CA1074027A - Process for the treatment of activated sludge - Google Patents

Abstract

F.W. Herrick - 20 PROCESS FOR THE TREATMENT OF ACTIVATED SLUDGE

Abstract of the Disclosure A process for the separation and removal of solids from a low solids content activated sludge from a secondary treatment system. The sludge, containing water-filled bio-logical cells, is acidified with a strong mineral acid to reduce the pH to less than 4, preferably less than 3 and is then heated at atmospheric pressure to a temperature of from 70 to 100°C, preferably 80 to 95°C, for a time necessary to rupture by hydrolysis the biological cells and release the bulk of the water therefrom while solubilizing only minor proportions of the sludge solids. The dewatered sludge is then separated from the water by filtration, flotation or other known means.

Description

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- 2 - F.W. Herrick - 20 This invention relates to a process for the conditioning of activated sludge from a secondary biological treatment to separate and remove the bulk of the solids content therefrom.
In the treatment of industrial or municipal wastes, a S major problem involves the separation of solids from water in activated sludge. After sewage or industrial wastes are permitted to settle for "primary solids" removal, the liquid portion is secondarily treated to reduce the BOD (biological oxygen demand). The resulting low ~olids content biological material is "secondary activated sludge". The microorganisms or bacteria present in secondary activated sludge form bio-logical cells containing over 95~ water. The membranes of these biological cells must in some way be ruptured to separate the solids from the overall gelatinous biomass in order to dewater the sludge. While many methods are known for accomplish-ing this dewatering ~tep, such as compaction, forced fil-tration, heating under pressure or the addition of chemicals to the sludge, none has proven economically practical.
U.S. patent 1,543,939 describes a proce~s of treating activated sludge in sewage with sulfur di~xide, either hot or cold, to sterilize and dewater the sludge. This patent issued many years before the development of primary solid~ removal and thus the activated sludge of this patent contains both : primary sewaqe solids, such as paper fibers, and the secondary biological growth. The fibrous material of the primary sludge was necessary for the purpo~es of the patent to form a fibrous sheet used in the filtration and removal of water.
Canadian patent 938,743 discloses a process for digesting activated sludge with aqueou~ S02 under elevated pressures at temperatures of from 122-164C (252-328F). The patent indicates that these conditions yield a fast filtering, insoluble sludge residue and a liquid portion containing a large fraction of iO7~ 's~

- 3 - ~.W. Herrick - 20 sol~bilized sludge solids. A major purpose of the process of this Canadian patent i~ the solubilization of as much as possible of the sludge solids, preferably from 50-75% of the original solids in the ~ludge, for use ultimately as a protein source.
This process however is relatively costly because it requires autoclave equipment to achieve the pressures and temperatures employed. Moreover, it produces a liquid portion with high biological oxygen demand tBOD) which creates a disposal problem.
U.S. patent 3,300,402 discloses a process for chemically oxydizing activated sludge with chlorine to preven~ putrefaction of the sludge during further processing in waste treatment plants.
Effective chlorination treatment is stated to produce an effluent having a pH in the range of 2.5 to 6.5, while oxidation is carried out at room temperature or possibly as high as 110F
(43C). Although a method of sanitizing the sludge is provided, the chlorination treatment may also form chlorinated organic compounds that are potentially toxic and deleterious to the environment or further use of the sludge product.
It is accordingly a primary object of the presen^ invention to provide a simple, inexpensive but effective conditioning process for dewatering secondary activated sludge.
It is an additional object of this invention to provide a process which produ~es a rapid five to ten fold concentration of sludge solids which are easily separated and conveniently disposed of by known techniques.
It is still an additional object of this invention to provide a conditioning process for dewatering activated sludge which solubilizes only minor proportions of the sludge solids and produces a liquid effluent of low BOD.
The foregoing and other objects of the invention are achieved by acidifying, with a strong mineral acid, secondary activated sludge containing water-filled biological cells to reduce the pH to less than 4, heating the acidified sludge at ' .', : . ' ', ~ :

atmospheric pressure to a temperature in the range of 70 to 100C
for a time sufficient to rupture by hydrolysis the biological cells thereby releasing the bulk of the water from the cells without solubilizing major proportions of the sludge solids and separating the dewatered sludge from the water.
The dewatered sludge produced by the present process may be conveniently disposed of by known methods, such as by burning or by conversion to a useful by-product such as a fertilizer or protein feed for animals. The liquid or water portion is of sufficiently low BOD so that it may be recycled to the secondary system without adverse effects. The condition-ing process of the present system is useful for the treatment of secondary activated sludge from any source including that from municipal or industrial wastes. It is particularly adapted for use with activated sludge produced by the secondary treat-ment of the effluent of a sulfite pulping operation. The term "conditioning" is used herein in accordance with its commonly accepted meaning in the field of waste treatment to indicate a modification of activated sludge that facilitates dewatering, filtration and recovery of dewatered solids.
In accordance with the invention there is provided a process for the separation and removal of solids from a low solids content secondary activated sludge, said sludge containing water-filled biological cells, comprising acidifying the secondary sludge with a strong mineral acid to reduce the pH
thereof to less than 4, heating the acidified sludge at atmospheric pressure to a temperature in the range of 70 to 100C
for a time necessary to rupture by hydrolysis the biological cells and release the bulk of the water therefrom, the time at pH and temperature of the sludge being insufficient to solubilize .
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more than 6% of the organic solids content of the original sludge, and separating the dewatered sludge from the water.
In practicing the invention, the pH of activated sludge, normally above 7, is reduced to below 4 by the addition to the activated sludge of a strong mineral acid. The secondary treat-ment activated sludge from a sulfite pulping operation generally has a low solids content - about 2-5% - and a pH of approximately 8. The quantities of activated sludge which are continuously produced in a sulfite mill secondary treatment system are normally of such a large magnitude that the cost of acids used for reduction of pH from 8 to less than 4 are of critical importance. For this reason, it is necessary to use a strong mineral acid, a particularly preferred acid being sulfuric which is both inexpensive and highly effective. However, other acids may be used including hydrochloric, phosphoric and nitric acid.

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- 5 ~ F . W . Herrick - 20 A preferred pH of the activated sludge before heating is between 2 and 3. In this range, dewatering takes place rapidly upon heating without solubilizing large amounts of solids. It has been found that at pH's above 4, insufficient coagulation occurs while at pH's less than 2, the proportion of dissolved organic solids is increased and thus also the BOD level of the water effluent. Moreover, pH's of less than 2 require more acid and are adverse to the economics of the process.
After acidifying, the activated sludge is rapidly heated to a temperature of 70 to 110C, preferably 80-95C, and even more preferably from 90-95C. Dewatering of the activated sludge occurs very rapidly -within less than sixty minutes and in a matter of seconds under optimum conditions of pH and temperatures.
During this time, from 70 to 90% of the water or fluid volume within the original biomass is released. The peak de~ree of dewatering, as measured by filterability, is achieved after a very short digestion period and extension beyond this period dissolves a larger proportion of organic solids without improving filterability. The specific amount of time will vary ~ith pH, temperature and type and solids content of the sludge. Digestion should be stopped before 2 to 3%, or 6~ at most, of the origin~l sludge organic solids are solubilized. This will normally occur in less than 30 minutes, and frequently considerably less, for example, one minute or less. (The Percent of solubilized organic solids as used herein indicates the increment of soluble solids that is assignable to the acid-heat treatment. It excludes organic soluble solids present in the original sludge and soluble mineral solids contributed by the acid.) The dewatered sludge will frequently float to the top upon heating and may be separated by gravity differential between the thickened sludge and the water. Flotation or air separation (use of air bubbles to aid flotation) of treated sludge is best done with little or no agitation. The latter tends to break up - 6 - F.W. Herrick - 20 agglomerates and cause the sludge to disperse or settle. If flotation does not occur, or if agitation i~ used, separation may be effected by filtration, by screening or by other well known mechanical separation techniques. Screening to separate the coagulated or agglomerated conditioned sludge was found effective in all cases. We have found that screening throllgh a 300 mesh screen yields a relatively clear filtrate; screening through a 100 mesh screen results in passage of a minor amount of fine material into the filtrate. Best filtration rates are achieved if filtration occurs while the treated mass is still hot and prior to any neutralization. The addition of flocculants to the treated sludge is also effective for further dewatering and increasing the filtration rates of the treated sludge.
The following examples illustrate the practice of the invention. Filtration rate is a function of the solids content of a given sludge material and thus where comparative data is given, the data compares results for samples of activated sludge having the same solids content.
Example I
Three samples of 100 ml of activated sludge, produced by secondary treatment of sulfite mill effluents, having a solids content of 2.5% and a pH of 8, were each treated with about 0.5 ml of 98% H2S04 to obtain a pH in the range of 2 to 2.4. The three samples were heated to 90C. After 15 seconds, the samples showed rapid shrinkage or coagulation of sludge and dewatering. A vacuum filtration, through a Buchner funnel and a 9-cm No. 42 filter paper for 5 minutes, was used as a measure of filterability. Filtration yielded wet sludge cakes containing 21.7 to 24.4% solids. One treated sample was cooled to 60C and filtered. Filtration rate was somewhat slower and wet filter cake had a solids content of 18.5%. A COD (chemical oxygen demand) value of 1190 ppm was obtained on the three filtrates.

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_ 7 F. W. Herrick- 20 Example 2 An 800 ml volume of sludge (of Example One) was treated with 4 ml of concentrated H2SO4 and heated at 90 C for 10 minutes. Filtration through a Iarger Buchner funnel and an 18.5 cm No. 41 paper required 20 minutes to yield a filter cake at 24% solids. Filtrate volume was 660 ml in 10 minutes (82.5%
dewatering) and 710 ml in 20 minutes or 8g.7% dewatering of the original sludge.
The light amber colored filtrate had a pH of 2.4 and COD was 1770 ppm. Filter cakes were light brown in color and had the appearance of wet Torula yeast cake.

Example 3 These tests were carried out on municipal secondary activiated sludge from the city of Renton, Washington. The settled biomass, as obtained, had a solids content of 1%. This voluminous, watery product was typical of that produced in winter and spring operation, during the rainy season. A standard filtration test was based on the use of a small Buchner funnel and vacuum filter flask apparatus. All filtrations were carried out using two layers of Whatman (trademark) No. 1 filter paper having a diameter of 11 cm. The filtrate volume collected in exactly 40 seconds was recorded for replicate samples. Each test was ; based on the use of a 250 ml volume of biomass at the solids concentration given.
Acid treatment was with concentrated sulfuric acid and thorough mixing. Heat treatments required about 1 minute to reach 90 C. In tests 5 to 8, the biomass was concentrated to 2.4% total solids by centrifuging.

Filtration Rate ml/40 sec. for Biomass Conditionin~ Treatment at Solids Content pHTemperature2 C. Test No. 1%Test No. 2 4%
No Treatment 1 47 5 8 No Acid 90 2 23 6 2 2No Heat 3 68 7 8 The advantageous filtration rate obtained by both acid treatment (pH 2) and heating to 90 C are noted in Tests No. 4 and 8.

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Z t 8 - F.W. Herrick - 20 Example 4 These tests were carried out on activated sludge from the secondary treatment of sulfite pulp mill effluents, by means of an intensive aerated (air) sy~tem. The raw settled biomass varied from week to week in solids contant. The test procedure of Example 3 was used. Filtrativn rates for treated biomass were found to be variable according to the character of the microorganisms present.

Conditioning Treatment Filtration Rate ml/40 sec. for Tempera- Biomass at Solids Content pH ture, C. Test No. 1~5% Te~t No. 2.4 10No Treatment 1 46 5 22 No Acid 90 2 8 6 5 2 No Heat 3 68 7 32 Examples 5-8 In these examples, the sludge of Example 4 was used with a variety of strong mineral acids. All samples were acidified, heated to 90C, and filtered as in Example 3. The results were as follows:

FI~TRATION RATE
EXAMPLE ACID pH Ml~40 sec.
Sulfuric 2 122 6 Hydrochloric 2 122 7 Nitric 2 124 8 Phosphoric 2 130 Example 9 Activated sludge containing 4.2% solids (pH 7.0) waR
obtained from the secondary treatment of sulfite pulp mill effluents via an intensive oxygen system. Samples of 250 ml each were acidified to pH 2.2 with 0~4 ml (0.73 g) of concen-trated sulfuric acid, heated to 90-95C and digested at this temperature for varying times. The filtration rate of the conditioned sludge was determined as in Example 3. Filtration

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- 9 - F.W. Herrick - 20 was continued to completion and the volume of filtrate was measured. Filtrates were analyzed for soluble solids content and COD. Filter cakes were weighed and analyzed for wet and dry solids content. A control ~ample (No. 1) of sludge, without acidification or heat treatment, was processed in the same manner.

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2'~t - 11 - F.W. Herrick - 20 The above data was used to calculate the amount of sludge solids (organic material) dissolved by a given treatment, but excluding the soluble solids present in the control (untreated) sample filtrate and the soluble (mineral) solids represented by the sulfuric acid used for conditioning. Total sample solids in these tests were determined as the sum of soluble solids and dry filter cake solids (column C and F). In the calculated data below, Column G represents the overall soluble material in a filtrate, calculated as percent of the total sample solids content. This value includes the soluble material present in filtrates of the untreated sludge (sample 1) and the sulfuric acid used to condition samples 2 to 5. In the case of sample 2, it was found that 50% of the soluhle material in the filtrate was account~d for by sulfuric acid, representing about 80% recovery of this reagent in the filtrate. To correct for sulfuric acid content, one-half of the soluble solids content for sample 2 was subtracted from the column G values for sample~
2 to 5 to ~btain the (corrected) soluble organic solids (column H). Since the soluble organic solids for sample 1 corresponds to material that is already in solution, if this value is subtracted from all the values in col D H, the remainder represents the organic material that is actually solubilized by the conditioning treatment (column I). In Sample 5, the digestion time was excessive and therefor resulted in over 6~ dissolved sludge.
Another method for estimating organic content is to analyze for COD. The values in column J were calculated by dividing the weight of COD determined for each filtrate by the tota' sample solids weight. The relative amount of COD represent-ed by organic material that is solubilized by a given condition-ing treatment may then be obtained by subtracting the value for sample 1 from those for samples 2 to 5 (column K). It is .

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- 12 - F.W. Herrick - 20 evident that a sludge conditioning treatment at pH 2.2 for 1 minute at 90-95~C results in minimal solubilization of sludge solids, while dewatering and filtration rates are advantageously high. Extending the digestion time at 90-95C, while increasing filtration rate to some extent, also results in progressive acid hydrolysis of the sludge to produce soluble organic material.

Calculated Values Based on Total Sample Solids _________ ___________ ____________ _________, .______ _____________ % Soluble ~ ~ COD
% SolubleOrganic Dissolved % Dissolved Sam~ Solids Solids Sludge ~ Sludge 1 4.05 4.05 ~.00 3.36 0.00 2 8.85 4.43 0.38 3.19 0.00 3 11.67 7.24 3.19 4.92 1.56 4 14.01 9.58 5.53 6.66 3.30 16.23 11.18 7.13 ~ 7.96~ 4.60 _________________________________ ___ _____ _____________ Example 10 - Example 9 was repeated but sample pH was adjusted to 3.0 by addition of 0.25 ml (0.46 g) of sulfuric acid. In calculating the value for soluble organic solids (column H) the determined weight of soluble solids (colum~ C) was corrected by subtracting 0.37 g, corresponding to the sulfuric acid analysi~ of samples 2 and 3. The results below show that at pH 3.0, the filtration rate for conditional sludge increases with digestion time, but even after 60 minutes dige~tion the filtration rate is not as high as obtained by 1 minute dige~tion at pH 2.2 (Example 9) while the amount of 301ubilized sludge is considerably greater.

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- 14 - F.W. Herrick - 20 .. _ _ Calculatell Yalues Based Dn Total Sam ?le Sol ~ COD

% Soluble Organic Dissolved% Dissolved

5 SampleSolids Solids SlIudge ~ Slud~e 1 4.05 4.05 0.00 3.36 0.00 2 10.49 7.44 3.39 4.41 1.05 l 3 12.46 9.22 5.17 6 01 2.65 Example 11 Example 9 was again repeated holding digestion time to one minute, but ~arying the pH from 2.0 to 3.5 These results clearly indicate the importance of pH to filterability. From the filtrate COD data for these tests it was evident that only small amounts of sludge were dissolved. The control filtrate COD value from Example 9, sample 1, column J was subtracted from the COD values calculated in these tests to obtain the net COD
values for dissolved sludge lColumn X).

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- 16 - F.W. Herrick - 20 - Calculated Values Based On Sludge Filter CakeTotal Sam le Solids Wet Cake Dry % Soluble %Di~solved SamPle Sol ds, % Weight g~ Sol ds COD Sludge _ 1 33.5 11.29 11.05 3.910.55 2 29.6 11.62 9.41 3.310.00 3 27.2 11.73 8.37 3.080.00 l 9 ___ 11.34 _ _ _ 3.810.45 Example 12 The activated sludge of Example 9 wa~ treated with ~ulfur dioxide ~S~2) at elevated temperatures and pressures as described in Canadian Patent 938,743. The amount of S02 addition, as given below was based on total sample 5sludge) solids. Sludge samples were also treated with sulfuric acid at pH 2Øunder comparable conditions. Workup and analyses in these tests were the same as used in Example 9. The filtrate COD values corresponding to sludge dis~olved by any given treatment were computed as described in Example 11. Pre~sures were about 3~ and 69 psi at 125C and 150C re8pectively.

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Calculate~ Values Base~ On Slu dge Filter ~ ake _ __ 35~:LLI~8~c~2~LLa~
% COD
Wet Cake Dry~ Soluble % Dissolved 5 ~e~ ~ Solids CODSludge 1 15.4 13.03 6.46 4.14 0.78 2 8.9 9.64 8.29 5.24 1.88 3 26.4 9.94 19.25 10.76 7.40 4 11.4 10.83 8.80 5.29 1.93 15.6 11.23 11.27 7.32 3.96

6 12.6 11.00 12.40 8.14 4.78

7 31.3 9.55 24.84 14.27 10.91 The above results again clearly show that pH iB critical and that higher temperatures and pressure reactions do not improve filtration rate a3 much as conditioning at lower pH.
These results also show that low pH combined with 30 minutes digestion at 125 to 150C results in undesirable excessive hydrolysis of the sludge to produce soluble organic matexial as determined by COD analysis, in contrast to very low dissolution of sludge when con~itioning i8 performed at pH 2 for 1 minute at 90-95C (Example 9).
In all tests as described in Examples 9 to 12, conditioning of sludge at pH 2 and heating to 90C or higher, not only resulted in fast filtration rates, but the sludge cakes that were obtained by vacuum filtration generally contained more than 30 solids. This property is economically significant in industry.
This advantageous property was not observed under conditions where pH was higher than 3Ø Moreover, when filtration rate was above 30 ml/40 sec., only a few minutes were required to complete the vacuum filtration and obtain a high solids filter cake. When filtration rate was in the range of 2 to 10 ml/40 sec., filtration required one hour or more and filter cakes were low in ~07~1z .~

- 19 - F.W. Herrick - 20 solids content.
Example 13 These tests were carried out on activated sludge from the secondary treatment of sulfite pulp mill effluents by means of intensive oxygen or aeration systems. The se~tled biomass products were found to be quite variable in solids content and other physical properties, depending on operating conditions and effluent (feed) compositions. The test procedure of example 3 was used. Treatment pH was varied from 2 to the pH measured for the biomass sample used. Heat treatment was carefully controlled for all test samples by heating rapidly to 90-92C, followed by immediate filtration.
The acid reagent used in all tests was sulfuric acid, sufficient to lower pH to the value given.

FILTRATION RATES OF TREATE~ BIOMASS

Oxygen System, 2.7% Solids Air System, 1.7~ Solids Test Rate, ml/ Test Rate, m /
No. ~ 40 sec. No. pH 40 sac.
.
1 2.1 114 6 2.1 230 2 3.1 78 7 3.1 122 3 4.0 23 8 3.9 88 4 5.0 16 9 7.~ 14 5 7.7 8 Tests 5 and 9 were conducted at the natural pH of the biomass.
It is evident that acidi~ication of biomass to a pH of less than 4, followed by heating to 90C, increases ~iltration rate very sharply.
Example 14 Activated sludge from secondary treatment of sulfite pulp mill effluents via an intensive oxygen system was used in these tests. The filtration test procedure was the same as described 7~V'~

- 20 - F.W. Herrick - 20 in Example 3. Samples 1 to 5 were acidified with ~ulfuric acid to the pH noted below, heated rapidly to the temperature given and filtered immediately. Samples 6 to 9 were treated in a similar manner, except that temperature was maintained for 30 minutes prior to filtration.

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- 22 - F.W. Herrick - 20 It is evident that acldification to a pH as low as 2 is not adequate in producing the desired fast filtration rates unless heating to about 80C is also provided.
Example 15 Activated sludge from the same process as tested in Example 14 was used. A fxesh sample of thi5 material contained 2.4% solids and had a pH of 8.4. A one (1) liter volume of this material was treated with 1.7 ml (3.1 g) of concentrated sulfuric acid to reduce pH to 2.3. This sample was heated rapidly to 90C and then filtered. Filtration rate, measured by the procedure given in Example 3 was 98 ml/ 40 ~ec. Total filtrate volume was 890 ml. The chemical and biological oxygen demands (COD, BOD) of this filtrate were 1400 and 533 ppm, respectively. The filter cake of acid treated biomass 15 contained 23% solids and had a dry weight of 19.2 g. Acid and heat treatment resulted in almost 90~ dewatering of the original bio~ass, while producing soluble COD solids only to the extent of 2.0% of the original solids content. To obtain this value, the filtrate COD, expressed as percent of ~otal sample solids was corrected by subtracting 3.36~, which corresp-onds to the COD of untreated sludge filtrate, calculated on the 8ame basis a~ in Example 9.
A second one tl) liter volume of activated sludge was treat~d as above but with sulfur dioxide instead of sulfuric acld. The S02 was bubbled into the biomass until a pH of 2.2 was obtained which required 7 g. or 29.2% of sludge solids.
After heating the filtration rate was 91 ml/40 sec. Total filtrate volume was 875 ml. COD and BOD analyses for thiq filtrate were 145n and 823 ppm, respe~ctively. Filter cake solids content was 23% and dry weight was l9g. The weight f S2 reagent required to match the effectiveness of sulfuric acid was over twice as much. The BOD of the filtrate , - . . - . .
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- 23 - F. W. Herrick - 20 from the SO2 treatment was also appreciably higher than that obtained using sulfuric acid.

Example 16 An 8.1 volume of the same activated sludge as used in Example lS, containing 2.4% solids was treated with 13 ml of conc. ~2SO4 while stirring vigorously to obtain a pH of 2.1. The acidified sludge was heated to 90C during 30 min-utes and filtered immediately through Whatman (trademark) No. 1 paper in a large Buchner funnel-vacuum filtration apparatus. Total filtrate volume was 6925 ml, containing about 0.5% solids. The BOD and COD of this filtrate were 650 and 1710 ppm, respectively. The filtrate was concentra-ted by vacuum evaporation to about 8% solids to facilitate chemical analysis. The wet filter cake was diluted with lS water to form a slurry that was Rpray dried using an inlet air temperature of 232C. The spray dried sludge solids were light brown in color. The concentrated filtrate and the dry sludge filter cake were analyzed for chemical components as summarized in Tables I and II.

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- 24 - F.W. ~errick - 20 TA~LE I

Analysis of Conditioned Sludge and Filtrate _ Spray Dried Analysis Percent, calculated on a Sludge Filte dry basis, except solids content. Filtrate Cake r _ Oven dry solids 7.98 93.5 Sodium _ O.29 Pota~ium 2.0 0.05 Ash 41.6* 5.2 Sulfur 17.8* 1.3 Nitrogen 4.43 8.07 ; Phosphorus 0.4 1.03 Galacto~ 1.4 0.4 Glucose 5.8 10.6 Mannose 1.3 O.7 Arabinose 0.1 0.1 Xylo8e 0.3 0.3 Monom~ric sugars O.O O.O
Total carbohydrate 8.9 12.1 . - ~ _ .: * Mostly Sul$uric Acid lo~7~ t - 25 - F.W. Herrick - 20 TABLE II

Protein and Amino Sludge and Filtrate and Toru a Yeast _ _ Analysis, Percent Spray Dried Torula Calculated on a dry Sludge Filter Yea~t Basis Filtrate Cake .
Nitrogen 4.43 8.07 6.58 Crude Protein 27.7 50.4 41.1 (% N x 6.25) _ Amino Acids:
Alanine 0.23 3.47 2.09 Arginine 0.12 2.84 2.72 Aspartic Acid 0.39 4.01 3.23 Cy~tine/2 N.D. N.D. N.D.
Glycine 0 67 2 12 1 42 Hi~tidine 0.04 0.79 0.81 Iqoleuclne 0.06 1.89 1.74 Leucine 0.09 3.67 2.48 Ly~ine 0 55 20 91 0 46 Phenylalanine 0.29 2.27 1.77 Proline 0.10 1.67 1.17 Serine 0.10 1.72 1.64 Threonine 0.11 2.21 1.94 Tryptophan N.D. N.D. N.D.
Tyrosine N.D. 1.68 1.60 Valine 0.09 2.96 2.12 Total 3.31 39.67 31.97 N.D. - Not determined.

~)7~L~
- 26 - F.W. Herrick - 20 From the tabulated chemical analysis data it i~ evident that soluble solids in the filtrate are compo~ed of a ma~or amount of sulfuric acid (17.8% S = 54.5~ H2S04~, a lesser amount of carbohydrate polymer (8~9~) and a small amount of protein (3.31% amino acids). Qualitative analysis also indicated the presence of amino sugars or their polymers (chitin) which would account for the substantial nitrogen content. The crude protein value of 27.7% for the filtrate dissolved solids, obtained by multiplying % N x 6.25, is obviously approximate in this case since the sum of the readily analyzable amino acids (Table II) is quite low.
The dry conditioned sludge was rich in protein and some-what better in this regard than Torula yeast grown on sugars present in spent sulfite liquor. The dry sludge was composed of a ma~or amount of protein (40 + ~ and a lesser amount of carbohydrate polymer (12.1%). Qualitative analysis on hydrolyzates also indicated the presence o~ amino sugars. The above analysis indicates that the cQnditioned sludge is useful as an animal feed. The conditions of the process al30 result in a sterile product with respect tQ biological activity.

Claims (12)

- 27 - F.W. Herrick - 20 THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the separation and removal of solids from a low solids content secondary activated sludge, said sludge containing water-filled biological cells, com-prising acidifying the secondary sludge with a strong min-eral acid to reduce the pH thereof to less than 4, heating the acidified sludge at atmospheric pressure to a temperature in the range of 70 to 100°C for a time necessary to rupture by hydrolysis the biological cells and release the bulk of the water therefrom, the time at pH and temperature of the sludge being insufficient to solubilize more than 6% of the organic solids content of the original sludge, and separating the dewatered sludge from the water.

2. The process of Claim 1 in which the sludge is acidified to a pH of from 2 to 3.

3. The process of Claim 1 in which the acidified sludge is heated to a temperature of from 80-95°C.

4. The process of Claim 3 in which the acidified sludge is heated to from 90-95°C.

5. The process of Claim 1 in which the sludge is acidified with a strong mineral acid selected from the group consisting of sulfuric, hydrochloric, nitric and phosphoric acid.

6. The process of Claim 5 in which the acid is sulfuric acid.

7. The process of Claim 1 in which the time is less than about one minute.

8. The process of Claim 1 in which the dewatered sludge is separated from the water by filtration.

- 28 - F. W. Herrick - 20

9. The process of Claim 1 in which the dewatered sludge is separated by gravity differential between the dewatered sludge and the water.

10. The process of Claim 8 in which the separation occurs while the sludge is still acidified and heated.

11. The process of Claim 1 in which the activated sludge is produced by the secondary treatment of sulfite mill effluent.

12. A process for the separation and removal of solids from a low solids content secondary activated sludge, said sludge containing water-filled biological cells, com-prising acidifying the sludge with sulfuric acid to reduce the pH thereof to from 2 to 3, heating the acidified sludge at atmospheric pressure to a temperature in the range of 80 to 95°C for a time nec-essary to rupture by hydrolysis the biological cells and release over 70% of the water therefrom, the time at pH and temperature of the sludge being insufficient to solubilize more than 6% of the organic solids content of the original sludge, and separating the dewatered sludge from the water.