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Treatment of raw domestic sewage in an UASB reactor
War. Res. Vol. 23, No. 12, pp. 1483-1490, 1989 Printed in Great Britain. All rights reserved
0043-1354/89 $3.00 +0.00 Copyright © 1989 Pergamon Press pie
T R E A T M E N T OF RAW DOMESTIC SEWAGE IN A N UASB REACTOR R. A. BARBOSA and G. L. SANT'ANNA JR COPPE/Universidadc Federal do Rio de Janeiro, C.P. 68502, 21945, Rio de Janeiro, Brazil (First received September 1988; accepted in revised form May 1989) Abstract--The treatment of raw domestic sewage at ambient temperatures in an upflow anaerobic sludge blanket (UASB) reactor with a volume of 1201. and a height of 1.92 m was studied. The sewage had an average BOD 5 of 357 mg 1- ~ and COD of 627 mg 1- t. Approximately 75% of the organic materials were in the suspended fraction. The sewage temperature ranged from 18 to 28°C during the experimental period. The reactor operated continuously for 9 months and assessed self-inoculation and raw domestic sewage purification. The unit was started without inoculum and ran during the entire experimental period with a hydraulic retention time of 4 h. During the experiment, a sludge bed build-up was observed. At the end of the experimental period, the predominance of spherical granular particles up to 6--8 mm in diameter was evident. After a 4-month operation, it was observed that the inoculation/acclimatization steps had been concluded. Removal efflciencies of BOD 5 = 78%, COD = 74% and TSS = 72% were obtained. A typical gas production factor of 801 kg-~ COD added was observed and the CH 4 content of the biogas was 69%. Key words--anaerobic treatment, sewage treatment, anaerobic reactor, UASB reactor, biodigestion, biological treatment
NOMENCLATURE
BODs = biochemical oxygen demand (5 days) (mg 1-l) COD ffi chemical oxygen demand (mg l -I) HRT = hydraulic retention time (liquid reactor volume/ flow rate) PE = populational equivalent SVI = sludge volume index (ml g-i) TSS = total suspended solids (rag 1-~) TS -- total solids (mg l -l) FE = filtered effluent FI = filtered influent TE = total effluent (non-filtered) TI = total influent (non-filtered) SS = suspended solids. (TI-TE), (TI-FE) and (FI-FE) are associated with BOD and COD removal efflciencies, calculated based on filtered and non-filtered samples.
INTRODUCTION The rapid development of the upflow anaerobic sludge blanket (UASB) reactor, at first for treating wastewaters with high and intermediate organic concentration stimulated the first investigations for applying this reactor to the treatment o f wastewaters with low organic concentration at low process temperatures. A review of recent published works of U A S B reactor performance on sewage treatment will be presented in this Introduction. As the efficiency o f the treatment process is usually expressed by C O D and B O D removal [(influent-effluent)/influent concentration], these parameters may be measured using filtered or non-filtered samples. T o avoid misunderstanding, the symbols ( F I F E ) and ( T I - T E ) are used here, as defined in the Nomenclature, to clarify which
type of concentration is being used to calculate the removal efficiency. The U S A B performance in the treatment of raw domestic sewage under tropical climate conditions was studied in The Netherlands by Lettinga et al. (1981), in a 6 m 3 pilot reactor inoculated with digested sewage sludge at a controlled temperature o f 20°C and an H R T of 8-48 h. They obtained C O D removal efficiencies ( T I - T E ) of 75% and gas production of 0.115 N m 3 of gas k g - l C O D added, with an H R T of 8 h. In a second set of experiments performed in the same pilot plant (Grin et al., 1983), the H R T was fixed at 8 h and the temperature ranged from 9.5 to 19°C. The results demonstrated that the C O D removal efficiencies and gas production were higher between 15 and 19°C within the range studied. Lettinga et al. (1983), for the purpose of assessing the feasibility of applying the U A S B reactor to raw domestic sewage under moderate climate c o n d i t i ~ s , studied the performance of a 120-1. reactor inoculated with active granular anaerobic sludge (grown~\,~on wastewaters from beet sugar production), under temperature conditions of 8-20°C and H R T of 24-8 h. They obtained C O D removal efficiencies ( T I - T E ) o f 65-90% ( H R T = 1 2 h ) and gas production of 100-2201. of gas kg -l C O D added, F o r the purpose of assessing the feasibility o f the U A S B reactor self-inoculation when applied to raw domestic sewage, Grin et al. (1983) followed up a 118-1. non-inoculated reactor performance, submitted to temperature conditions of 19--23°C and H R T of 40-24 h. They observed that after 3-4 months the C O D removal was 60% which gradually rose to 75% after 5-6 months. The results clearly showed the
1483
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R.A. BARBOSAand G. L. SANT'ANNAJR
possibility of starting up a U A S B reactor for raw domestic sewage treatment with no inoculation under temperature conditions 1>20°C. Schellinkhout et al. (1985) and Kooijmans et aL (1986) reported the results of operating a 64-m 3 U A S B reactor for raw domestic sewage treatment in Cali, Colombia (a typically tropical area). A small amount of inoculum, 1 m 3 of digested cattle manure, was used in this experiment and the sewage temperature was 24-26°C. Removal efficiencies ( T I - F E ) of C O D = 75-82% and B O D s = 78-85% were obtained with a H R T of 3-3.5 h. These efficiencies were maintained when the process was submitted to a H R T variation of 2.2 h during 12 h of daylight time and 6 h of night time. Schellinkhout et al. (1988), reported the results of the operation of a 35 m 3 U A S B reactor for raw domestic sewage treatment in Bucaramanga, Colombia. The sewage temperature was 23-24°C and the H R T imposed was 5.2 h. They obtained removal efficiencies (TI-TE) of B O D = 80%, C O D = 66% and TSS = 69%. Vieira (1984, 1988), and Vieira et al. (1986) reported several results of experiments with U A S B reactors applied to the treatment of domestic sewage carried out by C E T E S B (S~o Paulo State Environment Control Agency, Brazil). In the operation of a 1061. reactor inoculated with granular anaerobic sludge (grown in settled domestic sewage, at 35°C, in a reactor previously inoculated with digested sewage sludge), applied to raw sewage, at an average temperature of 20°C during winter and 23°C during summer, and H R T of 4 h, B O D removal efficiencies of 69% ( T I - T E ) were observed. C O D removal ( T I - T E ) was 60 and 65% during winter and summer, respectively. Suspended solids removal was 69% during this period. Gas production of 100 and 119 NI gas kg -~ C O D added was obtained during winter and summer, respectively. In the operation of a 120m 3 U A S B reactor inoculated with digested sewage sludge, at a temperature of 21-25°C and H R T of 4.7, BOD5 removal efficiencies ( T I - T E ) of 61% were attained. C O D removal (TI-TE) was 50% and TSS 73%. Gas production of 121 NI g a s k g -~ C O D added was obtained. Man et al. (1986) investigated the performance of three U A S B reactors of different volumes (0.12, 6 and 20 m3). The reactors were started using granular seed sludge. They observed that the anaerobic treatment of sewage (from Bennekom Village, The Netherlands), COD: 500-700 mg I ~, in the pilotscale granular U A S B reactors at an H R T of 7-12 h and at temperatures in the range of 12-18°C, provided a C O D removal (TI-TE) of 40-60%. Even at temperatures as low as 7-8°C C O D efficiencies ( T I - T E ) of 45-65% were obtained at an H R T of 9-14 h. These results demonstrated that anaerobic treatment is still fairly efficient at temperatures as low as 7-8°C, although the H R T should exceed 9 h under these conditions. In a second set of experiments performed in the 20 m 3 U A S B reactor, Man et al. (1986) treating a very dilute and septic sewage (from
Bergambacht Village, The Netherlands), COD: 3 5 0 m g l -~ and C O D / B O D : 3.5, observed that high efficiencies could not be achieved at an H R T below 12 h and temperature in the range of 6-17°C. In a third set of experiments performed in the 2 0 m 3 U A S B reactor inoculated with granular seed sludge, Man et al. (1988) treating sewage (from Lelystad, The Netherlands), COD: 740-1280 m g l ~, observed that C O D treatment efficiencies ( T I - T E ) obtained declined continuously from 48 to 16% and the same applied for the gas production which dropped from 80 to 31 NICH4 kg ~ C O D added. The methanogenic activity of the granular sludge in the reactor dropped to approx. 4% of its original activity. The reactor operated with H R T of 13-14h and a temperature range of 10-1Y'C. The causes of this activity loss were points of further extensive investigations. Nobre et al. (1987) described that BOD removal (TE-TI) of 78% and C O D removal (TE-TI) of 73% were attained in a 3.7 m 3 U A S B reactor inoculated with sewage digested sludge and operated with an H R T of 10-18 h at a temperature range of 24-26 C. Monroy et al. (1988) reported the results of the operation of a 1101. U A S B reactor for raw domestic sewage treatment. The reactor was inoculated with anaerobically batch-adapted activated sludge. The sewage temperature was 12-18 C and the H R T was 18 h. They obtained removal efficiencies (TI-TE) of C O D = 6 5 % and T S S = 7 3 % . At the time the experimental work reported herein was performed, little published data was available concerning the treatment of raw sewage in U A S B reactors, mainly at variable temperature and there was no detailed study on the start-up procedure without innoculation. The lack of knowledge in this area motivated the study which we present in this paper. MATERIALS AND M E T H O D S
The experimental unit was installed at the Ponta do Leal Pumping Station, of the Water and Sanitation Company of Santa Catarina State (CASAN), in Florian6polis. The pumping station received sewage from a typical residential area. The main characteristics of the sewage fed to the experimental unit are given in Table 2. The experimental set-up is illustrated schematically in Fig. 1. The sewage coming from a branch of the discharge line was thrown directly into a plastic-net basket immersed in a 150-1. holding tank. This net (2.25 mm 2 square holes) prevented coarse solids from entering the reactor. Liquid in the holding tank was continuously mixed by recirculation and renewed with the use of an overflow apparatus. The sewage mean retention time in the holding tank was 27 min. From this tank the sewage was pumped upward (using a peristaltic pump) to a small reservoir, where it would go by gravity to the reactor bottom. The reactor, built of PVC and plexi-glass, had a capacity of 1201. and was 1.92 m high. Plexi-glass windows, diametrically opposite and distributed along the reactor, made it possible to visually inspect the sludge bed dynamic behaviour and the solid/liquid/gas separating device provided at its top. The internal separator was of the inverted cone type, as depicted in Fig. I. The gas produced was measured by liquid displacement (NaCI saturated aqueous solution, acidified with HCI) in a Mariotte
This work
M o n r o y et al. ( 1 9 8 8 ) M a n et al. ( 1 9 8 8 )
N o b r e et aL ( 1 9 8 7 ) Vieira (1988) S c h e l l i n k h o u t et aL ( 1 9 8 8 )
M a n et al. ( 1 9 8 6 )
V i e i r a et al. ( 1 9 8 6 )
G r i n et al. ( 1 9 8 3 ) Vieira (1984) S e h e l l i n k h o u t et al. ( 1 9 8 5 )
L e t t i n g a et aL ( 1 9 8 3 )
G r i n et al. ( 1 9 8 3 )
L e t t i n g a eS al. ( 1 9 8 1 )
Reference
19-28
23 12-18 -7-8 -24-26 21-25 23-24 -12-18 10-15 4
8 8 8 12 12 24 4 4-8 3-3.5 4 4 7-12 -9-14 -10-18 4.7 5.2 -18 13 14
15 - 1 9 11-12 9. 5 - 1 0 8-20 8-20 19-23 35 24-26 24-26
20
18 24 8
HRT (h)
20 20 20
Temp. CC)
627 (357)
500 400 500 500 300 460 341 ( 1 3 7 ) 267 267 424 (195) 406 (180) 500-700 (250-350) 500-700 (250-350) 660(300) 265(150) 430-520 (190-220) 465 740-1280
550 51)0-550 500
376
----. -88 215 215 188 191 -. -. -123 200-250 . 154 --
-. --
TSS ( m g l i)
.
.
.
.
.
.
.
78
---
78 61 80
--
-72 --69 69 50-70
-----
--
--
BOD (TI-TE)
of published results--UASB
CODTt (BODTI) ( m g l i)
T a b l e I. S u m m a r y
.
.
.
.
.
COD
91
--
-88 83-85 78-85 85 84 -. -. ---. --
----
--
--
--
BOD (TI-FE)
BOD,
.
.
.
48-16 74
65
73 50 66
45-65
60 65 --60 65 40~0
40-55 30-50 30 66.5
--
55-75
COD (TI TE)
.
.
.
-89
65-80 55-70 55 65-90 50-60 70 83 83-85 78-85 82 83 . . . . 85-91 --. --
-70 75
COD (TI-FE)
and TSS removals (%)
reactors for sewage treatment
.
.
-72
-73 69 . 73
.
.
------61 70 -69 69
----
TSS
.
.
80-31 (CH4) 80
--
140 121 --
.
90 50 100-200 -115 118 --100 119 .
130
I 15
150-200 160
Gas NI kg- i COD added
-18 ( T S S )
--
-2 (TSS) --
9.6 ( S T ) 8.5 ( S T ) 13.4 ( S T ) 5.0-8.6(ST) ---7.3 ( T S S ) ----
9.9 (ST)
-7.9 ( S T )
Excess sludge kg ( ) PE i yr '
Gas and sludge production
?,
,.q
g
a
>
g~
O
a
¢a
1486
R . A . BARBOSAand G. L. SANT'ANNA JR
Table 2. Characteristics of the sewage fed to the UASB reactor
Table 3. List of monitored process variables
Range or mean value
Variable Temperature CC) pH Volatile acids (mgl ~) (as acetic acid) Total alkalinity (mg CaCO 31 ~) BODTI (rag 1- ]) BODF[ (rag I t) COD-r] (mg I i) CODFI (mg I ') TS (mgV i) TSS (mgl i) VSS (mgl ]) N-total Kjeldahl (mg N I ~) N-ammonium (mg N 1-]) P-total (mg PI ~) Oil and grease (mgl 1) Surfactants (mg MBAS 1 ~) Sulfate (rag SOl- 1 ~)
Variable
18-28 6.2 7.6 39
Volatile acids Total alkalinity BOD total BOD filtered COD total COD filtered TS TSS VSS N-ammonium N-total Kjeldahl P-total Oil and grease
198 357 94 627 151 872 376 297 54 30 9.9 93 1.8 124
type collector, so as to keep a constant pressure in the reactor of 210 m m of water column. Besides the basic operating variables (flow, temperature, pH and gas production), which were checked daily on site, a physical-chemical monitoring program was designed, whose parameters and analytical frequencies are displayed in Table 3. The reactor influent and effluent samples were composed o f grab samples collected every hour over a 24 h period. During the collection period, the samples were stored at 4°C. The physical-chemical analyses used were as recommended by Standard Methods for the Examination of Water and Wastewater (APHA, 1985). Gas composition was determined by gas chromatography (columns: chromosorb 102 and molecular sieve 5A; column temperature: 90°C; carrier gas: He; injection temperature: 100°C; detector temperature: I10°C). The specific methanogenic activity of sludge samples was determined according to the method used by Zeeuw (1984). The bacterial morphology of the pellets was examined by means of scanning electron microscope Jeol 25 SII. The
Frequency
Sample, c = composite g = grab
Each 15 days Each 15 days 2-3 per week 2 3 per week 2 3 per week 2-3 per week 2 3 per week 2 3 per week 2 3 per week Weekly Weekly Weekly Weekly
g g c c c c c c c c c c c
pellets examined were prepared according to the method described by Ross (1984) and Lane (1986). The reactor start-up was without inoculum and was fed with domestic sewage. The system operated with a hydraulic retention time of only 4 h, which was continued until the end of the experimental period. The experiment was conducted at the ambient temperature of the sewage. During the whole period the anaerobic reactor temperature ranged from 19 to 28°C. The results from approx. 9 m o n t h s continuous operation are reported. RESULTS AND DISCUSSION Although the operation was started without inocul u m , solids were o b s e r v e d in t h e r e a c t o r f r o m t h e very b e g i n n i n g . T h e solids b u i l d u p w a s p r o g r e s s i v e : c o m p a c t , s p h e r i c a l b a c t e r i a l g r a n u l e s a b o u t I m m in d i a m e t e r were o b s e r v e d a f t e r 1 m o n t h o f o p e r a t i o n . The incidence of granular formation was progressive. At the end of the 9-month operation, spherical g r a n u l e s u p to 8 m m d i a m e t e r were e s t a b l i s h e d . It w a s also o b s e r v e d t h a t , in t h e c o u r s e o f time, t h e
t
11
rl.e.~-
d~-
8
,,j*
¢-h-
b~
1 - ~ IM.ETi 2-PLASTIC NET ~ i 5..HOLDING TANK; 4-OMER F10W ; 5 , 6 , 1 3 - PUMPS ; 7-RESERVOIR 1 8 - ~ i 9-1N~ SEPARA'R~ i 10 -EFFLUENTi 11 -GAS HOLDER i 12-LIQUID CONTAINER ; a - i - SAMPLING TAPS. Fig. 1. Schematic diagram of the experimental set-up.
Treatment of raw domestic sewage in an UASB reactor
1487
~APRI M~l I JUN I JU- I AUG I SB' IOCr I NOViOB~ i
I
60. 40 2~ 8O
,
'
I REMOVAL (%)
TSS
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I
I
I
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20 mm
50
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.
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Reactor performance Reactor performance was assessed on the basis of BOD 5, COD and suspended solids (TSS) removal. In the case of BOD and COD, the removal was expressed in three ways: based on (i) total influent and total effluent; (ii) filtered influent and filtered effluent; and (iii) total influent and filtered effluent. The first represents the indicator (BOD or COD) overall removal, comprising the soluble substances and the suspended material contributions, thus reflecting the organic matter removal by physical action and biological decomposition. The second expresses the soluble organic substances removal, predominantly representative of the biological action. The third
Fig. 3. Scanning electron micrograph of a granule (magnification 45 x ).
.
.
,
- -
. __,,.r - - _ ,
Fig. 2. Anaerobic granules--shape and size.
granular formation started to prevail over the flocculent shapes of anaerobic sludge. At the end of the experiment, it was verified that the reaction compartment (height: 1.42 m) was completely full of sludge, with no harm to the final effluent quality. Figure 2 shows the shape and the size of the sludge granules. Figure 3, a scanning electron micrograph, illustrates the uniform aspect of the external surface of these granules.
(O,o
. . . . . . . . .
--I-1213
, ,,-, ~,~.UTOTAL..FILTEI.ED
/ .___: .... F--BOD RE]VI(NALS (%l -'-
TOTAL-FIL~
I
I
1415161718191 MONTH OF 00B~ATION
Fig. 4. Average monthly results of BOD, COD and TSS removals.
represents a treatment potential, which could be attained if an effective posterior sepration of suspended solids were implemented. A gradual increase in these three ways of BOD and COD removal, as well as TSS removal, was observed during the first 4 months of operation. This is consistent with the anaerobic sludge bed growth in the reactor. Figure 4 shows the average monthly results indicative of the UASB reactor performance (BOD, COD and TSS removal). A remarkable increase in these parameters during the first 4 months (March-June) is clearly visible, particularly the filtered BOD removal, representative of the soluble organic substances, which increased from about 2 to 57% during this period. These data show the increasing biological degradation potential of the.UASB reactor. After the fourth month of operation, reactor performance showed a gradual improvement, with lower ranges of removal increment. This fact indicates that after the first 4 months the inoculation phase was virtually completed (sludge bed height: 50 crn). This fast evolution of the reactor sludge bed was associated with the high suspended organic matter content found in the sewage that was used for this research (CODss=76% of CODT0. As reported by Shellinkhout et al. (1983), the high solid content in the sewage is important for the speed up of the reactor self-inoculation phase. Figure 5 gives the experimental results concerning the reactor influent and effluent BOD values, as well as the BOD removal values. The temperatures during testing are also presented in Fig. 5, ranging from 19 to 28°C. During the final 5 months of operation (July-December) the following average results were obtained: BOD removal--total: 78%, filtered: 65% and based on the total influent and filtered effluent: 91%. These results are doubtlessly satisfactory, as they were obtained with a hydraulic retention time of
1488
R.A. BARBOSAand G. L. SANT'ANNAJR
4 h with no temperature control. For COD removal the following average results were observed: COD removal--total: 74%, filtered: 52%, based on the total influent and filtered effluent: 89%, and COD/gas conversion factor: 80 NI gas kg-' COD added. Gas chromatographic analyses indicated that the gas contained 69% CH4, 26% N 2 and 5% CO2. The suspended solids in the reactor effluent were low and independent of the usual variations observed in influent suspended solids content. An average suspended solids removal of 72% was attained during the final operation period (July-December). During the experimental period, the contents of total Kjeldahl nitrogen, ammonium nitgrogen, total phosphorus, and oils and grease in the reactor influent and effluent were also measured. The average removal of these is listed in Table 4. A higher ammonium nitrogen concentration was found in the effluent in contrast to the domestic sewage content. This is explained by the biological conversion of organic nitrogen into ammonium nitrogen during the nitrogeneous organic matter degradation.
When the results of this work are compared to those reported in the literature (Table 1), it is possible to conclude that the COD, BOD and TSS removal efficiencies are higher or comparable to the best results attained by other investigators. The gas production obtained in this work in comparison with those reported (Table 1) demonstrated that although the high organic content of the sewage used and the removal efficiencies verified in this work, the gas production factor (80 NI gas kg -1 COD added) was low. Therefore, it appeared that physical action, involving several mechanisms of undissolved organic matter retention in the anaerobic reactor assumed an important role in the investigated system. However, the biodegradation rate of the undissolved organic matter retained was reduced, promoting direct effects on the gas production (liquefaction as the rate limiting step of the process). It is important to note that the sewage used in this work presented a high content of suspended organic matter (CODss/CODTI = 0.76). Few reports give the percentage of total COD which is associated with suspended organic solids in the sewage. Lettinga et al. (1981) and Grin et al. (1983) report a ratio of 0.35 and Schellinkhout et al. (1985) give a ratio of 0.58 (CODss/CODTI). The high content of suspended solids in the sewage used in this work (influent TSS = 376 mg/l) and the high capacity of solids retention in the UASB reactor were responsible for the pronounced rate of sludge accumulation in the reactor (18 kg TSS PE- ~y r ~) which was greater than the values obtained by other workers listed in Table 1. This fact is confirmed by a COD balance for the UASB reactor on the basis of the experimental data concerning the COD removal rate (from average CODTt and total COD removal efficiency) and CODCH4 production rate (total volume of methane produced in the process=measured volume of CH4 + volume of dissolved methane gas leaving the system with the effluent solution). Hence, from 362 g COD removed per day, 125 g per day were removed biochemically (converted into methane) and 237 g COD per day were removed by physical action, accumulating in the reactor: this represents 35 and 65% of the total COD reduced in the system, respectively. These results suggest that the COD was physically, rather than metabolically, removed. Moreover, considering that 112 g CODFI were introduced per day into the anaerobic reactor, we conclude that methane was also produced from undissolved organic matter biodegradation.
Table 4. Removalof other pollutantsin the UASB reactor Variable Removal (%) N-total Kjeldahl 26 N-ammonium P-total 49 Oil and grease 22
Table 5. Sludge settleability and specific methanogenicactivity results TSS SVI Specificactivity Sample (mgl-t) (mlg ITSS) (kgCOD-CH4kg-tVSSd ]) A 65,000 12 0.17 B 53,200 16 C 50,700 t7 0.14
MONTH IV~I dUN
JUL
AUG SEP
OCT
80 CoO 5O 4O. ~0. 2O 10.
I Nlil ' ,.
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IO0 6O0 5OO
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Fig. 5. Influent and effluent BOD changes during the test period.
Treatment of raw domestic sewage in an UASB reactor
1489
CONCLUSIONS
It was possible to start up the UASB reactor without using inoculum and reaching satisfactory performance after 4 months of operation, with a hydraulic retention time of 4 h at temperatures ranging from 19 to 28°C. The high suspended content of the sewage used in this work and the reactor solids retention potential contributed to the success of self-inoculation. This result is very important, because the usual inoculation procedure with digested sludge can thus be disregaded. Inoculation is a costly operation and is usually difficult to execute, due to Fig. 6. Scanning electron micrograph showing bacterial filaments (magnification 3000 x ).
Sludge
characteristics
The settling behaviour of the sludge was measured by the sludge volume index (SVI). The results of tests carried out in the eighth month of reactor operation are shown in Table 5. Samples A, B and C are composite samples from taps (a, b), (d, e) and (g, h) respectively (Fig. 1). The values of SVI reported by Vieira (1984) (25 ml gg’) and Nobre et al. (1987), (2s50ml g-l), are higher than the data shown in
Table 4. This illustrates the excellent settling characteristics of the sludge produced in the UASB reactor as described herein, which is comparable with similar values reported by Kooijmans et al. (1986) of 11 ml g-l. The specific methanogenic activity of sludge samples A and C, shown in Table 5, are compatible with the published results of Lettinga et al. (1981), Grin et al. (1983) and Schellinkhout et al. (1985) which are in the range of 0.1-0.25 kg COD-CH, kg-’ VSS d-’ and being significantly higher than the specific activity of digested sewage sludge. As aforementioned the sludge presented a granular composition (Figs 2 and 3). The granules observed in detail (scanning electron microscopy) showed a heavy structure with an appreciable quantity of inert material and a variety of morphotypes (coccus, coccus chains, sarcinas and multicellular rod-shaped filaments). These agglomerations of filaments, as shown in Fig. 6, seem to have an important role in the structural formation of the granules. The filaments were mainly present in the inner part of the granules. It was also observed that the bacterial cells were linked by a net of very thin threads similar to a spiderweb, which also seem to contribute to the structural integrity of the granules. A similar thin net was also observed by Ross (1984) in granules from an UASB reactor used in the treatment of industrial wastewater, who suggested that the net was formed by extracellular polymers which play a fundamental role in the mechanism of bioflocculation and bacterial agglomeration.
the lack of conventional treatment plants that have good quality anaerobic sludge available and especially in the developing countries. A pronounced granulation in the reactor anaerobic sludge was observed. This result is also significant as it promotes a high sludge settleability, representing a potential that could reduce the hydraulic retention time even more under routine conditions or during hydraulic overload. The UASB reactor used for domestic sewage treatment as reported herein, with no temperature control (19-28”(Z) and HRT of 4 h, attained the following removal efficiencies: BOD, 78% (based on the total influent and total effluent); COD, 74% (based on the total influent and total effluent) and TSS, 72%. The gas production factor was 80 Nl gas kg-’ COD added and the methane content of the gas was 69%. The high content of slowly degradable undissolved organic matter of the sewage used in this experiment and the high capacity of solids retention in the UASB reactor, promoted an excess sludge production estimated to be 18 kg TSS PE-’ yr-‘. The SW of the anaerobic sludge was 12-17 ml g-’ TSS which represents a high settleability. The methanogenic activity of the sludge was between 0.14 and 0.17 kg COD-CH, kg-’ VSS d-‘. The reactor operation did not result in intensive foaming at the free surface of the top separator. Unpleasant odours were not detected. It can be stated that the reactor had no operational problems during the 9-month run. Acknowledgements-The authors would like to thank the technical staff of FATMA and CASAN (Santa Catarina State Agencies) for its valuable cooperation. REFERENCES
American Public Health Association (1985) Standard Metho&for
the Examination
of Water and Wastewater,
16th
edition. Washington, D.C. Grin P. C., Roersma R. E. and Lettinga G. (1983) Anaerobic treatment of raw sewage at lower temperatures. Proc. European Symposium on Anaerobic
Wastewater
(AWWT), pp. 335-347. Noordwijkerhout,
Treatment
The Nether-
lands. Kooijmans J. L., Lettinga G. and Van Velsen A. F. M. (1986) Application of the UASB process for treatment of domestic sewage under sub-tropical conditions, the Cali case. Proc. Anaerobic Treatment a Grown-Up Technology (AQUATECH 86), Amsterdam, The Netherlands.
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R.A. BARBOSAand G. L. SANT'ANNAJR
Lane A. G. (1986) Start-up, operating requirements and granule formation during upflow sludge bed treatment of a strong food processing efliuent. Envir. Technol. Lett. 7, 555-564. Lettinga G., Roesma R. and Grin P. (1983) Anaerobic treatment of raw domestic sewage at ambient temperatures using a granular bed U A S B reactor. Biotechnol. Bioengng 25, 1701-1723. Lettinga G., Roesma R., Grin P., Zeeuw W., Hulshoff Pol L., Velsen L. V., H o b m a S. and Zeeman G. (1981) Anaerobic treatment of sewage and low strength wastewaters. In Anaerobic Digestion 1981, pp. 271-291. Elsevier Biomedical Press, Amsterdam. de Man A. W. A., Rijs G. B. J., Lettinga G. and van Starkenburg W. (1988) Anaerobic treatment of sewage using a granular sludge bed UASB-reactor. Proc. Fifth International Symposium on Anaerobic Digestion, pp. 735-738. Bologna, Italy. de Man A. W. A., Grin P. C., Roesma R. E., Grolle K. C. F. and Lettinga G. (1986) Anaerobic treatment of municipal wastewater at low temperatures. Proc. Anaerobic Treatment a Grown-up Technology (AQUATECH 86), Amsterdam, The Netherlands. Monroy O., Noyola A., Ramirez F. and Guyot J. P. 0988) Anaerobic digestion and water hyacinth as a highly efficient treatment process for developing countries. Proc. Fifth International Symposium on Anaerobic Digestion, pp. 747-751. Bologna, Italy. Nobre C. A. and Guimaries M. O. (1987) Experimentos em
digestfio anaer6bia de esgotos urbanos. Revista DAE 47, 75-85. Ross W. R. (1984) The phenomenon of sludge pelletisation in the treatment of a maize processing waste. Bat. S.A. 10, 197-204. Schellinkhout A., Jakma F. F. G. M. and Forero G. E. (1988) Sewage treatment: the anaerobic way is advancing in Colombia. Proc. Fifth International Symposium on Anaerobic Digestion, pp. 767-770. Bologna, Italy. Schellinkhout A., Lettinga G., Van Velsen L., Kooijmans J. L. and Rodrignez G. (1985) The application of the UASB reactor for the direct treatment of domestic wastewater under tropical conditions. Proc. Anaerobic Treatment of Sewage Seminar, Massachussets, U.S.A. Vieira S. M. M. (1984) Tratamento de esgotos por digestores anaer6bios de fluxo ascendente. Revista DAE 44, 139. Vieira S. M. M. (1988) Anaerobic treatment of domestic sewage in Brazil--research results and full-scale experience. Proc. Fifth International Symposium on Anaerobic Digestion, pp. 185 196. Bologna, Italy. Vieira S. M. M. and Souza M. E. (1986) Development of technology for the use of the UASB reactor in domestic sewage treatment. Proc. IA WPRC--lnternational Seminar on Anaerobic Treatment in Tropical Countries, Silo Paulo, Brazil. de Zeeuw W. J. (1984) Acclimatization of anaerobic sludge for UASB reactor start-up. Ph.D. thesis, Agricultural University, Wageningen, The Netherlands.
0043-1354/89 $3.00 +0.00 Copyright © 1989 Pergamon Press pie
T R E A T M E N T OF RAW DOMESTIC SEWAGE IN A N UASB REACTOR R. A. BARBOSA and G. L. SANT'ANNA JR COPPE/Universidadc Federal do Rio de Janeiro, C.P. 68502, 21945, Rio de Janeiro, Brazil (First received September 1988; accepted in revised form May 1989) Abstract--The treatment of raw domestic sewage at ambient temperatures in an upflow anaerobic sludge blanket (UASB) reactor with a volume of 1201. and a height of 1.92 m was studied. The sewage had an average BOD 5 of 357 mg 1- ~ and COD of 627 mg 1- t. Approximately 75% of the organic materials were in the suspended fraction. The sewage temperature ranged from 18 to 28°C during the experimental period. The reactor operated continuously for 9 months and assessed self-inoculation and raw domestic sewage purification. The unit was started without inoculum and ran during the entire experimental period with a hydraulic retention time of 4 h. During the experiment, a sludge bed build-up was observed. At the end of the experimental period, the predominance of spherical granular particles up to 6--8 mm in diameter was evident. After a 4-month operation, it was observed that the inoculation/acclimatization steps had been concluded. Removal efflciencies of BOD 5 = 78%, COD = 74% and TSS = 72% were obtained. A typical gas production factor of 801 kg-~ COD added was observed and the CH 4 content of the biogas was 69%. Key words--anaerobic treatment, sewage treatment, anaerobic reactor, UASB reactor, biodigestion, biological treatment
NOMENCLATURE
BODs = biochemical oxygen demand (5 days) (mg 1-l) COD ffi chemical oxygen demand (mg l -I) HRT = hydraulic retention time (liquid reactor volume/ flow rate) PE = populational equivalent SVI = sludge volume index (ml g-i) TSS = total suspended solids (rag 1-~) TS -- total solids (mg l -l) FE = filtered effluent FI = filtered influent TE = total effluent (non-filtered) TI = total influent (non-filtered) SS = suspended solids. (TI-TE), (TI-FE) and (FI-FE) are associated with BOD and COD removal efflciencies, calculated based on filtered and non-filtered samples.
INTRODUCTION The rapid development of the upflow anaerobic sludge blanket (UASB) reactor, at first for treating wastewaters with high and intermediate organic concentration stimulated the first investigations for applying this reactor to the treatment o f wastewaters with low organic concentration at low process temperatures. A review of recent published works of U A S B reactor performance on sewage treatment will be presented in this Introduction. As the efficiency o f the treatment process is usually expressed by C O D and B O D removal [(influent-effluent)/influent concentration], these parameters may be measured using filtered or non-filtered samples. T o avoid misunderstanding, the symbols ( F I F E ) and ( T I - T E ) are used here, as defined in the Nomenclature, to clarify which
type of concentration is being used to calculate the removal efficiency. The U S A B performance in the treatment of raw domestic sewage under tropical climate conditions was studied in The Netherlands by Lettinga et al. (1981), in a 6 m 3 pilot reactor inoculated with digested sewage sludge at a controlled temperature o f 20°C and an H R T of 8-48 h. They obtained C O D removal efficiencies ( T I - T E ) of 75% and gas production of 0.115 N m 3 of gas k g - l C O D added, with an H R T of 8 h. In a second set of experiments performed in the same pilot plant (Grin et al., 1983), the H R T was fixed at 8 h and the temperature ranged from 9.5 to 19°C. The results demonstrated that the C O D removal efficiencies and gas production were higher between 15 and 19°C within the range studied. Lettinga et al. (1983), for the purpose of assessing the feasibility of applying the U A S B reactor to raw domestic sewage under moderate climate c o n d i t i ~ s , studied the performance of a 120-1. reactor inoculated with active granular anaerobic sludge (grown~\,~on wastewaters from beet sugar production), under temperature conditions of 8-20°C and H R T of 24-8 h. They obtained C O D removal efficiencies ( T I - T E ) o f 65-90% ( H R T = 1 2 h ) and gas production of 100-2201. of gas kg -l C O D added, F o r the purpose of assessing the feasibility o f the U A S B reactor self-inoculation when applied to raw domestic sewage, Grin et al. (1983) followed up a 118-1. non-inoculated reactor performance, submitted to temperature conditions of 19--23°C and H R T of 40-24 h. They observed that after 3-4 months the C O D removal was 60% which gradually rose to 75% after 5-6 months. The results clearly showed the
1483
1484
R.A. BARBOSAand G. L. SANT'ANNAJR
possibility of starting up a U A S B reactor for raw domestic sewage treatment with no inoculation under temperature conditions 1>20°C. Schellinkhout et al. (1985) and Kooijmans et aL (1986) reported the results of operating a 64-m 3 U A S B reactor for raw domestic sewage treatment in Cali, Colombia (a typically tropical area). A small amount of inoculum, 1 m 3 of digested cattle manure, was used in this experiment and the sewage temperature was 24-26°C. Removal efficiencies ( T I - F E ) of C O D = 75-82% and B O D s = 78-85% were obtained with a H R T of 3-3.5 h. These efficiencies were maintained when the process was submitted to a H R T variation of 2.2 h during 12 h of daylight time and 6 h of night time. Schellinkhout et al. (1988), reported the results of the operation of a 35 m 3 U A S B reactor for raw domestic sewage treatment in Bucaramanga, Colombia. The sewage temperature was 23-24°C and the H R T imposed was 5.2 h. They obtained removal efficiencies (TI-TE) of B O D = 80%, C O D = 66% and TSS = 69%. Vieira (1984, 1988), and Vieira et al. (1986) reported several results of experiments with U A S B reactors applied to the treatment of domestic sewage carried out by C E T E S B (S~o Paulo State Environment Control Agency, Brazil). In the operation of a 1061. reactor inoculated with granular anaerobic sludge (grown in settled domestic sewage, at 35°C, in a reactor previously inoculated with digested sewage sludge), applied to raw sewage, at an average temperature of 20°C during winter and 23°C during summer, and H R T of 4 h, B O D removal efficiencies of 69% ( T I - T E ) were observed. C O D removal ( T I - T E ) was 60 and 65% during winter and summer, respectively. Suspended solids removal was 69% during this period. Gas production of 100 and 119 NI gas kg -~ C O D added was obtained during winter and summer, respectively. In the operation of a 120m 3 U A S B reactor inoculated with digested sewage sludge, at a temperature of 21-25°C and H R T of 4.7, BOD5 removal efficiencies ( T I - T E ) of 61% were attained. C O D removal (TI-TE) was 50% and TSS 73%. Gas production of 121 NI g a s k g -~ C O D added was obtained. Man et al. (1986) investigated the performance of three U A S B reactors of different volumes (0.12, 6 and 20 m3). The reactors were started using granular seed sludge. They observed that the anaerobic treatment of sewage (from Bennekom Village, The Netherlands), COD: 500-700 mg I ~, in the pilotscale granular U A S B reactors at an H R T of 7-12 h and at temperatures in the range of 12-18°C, provided a C O D removal (TI-TE) of 40-60%. Even at temperatures as low as 7-8°C C O D efficiencies ( T I - T E ) of 45-65% were obtained at an H R T of 9-14 h. These results demonstrated that anaerobic treatment is still fairly efficient at temperatures as low as 7-8°C, although the H R T should exceed 9 h under these conditions. In a second set of experiments performed in the 20 m 3 U A S B reactor, Man et al. (1986) treating a very dilute and septic sewage (from
Bergambacht Village, The Netherlands), COD: 3 5 0 m g l -~ and C O D / B O D : 3.5, observed that high efficiencies could not be achieved at an H R T below 12 h and temperature in the range of 6-17°C. In a third set of experiments performed in the 2 0 m 3 U A S B reactor inoculated with granular seed sludge, Man et al. (1988) treating sewage (from Lelystad, The Netherlands), COD: 740-1280 m g l ~, observed that C O D treatment efficiencies ( T I - T E ) obtained declined continuously from 48 to 16% and the same applied for the gas production which dropped from 80 to 31 NICH4 kg ~ C O D added. The methanogenic activity of the granular sludge in the reactor dropped to approx. 4% of its original activity. The reactor operated with H R T of 13-14h and a temperature range of 10-1Y'C. The causes of this activity loss were points of further extensive investigations. Nobre et al. (1987) described that BOD removal (TE-TI) of 78% and C O D removal (TE-TI) of 73% were attained in a 3.7 m 3 U A S B reactor inoculated with sewage digested sludge and operated with an H R T of 10-18 h at a temperature range of 24-26 C. Monroy et al. (1988) reported the results of the operation of a 1101. U A S B reactor for raw domestic sewage treatment. The reactor was inoculated with anaerobically batch-adapted activated sludge. The sewage temperature was 12-18 C and the H R T was 18 h. They obtained removal efficiencies (TI-TE) of C O D = 6 5 % and T S S = 7 3 % . At the time the experimental work reported herein was performed, little published data was available concerning the treatment of raw sewage in U A S B reactors, mainly at variable temperature and there was no detailed study on the start-up procedure without innoculation. The lack of knowledge in this area motivated the study which we present in this paper. MATERIALS AND M E T H O D S
The experimental unit was installed at the Ponta do Leal Pumping Station, of the Water and Sanitation Company of Santa Catarina State (CASAN), in Florian6polis. The pumping station received sewage from a typical residential area. The main characteristics of the sewage fed to the experimental unit are given in Table 2. The experimental set-up is illustrated schematically in Fig. 1. The sewage coming from a branch of the discharge line was thrown directly into a plastic-net basket immersed in a 150-1. holding tank. This net (2.25 mm 2 square holes) prevented coarse solids from entering the reactor. Liquid in the holding tank was continuously mixed by recirculation and renewed with the use of an overflow apparatus. The sewage mean retention time in the holding tank was 27 min. From this tank the sewage was pumped upward (using a peristaltic pump) to a small reservoir, where it would go by gravity to the reactor bottom. The reactor, built of PVC and plexi-glass, had a capacity of 1201. and was 1.92 m high. Plexi-glass windows, diametrically opposite and distributed along the reactor, made it possible to visually inspect the sludge bed dynamic behaviour and the solid/liquid/gas separating device provided at its top. The internal separator was of the inverted cone type, as depicted in Fig. I. The gas produced was measured by liquid displacement (NaCI saturated aqueous solution, acidified with HCI) in a Mariotte
This work
M o n r o y et al. ( 1 9 8 8 ) M a n et al. ( 1 9 8 8 )
N o b r e et aL ( 1 9 8 7 ) Vieira (1988) S c h e l l i n k h o u t et aL ( 1 9 8 8 )
M a n et al. ( 1 9 8 6 )
V i e i r a et al. ( 1 9 8 6 )
G r i n et al. ( 1 9 8 3 ) Vieira (1984) S e h e l l i n k h o u t et al. ( 1 9 8 5 )
L e t t i n g a et aL ( 1 9 8 3 )
G r i n et al. ( 1 9 8 3 )
L e t t i n g a eS al. ( 1 9 8 1 )
Reference
19-28
23 12-18 -7-8 -24-26 21-25 23-24 -12-18 10-15 4
8 8 8 12 12 24 4 4-8 3-3.5 4 4 7-12 -9-14 -10-18 4.7 5.2 -18 13 14
15 - 1 9 11-12 9. 5 - 1 0 8-20 8-20 19-23 35 24-26 24-26
20
18 24 8
HRT (h)
20 20 20
Temp. CC)
627 (357)
500 400 500 500 300 460 341 ( 1 3 7 ) 267 267 424 (195) 406 (180) 500-700 (250-350) 500-700 (250-350) 660(300) 265(150) 430-520 (190-220) 465 740-1280
550 51)0-550 500
376
----. -88 215 215 188 191 -. -. -123 200-250 . 154 --
-. --
TSS ( m g l i)
.
.
.
.
.
.
.
78
---
78 61 80
--
-72 --69 69 50-70
-----
--
--
BOD (TI-TE)
of published results--UASB
CODTt (BODTI) ( m g l i)
T a b l e I. S u m m a r y
.
.
.
.
.
COD
91
--
-88 83-85 78-85 85 84 -. -. ---. --
----
--
--
--
BOD (TI-FE)
BOD,
.
.
.
48-16 74
65
73 50 66
45-65
60 65 --60 65 40~0
40-55 30-50 30 66.5
--
55-75
COD (TI TE)
.
.
.
-89
65-80 55-70 55 65-90 50-60 70 83 83-85 78-85 82 83 . . . . 85-91 --. --
-70 75
COD (TI-FE)
and TSS removals (%)
reactors for sewage treatment
.
.
-72
-73 69 . 73
.
.
------61 70 -69 69
----
TSS
.
.
80-31 (CH4) 80
--
140 121 --
.
90 50 100-200 -115 118 --100 119 .
130
I 15
150-200 160
Gas NI kg- i COD added
-18 ( T S S )
--
-2 (TSS) --
9.6 ( S T ) 8.5 ( S T ) 13.4 ( S T ) 5.0-8.6(ST) ---7.3 ( T S S ) ----
9.9 (ST)
-7.9 ( S T )
Excess sludge kg ( ) PE i yr '
Gas and sludge production
?,
,.q
g
a
>
g~
O
a
¢a
1486
R . A . BARBOSAand G. L. SANT'ANNA JR
Table 2. Characteristics of the sewage fed to the UASB reactor
Table 3. List of monitored process variables
Range or mean value
Variable Temperature CC) pH Volatile acids (mgl ~) (as acetic acid) Total alkalinity (mg CaCO 31 ~) BODTI (rag 1- ]) BODF[ (rag I t) COD-r] (mg I i) CODFI (mg I ') TS (mgV i) TSS (mgl i) VSS (mgl ]) N-total Kjeldahl (mg N I ~) N-ammonium (mg N 1-]) P-total (mg PI ~) Oil and grease (mgl 1) Surfactants (mg MBAS 1 ~) Sulfate (rag SOl- 1 ~)
Variable
18-28 6.2 7.6 39
Volatile acids Total alkalinity BOD total BOD filtered COD total COD filtered TS TSS VSS N-ammonium N-total Kjeldahl P-total Oil and grease
198 357 94 627 151 872 376 297 54 30 9.9 93 1.8 124
type collector, so as to keep a constant pressure in the reactor of 210 m m of water column. Besides the basic operating variables (flow, temperature, pH and gas production), which were checked daily on site, a physical-chemical monitoring program was designed, whose parameters and analytical frequencies are displayed in Table 3. The reactor influent and effluent samples were composed o f grab samples collected every hour over a 24 h period. During the collection period, the samples were stored at 4°C. The physical-chemical analyses used were as recommended by Standard Methods for the Examination of Water and Wastewater (APHA, 1985). Gas composition was determined by gas chromatography (columns: chromosorb 102 and molecular sieve 5A; column temperature: 90°C; carrier gas: He; injection temperature: 100°C; detector temperature: I10°C). The specific methanogenic activity of sludge samples was determined according to the method used by Zeeuw (1984). The bacterial morphology of the pellets was examined by means of scanning electron microscope Jeol 25 SII. The
Frequency
Sample, c = composite g = grab
Each 15 days Each 15 days 2-3 per week 2 3 per week 2 3 per week 2-3 per week 2 3 per week 2 3 per week 2 3 per week Weekly Weekly Weekly Weekly
g g c c c c c c c c c c c
pellets examined were prepared according to the method described by Ross (1984) and Lane (1986). The reactor start-up was without inoculum and was fed with domestic sewage. The system operated with a hydraulic retention time of only 4 h, which was continued until the end of the experimental period. The experiment was conducted at the ambient temperature of the sewage. During the whole period the anaerobic reactor temperature ranged from 19 to 28°C. The results from approx. 9 m o n t h s continuous operation are reported. RESULTS AND DISCUSSION Although the operation was started without inocul u m , solids were o b s e r v e d in t h e r e a c t o r f r o m t h e very b e g i n n i n g . T h e solids b u i l d u p w a s p r o g r e s s i v e : c o m p a c t , s p h e r i c a l b a c t e r i a l g r a n u l e s a b o u t I m m in d i a m e t e r were o b s e r v e d a f t e r 1 m o n t h o f o p e r a t i o n . The incidence of granular formation was progressive. At the end of the 9-month operation, spherical g r a n u l e s u p to 8 m m d i a m e t e r were e s t a b l i s h e d . It w a s also o b s e r v e d t h a t , in t h e c o u r s e o f time, t h e
t
11
rl.e.~-
d~-
8
,,j*
¢-h-
b~
1 - ~ IM.ETi 2-PLASTIC NET ~ i 5..HOLDING TANK; 4-OMER F10W ; 5 , 6 , 1 3 - PUMPS ; 7-RESERVOIR 1 8 - ~ i 9-1N~ SEPARA'R~ i 10 -EFFLUENTi 11 -GAS HOLDER i 12-LIQUID CONTAINER ; a - i - SAMPLING TAPS. Fig. 1. Schematic diagram of the experimental set-up.
Treatment of raw domestic sewage in an UASB reactor
1487
~APRI M~l I JUN I JU- I AUG I SB' IOCr I NOViOB~ i
I
60. 40 2~ 8O
,
'
I REMOVAL (%)
TSS
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I
I
I
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.
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..... j r "---a ~____J
Reactor performance Reactor performance was assessed on the basis of BOD 5, COD and suspended solids (TSS) removal. In the case of BOD and COD, the removal was expressed in three ways: based on (i) total influent and total effluent; (ii) filtered influent and filtered effluent; and (iii) total influent and filtered effluent. The first represents the indicator (BOD or COD) overall removal, comprising the soluble substances and the suspended material contributions, thus reflecting the organic matter removal by physical action and biological decomposition. The second expresses the soluble organic substances removal, predominantly representative of the biological action. The third
Fig. 3. Scanning electron micrograph of a granule (magnification 45 x ).
.
.
,
- -
. __,,.r - - _ ,
Fig. 2. Anaerobic granules--shape and size.
granular formation started to prevail over the flocculent shapes of anaerobic sludge. At the end of the experiment, it was verified that the reaction compartment (height: 1.42 m) was completely full of sludge, with no harm to the final effluent quality. Figure 2 shows the shape and the size of the sludge granules. Figure 3, a scanning electron micrograph, illustrates the uniform aspect of the external surface of these granules.
(O,o
. . . . . . . . .
--I-1213
, ,,-, ~,~.UTOTAL..FILTEI.ED
/ .___: .... F--BOD RE]VI(NALS (%l -'-
TOTAL-FIL~
I
I
1415161718191 MONTH OF 00B~ATION
Fig. 4. Average monthly results of BOD, COD and TSS removals.
represents a treatment potential, which could be attained if an effective posterior sepration of suspended solids were implemented. A gradual increase in these three ways of BOD and COD removal, as well as TSS removal, was observed during the first 4 months of operation. This is consistent with the anaerobic sludge bed growth in the reactor. Figure 4 shows the average monthly results indicative of the UASB reactor performance (BOD, COD and TSS removal). A remarkable increase in these parameters during the first 4 months (March-June) is clearly visible, particularly the filtered BOD removal, representative of the soluble organic substances, which increased from about 2 to 57% during this period. These data show the increasing biological degradation potential of the.UASB reactor. After the fourth month of operation, reactor performance showed a gradual improvement, with lower ranges of removal increment. This fact indicates that after the first 4 months the inoculation phase was virtually completed (sludge bed height: 50 crn). This fast evolution of the reactor sludge bed was associated with the high suspended organic matter content found in the sewage that was used for this research (CODss=76% of CODT0. As reported by Shellinkhout et al. (1983), the high solid content in the sewage is important for the speed up of the reactor self-inoculation phase. Figure 5 gives the experimental results concerning the reactor influent and effluent BOD values, as well as the BOD removal values. The temperatures during testing are also presented in Fig. 5, ranging from 19 to 28°C. During the final 5 months of operation (July-December) the following average results were obtained: BOD removal--total: 78%, filtered: 65% and based on the total influent and filtered effluent: 91%. These results are doubtlessly satisfactory, as they were obtained with a hydraulic retention time of
1488
R.A. BARBOSAand G. L. SANT'ANNAJR
4 h with no temperature control. For COD removal the following average results were observed: COD removal--total: 74%, filtered: 52%, based on the total influent and filtered effluent: 89%, and COD/gas conversion factor: 80 NI gas kg-' COD added. Gas chromatographic analyses indicated that the gas contained 69% CH4, 26% N 2 and 5% CO2. The suspended solids in the reactor effluent were low and independent of the usual variations observed in influent suspended solids content. An average suspended solids removal of 72% was attained during the final operation period (July-December). During the experimental period, the contents of total Kjeldahl nitrogen, ammonium nitgrogen, total phosphorus, and oils and grease in the reactor influent and effluent were also measured. The average removal of these is listed in Table 4. A higher ammonium nitrogen concentration was found in the effluent in contrast to the domestic sewage content. This is explained by the biological conversion of organic nitrogen into ammonium nitrogen during the nitrogeneous organic matter degradation.
When the results of this work are compared to those reported in the literature (Table 1), it is possible to conclude that the COD, BOD and TSS removal efficiencies are higher or comparable to the best results attained by other investigators. The gas production obtained in this work in comparison with those reported (Table 1) demonstrated that although the high organic content of the sewage used and the removal efficiencies verified in this work, the gas production factor (80 NI gas kg -1 COD added) was low. Therefore, it appeared that physical action, involving several mechanisms of undissolved organic matter retention in the anaerobic reactor assumed an important role in the investigated system. However, the biodegradation rate of the undissolved organic matter retained was reduced, promoting direct effects on the gas production (liquefaction as the rate limiting step of the process). It is important to note that the sewage used in this work presented a high content of suspended organic matter (CODss/CODTI = 0.76). Few reports give the percentage of total COD which is associated with suspended organic solids in the sewage. Lettinga et al. (1981) and Grin et al. (1983) report a ratio of 0.35 and Schellinkhout et al. (1985) give a ratio of 0.58 (CODss/CODTI). The high content of suspended solids in the sewage used in this work (influent TSS = 376 mg/l) and the high capacity of solids retention in the UASB reactor were responsible for the pronounced rate of sludge accumulation in the reactor (18 kg TSS PE- ~y r ~) which was greater than the values obtained by other workers listed in Table 1. This fact is confirmed by a COD balance for the UASB reactor on the basis of the experimental data concerning the COD removal rate (from average CODTt and total COD removal efficiency) and CODCH4 production rate (total volume of methane produced in the process=measured volume of CH4 + volume of dissolved methane gas leaving the system with the effluent solution). Hence, from 362 g COD removed per day, 125 g per day were removed biochemically (converted into methane) and 237 g COD per day were removed by physical action, accumulating in the reactor: this represents 35 and 65% of the total COD reduced in the system, respectively. These results suggest that the COD was physically, rather than metabolically, removed. Moreover, considering that 112 g CODFI were introduced per day into the anaerobic reactor, we conclude that methane was also produced from undissolved organic matter biodegradation.
Table 4. Removalof other pollutantsin the UASB reactor Variable Removal (%) N-total Kjeldahl 26 N-ammonium P-total 49 Oil and grease 22
Table 5. Sludge settleability and specific methanogenicactivity results TSS SVI Specificactivity Sample (mgl-t) (mlg ITSS) (kgCOD-CH4kg-tVSSd ]) A 65,000 12 0.17 B 53,200 16 C 50,700 t7 0.14
MONTH IV~I dUN
JUL
AUG SEP
OCT
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IO0 6O0 5OO
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Fig. 5. Influent and effluent BOD changes during the test period.
Treatment of raw domestic sewage in an UASB reactor
1489
CONCLUSIONS
It was possible to start up the UASB reactor without using inoculum and reaching satisfactory performance after 4 months of operation, with a hydraulic retention time of 4 h at temperatures ranging from 19 to 28°C. The high suspended content of the sewage used in this work and the reactor solids retention potential contributed to the success of self-inoculation. This result is very important, because the usual inoculation procedure with digested sludge can thus be disregaded. Inoculation is a costly operation and is usually difficult to execute, due to Fig. 6. Scanning electron micrograph showing bacterial filaments (magnification 3000 x ).
Sludge
characteristics
The settling behaviour of the sludge was measured by the sludge volume index (SVI). The results of tests carried out in the eighth month of reactor operation are shown in Table 5. Samples A, B and C are composite samples from taps (a, b), (d, e) and (g, h) respectively (Fig. 1). The values of SVI reported by Vieira (1984) (25 ml gg’) and Nobre et al. (1987), (2s50ml g-l), are higher than the data shown in
Table 4. This illustrates the excellent settling characteristics of the sludge produced in the UASB reactor as described herein, which is comparable with similar values reported by Kooijmans et al. (1986) of 11 ml g-l. The specific methanogenic activity of sludge samples A and C, shown in Table 5, are compatible with the published results of Lettinga et al. (1981), Grin et al. (1983) and Schellinkhout et al. (1985) which are in the range of 0.1-0.25 kg COD-CH, kg-’ VSS d-’ and being significantly higher than the specific activity of digested sewage sludge. As aforementioned the sludge presented a granular composition (Figs 2 and 3). The granules observed in detail (scanning electron microscopy) showed a heavy structure with an appreciable quantity of inert material and a variety of morphotypes (coccus, coccus chains, sarcinas and multicellular rod-shaped filaments). These agglomerations of filaments, as shown in Fig. 6, seem to have an important role in the structural formation of the granules. The filaments were mainly present in the inner part of the granules. It was also observed that the bacterial cells were linked by a net of very thin threads similar to a spiderweb, which also seem to contribute to the structural integrity of the granules. A similar thin net was also observed by Ross (1984) in granules from an UASB reactor used in the treatment of industrial wastewater, who suggested that the net was formed by extracellular polymers which play a fundamental role in the mechanism of bioflocculation and bacterial agglomeration.
the lack of conventional treatment plants that have good quality anaerobic sludge available and especially in the developing countries. A pronounced granulation in the reactor anaerobic sludge was observed. This result is also significant as it promotes a high sludge settleability, representing a potential that could reduce the hydraulic retention time even more under routine conditions or during hydraulic overload. The UASB reactor used for domestic sewage treatment as reported herein, with no temperature control (19-28”(Z) and HRT of 4 h, attained the following removal efficiencies: BOD, 78% (based on the total influent and total effluent); COD, 74% (based on the total influent and total effluent) and TSS, 72%. The gas production factor was 80 Nl gas kg-’ COD added and the methane content of the gas was 69%. The high content of slowly degradable undissolved organic matter of the sewage used in this experiment and the high capacity of solids retention in the UASB reactor, promoted an excess sludge production estimated to be 18 kg TSS PE-’ yr-‘. The SW of the anaerobic sludge was 12-17 ml g-’ TSS which represents a high settleability. The methanogenic activity of the sludge was between 0.14 and 0.17 kg COD-CH, kg-’ VSS d-‘. The reactor operation did not result in intensive foaming at the free surface of the top separator. Unpleasant odours were not detected. It can be stated that the reactor had no operational problems during the 9-month run. Acknowledgements-The authors would like to thank the technical staff of FATMA and CASAN (Santa Catarina State Agencies) for its valuable cooperation. REFERENCES
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