CA1087330A - Waste treatment process - Google Patents

Abstract

WASTE TREATMENT PROCESS
ABSTRACT

A process for dewatering aqueous waste sludges containing suspended, organic matter which involves the sequential steps of treating the sludge with lime, neu-tralizing with an acid and filtering under pressure.
Solids contents of the filter cake in the range of 60-70 weight percent can easily be obtained at relatively mod-erate pressures of 10-12 atmospheres.

--i--

Description

10873;~

BACKGROUND OF THE; INV~NTION

The pollution free disposal of various dilute aqueous waste streams containing solids of high organic content is a pro-blem facing both industry and municipalities. In general, local ordinances do not permit the direct disposal of such waste streams into the immediate environment. As a result, the wastes must be treated to remove the solids from the water phase or to concentrat the waste stream sufficiently to permit transport of the solids or~
the concentrated sludges to a remote treatment facility or landfil area,-whére safe disposal can be made wlthout causing health haz-ards.
A serious problem in these circumstances is the extreme difficulty encountered in concentration of the dilute wastes which do not settle or filter readily. Some improvement in settling rat~
is had by adjusting the pH to a value in the al~aline range, e.g.
at a pH of ll. Chemical precipitation has also been suggested for the purification of sewage. This involves the use of a compound such as alum which forms a gelatinous precipitate under alkaline conditions and which acts as an absorbent f~r the impurities. A
coagulant or flocculating agent is-usually added to increase the partlcle size of the impurities. After settling, the supernatant liquid phase must be removed and neutralized in a separate zone prior to discharge. However, it has often been found that even with such treatments the sludges settle poorly and as a consequenc the clarified water phase still contains suspended solids in con-centrations above the maximum allowable limit. This has been found to be the case in e.g. the treatment of polymer plant efflu-ents especially those from suspension polymerization plant opera-- tions. These effluents, which contain organic suspended solids in '~ ' . . ' ' .
. -1-~ , ~ , ~ 1087330 the form of a mixture polymer and suspension agent are usually treated in a series of steps including bio-oxidation and chemical precipitation. Even with this treatment the soLids still do not settle well enough to permit the final disposal of a neutral clar-ified water phase of acceptable quality or turbidity.
The sludges remaining from any one of the prior art set-tling techniques usually contain solids at a concentration of at most 5 percent by weight, and the ultimate disposal of these sludges is still a problem. Filtration has not ~een a solution since sludges thus conditioned can only be dewatered by filtration with extreme difficulty, and-the solids content of the sludge afte dewatering is considerably less than 15 percent by weight, which precludes the disposal either by incineration or by transport of the sludge to a rem~te and safe disposal area as not economically -feasible. Furthermore, such sludges do not lend themselves to eas mechanical handling such as can be achieved with automatically dis charging filter presses. -It is therefore an object of the present invention to provide a process for the concentration of waste streams for the recovery of suspended solids at higher concentrations than hereto-fore possible.
Another object of the invention is to provide an econo-mically feasible process for the disposal of waste sludges. -It is also another object to provide a process for de-; 25 watering waste sludges to a solids content sufficiently high to enable incineration of the dewatered sludge.
It is further object to provide an economical waste treatment process of waste sludges wherein chemical consumption is ; kept to a minimum.

Further objects will become apparent from the following description and appended claims.
. ' . ~ .
THE INVENT I ON

In accordance with the present invention there is pro-5 - vided a process for dewatering a waste s~ream containing suspended solids, which Comprises mixing the waste stream with an amount of at least 15 percent by weight of particulate lime based on the solids concentration of the waste stream, said amount being in ex-cess of that being dissolved by the waste stream at ambient condi-tions, neutralizing the mixture to a pH in the range of about 6 to about 8 with an acid selected from sulfuric acid, sulfurous acid or carbonic acid, and filtering the neutralized mixture under pres-sure.
~he waste stream or sludge of this invention is typically one that has been treated in a bio-lagoon and optionally been sub-jected to settling in a settling or clarifying tank. However, it is within the scope of this invention to treat various other dilut~
waste streams containing suspended material of high organic conten~, and which would provide health hazards to the environment if dis-posed of without treatment. Usually the waste streams or sludges contain about 0.01 to about 5 percent by weight of, suspended solids, and they are commonly substantially neutral, e.g. pH in the range of 6-8.
After determination of the concentration of suspended solids in the waste stream to be treated, an appropriate quantity of finely divided lime is added thereto. ~ certain amount of lime ; is soluble in the waste stream, the solubility depending upon suCh ~factors as e temperaturc, initial Ph and dissolved solids of the~

_3_ " . ' ' ' . .
;~ .
.. . .

~ 10 !37330 waste stream,the form of lime used, i.e. Cao or Ca(O~)2, etc The appropriate quantity to be added to the waste stream is then the sum-of the soluble portion and the portion which is based on the weight of suspended solids in the waste stream to be treated. The latter portion should, as mentioned above, be at least 15 percent by weight of the weight of the suspended solids, and preferably at least 50 percent by weight. There is no critical upper limit for the invention to work in the intended manner, however, for practical and economic reasons it has been found that about 200 percent is a reasonable upper limit. Of calcium oxide and calcium hydroxide, the former is preferred.
After the aforementioned addition, the system becomes alkaline and flocculation sets in. It has been found that it is not necessary to add an~ additional precipitants or flocculating agents to the system, although such compounds can indeed be added to further promote the separation of solids. Examples of sùitable additives are alum, aluminum sulfate,ferric chloride or any other conventional precipitant known to be useful for this purpose, and/
or a flocculating agent such as poly(acrylic acid) or any other of the various well known flocculants available. When used, chemical precipitants are added in amounts typically from 5 to 5000 mg/lite of waste stream and flocculating agents generally from 1 to 100 mg/
- liter.
After the addition of the calcium oxide or hydroxide, the resulting sludge is neutralized to a pH in the range of about 6 to about 8 with the acid. The suitable acids for this purpose are sulfuric acid, sulfurous acid or carbonic acid (CO2). The neu-tralization reaction, in which the corresponding calcium salt is at least partially formed, is carried out with agitation and the re-. .

' 10~7330 .' sultant mixture is thereafter subjected to filtration, preferably under pressure.
It should be understood, that for the purpose of this invention, the particular manner in which the "pressure" is ap-plied is not important as long as a pressure differential is ob-tained across the filter cake. Thus, super atmospheric pressure can be applied upstream of the filter or a vacuum downstream of the filter. -Preferably, a pressure of at least about 0.5 atmos-pheres is employed and for best results a pressure of about 10 to about 15 atmospheres or even higher is used. There are many com-mercially available-filters, which operate at the aforementioned high pressures and are capable of processing large sludge volumes, e.g. the Dart Hoesch filter available from Dart Industries, Inc.
Temperature does not have any significant effect on the efficiency of the process, and the filtration can be carried out at any practical and convenient temperature.
By the addition of lime to a waste sludge followed by neutralization in accordance with this invention, a several fold increase in filtration rate can be achieved as compared to prior art methods, and sustained operations can be carried out on a com-mercial scale without interruptions caused by clogged filters.
Also, additives normally necessary in prior art processes, e.g.
chemical precipitants and flocculating agents, can be omitted or at least substantially reduced. The most important advantage, however, of the process of this invention is that the solids con-tent of the filter cake is considerably higher than what can be obtained by any prior art method. For instance, solids contents of 60-70~ by weight can easily be attained employin~ a relatively moderate filtration pressure of 10-12 atmospheres. In addition, ' ' . .

. . _ ..

~ 1087330 the filter cake resulting from operations in accordance with this-invention is considerably stiffer and more easily handled, e.g. in the removal from the filter. Still anather advantage of the inven-tion is that the total treatment can be carried out in one treat-ment zone, i.e. no separate facilities are necessary to provide a substantially neutral liquid discharge from the process.
One feature of the process of this invention, which coulc not be predicted, in fact, contradicts prior art experiences, is that the flocculated sludges after the acid neutralization are very stable. Employing conventional techniques, sludges and-polymer suspensions flocculated at a pH of 9 and above normally redisperse and are difficult to filter if they are neutralized to a pH in the range of about 6 to 8. Also the filter cakes obtained by conven-tional methoas are low in solids content and not easily handled and disposed of. ..
That the calcium compound is not merely functioning as a filter aid became quite apparent during the experimental program leading up to the invention. For instance, comparative tests on sludges, showed that wherein calcium oxide was added to a waste stream and the mixture then neutralized with sulfuric acid, the ; filtration rate was high and far superior to that obtained when ` merely adding the same quantity of calcium sulfate to the waste stream. Also, comparative tests with calcium oxide addition but with and without acid neutralization, showed that the filtration rate obtained with the neutralized sludge was significantly better than that of the non-neutralized sludge. The relatively good fil-tration rate obtained with calcium oxide treatment alone cannot be attributed to the resultant alkalinity of the sludge, since other tests with alkaline sludges containing no excess ~undissolved) ~ 1087330 calcium oxide, showed very poor filtration rates. Thus, calcium oxide or hydroxide possesses a unique ~uality of improving fil-tration rates, which cannot be explained by any obvious theories, and this improvement is further enhanced when the sludges contain-ing excess amounts of either one of these compounds are neutralizec before filtration.
Due to the high solids contents and the ease of handling of the filter cakes obtained in accordance with the process of thi~ ;
inYention, disposal thereof causes no particular problems, in that¦
transporting of the solids to a remote disposal area, e.g. for lan~
fill purposes, is now a feasible solution from an economical stand point.
A particularly advantageous manner of disposing of the filter cakes is to incinerate it, which now can easily be done due to the low water content of the recovered filter cake. In the in-cineration the organic material is burned off producing consider-able heat, that can be recovered as energy, e.g. steam. The in-çinerator residue may be recovered as a useful by-product of the ; process. When carbonic acid is the acid used in the neutraliza-tion, the residue, which mostly consists of calcium oxide, is ad-vantageously recycled to the process, thereby minimizing, if not ¦
wholly obviating, the need for fresh calcium oxide or hydroxide within the process. The filtrate, whlch is free from polluting solids,needs no further treatment due to its neutral pH, and can be discharged in any convenient manner.
In order to provide a better understanding of the inven-tion reference is had to the following examples.
The waste streams treated in these examples were various combined, biooxidated effluent streams from a commercial installa-30 - tion for the production of styrene-acrylonitrile (SAN) and acrylon _7_ -.~
'."

itrile-butadiene-styrene (ABS) resins using suspension polymeriza-tion techniques. The aforementioned waste streams contained an estimate of 0.05 to 0.5 weight percent of polymer based on the weight of suspended solids in said waste streams.
.

. . _ A series of five comparative experiments were carried to demonstrate the invention. In the first f,our experiments, each treatment was carried out on 3.3 liters of a dilute sludge con-, taining about 158 mg/liter of suspended solids. The sludge was obtained from a clarifier ~ank as the supernatant liquid phase and was a cloudy liquid containing visible suspended particles. The pH,of the sludge was 7.4.
In Example 1, the sludge received no treatment and was filtered under a vacuum of 0.5 atmosphere employing a 9.0 cm. dia-'- 15 meter Buchner funnel and a propylene filter cloth. The experiment ~, had to be interrupted due to clogging of the filter cloth and ex-. tremely low filtration rates.
In Example 2, calcium sulfate was added to the sludge in amount corresponding to 10 weight equivalents of the sludge solids ` 20 in excess of the solubility of the calcium sulfate ~2.0g gms. per ~;; liter of water). Thus, a total of 3.67 gm/1 of calcium sulfate was added to the sludge, which was stirred for 5 minutes before filtration. ' , As in Exa,mple 1, the test could not be completed due to an extremely low filtration rate and clogging of the filter.
Thus, adding calcium sulfate as an aid to the filtration was not -successful. , In Example 3, 10 weight,equivalents of calcium oxide was added instead of calcium sulfate, again in excess of the ~. .. ~

. 10~17330 solubility (1.2 gms. per liter of wate`r). A total of 2.78 gms./l of calcium oxide was used. A filtration rate of 18 gal./sq.ft./
hour was observed, demonstrating that calcium oxide addition is helpful in improving filtration rates. The pH of this filtrate was 13.1.
In Example 4, the procedure of Example 3 was repeated and after addition of the calcium oxide but prior to filtration the pH was adjusted to 7.2 with 25% sulfuric acid. The filtration rate was increased by more than 20 percent over that of Example 3.
The more concentrated sludge used in Example 5 contalned 5 gms, per liter of solids and was treated as in Example 4 except that only an excess of 1 weight equivalents of calcium oxide was used, i.e. a total of 6.2 qms./l of CaO was added. After pH ad-justment to 7.2 the treated sludge was filtered and the same fil-tration rate of 22 gal./sq.ft./hour was observed as in Example 4.
The experiment was repeated on a larger scale using a P-2.5 Model Dart Hoesch horizontal, fully automatic filter press. This unit filters, presses, washes and represses filter cakes at pressures up to lS atmospheres and discharges the thus separated solids in a completely automated operatlon. Its performance duplicates closel that of the commercial-sized units with total filter plate areas o up to at least 32 square meters. The filter cake of Example S had a solids content after pressing at 1 atmosphere of about 31 per- ?

2 cent by weight and was dewatered to 65 percent solids at 12 atmos-S pheres.
The filtrates from Examples 4 and 5 were extremely clear and had a turbidity of about 6 FTU (Formazine turbidity units), when measured in a HACH turbidimeter Model 2100A using standard techniques.
A summary of pertinent data from Examples 1-5 is pre-sented in Table I.

~ 1087330 ~' ~

~ ~ N

, ~ . , er~ . .

` ,~

~ ~

~ o 1' ~ ~ ~

S ~C ~a lZ

.. ~087330 ~X~PLES 6-9 Using the technique of the previous examples a sludge feed containing about 16,700 mg/liter suspended solids was treated in this set of experiments with approximately 0.20 weight equiva-5 . lents of lime, (either as calcium oxide or calcium hydroxide) base . on the weight of the suspended solids and in excess of the respec-tive amounts being dissolved by the sludges. The sludges also contained about 0.18 weight percent of added aluminum sulfate ~an-. . hydrous). The sludges were then neutralized either with sulfuric : acid or carbonic acid (dry ice) to a pH in the range of 7-7.5.
~` Filtration tests of 2-2.5 liter samples gave the results shown in Table II.

. ,' . , . .
. .' ,. .~
. ~

~ . . .. ... _ .

: 10~7330-. ~ ~ ,, . ~ ~ .
-, ,. . , .

I
. (d ~ . '.

~ o ~ o ~

E 1~
. U~ ~0 , ~1 ~ o o . .
~ U U U U

. ~ a) ., . , En'~ot`ooa~' ~î~ -, .. . _ '' ~087330 In all instances the filtrates were extremely clear and had turbidities not exceeding 1.5 FTU.
It is evident from the above data that excellent fil-tration rates are obtained even with quite small amounts of excess . calcium oxide or hydroxide additions, i.e. 0.2 weight equivalents : ` rather than 1.0 and 10 weight equivalents used in Examples 1-5.
Also, the data indicates the somewhat better results obtained, . when calcium oxide is used instead of calcium hydroxide.
.: While particular embodiments of this invention are il-lustrated above, it will be understood that the invention is ob-viously subject to variations and modifications without departing .
~ron its br der aspects.

. . ' .
. ', .
. , , '

Claims (12)

WHAT IS CLAIMED IS:

1. A process for dewatering a waste stream containing suspended solids which comprises a) mixing the waste stream with an amount of at least 15 percent by weight of particulate lime based on the solids concentration of the waste stream, said amount being in excess of that being dissolved by the waste stream at ambient conditions, b) neutralizing the mixture to a pH in the range of about 6 to about 8 with an acid selected from sulfuric acid, sulfurous acid or carbonic acid; and c) filtering the neutralized mixture under pressure to obtain a liquid filtrate and a solids filter cake.

2. A process according to claim 1 wherein said excess amount of lime mixed with the waste stream is at least 50 percent by weight.

3. A process according to claim 1 wherein the lime is cal-cium oxide.

4. A process according to claim 1 wherein the lime is cal-cium hydroxide.

5. A process according to claim 1 wherein the acid is sul-furic acid.

6. A process according to claim 1 wherein the acid is car-bonic acid.

7. A process according to claim 1 wherein the pressure is at least 0.5 atmospheres.

8. A process according to claim 1 wherein the pressure is between about 10 and about 15 atmospheres.

9. A process according to claim 1 wherein from about 5 to about 5000 mg/liter of a chemical precipitant is added to the waste stream prior to neutralization.

10. A process according to claim 1 wherein from about 1 to about 100 mg/liter of a flocculating agent is added to the waste stream prior to neutralization.

11. A process according to claim 1 wherein the filter cake is incinerated.

12. A process according to claim 11, wherein the acid is carbonic acid and calcium oxide resulting from the incineration of the filter cake is recycled to step (a).