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Denitrification with a bacterial disc unit
Water Researc:t Vol. 9, pp. 459 to 463. Pergamon Press 1975. Printed in Great Britain.
DENITRIFICATION WITH A BACTERIAL DISC UNIT T. R. DAVIESand W. A. PRETOR1US National Institute for Water Research of the Council for Scientificand Industrial Research, P.O. Box 395, Pretoria, South Africa
(Received 25 March 1974) Abstract--An enclosed rotating disc unit was operated anaerobically as a denitrifying system, with methanol as the hydrogen donor. As the bacterial population became established, denitrification rate increased by 1.5 mg NO~-N reduced m- 2h- 2, to a maximum rate of 260 mg NO3-N reduced m- 2h- 1. The C:N ratio necessary for complete denitrification was found to be 2.6:1. Optimum pH for denitrification lay in the range between pH 7.0 and 8.5. Q~o values were 1.38 between 10 and 30°C, -2.66 above 30°C and 13.06 below 10°C. INTRODUCTION
Various equipment designs and process modifications have been proposed for the complete or partial removal of nitrate from secondary sewage effluents or drainage waters. These include submerged biological filters and anaerobic or semi-anaerobic adaptations of the activated sludge process. Since biological denitrification is essentially an aerobic process occurring under anaerobic conditions, two problems are generally encountered. These are the production of a large microbial biomass, which eventually clogs the void spaces in submerged filters, and the evolution of nitrogen gas, which tends to adhere to particles of microbial floc, impeding the separation of biDmass in any type of gravity separation system. The known properties of cell retention and gas exchange of a rotating disc unit (RDU) could be used to largely overcome the problems of cell retention and separation by enclosing such an RDU with an airtight cover, thus changing a normally aerobic system into an anaerobic unit suitable for denitrification. This paper describes the denitrification performance of an anaerobic RDU, with respect to carbon requirement, pH and temperature.
The experimental unit The experimental unit consisted of a disc unit and controllers for the feed rate, pH and temperature. The disc unit. The RDU contained 43 parallel perspex discs mounted 3 mm apart on a horizontal shaft. The discs were 406 mm in diameter and 1.5 mm thick, giving a total surface area of 11.094 m z, or a total surface available for microbial attachment (including the wetted surface area of the container) of 11"25 m z. The liquid volume of the disc system without discs was 21-5 1., giving a surface area to container volume of 523-3 m 2 m-3. With the discs about 45% submerged, the liquid volume was 14-25 1., resulting in a surface area to liquid volume ratio of 789.5 m z m -3. The disc i
Co3. Feed pump, pH and temperature controllers. Variable speed pumps were used for feeding the nitrified effluent and the electron donor (methanol). The pH was controlled by means of a titrometer type pH meter (PHM28b/TTT1 lb, Radiometer) with automatic alkali or acid additions to control the pH within limits of 0.1 pH units over a range of 2-12. Temperature was maintained by means of a thermostatic control unit (K2RD, MGW Lauda) in the recirculation line of the disc unit. ,The temperature could be controlled with I°C of any selected point in the range between 5 and 30°C. Because of the relatively high feed rates eventually involved, it was necessary to precool the feed to 4°C in order to maintain any particular temperature below ~0°C. The feed The nitrified feed. The feed consisted of settled effluent
MATERIALS
w.R, 914
unit was enclosed by an airtight cover (Fig. 1). The discs were rotated at a constant speed of 6 rev min- 1. To facilitate mixing of the liquid being treated and control of pH and temperature, the system was operated as a completely mixed unit, the contents of the unit being constantly recycled through the various controller units and back to the reaction vessel at a rate of one volumetric displacement every 3 min by means of a magnetic drive pump (MDX3, March Mfg.
from trickling filters (humus tank effluent) collected from the Pretoria sewage works. Large suspended solid particles were removed from this liquid by screening through a finely woven cotton cloth. In order to maintain the concentration of nitrate nitrogen at a constant level, the concentration was raised to 60 mg 1- ~ by the addition of KNO3. The electron donor. Methanol was used as the electron donor, being added directly into the feed stream in undiluted form by means of a micro-metering pump (DCL series II, F.A. Hughes & Co. Ltd).
Method of operation
459
The mode of operation could be divided into four
460
T.R. [)ArIEs and W. A. PRErORIUS
Methanol pump I
q
I
[ Rotating disc unit
Feed
Effluent
Feed
puml[ J
Recycle
pH I 1 Temperature control control
Circulation pump 1
I
1
I
Fig. I. Schematic representation of the denitrifying rotating disc unit. phases, namely adaptation of the unit to denitrification, determining the smallest carbon to nitrogen ratio necessary to achieve complete denitrification and ascertaining the effect of first pH and then temperature on the rate of denitrification. Adaptation was accomplished by inoculating the unit with cells taken from an actively denitrifying laboratory scale packed column unit. With the C- N ratio constant, the pH fixed at 7.0 and the temperature maintained at 20~C, the feed rate was gradually increased in order to maintain an approximately constant nitrate concentration in the effluent of 10 mg NO3-N 1- 1. This process continued until a maximum rate of denitrification was achieved and maintained for several hydraulic displacements. The discs were then removed and scraped clean of cells to determine the volume and mass of cells on each disc. For the other experiments the R D U was readapted for denitrification, as described above, until a comparable maximum denitrification rate was achieved. The hydraulic residence time was maintained at 20 min, the pH at 7.0 and the temperature at 20~C, while the methanol feed rate was changed to vary the C : N
ratio between 1:1 and 10:1 in order to establish the optimum ratio for C : N needed to achieve maximum denitrification. Once the optimum C : N ratio for denitrification had been established, the effect of pH could be determined. Methanol was added at a C : N ratio of 3 : l, the temperature maintained at 2Z~C and the feed supplied at a rate of 57 1. h - 1, resulting in a hydraulic residence time [HRT) of 15 min. The pH was initially raised from 7 to 10 in stages of 0-5 pH units. Samples for analysis were taken once the nitrate concentration in the effluent showed the unit to be in a steady state. At pH 10 the cells began to detach themselves from the discs, producing a very turbid effluent. The p H was therefore restored to 7 and the unit was allowed to recover overnight. The following day activity in the unit was found to be unimpaired. The pH was then lowered from 7 to 4"5 in stages of 0'5 pH units and samples taken for analysis. At pH 4.5 denitrification had ceased and the pH was consequently raised to 7.0 to allow the system to recover. The effect of temperature variations on the rate of denitrification was studied in a similar way. Temperature was raised or lowered in stages of 5°C while the
'~E 50O
~
200
I00
a 0
50
IO0
150 Time,
200
h
Fig. 2. Adaptation of the rotating disc unit to denitrification with methanol as the hydrogen donor.
Denitrification with a bacterial disc unit
461
,,. ~la 5 0 0
7 ,'m 2 0 0 0 Z
C,N 2"6
'
o ~ IO0 -~. ."2_ m i I
i 3
2 C:N
L 4
.5
ratio
Fig. 3. Determination of the C : N ratio required for maximum denitrification with methanol as the hydrogen donor. Table 1. Analysis of the supernatant fluid from the centrifuged influent to and effluent from the RDU, operated at 15 min hydraulic residence time, 20°C, pH 7.0. All results in mg NI-1
In Out
NO3
NO2
NHa
Kjeldahl N
56.0 8-8
0 1.74
0 0
11"6 5.3
pH was maintained at 7-0 and the flow rate remained constant at 57 I. h - 1. Methanol was supplied at a C : N ratio of 3 : 1.
Chemical analyses For routine monitoring of the system the nitrate concentration in the effluent was measured with a specific ion meter (Model 407, Orion). Once steady state conditions had been established in the unit, samples were analysed by an AutoAnalyzer (Technicon). Nitrate and nitrite were determined by the hydrazine reduction method of Kamphake et al. (1967), while ammonia and Kjeldahl nitrogen were measured as described by Harwood and Huyser (1970a,b). The weight of washed solids was measured gravimetrically after drying to constant weight at 104°C.
2
50
/
/
./
\
\
•
RESULTS
After inoculation of the R D U , the rate of denitrification increased linearly until it reached a maximum of 260mg N O 3 - N reduced m -2 h -1 after 174 h (Fig. 2). The average dry wt of cells was 7 g of cells per m 2 disc area, which corresponds to a rate of 37 mg N O ~ - N reduced g - 1 cells h - 1. An investigation into the species of nitrogen present in the influent entering and effluent leaving the R D U showed that nitrate was reduced all the way to nitrogen gas (Table l). The small quantities of nitrite measured in the effluent can be assumed to be in the process of reduction to molecular nitrogen. There was a direct relationship between the mass of carbon supplied to the mass of nitrogen removed, as shown in Fig. 3. The optimum C : N ratio for denitrification with methanol was found to be 2.6 : 1. No improvement in denitrification could be effected by increasing the C "N ratio beyond this value. The concentration of organic carbon present in the humus tank effluent was 5 mg l-1, and did not significantly alter the C : N ratio. The effect of pH on the rate of denitrification is shown in Fig. 4. Rates of denitrification were expressed as a percentage of the maximum rate of 260 mg N O a N reduced m - 2 h - x. The curve connecting the points in this figure was fitted by eye. The influence of temperature on the rate of denitrification has been presented in the form of an Arrhenius plot (Fig. 5). F r o m the lines of best fit, three values of QlO could be calculated which describe the effect of temperature on the denitrification rates. These were 13-06 below 10°C, 1.38 between l0 and 30°C and -2"66 above 30°C. DISCUSSION AND CONCLUSIONS
o
5
i
6
I
I
7
8
I
9
L
I0
pH
Fig. 4. The effect of pH on the rate of denitrification.
When a rotating disc unit was converted to a denitrilying unit and inoculated with a culture previously adapted to the use of methanol as hydrogen donor in
462
T.R. DAVIESand W. A. PRETORIt!S 20%
50°C 6,
_~t~ /
~ ~
IO°C
y'14"6221-2"6905x ' r -0,9619, n-lO E a ~ = 5 3 2 6 . 7 9 4 col mole
E ~ = 1 8 1 6 6 7 4 3 6 col mole
\
--
y=756844 199765,~
E~ .:59555 5 COl mole
4
32
33
34
35
36
3?
I
X='~o x l O $
Fig. 5. The Arrhenius plot of the effect of temperature on denitrification; K = rate of the reaction; 7" = absolute temperature.
the denitrification process, denitrification rate increased by about 1.5 mg N O a - N reduced m-2 h-2, finally reaching a maximum rate of 260 mg N O g - N m -2 h- 1 after 174 h. At that stage the cell mass on the discs corresponded to 7 g m - 2 which would be comparable to about 5500 mg I 1 mixed liquor suspended solids in an activated sludge unit. The rate of denitrification of 37-6 mg N O g - N reduced g - ~ cells h- 1 correlated well with the rate of 35.5 mg NO~-N reduced g ' cells h - I recorded in activated sludge at 20°C (Moore and Schroeder, 1970). Although the theoretical C : N ratio for denitrification with methanol would be 1.9 : 1, the actual value obtained of 2'6 accords well with the findings of Dawson and Murphy (1972), who reported a ratio of approximately 2.75: 1. The ratio of 2.6:1 would necessitate the provision of 0.0088 ml of methanol for the reduction of 1 mg of nitrate-N. Figure 4 illustrates that the optimum pH for denitrification lies between 7 and 8-5, with 80 per cent of the maximum activity still recorded at pH 6-0 and 70 per cent of the maximum at pH 9"0. Exposure to pH 5 for two hours killed off the cells and the system did not recover. In the pH range between 6 and 8-5 generally encountered in domestic effluents, the effect of pH on the rates of denitrification closely resembled the pattern described by Delwicbe (1956). When the RDU was adapted at a temperature of 20°C, the population behaved in a different way to the pure culture of Pseudomonas denitr~cans, described by Dawson and Murphy (1972). They recorded a Qlo of 3.16 in the temperature range from 5 to 27°C. In the experiments reported here, the Qlo was only 1.38 between 10 and 30'C, while below 10°C it was 13-06. St. Amant and Beck (1970) made indirect reference to the existance of two Qlo values when they observed that the greatest drop in efficiency occurred when the temperature of the water was below 10°C. Stensel, Loehr and Lawrence (1973) suggested that temperature has no significant effect on the process of denitrification from 20 to 30°C, but when the temperature is
decreased below 10°C the biological activity decreases significantly. Above 30°C the denitrification rate decreased, having a Q lo of -2.66. It is apparent that the optimum temperature range for denitrification by organisms present in the RDU, when adapted at 22°C, lay between l0 and 30C. In this range temperature variations had a minimal effect on the rate of denitrification. The Q~0 of 1.38 recorded in this range was, however, too large to be due to an increased rate of diffusion. Gambill (1958) has stated that diffusivity is directly proportional to temperature, assuming the viscosity of the liquid is unchanged. This would result in an increase in diffusivity of less than 4 per cent over a 10°C increase in temperature. The increase in denitrification must therefore be due to increased enzyme activity. Once the optimum temperature has been passed, a small change in temperature would cause a relatively rapid decline in the rate of reaction (Stanier, Doudoroff and Adelberg, 1963). The severe inhibition of denitrification recorded below 10°C could be due to the combined effect of suppressed enzyme activity and reduced solute uptake. It can be concluded from the data presented here that the RDU could be converted into an efficient denitrification unit, particularly in situations where the influent water temperature does not fall much below 10°C (at 10"C the denitrification rate is 55 per cent of that at 30°C). Reaction rates have been recorded which compare favourably with those obtained using anaerobic filters. The shearing force exerted by the passage of the discs through water allows the discs to be closely spaced without clogging. The system cannot be blocked by bacterial growth and shortcircuiting is never experienced. The complete biological surface comes into contact with the water being treated during every revolution, ensuring efficient use of the complete surface area. It would be technically possible to submerge the discs completely, thus increasing the volume of water being treated at any moment. However, the denitrification reaction continues on the wetted surface after the cell layer has emerged from the water. As long
Denitrification with a bacterial disc unit as the speed of rotation is faster than the speed of the reaction, no saving could be accomplished by complete submersion. Corrosion of the bearings and less efficient evolution of nitrogen gas would both militate against complete submersion. A comparison of the figures for the operating costs of a rotating disc unit given by Antonie (1971) and the power consumption of activated sludge reveals that the R D U would require only 25-33 per cent of the power needed for activated sludge. With its mechanical simplicity and ease of operation, the R D U would be a system worth considering for denitrification. REFERENCES
Antonie R. L. (1971) Application of rotating disc process to municipal wastewater treatment. U.S. Envir. Protection Agency Res. and Monitoring, U.S. Govt. Printing Office. U.S. Wat. Pollut. Control Res. Set. 17050/BAM 11/71. Dawson R. N. and Murphy K. L. (1972a) Factors affecting biological denitrification of wastewater. In Advances in Wat. Pollut. Res. Proc. 6th Int. Conf. Jerusalem (Edited by Jenkins S. H.) 671 683.
463
Dawson R. N, and Murphy K. L. (1972b) The temperature dependency of biological denitrification. Water Res. 6, 71 83. Delwiche C. C. (1956) Denitrification. In A Symposium on Inorganic Nitrogen Metabolism (Edited by McElroy W. D. and Glass B.) Johns Hopkins, Baltimore, 233-259. Gambill W. R. (1958) Predict liquid diffusivities. Chem. Engng. June 30th, 7280. Harwood J. E. and Huyser D. J. (1970a) Automated Kjeldahl analyses of nitrogenous materials in aqueous solutions. Water Res. 4, 539-545. Harwood J. E. and Huyser D. J. (1970b) Automated analysis of ammonia in water. Water Res. 4, 695 704. Kamphake L. J., Hannah S. A. and Cohen J. M. (1967) Automated analyses for nitrate by Hydrazine Reduction. Water Res. I, 206. Moore S. F. and Schroeder E. D. (1970) An investigation into the effects of residence time on anaerobic bacterial denitrification. Water Res. 4, 685-694. St. Amant P. P. and Beck L. A. (1970) Methods of removing nitrates from water. J. Agric. Food Chem. 18, 78~788. Stanier R. Y., Doudoroff M. and Adelberg E. A. (1963) General Microbiology, 2nd edition. Macmillan, London. Stensel H. D., Loehr R. C. and Lawrence A. W. (1973) Biological kinetics of suspended growth denitrification. J. Wat. Pollut. Control Fed. 45, 249-261.
DENITRIFICATION WITH A BACTERIAL DISC UNIT T. R. DAVIESand W. A. PRETOR1US National Institute for Water Research of the Council for Scientificand Industrial Research, P.O. Box 395, Pretoria, South Africa
(Received 25 March 1974) Abstract--An enclosed rotating disc unit was operated anaerobically as a denitrifying system, with methanol as the hydrogen donor. As the bacterial population became established, denitrification rate increased by 1.5 mg NO~-N reduced m- 2h- 2, to a maximum rate of 260 mg NO3-N reduced m- 2h- 1. The C:N ratio necessary for complete denitrification was found to be 2.6:1. Optimum pH for denitrification lay in the range between pH 7.0 and 8.5. Q~o values were 1.38 between 10 and 30°C, -2.66 above 30°C and 13.06 below 10°C. INTRODUCTION
Various equipment designs and process modifications have been proposed for the complete or partial removal of nitrate from secondary sewage effluents or drainage waters. These include submerged biological filters and anaerobic or semi-anaerobic adaptations of the activated sludge process. Since biological denitrification is essentially an aerobic process occurring under anaerobic conditions, two problems are generally encountered. These are the production of a large microbial biomass, which eventually clogs the void spaces in submerged filters, and the evolution of nitrogen gas, which tends to adhere to particles of microbial floc, impeding the separation of biDmass in any type of gravity separation system. The known properties of cell retention and gas exchange of a rotating disc unit (RDU) could be used to largely overcome the problems of cell retention and separation by enclosing such an RDU with an airtight cover, thus changing a normally aerobic system into an anaerobic unit suitable for denitrification. This paper describes the denitrification performance of an anaerobic RDU, with respect to carbon requirement, pH and temperature.
The experimental unit The experimental unit consisted of a disc unit and controllers for the feed rate, pH and temperature. The disc unit. The RDU contained 43 parallel perspex discs mounted 3 mm apart on a horizontal shaft. The discs were 406 mm in diameter and 1.5 mm thick, giving a total surface area of 11.094 m z, or a total surface available for microbial attachment (including the wetted surface area of the container) of 11"25 m z. The liquid volume of the disc system without discs was 21-5 1., giving a surface area to container volume of 523-3 m 2 m-3. With the discs about 45% submerged, the liquid volume was 14-25 1., resulting in a surface area to liquid volume ratio of 789.5 m z m -3. The disc i
Co3. Feed pump, pH and temperature controllers. Variable speed pumps were used for feeding the nitrified effluent and the electron donor (methanol). The pH was controlled by means of a titrometer type pH meter (PHM28b/TTT1 lb, Radiometer) with automatic alkali or acid additions to control the pH within limits of 0.1 pH units over a range of 2-12. Temperature was maintained by means of a thermostatic control unit (K2RD, MGW Lauda) in the recirculation line of the disc unit. ,The temperature could be controlled with I°C of any selected point in the range between 5 and 30°C. Because of the relatively high feed rates eventually involved, it was necessary to precool the feed to 4°C in order to maintain any particular temperature below ~0°C. The feed The nitrified feed. The feed consisted of settled effluent
MATERIALS
w.R, 914
unit was enclosed by an airtight cover (Fig. 1). The discs were rotated at a constant speed of 6 rev min- 1. To facilitate mixing of the liquid being treated and control of pH and temperature, the system was operated as a completely mixed unit, the contents of the unit being constantly recycled through the various controller units and back to the reaction vessel at a rate of one volumetric displacement every 3 min by means of a magnetic drive pump (MDX3, March Mfg.
from trickling filters (humus tank effluent) collected from the Pretoria sewage works. Large suspended solid particles were removed from this liquid by screening through a finely woven cotton cloth. In order to maintain the concentration of nitrate nitrogen at a constant level, the concentration was raised to 60 mg 1- ~ by the addition of KNO3. The electron donor. Methanol was used as the electron donor, being added directly into the feed stream in undiluted form by means of a micro-metering pump (DCL series II, F.A. Hughes & Co. Ltd).
Method of operation
459
The mode of operation could be divided into four
460
T.R. [)ArIEs and W. A. PRErORIUS
Methanol pump I
q
I
[ Rotating disc unit
Feed
Effluent
Feed
puml[ J
Recycle
pH I 1 Temperature control control
Circulation pump 1
I
1
I
Fig. I. Schematic representation of the denitrifying rotating disc unit. phases, namely adaptation of the unit to denitrification, determining the smallest carbon to nitrogen ratio necessary to achieve complete denitrification and ascertaining the effect of first pH and then temperature on the rate of denitrification. Adaptation was accomplished by inoculating the unit with cells taken from an actively denitrifying laboratory scale packed column unit. With the C- N ratio constant, the pH fixed at 7.0 and the temperature maintained at 20~C, the feed rate was gradually increased in order to maintain an approximately constant nitrate concentration in the effluent of 10 mg NO3-N 1- 1. This process continued until a maximum rate of denitrification was achieved and maintained for several hydraulic displacements. The discs were then removed and scraped clean of cells to determine the volume and mass of cells on each disc. For the other experiments the R D U was readapted for denitrification, as described above, until a comparable maximum denitrification rate was achieved. The hydraulic residence time was maintained at 20 min, the pH at 7.0 and the temperature at 20~C, while the methanol feed rate was changed to vary the C : N
ratio between 1:1 and 10:1 in order to establish the optimum ratio for C : N needed to achieve maximum denitrification. Once the optimum C : N ratio for denitrification had been established, the effect of pH could be determined. Methanol was added at a C : N ratio of 3 : l, the temperature maintained at 2Z~C and the feed supplied at a rate of 57 1. h - 1, resulting in a hydraulic residence time [HRT) of 15 min. The pH was initially raised from 7 to 10 in stages of 0-5 pH units. Samples for analysis were taken once the nitrate concentration in the effluent showed the unit to be in a steady state. At pH 10 the cells began to detach themselves from the discs, producing a very turbid effluent. The p H was therefore restored to 7 and the unit was allowed to recover overnight. The following day activity in the unit was found to be unimpaired. The pH was then lowered from 7 to 4"5 in stages of 0'5 pH units and samples taken for analysis. At pH 4.5 denitrification had ceased and the pH was consequently raised to 7.0 to allow the system to recover. The effect of temperature variations on the rate of denitrification was studied in a similar way. Temperature was raised or lowered in stages of 5°C while the
'~E 50O
~
200
I00
a 0
50
IO0
150 Time,
200
h
Fig. 2. Adaptation of the rotating disc unit to denitrification with methanol as the hydrogen donor.
Denitrification with a bacterial disc unit
461
,,. ~la 5 0 0
7 ,'m 2 0 0 0 Z
C,N 2"6
'
o ~ IO0 -~. ."2_ m i I
i 3
2 C:N
L 4
.5
ratio
Fig. 3. Determination of the C : N ratio required for maximum denitrification with methanol as the hydrogen donor. Table 1. Analysis of the supernatant fluid from the centrifuged influent to and effluent from the RDU, operated at 15 min hydraulic residence time, 20°C, pH 7.0. All results in mg NI-1
In Out
NO3
NO2
NHa
Kjeldahl N
56.0 8-8
0 1.74
0 0
11"6 5.3
pH was maintained at 7-0 and the flow rate remained constant at 57 I. h - 1. Methanol was supplied at a C : N ratio of 3 : 1.
Chemical analyses For routine monitoring of the system the nitrate concentration in the effluent was measured with a specific ion meter (Model 407, Orion). Once steady state conditions had been established in the unit, samples were analysed by an AutoAnalyzer (Technicon). Nitrate and nitrite were determined by the hydrazine reduction method of Kamphake et al. (1967), while ammonia and Kjeldahl nitrogen were measured as described by Harwood and Huyser (1970a,b). The weight of washed solids was measured gravimetrically after drying to constant weight at 104°C.
2
50
/
/
./
\
\
•
RESULTS
After inoculation of the R D U , the rate of denitrification increased linearly until it reached a maximum of 260mg N O 3 - N reduced m -2 h -1 after 174 h (Fig. 2). The average dry wt of cells was 7 g of cells per m 2 disc area, which corresponds to a rate of 37 mg N O ~ - N reduced g - 1 cells h - 1. An investigation into the species of nitrogen present in the influent entering and effluent leaving the R D U showed that nitrate was reduced all the way to nitrogen gas (Table l). The small quantities of nitrite measured in the effluent can be assumed to be in the process of reduction to molecular nitrogen. There was a direct relationship between the mass of carbon supplied to the mass of nitrogen removed, as shown in Fig. 3. The optimum C : N ratio for denitrification with methanol was found to be 2.6 : 1. No improvement in denitrification could be effected by increasing the C "N ratio beyond this value. The concentration of organic carbon present in the humus tank effluent was 5 mg l-1, and did not significantly alter the C : N ratio. The effect of pH on the rate of denitrification is shown in Fig. 4. Rates of denitrification were expressed as a percentage of the maximum rate of 260 mg N O a N reduced m - 2 h - x. The curve connecting the points in this figure was fitted by eye. The influence of temperature on the rate of denitrification has been presented in the form of an Arrhenius plot (Fig. 5). F r o m the lines of best fit, three values of QlO could be calculated which describe the effect of temperature on the denitrification rates. These were 13-06 below 10°C, 1.38 between l0 and 30°C and -2"66 above 30°C. DISCUSSION AND CONCLUSIONS
o
5
i
6
I
I
7
8
I
9
L
I0
pH
Fig. 4. The effect of pH on the rate of denitrification.
When a rotating disc unit was converted to a denitrilying unit and inoculated with a culture previously adapted to the use of methanol as hydrogen donor in
462
T.R. DAVIESand W. A. PRETORIt!S 20%
50°C 6,
_~t~ /
~ ~
IO°C
y'14"6221-2"6905x ' r -0,9619, n-lO E a ~ = 5 3 2 6 . 7 9 4 col mole
E ~ = 1 8 1 6 6 7 4 3 6 col mole
\
--
y=756844 199765,~
E~ .:59555 5 COl mole
4
32
33
34
35
36
3?
I
X='~o x l O $
Fig. 5. The Arrhenius plot of the effect of temperature on denitrification; K = rate of the reaction; 7" = absolute temperature.
the denitrification process, denitrification rate increased by about 1.5 mg N O a - N reduced m-2 h-2, finally reaching a maximum rate of 260 mg N O g - N m -2 h- 1 after 174 h. At that stage the cell mass on the discs corresponded to 7 g m - 2 which would be comparable to about 5500 mg I 1 mixed liquor suspended solids in an activated sludge unit. The rate of denitrification of 37-6 mg N O g - N reduced g - ~ cells h- 1 correlated well with the rate of 35.5 mg NO~-N reduced g ' cells h - I recorded in activated sludge at 20°C (Moore and Schroeder, 1970). Although the theoretical C : N ratio for denitrification with methanol would be 1.9 : 1, the actual value obtained of 2'6 accords well with the findings of Dawson and Murphy (1972), who reported a ratio of approximately 2.75: 1. The ratio of 2.6:1 would necessitate the provision of 0.0088 ml of methanol for the reduction of 1 mg of nitrate-N. Figure 4 illustrates that the optimum pH for denitrification lies between 7 and 8-5, with 80 per cent of the maximum activity still recorded at pH 6-0 and 70 per cent of the maximum at pH 9"0. Exposure to pH 5 for two hours killed off the cells and the system did not recover. In the pH range between 6 and 8-5 generally encountered in domestic effluents, the effect of pH on the rates of denitrification closely resembled the pattern described by Delwicbe (1956). When the RDU was adapted at a temperature of 20°C, the population behaved in a different way to the pure culture of Pseudomonas denitr~cans, described by Dawson and Murphy (1972). They recorded a Qlo of 3.16 in the temperature range from 5 to 27°C. In the experiments reported here, the Qlo was only 1.38 between 10 and 30'C, while below 10°C it was 13-06. St. Amant and Beck (1970) made indirect reference to the existance of two Qlo values when they observed that the greatest drop in efficiency occurred when the temperature of the water was below 10°C. Stensel, Loehr and Lawrence (1973) suggested that temperature has no significant effect on the process of denitrification from 20 to 30°C, but when the temperature is
decreased below 10°C the biological activity decreases significantly. Above 30°C the denitrification rate decreased, having a Q lo of -2.66. It is apparent that the optimum temperature range for denitrification by organisms present in the RDU, when adapted at 22°C, lay between l0 and 30C. In this range temperature variations had a minimal effect on the rate of denitrification. The Q~0 of 1.38 recorded in this range was, however, too large to be due to an increased rate of diffusion. Gambill (1958) has stated that diffusivity is directly proportional to temperature, assuming the viscosity of the liquid is unchanged. This would result in an increase in diffusivity of less than 4 per cent over a 10°C increase in temperature. The increase in denitrification must therefore be due to increased enzyme activity. Once the optimum temperature has been passed, a small change in temperature would cause a relatively rapid decline in the rate of reaction (Stanier, Doudoroff and Adelberg, 1963). The severe inhibition of denitrification recorded below 10°C could be due to the combined effect of suppressed enzyme activity and reduced solute uptake. It can be concluded from the data presented here that the RDU could be converted into an efficient denitrification unit, particularly in situations where the influent water temperature does not fall much below 10°C (at 10"C the denitrification rate is 55 per cent of that at 30°C). Reaction rates have been recorded which compare favourably with those obtained using anaerobic filters. The shearing force exerted by the passage of the discs through water allows the discs to be closely spaced without clogging. The system cannot be blocked by bacterial growth and shortcircuiting is never experienced. The complete biological surface comes into contact with the water being treated during every revolution, ensuring efficient use of the complete surface area. It would be technically possible to submerge the discs completely, thus increasing the volume of water being treated at any moment. However, the denitrification reaction continues on the wetted surface after the cell layer has emerged from the water. As long
Denitrification with a bacterial disc unit as the speed of rotation is faster than the speed of the reaction, no saving could be accomplished by complete submersion. Corrosion of the bearings and less efficient evolution of nitrogen gas would both militate against complete submersion. A comparison of the figures for the operating costs of a rotating disc unit given by Antonie (1971) and the power consumption of activated sludge reveals that the R D U would require only 25-33 per cent of the power needed for activated sludge. With its mechanical simplicity and ease of operation, the R D U would be a system worth considering for denitrification. REFERENCES
Antonie R. L. (1971) Application of rotating disc process to municipal wastewater treatment. U.S. Envir. Protection Agency Res. and Monitoring, U.S. Govt. Printing Office. U.S. Wat. Pollut. Control Res. Set. 17050/BAM 11/71. Dawson R. N. and Murphy K. L. (1972a) Factors affecting biological denitrification of wastewater. In Advances in Wat. Pollut. Res. Proc. 6th Int. Conf. Jerusalem (Edited by Jenkins S. H.) 671 683.
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Dawson R. N, and Murphy K. L. (1972b) The temperature dependency of biological denitrification. Water Res. 6, 71 83. Delwiche C. C. (1956) Denitrification. In A Symposium on Inorganic Nitrogen Metabolism (Edited by McElroy W. D. and Glass B.) Johns Hopkins, Baltimore, 233-259. Gambill W. R. (1958) Predict liquid diffusivities. Chem. Engng. June 30th, 7280. Harwood J. E. and Huyser D. J. (1970a) Automated Kjeldahl analyses of nitrogenous materials in aqueous solutions. Water Res. 4, 539-545. Harwood J. E. and Huyser D. J. (1970b) Automated analysis of ammonia in water. Water Res. 4, 695 704. Kamphake L. J., Hannah S. A. and Cohen J. M. (1967) Automated analyses for nitrate by Hydrazine Reduction. Water Res. I, 206. Moore S. F. and Schroeder E. D. (1970) An investigation into the effects of residence time on anaerobic bacterial denitrification. Water Res. 4, 685-694. St. Amant P. P. and Beck L. A. (1970) Methods of removing nitrates from water. J. Agric. Food Chem. 18, 78~788. Stanier R. Y., Doudoroff M. and Adelberg E. A. (1963) General Microbiology, 2nd edition. Macmillan, London. Stensel H. D., Loehr R. C. and Lawrence A. W. (1973) Biological kinetics of suspended growth denitrification. J. Wat. Pollut. Control Fed. 45, 249-261.