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Physico-chemical treatment of municipal wastewater. Coagulation-flocculation
Water Research Vol. 12. pp. 35 to 40. Pergamon Press 1978. Printed in Great Britain.
PHYSICO-CHEMICAL TREATMENT OF MUNICIPAL WASTEWATER. COAGULATION-FLOCCULATION J. LEENTVAAR,
W. G. WERUMEUSBUNING and H. M. M. KOPPERS
Department of Waterpurification, Agricultural University, Wageningen, The Netherlands (Received 20 May 1976; in revised form 29 June 1977) Abstract--This article traces the possibilities of physico-chemical treatment of domestic sewage with particular attention to coagulation-flocculation processes. As coagulants the following have been used: ferric chloride, hydrated lime and alum. Different types of coagulant aids have been used too. Besides pilot-plant experiments, a large number of batch experiments has been carried out in order to determine the range of optimal doses of coagulants/coagulant aids/pH and so on. The fate of specific organic and inorganic components (proteins, detergents, low organic acids, phosphorus, nitrogen) as well as TOC, BOD and COD in this chemical treatment have been studied. The suspended and most of the colloidal fractions have efficiently been removed. A considerable fraction, which is defined as soluble, has been removed too. The significance of the findings for wastewater treatment processes have been discussed. extending phosphorus removal; better meeting of peak loadings; immediate start-up and shut-down; taking up less space; insensitive to toxic compounds in the wastewater.
INTRODUCTION
It is well known that a well operated mechanicalbiological wastewater treatment plant eliminates about 90% of the organic material based on COD. This is partially due to the fact that a small part of the compounds in the sewage is not biologically degradable and the growth of bacteria also creates a small amount of biologically inert matter. These compounds are called refractory (half-life time greater than two days) (Wuhrmann, 1972)~ Besides a diminution of the amount of total wastes discharged, physico-chemical treatment may offer a possibility to decrease the amount of "residual organic carbon" in the effluent (Roberts & Stumm, 1974). For the drinking water production from polluted riverwater and for the elimination of phosphorus from wastewater physico and/or chemical treatment methods are of great importance. Direct physico-chemical treatment of raw sewage without a biological step has been studied e.g. by Weber (1970); Zuckerman & Molof (1970); Cooper & Thomas (1974); Rebhun (1974). The first two studies have been carried out on low strength wastewater and the latter two reported about relatively strong municipal wastewater. This article refers to experiments carried out during more than two and a half years on physical-chemical treatment of medium strength domestic sewage.
The disadvantages are: the use of chemicals; more complicated processing; more checking apparatus; possibility of H2S-develo pmerit in carbon columns, when using fixed beds in downflow; higher energy consumption. For this reason Haskoning (consulting engineers), the Department of Water Purification of the Agricultural University and Norit N.V. (manufacturer of activated carbon) started a project in 1973 in order to develop a physico-chemical treatment plant for European municipal and industrial wastewaters. Anticipating the construction of a pilot-plant with a flow of 2 m 3 h - 1, which was completed in October 1975, preliminary experiments have been started on batch scale and in a small pilot-plant. All experiments have been carried out with sewage of the village Bennekom, which is of domestic origin. ANALYTICALPROCEDURES
BOD, COD, total and ammonia nitrogen, phosphates and detergents have been determined according to Standard Methods (1965). Proteins have been determined using a modified Folin method with albumine as a standard. Average influent-COD and - B O D s (mg O2/1) Volatile low organic acids have been determined on a gasliquid chromatograph. The soluble fraction of the sewage and the effluent has been defined as the filtrate after memWeber (1970) -about 50 brane filtration with a 0.45/zm filter. Zuckerman & Molof (1970) 195-568 105-164 Thus the experimentally defined soluble fraction conRebhun (1974) 1300 450 tains colloidal as well as soluble organic material. As Cooper & Thomas 0974) 1065 450 • polymers have been used: Superfloc Al00, CI00 and N100. Present study 460 171
The advantages of direct physico-chemical treatment compared with biological treatment of wastewaters are (Kossem 1974):
EXPERIMENTS
In the phase of the research reported here, chemical-flocculation-clarification has been studied as well in batch 35
36
J. LEENTVAAR, W. G. WERUMEUSBUNING and H. M. M. KOPPERS
Table 1. Chemical treatment of raw sewage with different ferric chloride doses and at different pH. Batch experiments
pH
Coagulant dose (g Fe/m 3)
Coagulant cost (f/m a)
5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 7.7 7.7 7.7 7.7
3.9 7.8 15.5 31.0 62.0 93.0 124.0 150.0 25.0 50.0 75.0 100.0
0.01 0.01 0.02 0.04 0.08 0.13 0.17 0.21 0.03 0.07 0.10 0.14
Before treatment TOC DOC 165 165 165 165 165 165 165 165 148 148 131 148
±63 ± 63 ± 63 ± 63 ±63 ±63 ± 63 ± 63 ± 31 ± 31 ±49 ± 31
73 73 73 73 73 73 73 73 124 128 107 135
After treatment TOC DOC
± 28 ± 28 ± 28 ± 28 ±28 ±28 ± 28 ± 28 ± 25 ± 28 ±20 ± 28
reactors as in a pilot-plant. During the initial period "jar test" scale studies have been performed to determine the general flocculation behaviour of the sewage. The results of the batch experiments have been used in deciding the experimental conditions to be used in the continuous-flow experiments. The costs given in the tables for the various doses of coagulant were calculated using the following costs: Ferric chloride (40°o w/w FeCl3 soln): f. 188.80/tonne ( = £44 = $76); Alum (98°~o w/w AI 2 (SO,) 3. 18H20): f. 295.00/tonne ( = C69 = S119); Lime (987~i w/w Ca(OH)2): f. 155.00/tonne ( = E36 = $63). BATCH EXPERIMENTS The batch experiments have been carried out in rectangular Plexiglas coagulation-flocculation tanks, which contained two litres of wastewater. The mixer consists of a stainless steel propeller with a known geometric design. The velocity gradients in the tank have been determined on the basis of the formula given by Liepe (1966). For rapid mixing during 60 s, in which the chemicals in liquid form are added to the sewage, the G-value is approximately 400 s - i (250 rpm). After this rapid mixing the sample is flocculated with a G-value of about 25 s - t (39 rpm) during 30 rain. Finally the suspension is allowed to settle during 30 rain and then a sample is taken at half height of the tank. In the sewage and the sample after settling the TOC, the COD, the B O D s and sometimes specific c o m p o u n d s are determined. Three flocculants have been used: ferric chloride, hydrated lime and alum. In some cases coagulant aids have been added.
138 ± 55 123 ± 4 6 1 0 2 ± 34 88 ± 28 7 0 ± 20 6 7 ± 24 63 ± 22 61 ± 24 126 ± 18 110 ± 16 77 ± 19 84 ± 20
73 ± 72 ± 70± 69± 64 ± 63 ± 57 ± 54 ± 102 ± 99 ± 63 ± 80 ±
0o removal Total Soluble
25 22 26 21 13 14 10 7 17 17 7 15
16 ± 12 24 ± 13 36 ± 14 45 ± 11 55 ± 11 58 ± 9 60± 8 62 ± 10 15 ± 1 24 ± 4 42 ± 6 42 ± 6
1 4 6 12 14 22 27 17 22 39 38
-± ± ± ± ± ± ± ± ± ± ±
0 3 4 10 13 14 4 I1 4 17 8
Results obtained with ferric chloride in batch experiments are summarized in Table 1. In this table the values for TOC, dissolved T O C (DOC) of the sewage, T O C and dissolved T O C (DOC) of the settled sample are given with their standard deviation (±). For every parameter there have been carried out at least 7 experiments. The percentage TOC- and DOC-reduction is the average of the percentage reduction of each separate experiment and consequently not from the average TOC-values before and after treatment respectively. The pH has been adjusted with sulphuric acid or sodium hydroxide to 5.25 ± 0.25 or 7.75 + 0.25. In Table 1 the coagulant cost are given too. In Fig. l the percentage T O C reduction has been plotted versus the pH at a fixed ferric cloride dose of 150 nag Fe/l. From this figure it is seen, that the o p t i m u m pH ranges from 4.5 to 6.5. In one experiment the residual iron concentration in the sample has been determined. The results are shown in Fig. 2. It appears that above a certain value (here 75 mg Fe/l added) a fixed percentage of the iron (about 84%) has been incorporated in the sludge. This value and percentage will be dependent on the total a m o u n t of pollution in the sewage. It has to be noted here that the addition of a small a m o u n t of polymer, either cationic, nonionic or anionic, to the, with o p t i m u m dose Fe coagulated suspension did not appear to have a significant effect on the elimination of TOC. With hydrated lime as coagulant, the coagulation-flocculation process is determined by the pH (Minton, 1973).
,I, I 0 0
8
I00
~ 9o ._c
80
60
40
20
0
c o
80
o
70
u c
60
5O
;
h
I
3
4
5
I
h
I
I
6
7'
8
9
pH
Fig. 1. Batch experiment with FeCI3 as coagulant. The pH versus the percentage T O C removal at an iron dose of 150 mg Fe/l.
25
5~0 mg
75
I00
125
Fe It
Fig. 2. A m o u n t of iron incorporated into the organic chemical sludge as percentage of the a m o u n t of coagulant used.
Physico-chemical treatment of municipal wastewater
// ~
37
~oo "1-
°
80(3 o "ID
400 ~
4o
Lg 2
2o
5
g
pH
0
Fig. 3. The alkalinity (meq/l), the calcium and m a g n e s i u m content (mg/l) in the s u p e m a t a n t after lime treatment at different pH.
"~-
I 9.0
I I0.0 pH
[ I1.0
I 12.0
Fig. 4. Required lime dose versus the pH. According to Black (1961) and Van Vuuren (1967) the formarion of Mg(OH)z fulfils an important role as coagulant aid. The wastewater used in this study contains on average 66.9 mg calcium and 5.5 nag m a g n e s i u m per liter. F r o m Fig, 3 it is seen that Mg(OH)2 starts to be formed at pH-values above about 10.2. Theoretically the magnesium concentration becomes less than 5.5 m g h at pH 10.45. However, the flocculation process runs optimal at a p H higher than 11.2. As the pH effects strongly the coagulation-flocculation process of wastewater with lime, a n u m b e r of experiments has been carried o u t in order to trace the effect of the pH on the removal of organic carbon. Table 2 demonstrates that the removal efficiency, especially of dissolved organic matter, increases at higher pH. The required dose of hydrated lime depends on the hardness/ alkalinity of the wastewater. The average alkalinity of the domestic sewage is 6.02 m e q HCO~/I. In Fig. 4 the required lime dose versus the pH has been plotted. I n order to improve the flocculation with lime, the effect of adding coagulant aids on the elimination of organic matter has been studied. The examined coagulant aids are ferric chloride, alum and organic polymers. The addition of small quantities of an inorganic coagulant to the wastewater beside lime leads to a substantial improvement of the quality of the settled sample. Table 2 shows a batch experiment in which has been added 0. 5 and 10mg Fe/l and 0, 5 and 1 0 m g AI/I at p H I 1.0 (with lime). The effect of polymers as coagulant aid on the TOC-removal with lime-flocculation at pH 11.0 is negligible. The polymers have an effect on the volume of chemical-organic sludge that arises. In those cases that alum (A12(SO4) 3.18H20) is used as coagulant, batch experiments have shown that in the range from 32 to 63 mg AI/I the TOC-removal does not differ
much. See Table 3. According to the measurements, the pH-range for optimal coagulation-flocculation with alum is rather wide and varies from pH 4.5 to 6.5. See Fig. 5. In flocculating sewage with alum it has been tried to raise the TOC-removal by adding poly-electrolytes. An anionic, a nonionic and a cationic polymer have been tested. Any significant improvement of effluent quality by adding polyelectrolytes has not been observed in those cases where the polymer together with the o p t i m u m alum dose has been added. Table 4 represents reduction data of specific c o m p o u n d s as proteins, NH4-N, Kjeldahl-N, anionactive detergents, volatile organic acids, phosphorus a n d of the C O D and B O D s by coagulation-floceulation of sewage with lime (at pH 11), iron (75 mg Fe/l, at pH 5.25) or alum (47 nag AI/I, at pH 5.50. These results have been obtained by batch experiments. Beside acetic acid and propionic acid have been determined: iso-butyric acid, butyric acid, iso-valeric acid and valeric acid. These c o m p o u n d s do play an inferior role and are to be found in concentrations less than 1.7 mg/l. The contribution to the raw sewage T O C of volatile organic acids is about 2 0 ~ In the sample after coagulation-flocculation-clarifieation this percentage is about 40%. PILOT-PLANT EXPERIMENTS In connection with the batch experiments continuous flow experiments have been carried out. Anticipating the construction of a pilot-plant with a flow of 2 m 3 h - 1 as mentioned before, preparatory experiments have been started with a small pilot-plant, as described earlier (De Bekker & Leentvaar, 1974). This pilot-plant has a capacity
Table 2. Chemical treatment of raw sewage with hydrated lime at different pH, with/without coagulant aid. Batch experiments
pH 9.0 9.5 10.0 10.5 11.0 11.5 12.0 11.0 11.0 11.0 11.0
Coagulant aid Coagulant tg/m 3) cost (f/m 3) -------5Fe 10Fe 5AI 10AI
0.02 0.05 0.07 0.08 0.09 0.11 0.19 0.10 0.11 0.11 0.13
Before treatment TOC DOC 135 + 44 135 + 44 135 _ 44 135 -t- 44 135 + 44 135 -I- 44 135 + 44 147+44 147 -I- 44 147+44 147 _+ 44
62 -t- 26 62 -I- 26 62 _ 26 62 -t- 26 62 _ 26 62 _+ 26 62 _+ 26 84-t-26 84 -t- 26 84+26 84 + 26
After treatment TOC DOC 81 _ 26 76 ___25 73 + 21 70 -t- 20 68 _ 22 62 _+ 22 54 + 18 68+1 67 + 2 76+2 57 _+ 1
52 + 30 54 + 28 53 -t- 24 55 + 25 55 + 25 53 _ 26 48 + 25 65-t-4 63 + 5 75+3 56 _ 4
% removal Total Soluble 38 + 11 43 + 8 45 _ 5 47 + 5 49 -I- 8 51 + 7 59 + 9 56+4 59 + 2 48-t-2 61 + 3
16 + 10 13 + 1 15 + 6 11 -t- 4 17 _ 4 15 + 2 23 + 3 23+10 25 + 7 10+4 33 + 3
J. LEENTVAAR, W. G. WERUMEUSBUNING and H. M. M. KOPPERS
38
Table 3. Chemical treatment of raw sewage with different alum dose. Batch experiments
pH
Coagulant dose (g Al/m 3)
Coagulantcost (f/m 3)
5.5 5.5 5.5 5.5 5.5 5.5 5.5
4.1 8.1 16.2 32.4 47.4 64.9 81.l
0.01 0.03 0.06 0.12 0.17 0.24 0.30
Before treatment TOC DOC 165 + 63 165+63 165 -+ 63 165 -t- 63 165 -+ 63 165 + 63 165 -+ 63
After treatment TOC DOC
73 -t- 28 73+28 73 4- 28 73 4-__28 73 -+ 28 73 + 28 73 + 28
of 0.06 m 3 h - ~ and consists of a coagulation-flocculation tank. a D o r t m u n d sedimentation tank, a multi-media sandfilter and an activated carbon column. See Fig. 6. After a few months an equipment for powdered activated carbon processes has been added. The average detention time of the wastewater in the coagulafion-flocculation tank is 23 min and in the sedimentation tank about 1.5 h. The rapid mixing of floceulants is obtained by injecting the chemicals into the sewage feed pipe. In the first flocculation-chamber the sewage stays on average 3 rain and is submitted to a G-value of about 64 s - t (92 rpm) by stirring with a propeller. In this flocculation-chamber is also placed the pH-electrode. In the second flocculation-chamber the sewage has an average detention time of 20 rain and is submitted to a G-value of circa 19 s-1 (13 rpm). Composite samples of the influent (raw wastewater) and of the sedimentation tank efltuent have been collected. These samples have been taken daily three times per hour. 100
80
132 __ 51 110+46 82 + 28 67 -I- 25 67 ___+23 64 + 22 66 + 22
% removal Total Soluble
72 + 28 72+20 70 -I- 20 57 -+ 6 55 -+ 2 59 + 5 55 + 2
19 _____14 3 3 + 16 48 -+ 12 59 -+ 10 59 -+ 9 60 -+ 8 60 + 10
1 5 22 24 19 24
This procedure has been executed for five days a week. In the continuous flow experiments three flocculants have been used: ferric chloride, hydrated lime and alum. The pilot-plant experiments on ferric chloride as coagulant have been carried out for 9 months. The ferric chloride used is a 41~o w/w FeCI3 solution. The pilot-plant studies have been started by adding a constant a m o u n t of ferric chloride to a constant flow influent. Soon it has turned out that the changes in the composition of the influent are so high that the floceulation often did not succeed, because of the low pH in the coagulation-flocculation tank when the sewage had been diluted with rainwater. This procedure has been left and has been changed over to a feed of ferric chloride related with the pH in the flocculation tank. That a m o u n t of flocculant has been dosed in the tank to reach a fixed pH-value. A presupposition of this way of adding ferric chloride is that the required feed of ferric chloride is proportional to the alkalinity of the sewage a n d that this magnitude is related to the concentration of pollution. Dependent on the pH and the alkalinity of the influent the dose of FeCl 3 ranges from 40 to 160rag Fe/l sewage at a fixed pH-value in the flocculation tank of about 5.3. This way of coagulation-flocculae
E
+0 -+ 3 -+ 8 -+ 5 -+ 7 + 5
f
60
c L) 0 I.-
40
2O
0 --%
3
I 4
I 5
I 6
t 7
I 8
9
pH Fig. 5. Coagulation-flocculation with alum in batch experiments. The pH versus the percentage T O C removal at an alum dose of 47 mg Al/1.
Fig. 6. Flow-scheme of the pilot-plant, a. influent; b. dosage of coagulant; c. coagulation-ttocculation-tank; d. D o r t m u n d sedimentation tank; e. sandfilter; f. activated carbon column.
Table 4. Removal of specific c o m p o u n d s with chemical treatment of sewage. Batch experiments
Component
Unit
BOD5 COD Proteins Ammonia Organic N Detergents Ortho-phosphate Total-phosphate Acetic acid Propionic acid
mg O 2 / 1 mg O 2 / 1 mg/l as albumine mg N/l mg N/1 mg/l (anion-active) mg P/I mg P/I mg/1 mg/l
Percentage removal with Raw wastewater Ferric chloride Hydrated lime Alum (total) 75 mg Fe/l at pH 11.0 47 mg AI/I 248 723 58 28 54 15 l0 31 44.5 8.5
-
63 72 63 16 6 56 99 96 0
59 69 62 18 10 47 99 94 12
l0
0
-
59 72 59 14 7 74 99 97 4 19
Physico-chemical treatment of municipal wastewater
Table 5. Amount and dry solid content of the chemicalorganic sludges produced at focculation of domestic sewage with ferric chloride, lime and alum
I00
SO
6o (.Ju O0 ~-o
39
.
X
~
Volume sludge (in 1) Coagulation-flocculation per liter with: sewage
x--TOC
'''x~
Dry solids content g per liter sludge
40
~,-- DOC
20
o
I
IO
5 mcj Fe / t
Fig. 7. Pilot-plant experiment with hydrated lime as coagulant at pH 11.0. The effect of ferric chloride as coagulant aid on the elimination of TOC and DOC.
tion is a rather expensive one. In the following experiments a constant amount of ferric chloride is dosed, in which the pH by means of NaHCOa and/or HzSO4 has been fixed at a pH of 5.3 or 7.7 in order to study the two parameters (pH and flocculant dose) independently. The efficiency of the coagulation-flocculation of sewage with iron-llI-chloride is higher at pH 5.3 (iron dosage on average 61 mg Fe/I) and the variation in percentage reduction is less than at pH 7.7. These values are:
pH 5.3 7.7
Percentage removal of Total Soluble Suspended solids organic carbon organic carbon (nag C/I) 54% + 14 50% + 24
0.069 0.025 0.065
5.0 25.0 4.5
I
I
o
Ferric chloride (75 mg Fe/1) Hydrated lime (at pH 11.0) Alum (47 mg AI/1)
27% + 12 19% + 10
93% _+ 4 77% + 11
The reduction of total phosphorus amounts in all cases to about 80%. When coagulating with lime the process is determined by the pH. These experiments have been executed for 9 months. In the continuous flow experiments the influent shows also a wide variation in organic carbon content too. The experiments have shown that the eliminated amount of organic carbon increases accordingly as the influent TOC rises, The addition of small quantities of iron-IlIchloride to the wastewater beside lime leads to some improvement of the effluent quality. See Fig. 7. In order to improve the coagulation-flocculation process also recirculation of sludge from the sedimentation tank to the flocculation basin has been practised. The theories about orthokinetic flocculation indicate that by increasing the volume fraction of the particles the formation of bigger floes is stimulated. In view of the wide variation in reduction with and without sludge recirculation in the flocculalion process with lime a conclusion cannot be drawn whether sludge recirculation is positive or not. The total reduction of organic carbon from the influent by coagula-
tion-flocculation with lime at pH 11.0 is 44% _+ 15; with lime at pH 11.0 and 5 mg Fe/i as coagulant aid this percentage is 51% +_ 13. The elimination of suspended solids is about 90%. For five months continuous flow experiments with alum as coagulant have been carried out. According to the results obtained from the batch experiments, the optimum dosage of alum in the pilot-plant is about 47 mg AI/I sewage. It is known that the alum-floc is very fragile. Therefore during two months 1.2 ppm of a cationic polymer has been added as a coagulant aid. No significant effect on the TOC-removal has been found however. With alum as coagulant it has also been studied whether recirculating chemical-organic sludge has a positive effect on the reduction of organic carbon. This has not been proved to be a significant success` The total reduction of TOC at a dosage of 47 mg AI/I at pH 5.9 is on average 58~ + 10. The reduction of dissolved organic carbon is 24Yo + 13. The removal of suspended solids is in all cases greater than 85~o and on average 94%. The three coagulants (ferric chloride, hydrated lime and alum) produce different amounts and types of chemicalorganic sludge. The quantity and the solid content of the sludge (without any treatment) that fows out of the sedimentation tank are given in Table 5. The results of the sludge treatment will be reported later. DISCUSSION
Based on the results obtained in this study the following conclusion can be drawn. The suspended a n d most of the colloidal fractions (in terms of T O C ) are efficiently removed from the raw sewage by coagulation-flocculation followed by settling. For ferric chloride, lime and a l u m the percentage T O C removed of this fractions does not differ much. The percentage reductions have been summarized in Table 6. A considerable fraction, which is defined as soluble, is removed too. F o r hydrated lime as coagulant the removal of dissolved organic c a r b o n (DOC) is a b o u t 17~o. F o r ferric chloride a n d alum these percentages are respectively 27 a n d 24%. In adding ferric chloride as coagulant aid to the flocculation with lime at pH
Table 6. Coagulation-flocculation experiments on pilot-plant scale
Coagulant Ferric chloride Hydrated lime Alum
Dose
pH
61 mg Fe/l 520 mg Ca(OH)2/I 47 mg AI/I
5.3 11.0 5.9
Percentage removal Suspended Soluble solids Total 27 17 24
93 90 94
54 44 58
40
_
J. LEENTVAAR,W. G. WERUMEUSBUNINO and H. M. M. KOPPERS pilot-plant have been started in October 1975. The results will be published in the future.
IO0I
,oFf/ *~ 20~// At 0
Acknowledgements---This study was carried out with partial support of Haskoning (consulting engineers) Nijmegen, Holland and of Norit N.V. (manufacturer of activated carbon) Amersfoort, Holland. We wish to acknowledge especially the contribution of Mr. W. C. van Lier (Norit) and Dr. G. Lettinga (Dept. of Waterpurification, Agricultural University). Acknowledgement is also due to Mr. T. S. J. Ywema, for his assistance in the experimental work.
co i
i
0.~0
0.20
Coagulant cost,
I
0.30 f/m 3
Fig. 8. The coagulant cost per m 3 sewage versus the removal of TOC. Batch experiments.
11.0, the removal percentage of D O C rapidly increases to circa 23yo. The resulting effluent of a coagulation-flocculation sedimentation plant consists to a large extent of soluble organics, which are easily biodegradable. Regarding the coagulant cost in relation to the T O C removal (Fig. 8) the use of iron-Ill-chloride as coagulant should be favourable. This study indicates that for overloaded conventional mechanical-biological sewage treatment plants a chemical treatment as the first step in a municipal wastewater treatment can be favourable, although the percentage removal is n o t so high as obtained by other investigators like Cooper & T h o m a s (1974). This study will be continued with particular attention to this subject. The results obtained on batch scale and in the small scale pilot-plant with a flow of 0.06 m 3 h - 1 are being checked in a larger pilot-plant, having a capacity of 2 m 3 sewage h - 1 a n d consists of a chemical treatment stage, a multimedia sand-filter a n d several activated c a r b o n columns. The chemical treatment stage has been designed as a plug flow reactor with six c o m p a r t m e n t s which are each individually stirred. The experiments with this
REFERENCES Bekker P. de & Leentvaar J. (1974) Investigations on physico-chemical methods of wastewater treatment (Dutch). H 2 0 7, 278-280. Black A. P. & Christman R. F. (1961) Electrophoretic studies of sludge particles produced in lime-soda softening. J. Am. Wat. W k s Ass. 53, 737-747. Cooper P. F. & Thomas E. V. (1974) Recent developments in sewage treatment based on physico-chemical methods. Wat. Pollut, Cont. 73, 505-520. Kossen N. W. F. (1974) Non-biological treatment of wastewater (Dutch). H 2 0 7, 234-236. Liepe F. (1966) Kennzahlen zur Bewertung von Riihrern. Chem. TechnoL 18, 230-235. Minton G. R. & Carlson D. A. (1973) Primary sludges produced by the addition of lime to raw wastewater. Water Res. 7, 1821-1847. Rehbun M. & Streit S. (1974) Physico-chemical treatment of strong municipal wastewater. Water Res. 8, 195-201. Roberts P. & Stumm W. (1974) Behandlung yon kommunalem Abwasser mit Aktivkohle. Gas-Eaux-Eaux us~es 54, 78-88. Standard Methods for Examination of Water and Wastewater (1965). 12th Edn. American Public Health Ass. Van Vuuren L R. J., Stander G. J., Henzen M. R., Meiring, R. G. J. & Van Blerk S. H. V. (1976) Advanced purification of sewage works effluent using a combined system of lime softening and flotation. Water Res. l, 463-474. Weber W. J., Hopkins Ch. B. & Bloom R. (1970) Physicochemical treatment of wastewater. J. Wat. Pollut. Control Fed. 42, 83-99. Wuhrmann K. (1972) La charge des eaux par les pollutants r+fractaires. FiJderation Europ6ischer Gewiisserschutz (19). 13-21. Zuckerman M. M. & Molof A. H. (1970) High quality reuse water by chemical-physical wastewater treatment. J. Wat. Pollut. Control Fed. 42, 437-456.
PHYSICO-CHEMICAL TREATMENT OF MUNICIPAL WASTEWATER. COAGULATION-FLOCCULATION J. LEENTVAAR,
W. G. WERUMEUSBUNING and H. M. M. KOPPERS
Department of Waterpurification, Agricultural University, Wageningen, The Netherlands (Received 20 May 1976; in revised form 29 June 1977) Abstract--This article traces the possibilities of physico-chemical treatment of domestic sewage with particular attention to coagulation-flocculation processes. As coagulants the following have been used: ferric chloride, hydrated lime and alum. Different types of coagulant aids have been used too. Besides pilot-plant experiments, a large number of batch experiments has been carried out in order to determine the range of optimal doses of coagulants/coagulant aids/pH and so on. The fate of specific organic and inorganic components (proteins, detergents, low organic acids, phosphorus, nitrogen) as well as TOC, BOD and COD in this chemical treatment have been studied. The suspended and most of the colloidal fractions have efficiently been removed. A considerable fraction, which is defined as soluble, has been removed too. The significance of the findings for wastewater treatment processes have been discussed. extending phosphorus removal; better meeting of peak loadings; immediate start-up and shut-down; taking up less space; insensitive to toxic compounds in the wastewater.
INTRODUCTION
It is well known that a well operated mechanicalbiological wastewater treatment plant eliminates about 90% of the organic material based on COD. This is partially due to the fact that a small part of the compounds in the sewage is not biologically degradable and the growth of bacteria also creates a small amount of biologically inert matter. These compounds are called refractory (half-life time greater than two days) (Wuhrmann, 1972)~ Besides a diminution of the amount of total wastes discharged, physico-chemical treatment may offer a possibility to decrease the amount of "residual organic carbon" in the effluent (Roberts & Stumm, 1974). For the drinking water production from polluted riverwater and for the elimination of phosphorus from wastewater physico and/or chemical treatment methods are of great importance. Direct physico-chemical treatment of raw sewage without a biological step has been studied e.g. by Weber (1970); Zuckerman & Molof (1970); Cooper & Thomas (1974); Rebhun (1974). The first two studies have been carried out on low strength wastewater and the latter two reported about relatively strong municipal wastewater. This article refers to experiments carried out during more than two and a half years on physical-chemical treatment of medium strength domestic sewage.
The disadvantages are: the use of chemicals; more complicated processing; more checking apparatus; possibility of H2S-develo pmerit in carbon columns, when using fixed beds in downflow; higher energy consumption. For this reason Haskoning (consulting engineers), the Department of Water Purification of the Agricultural University and Norit N.V. (manufacturer of activated carbon) started a project in 1973 in order to develop a physico-chemical treatment plant for European municipal and industrial wastewaters. Anticipating the construction of a pilot-plant with a flow of 2 m 3 h - 1, which was completed in October 1975, preliminary experiments have been started on batch scale and in a small pilot-plant. All experiments have been carried out with sewage of the village Bennekom, which is of domestic origin. ANALYTICALPROCEDURES
BOD, COD, total and ammonia nitrogen, phosphates and detergents have been determined according to Standard Methods (1965). Proteins have been determined using a modified Folin method with albumine as a standard. Average influent-COD and - B O D s (mg O2/1) Volatile low organic acids have been determined on a gasliquid chromatograph. The soluble fraction of the sewage and the effluent has been defined as the filtrate after memWeber (1970) -about 50 brane filtration with a 0.45/zm filter. Zuckerman & Molof (1970) 195-568 105-164 Thus the experimentally defined soluble fraction conRebhun (1974) 1300 450 tains colloidal as well as soluble organic material. As Cooper & Thomas 0974) 1065 450 • polymers have been used: Superfloc Al00, CI00 and N100. Present study 460 171
The advantages of direct physico-chemical treatment compared with biological treatment of wastewaters are (Kossem 1974):
EXPERIMENTS
In the phase of the research reported here, chemical-flocculation-clarification has been studied as well in batch 35
36
J. LEENTVAAR, W. G. WERUMEUSBUNING and H. M. M. KOPPERS
Table 1. Chemical treatment of raw sewage with different ferric chloride doses and at different pH. Batch experiments
pH
Coagulant dose (g Fe/m 3)
Coagulant cost (f/m a)
5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 7.7 7.7 7.7 7.7
3.9 7.8 15.5 31.0 62.0 93.0 124.0 150.0 25.0 50.0 75.0 100.0
0.01 0.01 0.02 0.04 0.08 0.13 0.17 0.21 0.03 0.07 0.10 0.14
Before treatment TOC DOC 165 165 165 165 165 165 165 165 148 148 131 148
±63 ± 63 ± 63 ± 63 ±63 ±63 ± 63 ± 63 ± 31 ± 31 ±49 ± 31
73 73 73 73 73 73 73 73 124 128 107 135
After treatment TOC DOC
± 28 ± 28 ± 28 ± 28 ±28 ±28 ± 28 ± 28 ± 25 ± 28 ±20 ± 28
reactors as in a pilot-plant. During the initial period "jar test" scale studies have been performed to determine the general flocculation behaviour of the sewage. The results of the batch experiments have been used in deciding the experimental conditions to be used in the continuous-flow experiments. The costs given in the tables for the various doses of coagulant were calculated using the following costs: Ferric chloride (40°o w/w FeCl3 soln): f. 188.80/tonne ( = £44 = $76); Alum (98°~o w/w AI 2 (SO,) 3. 18H20): f. 295.00/tonne ( = C69 = S119); Lime (987~i w/w Ca(OH)2): f. 155.00/tonne ( = E36 = $63). BATCH EXPERIMENTS The batch experiments have been carried out in rectangular Plexiglas coagulation-flocculation tanks, which contained two litres of wastewater. The mixer consists of a stainless steel propeller with a known geometric design. The velocity gradients in the tank have been determined on the basis of the formula given by Liepe (1966). For rapid mixing during 60 s, in which the chemicals in liquid form are added to the sewage, the G-value is approximately 400 s - i (250 rpm). After this rapid mixing the sample is flocculated with a G-value of about 25 s - t (39 rpm) during 30 rain. Finally the suspension is allowed to settle during 30 rain and then a sample is taken at half height of the tank. In the sewage and the sample after settling the TOC, the COD, the B O D s and sometimes specific c o m p o u n d s are determined. Three flocculants have been used: ferric chloride, hydrated lime and alum. In some cases coagulant aids have been added.
138 ± 55 123 ± 4 6 1 0 2 ± 34 88 ± 28 7 0 ± 20 6 7 ± 24 63 ± 22 61 ± 24 126 ± 18 110 ± 16 77 ± 19 84 ± 20
73 ± 72 ± 70± 69± 64 ± 63 ± 57 ± 54 ± 102 ± 99 ± 63 ± 80 ±
0o removal Total Soluble
25 22 26 21 13 14 10 7 17 17 7 15
16 ± 12 24 ± 13 36 ± 14 45 ± 11 55 ± 11 58 ± 9 60± 8 62 ± 10 15 ± 1 24 ± 4 42 ± 6 42 ± 6
1 4 6 12 14 22 27 17 22 39 38
-± ± ± ± ± ± ± ± ± ± ±
0 3 4 10 13 14 4 I1 4 17 8
Results obtained with ferric chloride in batch experiments are summarized in Table 1. In this table the values for TOC, dissolved T O C (DOC) of the sewage, T O C and dissolved T O C (DOC) of the settled sample are given with their standard deviation (±). For every parameter there have been carried out at least 7 experiments. The percentage TOC- and DOC-reduction is the average of the percentage reduction of each separate experiment and consequently not from the average TOC-values before and after treatment respectively. The pH has been adjusted with sulphuric acid or sodium hydroxide to 5.25 ± 0.25 or 7.75 + 0.25. In Table 1 the coagulant cost are given too. In Fig. l the percentage T O C reduction has been plotted versus the pH at a fixed ferric cloride dose of 150 nag Fe/l. From this figure it is seen, that the o p t i m u m pH ranges from 4.5 to 6.5. In one experiment the residual iron concentration in the sample has been determined. The results are shown in Fig. 2. It appears that above a certain value (here 75 mg Fe/l added) a fixed percentage of the iron (about 84%) has been incorporated in the sludge. This value and percentage will be dependent on the total a m o u n t of pollution in the sewage. It has to be noted here that the addition of a small a m o u n t of polymer, either cationic, nonionic or anionic, to the, with o p t i m u m dose Fe coagulated suspension did not appear to have a significant effect on the elimination of TOC. With hydrated lime as coagulant, the coagulation-flocculation process is determined by the pH (Minton, 1973).
,I, I 0 0
8
I00
~ 9o ._c
80
60
40
20
0
c o
80
o
70
u c
60
5O
;
h
I
3
4
5
I
h
I
I
6
7'
8
9
pH
Fig. 1. Batch experiment with FeCI3 as coagulant. The pH versus the percentage T O C removal at an iron dose of 150 mg Fe/l.
25
5~0 mg
75
I00
125
Fe It
Fig. 2. A m o u n t of iron incorporated into the organic chemical sludge as percentage of the a m o u n t of coagulant used.
Physico-chemical treatment of municipal wastewater
// ~
37
~oo "1-
°
80(3 o "ID
400 ~
4o
Lg 2
2o
5
g
pH
0
Fig. 3. The alkalinity (meq/l), the calcium and m a g n e s i u m content (mg/l) in the s u p e m a t a n t after lime treatment at different pH.
"~-
I 9.0
I I0.0 pH
[ I1.0
I 12.0
Fig. 4. Required lime dose versus the pH. According to Black (1961) and Van Vuuren (1967) the formarion of Mg(OH)z fulfils an important role as coagulant aid. The wastewater used in this study contains on average 66.9 mg calcium and 5.5 nag m a g n e s i u m per liter. F r o m Fig, 3 it is seen that Mg(OH)2 starts to be formed at pH-values above about 10.2. Theoretically the magnesium concentration becomes less than 5.5 m g h at pH 10.45. However, the flocculation process runs optimal at a p H higher than 11.2. As the pH effects strongly the coagulation-flocculation process of wastewater with lime, a n u m b e r of experiments has been carried o u t in order to trace the effect of the pH on the removal of organic carbon. Table 2 demonstrates that the removal efficiency, especially of dissolved organic matter, increases at higher pH. The required dose of hydrated lime depends on the hardness/ alkalinity of the wastewater. The average alkalinity of the domestic sewage is 6.02 m e q HCO~/I. In Fig. 4 the required lime dose versus the pH has been plotted. I n order to improve the flocculation with lime, the effect of adding coagulant aids on the elimination of organic matter has been studied. The examined coagulant aids are ferric chloride, alum and organic polymers. The addition of small quantities of an inorganic coagulant to the wastewater beside lime leads to a substantial improvement of the quality of the settled sample. Table 2 shows a batch experiment in which has been added 0. 5 and 10mg Fe/l and 0, 5 and 1 0 m g AI/I at p H I 1.0 (with lime). The effect of polymers as coagulant aid on the TOC-removal with lime-flocculation at pH 11.0 is negligible. The polymers have an effect on the volume of chemical-organic sludge that arises. In those cases that alum (A12(SO4) 3.18H20) is used as coagulant, batch experiments have shown that in the range from 32 to 63 mg AI/I the TOC-removal does not differ
much. See Table 3. According to the measurements, the pH-range for optimal coagulation-flocculation with alum is rather wide and varies from pH 4.5 to 6.5. See Fig. 5. In flocculating sewage with alum it has been tried to raise the TOC-removal by adding poly-electrolytes. An anionic, a nonionic and a cationic polymer have been tested. Any significant improvement of effluent quality by adding polyelectrolytes has not been observed in those cases where the polymer together with the o p t i m u m alum dose has been added. Table 4 represents reduction data of specific c o m p o u n d s as proteins, NH4-N, Kjeldahl-N, anionactive detergents, volatile organic acids, phosphorus a n d of the C O D and B O D s by coagulation-floceulation of sewage with lime (at pH 11), iron (75 mg Fe/l, at pH 5.25) or alum (47 nag AI/I, at pH 5.50. These results have been obtained by batch experiments. Beside acetic acid and propionic acid have been determined: iso-butyric acid, butyric acid, iso-valeric acid and valeric acid. These c o m p o u n d s do play an inferior role and are to be found in concentrations less than 1.7 mg/l. The contribution to the raw sewage T O C of volatile organic acids is about 2 0 ~ In the sample after coagulation-flocculation-clarifieation this percentage is about 40%. PILOT-PLANT EXPERIMENTS In connection with the batch experiments continuous flow experiments have been carried out. Anticipating the construction of a pilot-plant with a flow of 2 m 3 h - 1 as mentioned before, preparatory experiments have been started with a small pilot-plant, as described earlier (De Bekker & Leentvaar, 1974). This pilot-plant has a capacity
Table 2. Chemical treatment of raw sewage with hydrated lime at different pH, with/without coagulant aid. Batch experiments
pH 9.0 9.5 10.0 10.5 11.0 11.5 12.0 11.0 11.0 11.0 11.0
Coagulant aid Coagulant tg/m 3) cost (f/m 3) -------5Fe 10Fe 5AI 10AI
0.02 0.05 0.07 0.08 0.09 0.11 0.19 0.10 0.11 0.11 0.13
Before treatment TOC DOC 135 + 44 135 + 44 135 _ 44 135 -t- 44 135 + 44 135 -I- 44 135 + 44 147+44 147 -I- 44 147+44 147 _+ 44
62 -t- 26 62 -I- 26 62 _ 26 62 -t- 26 62 _ 26 62 _+ 26 62 _+ 26 84-t-26 84 -t- 26 84+26 84 + 26
After treatment TOC DOC 81 _ 26 76 ___25 73 + 21 70 -t- 20 68 _ 22 62 _+ 22 54 + 18 68+1 67 + 2 76+2 57 _+ 1
52 + 30 54 + 28 53 -t- 24 55 + 25 55 + 25 53 _ 26 48 + 25 65-t-4 63 + 5 75+3 56 _ 4
% removal Total Soluble 38 + 11 43 + 8 45 _ 5 47 + 5 49 -I- 8 51 + 7 59 + 9 56+4 59 + 2 48-t-2 61 + 3
16 + 10 13 + 1 15 + 6 11 -t- 4 17 _ 4 15 + 2 23 + 3 23+10 25 + 7 10+4 33 + 3
J. LEENTVAAR, W. G. WERUMEUSBUNING and H. M. M. KOPPERS
38
Table 3. Chemical treatment of raw sewage with different alum dose. Batch experiments
pH
Coagulant dose (g Al/m 3)
Coagulantcost (f/m 3)
5.5 5.5 5.5 5.5 5.5 5.5 5.5
4.1 8.1 16.2 32.4 47.4 64.9 81.l
0.01 0.03 0.06 0.12 0.17 0.24 0.30
Before treatment TOC DOC 165 + 63 165+63 165 -+ 63 165 -t- 63 165 -+ 63 165 + 63 165 -+ 63
After treatment TOC DOC
73 -t- 28 73+28 73 4- 28 73 4-__28 73 -+ 28 73 + 28 73 + 28
of 0.06 m 3 h - ~ and consists of a coagulation-flocculation tank. a D o r t m u n d sedimentation tank, a multi-media sandfilter and an activated carbon column. See Fig. 6. After a few months an equipment for powdered activated carbon processes has been added. The average detention time of the wastewater in the coagulafion-flocculation tank is 23 min and in the sedimentation tank about 1.5 h. The rapid mixing of floceulants is obtained by injecting the chemicals into the sewage feed pipe. In the first flocculation-chamber the sewage stays on average 3 rain and is submitted to a G-value of about 64 s - t (92 rpm) by stirring with a propeller. In this flocculation-chamber is also placed the pH-electrode. In the second flocculation-chamber the sewage has an average detention time of 20 rain and is submitted to a G-value of circa 19 s-1 (13 rpm). Composite samples of the influent (raw wastewater) and of the sedimentation tank efltuent have been collected. These samples have been taken daily three times per hour. 100
80
132 __ 51 110+46 82 + 28 67 -I- 25 67 ___+23 64 + 22 66 + 22
% removal Total Soluble
72 + 28 72+20 70 -I- 20 57 -+ 6 55 -+ 2 59 + 5 55 + 2
19 _____14 3 3 + 16 48 -+ 12 59 -+ 10 59 -+ 9 60 -+ 8 60 + 10
1 5 22 24 19 24
This procedure has been executed for five days a week. In the continuous flow experiments three flocculants have been used: ferric chloride, hydrated lime and alum. The pilot-plant experiments on ferric chloride as coagulant have been carried out for 9 months. The ferric chloride used is a 41~o w/w FeCI3 solution. The pilot-plant studies have been started by adding a constant a m o u n t of ferric chloride to a constant flow influent. Soon it has turned out that the changes in the composition of the influent are so high that the floceulation often did not succeed, because of the low pH in the coagulation-flocculation tank when the sewage had been diluted with rainwater. This procedure has been left and has been changed over to a feed of ferric chloride related with the pH in the flocculation tank. That a m o u n t of flocculant has been dosed in the tank to reach a fixed pH-value. A presupposition of this way of adding ferric chloride is that the required feed of ferric chloride is proportional to the alkalinity of the sewage a n d that this magnitude is related to the concentration of pollution. Dependent on the pH and the alkalinity of the influent the dose of FeCl 3 ranges from 40 to 160rag Fe/l sewage at a fixed pH-value in the flocculation tank of about 5.3. This way of coagulation-flocculae
E
+0 -+ 3 -+ 8 -+ 5 -+ 7 + 5
f
60
c L) 0 I.-
40
2O
0 --%
3
I 4
I 5
I 6
t 7
I 8
9
pH Fig. 5. Coagulation-flocculation with alum in batch experiments. The pH versus the percentage T O C removal at an alum dose of 47 mg Al/1.
Fig. 6. Flow-scheme of the pilot-plant, a. influent; b. dosage of coagulant; c. coagulation-ttocculation-tank; d. D o r t m u n d sedimentation tank; e. sandfilter; f. activated carbon column.
Table 4. Removal of specific c o m p o u n d s with chemical treatment of sewage. Batch experiments
Component
Unit
BOD5 COD Proteins Ammonia Organic N Detergents Ortho-phosphate Total-phosphate Acetic acid Propionic acid
mg O 2 / 1 mg O 2 / 1 mg/l as albumine mg N/l mg N/1 mg/l (anion-active) mg P/I mg P/I mg/1 mg/l
Percentage removal with Raw wastewater Ferric chloride Hydrated lime Alum (total) 75 mg Fe/l at pH 11.0 47 mg AI/I 248 723 58 28 54 15 l0 31 44.5 8.5
-
63 72 63 16 6 56 99 96 0
59 69 62 18 10 47 99 94 12
l0
0
-
59 72 59 14 7 74 99 97 4 19
Physico-chemical treatment of municipal wastewater
Table 5. Amount and dry solid content of the chemicalorganic sludges produced at focculation of domestic sewage with ferric chloride, lime and alum
I00
SO
6o (.Ju O0 ~-o
39
.
X
~
Volume sludge (in 1) Coagulation-flocculation per liter with: sewage
x--TOC
'''x~
Dry solids content g per liter sludge
40
~,-- DOC
20
o
I
IO
5 mcj Fe / t
Fig. 7. Pilot-plant experiment with hydrated lime as coagulant at pH 11.0. The effect of ferric chloride as coagulant aid on the elimination of TOC and DOC.
tion is a rather expensive one. In the following experiments a constant amount of ferric chloride is dosed, in which the pH by means of NaHCOa and/or HzSO4 has been fixed at a pH of 5.3 or 7.7 in order to study the two parameters (pH and flocculant dose) independently. The efficiency of the coagulation-flocculation of sewage with iron-llI-chloride is higher at pH 5.3 (iron dosage on average 61 mg Fe/I) and the variation in percentage reduction is less than at pH 7.7. These values are:
pH 5.3 7.7
Percentage removal of Total Soluble Suspended solids organic carbon organic carbon (nag C/I) 54% + 14 50% + 24
0.069 0.025 0.065
5.0 25.0 4.5
I
I
o
Ferric chloride (75 mg Fe/1) Hydrated lime (at pH 11.0) Alum (47 mg AI/1)
27% + 12 19% + 10
93% _+ 4 77% + 11
The reduction of total phosphorus amounts in all cases to about 80%. When coagulating with lime the process is determined by the pH. These experiments have been executed for 9 months. In the continuous flow experiments the influent shows also a wide variation in organic carbon content too. The experiments have shown that the eliminated amount of organic carbon increases accordingly as the influent TOC rises, The addition of small quantities of iron-IlIchloride to the wastewater beside lime leads to some improvement of the effluent quality. See Fig. 7. In order to improve the coagulation-flocculation process also recirculation of sludge from the sedimentation tank to the flocculation basin has been practised. The theories about orthokinetic flocculation indicate that by increasing the volume fraction of the particles the formation of bigger floes is stimulated. In view of the wide variation in reduction with and without sludge recirculation in the flocculalion process with lime a conclusion cannot be drawn whether sludge recirculation is positive or not. The total reduction of organic carbon from the influent by coagula-
tion-flocculation with lime at pH 11.0 is 44% _+ 15; with lime at pH 11.0 and 5 mg Fe/i as coagulant aid this percentage is 51% +_ 13. The elimination of suspended solids is about 90%. For five months continuous flow experiments with alum as coagulant have been carried out. According to the results obtained from the batch experiments, the optimum dosage of alum in the pilot-plant is about 47 mg AI/I sewage. It is known that the alum-floc is very fragile. Therefore during two months 1.2 ppm of a cationic polymer has been added as a coagulant aid. No significant effect on the TOC-removal has been found however. With alum as coagulant it has also been studied whether recirculating chemical-organic sludge has a positive effect on the reduction of organic carbon. This has not been proved to be a significant success` The total reduction of TOC at a dosage of 47 mg AI/I at pH 5.9 is on average 58~ + 10. The reduction of dissolved organic carbon is 24Yo + 13. The removal of suspended solids is in all cases greater than 85~o and on average 94%. The three coagulants (ferric chloride, hydrated lime and alum) produce different amounts and types of chemicalorganic sludge. The quantity and the solid content of the sludge (without any treatment) that fows out of the sedimentation tank are given in Table 5. The results of the sludge treatment will be reported later. DISCUSSION
Based on the results obtained in this study the following conclusion can be drawn. The suspended a n d most of the colloidal fractions (in terms of T O C ) are efficiently removed from the raw sewage by coagulation-flocculation followed by settling. For ferric chloride, lime and a l u m the percentage T O C removed of this fractions does not differ much. The percentage reductions have been summarized in Table 6. A considerable fraction, which is defined as soluble, is removed too. F o r hydrated lime as coagulant the removal of dissolved organic c a r b o n (DOC) is a b o u t 17~o. F o r ferric chloride a n d alum these percentages are respectively 27 a n d 24%. In adding ferric chloride as coagulant aid to the flocculation with lime at pH
Table 6. Coagulation-flocculation experiments on pilot-plant scale
Coagulant Ferric chloride Hydrated lime Alum
Dose
pH
61 mg Fe/l 520 mg Ca(OH)2/I 47 mg AI/I
5.3 11.0 5.9
Percentage removal Suspended Soluble solids Total 27 17 24
93 90 94
54 44 58
40
_
J. LEENTVAAR,W. G. WERUMEUSBUNINO and H. M. M. KOPPERS pilot-plant have been started in October 1975. The results will be published in the future.
IO0I
,oFf/ *~ 20~// At 0
Acknowledgements---This study was carried out with partial support of Haskoning (consulting engineers) Nijmegen, Holland and of Norit N.V. (manufacturer of activated carbon) Amersfoort, Holland. We wish to acknowledge especially the contribution of Mr. W. C. van Lier (Norit) and Dr. G. Lettinga (Dept. of Waterpurification, Agricultural University). Acknowledgement is also due to Mr. T. S. J. Ywema, for his assistance in the experimental work.
co i
i
0.~0
0.20
Coagulant cost,
I
0.30 f/m 3
Fig. 8. The coagulant cost per m 3 sewage versus the removal of TOC. Batch experiments.
11.0, the removal percentage of D O C rapidly increases to circa 23yo. The resulting effluent of a coagulation-flocculation sedimentation plant consists to a large extent of soluble organics, which are easily biodegradable. Regarding the coagulant cost in relation to the T O C removal (Fig. 8) the use of iron-Ill-chloride as coagulant should be favourable. This study indicates that for overloaded conventional mechanical-biological sewage treatment plants a chemical treatment as the first step in a municipal wastewater treatment can be favourable, although the percentage removal is n o t so high as obtained by other investigators like Cooper & T h o m a s (1974). This study will be continued with particular attention to this subject. The results obtained on batch scale and in the small scale pilot-plant with a flow of 0.06 m 3 h - 1 are being checked in a larger pilot-plant, having a capacity of 2 m 3 sewage h - 1 a n d consists of a chemical treatment stage, a multimedia sand-filter a n d several activated c a r b o n columns. The chemical treatment stage has been designed as a plug flow reactor with six c o m p a r t m e n t s which are each individually stirred. The experiments with this
REFERENCES Bekker P. de & Leentvaar J. (1974) Investigations on physico-chemical methods of wastewater treatment (Dutch). H 2 0 7, 278-280. Black A. P. & Christman R. F. (1961) Electrophoretic studies of sludge particles produced in lime-soda softening. J. Am. Wat. W k s Ass. 53, 737-747. Cooper P. F. & Thomas E. V. (1974) Recent developments in sewage treatment based on physico-chemical methods. Wat. Pollut, Cont. 73, 505-520. Kossen N. W. F. (1974) Non-biological treatment of wastewater (Dutch). H 2 0 7, 234-236. Liepe F. (1966) Kennzahlen zur Bewertung von Riihrern. Chem. TechnoL 18, 230-235. Minton G. R. & Carlson D. A. (1973) Primary sludges produced by the addition of lime to raw wastewater. Water Res. 7, 1821-1847. Rehbun M. & Streit S. (1974) Physico-chemical treatment of strong municipal wastewater. Water Res. 8, 195-201. Roberts P. & Stumm W. (1974) Behandlung yon kommunalem Abwasser mit Aktivkohle. Gas-Eaux-Eaux us~es 54, 78-88. Standard Methods for Examination of Water and Wastewater (1965). 12th Edn. American Public Health Ass. Van Vuuren L R. J., Stander G. J., Henzen M. R., Meiring, R. G. J. & Van Blerk S. H. V. (1976) Advanced purification of sewage works effluent using a combined system of lime softening and flotation. Water Res. l, 463-474. Weber W. J., Hopkins Ch. B. & Bloom R. (1970) Physicochemical treatment of wastewater. J. Wat. Pollut. Control Fed. 42, 83-99. Wuhrmann K. (1972) La charge des eaux par les pollutants r+fractaires. FiJderation Europ6ischer Gewiisserschutz (19). 13-21. Zuckerman M. M. & Molof A. H. (1970) High quality reuse water by chemical-physical wastewater treatment. J. Wat. Pollut. Control Fed. 42, 437-456.