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Removal of trihalomethane precursors from eutrophic water by dissolved air flotation
War. Res. Vol. 27, No. 1, pp. 41-49, 1993 Printed in Great Britain.All rightsreserved
0043-1354/93 $5.00+ 0.00 Copyright © 1993 PergamonPress Ltd
REMOVAL OF TRIHALOMETHANE PRECURSORS FROM EUTROPHIC WATER BY DISSOLVED AIR FLOTATION R. G ~ m t, C. SWARTZ2 and G. OV'r'RrNGA3 'Departmentof CivilEngineeringand Applied Mechanics,McGill University,Montreal,Quebec, Canada, H 3 A 2K6, 2Divisionof Water Technology, CSIR, P.O. Box 395 and 3Water Research Commission, P.O. Box 824, Pretoria0001, Republic of South Africa (First received April 1991; accepted in. revised form June 1992) Abstract--Batch, pilot and full-scale dissolved air flotation (DAF) tests were performed on a eutrophic water source to determine suitable physical and chemical (coagulation) conditions for the removal of trihalomethane precursors (THMP). The key parameters were identified as inorganic coagulant dose, pH control and possibly polymer addition. Factorial experiments on the pilot plant showed that 150mg/l ferric chlorideaddition and a pH of 5 would be appropriate; the need for polymer addition was dubious. Under these conditions, it was possible to reduce total (unfiltered) trihalomethane concentrations by over 80% to below 0.1 mg/l. When a full-scale DAF plant was operated under the conditions suggested by the pilot plant, THM reductions were not as dramatic, probably due to poorer chemical mixing conditions. An analysis of varions raw water quality parameters suggested that algae were responsible for a significant proportion of the precursor concentration. Dissolved organic carbon (DOC) was found to be a poor indicator of THMP concentration; furthermore, these THMPs were selectively removed from the overall DOC pool during the coagulation/DAF treatment process. Key words--trihalomethaue precursors, eutrophic waters, coagulation, dissolved air flotation, algae
INTRODUCTION
tests was conducted. An attempt was also made to use the collected data to demonstrate that the algae and their associated exocellular organic material (EOM) were major contributors to the precursor pool.
Eutrophic lakes, with their excess concentrations of algae and other biomass, are not ideal sources of potable water. Apart from taste and odour problems, interactions between chlorine and the various organics resulting from algal activity may produce undesirable products, such as trihalomethanes (THM), which could have negative long-term health implications (Briley et al., 1980; Hoehn et al., 1984; Oliver, 1983; Oliver and Shindier, 1980; Paimstrom et al., 1988). In many cases, the algae (or cyanophyta) will be naturally buoyant [many Microcystis spp, for example, exhibit this behaviour (Walsby, 1969)]. It has thus been found that dissolved air flotation (])AIr) is an appropriate separation technology (replacing gravity settling) for removing algae prior to filtration at water treatment plants (Bare et aL, 1975; Williams et al., 1985). However, the effectiveness of D A F at removing the trihalomethane precursors (THMP), as biomass and/or dissolved organic matter, has not been demonstrated. The objective of this study was to evaluate the D A F process in terms of its ability to remove T H M P from a eutrophic water body (the Hartbeespoort Dam, near Pretoria, Republic o f South Africa). In particular, physical and chemical parameters were assessed for their influence on the removal process, and optimization of the key parameters was attempted. Factorial experimental designs were employed in hatch and pilot tests to provide a statistical basis for the assessment. A limited range of full-scale
ANALYTICALAND STATISTICALM E T H O D S THMP concentrations were obtained (following filtration, when used, through Whatman GF/C glass microfibre filters) indirectly by chlorinating the samples under standard conditions, then performing THM analysis, as follows. The chlorine demand of the raw sample was first determined, using the sodium thiosulphate----starch/iodide end-point method (APHA et al., 1985); subsequent chlorine doses were then set such that a residual ofat least I mg/l 0 2 after 2 days reaction was available. A fresh ~mple was buffered at pH 7 with a phosphate buffer, then chlorinated at the appropriate dose with a l0 mg/1 dosing solution, and stored for 2 days at 20°C in the dark. The reaction was halted by adding a sulphite solution. Reagent blanks were prepared in a similar fashion. THM concentrations were evaluated by liquid-liquid extraction, followed by gas chromatography tudng an electron capture detector (Van Rensburg and Hassett, 1982). The detection limit of this method was approx, l/Ag/l. It should be noted here that to avoid confusion with usage of the terms "THM" and "THMP", since the former are measured directly and could be present in the final, chlorinated waters, whereas the latter would be present in the raw water and might be removed by the coagulation/DAF process, refe~e~e will henceforth be made to THM re. duction and THMP removal The term "trihalomethane formation potential (THMFP)" was not used in this paper because the potential for trihulomethane formation could be affected by process conditions such as pH, chlorination dose and contact time etc., whereas the empham here was on 41
42
R. G~nt et al.
removal of the organics which were precursors to the THMs. Dissolved organic carbon (DOC) on filtered rumples was measured by u.v. oxidation in a silica tube, with potassium permlphate catalyst (Van Steenderen and Lin, 1981). Chlorophyll-a was determined by 5-rain extraction of material, retained on glass-fibre filters, into 96% ethanol at 78°C, followed by spectrophotometric determination of the concentration (at 665 nm; see Sartory, 1982). Chemical oxygen demand (COD) and suspended solids (SS) were measured according to Method Nos 508C and 209C in Standard Methods (APHA et aL, 1985); phosphorus and TKN were performed according to the methodology suggested by Technicon (1975). The factorial experimental designs were based on procedures developed by Cochran and Cox 0957), but analyses of most of the 2~ designs were performed directly by a procedure in the "Statgraphics" computer package (STSC, 1988). Briefly, for 2' designs, n independent variables are set at two levels (either high or low values, with the high value indicated by the presence of a code letter) for each test, and the effect of each varmble is evaluated by analysis of variance, or by cumulative probability plots (Box et a/., 1978). With the latter technique, when a straight line is drawn through or between most points on the plot, such that it passes through or close to the zero effects, and 50 cumulative percent coordinates, respectively, then points lying at an appreciable distance off the line are indicative of effects which are sJ~if~ant. This factorial procedure thus allows one to evaluate statistically the impact of several variables applied simultaneously, as well as their interactions. The variabilityin influent quality (see below) would compromise the power of the statistical procedures, hence percentage removals have been used throughout to minimize this effect.
RAW WATERQUALITY The raw water was sampled for more detailed analysis at various times during the period of the study. Values are given in Table 1. Variability o f some parameters was relatively high; for example, the standard deviation for COD in 1986/1987 was 17.6
for a mean of 39 rag/l, and total (unfiltered) T H M P had a standard deviation o f 48.4 for a mean of 161 ~tg/l during February and March 1989.
Experimental methods
The objective o f the preliminary experiments was to screen a number of variables and their levels in a batch unit, and then to select those to be used in further pilot plant tests. Batch D A F tests were done using a 6-cylinder apparatus akin to the familiar "jar test" (Water Research Centre, 1977), but with the jars replaced by conical-bottomed cylinders, 0.1 m dia and 0.44 m tall, into which pressurized recycle water (at 400 kPa) could be fed. Flocculation of the sample could thus be achieved via chemical addition, as well as rapid and slow mixing (2rain at 100rpm, followed by 15 min at 20 rpm, respectively), before the addition of the air-laden "recycle" (in this case, pressurized tap water). The first set of batch factorial experiments (2s) tested the following variables: recycle ratio (6.4 and 12.8%), alum dose (0 and 6.5 m8/l as AP+), polymer dose [Percol 757 cationic polymer (Allied Colloids Ltd), 0 and 0.6 mg/l], bentonite dose (0 and 60 rag/l) and pH (6 and ~ 9 , where ~ 9 was the natural p H o f Hartbeespoort Dam). Only alum, polymer and pH had sJ~ificant effects. Thus a second set of batch tests was done, this time using a 33 factorial design (i.e. three variables at three levels), with the following variables: - - a l u m : 3.3, 6.5, 9.8 mg/l as Al3+ - - p o l y m e r : 0.3, 06, 0.9 mg/l - - p H : 5, 7, ~ 9 .
Table 1. Water quality from the HartbempoortDmn at differentperiods of the study (averagevahma) 1986-1987 Feb. + March 1988 April 1988 Feb. + Match 1989 Variable (17-23 samples) (4--7 samples) (1-3 samples) (3-3 samples) Bromide (m~) 0.63 ---(0.19) Chlorophylla ~ 1 0 ) 52,8 60.0 6.23 32.6 COD (ms/l) DOC (rag/l) Dominant algal 8peO~
pH SUgl~nU~dsolids (m~)
(45.4)
39.1 (17.6) 8.88 (2.63) Microcystb aeruKt~m (& Oocystb qrp in Oct.,Nov. and Dec.) 9.1 (8.4-10.3) 13.5 (8.6)
THMP 0ag/i)
--
TKN ~s/l)
868 (8.1)
Total pho~horm (~10)
408
• ~r~dity (NTU) NoW ~ i , , . in p . ~ t h m
--
(36.0) --
7.47 (0.81) Mk~'ystb ~ a
6.8 (0.42) Cryptomonas q~p, CycioteOar~neghMiana
9.7 (9.6-9.8) --
9.2
8.7 (8.5-9.0)
--
--
344
242
161
06.4)
03.D
(~.4)
718 (170) 318
(38:7)
(10.7)
(3.6)
d~,~tiom. ~
--
8.62 (I.06) M~oystb aer~Inoaa
(55)
12.7
.~ .HU~
(38,4)
12.t
for p~. ~
I,H4 271 3.0
(l.i) ~ .rap le Si,~.
~96 (345) 56
(162)
3.0
(0.89) ,.
THMP removal from eutrophic water by DAF
43
Table 2. Analysis of batch 33 factorial experiments. Analysis of variance for THMP removal Source
Degrees of freedom
Sum of squares
Mean square
F ratio
F,~ s~.
2 2 2 2
6 ! 3.1 1571 5.03 1032
306.6 785.5 2.5 516.0
32.67 83.72 0.267 54.99
5.14
4
222.8
4.53
Main effects Blocks Alum Polymer pH Interactiona Alum/polymer Alum/pH Polymer/pH Error (by subtraction) Total
55.7
5.94
4
233.3
58.3
6.22
4 6
28.3 56.3
7.1
0.756
26
Recycle ratio was constant at 12.8%, and no bentonite was used.
Results from batch tests Results of the 33 factorial tests arc shown in Table 2. Analysis of variance was used here because probability plots for 33 factorial experiments were not available on the "Statgraphics" package. The results indicate that alum and pH have statistically significant effects (F-ratio much higher than critical). Plots (not given herein) of two variables averaged over the third showed a linear increase in THM reduction with alum dose, but only a slightly higher reduction for pH below 7. Thus, the highest alum dose and lower pH would be desirable. The variation in polymer dose at the relatively high levels tested was not critical. All three variables were therefore chosen for testing in the pilot plant, but with polymer applied at zero and the middle dose only (see Table 3). The results obtained above for alum and pH effects were not unexpected. Inorganic coagulants such as alum and ferric chloride are known to be successful at removing THMP (Chadik and Amy, 1983), and optimum pH levels for these have been found in one study with humic material to be 5-6 for alum and 3-5 for ferric salts (Collins et aL, 1986). These authors also believed that the precursors themselves were affected by pH, due to acidic functional groups on the humic substances.
3762
Treatment Plant on the banks of the Hartbeespoort Dam, and run in parallel with the plant's full-scale DAF unit (see later in the paper). Raw water, drawn 1 m below the dam water surface, was pumped at 6.2 l/min through rapid-mix and slow-mix tanks, and through a baffled flocculation section before entering the flotation tank. The detention time, including a recycle flow of 10%, was 8.5 rain. The air:solids ratio (A:S), at a suspended solids concentration of 13 rag/l, and assuming 90% saturation of the recycle flow which was pressurized at 400 kPa at 25°C, was 0.45. Downward velocity in the clarification section of the DAF unit (i.e. undertow rate), including recycle, was ~ 7 mfa. The chemicals used included alum or ferric chloride (but contrary to the original plans, the latter was the only inorganic chemical to be used in the full-scale plant, hence it would be the only inorganic chemical included in the factorial experiments reported below), Magnafloc LT 24 (Alfied Colloids) cationic polymer (since Pereol 757 was no longer available in the country), and 1 N sulphuric acid. A Hewlett-Packard data logger/control system (Models 71B Portable Computer, 3421 Data Acquisition/Control Unit, and 9114 Disk Drive) was used to control pH, and monitor influent temperature, inttuent and flocculated pH, and influent and treated effluent turbidity.
Results from pilot plant tests PILOT PLANT
Equipment used The pilot plant DAF unit is shown diagramatically in Fig. 1. It was set up at the Schoemansville Water Table 3. Codes for pilot plant
Code A B C 0
L
Condition High FeCi3 d o s e - 150mg/! (as FeCI3) High polymer dose = 0.6 mg/l High initial pH - 9.0 Low variabbs condition: FeC13 - 50 mgl Polymer dine - 0 pH-5 Alum dora at 150 ~ (as alum)
Note: (1) for coda A. B, C and L, all ~ not mentioned were at the low setting given under code 0. (2) pH values are approximate; low final pH range 4.8--6.0 and high final pH nmge 6.7--8.6.
Codes for the pilot plant test conditions are shown in Table 3. Basic iniluent/efliuent results are plotted chronologically as paired histogram~ in Fig. 2 for total (i.e. unfiltered) THM and bromodichloroform (CHBrCIz) (both following chlorination), respectively. Percentage reductions for filtered THM, CHBrCI, and DOC are shown in Table 4. The tests were actually performed during the summer months of 1988 and 1989, and a chronological plot of the data would show a slight downward shift of concentrations of the above three variables in the (chlorinated) raw water. A sudden drop in CHBrCI, concentrations occurred for tests 10, 11 and 12 (i.e. conditions BC, C and AB; Fig. 2), which were performed immediately following dam overflow (at the end of March 1988) and concomitant loss of algal scum. Some marine algae have been shown to contain
44
R. Gmm et al. A d d remrveir
Effluent
FÁ••--
~
p~l
temperature
p14 prem,
~,.I /iL'l'-/I J~ll~l .~11 ;
~/,o,,/.
Addlllonll tlooeulatlon chunber
A_-~I--/V
I-~-J J / ~ I
/
~" From
'
//..o,.,,,,.
et.
i l ~ I ~,-,-~-,-~t 1~1 It L II , I 11 4o,.
IIowmlx
tank
~..,,.~,o
I I /.... I
m~l~/mr
Ill....
I I/~,:"~1
relefvOlrll IIKI
i if
"//I/////////////,,
I1".."~
e~.-,v I
[I
e--,,~,
"/,¢/////////////////////~////~
DAF tlmk
Feed relervolr
Fig. 1. Schematic of DAF pilot plant. substrate bromine at elevated concentrations compared to their water environment (Pallaghy et aL, 1983; Saenko et al., 1978), and similar behaviour with the algae in the dam would explain why their loss was associated with considerable reductions in the overall bromide concentrations (normally at ~0.63 mE/l). Table 4 shows that under certain conditions, such as codes B, ABC and AC, the pilot plant was able to achieve above 65% filtered THM reduction. Concentrations below the current US standard of 100/48/1 were often achieved (last 6 tests in Fig. 2), although
it should be realized that in reality, THM concentrations at the tap would be lower due to the less rigorous reaction conditions which would occur during normal water chlorination, and the fact that total, rather than filteredT H M values are shown in this figure. Removal of CHBrCI 2 (Fig. 2 and Table 4) is generally not as good, but this is to be expected, as it is not the brominated precursors which are being removed, but the organic pr~-ursors in general. Addition of chlorine to water containing bromides will oxidize these to bromine, which in turn is a more
Conoentration (pg/L) 850 300 I"1
260
R
200 150 100 5O 0
LBO LB
LG
LG
1 L
A
r-I
o
LBC LB B(~
CHBrCI2
I
G
AB
BC
B
0
ABO AC
I Total T H M
Fig. 2. Pilot plato total (-nflltmxl) THMs and C H E ~ .
I n f l m t and i~um~t ('m test i~itw~ce).
THMP removal from eutrophic water by DAF Table 4. Reductions (%) of filteredTHM, filtered CHBt'CI=and DOC in the pilotplant
99.9
Code
99.0
LBC LB LC LC L A
T H M (filtered)
LBC LB
21.3 39,3 32,0 21.1 28.9 53.6 50.0 6. ! 4 26.9
BC
44.4
C AB
---75.9 36.4 72.5 68.7
0
BC B C
ABC AC
C H B r C i 2 (filtered) 10.3 19.5 15.1 12.1 17.1 29.3 27.4 I i .0 18.0 70.0 --51.4 47.0 19.2 53.8 41.9
DOC --?+6.1 --54.2 60.4 9.30 26.7
24.4
45
95.0 °~a. 80.0 50.0
"5 2O.O E _ " 5.0
23.3 67.1 43.8 50.0 44.1 44.5 41.9
@
1.0 0.1 -I -20
I -10
I 0
I 10
I 20
I 30
Effects (%)
Fig. 3. Filtered THM percent reduction in the pilot plant. Normal probability plot for factorial analysis. powerful reactant than chlorine is in producing halogenated organics (Cooper et aL, 1985). Thus organics remaining after flocculation and D A F would tend to be scavenged first for brominated species. An analysis of the results of the eight factorial experiments (Tests A, 0, BC, AB, B, C, ABC and AC, shown in order in Fig. 2) based on percentage reduction, is shown in Table 5, and Figs 3 and 4 for filtered T H M and DOC, respectively. The first row in this table indicates the average percentage reduction of each component (such as filtered T H M , etc.) for all eight tests, and subsequent rows show the effects (above or below the average) for the various combinations of test parameters. Typical probability plots for the effects of these two factors are given in Figs 3 and 4; ringed points on these figures which lie at an appreciable distance from the line (drawn automatically by Statgraphics) through X ffi 0 are considered to be significant, and are underlined in the table. A high dosage of FeCI3 (i.e. 150 mg/l; code ,4,) and a low pH coupled with the presence of polymer (negative effects for high pH) are significant for improvements in filtered T H M reduction. Interestingly, DOC removal is affected significantly only by a low pH. The graphical methodology for assessing signllicance was not suitable for analysing the total T H M results, due to the high values of the higher-order interaction sum-of-sclnares (SS); see Table 6. In fact, Table 5. Analysisof pilot plant 23 factorial experiments Code Filtered THM DOC Averase reduction ( % )
A B AB C AC BC ABC
62.7
50.9
22.0 21.-"'6 4.1 -14.4 8.2 -15.1 -6.2
2,6 1.45 6.8 -14.1 -2.85 0.2 -4,85
Note: the tabulatedv~lumfor eodmA, B etc., L,~4"~eeffects above or ~ the avmmlc. Underlined values arc rinpd +- Fis, 3 ,J+d 4. Wlt 2711--0
none of the main effects were significant, but the low pH had the greatest observed effect and highest SS. It is not clear whether this lack of significance is due to 0aboratory) analytical problems when evaluating total THMs, or because particulate removal by DAF is inherently efficient, and would not be greatly improved by changes in the levels of the chosen factors (note that the low variable condition still involved the presence of the inorganic coagulant at 50rag/l). However it is clear that the dissolved THMPs were being removed more effectively at the high FeC13 dose and low pH. This is perhaps a more interesting result, as it indicates that the effectiveness of the overall D A F system extends beyond simple particulate removal.
Evidence for the contribution of algae to the precursor pool Figure 5 shows a strong correlation between influent total T H M P and p H (R =0.914). Additional stepwise regression with turbidity and DOC failed to improve the model significantly, however a high
99.0 n 96.0 • 80,0 O. 5o.o
20.0 ~
6.0 1.0 0.1
"I -15
I -10
I -.6
I 0
I 5
I 10
Effects (%) Fig. 4. D ( ~ percent nnnoval in the pilot plant. Normal probability plot for factorial analysis.
46
R. G ~
et al.
Table 6. Analysis of variance for total THM removal in the pilot plant* Observations (%)
Code 0 A B AB C AC BC ABC
Total effect
60.3 67.5 85.1 87.8 54.0 49.1
55.2 41.2 5O.4 -98.4 35.2
74.6
59.6
24.6
Degre~ of Sum of freedom squares F ratiot 381 212 317 1,210 155 303 444
-49.2
2.73
*For methodology see Cochran and Cox (1957). t F ratio = SSe~.t./SS~ F~es.l., ffi 161.5.
correlation was also observed between turbidity and pH (R : 0.806). pH levels in the dam are to a large extent regulated by algal activity; an increase in this activity would lead to higher pH values as well as increased turbidity. Thus, these two correlations provide circumstantial evidence that algae and their EOM are responsible for a significant portion of THMP in the dam. From the few paired data points which were taken during the pilot plant work, as well as hourly samples taken over a 24 h period, it appeared that influcnt THMP and chlorophyll-a were also well correlated (correlation coefficients of 0.88 and 0.66 for filtered and total THMPs for the paired data, and 0.83 for total THMP for a 24-h experiment).
FU~ALE
PLANT
Plant setup A diagram of the full-scale DAF system at Schcemansville is given in Fig. 6. Ferric chloride is added, together with polymer (Organofloc 863), and at times powdered activated carbon (PAC) (see Table 7), into the influcnt weir, and flocculation occurs in an unmixed basin. Thus, coagulation conditions are not as well managed as in the pilot plant, and this is reflected in poorer performance (see below). Each DAF unit has an area of 18.6 m 2, and a nominal detention time of ~ 9 rain at the test flow rate of 4.2 m3/min. The air dissolution system is similar to that of the pilot plant, and the underflow rate of the DAF unit was 8 m/h, including a recycle flow of 10% of feed.
Operating results from the full-scale plant
The full-scale plant was monitored during the period of the pilot plant tests, and had the same intiucnt. Conditions were set to be as close as possible to the pilot plant. However, physical limitations and the need to maintain a water supply to the community precluded the estabfishment of a comprehensive test protocol. Results for percentage reduction of total THM only, in test sequence, are shown in Fig. 7. It is clear from Fig. 7 that the full-scale plant performance is generally not as good as that of the pilot plant. Maximum unfiltered THM reductions Relationship between DOC and THMP were approx. 65% (compared with over 80% for the Previous researchers have suggested that, for vari- pilot plant), and this occurred, interestingly, at low ous reasons, DOC and THM levels may not corre- FeCI~ and polymer doses (test code 0). High total late, and that selective removal of THMP from the THM reduction was also experienced under test DOC pool may occur during coagulation processes condition BD (which included PAC), and high (Collins et aL, 1986; Hoehn et al., 1984; Oliver and filtered THM reduction (not graphed) occurred at Lawrence, 1979). In the present study, the regression condition A (high FeCI~). coefficient between filtered influcnt THMP and DOC It is to be expected that the full-scale plant would was low (0.57), which implies that only a portion of give poorer performance than the pilot plant, because the dissolved organics are responsible for THM. the former had no pH adjustment, neither were the Percent reductions for THM are often higher than for flocculation conditions as well controlled. From DOC (Table 4), confirming the results of the above visual inspection of the results, it does appear that researchers that selective removal of THMP can be high doses of polymer are unnecessary (condition expected. BX), and in fact may be detrimental to THM removal (it is known that the polymer itself could contribute 4OO to the THMPs---see Soponkanaporn and Gehr, 1989). The single test with PAC yielded an effluent THM concentration below 0.1 rag/l, but it was also 300 possible to achieve this level simply with high doses of FeCI3 and no polymer.
•
200
•
e
u
CONCLUSIONS
o "1 8.0
I 8.5
I g.o
pN
I g.5
I lO.0
Fig. 5. Reg~ufion of pilot plant influent total THM on pH.
Dissolved air flotation (DAF) has been found capable of removing trihalomethane precm~rs (THMP) from eutrophic waters to below the 100/Jg/l THM standard in the U.S.A. Desirable conditions for the case studied, based on a statistical ~nA!ysis of factorial experiments, appear to be 150 mg/l FeCI3 dosage and a pH of around 5. This low pH level is also advantageous for rapid chlorination to achieve
I
Redrculatedwmor
n pipe
Sludge drain
~'~ / / ~
Flow mater Air saturated water st 400 kPa
Fig. 6. Schematic of full-scale DAF system at Schoemansville.
Float wdve
Ranged branch . _ ~
~ed
Packing Is 25mm dis x 30ramlong d plasticpipe,randomly
/ T , /
/
~
t t
j r
/
/
/ / ~
w
Flocculated water Collector outflow pipe pipes
From air compressor (400 kPa) Pressure gauge Oiatdbutor
/
Reclrculationwater p i p e from air saturator
Micro - bubble rmT.zle
Pressure releasevalve
~" ~,
Dmwon pipes
Old settlingtank fined with concrete
~
pipes
1
Chain driven ~ scraper
CollectOr
Emuemto rind sand filter
Float stabikmllon gdd
I I~
Powdered carbon
Flash mixing vessel
--w-.,
- - coa0.1ar.
slurry
Large settlingtank converted to hydraulic ficcculationtank
.r
/'~
/
-~
"n
o
48
R. Gwat et al.
% reduction 70 60 60 40 30 20 10 0 BX
BX
BX
B
BX
BD
O
AB
A
Total THM Fig. 7. Total THM percent reduction in the full-scale plant (in test sequence).
A B BX D 0
Table 7. Codm for fuli-w.aleplant Condition High FeCI3 dote-. 100mffi (as FeCI~) robjmer dou- 0.6 m~ Polymer doae- 6.0 mg/l PAC dine - 10mg/I Low variable condition: FeCl3- 50rag/I Polymer dine - 0 PAC dme-O
disinfection, hence overall reaction times could be minimi:,ed, thus further reducing final THM concentrations in the delivered water. Naturally, the pH of this water would eventually have to be raised appropriately. Polymer addition would probably be unnecessary for total T H M P removal, although it may be required for removal of turbidity, etc. An analyms of the interaction of pertinent water quality parameters indicated that algae are likely to be major contributors to the total precursor pool. In addition, it was shown that DOC is not a good indicator of precursor concentration, and that THMPs are eelectively removed from the overall DOC pool. Full-w.41e teats were also encourasing, and it appcared that, provided the process was operated under the conditions suggested by the pilot plant tests, powered activated carbon (PAC) is probably unnecessary for T H M P ~e~oval. Acknow/edzements--Fundins for this project by the South African Water Research Commismon is gratefully acknowledpd. The first author was supported by a grant from the Fotmdation for Rmearch Development during the period of the study. This paper is published with the permission of the Director, Division of Water Technology, CSIR, and the Executive Director, Water Research Commission.
REFERENCES APHA, A W W A and W P C F (1985) Standard Methods for the Examination of Water and Wastewater, 16th edition. American Public Health Association,American Water Works Amc~iation and Water Pollution Control Federation, WaShington D.C. Bare W. F. R., Jones N. B. and Middlebrooks E. J. 0975) Alsae removal using dissolved air flotation. J. War. Pollut. Control Fed. 47, 153-169. Box G. E. P., Hunter W. G. and Hunter J. S. (1978) Statistics for Experiments, pp. 329-334. Wiley, New York. eriley K. F., Williams R. F., Longley K. E. and sorbert C. A. (1980) Triludomethane production from algal precursors. In Water Chlorination: Eneironmental Impact and Health Effects (Edited by Jolley R. L., Brunp W. A., Cummin s R. B. and Jacobs V. A.), Vol. 3. Ann Arbor Science, Ann Arbor, Mich. Chadik P. A. and Amy G. L. (1983) Removing trihalomethane precursors from various natund waters by metal con£tdA, ta. J. Am. War. Wks Ass. 751 532536. Cochran W. G. and Cox G. M. (1957) Exper/menbd Designs, 2nd edition, Chap. 5. Wiley, New York. Collin~ M. R., Amy G. L, and Stelink C. (1986) Molecular weight distribution, carboxylic acidity, and humic substances content of aquatic or nni¢ matter:, implicationa for removal during water treatment. Ewoir. $ci. Tecimol. 20~ 1028-1032. Cooper W. J., Zika R. G. and Steinlmuer M. S. (1985) Bromide-oxidant interactions and THM formarion: a literature review. J. Am. War. ~ Ass. 77, 116-121. Hoelm R. C., Dixon K. L., Malone J. K., Novak J. 1". and Randall C. W. (1984) Biologically induced variations in the nature and removability of THM precursors by alum treatment. J. Am. War. ~ Ass. 76, 134-141. Oliver B. G. (1983) Dihaloacetonitrilu in drinkin~ wxtm':. algae and fulvic acid u precursors. Envlr. $ci. Tedmal. 17, 80-83. Oliver B. (3. and Lawrtmce J. (1979) Haloforms in drlnkln_a watt. a study of pmcurmrJ and precun~r removal. J. Am. Wat. Wks Ass. 71, 161-163.
THMP removal from eutrophic water by DAF Oliver B. G. and Shindler D. B. (1980) Trihaiomethanes from the chlorination of aquatic algae. Envir. Sci. Techno/. 14, 1502-1505. Pallaghy C. K., Minchinton J., Kraft G. T. and Wetherbee R. (1983) Presence and distribution of bromine in Thysan. oclad/a densa (solieriaceae, Gigartinales), a marine red alga from the Great Barrier Reef. J. Phycol. 19, 204-208. palm.qrom N. S., Carlson R. E. and Cooke G. D. (1988) Potential linksbetweeneutrophicationand the formationof carcinogensin drinking water. Lake Reserv. Man. 4, 1-15. Saeuko G. N., Kravtsova Y. Y., Ivaneneko V. V. and Sheindko S. I. (1978) Concentration of iodine and bromine by plants in the Seas of Japan and Okhotsk. Mar. Biol. 47, 243-250. Sartory D. P. (1982) Spectrophotometric analysis of chlorophyll a in freshwater phyto-plankton. Technical Report TR 115, Department of Environmental Affairs, Prctoria, Republic of South Africa. Soponkanaporn T. and Gehr R. (1989) The degradation of polyelectrolytes in the environment: insights provided by size exclusion chromatography measurements. War. Sci. Technal. 21, 857-868.
49
STSC Inc. (1988) Statgraphics, Version 3.0. Statistical Graphics Corporation, Rockville, Md. Technicon (1975) Autoanalyzer methodology. Individual/ simultaneous determination of nitrogen and/or phosphorus in BD acid digests. Industrial method 329-74W. Techicon Industrial Systems, Tarrytown, N.Y. Van Rensburg J. F. J. and Hassett A. J. (1982) A lowvolume liquid-liqnidextraction technique. J. High Resoln Chromatogr. Chromatogr. Commun. 5, 574-576. Van Steendercn R. A. and Lin J.-S. (1981) Determination of dissolved organic carbon in water. Analyt. Chem. 53, 2157-2158. Waisby A. E. (1969) The permeability of blue-green algal gas-vacuoles' membranes to gas. Proc. R. Soc. Land. B 173, 235-255. Water Research Centre (1977) Notes on Water Research No. 13: Laboratory Coagulation Tests. Williams P. G., van Vuuren L. R. J. and van der Merwe P. J. (1985) Dissolved air flotation upgrades a conventional plant treating eutrophic water. Paper presented at the llth Federal Convention, Australian Water and Wastewater Ass., 28 April-3 May, Melbourne.
0043-1354/93 $5.00+ 0.00 Copyright © 1993 PergamonPress Ltd
REMOVAL OF TRIHALOMETHANE PRECURSORS FROM EUTROPHIC WATER BY DISSOLVED AIR FLOTATION R. G ~ m t, C. SWARTZ2 and G. OV'r'RrNGA3 'Departmentof CivilEngineeringand Applied Mechanics,McGill University,Montreal,Quebec, Canada, H 3 A 2K6, 2Divisionof Water Technology, CSIR, P.O. Box 395 and 3Water Research Commission, P.O. Box 824, Pretoria0001, Republic of South Africa (First received April 1991; accepted in. revised form June 1992) Abstract--Batch, pilot and full-scale dissolved air flotation (DAF) tests were performed on a eutrophic water source to determine suitable physical and chemical (coagulation) conditions for the removal of trihalomethane precursors (THMP). The key parameters were identified as inorganic coagulant dose, pH control and possibly polymer addition. Factorial experiments on the pilot plant showed that 150mg/l ferric chlorideaddition and a pH of 5 would be appropriate; the need for polymer addition was dubious. Under these conditions, it was possible to reduce total (unfiltered) trihalomethane concentrations by over 80% to below 0.1 mg/l. When a full-scale DAF plant was operated under the conditions suggested by the pilot plant, THM reductions were not as dramatic, probably due to poorer chemical mixing conditions. An analysis of varions raw water quality parameters suggested that algae were responsible for a significant proportion of the precursor concentration. Dissolved organic carbon (DOC) was found to be a poor indicator of THMP concentration; furthermore, these THMPs were selectively removed from the overall DOC pool during the coagulation/DAF treatment process. Key words--trihalomethaue precursors, eutrophic waters, coagulation, dissolved air flotation, algae
INTRODUCTION
tests was conducted. An attempt was also made to use the collected data to demonstrate that the algae and their associated exocellular organic material (EOM) were major contributors to the precursor pool.
Eutrophic lakes, with their excess concentrations of algae and other biomass, are not ideal sources of potable water. Apart from taste and odour problems, interactions between chlorine and the various organics resulting from algal activity may produce undesirable products, such as trihalomethanes (THM), which could have negative long-term health implications (Briley et al., 1980; Hoehn et al., 1984; Oliver, 1983; Oliver and Shindier, 1980; Paimstrom et al., 1988). In many cases, the algae (or cyanophyta) will be naturally buoyant [many Microcystis spp, for example, exhibit this behaviour (Walsby, 1969)]. It has thus been found that dissolved air flotation (])AIr) is an appropriate separation technology (replacing gravity settling) for removing algae prior to filtration at water treatment plants (Bare et aL, 1975; Williams et al., 1985). However, the effectiveness of D A F at removing the trihalomethane precursors (THMP), as biomass and/or dissolved organic matter, has not been demonstrated. The objective of this study was to evaluate the D A F process in terms of its ability to remove T H M P from a eutrophic water body (the Hartbeespoort Dam, near Pretoria, Republic o f South Africa). In particular, physical and chemical parameters were assessed for their influence on the removal process, and optimization of the key parameters was attempted. Factorial experimental designs were employed in hatch and pilot tests to provide a statistical basis for the assessment. A limited range of full-scale
ANALYTICALAND STATISTICALM E T H O D S THMP concentrations were obtained (following filtration, when used, through Whatman GF/C glass microfibre filters) indirectly by chlorinating the samples under standard conditions, then performing THM analysis, as follows. The chlorine demand of the raw sample was first determined, using the sodium thiosulphate----starch/iodide end-point method (APHA et al., 1985); subsequent chlorine doses were then set such that a residual ofat least I mg/l 0 2 after 2 days reaction was available. A fresh ~mple was buffered at pH 7 with a phosphate buffer, then chlorinated at the appropriate dose with a l0 mg/1 dosing solution, and stored for 2 days at 20°C in the dark. The reaction was halted by adding a sulphite solution. Reagent blanks were prepared in a similar fashion. THM concentrations were evaluated by liquid-liquid extraction, followed by gas chromatography tudng an electron capture detector (Van Rensburg and Hassett, 1982). The detection limit of this method was approx, l/Ag/l. It should be noted here that to avoid confusion with usage of the terms "THM" and "THMP", since the former are measured directly and could be present in the final, chlorinated waters, whereas the latter would be present in the raw water and might be removed by the coagulation/DAF process, refe~e~e will henceforth be made to THM re. duction and THMP removal The term "trihalomethane formation potential (THMFP)" was not used in this paper because the potential for trihulomethane formation could be affected by process conditions such as pH, chlorination dose and contact time etc., whereas the empham here was on 41
42
R. G~nt et al.
removal of the organics which were precursors to the THMs. Dissolved organic carbon (DOC) on filtered rumples was measured by u.v. oxidation in a silica tube, with potassium permlphate catalyst (Van Steenderen and Lin, 1981). Chlorophyll-a was determined by 5-rain extraction of material, retained on glass-fibre filters, into 96% ethanol at 78°C, followed by spectrophotometric determination of the concentration (at 665 nm; see Sartory, 1982). Chemical oxygen demand (COD) and suspended solids (SS) were measured according to Method Nos 508C and 209C in Standard Methods (APHA et aL, 1985); phosphorus and TKN were performed according to the methodology suggested by Technicon (1975). The factorial experimental designs were based on procedures developed by Cochran and Cox 0957), but analyses of most of the 2~ designs were performed directly by a procedure in the "Statgraphics" computer package (STSC, 1988). Briefly, for 2' designs, n independent variables are set at two levels (either high or low values, with the high value indicated by the presence of a code letter) for each test, and the effect of each varmble is evaluated by analysis of variance, or by cumulative probability plots (Box et a/., 1978). With the latter technique, when a straight line is drawn through or between most points on the plot, such that it passes through or close to the zero effects, and 50 cumulative percent coordinates, respectively, then points lying at an appreciable distance off the line are indicative of effects which are sJ~if~ant. This factorial procedure thus allows one to evaluate statistically the impact of several variables applied simultaneously, as well as their interactions. The variabilityin influent quality (see below) would compromise the power of the statistical procedures, hence percentage removals have been used throughout to minimize this effect.
RAW WATERQUALITY The raw water was sampled for more detailed analysis at various times during the period of the study. Values are given in Table 1. Variability o f some parameters was relatively high; for example, the standard deviation for COD in 1986/1987 was 17.6
for a mean of 39 rag/l, and total (unfiltered) T H M P had a standard deviation o f 48.4 for a mean of 161 ~tg/l during February and March 1989.
Experimental methods
The objective o f the preliminary experiments was to screen a number of variables and their levels in a batch unit, and then to select those to be used in further pilot plant tests. Batch D A F tests were done using a 6-cylinder apparatus akin to the familiar "jar test" (Water Research Centre, 1977), but with the jars replaced by conical-bottomed cylinders, 0.1 m dia and 0.44 m tall, into which pressurized recycle water (at 400 kPa) could be fed. Flocculation of the sample could thus be achieved via chemical addition, as well as rapid and slow mixing (2rain at 100rpm, followed by 15 min at 20 rpm, respectively), before the addition of the air-laden "recycle" (in this case, pressurized tap water). The first set of batch factorial experiments (2s) tested the following variables: recycle ratio (6.4 and 12.8%), alum dose (0 and 6.5 m8/l as AP+), polymer dose [Percol 757 cationic polymer (Allied Colloids Ltd), 0 and 0.6 mg/l], bentonite dose (0 and 60 rag/l) and pH (6 and ~ 9 , where ~ 9 was the natural p H o f Hartbeespoort Dam). Only alum, polymer and pH had sJ~ificant effects. Thus a second set of batch tests was done, this time using a 33 factorial design (i.e. three variables at three levels), with the following variables: - - a l u m : 3.3, 6.5, 9.8 mg/l as Al3+ - - p o l y m e r : 0.3, 06, 0.9 mg/l - - p H : 5, 7, ~ 9 .
Table 1. Water quality from the HartbempoortDmn at differentperiods of the study (averagevahma) 1986-1987 Feb. + March 1988 April 1988 Feb. + Match 1989 Variable (17-23 samples) (4--7 samples) (1-3 samples) (3-3 samples) Bromide (m~) 0.63 ---(0.19) Chlorophylla ~ 1 0 ) 52,8 60.0 6.23 32.6 COD (ms/l) DOC (rag/l) Dominant algal 8peO~
pH SUgl~nU~dsolids (m~)
(45.4)
39.1 (17.6) 8.88 (2.63) Microcystb aeruKt~m (& Oocystb qrp in Oct.,Nov. and Dec.) 9.1 (8.4-10.3) 13.5 (8.6)
THMP 0ag/i)
--
TKN ~s/l)
868 (8.1)
Total pho~horm (~10)
408
• ~r~dity (NTU) NoW ~ i , , . in p . ~ t h m
--
(36.0) --
7.47 (0.81) Mk~'ystb ~ a
6.8 (0.42) Cryptomonas q~p, CycioteOar~neghMiana
9.7 (9.6-9.8) --
9.2
8.7 (8.5-9.0)
--
--
344
242
161
06.4)
03.D
(~.4)
718 (170) 318
(38:7)
(10.7)
(3.6)
d~,~tiom. ~
--
8.62 (I.06) M~oystb aer~Inoaa
(55)
12.7
.~ .HU~
(38,4)
12.t
for p~. ~
I,H4 271 3.0
(l.i) ~ .rap le Si,~.
~96 (345) 56
(162)
3.0
(0.89) ,.
THMP removal from eutrophic water by DAF
43
Table 2. Analysis of batch 33 factorial experiments. Analysis of variance for THMP removal Source
Degrees of freedom
Sum of squares
Mean square
F ratio
F,~ s~.
2 2 2 2
6 ! 3.1 1571 5.03 1032
306.6 785.5 2.5 516.0
32.67 83.72 0.267 54.99
5.14
4
222.8
4.53
Main effects Blocks Alum Polymer pH Interactiona Alum/polymer Alum/pH Polymer/pH Error (by subtraction) Total
55.7
5.94
4
233.3
58.3
6.22
4 6
28.3 56.3
7.1
0.756
26
Recycle ratio was constant at 12.8%, and no bentonite was used.
Results from batch tests Results of the 33 factorial tests arc shown in Table 2. Analysis of variance was used here because probability plots for 33 factorial experiments were not available on the "Statgraphics" package. The results indicate that alum and pH have statistically significant effects (F-ratio much higher than critical). Plots (not given herein) of two variables averaged over the third showed a linear increase in THM reduction with alum dose, but only a slightly higher reduction for pH below 7. Thus, the highest alum dose and lower pH would be desirable. The variation in polymer dose at the relatively high levels tested was not critical. All three variables were therefore chosen for testing in the pilot plant, but with polymer applied at zero and the middle dose only (see Table 3). The results obtained above for alum and pH effects were not unexpected. Inorganic coagulants such as alum and ferric chloride are known to be successful at removing THMP (Chadik and Amy, 1983), and optimum pH levels for these have been found in one study with humic material to be 5-6 for alum and 3-5 for ferric salts (Collins et aL, 1986). These authors also believed that the precursors themselves were affected by pH, due to acidic functional groups on the humic substances.
3762
Treatment Plant on the banks of the Hartbeespoort Dam, and run in parallel with the plant's full-scale DAF unit (see later in the paper). Raw water, drawn 1 m below the dam water surface, was pumped at 6.2 l/min through rapid-mix and slow-mix tanks, and through a baffled flocculation section before entering the flotation tank. The detention time, including a recycle flow of 10%, was 8.5 rain. The air:solids ratio (A:S), at a suspended solids concentration of 13 rag/l, and assuming 90% saturation of the recycle flow which was pressurized at 400 kPa at 25°C, was 0.45. Downward velocity in the clarification section of the DAF unit (i.e. undertow rate), including recycle, was ~ 7 mfa. The chemicals used included alum or ferric chloride (but contrary to the original plans, the latter was the only inorganic chemical to be used in the full-scale plant, hence it would be the only inorganic chemical included in the factorial experiments reported below), Magnafloc LT 24 (Alfied Colloids) cationic polymer (since Pereol 757 was no longer available in the country), and 1 N sulphuric acid. A Hewlett-Packard data logger/control system (Models 71B Portable Computer, 3421 Data Acquisition/Control Unit, and 9114 Disk Drive) was used to control pH, and monitor influent temperature, inttuent and flocculated pH, and influent and treated effluent turbidity.
Results from pilot plant tests PILOT PLANT
Equipment used The pilot plant DAF unit is shown diagramatically in Fig. 1. It was set up at the Schoemansville Water Table 3. Codes for pilot plant
Code A B C 0
L
Condition High FeCi3 d o s e - 150mg/! (as FeCI3) High polymer dose = 0.6 mg/l High initial pH - 9.0 Low variabbs condition: FeC13 - 50 mgl Polymer dine - 0 pH-5 Alum dora at 150 ~ (as alum)
Note: (1) for coda A. B, C and L, all ~ not mentioned were at the low setting given under code 0. (2) pH values are approximate; low final pH range 4.8--6.0 and high final pH nmge 6.7--8.6.
Codes for the pilot plant test conditions are shown in Table 3. Basic iniluent/efliuent results are plotted chronologically as paired histogram~ in Fig. 2 for total (i.e. unfiltered) THM and bromodichloroform (CHBrCIz) (both following chlorination), respectively. Percentage reductions for filtered THM, CHBrCI, and DOC are shown in Table 4. The tests were actually performed during the summer months of 1988 and 1989, and a chronological plot of the data would show a slight downward shift of concentrations of the above three variables in the (chlorinated) raw water. A sudden drop in CHBrCI, concentrations occurred for tests 10, 11 and 12 (i.e. conditions BC, C and AB; Fig. 2), which were performed immediately following dam overflow (at the end of March 1988) and concomitant loss of algal scum. Some marine algae have been shown to contain
44
R. Gmm et al. A d d remrveir
Effluent
FÁ••--
~
p~l
temperature
p14 prem,
~,.I /iL'l'-/I J~ll~l .~11 ;
~/,o,,/.
Addlllonll tlooeulatlon chunber
A_-~I--/V
I-~-J J / ~ I
/
~" From
'
//..o,.,,,,.
et.
i l ~ I ~,-,-~-,-~t 1~1 It L II , I 11 4o,.
IIowmlx
tank
~..,,.~,o
I I /.... I
m~l~/mr
Ill....
I I/~,:"~1
relefvOlrll IIKI
i if
"//I/////////////,,
I1".."~
e~.-,v I
[I
e--,,~,
"/,¢/////////////////////~////~
DAF tlmk
Feed relervolr
Fig. 1. Schematic of DAF pilot plant. substrate bromine at elevated concentrations compared to their water environment (Pallaghy et aL, 1983; Saenko et al., 1978), and similar behaviour with the algae in the dam would explain why their loss was associated with considerable reductions in the overall bromide concentrations (normally at ~0.63 mE/l). Table 4 shows that under certain conditions, such as codes B, ABC and AC, the pilot plant was able to achieve above 65% filtered THM reduction. Concentrations below the current US standard of 100/48/1 were often achieved (last 6 tests in Fig. 2), although
it should be realized that in reality, THM concentrations at the tap would be lower due to the less rigorous reaction conditions which would occur during normal water chlorination, and the fact that total, rather than filteredT H M values are shown in this figure. Removal of CHBrCI 2 (Fig. 2 and Table 4) is generally not as good, but this is to be expected, as it is not the brominated precursors which are being removed, but the organic pr~-ursors in general. Addition of chlorine to water containing bromides will oxidize these to bromine, which in turn is a more
Conoentration (pg/L) 850 300 I"1
260
R
200 150 100 5O 0
LBO LB
LG
LG
1 L
A
r-I
o
LBC LB B(~
CHBrCI2
I
G
AB
BC
B
0
ABO AC
I Total T H M
Fig. 2. Pilot plato total (-nflltmxl) THMs and C H E ~ .
I n f l m t and i~um~t ('m test i~itw~ce).
THMP removal from eutrophic water by DAF Table 4. Reductions (%) of filteredTHM, filtered CHBt'CI=and DOC in the pilotplant
99.9
Code
99.0
LBC LB LC LC L A
T H M (filtered)
LBC LB
21.3 39,3 32,0 21.1 28.9 53.6 50.0 6. ! 4 26.9
BC
44.4
C AB
---75.9 36.4 72.5 68.7
0
BC B C
ABC AC
C H B r C i 2 (filtered) 10.3 19.5 15.1 12.1 17.1 29.3 27.4 I i .0 18.0 70.0 --51.4 47.0 19.2 53.8 41.9
DOC --?+6.1 --54.2 60.4 9.30 26.7
24.4
45
95.0 °~a. 80.0 50.0
"5 2O.O E _ " 5.0
23.3 67.1 43.8 50.0 44.1 44.5 41.9
@
1.0 0.1 -I -20
I -10
I 0
I 10
I 20
I 30
Effects (%)
Fig. 3. Filtered THM percent reduction in the pilot plant. Normal probability plot for factorial analysis. powerful reactant than chlorine is in producing halogenated organics (Cooper et aL, 1985). Thus organics remaining after flocculation and D A F would tend to be scavenged first for brominated species. An analysis of the results of the eight factorial experiments (Tests A, 0, BC, AB, B, C, ABC and AC, shown in order in Fig. 2) based on percentage reduction, is shown in Table 5, and Figs 3 and 4 for filtered T H M and DOC, respectively. The first row in this table indicates the average percentage reduction of each component (such as filtered T H M , etc.) for all eight tests, and subsequent rows show the effects (above or below the average) for the various combinations of test parameters. Typical probability plots for the effects of these two factors are given in Figs 3 and 4; ringed points on these figures which lie at an appreciable distance from the line (drawn automatically by Statgraphics) through X ffi 0 are considered to be significant, and are underlined in the table. A high dosage of FeCI3 (i.e. 150 mg/l; code ,4,) and a low pH coupled with the presence of polymer (negative effects for high pH) are significant for improvements in filtered T H M reduction. Interestingly, DOC removal is affected significantly only by a low pH. The graphical methodology for assessing signllicance was not suitable for analysing the total T H M results, due to the high values of the higher-order interaction sum-of-sclnares (SS); see Table 6. In fact, Table 5. Analysisof pilot plant 23 factorial experiments Code Filtered THM DOC Averase reduction ( % )
A B AB C AC BC ABC
62.7
50.9
22.0 21.-"'6 4.1 -14.4 8.2 -15.1 -6.2
2,6 1.45 6.8 -14.1 -2.85 0.2 -4,85
Note: the tabulatedv~lumfor eodmA, B etc., L,~4"~eeffects above or ~ the avmmlc. Underlined values arc rinpd +- Fis, 3 ,J+d 4. Wlt 2711--0
none of the main effects were significant, but the low pH had the greatest observed effect and highest SS. It is not clear whether this lack of significance is due to 0aboratory) analytical problems when evaluating total THMs, or because particulate removal by DAF is inherently efficient, and would not be greatly improved by changes in the levels of the chosen factors (note that the low variable condition still involved the presence of the inorganic coagulant at 50rag/l). However it is clear that the dissolved THMPs were being removed more effectively at the high FeC13 dose and low pH. This is perhaps a more interesting result, as it indicates that the effectiveness of the overall D A F system extends beyond simple particulate removal.
Evidence for the contribution of algae to the precursor pool Figure 5 shows a strong correlation between influent total T H M P and p H (R =0.914). Additional stepwise regression with turbidity and DOC failed to improve the model significantly, however a high
99.0 n 96.0 • 80,0 O. 5o.o
20.0 ~
6.0 1.0 0.1
"I -15
I -10
I -.6
I 0
I 5
I 10
Effects (%) Fig. 4. D ( ~ percent nnnoval in the pilot plant. Normal probability plot for factorial analysis.
46
R. G ~
et al.
Table 6. Analysis of variance for total THM removal in the pilot plant* Observations (%)
Code 0 A B AB C AC BC ABC
Total effect
60.3 67.5 85.1 87.8 54.0 49.1
55.2 41.2 5O.4 -98.4 35.2
74.6
59.6
24.6
Degre~ of Sum of freedom squares F ratiot 381 212 317 1,210 155 303 444
-49.2
2.73
*For methodology see Cochran and Cox (1957). t F ratio = SSe~.t./SS~ F~es.l., ffi 161.5.
correlation was also observed between turbidity and pH (R : 0.806). pH levels in the dam are to a large extent regulated by algal activity; an increase in this activity would lead to higher pH values as well as increased turbidity. Thus, these two correlations provide circumstantial evidence that algae and their EOM are responsible for a significant portion of THMP in the dam. From the few paired data points which were taken during the pilot plant work, as well as hourly samples taken over a 24 h period, it appeared that influcnt THMP and chlorophyll-a were also well correlated (correlation coefficients of 0.88 and 0.66 for filtered and total THMPs for the paired data, and 0.83 for total THMP for a 24-h experiment).
FU~ALE
PLANT
Plant setup A diagram of the full-scale DAF system at Schcemansville is given in Fig. 6. Ferric chloride is added, together with polymer (Organofloc 863), and at times powdered activated carbon (PAC) (see Table 7), into the influcnt weir, and flocculation occurs in an unmixed basin. Thus, coagulation conditions are not as well managed as in the pilot plant, and this is reflected in poorer performance (see below). Each DAF unit has an area of 18.6 m 2, and a nominal detention time of ~ 9 rain at the test flow rate of 4.2 m3/min. The air dissolution system is similar to that of the pilot plant, and the underflow rate of the DAF unit was 8 m/h, including a recycle flow of 10% of feed.
Operating results from the full-scale plant
The full-scale plant was monitored during the period of the pilot plant tests, and had the same intiucnt. Conditions were set to be as close as possible to the pilot plant. However, physical limitations and the need to maintain a water supply to the community precluded the estabfishment of a comprehensive test protocol. Results for percentage reduction of total THM only, in test sequence, are shown in Fig. 7. It is clear from Fig. 7 that the full-scale plant performance is generally not as good as that of the pilot plant. Maximum unfiltered THM reductions Relationship between DOC and THMP were approx. 65% (compared with over 80% for the Previous researchers have suggested that, for vari- pilot plant), and this occurred, interestingly, at low ous reasons, DOC and THM levels may not corre- FeCI~ and polymer doses (test code 0). High total late, and that selective removal of THMP from the THM reduction was also experienced under test DOC pool may occur during coagulation processes condition BD (which included PAC), and high (Collins et aL, 1986; Hoehn et al., 1984; Oliver and filtered THM reduction (not graphed) occurred at Lawrence, 1979). In the present study, the regression condition A (high FeCI~). coefficient between filtered influcnt THMP and DOC It is to be expected that the full-scale plant would was low (0.57), which implies that only a portion of give poorer performance than the pilot plant, because the dissolved organics are responsible for THM. the former had no pH adjustment, neither were the Percent reductions for THM are often higher than for flocculation conditions as well controlled. From DOC (Table 4), confirming the results of the above visual inspection of the results, it does appear that researchers that selective removal of THMP can be high doses of polymer are unnecessary (condition expected. BX), and in fact may be detrimental to THM removal (it is known that the polymer itself could contribute 4OO to the THMPs---see Soponkanaporn and Gehr, 1989). The single test with PAC yielded an effluent THM concentration below 0.1 rag/l, but it was also 300 possible to achieve this level simply with high doses of FeCI3 and no polymer.
•
200
•
e
u
CONCLUSIONS
o "1 8.0
I 8.5
I g.o
pN
I g.5
I lO.0
Fig. 5. Reg~ufion of pilot plant influent total THM on pH.
Dissolved air flotation (DAF) has been found capable of removing trihalomethane precm~rs (THMP) from eutrophic waters to below the 100/Jg/l THM standard in the U.S.A. Desirable conditions for the case studied, based on a statistical ~nA!ysis of factorial experiments, appear to be 150 mg/l FeCI3 dosage and a pH of around 5. This low pH level is also advantageous for rapid chlorination to achieve
I
Redrculatedwmor
n pipe
Sludge drain
~'~ / / ~
Flow mater Air saturated water st 400 kPa
Fig. 6. Schematic of full-scale DAF system at Schoemansville.
Float wdve
Ranged branch . _ ~
~ed
Packing Is 25mm dis x 30ramlong d plasticpipe,randomly
/ T , /
/
~
t t
j r
/
/
/ / ~
w
Flocculated water Collector outflow pipe pipes
From air compressor (400 kPa) Pressure gauge Oiatdbutor
/
Reclrculationwater p i p e from air saturator
Micro - bubble rmT.zle
Pressure releasevalve
~" ~,
Dmwon pipes
Old settlingtank fined with concrete
~
pipes
1
Chain driven ~ scraper
CollectOr
Emuemto rind sand filter
Float stabikmllon gdd
I I~
Powdered carbon
Flash mixing vessel
--w-.,
- - coa0.1ar.
slurry
Large settlingtank converted to hydraulic ficcculationtank
.r
/'~
/
-~
"n
o
48
R. Gwat et al.
% reduction 70 60 60 40 30 20 10 0 BX
BX
BX
B
BX
BD
O
AB
A
Total THM Fig. 7. Total THM percent reduction in the full-scale plant (in test sequence).
A B BX D 0
Table 7. Codm for fuli-w.aleplant Condition High FeCI3 dote-. 100mffi (as FeCI~) robjmer dou- 0.6 m~ Polymer doae- 6.0 mg/l PAC dine - 10mg/I Low variable condition: FeCl3- 50rag/I Polymer dine - 0 PAC dme-O
disinfection, hence overall reaction times could be minimi:,ed, thus further reducing final THM concentrations in the delivered water. Naturally, the pH of this water would eventually have to be raised appropriately. Polymer addition would probably be unnecessary for total T H M P removal, although it may be required for removal of turbidity, etc. An analyms of the interaction of pertinent water quality parameters indicated that algae are likely to be major contributors to the total precursor pool. In addition, it was shown that DOC is not a good indicator of precursor concentration, and that THMPs are eelectively removed from the overall DOC pool. Full-w.41e teats were also encourasing, and it appcared that, provided the process was operated under the conditions suggested by the pilot plant tests, powered activated carbon (PAC) is probably unnecessary for T H M P ~e~oval. Acknow/edzements--Fundins for this project by the South African Water Research Commismon is gratefully acknowledpd. The first author was supported by a grant from the Fotmdation for Rmearch Development during the period of the study. This paper is published with the permission of the Director, Division of Water Technology, CSIR, and the Executive Director, Water Research Commission.
REFERENCES APHA, A W W A and W P C F (1985) Standard Methods for the Examination of Water and Wastewater, 16th edition. American Public Health Association,American Water Works Amc~iation and Water Pollution Control Federation, WaShington D.C. Bare W. F. R., Jones N. B. and Middlebrooks E. J. 0975) Alsae removal using dissolved air flotation. J. War. Pollut. Control Fed. 47, 153-169. Box G. E. P., Hunter W. G. and Hunter J. S. (1978) Statistics for Experiments, pp. 329-334. Wiley, New York. eriley K. F., Williams R. F., Longley K. E. and sorbert C. A. (1980) Triludomethane production from algal precursors. In Water Chlorination: Eneironmental Impact and Health Effects (Edited by Jolley R. L., Brunp W. A., Cummin s R. B. and Jacobs V. A.), Vol. 3. Ann Arbor Science, Ann Arbor, Mich. Chadik P. A. and Amy G. L. (1983) Removing trihalomethane precursors from various natund waters by metal con£tdA, ta. J. Am. War. Wks Ass. 751 532536. Cochran W. G. and Cox G. M. (1957) Exper/menbd Designs, 2nd edition, Chap. 5. Wiley, New York. Collin~ M. R., Amy G. L, and Stelink C. (1986) Molecular weight distribution, carboxylic acidity, and humic substances content of aquatic or nni¢ matter:, implicationa for removal during water treatment. Ewoir. $ci. Tecimol. 20~ 1028-1032. Cooper W. J., Zika R. G. and Steinlmuer M. S. (1985) Bromide-oxidant interactions and THM formarion: a literature review. J. Am. War. ~ Ass. 77, 116-121. Hoelm R. C., Dixon K. L., Malone J. K., Novak J. 1". and Randall C. W. (1984) Biologically induced variations in the nature and removability of THM precursors by alum treatment. J. Am. War. ~ Ass. 76, 134-141. Oliver B. G. (1983) Dihaloacetonitrilu in drinkin~ wxtm':. algae and fulvic acid u precursors. Envlr. $ci. Tedmal. 17, 80-83. Oliver B. (3. and Lawrtmce J. (1979) Haloforms in drlnkln_a watt. a study of pmcurmrJ and precun~r removal. J. Am. Wat. Wks Ass. 71, 161-163.
THMP removal from eutrophic water by DAF Oliver B. G. and Shindler D. B. (1980) Trihaiomethanes from the chlorination of aquatic algae. Envir. Sci. Techno/. 14, 1502-1505. Pallaghy C. K., Minchinton J., Kraft G. T. and Wetherbee R. (1983) Presence and distribution of bromine in Thysan. oclad/a densa (solieriaceae, Gigartinales), a marine red alga from the Great Barrier Reef. J. Phycol. 19, 204-208. palm.qrom N. S., Carlson R. E. and Cooke G. D. (1988) Potential linksbetweeneutrophicationand the formationof carcinogensin drinking water. Lake Reserv. Man. 4, 1-15. Saeuko G. N., Kravtsova Y. Y., Ivaneneko V. V. and Sheindko S. I. (1978) Concentration of iodine and bromine by plants in the Seas of Japan and Okhotsk. Mar. Biol. 47, 243-250. Sartory D. P. (1982) Spectrophotometric analysis of chlorophyll a in freshwater phyto-plankton. Technical Report TR 115, Department of Environmental Affairs, Prctoria, Republic of South Africa. Soponkanaporn T. and Gehr R. (1989) The degradation of polyelectrolytes in the environment: insights provided by size exclusion chromatography measurements. War. Sci. Technal. 21, 857-868.
49
STSC Inc. (1988) Statgraphics, Version 3.0. Statistical Graphics Corporation, Rockville, Md. Technicon (1975) Autoanalyzer methodology. Individual/ simultaneous determination of nitrogen and/or phosphorus in BD acid digests. Industrial method 329-74W. Techicon Industrial Systems, Tarrytown, N.Y. Van Rensburg J. F. J. and Hassett A. J. (1982) A lowvolume liquid-liqnidextraction technique. J. High Resoln Chromatogr. Chromatogr. Commun. 5, 574-576. Van Steendercn R. A. and Lin J.-S. (1981) Determination of dissolved organic carbon in water. Analyt. Chem. 53, 2157-2158. Waisby A. E. (1969) The permeability of blue-green algal gas-vacuoles' membranes to gas. Proc. R. Soc. Land. B 173, 235-255. Water Research Centre (1977) Notes on Water Research No. 13: Laboratory Coagulation Tests. Williams P. G., van Vuuren L. R. J. and van der Merwe P. J. (1985) Dissolved air flotation upgrades a conventional plant treating eutrophic water. Paper presented at the llth Federal Convention, Australian Water and Wastewater Ass., 28 April-3 May, Melbourne.