Chemical pretreatment before biological treatment in sewage plants

Water Research Pergamon Press 1973. Vol. 7, pp. 227-247. Printed in Great Britain

CHEMICAL PRETREATMENT BEFORE BIOLOGICAL TREATMENT IN SEWAGE PLANTS BERNT ERICSSON VBB-Vattenbyggnadsbyr/~n, and The Royal Institute of Technology, Stockholm, Sweden

Abstract--At present the authorities in Sweden seem to favour chemical post-treatment after biological treatment. Interest in simultaneous chemical and biological treatment has diminished considerably since the successful operation of chemical pretreatment with alum before biological treatment in Stockholm. The pre-precipitation method seems to have both advantages and disadvantages, e.g. removal of poisonous and disagreeable material respectively increased plant supervision owing to more variable influent flow and concentrations compared with the effluent at a plant without chemical precipitation. Test operation with pre-precipitation is described in two cases in the paper. The assumptions for successful operation with pre-precipitation were quite different, and the results show that the content of BOD after pre-sedimentation is the most critical factor in the activated sludge process. In addition to pH, the content of soluble phosphorus in sewage after pre-sedimentation seems to be more important than total phosphorus. The influence of low concentrations of BOD and soluble phosphorus has been tested in a laboratory model of the activated sludge process. INTRODUCTION WAVER preservation in Sweden during the last two decades has undergone a rapid development. In m a n y watercourses in Sweden the high content of nutrients, especially of phosphorus, required measures for phosphorus precipitation in sewage to reduce the algal blooms, vegetation turbidity and excessive weed growth (eutrophication). On July 1, 1971 we had 80 chemical treatment works in operation. It has been estimated that about half the population of built-up areas in Sweden will be connected to chemical treatment works within a b o u t 3 yr. In sewage plants with biological treatment the chemicals for precipitation of phosphates can be added according to the following three alternatives: I. Pre-precipitation II. Simultaneous chemical precipitation and biological treatment III. Post-precipitation. At present the authorities in Sweden seem to favour chemical post-treatment after biological treatment. The reasons for advocating the more expensive post-treatment process in old biological treatment works are as follows: 1. N o interference with the activated sludge process and a more regular quality of the effluent especially in respect to organic matter, biological parasites such as tapeworms and heavy metals. 2. Better possibilities of obtaining a higher purification by pH-adjustment and flocculation. 3. Chemical sludge separated from other sludges requiring stabilization. Pre-precipitation with alum ( 8 ~ A1) started in June 1969 in a sewage plant at Loudden, Stockholm, designed for 30,000 people. Since the successful operation of 227

228

BER:crE~cssox

chemical pre-treatment, interest in simultaneous chemical and biological treatment has diminished considerably in Sweden. The pre-precipitation process is now in operation at four additional sewage treatment works in Stockholm (HOK~VAtL, 1971). Pre-precipitation studies have been reported in literature by many authors. In the U.S.A. lime is used most as pre-precipitant (ALBER-rSON and SHERWOOD, 1969; SCHMtD and MCKINNEY, 1969; and others). Trial operation with pre-precipitation was performed in the sewage plant at Botkyrka near Stockholm designed for 20,000 people, in the middle of 1971, and in the sewage plant at Simsholmen in JrnkSping designed for 100,000 people, at the beginning of 1970. The biological treatment work at Botkyrka is planned to be in operation only until 1974 when the large treatment work at Himmersfj~irden, now under construction, will replace some small sewage plants. Consequently the purpose of the trial operation in Botkyrka sewage plant was to examine the possibilities for phosphorus removal by pre-precipitation for the remaining operation time. In the case of the biological treatment work of Simsholmen, situated on Lake Vetter in the south of Sweden, the problem was not the same. Chemical precipitation was planned, and the experiment in this case was intended to compare pre-precipitation and post-precipitation. VBB--Vattenbyggnadsbyrfin has been commissioned to work out convenient methods, choice of chemicals and the extent of the experimental work. Experimental work on pre-precipitation on a laboratory scale has also been carried out recently at the Department of Water Supply and Sewerage and Water Chemistry of the Royal Institute of Technology in Stockholm. The results of the trial operation and the laboratory work have been described below. EXPERIMENTAL RESULTS FROM BOTKYRKA SEWAGE PLANT

Reference period The reference period ranged over 1 working-day (11/18-11/19 1970) and one holiday

(12/5-12/6 1970) between 0800 and 0800 hours. During the testing periods some samples taken manually at random, some daily samples selected by automatic samplers were taken out. The average values of the analyses together with variation intervals within brackets are compiled in TABLE 1 for the influent, the sewage after pre-sedimentation and the effluent. At Botkyrka sewage plant the anaerobically stabilized primary and excess sludge is centrifuged, and the reject from the centrifuge, about 5 m 3 h-1, is recycled to the pre-aeration tank between 0800 and 2000 hours. According to the analysis results during this time the contents of suspended matter, BOD7, and total phosphorus in the influent increased on November 18 by 5 mg 1-1, 5 mg 02 1S1 and 1.2 mg P 1-1, respectively, on the average (cp. TABLE 1). The corresponding values on December 5 between 0800 and 1700 hours were 8 mg 1-x, 5 mg 02 1-~ and 0.7 mg P 1-1. The analysis results in TABLE 1 show that the phosphorus removal during the presedimentation is slight (surface loads: 2.0 m 3 m -2 h -~ on 11/18-11/19 and 2.4 m 3 m -2 h -1 on 12/5-12/6"). The reduction of BODy during the pre-sedimentation is lower than normal, only about 20 per cent, due to the comparatively high surface load. In the whole sewage plant the reduction of BOD7 and total phosphorus were 56 and 31 per cent respectively during the working-day and 64 and 31 per cent respectively during the holiday. The low BOD-reduction might depend on nitrification in

Chemical Pretreatment Before Biological Treatment in Sewage Plants

229

TABLE 1.

RESULTS OF ANALYSES DURING THE REFERENCE PERIOD AT BOTKYRKA WITH VARIATION INTERVALS IN BRACKETS

Wednesday- SaturdayThursday Sunday 11/18-11/19 12/5-12/6

Influent (excluding reject from centrifuge) Flow (m 3 h - t) pH BODy (total) (mg Oz 1- t) BOD7 (soluble) (mg Oz 1-1) Phosphorus (total) (mg P 1-t) Phosphorus (soluble) (rag P 1-t) Suspended matter (mg 1- t) DO. loss of ignition (mg l - t )

331 (158--460) 7-3 (6-9-7"6) 211 (115-301) 57 7.0 (3.6-9.8) 4.4 (2.2-7.8) 73 60 (82~)

405 (202-546) 7.1 (6.9-7.1) 154 (31-205) 49 3"2 (0-98-5.8) 2.1 (0"88-5"12) 58 51(88 ~ )

7.3 (7.1-7.4) 172 70 8"6 5-4

7.0 (6.8-7-1) 130 59 3'9 3-0

7.3 (7-0-7.4) 94 35 4'8 3.8

7.1 (6-9-7.1) 56 12 2-2 2.1

Sewage after pre-sedimentation pH BOD7 (total) (mg O2 I-t) BODy (soluble) (mg Oz 1-1) Phosphorus (total) (mg P 1- t) Phosphorus (soluble) (rag P 1-t)

Effluent pH BOD7 (total) (rag Oz 1-t) BODy (soluble) (mg Oz 1-1) Phosphorus (total) (rag P 1-1) Phosphorus (soluble) (mg P I-t) Suspended matter (rag 1- t)

11

7

Working doy --

Holidoy

15C

% Io0

5¢

1 0730

T I 1130

l I 1530

I 1930 "rime

I

I 2330

I

I t 0330

[ 0730

FIG. 1. Effluent flow variations, expressed in ~ of the daily mean value during working-day

and holiday, respectively (2 h mean values).

230

BERNT ERICSSON

Working day - - HolidQy

200

i

150

%

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100

5o m

07:550

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1130

1530

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I 1930

Itl

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Time FIG. 2. Phosphorus flow (kg P h - z) variations, expressed in ~o of the daily mean value during working-day and holiday, respectively (2 h composite samples).

Working day

200

Holiday

150

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FiG. 3. BODy flow (kg 0 , h-1) variations, expressed in % of the daily mean valu¢ during working-day and holiday, respectively (2 h composite samples).

Chemical Pretreatment Before Biological Treatment in Sewage Plants

231

unfiltered samples during the BOD-analyses o f effluent samples. As is shown in TABLE 1, the suspended BOD 7 in the effluent is remarkably high compared with the content of suspended matter. This is an indication of a higher degree of nitrification in the BOD-bottles for unfiltered samples than for filtered samples. A further investigation of this question is discussed on p. 234. In FIG. 1 the percentual deviations in the effluent flow from the average value (equal to 100 per cent) are shown for the working-day and the holiday. Analogous diagrams have been drawn showing the variations in phosphorus flow, FIG. 2, and in BOD 7 t].ow, FIG. 3, in the influent. The phosphorus and BOD7 flow are expressed in kg P h -~ and kg 02 h -~, respectively. The analyses of sludge samples from the aeration tanks and the return-sludge were carried out on samples taken out every four hours. The mean value of sludge volume index was 300 ml g - x during the working-day and 390 ml g-1 during the holiday. The corresponding mean values of suspended matter were 2000 and 1800mg 1-~, respectively. The calculation of the sludge production was based on the increase in the content of suspended matter in the aeration tanks, when no excess activated sludge was taken out. It was not possible to measure the excess activated sludge separately. The sludge production was estimated at 1.1 kg suspended matter per kg BODsreduction during the working-day and 0.8 during the holiday. The corresponding sludge ages were estimated to be 3.5 and 3.9 days, respectively. The calculated values are uncertain, partly owing to floating sludge, which is separated with a partition wall in the clarification tank, partly owing to the nitrification, which might occur in the BOD-test. However, the values of the sludge age are normal for a conventional activated sludge process. Besides the testing and analysing mentioned above, metal analyses were also performed. The results are compiled in TABLE 2. TABLE 2.

Date 11/18-il/19 (0800-0800)

Sample Influent

METAL ANALYSES IN SEWAGE SAMPLES AT BOTKYRKA

Calcium Zinc (mg Ca 1- t) (mg Zn 1-1) Total Soluble Total Soluble

Iron Aluminium (mg Fe 1-1) (mg AI 1- l) Total Soluble Total Soluble

36

36

0.27

0-25

1.1

0.4

0.45

0.24

Pre-settled sewage 36 Effluent 38 11/18 Reject from (0800-2000) centrifuge 19

37 37

0.35 0.19

0.30 0.18

1.I 0.4

0.4 0-2

0.45 0.15

0.17 0-11

14

0"10

0"04

0"7

0.2

0.26

0"10

As reported, the total hardness is 100-125 mg 1- ~ CaCOa at Botkyrka. The conditions for a very high phosphorus reduction in the activated sludge process seem to be lacking depending not only on the relatively low content of metals in the influent compared with the conditions at, for example, San Antonio according to JENKINS and ME,tAg (1968) but also on the dimensioning and the operation of the activated sludge process, which is not suitable for storing phosphorus in the bacteria cells, so-called "luxury consumption" (LEv~ and SHAPIRO, 1965; BAROMAN et aL, 1970).

BER3rf Etucssoy

232

Experiments on chemical precipitation on laboratory scale During working-days on 12/7-12/9 flocculation and sedimentation tests were carried out with a 60-I. influent sewage sample in a long-tube of 29 cm dia. and 150 cm height. The gate-stirrer (35 rev. m i n - 1) was connected for 20 rain after the addition of chemicals. After sedimentation samples o f the clarified water were taken out corresponding to the simulated surface loads 3.0, 2.0 and 1.0 m 3 m -2 h -1. The results

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FiG. 4. The reduction of total phosphorus vs. chemical dosage with the surface load as parameter (1-0-3-0 m 3 m -2 h - t ) .

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FIG. 5. The reduction of total phosphorus vs. the dosage of slaked lime with the surface load as parameter (1.O-3-0 m ~ m - " h-L).

Chemical Pretreatment BeforeBiologicalTreatment in Sewage Plants

233

with alum and ferric chloride as precipitants are illustrated graphically in FIG. 4 and with slaked lime in FXG. 5. In FIG. 4 equivalent amounts of alum and ferric chloride are marked on the abscissa and mg 1- ~ total phosphorus (unfiltered samples) on the ordinate. The influence of the surface load is apparently small in the precipitation with ferric chloride. This is of course an advantage at Botkyrka sewage plant with respect to the big variations in the influent flow owing to the operation of the pumps. According to the experimental results, which indicate rather bad precipitation conditions, the following approximate chemical dosages are required to reduce the concentration of soluble phosphorus to about 1 mg P 1-~: 120-150 mg 1-1 alum, AI 2 (SO,)3 x 18 H20 (pH 6"8), 90 mg 1- t ferric chloride, FeC13 × 6 H20 (pH 7.1), 150 mg 1-~ slaked lime, Ca(OH)2 ( p n 9.6). The values of flocculation-pH have been stated in brackets above. The results show that precipitation in the case of soluble phosphorus is more pH-sensitive in precipitation with alum than with ferric chloride. According to FIG. 4 the flocculation and sedimentation is feasible within a broader optimum pH-interval in precipitation with ferric chloride compared to alum. This result is in accordance with earlier experimental works on post-precipitation (ERICSSON, 1971). The best pH-range in precipitation with alum is usually reached around pH 6.0, but the width of the optimum pH-range is influenced by the composition of the sewage, alum dose, flocculation and sedimentation. The surface load in the pre-sedimentation tank is at average flow at Botkyrka sewage plant about 2 m 3 m -2 h -~. At this surface load the total BOD7 is reduced, according to the precipitation experiments, by about 40-65 ~ at a dosage of 90-150 mg 1-x alum, 65-80 ~o at a dosage of 90-150 mg 1-~ ferric chloride and 65-70 ~o at a dosage of 150-250 mg 1- ~ slaked lime. Without addition of chemicals the corresponding reduction was 40--45 per cent. On the basis of the results of the experiments and the analyses described above ferric chloride was chosen as the precipitant for the subsequent trial operation with pre-precipitation. Ferric chloride is supplied as a concentrated solution containing 70 ~ FeC13.6H20 by weight. As an alternative precipitation method in this case the simultaneous chemical and biological treatment with the cheap waste product, ferrous sulphate FeSO4.7H20 ("copperas") may be mentioned. This process has given good results in Finland and Sweden at small sewage treatment works. The ferrous iron, Fe 2÷, is oxidized during the aeration in the activated sludge process to the ferric state, which has also been shown by laboratory experiments carried out earlier. The storing of ferrous sulphate in silo causes, however, certain problems.

Trial operation with pre-precipitation at Botkyrka The trial operation included one control period (6/1-6/7 1971) and one experimental period (6/8-6/24 1971) with a dosage of 90 g FeCI3.6H20 as ferric chloride solution (70 ~) per m 3 influent, according to the planning, before the pre-aeration tank but after mechanically cleaned screen. The chemical dosage was adjusted at 0800 and 2200 hours based on the day and night flow and nearest corresponding day and night,

234

BERberEaicsso.~

respectively. The variations in the flow and the ferric chloride dosage are shown on FIG. 6. The average daily flow during the control period was 250 m 3 h - i (227-264 m 3 h - 1) and during the test period 300 m 3 h -1 (246-358 m 3 h - l ) . Owing to the pumping frequency to the sewage plant the surface load in the pre-sedimentation tanks during pumping periods will be about twice the average value. The mean value of the chemical dosage during the test period was 84 g FeC13.6H20 m -3. The comparatively wide variations in the ferric chloride dosage depend on deviations between the real and the calculated flow and also on irregularities and very occasionally pumping interruptions. During the time 6/15-6/22 an operation interference arose in the clarification tank through leakage adjacent to an outlet pipe for clarified water to the outlet channel. The analysed effluent thereby acquired a considerably higher turbidity and residual content of suspended phosphorus. During the control period and the test period the daily samples were taken out with atttomatic samplers on Tuesday-Wednesday and Thursday-Friday (working-days) and also on Saturday-Sunday (holidays) between 0800 and 0800 hours. In addition samples have been taken from the aeration tanks, the outlet of the aeration tanks, and the return-sludge at 1000 and 1500 hours for the determination of dissolved oxygen content, sludge activity, settled volume and suspended matter. During certain days the redox potential in the aeration tanks was also measured. During the control period the contents of suspended matter, BOD 7 and total phosphorus were as follows:

Working-days p-sed Susp. matter (mg 1-~) 93 BOD7 (total) (mg 02 1-1) 146 BOD7 (soluble) (mg 02 1-1) 57 Phosphorus (total) (mg P l -t) 4.7 Phosphorus (soluble) (mg P 1- t) 2-5

Holidays

The whole period

effl.

p-sed

effl.

p-sed

effl.

36 55 16 3"4 2"5

70 146 73 5.5 4-2

15 35 17 3"8 2.7

88 146 62 4.9 3"0

31 49 16 3"5 2"5

p-sed = after pre-sedimentation. effl. = effluent. The reduction of BOD7 in the plant during the control period was 80 per cent during working-days and 91 per cent during holidays. The interference of nitrification in the BOD-tests during the control and test period was minor compared with the reference period. This was shown according to WOOD and Mov,gls (1966) and MONTGOMERY and BORNE (1966) by the addition of thio-allyl-urea as nitrification inhibitor. The total BODT-values concerning pre-settled sewage decreased by 11 per cent and by 23 per cent for effluent on the average compared with the BODT-analyses without inhibitor. The ratio BODT:BODs in samples of pre-settled sewage without inhibitor was 1.22 on the average, which is normal. The relationship between BOD7 and Chemical Oxygen Demand (COD) is shown on FIG. 7. As the BOD test is not suitable if a quick analysis or continuous measurement is required, it is important to measure the correlation between COD and BOD at each separate plant.

Chemical Pretreatment Before Biological Treatment in Sewage Plants

500

-

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400 --

FI

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-

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Ferric chloride dos.

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120 E rio o

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15 14 15 16 17 18 19 20 21 22 2324 D o t e , June 1971

FiG. 6. The variations in the influent flow (measured as effluent flow) and the ferric chloride dosage as well as the conditions of precipitation during the control and test period. T h e m e a n values o f the contents o f s u s p e n d e d m a t t e r , BOD7 a n d t o t a l p h o s p h o r u s d u r i n g the test p e r i o d are c o m p i l e d below. Working-days

Susp. matter (mg 1-t) BOD7 (total) (mg Oz 1-1) BOD~ (soluble) (mg 02 l - t ) Phosphorus (total) (mg P 1-1) Phosphorus (soluble) (mg P 1-t)

p-sed

effl.

82 128 63 3-1 1"4

30 41 16 1-2 0.65

Holidays p-sed 56 99 48 2.3 1.1

The whole period

effl.

p-sed

effl.

18 48 15 0.34 0.16

78 120 58 2"9 1.3

28 43 15 0.95 0-51

p-sed = after pre-sedimentation. effl. = effluent. T h e influence o f the leakage in t h e clarification t a n k is very obvious, as is evident f r o m the d a t a concerning the effluent on w o r k i n g - d a y s given below. Leakage did Leakage did not occur occur Susp. matter (rag 1-t) BODy (total) (rag 0 2 1 - t ) BOD7 (soluble) (mg 02 1- t) Phosphorus (total) (nag P 1-~) Phosphorus (soluble) (rag P 1-t)

13 26 15 0"68 0"54

53 62 17 1"95 0-82

236

BERNT ER~CSSOX

× Tnfluent

Sewage after p r e - s e d i m e n ~ c Effluent × / K Con~'rol period ( 6 / I - - 6 / 8 ) ~ x /

400

S

~,

3OO

x

x

E E 0 ~J

200

100

/ T I00

300

200 BOD,

,

mg

L-r

FIG. 7. Chemical Oxygen Demand (COD) vs. biochemical oxygen demand (BOD~) during the trial operation at Botkyrka sewage plant.

Based on the BOD-reduction in the activated sludge process and the ratio of about 90:1 between reduced BOD 5 and assimilated phosphorus during the bacteria metabolism, the normal biological phosphorus reduction can be calculated. During the control period this uptake of phosphorus will be about 1.0 mg P 1- ~ and during the test period about 0.7 mg P 1-:. The soluble phosphorus in pre-settled sewage should be easier to assimilate by the micro-organisms than the suspended phosphorus. The soluble phosphorus after chemical pretreatment is, obviously, present in sufficient amount in this respect. The chemical pre-treatment with ferric chloride in comparison with the control period and regarding the actual leakage has given a substantial lower content of total phosphorus, BOD7 and suspended matter in the et~uent. The concentration of total iron in the effluent during the test period did not increase except on the day when the leakage occurred (2.5 mg Fe 1-1 on 6/15-6/16). The results of the analyses of samples taken from the aeration tanks and the outlet of the aeration tanks have been summarized in TABLE 3. From the table it is evident TABLE 3. ANALYSES OF THE ACTIVATED SLUDGE PROCESS

Suspended matter (mg 1- I )

DO. loss of ignition (Yo)

Settled matter (ml l - l )

SVI (rrd g - l )

pH

2770 2490

86 76

660 290

250 120

6.7 6-8

Outlet of the aeration tanks Control period 2570 Test period 2340

86 77

640 260

255 115

6"8 6"8

Aeration tanks Control period Test period

Chemical Pretreatment Before Biological Treatment in Sewage Plants

237

that the chemical pre-precipitation has had a favourable effect on the sludge volume index and has very slightly affected the pH-values in the aeration tanks. In this respect it may be mentioned that pH will rise slightly after the addition of ferric chloride in the pre-aeration tank owing to carbonic acid stripping. As far as the activity of the sludge is concerned, the oxygen and dehydrogenase activity decreased a little during the test period with chemical pre-precipitation compared with the control period (see FIG. 8). The redox potential, i.e. the oxidation capacity, increased in the aeration tanks during the test period. .= E -24-~) 2.2 -~

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Chemical pretreatment with Fe CL3 I I I I I I I I 1 I I I I I I I I ! I I ! I 2 34 5 (5 7 8 91011 1213 141.516171B I9 20 212~ Dote, June 1971

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Date June 1971

Fla. 8. Sludge activity measurements during the trial operation at Botkyrka sewage plant.

FIGLrl~ 9 shows the contents of iron and phosphorus in sludge from the outlet of the aeration tanks, expressed in percentage of suspended matter. As might have been expected, the amount of iron increased more than that of phosphorus. This is associated with the incomplete separation of the chemical sludge in the pre-sedimentation tanks. It has not been possible to measure the amount of excess activated sludge taken out during the trial periods. However, this amount does not depend only on the biological

238

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8 9 I0 II 12 13 14 15 16 17 18 1920 21 27' Date, dune 1971

FIG. 9. The amounts of iron and phosphorus expressed in ~ of suspended solids in the outlet of the aeration tanks, during the pre-precipitation with ferric chloride at Botkyrka sewage plant. sludge production but also on the amount of inorganic, suspended solids in presettled sewage and the sludge flight from the clarification tanks. Although the separation of the suspended matter in the pre-sedimentation tanks was improved through the admixture of ferric sludge according to the tabulations already shown, the content of inorganic suspended solids in pre-settled sewage increased from about 19 to 25 mg 1- t during working-days. As has also been pointed out, the content of suspended matter in the effluent was lower during the test period than the control period. These two changes consequently contribute to more sludge in the aeration tanks contrary to the biological sludge production, which decreases during the test period owing to the lower BOD-reduction in the activated sludge process. The amount of dry matter in the mixed sludge taken out for digestion during the control and test period was 4.0 and 3.9 per cent respectively (loss of ignition 74 and 70 per cent respectively). The thickening characteristics of the raw sludge did not apparently change appreciably after pre-precipitation with ferric chloride. The amount of mixed sludge during the test period corresponded to a digestion time of about 10 days. Even when the operation of the sewage plant is normal, the detention time of digestion is small depending on overload. Facilities to dry the sludge after centrifuging have now been installed. In the pre-precipitation process with ferric salts special attention should be paid to the question whether or not phosphorus release from the chemical sludge occurs during the anaerobic stabilization. THOMAS (1967) has stressed that no phosphorus release will occur in the digestion process with ferric sludge. However, reduction of suspended ferric iron to the ferrous state seems to be probable on a larger or smaller scale depending on the operation conditions. The possibility of phosphorus release accordingly also exists, owing to competing anions, e.g. sulfide, and the solubility of ferrous phosphate in the neutral pH-region. This was investigated in the trial operation at Botkyrka by analysing the reject from the centrifuging of the digested sludge. The concentration of soluble phosphorus during the control and test period was, on average, 73 and 59 mg P 1-~ respectively. Consequently phosphorus release out of

Chemical Pretreatment Before Biological Treatment in Sewage Plants

239

ferric sludge did not occur at all but, on the contrary, some precipitation or adsorption of phosphorus released from biological sludge did occur. CONCLUDING

REMARKS

REGARDING THE EXPERIMENTAL AT BOTKYRKA

WORK

The trial operation with pre-precipitation at Botkyrka sewage plant during the period 6/8-6/24 1971 is short, and definite conclusions may not be drawn. The results of the operation were, however, generally good and indicate that the total phosphorus content in sewage effuent with a uniform dosage at about 90 mg 1-1 FeC13.6H20 can be reduced to about 0.5 mg P 1-1. This presupposes, however, a more uniform chemical dosage than that of the trial operation. The chemical dosage should be regulated by the pumping operation of the sewage from the pumping station in order to attain an accurate dosage. At present the pumping operation will give about twice the surface load in the pre-sedimentation tanks based on the average flow. This ratio will be decreased by turning the pumping wheel to reduce the pumping capacity and. consequently also the flow peaks. No adverse effect was observed on the activated sludge or digestion process regarding the BOD-removal and the decomposition of organic matter, respectively, but the activity of the sludge was somewhat decreased. The amount of mixed sludge taken out for anaerobic stabilization will increase, of course, in the case of chemical preprecipitation, in the order of magnitude 45 g dry matter m- 3 sewage. No phosphorus leakage from the chemical sludge occurred during digestion. The concentration of soluble phosphorus in the supernatant, on the other hand, was lower during the test period than the control period. The use of a cationic polymer, Praestol 444K, at the centrifuging of digested sludge containing ferric sludge did not seem to work as well as without chemical sludge. Other polymers, e.g. anionic polymers, will also be tested when a suitable opportunity arises. Based on the results, operation with chemical pre-precipitation using ferric chloride (commercial quality 2, made by chlorination of spent pickling liquor) started at Botkyrka sewage plant on April 12, 1972. The daily flow was initially about 50 per cent higher than the yearly average value (8500 m 3 day- 1). The chemical dosage was regulated at 50 g m-3 FeCI3.6H20 automatically, based on the flow during the gotime of the pump at the pumping station. During the first month after the start, the reduction of total phosphorus and BOD7 were both 90-95 per cent. The operation of the plant was markedly improved by the pre-precipitation. The concentration of total phosphorus and iron in the effluent varied between 0.25-0.50 mg P 1-1 and 0.2-1.5 mg Fe 1-1, respectively. The high phosphorus reduction with the comparatively small ferric chloride dose probably depends mostly on the proper chemical dosage mentioned above but also on the dilution of the sewage by a larger amount of storm-water than normal. CHEMICAL

PRE-PRECIPITATION

AT JONKt~PING

Introductory remarks The trial operation with chemical pre-precipitation using alum in Jrnkrping sewage plant was carried out during short periods in February and March 1970. The w.R. 7l 1-2---Q

240

BER.'cr ERICSSON

pre-precipitation had to be stopped owing to interference with the activated sludge process. For this reason the results will be discussed very briefly below.

Operation of the plant without chemical pre-precipitation Analyses for operation control during the year 1969 are shown in TABLE 4. The amount of excess activated sludge taken out was estimated to be 740 kg suspended matter day-1. If suspended matter in the effluent is included, the total amount of excess sludge was about 1700 kg day -~, corresponding to only 0-33 kg suspended matter kg -~ BOD~-reduction. In 1970 and 1971 the sludge production in the activated sludge process was even smaller. The remarkably low sludge production is an important feature as far as chemical pre-precipitation is concerned. This may be due to the admixture of industrial water which is evident from the ratio of BODs : P equal to 52:1 in the influent compared with the normal ratio of about 20:1 in Sweden, and the interference of heavy metals, e.g. zinc, with the activated sludge process. However, this question has not been investigated in detail. TABLE 4. OPERATION CONTROL ANALYSES IN 1969

[nfluenl Flow (m 3 h -1) BOD5 (total) (mg 02 1- t) Phosphorus (total) (mg P I- t) Suspended matter (rag 1-1) DO. loss of ignition (rag I-t)

1770 230 4"4 119

85

(72 %)

140 61 38

(62%)

Sewage after pre-sedimentation BODn (total) mg (02 1-1) Suspended matter (rag 1-~) DO. loss of ignition (mg t-t)

Clarified sewage BODs (total) (mg 02 l -t) Phosphorus (total) (mg P 1-1) Suspended matter (mg 1-t) DO. loss of ignition (rag 1-t)

17 2.2 23 13

(57 %)

Aeration tanks Suspended matter (mg 1-1) Settled matter (ml 1-t) Sludge volume index (ml g-1) Sludge load (kg BOD5 kg susp. matter-i day-~) Sludge age (days)

1900 140 75 0"56 6

Return sludge Suspended matter (mg 1-1) Settled matter (ml 1-1)

4460 450

Laboratory experiments and trial operation The variations of the influent flow, measured as effluent flow, the content of phosphorus (mg P 1-x) in the influent are shown on FIG. 10. Based on this diagram, the phosphorus flow during the day between 0800-0100 hours will m a k e a total of about .185 kg P, and during the night (0100-0800 hours) about 16 kg P. The dosage of alum .was calculated on an average flow of 1770 m 3 h -1 during the day and 1430 m 3 h -1

241

Chemical Pretreatment Before Biological Treatment in Sewage Plants

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FIG. 10. The variations of flow, phosphorus content and phosphorus flow on a reference day at the sewage plant at JSnkSping.

during the night. The corresponding surface loads were about 1"1 and 0-9 m 3 m -2 h- *, respectively. According to the laboratory experiments preceding the test operation with pre-precipitation, the addition of 150 g m-3 alum (8 ~o AI) is needed to obtain about 75 per cent reduction of the total amount of suspended matter or about 65 per cent of the total amount of aluminium. With this alum dose, however, there is a risk of obtaining too high reductions of BOD and soluble phosphorus with respect to the activated sludge process. Accordingly, the addition of a flocculating agent, Purifloc A23, was recommended in order to improve the separation of the chemical sludge at a lower alum dosage. During the whole test period the influent flow showed very strong pulsating variations depending on the tyristor adjustment (SCR) at a pumping station which was out of order. Consequently the pumps were working on nominal numbers of revolutions in a stop--go system. This gave rise to troublesome currents in the sedimentation and flushing effects in the sedimentation tanks during maximum flow. In the test periods in March the variations of the flow were considerably suppressed by using outlet flutes with holes instead of overflow weir (see Fzo. 11). The damping of the

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FIG. 12. The damping effect with flow equalization according to FIG. 11, calculated by computer. A: overflow weir. B: outlet flutes with holes. flOW from the pre-sedimentation tanks, calculated by computer, is shown on FIG. 12. During the first test period in March the chemical dosage, was on the average, 119 mg 1-~ alum q- 0.48 mg 1-1 Purifloc A23 during the day and 85 mg 1-~ alum q0.35 mg 1-1 Purifloc A23 during the night. The average flow on day and night was 2020 and 1040 respectively. The variation of the surface load in the pre-sedimentation tanks, based on FIG. 12 and the average flow, was 0-8-1.6 m 3 m -2 h - I in the day (0800-0100 hours). The reduction of BOD5 and total phosphorus during the test period was, on the average, 93 and 83 per cent respectively, corresponding to 11 mg 02 1-1 and 0.61 mg P 1- ~ in the effluent. The average content of suspended matter in effluent was 10 mg 1-1. Analysis of pre-settled sewage during the test period gave the following mean values with variations in brackets: Suspended matter (nag 1- x) Phosphorus (total) (mg P 1-t) Phosphorus (soluble) (mg P 1-1) BODs (total) (mg 02 I-1)

66 2.5 1.0

73

(39-84) (1.8-3.7) (0-59-1.8) (60-80)

In comparison with TABLe 4 the content of suspended matter in pre-settled sewage is about the same, but BOD5 has been reduced by approximately 50 per cent. The sludge age of the activated sludge process was estimated to be about 3 days. However, the content of suspended matter decreased from 1030 to 480 mg 1-1 in the aeration tanks, and the sludge volume index increased from 145 to 200 ml g-1 after 1 week. This makes the operation ofthe activated sludge process more difficult regarding sludge bulking and sludge flight and the trial operation was interrupted accordingly. The flora of micro-organisms changed during the test period, and among other things, the number of slipper animals decreased and filamentous bacteria of Sphaerotilus type were formed.

Chemical Pretreatment Before BiologicalTreatment in Sewage Plants

243

It was intended to adjust the alum dosage at 150 and 75 g m-3 day and night respectively during the second test period in March. Since dosage regulation guided by the flow was lacking, the average alum dosages became as high as 170 and 140 day and night respectively. The pre-precipitation process was stopped after 10 days for the same reason as in the previous test period. CONCLUDING REMARKS REGARDING THE EXPERIMENTAL WORK AT JONKOPING The trial operation in J6nk6ping sewage plant has been of too brief duration to draw definite conclusions. The results show, however, that the pre-precipitation process is not well suited to the present conditions at the plant mainly because of sludge depletion in the aeration tanks. This depends not only on about 50 per cent lower BOD-content in pre-settled sewage during the pre-precipitation with alum compared with normal operation, but also on the extremely low sludge production in the activated sludge process. An attempt has been made to maintain a sludge age of about 3 days during the pre-precipitation, and the calculated values varied between 2 and 3 days. It will be difficult to operate the activated sludge process properly without sludge bulking and sludge flight in the case of such a low content of MLSS as during the test periods, i.e. below 500-1000 mg 1-1. Another reason for stopping the trial operation at J6nk6ping was the difficultyin obtaining the intended reduction of total phosphorus, corresponding to 0-5 mg P I-1 or less, in the effluent. THE INFLUENCE OF pH, BOD, SOLUBLE AND TOTAL PHOSPHORUS ON THE ACTIVATED SLUDGE PROCESS

Theory Using pre-precipitation, three parameters are of great significance to the activated sludge process. These are pH, BOD and phosphorus content. Soluble phosphorus, which is easier to assimilate by micro-organisms than suspended phosphorus, is more important than total phosphorus. Experiments on heterotrophic bacteria (HARTMANand LAUBENBERGER,1968) and nitrification bacteria (HULTMAN, 1971) have shown that the growth rate of the bacteria, considering the rate limiting substrate S, may be written: /z(S, pH) ~

tzmax ~pn o~o (K, + S) [1 + kl (10 (p"'pn°p`) -- 1)]

where/z(S, pH) -----growth function Ks = value of S at tz(S) = ~mJ2 k, ----- constant pHop t = optimal pH. The decrease of the activity is obvious at pH less than about 6.5 or higher than 9-9.5. The lowest BOD-content in pre-settled sewage for steady state may be calculated according to the following balance equation, if the content of suspended inorganic and inert organic solids in the pre-settled sewage is neglected and complete mixing is assumed:

tzn,tXV = Q Yapp(So- S ) = OXout where tz.ot = net growth rate = 1/G (G = sludge age) X = suspended solids content (dX]dt = tz.,tX)

224

BERN"I" ERICSSON

Y,vp= apparent sludge yield

So = substrate concentration in pre-sedimented sewage S = substrate concentration in aeration tanks and effluent Q = influent flow rate Xout= suspended solids in effluent. If the residence time 0 is introduced (0 = V/Q) the formula above will be:

XO G

--

Yao. ( S o -

S) =

Xou,.

It is evident from calculation examples with the formula above, that the content of MLSS in the aeration tanks can vary greatly depending on the operation conditions of the activated sludge process. It may be mentioned in this connection that trial operation with pre-precipitation at a higher sludge age than 3 days (about 6 days) was also tested at J6nk6ping without any success. The content of phosphorus, which is required to build up protoplasm, varies according to the type of micro-organisms. The cell mass of bacteria usually has a higher phosphorus content than that of fungi. The latter often have bad sedimentation characteristics, which may give rise to sludge bulking (JONES, 1965). The common values of the phosphorus content in activated sludge vary between 1 and 3 per cent. The organic substrate generally limits the growth rate of the activated sludge since nutrients and oxygen are supplied in excess. With pre-precipitation, however, phosphorus may become growth-rate limiting for the activated sludge in the case of too high phosphorus reduction in the pre-treatment stage. In pre-settled sewage after pre-precipitation, the phosphorus is largely in a suspended state. It is possible that suspended phosphorus is not as amenable as soluble phosphorus to the microorganisms in the formation of protoplasm. In such a case the reduction of soluble phosphorus should not exceed a certain limit, depending on the BOD-content. The ratio of BODs (reduced): P in the activated sludge process is usually 90:1. The reduction of soluble phosphorus in effluent with alum and ferric chloride has been described, in literature, in adsorption isotherms (LEA et al., 1954; HENRIKSEN, 1962, 1963; ERICSSON, 1967). In the effluent most of the phosphorus is in an orthophosphate state since poly-phosphates are, to a great extent, hydrolysed by extracellular enzymes to ortho-phosphate or, in smaller amounts, taken up by the cells (ERICSSON, 1967). The laboratory experiments also indicated that on a larger scale the poly-phosphates, forming an integral part of washing-material as builders, would be co-precipitated with ortho-phosphate in the influent. TENNEY and STUMM (1965) showed that an excess of the metal coagulant (A1, Fe), computed on the basis of molar equivalents, is required to precipitate condensed phosphate. Otherwise soluble complexes of metal polyphosphates will be formed.

Laboratory experintents In order to investigate the influence of low BOD-content in the influent on the activated sludge process, and the availability of suspended phosphorus for the microorganisms, laboratory experiments involving continuous operation of two identical laboratory models of the activated sludge process (FIG. 13) were carried out in three periods of about 10 days each. The purification effect with regard to COD as well as

Chemical Pretreatment Before Biological Treatment in Sewage Plants

245

Fiocc ulg tlon

vessel

Substrote

Aeration tanks

Sedimentation tank Air Effluent

R e t u r n .sludge

Excess activated

sludge

FIG. 13. A model of the activated sludge process for continuous operation on a laboratory scale, equipped with a flocculation vessel before the inlet.

B a D , MLSS and turbidity in the effluent were measured. The organic substrate was prepared synthetically, containing among other things, casein hydrolysate (DIFCO) and meat extract with 150 mg 1-I B a D 7. The concentration of BaD7 and soluble phosphorus was regulated between about 50--150 and 0-3.5, respectively, by dilution and pre-precipitation with 125 mg 1-1 alum or 125 mg 1-1 FeC13.6H20 (see Fla. 13). Results and discussion

Due to the brief experimental periods and variations in the amount of sludge in the sedimentation tanks, no definite conclusions may be drawn from the experiments. The results clearly indicate, however, that the concentration of BaD7 and soluble phosphorus in pre-settled sewage after pre-precipitation must not be too small and should be kept under control. When the phosphorus was supplied to the activated sludge process in suspended instead of soluble state, the BOD~-content, but not COD, in the effluent increased a little. The turbidity in the effluent also increased when the content of suspended phosphorus instead of soluble phosphorus was high and BOD~ low. The application of the laboratory results at sewage plants is also made more difficult by the differences in the organic substrates. SUMMARY

Test operation with pre-precipitation on a technical scale has been performed with the addition of ferric chloride and alum at Botkyrka and J6nk6ping, respectively. In the test operation at Botkyrka in June 1971, no adverse effect was observed on the activated sludge or digestion process regarding the BaD-removal and decomposition

246

BERNTERICSSON

of organic matter, respectively, but the activity of the sludge was somewhat decreased. According to the results the total phosphorus content in sewage effluent with a uniform dosage of about 90 mg I-1 FeC13.6H20 can be reduced to about 0"5 mg P 1-i Based on the results, operation with chemical pre-precipitation using ferric chloride started in April 1972. The results hitherto are even more promising, and the operation of the plant has been improved. At J6nk6ping on the other hand, the test operation must be discontinued owing to the decrease of suspended matter in the aeration tanks to about 500 mg 1-1 only. The reason was not only the low BOD-content in sewage to the aeration tanks but also very low sludge yield coefficient observed at the treatment plant even without proprecipitation. The high industrial supply of BOD as well as industrial impurities with detrimental action on the activated sludge process may be the explanation for this. Also the short-time variations of the influent flow should be mentioned, as this will increase the surface load in the pre-sedimentation tanks during go-time of the pumps and make a proper chemical dosage more difficult. With a low sludge content in the aeration tanks it will be more difficult to operate the activated sludge process. Successful operation of pre-precipitation presupposes sufficient degradable organic carbon, no phosphorus-limitation of the biological process and accurate control of the amount of wasted sludge in order to maintain sufficient active bacterial solids in the aeration tanks. None of these assumptions was satisfactorily achieved at J6nk6ping. The critical values as well as the effect of the activated sludge operation can be calculated theoretically under ideal conditions. As far as the biological sludge production, determined of BOD, is concerned it must be borne in mind that an efficient sludge separation is important and that the amount of phosphorus removed from the biological step depends exclusively on the amount of excess sludge taken out from the plant. Laboratory experiments show the adverse effect of the chemical pro-treatment process on the purification when BOD and soluble phosphorus are excessively reduced too much with regard to the biological sludge production. The results emphasize that the content of soluble phosphorus instead of total phosphorus, besides BOD, in sewage after pre-sedimentation should be kept under control. Another important parameter in the case of lime precipitation is pH. At some sewage treatment plants, e.g. Botkyrka, none of these parameters will deteriorate the operation of the activated sludge process in the case of chemical pre-precipitation due to the composition of the influent. REFERENCES ALBERTSONO. E. and SHERWOODR. J. (1969) Phosphate extraction process. J. Water Pollut. Control Fed. 42, 1467-1490. BARGMANR. D., BETZ J. M. and GARBERW. F. (1970) Nitrogen-phosphate relationships and removals obtained by treatment processes at the Hyperion treatment plant. Proceedings 5th International Conference on Water Pollution Research, San Francisco, Calif. ERICSSONB. (1967) Niirsaltrecluktion vicl avloppsverk. Vatten 23, 92-102. ERICSSONB. (1971) Some problems connected with phosphorus removal at sewage plants. Proceedings Scandinavian Congress of Chemical Engineering, Vol. 4, Copenhagen, Denmark. HARTMANN L. and LAUBENBERGERG. (1968) Toxicity measurements in activated sludge. J. san#. Engng Div. ASCE 94, SA2, 247-256. HEN'V.lC.SENA. (1962, 1963) Laboratory studies on the removal of phosphates from sewage by the coagulation process. Schweiz. Z. Hydrol. 24, 253-271 ; 25, 380-396. HUL'rMAN B. (19"/1) Studies on biological nitrogen reduction. Proceedings Scandinavian Congress of Chemical Engineering, Vol. 4, Copenhagen, Denmark.

Chemical Pretreatment Before Biological Treatment in Sewage Plants

247

H6KERVALL E. (1971) Pre-precipitation--trials at the sewage treatment plants in Stockholm. E/ft. Water Treat. J. 11, 551-553. JENKINS D. and MENAR A. B. (1968) The pathways of phosphorus in biological treatment processes. Proceedings 4th International Conference on Water Pollution Research. Prague, Czechoslovakia. JONES P. H. (1965) The effect of nitrogen and phosphorus compounds on one of the microorganisms responsible for sludge bulking. Proceedings 20th Industrial Waste Conference. Purdue University, pp. 297-315. LEA W. G., ROHLICH S. A. and KArZ W. J. (1954) Removal of phosphates from treated sewage. Sew. Ind. Wastes 26, 261-275. LEVlN G. V. and SHAPmO J. (1965) Metabolic uptake of phosphorus by wastewater organisms. J. Water Pollut. Control Fed. 37, 800-821. MONTGOMERY H. A. C. and BORNE ]~. J. (1966) The inhibition of nitrification in the BOD test. J. Proc. Inst. Sew. Purif. Part 4, 357-368. SCHMID L. A. and MCKINNEY R. E. (1969) Phosphate removal by a lime-biological treatment scheme. J. Water Polhtt. Control Fed. 41, 1259-1276. TENNEY M. W. and STUM~i W. (1965) Chemical flocculation of microorganisms in biological waste treatment. J. Water Pollut. Control Fed. 37, 1370-1378. THOMAS E. A. (1967) Die Phosphat-Hypertrophie der Gew/isser. Chemisch Weekblad 63, 305-319. WOOD L. B. and MORRIS H. (1966) Modifications to the BOD test. J. Proc. Inst. Sew. Purif. Part 4, 350-356. DISCUSSION Could the degree of treatment be reduced so that the ratio is raised to facilitate treatment by activated sludge, or does the use of alum not lend itself to such regulation? What quantities of primary-chemical sludge are produced by the use of alum and ferric chloride, what are the properties of such sludges with regard to dewatering, and the appropriate costs of dewatering? Waterhouse & Partners, E. WAL'rON Rowlands Gill, Co. Durham.

Repty According to the laboratory tests only about 20 per cent of the chemical sludge, measured as alumininm with atomic absorption spectrophotometer, will be separated in the pre-sedimentation tanks at Simsholmen-sewage treatment plant with 125 mg 1-L alum (8 per cent AI). In order to obtain a better sludge separation effect without increasing the alum dosage Purifloc A23 was added in addition to alum. The lowest daily mean value during the trial operation was 90 mg 1-t alum and 0.36 mg 1-t Purifloc A23. With this dosage it was, however, not possible to attain 0.5 mg P 1-~ or lower in the effluent, which was the purpose of the experimental work. As facilities for a proper chemical dosage were lacking and the period of trial operation was short, definite conclusion cannot be drawn. The increase in mixed primary and excess sludge in the case of chemical preprecipitation with alum or ferric chloride may be estimated to be about 30 per cent. The distribution of the chemical sludge in the primary and excess sludge, respectively, depends on the sludge separation in the presedimentation tanks. As the results of the admixture of the chemical sludge the mixed primary and excess sludge will be more difficult to dewater. The thickening properties are not as good as those occurring without chemical sludge. Experience in Sweden is limited to the mechanical dewatering of mixed sludge containing chemical sludge with centrifuges, filter-band presses or vacuum-filters. The amount of polyelectrolyte to attain about 15 per cent dry matter in the sludge cake and 0'05 per cent in the reject-water in the case of mixed sludge from a treatment plant with pre-precipitation with alum may be estimated to be about 5 kg ton - t dry matter. If the operation time is 40 h a week the estimated cost will be about 200 Sw kr ton -a dry matter (1 Sw kr = 27 cents). The chemical sludge, however, has to be stabilized in the case of pre-precipitation. The total sludge treatment costs for a large treatment plant (100,000 people)--including thickening, anaerobic digestion, mechanical dewatering and tipping--may be estimated to be about 300 Sw kr t o n - ~ dry matter. B. ERICSSON