The Application of ORP in Activated Sludge Wastewater Treatment Processes. Baikun Li and Paul L. Bishop*

Transcription

1 ENVIRONMENTAL ENGINEERING SCIENCE Volume 18, Number 5, 2001 Mary Ann Liebert, Inc. The Application of ORP in Activated Sludge Wastewater Treatment Processes Baikun Li and Paul L. Bishop* Department of Civil and Environmental Engineering University of Cincinnati Cincinnati, OH ABSTRACT This study researched, at full and pilot scale, the potential for using ORP to control the operation of activated sludge aeration tanks. Results indicate that ORP values can clearly show the COD removal occurring along the length of an aeration tank; ORP values increased dramatically as organic matter was removed. Based on the experimental data obtained on site during more than a half year of study, the ORP and COD in the aeration tank effluent followed a good linear relationship. Because ORP control is much easier and quicker than using COD measurements, the DORP of the influent and effluent and the ORP values of the effluent could be used to in situ indicate the pollutant removal efficiency and the effluent quality, respectively. ORP changes occurring under different COD and DO concentrations were also examined using a pilot column. The pilot column data confirmed the results from the aeration tanks. The ORP values of the mixed liquor increased during biodegradation of the organic matter, and then leveled off when biodegradation was finished. However, under high COD (higher than 1000 mg/l) loading conditions, the biodegradation continued throughout the whole aeration cycle, and the ORP values continued increasing throughout the cycle. The data from systems operating under several DO conditions indicated that supplying a very high DO concentration (higher than 6.0 mg/l) is not necessary for biodegradation. Key words: ORP regulation; activated sludge; wastewater treatment processes; aeration tanks; dissolved oxygen (DO); COD removal INTRODUCTION OXIDATION-REDUCTION POTENTIAL (ORP) is used to determine the oxidizing or reducing properties of solutions. ORP is the electromotive force developed when oxidizers or reducers are present in aqueous solution. Oxidizers are chemical species that accept electrons, while reducers are chemical species that donate electrons. For biological wastewater treatment systems, free oxygen, nitrite, and nitrate are usually the oxidizer species in aeration tanks, and the biomass and many organic pollutants are typically the reducers. ORP can be a useful on-line control parameter in wastewater treatment processes that reflects the potential for pollutant removal. Previous investigations on ORP control indicate that the ORP time profile in the nitrifi- *Corresponding author: Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH Phone: ; Fax: ; Paul.Bishop@UC.edu 309

2 310 LI AND BISHOP cation and denitrification process can be a valuable tool for achieving consistent biological nutrient removal (BNR). Compared with dissolved oxygen (DO) control, ORP control has the following advantages: 1. The recent need for enhanced nitrogen and phosphorus removal has often led to the adoption of using anoxic and anaerobic selector zones, where no dissolved oxygen is present. This makes control using DO measurements useless. But, an ORP value is still available. Simultaneous nitrification and denitrification can occur when the ORP is mv (Ag/AgCl), even though DO may be only mg/l (Moriyama et al., 1993). It is clear that the system ORP is potentially a more precise measurement of the extent of anaerobicity or aerobicity (Lie and Welander, 1994; Lo and Yu, 1994). 2. Compared with the DO time profile in the aeration tank, which is often erratic, the ORP time profile is generally smooth and gradual (Charpentier et al., 1989). This makes the ORP time profile relatively easy to research and observe. 3. Unlike DO, which can only show dissolved oxygen concentrations in the aeration tank bulk liquid, the ORP regulation strategy is more in tune with the process dynamics, because it clearly reflects which electron acceptors the microbes need under different conditions. In activated sludge systems, many biological substances, such as enzymes and vitamins, and most metabolic processes are redox systems and correlate strongly with ORP values. As an indicator of the redox status of wastewater, the ORP value can indicate the comprehensive redox effect in wastewater, which varies with COD, DO, and NO 3 2 /NH 4 1 concentrations. ORP real-time control in a full-scale plant not only provides for carbonaceous and nitrogenous material removal with good performance, but it can also conserve 20% of the energy cost (Charpentier and Florentz, 1987). However, some attention should be paid when using and interpreting ORP measurements: 1. The measured ORP value of mixed liquor cannot indicate all redox couples present in the solution. This is especially true for wastewater, where some redox reactions occur very slowly and may not reach equilibrium for a very long time (Pankow, 1991). Consequently, there may be redox couples present in wastewater that do not act completely (at equilibrium) on the platinum electrode of the ORP probe during measurement. This may cause difficulties in interpreting ORP values. Some researchers have suggested using a combination of ORP and other parameters (ph, DO, and conductivity) for on-line monitoring of nitrogen and phosphorus removal processes (Spagni et al., 2001). It is believed that these could more reliably reflect the treatment process. 2. The ORP probe can easily become dirty or poisoned, and give erroneous readings. To maintain good performance of the ORP probes permanently installed in an aeration tank, careful cleaning and calibration should be performed at least once per week. This may be difficult, and may pose an obstacle for the use of ORP control in aeration tanks. Even with these caveats, ORP control still presents advantages over DO control, particularly with respect to indicating pollutant removal and water quality. A broad increased interest in the use of ORP for realtime process control of wastewater treatment systems has occurred over the past 20 years. Achieving ORP control in a wastewater treatment plant has economic importance, but until now, most research on ORP control has been conducted only at lab scale or pilot scale (Eilbeck, 1984; Koch and Oldham, 1984; Rimkus, 1985; Rittmann and Yamamoto, 1985; Sekine, 1985; Charpentier and Florentz, 1987; Heduit and Thevenot, 1989; Peddie and Mavinic, 1990; de la Menardiere et al., 1991; Sasaki and Yamamoto, 1993; Schon and Geywitz, 1993; Wareham and Mavinic, 1994; Yu and Lo, 1994; Chang and Hao, 1996; Bertanza, 1997; Demouline and Goronszy, 1997; Yu and Liaw, 1997; Paul, 1998; Zhao, 1999), and most of this research concentrated on the simultaneous nitrification denitrification process. Although ORP control for COD removal has been monitored in a few research projects (Yu and Lo, 1994; Charpentier et al., 1998), the ORP and COD relationship in aeration tanks has not as yet been thoroughly examined. Most aeration systems are still controlled using dissolved oxygen, which cannot reflect the real biochemical status in the aeration tanks. At times, DO in aeration tanks can reach 6 9 mg/l, which is not necessary for biodegradation, and may not necessarily be indicative of a stabilized waste. In this field-scale research, organic removal from aeration tank mixed liquor was measured with respect to ORP along the length of the aeration tanks. The relationship between ORP and COD in the aeration tank effluent was established to probe the potential of using ORP values to indicate water quality. If this relationship is established, the operators can determine the treatment efficiency and effluent quality by just measuring ORP values along the aeration tanks. This saves time, and is more efficient than measuring COD or BOD 5. Based on the ORP readings obtained, operator could immediately adjust aeration airflow rates and other operational factors.

3 ORP IN WASTE TREATMENT PROCESSES 311 The effect of DO concentration on COD removal and ORP increase along the aeration tanks was also researched. In a pilot-scale study, the ORP profiles resulting from different COD and DO conditions were monitored. This research may help other wastewater treatment plants to reach optimal operation using cost-effective ORP control. MATERIALS AND METHODS Plant description The Mill Creek Wastewater Treatment Facility is the largest of 26 treatment plants operated by the Metropolitan Sewer District of Greater Cincinnati (MSD). The source of the influent to the wastewater treatment facility is composed of 70% industrial wastewater and 30% municipal sewage. The plant has the design capacity to treat 120 MGD flow and a peak flow capacity of 300 MGD. The wastewater treatment process consists of primary sedimentation, secondary activated sludge treatment, and chlorination. There are six equally sized aeration tanks that provide detention time and bubble diffuser aeration for the mixed liquor. The West #1 tank was chosen as the experimental site (Fig. 1). The aeration tanks are designed as plug flow reactors, and each tank is folded into three passes that are each 110 m (L) m (W) m (D), with a volume of 18,896 m 3 (5 million gallons). Hydraulic retention time (HRT) in each aeration tank at design flow is 6 h. The COD range for the aeration tank influent during this study was mg/l. The ph in the aeration tanks was neutral ( ), and MLSS was 2,000 3,000 mg/l. The ammonium concentration in the aeration tank influent was mg NH 3 - N/L; minimal or no nitrification or denitrification occurs in the aeration tanks. Currently, aerator operation at the Mill Creek plant depends on DO control. Intermittent aeration has been employed in the aeration tanks since summer 1998 (Aman and Bishop, 1999). The wastewater is aerated for a period, and then the air supply is greatly decreased for a short period so that the bacteria effectively use the supplied air. The ideal operation appears to be aeration with alternating aerator ON and OFF cycles of 30 min, respectively. In the beginning of each aeration tank, there is an anoxic selector. The selector is used so that micro-organisms can sorb the nutrients in the wastewater quickly, produce a better settling sludge, inhibit the growth of filamentous bacteria, and regenerate alkalinity for ph control. Based on our experimental data, most of the biodegradation occurs in the first 3 h of aeration; in the second half of the tanks, no apparent macrobiodegradation (in the bulk liquid) was observed. There appears to be two kinds of biodegradation in the aeration tanks: macrobiodegradation (consisting of the adsorption of large particles on the surfaces of activated sludge floc and the biodegradation of easily biodegradable substrates), which is accomplished very quickly in the treatment (Bitton, 1994), contributes to pollutant removal from the wastewater, and can be indicated by COD measurements; and microbiodegradation, which occurs inside the activated sludge floc, further consumes the absorbed pollutants from the macrobiodegradation, and cannot be indicated by bulk-liquid COD measurements. It is likely that the macrobiodegradation (sorption of pollutants) was typically finished in the first half of the aeration tanks; biodegradation of pollutants sorbed by the ac- Figure 1. Operation mode of West #1 aeration. ENVIRON ENG SCI, VOL. 18, NO. 5, 2001

4 312 LI AND BISHOP tivated sludge dominated in the second phase of the treatment. This contributed to the thorough removal of pollutants and the establishment of a good-quality effluent. Experimental phases ORP, DO, and ph were periodically measured in situ in the West #1 aeration tank. There were 10 sampling points along the aeration tank (shown in Fig. 1), approximately one per 30 min of hydraulic retention time. The influent sample was taken at the location where the settled primary effluent mixes with the recycled activated sludge. The samples were collected chronologically from the locations as the plug flow moved though the aeration tank, i.e., the effluent sample was taken 6 h after the influent sample. This prevented problems associated with temporal changes of the influent characteristics. The frequency of sampling was twice per week, and the sampling time was from 10:00 a.m. to 4:00 p.m. The experimental period was from December 1999 to July ORP control was also tested in a pilot column (D 5 30 cm, H m; see Fig. 2). The HRT and aeration shifts for the pilot column were the same as in the West #1 tank, that is 6 h aeration time with alternating 30-min air-off/air-on periods. During the air-off period, airflow was not totally shut down; a small amount of air was supplied to control the DO between 0.5 to 0.8 mg/l to mix the activated sludge. A Kings flow meter was used to control the airflow into the column. DO concentration in the mixed liquor was measured in situ with an Orion DO probe. The pilot column was run as a batch reactor, with no additional recycling of activated sludge into the column. Wastewater was pumped from the influent (after mixing with the returned activated sludge) of the West #1 tank to the pilot column at the beginning of the cycle. Sampling ports located at 25-cm intervals along the depth of the column were used to draw samples once per 30 min. No significant COD difference was observed among the samples from the different depths. Samples for complete analyses were collected from the sampling port at the 0.7 m depth beneath the surface. Analytical methods A model 1230 Orion multifunction portable meter was used to measure on site the DO, ph, and ORP in the aeration tanks. The Orion ORP sensor (Model No. 9678) is an epoxy body with a sleeve junction, and combines a platinum redox and Ag/AgCl reference electrode in one body. The ORP electrode was cleaned with deionized water and calibrated with freshly prepared potassium ferocyanide/potassium ferrictanide/potassium fluoride buffer solution [ORP values: 234 and 300 mv (Ag/AgCl, Orion Company (1997) ORP electrode manual] before every use. The ORP values reported here are the standard hydrogen redox values (Eh), which were converted from the measured ORP values (Ag/AgCl) by added 200 mv. After the wastewater sample was collected from an aer- Figure 2. Schematic diagram of the pilot column system.

5 ORP IN WASTE TREATMENT PROCESSES 313 ation tank, it was immediately put into a cooler and allowed to settle for 5 min. The supernatant was then filtered through a glass fiber membrane and transferred to a 50-mL centrifuge vial. In this way, the loss of COD after sample collection was prevented. The centrifuge vial was stored in a refrigerator; COD was measured on the same day of sampling using the Hach procedure. Acetate membranes (pore size 0.45 mm) were used to filter wastewater samples before analysis to avoid the effect of suspended solids on COD measurements. Duplicate measurements were made for each sample. RESULTS AND DISCUSSION Observations and ORP recordings in the West #1 aeration tank ORP measurements in wastewater. A redox reaction is a reaction involving transfer of electrons. The ORP value is a measure of the tendency of a given system to donate or receive electrons. Theoretically, it describes the equilibrium position for all redox pairs in a given system. This concept is well established for pure and equilibrium solutions in chemistry. How can this concept be applied to dynamic biological systems? In the mixed liquor in the aeration tanks, many inorganic and organic compounds, metals, and biomass are present. There are many biochemical reactions occurring concurrently. By strict definition, the ideal redox equilibrium does not exist in the wastewater. So, we should make sure that the measured ORP data represents the redox status of actual wastewater in aeration tanks. Some authors (Yu and Bishop, 1998; Yu, 2000) have pointed out that the ORP of a system containing mixed microbial species, such as wastewater biofilm or activated sludge, is an overall result of the microbial and chemical activities of most components present during testing. The ORP value can be used to comprehensively represent the redox potential level of most biochemical reactions in the system. Many environmental factors, such as the inorganic/organic compounds present, dissolved oxygen, ph, or other redox species can affect the ORP values of wastewater. In wastewater, changes in the pollutant concentration (ranging from mg/l COD in this study) would be the predominant factor in initiating ORP changes, rather than DO concentration changes (changing from mg/l), under most circumstances. From the ORP definition, the higher are the concentrations of reductive compounds, the lower will be the ORP values of the system. In aeration tanks, pollutants (reductive compounds) are biodegraded by micro-organisms. The decrease of pollutant concentrations can be reflected by the ORP values along the aeration tank. Because many biochemical reactions occur simultaneously in wastewater, the ORP readings may take some time to stabilize in the dynamic system. This requires that the ORP probe should be sensitive enough to keep up with the reaction rate. ORP measurements in the West #1 tank influent and effluent showed that it took s for the ORP readings to stabilize. Generally speaking, it took less time for ORP readings to stabilize in the aeration tank effluent than in the aeration tank influent. Most biodegradation has already been completed in the effluent. Only a small amount of pollutants remain; the biochemical reaction rate is slow, and the system is almost in equilibrium. In the aeration tank influent, with many pollutants present, the bulk wastewater is in a reducing redox status. The biochemical reaction rate is comparatively high and the ORP probe takes time to come to equilibrium with the reaction rate in the mixed liquor. ORP change along the length of the West #1 aeration tank. Experiments were performed to determine ORP changes that occur along the length of the West #1 tank. Based on the experimental data (Fig. 3), it is evident that ORP values (Eh) increase as COD is removed along the aeration tank. Figure 3 is a plot of ORP, DO, and COD changes along the length of West #1 tank on December 10, 1999, and is representative of typical profiles. COD in the aeration tank influent was 100 mg/l, while the influent ORP was 180 mv. As COD was removed along the West #1 tank, the ORP increased accordingly. Effluent COD decreased to 30 mg/l in the aeration tank effluent, while ORP increased to over 350 mv, a relatively high oxidizing status. The data accumulated over a half year of monitoring has demonstrated that ORP values can be used to reflect the COD removal in the aeration tanks. As speculated in the Introduction, in the first 3 h of treatment, when sorption and macrobiodegradation of pollutants were dominant, the ORP values greatly increased. In the latter part of the aeration tank, where organic stabilization within activated sludge flocs dominated, ORP values did not increase further. This indicates that sorption and biodegradation in the macroenvironment contribute to changes in the redox status of the wastewater from a low oxidizing condition to a more oxidizing one, leading to the dramatic increase in ORP values along the aeration tanks that were observed. In the second 3 h of treatment, the micro-organisms within the activated sludge fluc biodegraded pollutants that had been adsorbed in the first 3 h of treatment. This microbial biodegradation did not lead to further significant ORP increases in the mixed liquor, because in the macroenvironment in the aeration tanks, pollutant removal was already complete and the redox status had ENVIRON ENG SCI, VOL. 18, NO. 5, 2001

6 314 LI AND BISHOP Figure3. COD, DO, and ORP in West #1 Aeration Tank at the Mill Creek plant (the shaded zones are air-off zones; the blank ones are air-on zones). reached equilibrium. The bulk liquor ORP values stabilized. It is also seen in Fig. 3 that, upon reaching air-on zones (DO was higher than 1.0 mg/l) in the first part of the West #1 tank, the ORP increased sharply and COD dropped dramatically to 38 mg/l. The plentiful oxygen in the wastewater leads to macrobiodegradation and causes the ORP to increase. In the second half of the tank ( h), COD decreased from 38 mg/l to 30 mg/l in the effluent. Although air-off and air-on zones still alternated and DO fluctuated from mg/l, ORP values did not increase further. Figure 3 shows that ORP values are closely related to the COD concentration in the wastewater, while dissolved oxygen plays an indirect role by sustaining biotransformation rates of COD that alter the ORP values. When COD removal from the bulk liquid is completed in the aeration tank, even though the organics may not be fully stabilized, DO has little influence on ORP values in the bulk wastewater. DO effect on ORP and COD in the aeration tank. Three DO concentration levels were observed in the West #1 tank: low DO ( mg/l), medium DO ( mg/l), and high DO ( mg/l). The effect of DO concentration on ORP and COD in the West #1 tank is shown in Fig. 4. The data were obtained from sampling points 1 10 along the length of the West #1 tank. It was found that when the DO concentrations were low, ORP values were low in the aeration tank; never more than 250 mv. The lowest ORP reading was 50 mv. Under Figure 4. ORP and COD at different DO concentrations in aeration tank.

7 ORP IN WASTE TREATMENT PROCESSES 315 these conditions, the mixed liquor COD was never less than 50 mg/l, indicating that the biodegradation was not complete. When the DO concentration was in the medium range, ORP values clearly increased, compared with the low DO concentration condition. The highest ORP in the aeration tank when DO was in the medium range was 370 mv. The COD in the aeration tank effluent was often as low as 25 mg/l, indicating that under proper redox conditions, micro-organisms in activated sludge could efficiently biodegrade organic matter in the wastewater, leading to good water quality. The high DO concentration situation did not result in any further increase in ORP values in the aeration tank; the highest ORP value in the aeration tank was 360 mv. Aeration tank effluent quality did not improve appreciably in the high DO state when compared with that at medium DO concentration. Sufficient oxygen was available when DO was in the range of 2 5 mg/l. Based on Fig. 4, it was found that at medium DO concentration, and when the ORP of the aeration tank effluent was higher than 350 mv, the resulting COD was always lower than 50 mg/l, indicative of good effluent quality. The DO concentration also had an effect on the ORP increase along the aeration tank. Figure 5 shows the increase in ORP along the aeration tank. The DO presented in the figure is the DO concentration in the aeration tank effluent, which represents the highest DO achieved in the aeration tanks. The DORP value in the first 3-h period is the ORP value at the third hour in the West #1 tank minus the ORP of the aeration tank influent; the DORP between influent and effluent is the ORP value of the aeration tank effluent minus the ORP of the aeration tank influent. From Fig. 3, most of the organic sorption and COD removal from the macroenvironment was accomplished in the first part of the aeration tank, and the increase in ORP values was also almost finished in the first part. Figure 5 shows that the DORP values for the whole tank were only 5 20 mv higher than the DORP values in the first part of the tank, over the whole DO range. This indicates that ORP values, which represent the redox status of the wastewater, increased significantly only during the sorption and initial biodegradation phases. After sorption and biodegradation in that macroenvironment were finished, the biochemical reactions reached a balance in the bulk wastewater. The microenvironment biodegradation inside the activated sludge flocs does not cause ORP values in the bulk wastewater to further increase. Figure 5 also shows that, under low DO (0 1.0 mg/l) conditions, the increases in ORP values were generally not high, no more than 100 mv throughout the West #1 tank. When DO increased to 2 5 mg/l, the DORP values jumped to mv. This indicates that supplying sufficient DO is necessary for biodegradation and redox status improvement in the aeration tank. Higher DO (6 9 mg/l), though, did not lead to further ORP improvement in the West #1 tank. DO effect on ORP and COD in the aeration tank effluent. Figure 6 shows the effect of DO concentrations on ORP values and COD in the West #1 tank effluent. When the DO in the aeration tank effluent was low (0 1 mg/l), the effluent ORP was never higher than 250 mv, although effluent COD was sometimes as low as 43 mg/l. We found that low effluent COD associated with low ORP was observed in summer (T water C), while high effluent COD associated with low ORP was observed in winter (T water C). It has previously been reported that ORP readings steadily decrease when Figure 5. Effluent DO concentration and DORP values from influent to effluent in the West #1 aeration tank. ENVIRON ENG SCI, VOL. 18, NO. 5, 2001

8 316 LI AND BISHOP Figure 6. wastewater temperatures increase (Hetzler and Spielman, 1995). This is due to the lowered wastewater oxygen saturation capacity and the greater oxygen consumption by micro-organisms brought on by warmer conditions. Just as described above, without sufficient oxygen, pollutants would not be completely biodegraded along the aeration tanks, causing the redox status of the mixed liquor to not improve much. When the DO of the effluent was increased to 2 mg/l, ORP values were generally higher than 250 mv. When the effluent ORP value was higher than 350 mv, the corresponding CODs were all lower than 50 mg/l, even though DO ranged from 1 8 mg/l. This indicates that sufficient DO is necessary for pollutants to be efficiently biodegraded, and is helpful for achievement of improvements in the redox status of the mixed liquor. When the The effect of DO on COD and ORP values in the aeration tank effluent. pollutant macrobiodegradation is completed, and the oxygen concentration no longer has an effect on the redox status of the wastewater, DO is no longer a limiting factor. From Fig. 7 it can be seen that COD and ORP followed a linear relationship when DO was higher than 1 mg/l. This is very useful for operators. By measuring the ORP in the aeration tank effluent, which takes less than 3 min, the operator can get a good indication of the aeration tank effluent quality. This is much easier and more efficient than measuring COD to reflect the effluent quality. The relationships between ORP increase and COD removal efficiency in the West #1 aeration tank. The relationship between ORP increase in the West #1 tank and COD removal efficiency [defined as: (influent COD-ef- Figure 7. DORP values and COD removal efficiency in the West #1 aeration tank.

9 ORP IN WASTE TREATMENT PROCESSES 317 fluent COD)/influent COD] was also examined (Fig. 7). The results show that when DO is higher than 1 mg/l in the effluent, the increase in ORP values in the West #1 tank is always higher than 140 mv and is linear with COD removal efficiency (40 80%). The increasing ORP values (DORP, representing the redox status improvement) correlate well with the COD removal efficiency (pollutant biodegradation). When DO is lower than 1 mg/l in the effluent, however, this relationship is not good. With the low COD removal efficiency (not more than 50%), the pollutant concentration in the effluent was still high. This caused the redox status of the mixed liquor to not be stable, and DORP could not be used accurately. The resulting DORP values were no more than 100 mv, indicating the redox status of the wastewater did not improve appreciably. DORP values indicate the extent of the redox potential improvement along the aeration tank. The experimental data show that DORP values in the West #1 tank were mv, with COD removal efficiencies of 40 80%. This could be used to control the aeration tank operation. We recommend that the DORP values and ORP readings of the effluent be used together. DORP values are related with COD removal efficiencies along the aeration tanks, while effluent ORP readings indicate effluent quality. Based on our experiments in two other municipal wastewater facilities in Cincinnati and an industrial wastewater treatment plant (data not shown here), the relationship between ORP values and wastewater COD concentration is constant, but is somewhat specific to the system in which they are measured. This is probably caused by the differing incoming wastewater qualities and buffer capacities. When ORP and DORP values are used in aeration tank operational control, measurements should be done to obtain the ORP values specific to that treatment facility (Li and Bishop, 2001). Observations and ORP recordings from a pilot column ORP changes under different DO concentrations. Changes in ORP due to varying DO concentrations were examined using the pilot column. The DO in the pilot column was changed by adjusting the airflow rate from an air compressor. The intermittent aeration cycle (30 min air-on/30 min air-off) used in the full-scale aeration tanks was also used here. Six DO concentrations were used for the air-on periods: 1.5, 2.5, 4, 6, 8, and 9 mg/l. The airon/air-off DO target concentrations were met within 1 2 min after switching to a new setting. The HRT of each complete cycle was 6 h, the same as in the West #1 tank. The influent to the pilot column was pumped from the West #1 aeration tank influent, after having been mixed with the returning activated sludge from the secondary settling tanks. Influent ORP values were mv. The resulting data are shown in Fig. 8. It is observed that the ORP increased less than 120 mv over the complete cycle when DO was 1.5 mg/l, while ORP increased mv when DO was higher than 4 mg/l. DO concentrations of 6, 8, and 9 mg/l did not result in higher ORP values. These data corroborate the findings presented earlier on the effect of DO on ORP increases in the West #1 tank (see Fig. 5). Again, for appropriate redox potential improvement in the aeration tank (DORP values higher than 140 mv), DO should be maintained Figure 8. The effect of DO on DORP values in the pilot column. ENVIRON ENG SCI, VOL. 18, NO. 5, 2001

10 318 LI AND BISHOP Figure 9. above 1 mg/l to provide sufficient oxygen for micro-organisms for biodegradation, but a DO higher than 6 mg/l will not further improve the redox status of the wastewater. Sufficient oxygen is available when DO is mg/l. ORP changes under different COD concentrations. Eight influent COD concentrations were established in the pilot column. CODs of 42, 86, and 121 mg/l were obtained by sampling from the West #1 tank influent on different days. Higher COD values of 250, 420, 759, 1105, and 1469 mg/l were achieved by adding sodium acetate plus a small amount of the essential nutrients NaH 2 PO 4, Na 2 HPO 4, MgSO 4 to samples from the West #1 influent. DO was kept at 3 4 mg/l in the air-on zones, while in air-off zones, the DO was mg/l. The HRT was 6 h in each cycle. Figure 9 shows the COD removals achieved under these operating conditions, while the effect of influent COD concentrations on the changes in ORP over the whole cycle are shown in Fig. 10. Figure 9 shows that when influent COD varied from mg/l, essentially complete COD removal was accomplished within the first 3 h. Soluble COD remaining at the end of the first 3-h period was mg/l. COD in the wastewater did not further decrease during the second 3 h. With influent COD ranging from 420 to 759 mg/l, COD dropped to mg/l after the first 3-h treatment period; COD of the final effluent was mg/l. However, COD removal was not completed in the first 3 h when the influent COD was 1,105 or 1,469 mg/l. At the end of the first 3-h aeration period, the CODs were still 557 and 961 mg/l, respectively. During the second 3 h, COD removal continued and ORP continued to increase. The final effluent CODs for these two high COD COD removal in the pilot column under different influent COD conditions. influents were 41 and 435 mg/l, respectively, indicating that COD biodegradation was essentially completed for the influent COD of 1,105 mg/l, but not when the influent COD was 1,469 mg/l. From Figure 10, it can be seen that the DORP values increased with increasing influent COD ( mg/l). This followed the trend observed in the West #1 tank: the more the total COD removal (influent COD minus effluent COD) is, the higher the DORP values are. Then the DORP values dropped for the two high influent COD wastewaters (1,105 mg/l and 1,469 mg/l), probably because the sorption and biodegradation of pollutants was not yet complete. The CODs at the end of the first 3 h were much higher than those of the previous six influents. Under these two high COD conditions, the redox status of the final wastewater effluent was not appreciably improved over that of the influent. It was also found that, in the influent pumped directly from the West #1 tank (COD 5 42, 86, and 121 mg/l), the DORP values for the whole 6-h cycle were only 5 15 mv higher than the DORP values during the first 3 h of treatment. This again indicates that little further biodegradation in the macroenvironment occurred in the second 3-h period, and the system had almost reached redox equilibrium. In the influent supplemented by sodium acetate, the differences between the ORP changes during the whole cycle and the first 3-h period were more than 30 mv. This is probably due to the larger amount of COD present and the longer time required for processing. For the highest influent COD (1,468 mg/l), where COD removal in the macroenvironment continued for the whole 6-h cycle, DORP values for the whole cycle were 50 mv higher than those for the first 3 h of treatment. As discussed earlier, DORP values reflect the redox status improvement of wastewater and the extent to which

11 ORP IN WASTE TREATMENT PROCESSES 319 Figure 10. biodegradation has proceeded. If the biodegradation is still underway in the macroenvironment, it should be reflected in the ORP increase. Combining the data from the eight influent COD concentrations, it can be preliminarily concluded that, compared with the biodegradation that still may be continuing within the activated sludge flocs, pollutant removal from the macroenvironment contributes the most to the ORP increase. CONCLUSIONS 1. Compared with a wastewater s DO concentration, its ORP value represents a more comprehensive status of the wastewater under equilibrium conditions. Our data shows that the effluent ORP value is related to the COD concentration of the wastewater, and that the increasing ORP value (DORP) along the tank length is indicative of the COD removal percentage along the tank. ORP can be used to better control the operation of activated sludge treatment processes than can dissolved oxygen concentration. 2. Based on experiments at the West #1 aeration tank, it was found that when DO in the aeration tank effluent was lower than 1 mg/l, the wastewater s ORP values were always lower than 250 mv. When DO was higher than 1 mg/l, ORP values in the aeration tank effluent were from mv. When ORP was higher than 350 mv, the corresponding COD of the effluent was always below 50 mg/l. To maintain a good effluent quality and to have sufficient DO in the aeration tank effluent, the ORP of the effluent should be kept higher than mv. 3. ORP increases (DORP values) during activated sludge The effect of COD on DORP values in the pilot column. treatment represent the extent of COD removal. Sorption and biodegradation in the macroenvironment change the redox status along the aeration tank, which is reflected in significant ORP increases. Biodegradation of organics stored within the activated sludge floc produces only small additional changes in bulk liquid ORP in the second half of the aeration tanks. Based on the data available so far, an ORP increase of mv along the length of an aeration tank is indicative of efficient biodegradation. 4. Although ORP is closely tied to COD concentration in the mixed liquor, the dissolved oxygen concentration plays an indirect role in ORP values when the wastewater pollutant concentration is high. Sufficient oxygen is necessary to sustain biodegradation along the aeration tank and to improve the redox status of the wastewater. In the aeration tank effluent, where pollutant concentrations are lower, residual COD is the controlling factor for ORP values. There is a good linear relationship between COD and ORP in the West #1 tank effluent. Based on our experiments, we suggest a better way to control aeration tank operation: the DORP between the aeration influent and effluent and the ORP value of the aeration tank effluent can be used together to provide guidance for operators to quickly estimate the treatment efficiency and effluent quality. Because the wastewater quality varies among different treatment facilities, experiments should be performed to obtain a reliable ORP-COD relationship specifically for a given treatment facility. 5. Pilot column experiments on the effect of DO concentrations on ORP increase demonstrated that ORP values did not increase appreciably when DO was below 1.5 mg/l, while the DORP values could reach mv when DO was higher than 3.6 mg/l. DO ENVIRON ENG SCI, VOL. 18, NO. 5, 2001

12 320 LI AND BISHOP concentrations higher than 6 mg/l did not contribute to further ORP increases. Experiments on the effect of COD loadings on ORP increase indicated that the rate of ORP increase increased proportionally with influent COD. This corresponded with the data from the West #1 aeration tank, indicating that the increasing DORP values can be an indicator of COD removal efficiency. When influent COD was very high (higher than 1,000 mg/l), COD removal was not complete during the aeration cycle (6 h), and the DORP values dropped. In the high COD influent (supplemented with sodium acetate), the difference between the ORP increase for the whole 6-h cycle and for the first 3-h period was mv. 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