COLOUR REMOVAL FROM BLEACHED KRAFT-PULP WASTE WATERS

COLOUR REMOVAL FROM BLEACHED KRAFT-PULP WASTE WATERS J. BEBIN, P. BOULENGER and J.C. BOURDELOT 5, Boulevard du General de Gaulle, 92 Rueil Malmaison, France

INTRODUCTION The paper-pulp industry produces 10% of the pollution of all the French industries. The effluents are strongly coloured, and high in content of non-biodegradable products which are mostly due to the presence of lignin upon which biological treatment has almost no effect. In order to solve this problem, it is necessary to rely on new techniques. So far only coagulation with lime and precipitation have been applied on an industrial scale. We describe in this paper a new process of colour removal with aluminium sulphate coagulation, taking as an example waste water from the manufacture of bleached Kraft pulp.

CHARACTERISTICS OF THE EFFLUENTS Waste water from bleaching of the pulp represents the main source of pollution, i.e. nearly 65% of the COD, and 60% of the colour. Table 1 shows an analysis of bleaching waters at each stage of the process: Table 1. Characteristics of the waste waters from bleaching section Chlorination pH COD Colour Volume

mg/1 kg/t mg Pt/1 kg Pt/t m 3 /t

Sodation 9 to 11 2300 to 4500 30 to 50 3000 to 6000 30 to 40 8 to 13

1.5 to 2.2 450 to 700 25 to 40 300 to 600 25 to 35 48 to 56

Chlorinosodation 2 to 2.5 1000 to 1500 60 to 90 900 to 1600 55 to 75 55 to 70

ANALYTICAL METHODS Standard techniques were used. Colour was measured with reference to the platinum-cobalt scale after filtration and neutralization by pH 7 ± 0.2, using a spectrophotometer at the wave length of 387,5 ιτιμ. AI and Ca were also noted, for information only.

595

596

J. Bebin, P. Boulenger and J.C. Bourdelot COLOUR REMOVAL WITH LIME

It was assumed that colour removal with lime would result from a chemical reaction where enolic and phenolic groups of the structures which themselves are bound with chromophoric groups, would combine with lime within an alkaline medium, and produce insoluble organic calcium compounds, whereas the molecular weight of the solids contained in the spent liquor would be of importance.

LIME KILN

VACUUM FILTER

Fig. 1. Treatment of Bleaching Effluents with Lime - Flow Sheet

However, settlement of these precipitates is rather poor, and for the economics of the process it is essential to precipitate also the calcium ion Ca++ which remain in solution. This is possible by lowering near to neutral the actual value of pH, with carbonation by flue gases containing C0 2 . Calcium precipitates as calcium carbonate, giving a dense floe, and the resulting action leads to an improvement of the decoloration effect on the settled water. The prior addition of lime carbonate, which is available, to the stored raw pulp, improves the clarification and also the quality of the settled sludge. The treatment removes up to 90% of the colour and 70% of the COD. The sludge contains a high percentage of material with low BTU value, and therefore, when incinerating the sludge for lime recovery, large amounts of added fuel are necessary.

Colour Removal from Bleached Kraft Pulp Waste Waters

597

ALUMINIUM SULPHATE PROCESS By this process coloured substances are removed as an aluminium organic precipitate, which is settled and incinerated in order to recover the aluminium. The results are given of a laboratory study carried out for many months in a modern pulp bleached Kraft factory. Laboratory procedures. Optimum conditions for the precipitation of organic salts Precipitation with commercial aluminium sulphate, A1 2 (S0 4 ) 3 , 18 H 2 0, occurs in an acid medium in a well defined pH zone close to 4.1. Above and below this zone, whatever the quantity of added aluminium, the reaction is not apparently improved. Table 2. Precipitation at different initial pH values initial pH optimum precipitation pH Colour (mg Pt/1) reduction % sludge volume after 30 mins in %

8.0 3.7 1780 63

9.2 3.85 1680 65

10.25 4.1 1100 77

11.2 5.2 2400 50

12.0 10.95 4800 0

52

53

63

32.5

0

The required dose of aluminium sulphate to attain the optimum pH depends on various factors amongst which the initial pH of the effluent to be treated and its buffering capacity; the dose might vary from 2000 to 4000 ppm. Definition of optimum conditions of precipitation of aluminium hydroxide The efficiency of the reaction is limited and a large quantity of aluminium ions as Al3+ still remain in solution. The second stage aims at precipitating it, in order to recover it later. When the pH rises, an aluminium hydroxide Al(OH)3 "floe" appears; when precipitated a complementary reduction in COD and quite an important colour removal take place. The optimum pH is observed at 5.4.

T \ 4000f | J ^ 3000-

30

2000-

20 Colour

1000-

10 reduction: 79% PH

Fig. 2. pH of Precipitation of Aluminium Organic Salts

0

598

J. Bebin, P. Boulenger and J.C. Bourdelot 4^

l§ ioofe Colour

90 .-JLQD

80 +

■30 70·

20

_^f-

60

110 50-

4

5

6

7 pH

Fig.3 pH of Precipitation of Aluminium Hydroxide

The efficiency of this second precipitation appears to be an increasing function of Al3+ content in excess after the first phase. It is possible to recover all of the aluminium ions by adding aluminium sulphate, before adjusting the pH value, but the process is somewhat lengthened. This observation makes it possible to accept, in the first stage, an excess of aluminium sulphate and consequently brings down the initial pH near to 3.8. Subsequent addition of soda or lime gives a decolorised effluent, the residual Al3+ content of which effluent does not exceed 5 mg/1. Table 3. Effect of aluminium sulphate on the dosage sodation effluent pH colour = 4450 mg Pt/1 aluminium sulphate organic salts precipitation hydroxide precipitation colour removal efficiency discharged Al3+

ppm pH pH %

3900 4.20 5.40 92.9 8

8.70

7300 3.95 5.40 97.8 0

Total aluminium recovery is expensive but a satisfactory result can be obtained by increasing from 30 to 35% the basic quantity of reagent, from the point of view of recovery and purification. Application of process The process previously described directly applies to the sodation effluent when the pH is reduced to 3.8 by adding the aluminium sulphate. However, the pH of the mixed chlorination-sodation flows, 2 to 2.5, is below the optimum precipitation pH. Thus the process has to be modified as follows by addition of an arbitrary aluminium sulphate dose which would not bring a change over the pH initial

Colour Removal from Bleached Kraft Pulp Waste Waters

599

value, and then adding alkali to increase the pH value to 5.4. Simultaneous precipitation of the organic matter and excess aluminium hydroxide occurs. This double precipitation can take place in the same tank divided in two compartments, provided there is ample contact time between the two successive applications of reagent doses. Sludge characteristics Organic salts and aluminium hydroxide are precipitated as a sludge of high water content; after 2 hours settlement and polyelectrolyte addition, it would represent a volume of 0.2 m3 with 3 kg of dry matter per cu.m of treated waste. The utilization of lime instead of soda to raise the pH value has the advantage of cheapness: it also improves sludge settlement. However, the presence of calcium ion in this type of sludge requires special attention. Care should be taken that the dose of Ca does not result in the solubility value of calcium sulphate being exceeded. Recovery of aluminium sulphate from settled sludge Our work shows the impossibility of recovering the aluminium from the sludge by concentration and acidification. A bad redissolution of aluminium and a reversion of colour is noted. To recover the aluminium the drawn off sludge is thickened, in order to raise the suspended matter concentration to 30-40 g/1. Further dewatering, by centrifugation and conditioning by anionic polyelectrolyte will bring the dry matter content up to 18 to 23%. Incineration Incineration of the dewatered sludge at the optimal temperature of 750°C requires some additional fuel. The residue is a white powder consisting mainly of alumina, A1 2 0 3 , and various impurities (especially Ca or Na). The alumina is redissolved in sulfuric acid to reform aluminium sulphate. Very little insoluble residue remains. Table 4. Aluminium sulphate recovery from the sludge 1. Precipitation introduced aluminium sulphate discharged aluminium sulphate with effluent

kg/m3 %

:

3.72 2.8

2. Incineration of the Sludge sludge weight in D.S. mineral matter content

kg/m3 %D.S.

: :

2.60 27.7

3. Aluminium Sulphate Recovery H 2 S0 4 weight recovered aluminium sulphate

kg/m3 %

:

1.85 93

Colour removal efficiency from pulp bleaching waste water The average percentage colour removal varies from 90 to 95%. In certain cases, higher efficiencies could be obtained, but at the price of a costly aluminium sulphate overdose. Proper doses are a function of the pH and of the concentration of coloured matters: for bleaching waste water from soda addition alone : 2500 to 4000 mg/1 for mixed bleaching waste water : 1800 to 3000 mg/1

600

J. Bebin, P. Boulenger and J.C. Bourdelot Table 5. Treatment by aluminium sulphate Colour, COD, BOD efficiency Sodation

Chlorinosodation

raw effluent characteristics : PH colour mg Pt/1 COD mg/1 BOD mg/1

10.8 3350 2400 430

2.4 1100 1090 -

treated effluent characteristics : PH colour mg Pt/1 COD mg/1 BOD mg/1

5.4 290 600 260

5.4 95 450 -

obtained efficiencies : colour % COD % BOD %

91.5 75 39.5

91.4 59 -

Efficiency in COD and BOD removal The BOD is not very much affected by chemical treatment. The COD reduction tends to show that a part of the colour removal is not only due to the removal of all the molecules responsible for the COD, but also some transformation of certain of these molecules. Reagent and energy consumption Table 6. Aluminium sulphate treatment. Power and reagent consumption Sludge production Mineral matter content

kg/t of pulp % D.S.

Reagent consumption . technical aluminium sulphate . lime . organic polymeres . sulphuric acid

kg/t of pulp

Power consumption . electrical . thermal

40 20 to 30 3.8 2 0.078 30

kWh/tofpulp thermies/t of pulp

See Fig. 4: aluminium sulphate process — basic diagram.

2 75.5

Colour Removal from Bleached Kraft Pulp Waste Waters

Effluent to

601

SETTLING

PRECIPITATION

be trea-ted

+ i < n v i f l n .  4 +    +i+ +#

A

THICKENING TANK

ot

+

injection|

t

CoSO^ fventuol

ALUMINIUM

SULPHATE

TALUMINIUM [RECOVERY

I

2SO4+H2O

.

J

1

!

1

+

T

\^~)

|

dump

I

«^JX^INCINERATION

Fig. 4. Treatment of Bleaching Effluents with Aluminium Sulphate - Flow Sheet

COMPARATIVE COST OF TREATMENT BY ALUMINIUM SULPHATE AND LIME Aluminium Sulphate Lime Characteristics Cost per ton of Characteristics Cost per ton of pulp in Dollars pulp in Dollars Investments . Equipment (clarifiers, sludge treatment, regulation units) . Civil Engineering Total Depreciation

355,000 65,000 420,000 on 10 years

0.57

Power . electrical (Dollars/kWh) . thermal (Dollars/thermie)

0.0145 0.0027

0.0273 0.21

Chemicals aluminium sulphate . lime . coagulation aid products sulphuric acid Labour Maintenance TOTAL WITHOUT DEPRECIATION

710,000 55,000 765,000 on 10 years

1.04 0.30 0.642

0.20 0.031 0.187 0.642 4% of annual capital

0.16 0.153 2.175 1.61

4% of annual capital

0.16 0.28 2.42 1.38

602

J. Bebin, P. Boulenger and J.C. Bourdelot

The respective estimated total costs of colour removal by either of these processes are given for a modern factory with a 300 tons/day production of bleached Kraft pulp. These calculations are based on a discharge of 13 m3 per ton of pulp, for a sodation effluent with the following characteristics: pH = 10.2 Colour removal: Reduction in COD:

colour = 3700 mg/Pt/1 COD = 2300 mg/1 Aluminium sulphate 93%, Lime 88% Aluminium sulphate 77%, Lime 69.5%

From these figures it appears that the cost including depreciation of the alumina process is very competitive with lime treatment and, moreover, it gives better purification. The breakdown of running and capital costs differs greatly. Chemicals, in one case, account for 50% of the total cost, and nothing in the other. The cost of the lime process is mainly due to energy consumption and equipment depreciation.

CONCLUSION Colour removal of paper-pulp effluents with aluminium sulphate is competitive with lime treatment. It gives efficient decoloration of the water. The pH of the effluent is then low enough to allow direct polishing treatment on granular activated carbon to be carried out and for further re-usal as make up water. Most of the colour and the non-biodegradable COD can be removed by this treatment applied separately to waste water from the bleaching section of a Kraft pulp factory; the other wastes are amenable to efficient biological treatment and can be treated separately.

REFERENCES 1 LESZCZYNSKI, C.Z., "Usuwanie barwy sciekow posiarczanowych" - Przegl. Papiern 26, No. 7,221-227(1970). 2 National Council of the Paper Industry for Air and Stream Improvement - Tech. Bull. No. 199 - N e w York (1967). 3 National Council of the Paper Industry for Air and Stream Improvement - Tech. Bull. No. 203 - New York (1967). 4 National Council of the Paper Industry for Air and Stream Improvement - Tech. Bull. No. 239 - N e w York (1970). 5 O'DONNELL, A.F. and KING, P.W., "The lime kiln in the Kraft recovery cycle" Appita 23, No. 6, 434-436 (1970). 6 THIRUMURTHI, D., McKENNA, G. and BOWN, H.G., "BOD and colour removal from Kraft mill wastes" - Water & Sewage Works 116, No. 12, 491-494 (1969). 7 US Patent Nr. 3, 120,464(1964).

Discussion by Matthew Gould As the authors point out, the only commercial processes currently in operation for kraft color removal are variants of the lime precipitating process originally proposed by Berger et al 1 ' 2 of the National Council of Air and Stream Improvement. Over the years laboratory work has been conducted by many researchers into the feasibility of an aluminum sulphate based precipitation process akin to lime discolorization 3 ' 4 » 5 . It is the opinion of this discusser that, although the various proposals are technically feasible, none of them are economically attractive unless a pulp mill is fortunate enough to be adjacent to an alum manufacturing plant6'7. The caustic extract, which is usually the second stage effluent of a conventional bleached kraft mill, is correctly identified as the main cause of color in the mill effluent. Caustic extract A.P.H.A. color values from 5000 to 20000 Pt-Co units are common, varying with wood species and both digester and bleach plant operating conditions. Treating caustic extract before dilution with less colored streams achieves the maximum color reduction, combined with minimal chemical consumption at lowest capital cost. The paper discusses the status of color removal with lime. However, without the benefit of acquaintance with the most recent studies, the information given is badly outdated. In particular, reference is made to errors introduced in color measurements by prefiltration 8 . The assumption made as to the mechanism of the lime precipitation process must surely be beyond a shadow of doubt, after the classic research by C. Dence, et al 9 . It is also disappointing that only the early Berger work is reported, invalidating the economic conclusions from the comparison of the alum and lime processes. This will be examined in more detail later. B.O.D. removal efficiences at around 40% are comparable for both lime and alum treatment. Laboratory studies confirm however, that slightly higher color removal is possible with massive alum doses in range of 2000 - 4000 p.p.m. This dosage in itself would represent a significant materials handling problem for the 600 - 1000 t.p.d. kraft mill, which is more typical of current industry practice, than the 300 t.p.d. example cited in the paper. At an effluent discharge of 13 m 3/T of pulp even the tiny 300 t.p.d. mill would process 11 — 17 ton of alum, on a dry basis, each day and be faced with dewatering four times this weight as dewatered sludge: assuming of course, that the notoriously sticky hydrated sludge can be thickened and dewatered to 25% solids successfully on a continuing basis. For the 1000 t.p.d. mill the problem would become correspondingly greater. The writer having gone through the agonies of evaluating centrifuges, vacuum filters and presses for handling difficult sludges, must emphasize once again that the transition from theory to practice is often fraught with misadventure. The prospect of having to install and operate a rotary or fluidized bed kiln, specifically for alum regeneration, will deter many mill managements, not to mention an additional sulphuric acid handling and dissolving system. This touches on a related aspect. Lime kiln operation and lime slaking and clarification are part of standard kraft pulp mill practice and, human nature being frail as it is, lime color removal is less frightening to most plant operators and management than the prospect of a new process that has seemingly little in common with pulp production. The prospect of calcinating alum at lower temperatures than lime is attractive, but the statement inferred in the middle of P.3 of the paper that the B.T.U. value of the organic matter associated with lime would have a lower B.T.U. value than essentially similar material associated with alum is unacceptable! One would also feel that the much lower moisture content of dewatered lime (50 - 60% solids) would more than offset any benefit from lower kiln operating temperatures for the alum sludge (18 - 23% solids). The report of the anomalous behavior of alum floe exhibiting only partial precipitation on initial pH depression to 3.8 and completing precipitation on restoring to pH 5.4 is of particular interest. One wonders however, if the apparently critical pH values and the need for a dual clarifier will tend to deter potential users from considering color removal with alum. Reliable and sophisticated control systems would appear an absolute necessity. Finally, I would refer to current practice with lime discolorization. There is one partial and three complete systems in operation in the U.S.A. Operation of the partial process, C. Davis °, is on total effluent of an unbleached mill and to date precipitated lime solids have been discarded. Dosage is in the 1500 p.p.m. lime range as contrasted with 15000 p.p.m. values used in the original massive lime process l ' 2 . The remaining systems all selectively treat caustic extract in bleached kraft mills and use lime in the 1500 - 3000 p.p.m. range 1 2 ' 1 3 . The two Georgia-Pacific mills I am familiar with, use a patented lime dewatering process and calcine the sludge through existing rotary kilns π ' 12 > 14 . A comparison of the actual operating cost data of these two mills with the cost projections in the paper shift the economics substantially in favor of lime and appear discouraging to future development of alum as a decoloring means in kraft mill applications. It should be noted that recarbonization is not practiced at these mills. The additional operating and capital costs of recarbonization are avoided as the lime lost in solution in the decolorized effluent is needed for pH correction in the final waste water from these mills. 603

604

Discussion REFERENCES

1 National Council for Air and Stream Improvement, Technical Bulletin No. 157 (1962). 2 U.S. Patent No. 31120,464(1964). 3 National Council for Air and Stream Improvement, Technical Bulletin No. 157 (1962) Ref: 11, 21, 34,35,36,37,38,43,46. 4 TYLER, M.A. & FITZGERALD, A.D. Review of color reduction technology in pulp and paper effluents, 58th Annual general meeting, Technical Section (1971) Canadian Pulp & Paper Association. 5 U.S. Patent No. 2,846,811(1958). 6 FULLER, R.R. Color removal from kraft effluents, Southern Pulp & Paper Manufacturer, September 10, 1971. 7 U.S. Patent No. 3,627,679(1971). 8 National Council for Air and Stream Improvement, Technical Bulletin No. 253 (1971) Instigation into improved procedures for measurement of mill effluent and receiving water color. 9 National Council for Air and Stream Improvement, Technical Bulletin No. 239 (1970) Mechanisms of color removal in the treatment of pulping and bleaching effluents with lime. 10 DAVIS Jr., C.L. TAPPI, Vol. 2, Page 1923 (October 1969) Tertiary treatment of kraft mill effluent including chemical coagulation for color removal. 11 GOULD, M. Physio-chemical treatment of kraft mill effluent, 25th Industrial Waste Conference, Purdue University, 1970. 12 U.S. Patent No. 3, 531, 370 (1970). 13 U.S. Patent No. 3,639,206. 14 GOULD, M. New lime process for removal of color from kraft mill effluent 8th Annual TAPPI Environmental Conference (1972). Discussion by R. L. Sanks* At the present time the most promising processes for the removal of color from kraft bleach wastes are: massive lime precipitation, carbon sorption, oxygen bleaching (which produces no color), ion exchange, and reverse osmosis. In the massive lime process patented by Berger, Gehm, and Herbet , the color is removed.by precipitation using the entire lime budget of the plant. The precipitated lime is then dewatered and used to recausticise the green liquor. The color bodies dissolve in the white liquor and are burned with the black liquor in the recovery furnace. The lime mud is recalcined in the kiln as usual. The advantages of the process are: little extra energy is needed, there is no interference from the color bodies, the loss of chemicals is small, and the process integrates easily into the existing pulp process so the only additional equipment is an extra sedimentation basin and, usually, a vacuum filter. The process is flexible. For example, a recarbonation step and a second sedimentation basin can be added to improve lime recovery. There are several difficulties in the reuse of bleach plant waste water including (in order of importance) slime, suspended solids, color, total dissolved solids, foaming, corrosion (due mostly to chlorides) and scale. Therefore, greater refinement such as, for example, sorption, ion exchange, or reverse osmosis is needed. Either granular or powdered carbon can be used for polishing the liquor after massive lime precipitation. A promising new development for treating either raw or pretreated liquor is the manufacture of carbon by "scorching** (partially burning) the black liquor to produce carbon, utilizing the carbon for the sorption of color, and then completely burning the carbon and sorbed organics for heat recovery. Ion exchange can also be used to polish the pretreated liquor, and the Rohm and Haas Company2 has developed a resin to decolor raw wastes. Demineralization can be accomplished by ion exchange or reverse osmosis. Pretreatment to protect membranes in the reverse osmosis process may not be necessary3. Color Removal with Lime At Montana State University, recycling and reuse of wastes is under study using bleach waste waters obtained from the Hoerner-Waldorf Corporation Pulp Plant in Missoula, Montana. Good housekeeping and extensive counter-current flow results in effluent volumes only one-third of those the authors report, and the color is approximately five times as great. Contrary to the authors' experience with poor settling of lime precipitates, the writer's experience (substantiated by many others) shows good settlement and color removals up to 95 percent or more. * Professor of Civil Engineering and Engineering Mechanics, Montana State University, Bozeman, Montana.

Discussion

605

The authors objected to the large addition of fuel necessary to incinerate sludge for lime recovery. In American practice as previously described, the "sludge" is just a portion of the slaked lime and is reburned only after it has become lime mud. The extra energy required is almost negligible. Another objection to the use of the massive lime process (not given in the paper) was the fear that an increase of chlorides would result in excessive corrosion. On this supposition the designers were willing to separate the lime sludge recovery process from the rest of the mill, and thereby to add all of the equipment shown in Figure 1, but by integrating massive lime precipitation into the mill process, the only equipment really needed is settling tank No. 1 and perhaps the vacuum filter. During a 16-month demonstration project by International Paper Company 4 at Springhill, Louisiana, it #as shown that green liquor normally contains approximately 80 mg/1 chloride. The addition of the massive lime process added only 20 mg/1 to give a total of 820 mg/1. This increase of chloride is negligible. If the increase of chloride in the lime sludge is feared, it would be easy to wash the sludge on the vacuum filter to eliminate all but traces of chloride. Hence, it seems unreasonable to base costs for massive lime precipitation on a non-integrated process, because integration is the major appeal of massive lime precipitation. On the other hand, the cost of alum precipitation must be based upon all of the process equipment shown in Figure 4, because it cannot be integrated into the pulping process. Alum Recovery Apparently the authors were able to recover only 92 percent of the alum in the laboratory. Considering the massive doses used it appears that about 500 mg/1 of alum is not recovered and this, together with the sulfuric acid required, represents a significant cost. Furthermore, laboratory experience with alum sludge is deceptive. Alum sludge is quite difficult to treat under full-scale conditions. Costs The comparative costs should have been based on the most economical feasible practice for each process. It is regrettable the authors did not give complete information on sizes of reactors and all other factors used in estimating comparative costs. It is not easy to compare costs in one country with costs in another unless these data are provided.

REFERENCES 1 BERGER, H.F., GEHM, H.W. and HERBET, A.J., "Decolorizing kraft waste liquors" - U.S. Patent 3,129,464 (Feb. 4, 1964). 2 Rohm and Haas Company - "Decolorization of kraft pulp bleaching effluents using Amberlite XAD-8 polymeric adsorbent" - Philadelphia (1967). 3 WILEY, A.J., AMERLAAN, A.C.F. and DUBEY, G.A., "Application of reverse osmosis to processing of spent liquors from the pulp and paper industry" - TAPPI 50, No. 9, 455-460 (1967). 4 International Paper Company - "Evaluation-demonstration of the massive lime process for the removal of color from kraft pulp mill wastes" - EPA Project No. 12040 DYD (in press). Reply The essential part of the criticism made by my two discussers is based on an economical comparison between lime and alum processes. However, Mr Gould surprises me by refering to Dence's studies on lime precipitating process since I gave a summary of the conclusions of this study and included it as a reference. It is a matter of fact that if just a partial color removal from bleached kraft pulp waste waters is wanted, it is possible to simplify the lime process, avoiding recarbonation, integrating calcination of sludge in the lime manufacturing works and so obtaining a very competitive treatment cost. But in our case, the research which a French group of papermills asked us to carry out had to be a very elaborate process allowing — thanks to final treatment with activated carbon - a high quality water fit to be reused to be obtained. In these conditions, we devised a process giving very high color removal efficiency, more than 90% both with the sodation effluent and the chlorination - sodation mixed effluent. The favourable pH for treatment with activated carbon being near 5,4 one sees that the comparison can be done only with a very complete lime process including recarbonation step. In contrast to American practice French pulp mill owners asked us not to integrate sludge calcination systems with devices existing in the mill for fear of corrosion by chlorides. So in answer to Mr. Sanks we employed the following materials:

606

Discussion

For the alum process:- one floculation device with pH regulation; - one scraped sedimentation tank, calculated on a load of 0,4 - 0,5 m 3 / m 2 / h r ; - one scraped thickening tank calculated on a 40 kg SM/m 2 /day basis; - two centrifuges, ,0 43 cm continute decanters each allowing to treat 300 kg SM/hr, - one rotary kiln treating 2300 kg of wet solids (25% dryness) per hour at 750°C with dust washing of fume, - one alum recovery unit with reagents tanks, dosering pump, stirrer, etc. For the lime process: - one lime dosing device; - one scraped sedimentation tank, calculated on a load of 1,2 - 1,5 m 3 / m 2 / h ; - one 6000 m 3 high-pressure blower for injection of dry flue gases rich of CO2, Counterpressure : 4,5 m of water; - one second scraped sedimentation tank calculated on a load of 0,8 - 1 m 3 / m 2 / h ; - two rotary vacuum filters with accessories, calculated on a 25 kg dry matters/m 2 /h basis; - one rotary kiln treating 5500 kg DM/hr at 1000°C. it is to be noted that contrary to Mr. Gould's belief, we did not think that the alum sludge organic solids has a calorific power higher than the sludge coming from lime treatment, but only that alum sludge are richer in organic solids (70%) than lime sludge (25%). This is why it is possible to approach autocombustibility with a dryness lower than that obtainable with lime sludge. As for the working expenses, 92% efficiency for alum recovery is an estimated one, for we obtained in the laboratory easy 100% recovery. We hope to reach 97% recovery. It will be pointed out that we calculated the sedimentation tank to be more efficient with lime than alum. But from our experience and that of French pulp mill owners, this can only be applied if treatment with a lime and Ca carbonate mixture is used. We have had to use 5500 ppm lime and 8000 ppm Ca carbonate doses in sodation effluent - 3000 and 4000 ppm in chlorination - sodation mixed effluent, and obtained only respectively 86 and 83% color reduction in these two effluents. So we are far from the 95% efficiency mentioned by Mr Sanks. We think that the economical comparison, favourable to alum process, applies when a high color removal efficiency is wanted. We were recently informed that in the USSR in Brask near by the Baikal Lake, of an alum process similar to ours. They required a very high efficiency process to preserve their lake and have retained the alum process.