could be used to advantage as was the case for ammonia. Such a microbiological system could consist of a single or of multiple purification stages. I
The pre-treated effluent is then conveyed to a first aeration tank at a to~al solids content preferably in the range of 1 to 3%; for most spent liquor effluents, this involved a B.O.D. range of 3,000-11,000 ppm. Such an aeration tank is con-ventional and well known to those skilled in the art and neednot be discussed herein; however, the loading rate is preferably in the range of 200 to 500 lbs. B.O.D. per 1,000 cubic feet per day. In the first aeration tank, the mixed liquor suspended solids is preferably in the range of 4,000 to 10,000 ppm. In this respect, the term "BOD" is an art recognized term meaning the biological oxygen required which represents that amount of -~ oxygen which is consumed by the micro organisms for decomposition of the organic impurities in a given effluent water within five days. The BOD value is commonly used to express the amount of decomposable substance in an effluent water. The detention time in the aeration tank will naturally depend on the degree of puri-fication desired therein. Generally, in the process of the present invention, a detention time in the range of 18 to 36 hours has been found to be suitable.
It has been found desirable to minimize the amount of protozoa in the first aeration tank. To this end, the life conditions in the aerating tank are controlled such that bacteria but not the protozoa can freely develop. This may be done by controlling, among other parameters, the dissolved oxygen content to an amount preferably not greater than 1.5 ppm. Thus, under such conditions, the organic material in the effluent is efficiently and continuously aerated and decomposed by the activated sludge.
~04~667 Subsequently, the effluent is removed from the first aeration tank and conveyed to a first sedimentation tank for `
sedimentation of the sludge. The loading rate to the first sedimentation tank should preferably be below 45 pounds total ! solids per sq. ft. per day. Other sedimentation operatingparameters are well known to those skilled in this art.
From the first sedimentation tank, a first portion of the sedimentary part of the biological sludge is removed and returned to the first aerating tank in order to maintain the proper sludge concentration therein.
A second portion of the sludge from the first sedimen- ~
tation tank is removed in order to maintain the sludge in ~ -proper balance and avoid a build-up of the same therein. This second portion then becomes what will be called "surplus sludge"
which will be further treated as discussed hereinafter.
A portion of the partially sedimented first treated effluent is then removed in order to maintain a proper liquid ` balance and which contains a minor portion of sediment. This treated effluent is then conveyed to a second aerating tank.
The effluent water from the first sedimentation tank pre-dominantly contains the bacteria which cannot be sedimented.
In the second aerating tank, the mixed liquor sus-pended solids should be in the range of 3,000 to 5,000 ppm.
In contrast to the first aerating tank, the second aerating tank should contain a dissolved oxygen content as high as possible and preferably greater than 2 ppm. The higher oxygen concentration will maintain the highest possible protozoa and lowest possible bacterial content. Because approximately one-half the B.O.D. has been removed in the first stage, the B.O.D.
loading rate in the second stage aeration tank should be in the range of one-hal~ the range in the first stage, i.e. 100 -250 lbs. B.O.D. per 1,000 cubic feet per day. The detention , .. .. . . . . .
", ' : . , '; . . '''' ' , . . . .
~Q496~i7 time in the second aerating tank will depend on the desired BOn of the purified efEluent; in practice, a time in the range of 18 to 36 hours has been found suitable. As aforementioned, the percent protozoa should be as high as possible and the percent of microbacteria as low as possibie. Filamentous organisms should be avoided if possible.
Subsequently, the thus treated effluent is removed to a second sedimentation tank.
In the second sedimentation tank, the loading rate is preferably not greater than 50 pounds per sq. ft. per day.
Following sedimentation, a first portion of the sludge is retuxned to the second aerating tank to maintain the required mixed liquor suspended solids. A further portion o~ the sludge is continuously removed to form a second part of the surplus sludge. Finally, the second treated effluent is removed therefrom to form the biologically purified effluent.
According to the present invention, the surplus sludges obtained from both the first and second sedimentation tanks are then employed in the digestion step of a pulping process.
As one particular example, the surplus sludge may be mixed with softwood chips and a sodium bisulfite cooking liquor and the mixture cooked under normal mill digestion conditions.
The spent liquor from the digestion step is then employed as the raw effluent fed to the pre-purification or the first aerating tank. The digestion step in the pulp process is well known per se and need not be discussed in greater detail herein. The amount of the surplus sludge injected into the ;~
digestion phase of the pulping process may be varied depending - on the final product desired. Generally, it has been ~ound that up to about ~ive percent (5% dry basis) of sludge to wood charged to the digestion may be employed without any significant loss in the physical properties of the pulp. For different . -,.. , , : ,............ . ..
'~. ' ' , ' '' ' ' : . ' 9667 - `
sludges and pulp quality, more sludge can, of course, be ~ ~ -injected into the digestion step.
As a result of the use of the surplus sludge in the digestion or cooking phase of the pulping process, it has been found that a portion of the digested sludge is retained as part of the pulp yield. A second portion of the sludge is discharged as a ventable gas during the digestion and blowing steps while a third portion appears in the raw effluent to form a part of the BOD component of the effluent. In this respect, it has been found that in certain cases and depending on the amount of sludge added, it is possible for the BOD
component of the effluent to increase by about 5%. Still further, it has been found that the third portion provides special bionutrient value to the effluent in the form of such materials as phosphorus and that part of the nitrogen which is not vented as a gas during the digestion step. These elements are required by the micro organisms for proper breeding/growth.
In fact, even with the recycling of the sludge through the ~-digestion step, further additions of nitrogen-and phosphorus should be made on a continuous basis to maintain proper bio- ;~logical acti~ity.
It has been found that the above process can operate fairly effectively over a substantial temperature range -e.g. 40F. to 150F. The heat derived from the raw effluent as it leaves the pulping plant together with the heat formed during the growth and activity of the micro organisms and the continuous nature of the process serves to maintain the process in an operable temperature range in most locations. However, whenever possible it is preferable to operate at the higher temperature level; this, for example, aids the sedimentation.
`' ~
966~7 Surprisingly, it has heen found that the disposal of reasonable amounts of the surplus sludge in the digestion step of the pulping process does not significantly adversely effect the quality of the pulp produced. The process may be used in treating many other wastes. The mechanical pulping of wood chips is increasingly used and the large quantities of BOD
material are being released into pulp mills effluents. These BOD materials are treatable by the above process. Similarly, small Kraft and other chemical or semi-chemical pulping mills which can not economically recover the chemicals of their spent liquors by combustion processes (which recovers the heat value thereof) can use the present process to reduce the BOD of the effluent. In fact even those chemical pulping processes using chemical and heat recovery could still use the present process to advantage.
he following examples are illustrative of the ~ -- invention.
An effluent from the blow pits (including the wash-ings) of a high yield sodium-base sulfite pulping plant was fed to a first aerating tank at a BOD loading rate of 300 pounds/
l,000 cu.ft/day. The effluent after pre-treatment had a percent solids (total) of 1.7; a pH of 7.5; and a BOD of 6,500 ppm. The mixed liquors suspended solids in the tank sludge of the first aeration tank were 7,000 mg/litre; the dissolved oxygen 1 ppm;
the detention time was 32 hours. It was found that the micro organism composition was over 80% bacteria and the aeration was accomplished by bubbling air through the effluent in the tank.
The treated portion of the aerated and decomposed effluent was continuously removed from the first aeration tank and sent to a first sedimentation tank at a solids loading rate of 40 pounds/sq.ft./day.
_ g _ .
~ . ?
: . , ~04g6~7 After sedimentation, a first portion of the sedimented sludge was returned to the first aerating tank to maintain the required mixed liquor suspended solids and to maintain the sludge in balance. A second portion was continuously removed to form a partlof the surplus sludge while to maintain a proper liquid balance, a portion of the effluent was con-tinuously removed and subjected to treatment in a second aerating tank.
In the second aerating tank, the organic material was subjected to aeration and decomposition by an activated con-centrated sludge at a BOD loading rate of 300 pounds BOD/
1,000 cu. ft./day under the following conditions:
` Mixed liquor suspended solids - 3,000 mg/litre dissolved oxygen - 3 ppm pH - 7.5 Detention time - 32 hours The organism composition was over 80% protozoa.
The second treated effluent was continuously removed to a second sedimentation tank at a solids loading rate of 40 pounds/sq. ft./day and following sedimentation, a first part of the sedimented sludge was returned to the second aerating tank to maintain the desired mixed liquor suspended solidsO A
second portion of the sludge was removed to form a second sur-plus sludge while the treated effluent was continuously re-- moved as a biologically purified effluent.
The two parts forming the surplus sludge were mixed with softwood chips and the sodium bisulfite cooking liquor and the mixture was cooked under normal mill digestion condi-tions as set forth in Examples 2 and 3.
~(~4~6i6~
Softwood chips with and without surplus sludge were cooked in digesters to produce a high yield sodium base bisul-fite pulp. Samples of pulp and spent liquor were collected and tested. Cooked chlps were disintegrated in a Cowles' dissolver,washed in acentrifuge and pulp yield determined.
Pulps were refined in a PFI mill and tested. The results are summarized in Table II below:
TABLE II
CONTROL WITH SLUDGE
Sludge, ~ O.D. wood basis nil 2.5 Yield, Pulp ~ 73.9 73.8 BOD, ppm 34,158 33,750 Burst factor 49 50 ; Tear factor 91 89 Brightness, ~ 48.7 48.2 As may he seen from the above, the results show that ~ ~
the surplus sludge incorporated during digestion has not affected ~ `
` the pulp's quality and surprisingly, there has not been a recorda~le increase in the BOD of the spent liquor, any small increase in BOD being masked by other variations.
Softwood chips with and without sludge were cooked (three runs each) in a digester to produce a high yield sodium-base ~isulfite pulp. After, the cooking digesters were cooled overnight and spent liquor drained and measured. Effluent liquor was tested for BOD, nitrogen and phosphorus. Cooked ~ chips were disintegrated, washed and tested. The results of `
; the averages of the three runs are shown in Table III.
': : ` ' ' , :1~34~ 7 TABLE III
CONTROL WITH SLUDGE
Sludge, % O.D. wood basis nil 3.8 Pulp Yield, ~ O.D.70.7 71.8 BOD,ppm ! 32,000 33,000 Nitrogen, ppm - 29 Phosphorus, ppm 5 69 Burst factor 41 45 Tear factor 106 104 Brightness, % 45.3 46.6 As will be seen, the 3.8% of sludge does not have a significant effect on the pulp quality. There is a slight increase in the BOD of the effluent liquor. It was noted that 20 to 25~ of phosphorus was recovered in the pulping process.
Hardwood chips tmix of birch, maple and poplar) with and without sludge were cooked in a digester to produce a neutral sulphite semi-chemical (NSSC) pulp. The pulp and liquor were tested and the results are as shown in Table IV.
- TABLE IV
CONTROL WITH SLUDGE
; Sludge ~ O.D. wood basis nil 3.0 Pulp Yield, ~ 70.7 71.8 BOD ppm 26,330 29,000 Burst factor 29 27 Stretch % 2.01 2.06 CMT, lbs. 52 48 Softwood chips tspruce and balsam) with and without sludge were cooked in a digester to produce a Kraft pulp. The : : . ., ~ . .
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~: . . .
. ~ ;
::
~, :: . . :~ :~ ' 10~
pulp was tested and results are shown in Table V.
TABLE V
CONTROL -WITH SLUDGE
Sludge % O.D. Wood basis nil 3.0 Pulp yield, % 47.4! 49.2 K no 18.321.3 Burst factor 77 71 Tear factor 122 121 Fold (1.5 Kg) 550 543 Brightness, % 25.7 230 BOD, ppm 48,00053,700 The results shown in t~ above tables suggest that the sludge might have a slightly greater effect on the pulp . .; , .
when cooking at the lower yields. Also, the effect on BOD ~ ~
.
appears to be greater when the pH of the cooking conditions is closer to neutral. However, some of these effects can be corrected by adjustment in the cooking conditions.
It will be understood that the above examples are ~ ~;
for illustration only and are not limiting of the invention.
Also, it will be understood that changes and modifications may be made to the above-des~ribed embodiments without departing from the spirit and scope of the irvention.
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.: ' :
: : -.............................. - .
Many systems for t~e biological purification of domestic sewage have been proposed. In a very few cases, these systems have been adapted to the treatment of pulp and paper mill effluents; however, the prior art proposed systems do not provide adequately for the disposal of the surplus sludge. This, to a great extent, defeats the - original purpose of attempting to dispose of the contaminatling organic material in the effluent to be treated.
In the prior art of treating domestic and other effluent water, there has been proposed the two stage activated sludge process which is a continuous operation.
In such a system, each stage comprises an aeration tank and a secondary settling tank. The return sludge circulates only within its own stage i.e. part of the sludge from each settling tank is recirculated through its prior aeration tank.
The effluent from the settling tank of the first stage serves as feed for the aeration tank of the second stage and the effluent from the settling tank of the second stage constitutes -the puri~ied effluent. In the aerating tank, the effluent water is aerated in the presence of micro organisms - i.e.
activated sludge. Due to the bacterial activity, the organic ;~
; impurities present in the effluent water are assimilated ; and decomposed. By controlling the oxygen content of each ;~ stage the nature/type of micro organism can be controlled so that the organisms of the second stage can feed on those of the first stage. The sludge in the settling tanks that is not recirculated, builds up and must be withdrawn as surplus sludge.
, .
96~7 While th~ above process has been used for the treatment of domestic effluent water, even in this field pro-blems have been encountered. In particular, the wrong type of micro organism or an improper balance in the various types of oFganism may cause difficulties in the sedimentation or settling tanks. In other words, to work at full efficiency, optimum conditions must be maintained in order to maintain the proper type of organisms at their optimum level. As will be understood by those skilled in this art, the life conditions of the various groups of organisms are markedly dif~erent and the proper conditions for each application have to be worked out.
It is an object of the present invention to provide -~
- a method to purify effluent waters and in particular, pulp and -paper mill effluent waters.
It is a further object of the present invention to ~ , :
provide a method to purify pulp and paper mill effluents which disposes of the surplus sludge in an economic and efficient ` manner.
In one aspect of the present invention, there is pro-vided a process for the disposal of surplus sludge which com-prises injecting said sludge into a digestion step of a pulping -operation whereby, after said sludge is digested, a first portion thereof is retained as a part of the pulp yield, a second portion is digested and discharged as part of a spent liquor from said pulping operation. The surplus sludge for such a disposal process could come from a variety of sources, e.g. chemical or mechanical processes steps; microbiological purification processes including lagooning and other similar processes involving sedimentation :;
- techniques, etc.
In a further aspect of the present invention, there is provided a process for the disposal of surplus sludge ~rom a system which microbiologically purifies the spent liquor effluent from a pulping process by means of an activated sludge, :. . .
.
comprising the step of injecting said surplus sludge into a digestion step of said pulping process whereby, after said sludge is digested, a first portion of said digested sludge .is retained as part of the pulp yield, a second portion is discharged as a ventable gas, and a third portion is rec'ycled through said system to serve as a nutrient for microbiological purification by said activated sludge.
In a further aspect of the present invention, there is provided a process for the microbiological purification of : 10 spent pulping liquor effluent and other raw effluents from a pulp and paper mill which employs a wood digestion process, -comprising the steps of continuously feeding raw effluent into a first aerating tank at a desired loading rate, aerating and decomposing in a first stage the organic material in said ~`
effluent using an activated concentrated sludge to thereby form a first treated effluent, sedimenting said first treated ; effluent to form a treated first effluent, recycling a first portion of the sedimentary part of the biological sludge from said first sedimenting step to said first aerating stage, taking a second portion of the sedimented sludge to form a first sur- . :
plus sludge portion, aerating and decomposing the organic material in said treated first effluent to form a second treated effluent, : sedimenting said second treated effluent, to form a biologically purified effluent, recycling a first portion of the sedimentary : part of the biological sludge from sedimenting step to said sec-ond aerating stage, removing a second portion of the sedimentary sludge to form a second surplus sludge portion, collecting said first and second surplus sludge portions and subjecting them to a digestion step of a wood digestion process.
In greater detail, the treatment of the raw effluent -from pulp and paper mills desirably includes the preparation, ~if required, of the raw effluent prior to feeding the same :
to a first aerating/decomposition tank. Thus, prior to ~, 4g~
feeding the raw effluent to the first aerating tank and depending on the nature of the effluent to be treated, it has been found that particular conditions should be established in this preliminary treatment step. In such a preliminary treatment step, the removal or neutralization of coarse sus-pended solids and other substances such as free sulphur dioxide which could seriously affect proper subsequent microbiological ~;
growth of the bacteria is accomplished. This pre-purification ! may be accomplished by conventional means well known to those -skilled in the related arts. As will be readily understood by those skilled in the art, in many situations, the coarse suspended solids will automatically be removed from the raw effluent due to the nature of the pulping operation - i.e. the blowing of digested pulp into blow pits where the raw effluent is drained off and the pulp mat so formed will itself filter most of the suspended solids.
Also, the pre-purification step may include a pH
adjustment. Preferably, the pH may be adjusted by treating the effluent with a suitable alkali to achieve a pH range of between 7 to 9. This will automatically neutralize any free sulphur dioxide and as has been found, this range will discourage the formation of undesirable filamentous bacteria.
In the case of acid digestion processes, substances such as free sulphur dioxide which are present in the cooking liquor -will be removed to a large extent during the blowing operation. -Many suitable alkalis are known for treating the effluent -e.g. lime, sodium carbonate, sodium hydroxide, ammonia, magnesium dioxide, etc. Use of ammonia is advantageous in that ~ ;
it will pro~ide the nitrogen necessary for proper microbiolo-gical growth as will be discussed in greater detail hereinafter.
In the case of an alkaline digestion process, such as the Kraft and Soda processes, the spent liquor effluent will already be . .
_ 4 _ `. .' ' , :, ~ ' ' .. . . .
alkaline and if it is necessary to adjust the effluent to a lower p~ the use of inexpensive acidic phosphorous substances could be used to advantage as was the case for ammonia. Such a microbiological system could consist of a single or of multiple purification stages. I
The pre-treated effluent is then conveyed to a first aeration tank at a to~al solids content preferably in the range of 1 to 3%; for most spent liquor effluents, this involved a B.O.D. range of 3,000-11,000 ppm. Such an aeration tank is con-ventional and well known to those skilled in the art and neednot be discussed herein; however, the loading rate is preferably in the range of 200 to 500 lbs. B.O.D. per 1,000 cubic feet per day. In the first aeration tank, the mixed liquor suspended solids is preferably in the range of 4,000 to 10,000 ppm. In this respect, the term "BOD" is an art recognized term meaning the biological oxygen required which represents that amount of -~ oxygen which is consumed by the micro organisms for decomposition of the organic impurities in a given effluent water within five days. The BOD value is commonly used to express the amount of decomposable substance in an effluent water. The detention time in the aeration tank will naturally depend on the degree of puri-fication desired therein. Generally, in the process of the present invention, a detention time in the range of 18 to 36 hours has been found to be suitable.
It has been found desirable to minimize the amount of protozoa in the first aeration tank. To this end, the life conditions in the aerating tank are controlled such that bacteria but not the protozoa can freely develop. This may be done by controlling, among other parameters, the dissolved oxygen content to an amount preferably not greater than 1.5 ppm. Thus, under such conditions, the organic material in the effluent is efficiently and continuously aerated and decomposed by the activated sludge.
~04~667 Subsequently, the effluent is removed from the first aeration tank and conveyed to a first sedimentation tank for `
sedimentation of the sludge. The loading rate to the first sedimentation tank should preferably be below 45 pounds total ! solids per sq. ft. per day. Other sedimentation operatingparameters are well known to those skilled in this art.
From the first sedimentation tank, a first portion of the sedimentary part of the biological sludge is removed and returned to the first aerating tank in order to maintain the proper sludge concentration therein.
A second portion of the sludge from the first sedimen- ~
tation tank is removed in order to maintain the sludge in ~ -proper balance and avoid a build-up of the same therein. This second portion then becomes what will be called "surplus sludge"
which will be further treated as discussed hereinafter.
A portion of the partially sedimented first treated effluent is then removed in order to maintain a proper liquid ` balance and which contains a minor portion of sediment. This treated effluent is then conveyed to a second aerating tank.
The effluent water from the first sedimentation tank pre-dominantly contains the bacteria which cannot be sedimented.
In the second aerating tank, the mixed liquor sus-pended solids should be in the range of 3,000 to 5,000 ppm.
In contrast to the first aerating tank, the second aerating tank should contain a dissolved oxygen content as high as possible and preferably greater than 2 ppm. The higher oxygen concentration will maintain the highest possible protozoa and lowest possible bacterial content. Because approximately one-half the B.O.D. has been removed in the first stage, the B.O.D.
loading rate in the second stage aeration tank should be in the range of one-hal~ the range in the first stage, i.e. 100 -250 lbs. B.O.D. per 1,000 cubic feet per day. The detention , .. .. . . . . .
", ' : . , '; . . '''' ' , . . . .
~Q496~i7 time in the second aerating tank will depend on the desired BOn of the purified efEluent; in practice, a time in the range of 18 to 36 hours has been found suitable. As aforementioned, the percent protozoa should be as high as possible and the percent of microbacteria as low as possibie. Filamentous organisms should be avoided if possible.
Subsequently, the thus treated effluent is removed to a second sedimentation tank.
In the second sedimentation tank, the loading rate is preferably not greater than 50 pounds per sq. ft. per day.
Following sedimentation, a first portion of the sludge is retuxned to the second aerating tank to maintain the required mixed liquor suspended solids. A further portion o~ the sludge is continuously removed to form a second part of the surplus sludge. Finally, the second treated effluent is removed therefrom to form the biologically purified effluent.
According to the present invention, the surplus sludges obtained from both the first and second sedimentation tanks are then employed in the digestion step of a pulping process.
As one particular example, the surplus sludge may be mixed with softwood chips and a sodium bisulfite cooking liquor and the mixture cooked under normal mill digestion conditions.
The spent liquor from the digestion step is then employed as the raw effluent fed to the pre-purification or the first aerating tank. The digestion step in the pulp process is well known per se and need not be discussed in greater detail herein. The amount of the surplus sludge injected into the ;~
digestion phase of the pulping process may be varied depending - on the final product desired. Generally, it has been ~ound that up to about ~ive percent (5% dry basis) of sludge to wood charged to the digestion may be employed without any significant loss in the physical properties of the pulp. For different . -,.. , , : ,............ . ..
'~. ' ' , ' '' ' ' : . ' 9667 - `
sludges and pulp quality, more sludge can, of course, be ~ ~ -injected into the digestion step.
As a result of the use of the surplus sludge in the digestion or cooking phase of the pulping process, it has been found that a portion of the digested sludge is retained as part of the pulp yield. A second portion of the sludge is discharged as a ventable gas during the digestion and blowing steps while a third portion appears in the raw effluent to form a part of the BOD component of the effluent. In this respect, it has been found that in certain cases and depending on the amount of sludge added, it is possible for the BOD
component of the effluent to increase by about 5%. Still further, it has been found that the third portion provides special bionutrient value to the effluent in the form of such materials as phosphorus and that part of the nitrogen which is not vented as a gas during the digestion step. These elements are required by the micro organisms for proper breeding/growth.
In fact, even with the recycling of the sludge through the ~-digestion step, further additions of nitrogen-and phosphorus should be made on a continuous basis to maintain proper bio- ;~logical acti~ity.
It has been found that the above process can operate fairly effectively over a substantial temperature range -e.g. 40F. to 150F. The heat derived from the raw effluent as it leaves the pulping plant together with the heat formed during the growth and activity of the micro organisms and the continuous nature of the process serves to maintain the process in an operable temperature range in most locations. However, whenever possible it is preferable to operate at the higher temperature level; this, for example, aids the sedimentation.
`' ~
966~7 Surprisingly, it has heen found that the disposal of reasonable amounts of the surplus sludge in the digestion step of the pulping process does not significantly adversely effect the quality of the pulp produced. The process may be used in treating many other wastes. The mechanical pulping of wood chips is increasingly used and the large quantities of BOD
material are being released into pulp mills effluents. These BOD materials are treatable by the above process. Similarly, small Kraft and other chemical or semi-chemical pulping mills which can not economically recover the chemicals of their spent liquors by combustion processes (which recovers the heat value thereof) can use the present process to reduce the BOD of the effluent. In fact even those chemical pulping processes using chemical and heat recovery could still use the present process to advantage.
he following examples are illustrative of the ~ -- invention.
An effluent from the blow pits (including the wash-ings) of a high yield sodium-base sulfite pulping plant was fed to a first aerating tank at a BOD loading rate of 300 pounds/
l,000 cu.ft/day. The effluent after pre-treatment had a percent solids (total) of 1.7; a pH of 7.5; and a BOD of 6,500 ppm. The mixed liquors suspended solids in the tank sludge of the first aeration tank were 7,000 mg/litre; the dissolved oxygen 1 ppm;
the detention time was 32 hours. It was found that the micro organism composition was over 80% bacteria and the aeration was accomplished by bubbling air through the effluent in the tank.
The treated portion of the aerated and decomposed effluent was continuously removed from the first aeration tank and sent to a first sedimentation tank at a solids loading rate of 40 pounds/sq.ft./day.
_ g _ .
~ . ?
: . , ~04g6~7 After sedimentation, a first portion of the sedimented sludge was returned to the first aerating tank to maintain the required mixed liquor suspended solids and to maintain the sludge in balance. A second portion was continuously removed to form a partlof the surplus sludge while to maintain a proper liquid balance, a portion of the effluent was con-tinuously removed and subjected to treatment in a second aerating tank.
In the second aerating tank, the organic material was subjected to aeration and decomposition by an activated con-centrated sludge at a BOD loading rate of 300 pounds BOD/
1,000 cu. ft./day under the following conditions:
` Mixed liquor suspended solids - 3,000 mg/litre dissolved oxygen - 3 ppm pH - 7.5 Detention time - 32 hours The organism composition was over 80% protozoa.
The second treated effluent was continuously removed to a second sedimentation tank at a solids loading rate of 40 pounds/sq. ft./day and following sedimentation, a first part of the sedimented sludge was returned to the second aerating tank to maintain the desired mixed liquor suspended solidsO A
second portion of the sludge was removed to form a second sur-plus sludge while the treated effluent was continuously re-- moved as a biologically purified effluent.
The two parts forming the surplus sludge were mixed with softwood chips and the sodium bisulfite cooking liquor and the mixture was cooked under normal mill digestion condi-tions as set forth in Examples 2 and 3.
~(~4~6i6~
Softwood chips with and without surplus sludge were cooked in digesters to produce a high yield sodium base bisul-fite pulp. Samples of pulp and spent liquor were collected and tested. Cooked chlps were disintegrated in a Cowles' dissolver,washed in acentrifuge and pulp yield determined.
Pulps were refined in a PFI mill and tested. The results are summarized in Table II below:
TABLE II
CONTROL WITH SLUDGE
Sludge, ~ O.D. wood basis nil 2.5 Yield, Pulp ~ 73.9 73.8 BOD, ppm 34,158 33,750 Burst factor 49 50 ; Tear factor 91 89 Brightness, ~ 48.7 48.2 As may he seen from the above, the results show that ~ ~
the surplus sludge incorporated during digestion has not affected ~ `
` the pulp's quality and surprisingly, there has not been a recorda~le increase in the BOD of the spent liquor, any small increase in BOD being masked by other variations.
Softwood chips with and without sludge were cooked (three runs each) in a digester to produce a high yield sodium-base ~isulfite pulp. After, the cooking digesters were cooled overnight and spent liquor drained and measured. Effluent liquor was tested for BOD, nitrogen and phosphorus. Cooked ~ chips were disintegrated, washed and tested. The results of `
; the averages of the three runs are shown in Table III.
': : ` ' ' , :1~34~ 7 TABLE III
CONTROL WITH SLUDGE
Sludge, % O.D. wood basis nil 3.8 Pulp Yield, ~ O.D.70.7 71.8 BOD,ppm ! 32,000 33,000 Nitrogen, ppm - 29 Phosphorus, ppm 5 69 Burst factor 41 45 Tear factor 106 104 Brightness, % 45.3 46.6 As will be seen, the 3.8% of sludge does not have a significant effect on the pulp quality. There is a slight increase in the BOD of the effluent liquor. It was noted that 20 to 25~ of phosphorus was recovered in the pulping process.
Hardwood chips tmix of birch, maple and poplar) with and without sludge were cooked in a digester to produce a neutral sulphite semi-chemical (NSSC) pulp. The pulp and liquor were tested and the results are as shown in Table IV.
- TABLE IV
CONTROL WITH SLUDGE
; Sludge ~ O.D. wood basis nil 3.0 Pulp Yield, ~ 70.7 71.8 BOD ppm 26,330 29,000 Burst factor 29 27 Stretch % 2.01 2.06 CMT, lbs. 52 48 Softwood chips tspruce and balsam) with and without sludge were cooked in a digester to produce a Kraft pulp. The : : . ., ~ . .
. , , : . . . :
~: . . .
. ~ ;
::
~, :: . . :~ :~ ' 10~
pulp was tested and results are shown in Table V.
TABLE V
CONTROL -WITH SLUDGE
Sludge % O.D. Wood basis nil 3.0 Pulp yield, % 47.4! 49.2 K no 18.321.3 Burst factor 77 71 Tear factor 122 121 Fold (1.5 Kg) 550 543 Brightness, % 25.7 230 BOD, ppm 48,00053,700 The results shown in t~ above tables suggest that the sludge might have a slightly greater effect on the pulp . .; , .
when cooking at the lower yields. Also, the effect on BOD ~ ~
.
appears to be greater when the pH of the cooking conditions is closer to neutral. However, some of these effects can be corrected by adjustment in the cooking conditions.
It will be understood that the above examples are ~ ~;
for illustration only and are not limiting of the invention.
Also, it will be understood that changes and modifications may be made to the above-des~ribed embodiments without departing from the spirit and scope of the irvention.
~ ' ~
.: ' :
: : -.............................. - .
Claims (12)
1. A process suitable for the disposal of surplus sludge effluent from a pulping process, comprising the step of injec-ting said surplus sludge into the digestion step of the pulping process whereby said sludge is digested, a first portion of the digested sludge being retained as a part of the pulp yield, a second portion of the digested sludge being discharged as a ventable gas, and a third portion of the digested sludge being employed as a nutrient for the microbiological purification of liquor effluent with an activated sludge.
2. A process for the microbiological purification of organic material containing spent pulping liquor effluent from a pulp and paper mill employing a wood digestion process compri-sing the steps of:
(A) aerating and decomposing in a first aerating stage the organic material in said effluent with using an activated concentrated sludge having a desired sludge concentration to form a first treated effluent;
(B) conveying said first treated effluent to a first sedi-mentation stage to form a sedimented sludge and a treated first effluent;
(C) removing from said first sedimentation stage (a) a first portion of the sedimented sludge and returning said first portion to the first aera-ting stage to maintain said desired sludge con-centration therein; and (b) a second portion of the sedimented sludge to form a first part of a surplus sludge;
(D) aerating and decomposing in a second aerating stage said treated first effluent from said first sedimentation stage to form a second treated effluent;
(E) conveying said second treated effluent to a second sedimentation stage to form a sedimented sludge and a microbiologically purified effluent;
(F) removing from said second sedimentation stage (a) a first portion of the sedimentation sludge and returning said first portion to the second aerating stage;
(b) a second portion of the sedimented sludge to form a second part of a surplus sludge; and (c) the microbiologically purified effluent;
and then (G) collecting said first and second surplus sludge parts and subjecting them to a digestion step of a wood pulping operation.
(A) aerating and decomposing in a first aerating stage the organic material in said effluent with using an activated concentrated sludge having a desired sludge concentration to form a first treated effluent;
(B) conveying said first treated effluent to a first sedi-mentation stage to form a sedimented sludge and a treated first effluent;
(C) removing from said first sedimentation stage (a) a first portion of the sedimented sludge and returning said first portion to the first aera-ting stage to maintain said desired sludge con-centration therein; and (b) a second portion of the sedimented sludge to form a first part of a surplus sludge;
(D) aerating and decomposing in a second aerating stage said treated first effluent from said first sedimentation stage to form a second treated effluent;
(E) conveying said second treated effluent to a second sedimentation stage to form a sedimented sludge and a microbiologically purified effluent;
(F) removing from said second sedimentation stage (a) a first portion of the sedimentation sludge and returning said first portion to the second aerating stage;
(b) a second portion of the sedimented sludge to form a second part of a surplus sludge; and (c) the microbiologically purified effluent;
and then (G) collecting said first and second surplus sludge parts and subjecting them to a digestion step of a wood pulping operation.
3. The process of claim 2 including the step of pre-treating said organic material containing effluent to remove coarse suspended solids.
4. The process of claim 3 additionally including the step of adjusting the pH of said organic material containing effluent prior to step (A) to have a pH of between 7 to 9.
5. The process of claim 2 wherein said organic material containing effluent is fed to step (A) at a loading rate of between 200 and 500 pounds B.O.D. per 1,000 cu. ft. per day and said organic material containing effluent has between 1% to 3%
total solids.
total solids.
6. The process of claim 2 wherein said organic material containing effluent is aerated in step (A) for between 18 to 36 hours so as to provide a dissolved oxygen content in said effluent of less than 1.5 ppm and the mixed liquor suspended solids in step (A) is in the range of 5,000 - 10,000 ppm.
7. The process of claim 2 wherein said effluent from step (A) is conveyed to said first sedimentation stage at a loading rate less than 45 pounds of total solids per sq. ft.
per day.
per day.
8. The process of claim 2 wherein said effluent in step (D) is aerated for between 18 to 36 hours so as to provide a dissolved oxygen content in said effluent of greater than 2 ppm, and the mixed liquor suspended solids in step ID) is in the range of 3,000 to 5,000 ppm.
9. A process for the microbiological purification of organic material containing spent pulping liquor effluent from a pulp and paper mill employing wood digestion process comprising the steps of:
(A) pre-treating said effluent to remove coarse suspended solids and adjusting the pH and chemical composition of the effluent;
(B) aerating and decomposing in a first aerating stage the organic material in the pre-treated effluent with an activated concentrated sludge, having a desired sludge and oxygen concentration and micro-organism composition, to form a first treated effluent;
(C) conveying said first treated effluent to a first sedi-mentation stage to form a sedimented sludge and a treated first effluent;
(D) removing from said first sedimentation stage (a) a first portion of the sedimented sludge and returning said first portion to said first aera-ting stage to maintain said desired sludge con-centration therein; and (b) a second portion of the sedimented sludge to form a first part of a surplus sludge;
(E) aerating and decomposing in a second aerating stage said treated first effluent with an activated concen-trated sludge and under conditions to maximize the content of protozoa and minimize the content of bacteria and to form a second treated effluent;
(F) conveying said second treated effluent to a second sedimentation stage to form a sedimented sludge and a microbiologically purified effluent;
(G) removing from said second sedimentation stage (a) a first portion of said sedimented sludge and re-turning said first portion to said second aerating stage;
(b) a second portion of said sedimented sludge to form a second part of a surplus sludge; and (c) the microbiologically purified effluent;
and then (H) collecting said first and second surplus sludge parts and injecting said surplus sludge in a digestion step of a wood pulping process to digest said sludge and form a wood pulp whereby a first portion of said digested sludge is retained as a part of the pulp yield, a second portion of said digested sludge is discharged as a ventable gas, and a third portion of said digested sludge is employed as a part of said spent pulping liquor effluent to serve as a nutrient in the microbiological purification operation.
(A) pre-treating said effluent to remove coarse suspended solids and adjusting the pH and chemical composition of the effluent;
(B) aerating and decomposing in a first aerating stage the organic material in the pre-treated effluent with an activated concentrated sludge, having a desired sludge and oxygen concentration and micro-organism composition, to form a first treated effluent;
(C) conveying said first treated effluent to a first sedi-mentation stage to form a sedimented sludge and a treated first effluent;
(D) removing from said first sedimentation stage (a) a first portion of the sedimented sludge and returning said first portion to said first aera-ting stage to maintain said desired sludge con-centration therein; and (b) a second portion of the sedimented sludge to form a first part of a surplus sludge;
(E) aerating and decomposing in a second aerating stage said treated first effluent with an activated concen-trated sludge and under conditions to maximize the content of protozoa and minimize the content of bacteria and to form a second treated effluent;
(F) conveying said second treated effluent to a second sedimentation stage to form a sedimented sludge and a microbiologically purified effluent;
(G) removing from said second sedimentation stage (a) a first portion of said sedimented sludge and re-turning said first portion to said second aerating stage;
(b) a second portion of said sedimented sludge to form a second part of a surplus sludge; and (c) the microbiologically purified effluent;
and then (H) collecting said first and second surplus sludge parts and injecting said surplus sludge in a digestion step of a wood pulping process to digest said sludge and form a wood pulp whereby a first portion of said digested sludge is retained as a part of the pulp yield, a second portion of said digested sludge is discharged as a ventable gas, and a third portion of said digested sludge is employed as a part of said spent pulping liquor effluent to serve as a nutrient in the microbiological purification operation.
10. The process of claim 9 wherein said effluent is con-veyed to said first aerating stage at a loading rate of 200 to 500 pounds B.O.D. per 1,000 cu. ft.per day and having 1 to 3%
total solids and a pH in the range of between 7 to 9.
total solids and a pH in the range of between 7 to 9.
11. The process of claim 9 wherein the dissolved oxygen content in said first treated effluent is less than 1.5 ppm and in said second treated effluent, greater than 2 ppm.
12. A process suitable for the disposal of surplus sludge from a system which microbiologically purifies spent pulping liquor effluent from a wood pulping process comprising the step of injecting said surplus sludge into the digestion step of the wood pulping process whereby said sludge is digested and a wood pulp is formed, a first portion of said digested sludge being retained as a part of the pulp yield, a second portion of said digested sludge being discharged as a ventable gas, and a third portion of said digested sludge being recycled through said system to serve as a nutrient for the microbio-logical purification of spent pulping liquor effluent with an activated sludge.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA242,540A CA1049667A (en) | 1975-12-22 | 1975-12-22 | Disposal of biological sludge from pulp mills |
| US05/701,611 US4030968A (en) | 1975-12-22 | 1976-07-01 | Disposal of paper pulp mill sludge |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA242,540A CA1049667A (en) | 1975-12-22 | 1975-12-22 | Disposal of biological sludge from pulp mills |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1049667A true CA1049667A (en) | 1979-02-27 |
Family
ID=4104837
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA242,540A Expired CA1049667A (en) | 1975-12-22 | 1975-12-22 | Disposal of biological sludge from pulp mills |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1049667A (en) |
-
1975
- 1975-12-22 CA CA242,540A patent/CA1049667A/en not_active Expired
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