Some operational characteristics of a bacterial disc unit

Water Research Pergamon Press 1971. Vol. 5, pp. 1141-1146. Printed in Great Britain

SOME

OPERATIONAL BACTERIAL

CHARACTERISTICS DISC

OF

A

UNIT

W. A. PRETORIUS National Institute for Water Research of the South African Council for Scientific and Industrial Research, P.O. Box 395, Pretoria, South Africa (Received 23 June 1971)

Abstract--Anaerobically pretreated sewage was treated by an aerobic bacterial disc unit. Owing to the plug-flow nature of the system, different microfloral populations developed on the different discs. The maximum weight of biomass obtained was 45 g m - 2 of the wetted surface. Maximum COD removal was 0.49 g COD g-t biomass day-t. Practically all nitrogen was converted to nitrate; maximum nitrification rate was 0'138 g NO a-N g - t biomass day- 1. INTRODUCTION WHEN the removal of ammonia from anaerobically pretreated raw sewage was investigated by means of a rotating algal disc unit (HEMENS and STANDER,1969) it was found that bacteria quickly overgrew the benthic algae on the discs, rendering the unit incapable of rernoving ammonia. This was probably due to the inability of the bacteria to raise the p H of the water to a level where physical stripping of ammonia takes place. Although rotating bacterial disc units have been used before (GLOYNA et aL, 1952; HARTMAN, 1965; KERSHAW,1970) relatively little is known regarding the Chemical Oxygen Demand (COD) removal and ammonia oxidising capabilities of such systems. The simplicity of converting an algal disc unit into a bacterial disc unit as well as the fact that this system differs from other aerobic systems in that only the biomass on the discs is in effect aerated, initiated an investigation into the operational properties o f such a unit. This paper describes some of the results obtained from a laboratory size bacterial disc unit operated on anaerobically pretreated domestic sewage. MATERIALS AND METHODS The disc unit

The rotating disc unit (FIG. 1) was basically the same as used by HEMENSand STANDER (1969). The unit consisted of nine parallel PVC discs, 40 cm in diameter, set 6.3 c m apart on a horizontal shaft. The discs were separated from each other by notched baffles in a V-shaped tank. To increase the surface area of the discs, as well as to provide a holdfast for the microbial film, each disc was perforated with 1335 holes, 0.6 cm in diameter and distributed at random. The total available surface area (including the sides o f the holes) was then 3360 cm 2 (compared to 2605 cm 2 for unperforated discs). The discs were rotated at a constant speed of 6.75 rev m i n - 1 by means of a Framco motor (Fractional H.P. Motors Ltd., Enfield, England). Although the rotation of the discs created a completely mixed flow pattern in each individual disc compartment, there was no back mixing from one compartment to another, so that the overall flow was essentially "plug-flow". The unit was operated in a constant temperature room at 20°C. 1141

1142

W.A. PRETORIUS

Operational procedures The feed was obtained from a laboratory size anaerobic digester unit operated on raw sewage (PRETORIUS, 1971) Apart from a small inoculum from an activated sludge unit applied initially, the microflora was allowed to build up naturally according to a feed programme as summarized in TABLE 1. TABLE 1. FEEDING PROGRAM OF ROTATING DISC UNIT

Phase

Overall residence time (h)

Days at steady state

Separate sets of samples

Remarks

I

16

21

7

Adaptation period

II

13

39

8

Ill

11

6

2

Microscopical observations and bacterial counts When the discs were covered with a microbial layer, micro-photographs of the dominant bacterial species were made with a Zeiss Photomicroscope. On two occasions during Phase II, total counts on nutrient agar plates, incubated at 20°C for 4 days, were done, as well as faecal Escherichia coli counts by means of the membrane filter technique.

Sludge handling Owing to the rapid growth-rate on the first disc, it was found necessary to withdraw sludge (microbial cells) periodically from the first disc compartment. The sludge which accumulated at the bottom of the other compartments was removed and weighed after 66 days of operation. At this stage the experiment was terminated and the biomass accumulated on each disc was removed and weighed.

Chemical sampling and analysis When possible, samples were taken twice weekly from each compartment and analysed on an Auto-analyser (Technicon, Tarry Town, New York). The COD was determined by the method ADELMAN (1968); ammonia by the method o f HARWOOD and HUYSER (1970); orthophosphate by the method o f MURPHY and RILEY (1962) and ABS by means of the method of Standard Methods (1965). Nitrates and nitrites were determined by the methods of KAMPnAKE et al. (1967). Total and volatile solids were determined gravimetrically at 104°C and 600°C respectively. During Phase II operation, dissolved oxygen was determined four times by means o f a Beckman Model 777 Laboratory Oxygen Analyser (Bechman Instrument Inc., Fullerton, California). RESULTS

Microbiological After twenty-one days a good biological film had developed on all the discs. A distinct difference was observed in the nature o f the microbial population on different discs, evidently due to the plug-flow nature of the unit.

FIG. 1. Laboratory scale bacterial disc unit.

(Facing p. 1142)

FIG. 2.(a) Filamentous growth of bacteria on first disc. (b) Lower power dark field magmfication of filamentous bacteria showing the composition of a single strand. (c) High power phase contrast magnification of filamentous bacteria showing Beggiatoa and Sphaerotilus species. (d) Cluster of ciliates amongst filamentous bacteria. (e) Zoogloeal mass with five filamentous cells present on successive discs. (f). Trapping devices of nematode trapping Athrobotrys species.

Some Operational Characteristics of a Bacterial Disc Unit

1143

The first disc nearest to the feeding point, and to a lesser degree the second disc, was covered with a thick filamentous growth (see FIG. 2a). This consisted o f Sphaerotilus and Beggiatoa species interwound (see FIG. 2b). The individual cells present in the sheath of Sphaerotilus as well as the sulphur granules present in Beggiatoa are shown on FIO. 2c. Protozoa, including amoeba and ciliates, especially Paramecium species, were very weLl represented (see FIG. 2d). Fungi were absent from the first two discs, although m a n y nematodes were present. A marked change occurred in the form of the growth on the third to the ninth discs; the filamentous growth changed to a slimy indefinable film. Microscopically this growth consisted of very fine filamentous cells together with a zoogloeal mass (see FIo. 2e). A loop-forming fungus species, probably belonging to the nematode-trapping genus Athrobotrys (see FIG. 2f), was very well represented on these discs. The total and E. coli counts on the influent and effluent are shown in TABLE 2. TABLE 2. ]~UMBER OF BACTERIA PRESENT IN INFLUENT AND EFFLUENT FROM ROTATING DISC UNIT*

Faecal E. coli (number ml- 1)

Total count (number ml- 1)

Sample number

Influent

Effluent

Influent

Effluent

1

16 x

105

13 X 103

17.3 X 10a

3

2

12 X 105

16 x 103

23 X I03

60

* Each count represents the average of three replicates. There was a very slow increase in the weight of the biomass on the third to last discs. In the case of the first two discs the biomass tended to build up to a certain point, at which it would then become detached from the disc surface and accumulate at the bottom of the compartment. The minimum weight of the biomass left on the discs immediately after the rest had slipped off, was 2.0 g, and the maximum 15.0 g, with an average of 8.2 g. When the discs were scraped, the suspended solid content o f the harvested biomass was 3"6 per cent. The average of the daily withdrawn sludge which TABLE 3. DRY WEIGHTS OF BIOMASS PRESENT IN THE ROTATING DISC UNIT

Disc number

Biomass on discs (g) Volatile Biomass settled in compartment (g) Volatile W.R. 5/12--B

1

2

3

4

5

6

7

8

9

6.8

11.8

11.8

8.0

5.4

3.6

3'0

2.4

1-6

94.1

88.1

80.0

82.5

85.2

78.0

80.0

75.0

75.0

0.8

7"5

3"5

1'4

1.4

1"8

1"4

0-6

0'6

93.8

89'3

85.7

85.7

71.4

77.8

71.4

66.7

75.0

1144

W . A . PRETORIUS

I

2

4

5

2 0 0 ~ 1

I

Compartments 5

I

7

6

I

I

8

9

•1

I

I

I \ T~

E

100

O~O~®...."

--

d o o

I 0

1

I

I

I

I

I

I

I

I

I

I

I

I

2

3

4

5

6

7

8

9

I0

It

12

Time,

]

13

h

FIG. 3. COD removal by a disc unit.

Compartments I

2

3

'1

50

4

I

5

I

6

1

I

7

8

i

9

I

I

"-------e~B 40

;''~'A

~ A

~

N0$/o------.-o~o

~

T.j 30

E 20

10

' , ~ × T

0

I

2

3

t-x

:

xI

4

5

6

Time,

Ix

,,

7

8

x ',

j~--

9

_~,~i.~.,.

10

II

12

13

h

FIo. 4. Nitrogen transformations through a disc unit. Compartments 1

2

I0

I

3

4

I

I

5

6

I

1

7 I

8

9

I

I

Dissolved O~.z& / ' 7~

A.___-.--~A X ® o

I

I

I

I

I

I

1

I

I

I

I

I

I

2

3

4

5

6

7

8

9

I0

II

12

Time,

13

h

FIO. 5. Distribution of dissolved oxygen and A.B.S. concentrations through a disc unit.

Some Operational Characteristics of a Bacterial Disc Unit

1145

detached from the first disc was 0.53 g, contained in approximately 200 ml water. The weights of the biomasses on each disc, as well as at the bottom of each compartment at the end of the experiment, are given in TABLE 3. It can be seen that practically all the biomass was produced on the first two discs. The removal of C O D is shown in FIG. 3, the transformations of nitrogen in FIG. 4 and the average oxygen levels and ABS concentrations in FIG. 5. The various rates of removals and transformations are shown in TABLE 4. Although the total phosphate TABLE 4.

Removals and transformation rates g COD g-t cells day -1 g COD m -2 day -1 g NHa-Ng-lcellsday -1 g

NOa-N g-t cells day -t

RATESOF COD REMOVALANDNITROGENCONVERSIONS

Disc number 5 6

1

2

3

4

0"49

0"17

0.09

0'04

0-07

7-7

4"1

1.18

1.43

12.7

7

8

0"06

0.07

--

0-84

0.84

--

9

--

0.016 0.0124 0.043 0"042 0.0896 0"0414 0.033 0.006

0

0.005 0.014 0.032 0.035 0-138 0.04

--

--

--

concentration decreased slowly from 13"2 to 10"6 mg 1-1, the orthophosphate concentration remained more or less constant at 10.0 mg 1-1. DISCUSSION F r o m a microbiological point of view the rotating disc unit provided an excellent means for ecological studies o f biologically based purification processes. Owing to the plug-flow nature of the process, the microbial population tends to differentiate itself from disc to disc. In this respect the bacterial flora change from rapidly growing Sphaerotilus species and sulphur-containing Beggiatoa species on the first disc, to highly aerobic nitrate-forming bacteria and fungi on the successive discs. The absence o f any nematodes in the effluent may be associated with the presence of the carnivorous genus Athrobotrys (PiPEs, 1965). At all times the rotating disc unit gave a clear effluent, which is verified by the relatively low total and E. coli counts (see TABLE 2), but a brownish colour, typical of biological filter effluents, was always present. As a result of normal biomass loss due to detachment from the first disc, the average b!omass age of the first disc was 15.5 days. The maximum weight of biomass which could be maintained on a disc was 45 g m - 2, which corresponded in this unit to a mixed liquor suspended solids concentration of 3.7 g 1-1. The maximum rate of C O D removal occurred on the first disc, namely 0"49 g C O D g - 1 cells d a y - 1 (see TABLE 4), which compares favourably with activated sludge units. Although the unit was poor in overall nitrogen removal, probably owing to the prevention of denitrification by the high oxygen concentration maintained, the unit was found to be highly effective in converting ammonia N to nitrate N. Complete

1146

W.A. PRETORIU$

conversion t o o k place in a b o u t 8"5 h (see FIG. 3). A l t h o u g h ABS removal was good, there was practically no p h o s p h o r o u s removal. A l t h o u g h the flow rate during Phase I I I (TABLE 1) WaS a b o u t 15 per cent higher than Phase II, the chemical data followed a similar pattern. W h e n c o m p a r e d on a time basis, the curves were o f the same configuration, except for a 15 per cent lag in efltuent quality o f the different compartments during phase III. This indicated a zero-order type o f reaction where reaction rate (within limits) was independent f r o m concentration. SUMMARY W h e n a bacterial disc unit was operated on anaerobically pretreated raw sewage it was f o u n d that: (1) " b u l k i n g " did not occur; (2) owing to its plug-flow nature, different microfloral populations, each performing a different function, developed o n the different discs; (3) practically all nitrogen was converted to nitrate; (4) the m a x i m u m weight o f biomass obtained was 45 g m - 2 o f wetted surface; (5) m a x i m u m C O D removal was 0.49 g C O D g - t biomass d a y - ~ ; (6) m a x i m u m nitrification was 0.138 N O a - N g - 1 biomass d a y - t; (7) within the C O D range operated the reaction rates seemed to be zero order. Acknowledgement--The technical help of Mr J. H. NELLand Miss P. I. PALMERare gratefully acknow-

ledged. REFERENCES ADELMANM. H. (1968) Automated measurement of COD. Water Wastes Engng 5 (6), 52-55. GLOYNAE. F., COMSTOCKR. F. and RENNC. E. (1952) Rotary tubes as experimental trickling filters. Sewage Ind. Wastes 24, 1355-1357. HARTMANNH. (1965) The Bio-Disc Filter. Oesterreich. Wasswirtsch. 11/12. HARWOODJ. E. and HUYSERD. J. (1970). Automated analysis of ammonia in water. Water Research 4, 695-704. HEMENS J. and STANDERG. J. (1969). Nutrient removal from sewage effluents by algal activity. Advances in Water Pollution Research, pp. 701-715. Proceedings of the 4th International Conference, Prague 1969. (Edited by S. H. JENKINS)Pergamon Press, Oxford. KAMPHAKEL. J., HANNAHS. A. and COHENJ. M. (1967) Automated analyses for nitrate by Hydrazine Reduction Water Research 1, 206. KERSHAWM. A. (1970) Waste Water Treatment. Process Biochem. 13-16. MURPHYJ. and RILEYJ. P. (1962) A modified single solution method for the determination of phosphate in natural waters. Analyt. chim. acta 27, 31-36. PIPES W. O. (1965) Carnivorous plants in activated sludge. Proeeedings of the 20th Ind. Waste Conf., Purdue University, Lafayette, Indiana. Engineering Extension Series, No. 118, pp. 647-656. PRETORIUSW. A. (1971) Anaerobic digestion of raw sewage. Water Research 5, 681-687. Standard Methods for the Examination of Water and Waste Water. 12th Edn (1965) American Public Health Association, Inc.