Rotary biological contractor operation with water action

Environment International, Vol. 18, pp. 403-408, 1992 Printed in the U.S.A. All fights reserved.

0160-4120/92 $5.00 + .00 Copyri~at © 1992 PergamonPress Ltd.

ROTARY BIOLOGICAL CONTACTOR OPERATION WITH WATER ACTION

Ismail I. Esen and Sadequr R.A. Ashraf Departmentof Civil Engineering,KuwaitUniversity,P.O. Box 596g Safat, 13060 Kuwait Nadai S. Puskas Human Protection and Modern Process Technology Ltd., Hataror ut 39, Budapest 1122, Hungary

E1 9112-189 M (Received 6 December 1991; accepted 20 March 1992)

A model rotary biological contactor (RBC) system was developed such that rotation of the unit was achieved by the weight of the influent sewage and recycled treated wastewater. The RBC system developed was intermittently fed by raw sewage 8 h/d. Results of the investigations showed a significant reduction in biochemical oxygen demand (BED). The dissolved oxygen levels were higher, and power requirements were lower than conventional RBC systems. The recommended values for hydraulic loading and rotational speed are 0.035 m3/d.m 2 and 2 rpm, respectively, The corresponding organic loading was about 0.01 kg/d.m2 as settled BeD.

INTRODUCTION In its simplest form, a rotating biological contactor (RBC) consists of parallel circular disks attached perpendicular to a horizontal shaft which passes through their centers. The entire assembly is placed into a tank with the shaft slightly above the surface of the liquid so that the disks are approximately half immersed. Microorganisms grow on the surface of the disks and rotation of the shaft brings them into contact with the liquid allowing the digestion of the organic matter. Although microbial growth results from this substrate utilization, the rotation of the disks through the liquid provides a constant shear force which causes continual sloughing of the culture, thereby maintaining a more or less constant film thickness. The rotation of the disk also mixes the liquid which keeps the stripped biomass in suspension and allows it to be carried from the reactor by the effluent. Aeration of the culture is accomplished by two mechanisms. As a point on the disk rises

above the liquid surface, a thin film of liquid remains attached to it and oxygen is transferred to that film as it passes through the air. In addition, a certain amount of air is entrained by the bulk of the liquid duo to the turbulence caused by the rotation of the disks (Antonie 1976; Grady and Lim 1980). Performance of a RBC system is affected by the influent flow rate, rotational speed of the disks, variations in hydraulic and organic loading rates, and temperature (Antonie 1970; Ball 1983; Clark et al. 1978; Del Borghi et al. 1985; Ellis and Banaga 1976; Filion et al. 1979; Friedman et al. 1979). It is generally not necessary to recycle the treated effluent or the settled sewage. Several kinetic and hydraulic transport models are available for the analysis of the removal of organic material in RBC units (Grady and Lim 1980). The dissolved oxygen (DO) levels in RBC units are generally low, occasionally affecting their performance. The adverse effects of low DO levels can

403

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be overcome by supplemental aeration (Bintaja et al. 1976; Surampalli et al. 1984). The rotation of the disks is achieved by electrical motors coupled to gear boxes or belt and pulley arrangements. Long-term operation of such systems is prone to mechanical failure, and RBC units may remain unoperational for extended periods of time, especially in developing countries. For these reasons, a model RBC system was developed in which the RBC unit was rotated by the weight of the influent sewage and portion of the treated wastewater which was recirculated. Such a system would have the additional advantage of aerating the wastewater. EXPERIMENTAL

The experimental investigations were carried out at the site of the Kuwait Institute for Scientific Research Pilot Plant within the premises of the Ministry of Public Works, Ardiya Wastewater Treatment Plant in Kuwait. The model RBC system consisted of an oil trap, a primary sedimentation tank, the RBC Unit, a holding tank and a secondary sedimentation tank operating in series. In each test, wastewater was taken from the oil trap to the primary sedimentation tank at a prescribed rate. The effluent of the primary sedimentation tank was intermittently fed to the RBC Unit for 8 h/d between 8 am and 4 pm during which time treated effluent was recirculated from the outlet zone of the RBC unit to the driving mechanism by a pump. During the remaining 16 h, sewage was not supplied to the system, and a second pump provided recirculation between the inlet zone of the secondary sedimentation tank and the driving mechanism of the RBC unit. Thus, during night operation, the secondary sedimentation tank acted like a holding tank. Sludge was withdrawn from the sedimentation tanks at the end of each 8-h daytime operation. The

flow diagram for the model RBC system is shown in Fig. 1. The primary and secondary sedimentation tanks were rectangular in shape and had effective volumes of 0.84 and 0.42 m 3, respectively. The RBC unit consisted of three elements: the semicircular tank; the rotating part; and the driving mechanism. The semicircular tank had a diameter of 0.80 m with straight extensions of 0.15 m to provide support for the slots of the rotating shaft. The total length of the tank was 1.10 m of which 0.90 m was used to house the rotating part. The remaining part of length 0.20 m was the outlet region. The effective volume of the tank was 0.160 m~. The rotating part consisted of 21 circular disks of diameter 0.68 m and 20 disks of diameter 0.20 m. Each disk had a thickness of 20 ram. The larger and smaller disks were alternately placed on a horizontal shaft with a diameter of 27 mm. This gave a total effective surface area of 13.6 m 2. The ends of the shaft were attached to bearings for easy rotation, and the entire assembly was placed on the semicircular tank with the shaft slightly above the surface of the liquid. Thus, at any given instant about 40% of the disks were immersed in wastewater. By adjusting the height of the outlet pipe in the outlet zone, the depth of immersion could be varied, if needed. The driving unit was placed directly above the rotating unit. The driving unit consisted of four circular metal sheets of diameter 0.68 m and thickness 0.5 mm attached perpendicularly to a horizontal shaft. The distance between each sheet was 0.20 m. In each gap, eight containers were made from 0.20 x 0.36 m metal sheets welded radially with the circular plates. Half of the opening of each container was closed to retain the inflowing water. Between each adjacent wedge-shaped container, there was an offset of 22.5 ° . Rotating and driving units were connected by a belt

Figure 1. Flow diagram for the model RBC system.

RAWSEWAGE r I OIL TRAP

OVERFLOW

SED. ~ RBC UNIT ~ TANK I ~ L I

SLUD~GE' II

~PU(,~

~

PUMP 2

SED. ITANK /

I

SLUDGE

~

EFFWENT

Rotary biological contactor

405

and pulley system such that one revolution of the driving mechanism resulted in two revolutions of the rotating unit. Rotation of the disks was achieved by directing the influent wastewater and the recirculated effluent into the containers of the driving mechanism. With the feeding and recirculation rates applied in this study, angular speeds of up to 5 rpm could be obtained for the rotating unit. Figure 2 shows the details of the driving and rotating units. During the preliminary tests, the rotational speed was varied between 1 and 4 rpm, and the influent flow rate was varied between 1 and 6 L/rain. With the present system, it was not possible to keep the rotational speed steady and constant for extended operation of the RBC unit for rotational speeds less than 2 rpm. At low rotational speeds, the biofilm (which was exposed to the high ambient temperatures prevailing in Kuwait) occasionally dried up. Also, influent flow rates greater than 3 L/min resulted in overloading of the system. For these reasons, it was decided to perform nine experiments with rotational speeds of 2, 3 and 4 rpm, and influent flow rates of 1, 2 and 3 L/min. These flow rates corresponded to hydraulic loadings of 0.035, 0.071 and 0.106 m3/day.m 2, respectively. Duration of each test was about two weeks. Initially, both the supernatant liquid and the sludge from the secondary sedimentation tanks were recirculated. It was observed, however, that when the

sludge was recirculated, the solids did not reattach themselves to the rotating disks and settled in the semicircular tank. This has resulted in poor quality effluent, and, at the same time, slowed down the rotation of the disks. For this reason, only liquid was recirculated between the secondary sedimentation tank and the RBC unit. RESULTS AND DISCUSSION

The summary of test results are given in Table 1, where the experiments are designated by two numerals: the first numeral indicates the influent flow rate in liters per m i n u t e , and the s e c o n d numeral indicates the rotational speed in revolutions per minute. The influent flow rate and recirculation rate are indicated by the symbols QI and QR, respectively. The total flow rate QT is the sum of Q1 and QR, and the recirculation ratio is given as the ratio of QR to QI. The filtrable BOD values listed represent the 8-h composite values for raw sewage (RS); and measurements made with the samples taken from the primary sedimentation tank (PST) and secondary sedimentation tank (SST) at 2 pm. The average BOD value for the final two successive days of each experimental run are reported. Table 1 also lists the average temperature of water in the RBC unit for each experimental run, and the BOD removals for the total system (raw

Fig. 2. Driving and rotating parts for the RBC unit,

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Water Containers Circular Plate

DRIVING PART I

Bio _disks

ROTATING PART )

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Table 1. S u m m a r y o f test results.

Rotational

Speed

BOD (mg/L) RS PST SST

(rpm) (6)

(7)

(8)

6.8 12.1 13.7

2.0 3.0 4.0

345 383 355

236 315 308

7.i 12.1 15.9

2.6 5.1 7.0

2.0 3.0 4.0

380 329 367

7.6 12.8 15.2

1.5 3.3 4.1

2.0 3.0 4.0

339 335 365

Exp. No.

Flow Rates QI QR

(L/min) QT

Recirculation Ratio

(1)

(2)

(3)

(4)

(5)

12 13 14

1.0 1.0 1.0

6.8 12.1 13.7

7.8 13.1 14.7

22 23 24

2.0 2.0 2.0

5.i i0.i 13.9

32 33 34

3.0 3.0 3.0

4.6 9.8 12.2

sewage--secondary sedimentation tank effluent) and primary settled sewage (primary sedimentation tank effluent--secondary sedimentation tank effluent). Variation of average filtrable BOD removal as a function of rotational speed is shown in Fig. 3. Figure 4 shows the variation of average filtrable BOD removal with influent flow rate. Variation of rotational speed with average total flow rate is shown in Fig. 5. The BOD removal efficiency for an RBC unit increases as the rotational speed of the disks is in-

100

I

I

90.--

Temp (*C)

System

Sewage

(i0)

(ii)

(12)

43 78 108

22 20 24

88 80 72

82 75 65

292 314 315

118 131 146

17 16 20

69 60 60

60 58 54

298 270 305

83 93 130

15 13 26

76 72 64

72 65 57

(9)

creased. This is basically due to increased mass transfer in the bulk liquid. The mass of substrate removed by the aerated sector also increases slightly (Grady and Lim 1980). The effect is more pronounced when the rotational speed is small. Considerable improvement in BOD removals for rotational speeds of up to 3.2 rpm were reported by Antonie (1976); however, beyond that, improvement was marginal. On the other hand, recirculation reduces the influent substrate concentration, but increases the hydraulic loading.

100

I

I

I

90

• Total system

o Settled sewage

-5

"5 80 >

> 0

8C

E

0

E

¢r c~ o m

I

o Total s y s t e m

o Settled sewage v

BOD Removal (%) Total Settled

0

70

0

7C

r'~

U..

~ 60

6C

5C

I 2

I 3 Rotational

I 4 Speed

(rpm)

Fig. 3. A v e r a g e B O D r e m o v a l as a f u n c t i o n o f rotational speed.

5C 0

I 1

I 2

InfluentFIow

I 3 Rate (L/min)

Fig. 4. Average B O D r e m o v a l as a f u n c t i o n o f i n f l u e n t f l o w rate.

Rotary biological contaetor

5

E O.

I

I

I

407

I

[

I

I

I

I

4

"U

o. 3

C O

O or"

1

I

0

I

I

I

I

I

I

I

10 Average Total Flow Rate (L/min)

I

20

Fig. 5. Variation of rotational speed with average flow rate.

Assuming that the kinetic properties remain constant, the BOD removal decreases with increasing recirculation ratios for substrate-limiting conditions. The reverse may be true for oxygen-limiting conditions (Ball 1983). Since in our studies, the rotation of the disks was achieved mainly by the weight of the recirculated water, higher rotational speeds imply more recirculation. Also, the high dissolved oxygen values measured in the RBC unit and secondary sedimentation tank indicated that the system was in the substrate-limiting state. The overall effect has been an almost linear reduction in BOD removal efficiency with increasing rotational speed for every influent flow rate investigated. As shown in Fig. 3, average BOD removal (average value corresponding to three influent flow rates) decreases linearly with increasing rotational speed and, hence, the recirculation ratio for both the total system and the primary settled sewage. For the reasons explained before, rotational speeds of less than 2 rpm were not investigated in this study. However, there are all the indications that the optimum BOD removal rate should occur for rotational speeds less than 2 rpm. This has the additional benefit of reducing the power requirement for pumping. As a preliminary design value, however, a rotational speed of 2 rpm will be recommended. The process of pumping water to the driving unit and then allowing water to fall freely into the RBC

tank resulted in high dissolved oxygen concentrations in the RBC unit and the secondary sedimentation tank. When all 2-pro measurements were considered regardless of the temperature, influent flow rate and recirculation ratio, it was observed that 95 % of the dissolved oxygen values were greater than 1 rag/L, 85% greater than 2 rag/L, and 50% greater than 4 mg/L. Thus, the system practiced is analogous to supplemental aeration which significantly enhances RBC performance. For each rotational speed, the BOD removal rate dropped with increasing flow rates of up to 2 L/min, but consistently increased at 3 L/rain. Average BOD removal (average value corresponding to three rotational speeds) as a function of influent flow rate is shown in Fig. 4. A safe value of 1 L/rain is recommended as the influent flow rate. This corresponds to a hydraulic loading of 0.035 m3/d.m 2 and an average organic loading of 0.01 kg/d.mZ as settled BOD. As shown in Fig. 5, the variation of rotational speed with total flow rate is almost linear for the range of flow rates investigated. When the driving mechanism is placed sideways with the RBC unit, the minimum pumping head would be approximately equal to the diameter of the disks. For an influent flow rate of 1 L/min, a rotational speed of 2 rpm and a recirculation rate of 7 L/min, the power requirement would be about 1 w. With pipe losses, a design value of 2 w is recommended for the present system. Thus, very little power is required. Removal of ammonia nitrogen was not within the scope of the present study. Similarly, kinetic analyses or scale-up studies were not made. CONCLUSIONS AND RECOMMENDATIONS

It is believed that the proposed system of rotating the biological contactor units by the weight of influent sewage and recirculated effluent is a viable alternative for the treatment of wastewater in small communities where sewage generation is not continuous. With the data available, the system can be operated at 2 rpm with an influent flow rate of 1 L/min. The corresponding recirculation rate is 7 L/min, and the h y d r a u l i c and BOD l o a d i n g rates are 0.035 m3/d.m 2 and 0.01 kg/d.m 2, respectively. With the present system, BOD removals of up to 88% can be achieved with a single RBC unit. High dissolved oxygen concentrations occuring in the RBC unit enhance its performance, and power requirements are minimal. It is suggested to test the proposed system with a full-scale pilot plant. Two RBC units with 3-4 m diameter disks should be operated in parallel. The

408

first unit should be operating as a control with fixed rotational speed of 2 rpm and hydraulic loading of 0.035 m3/d.m 2. The second should be operated at variable conditions. The removal of ammonia nitrogen should be investigated in addition to the BOD removal. The system should be further tested by operating two RBC units in series for an extended period of time. ~ This study was sponsored by the Kuwait University Research Council Grant No. EV 035. The experiments were performed at the Kuwait Institute for Scientific Research, Ardiya Pilot Plant. The authors express their gratitude to both institutions for their support and encouragement.

Acknowle&ment

REFERENCES Antonie, R.L. Response of the bio-disc process to fluctuating wastewater flow. In: Prec. 25th Industrial Waste Conference, Purdue University, Engineering Extension Series No. 137,427435; 1970. Available from: Purdue University, Lafayette, IN. Antonie, R.L. Fixed biological surfaces-wastewater treatment. Cleveland, OH: CRC Press; 1976.

I.I. Esen et al.

Ball, R.O. The effects of hydraulic variation on fixed-film reactor performance. In: Fixed film biological processes for wastewater treatment. Park Ridge, NJ: Noyes Data Corporation; 1983: 174-205. Bintaja, H.H.J.; Brunsmann, J.J.; Boelhouwer, C. The use of oxygen in a rotating disc process. Water Reg. 10: 561-565; 1976. Clark, J.H.; MoJeng, E.M.; Asano, M.T. Performance of a rotating biological contactor under varying wastewater flow. J. Water Pollut. Control Fed. 50: 896-911; 1978. Del Borghi, M.; Palazzi, E; Parisi, F.; Ferraiolo, G. Influence of process variables on the modelling and design of a rotating biological surface. Water Reg. 19: 573-580; 1985. Ellis, K.V.; Banaga, S.E.I. A study of rotating treatment units operating at different temperatures. J. Water Pollut. Control Fed. 75: 73-91; 1976. Filion, M.P.; Murphy, K.L.; Stephenson, J.E Performance of a rotating biological contactor under transient loading conditions. J. Water Pollut. Control Fed. 51: 1925-1933; 1979. Friedman, A.A.; Robbing, L.E.; Woods R.C. Effect of disk rotational speed on biological contactor efficiency. J. Water Pollut. Control Fed. 51: 2678-2690; 1979. Grady, C.P.L.; Lira, H.H.C., Jr. Biological wastewater treatment-theory and applications. Basel: Marcel Dekker; 1980: 755-785. Surampalli, R.Y.; Tekippe, R.C.; Baumann, E.R. The value of supplemental air in improving RBC performance. In: Prec. 2nd international conference on Fixed Biological Processes. Arlington, VA. Available from: U.S. Army Corps of Engineers, Washington, DC.