The biological unit was a trickling filter. The non-settled biological effluent was sub- jected to coagulation followed by rapid solid-liquid separation via dissolved ...
Environmentlnternationai Vol. 3, pp. 219-223. Pergamon Press Ltd. 1980. Printed in Great Britain
The Optimization of Wastewater Treatment via Combined Techniques* Part I1: Combined Biological--Dissolved Air Flotation Fatma A. EI-Gohary and Sohair E. Abo EI-Ela Env. Sc. Div. , Wat. Poll. Con. Lab. , National Research Centre, Dokki, Cairo, Egypt.
Optimum operating conditions for sewage treatment via combined biological-physico chemical treatment has been studied. The biological unit was a trickling filter. The non-settled biological effluent was subjected to coagulation followed by rapid solid-liquid separation via dissolved air flotation. The impact o f organic load on the over-aU efficiency of the system has been investigated using three different organic loads namely, 9.48, 20.0, and 27.85 g BODon-=d "1. The coagulants used were ferric chloride and alum. Calcium oxide was used as a coagulant aid with ferric chloride. Factors affecting the efficiency of dissolved air flotation process such as detention time, air solids ratio, and position of coagulant's addition have been investigated. The results obtained showed that the use of biological, extended physico-chemical technique leads to the removal of organic as well as inorganic contaminants. Moreover, sludge with high solids content can be obtained.
The operating conditions for the system are summarized in Table 1.
Introduction
Many municipal and industrial wastewater treatment facilities currently face the prospect o f being required to provide treatment efficiency greater than that normally attained by conventional biological treatment systems. To meet these requirements, more effective removal of both organic and inorganic contaminants is required (Weinberger, 1968; Eckenfelder, Jr., 1972; Eckenfelder, 1975; EI-Gohary, 1976). In the first part biological treatment of wastewater followed by chemical coagulation has been investigated. In this study, the application of dissolved air flotation instead of sedimentation has been used.
Table 1. Operating conditions Variables Hydraulic-load (1 m -2 d -1) BOD-load (g BODgn -2 d -1) BOD~ (mg 1-1) COD (mgO= l-l) Ammonia (mg N 1-1) Organic Nitrogen (rag N l -i) Nitrite (mg N 1-1) Nitrate (mg N 1-1) Total phosphate (mg P !q)
Load I
Value Load II
Load III
29.63 9.48 320 420 8.44 10.69 0 0 5.59
29.63 20 676 835 16.87 18.41 0 0 l 1.49
29.63 27.85 940 1402 25.30 28.86 0 0 16.16
Dissolved air flotation unit Material and methods A two-stage continuous laboratory-scale system was designed. The first one was a biological unit while the second stage was coagulation followed by dissolved air flotation unit.
Biological unit The biological unit was a laboratory design simulating a plastic packed trickling filter similar to that used in Part I. To study the impact o f organic load on the efficiency o f the combined system, three different organic loads, namely, 9.48, 20.0, and 27.85 g BODgn -2 d -t were investigated.
* Received l0 August 1979; received for publication 21 November 1979.
219
The main components o f this unit were: air compressor, a retention tank, and a flotation unit. The retention tank (Fig. l) was made o f copper. The inner surface was coated with a plastic layer. It was designed to withstand a pressure up to 10 atms. The pressure within the tank was regulated via a pressure gauge mounted on the exist line. The pressure applied during this investigation was kept constant at 4 atms. The flotation unit was made of a calibrated glass column, 66 cm length and 8 cm diameter. It was provided with several side arms for clarified effluent withdrawal. The pressurized air-water mixture was released from the retention tank to the flotation unit through a valve located at the bottom o f the column. The air-solids mixture rises to the surface, where it was skimmed off. The clarified effluent was withdrawn from the side arm, which was found to be most suitable.
220
Fatma A. El-Oohary and Sohair E. Abo EI-Ela Pressure
W~er rye, inlet Safety valve
Treatment via dissolved air flotation
[ ~ ~------------"~ K] I : ' , l e : l ! = " I'=l " ~
..,~Air
water
Rubber ring
Pressurization chamber
The non-settled biological effluent at the three different organic loads was subjected to dissolved air flotation. A pressure of 4 atms was used in the flotation cell. Optimum A/S ratio
~ Water level
check valve
I
t
r-~
Dimensions in mm Scale 1 : 50
Fig. I. Retention tank.
Factors affecting the dissolved air flotation process, such as detention period, air/solids ratio and position of coagulant addition have been investigated. Flotation was performed with and without coagulant addition. The coagulants examined were alum and ferric chloride. Lime was used as a coagulant aid and to adjust the pH value. During the study, extensive sampling programme was conducted to determine the physico-chemical characteristics of the raw waste, the effluent from the biological unit, and the effluent from the flotation unit at the different organic loads under investigation. Furthermore, sludge analysis were performed. Results and discussion
Biological treatment Average results of the efficiency of the biological treatment at the three different organic loads are shown in Fig. 2. The results obtained showed that the trickling filter performance was not affected by increasing the organic loads up to 27.85 g BODgn-=d -1. Considerable reduction in both B e D and COD have been achieved. On the other hand, the phosphate percentage removal values were only 51.9, 53.4, and 55% respectively for the three different organic loads examined.
The effectiveness of dissolved air flotation is believed to be highly dependent on the ratio of the weight of air released from solution to the weight of the solids treated. This is expressed as the dimensionless air: solids ratio (Metcalf and Eddy, Inc., 1972). This ratio which is the most important parameter used in the development of design criteria of dissolved air flotation system, was determined. The non-settled effluents of the biological unit were mixed with different volumes of pressurized air at constant solids content and detention period. The pressurized air was mixed with the wastewater in the flotation unit in a ratio ranging from 1/25 to 1/6. The detention period was about 15 mins. This experiment was repeated for the different organic loads examined. The A/S ratio was calculated according to the following equation (Eckenfelder, 1966). A/S
1.3 s= R(P- I) =
QS°
Where = air saturation, cm3.1-1 P= absolute pressure, atms. R = pressurized volume, 1. (2= waste flow, litre S,, = influent suspended solids, mg 1-1 The results obtained are illustrated graphically in Fig. 3. Available results tend to show that the optimum A / S ratio was 0.064, 0.034 and 0.037 for the three organic loads. These results are in agreement with those obtained by Eckenfelder (1966). However, it is less than that obtained by Reed et al. (1976), A / S = 0.12. An increase in the influent suspended solids from 136 to 216 mg 1-1, led to a decrease in the A / S ratio from 0.064 to S=
o Organic Load I x Organic Load ]1" D Organic Load ~1"
0.0~
100. r-i%Rof BOD 00-
o
I
• '/,,Rof COD
.o_ 00( 1/) "(3
o • %Rot P
O.Oz
U3
< 002 0
Fig. 2. Efficiency of biological treatment at the three different organic loads,
0
L
1
20
A
I
I
I
a
I
40 , i 60 80 mg.i-~ss of effluent
J
i 100
Fig. 3. Effect of air: solids ratio on the SS removal at the three different organic loads.
221
The optimization o f wastewater treatment - - 1I
0.034. However, further increase of the influent suspended solids from 216 to 360 mg 1-1 did not affect the A / S ratio. This may be due to the fact that the higher suspended solids content in wastewater give more chance for adhesion between solid particles and air surface, consequently, the liquid-solids separation rates increased.
o Organic Load I x Organic Load II o Organic Load g[
100 -
90
"5 80
Optimum detention time In order to determine the optimum detention time required for the best clarification, different detention periods ranging from 3 to 17 min have been examined. The pressurized air-water volume was kept constant at the pre-determined values. Available results showed that the optimum detention period required ranged from 10-12 min for the biological effluent obtained from the three different organic loads (Fig. 4).
E
~ 7c 6C
50
I 4
I 8
1 12
I 16
T i m e , minute
Fig. 4. Effect o f the three different organic loads on flotation detention time.
Addition of coagulants to the flotation system
Many trials have been carried out to achieve the best way of adding coagulants to the flotation system. As a result of this investigation, it was found that the removal of colloidal particles requires coagulation of the solids before applying the pressurized air: water mixture. Generally, the surface charge of the colloidal particle promotes adsorption of the surfactant, rendering the colloidal particle surface active. The passage of minute gas bubbles through the liquid creates a very large interfacial area on which the surface active particle may be adsorped and thus be removed from the solution (Reed, 1976; Mennel et al., 1974). Average results of the effect of the organic load on the performance and the efficiency of the combined system is shown in Table 2 when alum was used as a coagulant.
From these results, it can be seen that the efficiency of the combined system proved to be very promising. It was found that increasing the organic load from 9.48 to 27.85 g BODsm-=d-1 has no significant effect on the performance of the combined system. BOD removal was about 98% in all cases. Corresponding COD removal decreased slightly by only one percentage point at the highest organic load. Residual phosphate ranged from 0.33 to 0.7 mg P 1-1. Similar results have been obtained when ferric chloride was used (Fig. 5). The process proved to be particularly useful for the disposal of the sludge produced. The solids capture content was high as indicated by the sludge volume index. When alum was used, the sludge volume index was 68, 49, and 66, at the three different organic loads examined. Corresponding values of S.V.I., when ferric chloride was used, were 72.8, 82, and 65.
Table 2. Average results o f combined biological-coagulation dissolved air flotation treatment at the three different organic loads using alum. Organic-load gBODsm-Zd-t Variables Dose o f alum (mgAl3*1-1) ph Value BOD )mg l -t) C O D m g O 21-1 Total P h o s p h a t e m g P 14
Raw mg1-1
9.48 Final effl. mgl-: 070 R
Raw m g l -~
20.0 Final effl. mg1-1 070 R
Raw mgl-:
27.85 Final effl. m g l -t 070 R
7.9 320 402 5.59
6,82 6,3 3,99 18 0.7
7.6 676 835 11.5
9.09 5.5 15 26.4 0.98
7.4 940 1402 16.2
18.18 6.3 18.8 80.4 0.33
98.8 95.5 87.5
97.8 96.8 91.5
Sludge analysis Sludge volume produced
m3rt/-3
10× 10-3
19.6x 10-a
2 3 x 10-a
6.6 3.8 450 68 58
13 8.95 729 56 69
11.8 9.3 769 65 78
Sludge characteristics Sludge wt at 105°C g 1-t Sludge wt at 550°C g 1-: Sludge volume ml 1-1 S.V.1. 070 V O M
t 20
98 94 98
222
Fatma A. El-Ooharyand SohairE. Abo EI-EIa
Comparison between the efficiency of the biologicalcoagulation sedimentation and biological coagulation flotation techniques Comparison between the efficiency of the two techniques, investigated in Parts I and II, showed that the combined biological coagulation dissolved air flotation offered number of advantages over the combined biological coagulation-sedimentation process. Using the same coagulant doses and under the same operating loads, higher removal values of COD, BOD, and phosphate were achieved (Figs 6, 7, 8). Consideration of the results obtained at the highest organic load using 18.1.8 mg AP + 1-~ showed that residual BOD decreased from 61.43 mg 1-~ for the combined biological chemical treatment to 18.2 mg 1-~ when flotation under pressure was used. Furthermore, the
residual phosphate values, was reduced from 0.87 to 0.33 mg P 1-1. The use of flotation technique led to a reduction in the coagulant doses required. At an organic load of 20.0 g BODsm-2d -1, the dose of alum was reduced by 25o7o than that required for the combined biological coagulation sedimentation process. This may be due to the fact that in the case of flotation, it is not necessary to add additional amounts of chemicals for increasing the specific gravity of the floc as required sometimes for sedimentation. Moreover, the flotation technique produced a sludge with more solids content compared to that obtained when sedimentation was used. Further-
-% Removal - -Residual o Biological-Coagulation-Sedimentation x Biological- Coagulation- Flotation
100
o
% Removal of BOD
8rn
x % Removal of COD o % R e m o v a l of P
5-_ 100
9s
o"
too
~.._....._.....--------~
D
x
E / 90
;o ~:
,P
/ /
60 /
//
/
40
J
I
0
i
5
I
20
15
D
I
25
20
I
30
Organic Loads g. eODs.rfi2 a 1
8.0E
5
10
15
20
25
30
o~gao~ LoS. g. B%~
Fig. 5. Results of combined biological-chemicalcoagulation-flotation treatment at the threeorganic loads using ferric chloride.
2
Fig. 7. Comparison between the efficiency of the biological coagulation sedimentation and biologiC--coagulation flotation techr~ques (using alum).
% Removal Residual o Biological-C oagulat ion - Sedimentation x Biological-Coagulation-Flotation
-% Removal -- Residual o Biological-Coagulation- Sedimentation x Biologicm - Coagulat ion - Flotalion
100
106 140 / /
120
/ 0~
"a
&
o~
[. 30 8
1.0
E
3.9 .~
/ . / / / "//
I ~..~.. ~ ~ ~ ~
0.8 i~ 33
\\\
80
/
o 5 '~
\\
~...d///
0.4
8=
I S
I 10
I
15
I
20
I
25
I 30
Organic Load. g, BOD5.~'12 a!
Fig. 6,. Comparison between biological-coagulation sedimentation and biological-coagulation flotation techniques (using alum).
0
I 5
I 10
I 15
I 20
'
I 25
\"k
0.3
I 3O
t
0.2 0.I
Organic Load, g. BOO5 . r ~ c~1
Fig. 8. Comparison between biological-coagulation sedimentation and biological coagulation-flotation techniques (using alum).
The optimizationof wastewatertreatment -- II more, the flotation detention time was very short (10 - - 12min) compared to gravity settling (1 - 2 h). Conclusion From the available data it may be concluded that the use of dissolved air flotation in combination with biological treatment adds more advantages to the treatment performance and proved to be a very promising technique for wastewater treatment. The merits of the combined biological-physical-chemical treatment are: (a) Provide an effluent quality superior to conventional secondary effluent, in terms of organic and nutrients removal. Phosphate percentage removal up to 98070 was obtained equivalent to residual phosphorus o f only 0.3 mg P 1-1 at the highest organic load, namely 27.85 g BODs m-Zd-1. (b) Insure consistent effluent quality. (c) The process proved to be particularly useful for the sludge disposal. Sludge with high solid capture was obtained.
223 (d) In addition, the high-rate trickling filter system proved to be unaffected by organic surges. References Eckenfelder, W.W., Jr. (1972) Application o f New Concepts o f Physical-Chemical Wastewater Treatment. Sept. 18-22 Pergamon Press, Inc. Eckenfelder, W.W. (1975) Wastewater treatment design, Part II. Wat. Sewage Wks 122, 70. Eckenfelder, W.W. (1966) Industrial Water Pollution Control. McGraw-Hill, New York. EI-Gohary, F.A. (1976) Controlling pollutants by combined treatment. Waste Sewage Treatment. Mennei, M. et al. (1974)Treatment of primary effluent by lime precipitation and dissolved air flotation. J. Wat. Pollut. Control. Fed. 2470-2485. Metcalf and Eddy, Inc. (1972) Wastewater Engineering: Collection, Treatment, Disposal. McGraw-Hill series in Water Resources and Environmental Engineering, 299. Reed, S.W. and Woodward, F.E. (1976) Dissolved air flotation of poultry processing waste. J. Wat. Pollut. Control Fed. 48, Weinberger, L.F. (1968) Wastewater treatment for phosphorous removal. U.S. Spt. Interior,Fed Water Pollut. Contr. Admin. Prec. Conference on Pollution of Lake Michigan, Vol. 2, 812-877.
