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The feasible sequential control strategy of treating high strength organic nitrogen wastewater with


The feasible sequential control strategy of treating high strength organic nitrogen wastewater with sequencing batch bio?lm reactor
B-C. Cho*, C-N. Chang**, S-L. Liaw* and P-T. Huang*
*Graduate Institute of Environmental Engineering, National Central University, Chung-Li City, Taiwan 32054 **Graduate Insitute of Environmental Science, Tunghai University, Taichung City, Taiwan Abstract The bio-kinetics and feasible sequential control strategy of treating high strength organic carbon and nitrogen wastewater were investigated by conducting the ABS manufacturing wastewater in a series of Sequencing Batch Bio?lm Reactors (SBBRs). The on-line ORP, pH, and DO monitoring parameters were applied to identify the feature-points when ammoni?cation, nitri?cation, and denitri?cation ends. The carbonaceous matter removal kinetics in the anaerobic and aerobic reaction stages can be expressed by the Michaelis-Menten equation. High ef?ciency of organic carbon removal and organic nitrogen ammoni?cation in the anaerobic stage can eliminate the substrate competition and activation inhibition to nitrifying organisms in the following aerobic stage. In the sequencing nitrogen removal processes, the producing time and system ORP values of these feature-points have good function relationships with the in?uent COD loading rates of SBBR, which can be integrated into a set-point (set-time and set-ORP) sequential control strategy of nitrogen removal. The automatic control operation results revealed ORP was one of the major control parameters of the sequencing nitrogen removal process in SBBR system and high overall removal ef?ciency were obtained. Keywords ABS resins; automatic control; organic nitrogen; on-line monitoring; Oxidation-Reduction Potential (ORP); Sequencing Batch Bio?lm Reactor (SBBR); sequential control

Water Science and Technology Vol 43 No 3 pp 115–122 ? IWA Publishing 2001

Introduction

The ABS (Acrylonitrile-polyButadiene-Styrene) industry is booming in southeastern Asian countries, and these countries are becoming the leading ABS suppliers in the world. Major raw materials for manufacturing ABS resins include acrylonitrile and styrene; both are toxic and highly resistant to biological degradation. Field data show that the ABS wastewater contains high concentrations of COD, organic nitrogen, and ratios of TKN/TOC and Org-N/COD as well as high alkalinity. The high organic nitrogen contents combining with the aromatic structure of styrene indicated the refractory characteristics of biodegradation. Most of the wastewater treatment processes neglected the functional design of this high strength organic nitrogen removal in the existing plants, and cause serious environmental problems of eutrophication (Chang et al., 1997). Many papers had indicated that the biofilm nitrification/denitrification processes can encourage the reproductive capacity of nitrifying organisms and enhance the nitrification rate to achieve high total nitrogen removal efficiency (van Benthum et al., 1995; Ohashi et al., 1995; Cheng and Chen, 1994). The SBBR combined the operational advantages of biofilm reactor and sequencing batch reactor (SBR), which can maintain high biomass concentration, encourage cultures of slow growing organisms, and can obtain homogeneous biomass distribution throughout the reactor under the sequencing batch operation pattern (Kaballo and Wilderer, 1993). However, the conventional fixed-time sequential control approaches of SBR system always lead toward unnecessary resource consumption for maintaining the system’s performance.

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The oxidation-reduction potential (ORP) represents the basic chemical energy potential of an oxidation-reduction reaction. The activity of ORP is a major dominant factor of electronic potential. Therefore, many researchers applied ORP to monitor the overall reactions of biological and physicochemical processes. All indicated that ORP could reflect the deviations of reaction systems, predict the characteristics of different reaction stages (or states), and be a promising monitoring parameter for on-line control of these overall reactions (Charpentier et al., 1989; Jenkins and Mavinic, 1989; Paddie et al., 1990; Chang et al., 1996). Recently, many researchers explored more efficient and reliable automatic control approaches of biological nitrogen removal by monitoring ORP in SBR system (Plisson-saune et al., 1996; Nielsen and Onnerth, 1996; Hao and Huang, 1996; Yu et al., 1997). In this study, the biological degradation mechanisms and kinetics of organic carbon and nitrogen were investigated by conducting the high organic nitrogen content ABS wastewater to SBBR. The ORP, pH, and DO on-line monitoring parameters were selected to develop a feasible sequential control strategy of nitrogen removal.
Method and instruments

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This study was carried out in four sets of 5 L SBBR reactors maintained at 20?C, which consist of an aerator, mixer, plastic porous media (55.8% in volume), and fed with the ABS wastewater daily. The systems operated under the sequence of filling, anaerobic, aerobic, anoxic, reaeration, settling, and drawing stages made up each 24-hr cycle. The contact media was made of HDPE with 50~130 m2/m3 specific area and 97% porosity. The raw ABS wastewater characteristics were shown in Table 1. Four organic loading rates of 0.60, 0.15, 0.21, and 0.31 g COD/L/day (0.013, 0.032, 0.044, and 0.065 g DOC/L/d), and an organic nitrogen source with TKN/COD ratio of 0.12 were controlled in the fixed-time operated system. The mean conserved biomass in SBBR reactors was 5,963, 6,171, 6,253, and 6,484 mg VSS/L respectively. The parameters of ORP, pH, and DO were monitored with an on-line data acquisition system and connected with a microcomputer (Figure 1). Samples were periodically taken from reactors for analyses of + – – COD, DOC, TKN, NH4 –N, NO2 –N, NO3 –N following the procedures published in Standard Methods (APHA, 1995).

Results and conclusion
The substrate biodegradation mechanisms under ?xed-time control operated SBBR

For investigating the organic carbon and nitrogen biodegradation mechanisms and kinetics, the SBBR’s sequencing operation pattern was controlled by conventionally fixing the reaction time of anaerobic, aerobic, and anoxic stages. The substrate removal rates in each stage were obtained in Table 2. The variations of water quality parameters shown in Figure 2, which indicated the major biodegradation mechanisms of anaerobic reaction stage, included the hydrolysis and fermentation of carbonaceous organic matter and the ammonification of organic nitrogen. The COD and DOC removal rates and specific substrate utilization rates (qCOD and qDOC) of each reactor can be estimated and were shown in Figure 3. The COD and DOC removal rates can be maintained above 53.4% and 70.0% respectively by controlling the influent COD loading rate below 0.21 g COD/L/day. According to the DOC specific substrate utilization rate (qDOC) approaching to a maximum value with increasing influent loading rates, indicated that the ABS wastewater contained about 30% of non-biodegradation DOC, which were highly resistant to biological degradation by anaerobic heterotrophic bacteria.

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Figure 1 The apparatus of SBBR system Table 1 The characteristics of ?eld ABS wastewater
Parameters Units Range Mean

pH Alkalinity SS COD TOC BOD5 TKN Org-N NH3–N
– NO2 –N – NO3 –N

– mg/L as CaCO3 mg/L mg/L mg/L mg/L mg/L as N mg/L as N mg/L as N mg/L as N mg/L as N

6.8~7.4 6060~8570 330~460 6750~7620 4340~5180 830~1780 478~950 383~710 95~240 0~0.14 0~31 0.19 0.12 0.17

7.2 7220 358 7150 4760 1390 827 647 180 – –

BOD5/COD TKN/COD TKN/TOC

Table 2 The substrate removal ef?ciencies under ?xed-time control operated SBBRs
Reactors SBBR1 SBBR2 SBBR3 SBBR4

COD (%)

Anaerobic Aerobic Overall

60.2 9.3 83.0 71.4 16.0 99.1 52.6 8.2 94.7 22.1 50.9 88.9

56.6 20.0 87.1 71.0 21.0 96.0 52.4 21.0 81.0 29.9 49.3 86.0

53.4 25.1 85.6 70.0 23.6 97.2 34.7 36.7 77.8 15.4 58.2 89.0

38.9 31.3 86.6 41.0 50.0 92.3 – – 10.8 – – –
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DOC (%)

Anaerobic Aerobic Overall

Org-N (%)

Anaerobic Aerobic Overall

T-N (%)

Anaerobic Aerobic Overall

B-C. Cho et al. Figure 2 The carbon and nitrogen water quality parameter pro?les under ?xed-time control operated SBBR system with organic loading of 0.15 g COD/L/day Figure 3 The COD and DOC speci?c substrate utilization rates (qCOD and qDOC) and removal rates of various in?uent COD loading rates during the anaerobic stage of SBBR Figure 4 The COD and DOC speci?c substrate utilization rates (qCOD and qDOC) and removal rates of various in?uent COD loading rates during the aerobic stage of SBBR

The specific substrate utilization rates of COD and DOC were expressed by the MichaelisMenten equation: q=qmaxSe/(Ks+Se) (1)

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q = speci?c substrate utilization rate, mg COD/g VSS/hr or mg DOC/g VSS/hr qmax = maximum speci?c substrate utilization rate, mg COD/g VSS/hr or mg DOC/g VSS/hr Ks = half-saturation coef?cient, mg COD/L or mg DOC/L Se = ef?uent substrate concentration, mg COD/L or mg DOC/L. Based upon the observed bio-kinetic model, the maximum specific substrate utilization rates of COD and DOC (qCOD,max, qDOC,max) were 9.5 mg COD/g VSS/hr and 2.2 mg DOC/g VSS/hr, and the half-saturation coefficients of COD and DOC (Ks,COD, Ks,DOC) were 231.3 mg COD/L and 33.8 mg DOC/L respectively. The organic nitrogen ammonification was always accompanied with organic carbon biodegradation in the anaerobic reaction stage. Uncompleted ammonification occurred under high influent COD loading rate, which caused the inhibition of the nitrifying organism of the sequencing aerobic reaction stage. With controlling the influent COD loading rates below 0.21 g COD/L/day, the organic nitrogen ammonification efficiency reached about 34.7~52.6%. The major biodegradation mechanisms of the aerobic reaction stage were organic carbon and nitrogen oxidation/nitrification reactions. The COD and DOC removal rates and specific substrate utilization rates (qCOD and qDOC) were shown in Figure 4. Applying the Michaelis-Menten equation (1) to estimate the bio-kinetics of COD and DOC in the following aerobic reaction stage was shown in Figure 5, which indicated the residual organic carbon of anaerobic biodegradation complied with first-order degradation in low

B-C. Cho et al.

Figure 5 The bio-kinetics model estimation of organic carbon in aerobic reaction stage with Eq. (1)

Figure 6 The ORP, pH, and DO pro?les in settime control operated SBBR with organic loading of 0.15 g COD/L/day

substrate loading, and with zero-order degradation in high substrate loading. The DOC substrate utilization rate keeps constant at 0.138 L/g VSS/hr, and the maximum specific substrate utilization rate (qCOD,max) and the half-saturation coefficient (Ks,COD) of COD were 5.1 mg COD/g VSS/hr and 100 mg COD/L respectively, which caused the maximum COD specific substrate utilization rate to be less than that of the anaerobic reaction. It is obvious that the residual organic carbon of anaerobic biodegradation was difficultly decomposed by aerobic heterotrophic bacteria. But operating at a prolonged aeration time, it can oxidise these resistant organic matters and enhance both COD and DOC removal rates. Controlling the influent COD loading below 0.21 g COD/L/day can carry out the ammonia nitrogen nitrification significantly. Due to the internal biofilm dentrification happening, which + – caused the major type of nitrification to be nitritation (NH4 –N→NO2 –N) only. The total nitrogen and organic nitrogen removal rate were 68.8% and 56.3% respectively in the aerobic reaction stage, and the residual ammonia nitrogen concentrations were below 2 mg/L. – The anoxic reaction stage was denitrifying the nitrogen oxides (NOx –N) and organic carbon by facultative heterotrophic denitrifying bacteria. The residual COD and DOC of the aerobic stage can be an internal carbon and energy source of denitrifying bacteria. The high biomass density and speci?c oxygen uptake rates were maintained in SBBR system and caused low dis– solved oxygen and high NOx –N electronic activity achieved in the anoxic reaction stage – (Plisson-Saune et al., 1996). The NOx –N removal rate in this stage approached 78.0~92.9%. The overall removal rates of COD, DOC, organic nitrogen, total nitrogen achieved were in the ranges between 83.0~87.1%, 96.0~99.0%, 77.8~94.7%, and 86.0~89.0% respectively.
Characteristics of ORP, pH, and DO variations under ?xed-time control operated SBBR

Typical ORP, pH, and DO profiles with several well defined control points in a SBBR system were shown in Figure 6. In the filling and initial anaerobic reaction stage, due to the increaseof high alkalinity and reductive substrate the pH increased, ORP decreased, and DO depleted. An obvious bending-point A appeared in both ORP and pH profiles, which indicated organic acid compounds and CO2 were produced by anaerobic hydrolysis and fermentation. Due to no change of nitrogen oxidation states during organic nitrogen ammonification the ORP values tend to be stabilized. The maximum total nitrogen release can be determined in feature-point B based on the variation of analyzed water quality parameters. The DO profile exhibited a two-stage shape increase in the aerobic reaction stage. The first increase was due to the appearance of dissolved oxygen by aeration. The nitrification mechanism can be detected in the first stage by monitoring pH and ORP profiles. Due to the – – alkalinity consumption and oxidize nitrogen (NO2 –N and NO3 –N) increasing with nitrification, the pH decreased and ORP increased. The second increase in DO profile began at

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Table 3 The relationships of feature-point with in?uent loading rate and the setting operation functions of these points
Feature-points Producing time (min) and ORP value (mV) Setting operation functions

A

tA = 46.025 exp( 9.2745X); (R2 = 0.994)

ORPA= –210.12 Ln(X) – 613.61; To estimate the in?uent COD (R2 = 0.998) loading rate by monitoring the producing time and ORP value, and then to compute the producing time and ORP control point of B, C, and D.

B-C. Cho et al.
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B

tB = 2388.4 X –79.954; (R2 = 0.998) tC = 46.102 exp(8.8777X); (R2 =0.999) tD = 205

ORPB =–1373.8 X +30.254; (R2= 0.992) ORPC = –1587.4 X + 281.93; (R2 =0.992) ORPD = –2059.4 X +273.45; (R2 =0.999)

To determine the end of anaerobic reaction stage.

C

To determine the end of aerobic reaction stage.

D

To determine the end of anoxic reaction stage.

Table 4 The estimated control points of reaction stage and summary of operation results
Reaction stage Control points estimated and setting Operation results Accumulation time

Anaerobic Aerobic Anoxic Reaeration Settling Drawing

Time constraint ORP constraint Time constraint ORP constraint Time constraint ORP constraint Set t = 30 min Set t = 30 min

t ≥ 278 min ORP ≥ –176 mV t ≥ 175 min ORP ≥ 44 mV t ≥ 205 min ORP ≤ –35 mV 30 min 30 min

330 min –176 mV 195 min 44 mV 205 min –62 mV 120 min 880 min 910 min

330 min 525 min 730 min 850 min

Set t ≥ 2 hr; DO ≥ 3 mg/L

feature-point C, where the oxidized nitrogen concentration achieved to a maximum and the ammonia nitrogen decreased in a minimum value. This feature-point indicated the end of ammonia nitrogen oxidation by nitrifying bacteria and is termed the break point of nitrification (Hao and Huang, 1996; Yu et al., 1997). Less oxygen consumed matters and highoxidized nitrogen appeared after the feature-point C, the DO profile shape increased to higher level and the ORP approached stability. In the initial anoxic reaction stage, facultative heterotrophic bacteria decomposed the residual organic carbon of the aerobic reaction stage and released CO2, which caused the pH to decrease and ORP to increase sharply. The dissolved oxygen was consumed com– pletely, the NOx –N electronic activity achieved a maximum and the denitrification mechanism happened. The pH appeared to increase due to the alkalinity and OH– released during the period of denitrification. Theoretically, the ORP profiles have a knee-point occur at the – end of denitrification or at the minimum NOx –N electronic activity (Plisson-Saune et al., 1996), but it seemed difficult to detect in this study. Before the end of the anoxic reaction stage, it always appeared as a protrusive point (feature-point D) in the ORP profiles.
Simulation and comparison of set-time and set-ORP control operated SBBR

Based upon the producing time and ORP of feature-points A, B, C, and D can be predetermined the estimate of the influent COD loading rate X, and to generate a set-point (set-time and set-ORP) sequential control strategy of SBBR system by setting these point’s function as shown in Table 3. The variation of ORP, pH, DO and water quality parameters under set-point control operated SBBR were shown in Figure 7 and 8. The predicated time and ORP values of feature-point A were detected at 185 min and 215 mV respectively, and predicted the influent

Table 5 The removal characteristics of carbon and nitrogen substrates under automatic control operated SBBR system
Reaction stage Anaerobic Aerobic Anoxic Overall

Initial concentration (mg/L)

COD DOC Org-N NH3–N
– NOX –N

352 90 23 25 0.3 48.3 122 44 14 42 0.8 56.1 65.3 51.1 39.1 – – – 6.68 1.34 0.26 – – –

122 44 14 42 0.8 56.1 106 40 5 5 30.1 40.2 4.5 4.4 39.1 88.1 – 29.2 0.80 0.20 0.44 1.82 – –

106 40 5 5 30.1 40.2 15 4 3 4 3.2 7.2 25.8 40.0 8.6 2.4 92.6 58.1 4.26 1.68 0.09 0.05 1.26 1.54

15 4 3 4 3.2 7.2 ND ND 2 ND 2.1 4.1 100.0 100.0 91.3 100.0 93.0 92.8 3.71 0.95 0.22 0.44 0.30 0.52 B-C. Cho et al.

TN Terminal concentration (mg/L) COD DOC Org-N NH3–N
– NOX –N

TN Removal Rate (%) COD DOC Org-N NH3–N
– NOX –N

TN Speci?c substrate utilization rate (mg Sr/g VSS.hr) COD DOC Org-N NH3–N
– NOX –N

TN C/N ratios of denitri?cation

– COD/ NOX –N = 3.52 – DOC/ NOX –N = 1.32

COD loading rate was 0.15 g COD/L/day. The producing time and ORP values of featurepoints B, C, and D were estimated by using the predicted influent COD loading rate shown in Table 4. The results demonstrated that ORP was the major control parameter of the anaerobic and aerobic reaction stages. Theoretically, the denitrification knee-point can be detected and the predicated ORP value can control the end of anoxic reaction stage and it took a reaction time of 205 min. The overall reaction time of the anaerobic, aerobic, and anoxic stages was 850 min, which was more than the total estimated time 658 min, but can shorten by 36.8% the operation time by comparing it with conventional fixed-time control strategy. The removal characteristics of carbon and nitrogen substrates under set-point control operated SBBR systems were shown in Table 5. The COD and DOC removal rates were 65.3% and 51.1% respectively in the anaerobic reaction stage of set-point control operation strategy, which was more efficient than that in fixed-time control operation. The higher COD and DOC specific substrate utilization rates (6.68 mg COD/g VSS/hr and 1.34 mg DOC/mg VSS/hr respectively) detected and implied ORP could be the major control parameter of the anaerobic stage. Less COD and DOC removal rates and specific substrate utilization rates (1.22 mg COD/g VSS/hr and 0.23 mg DOC/mg VSS/hr respectively) in the aerobic reaction implied the residual organic carbon matters need longer oxidation times to decompose. Pre-oxidized organic carbon matters can be the internal carbon source of denitrifting bacteria, and be utilized completely in the anoxic reaction stage.

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B-C. Cho et al. Figure 7 The ORP, pH, and DO pro?les under setpoint control operated SBBR with predicted organic loading rate of 0.15 gCOD/L/day Figure 8 The carbon and nitrogen water quality parameter pro?les under set-point control operated SBBR with predicted organic loading rate of 0.15 gCOD/L/day

The ammonia nitrogen removal rate approached 88.1% in the aerobic reaction stage, and was removed completely by the following facultative heterotrophic denitrifying bacteria. – The overall organic nitrogen and NOx –N removal rates achieved were 91.3% and 93.0% respectively, and with the effluent total nitrogen concentration 4.1 mg/L.
Conclusion

30% of organic carbon matter contained in ABS wastewater belonged to bio-resistant organisms in the anaerobic stage, which can be oxidized by the following aerobic stage and become the internal carbon source of the denitrifying bacteria. The COD and DOC biokinetics can be expressed by the Michaelis-Menten equation. Organic nitrogen ammonification is the major reaction limiting factor of the nitrogen removal process. ORP can be used to develop a on-line sequential control system, which can find a proper reaction time of nitrogen removal in SBBR system.
References
APHA, AWWA and WEF (1995). Standard methods for examination of water and wastewater. 19th edn. Washington DC. Chang, C.N., Lin, J.G., Chao, A.C. and Liu, C.S. (1996). Modi?ed Nernst model for on-line control of the chemical oxidation decoloring process. Wat. Sci. Tech., 34(3–4), 151–157. Chang, C.N., Lin, J.G., Chao, A.C., Cho, B.C. and Yu, R.F. (1997). The pretreatment of acrylonitrile and styrene with ozonation process. Wat. Sci. Tech., 36(2), 263–270. Charpentier, J., Godart, H., Martin, G. and Mogno, Y. (1989). Oxidation Reduction Potential (ORP) regulation as a way to optimize aeration and C, N and P removal: experimental basis and various full-scale example. Wat. Sci. Tech., 21(10–11), 1209–1223. Cheng, S.S. and Chen, W.C. (1994). Organic carbon supplement in?uencing performance of biological nitri?cation in ?uidized bed reactor. Wat. Sci. Tech., 30(11), 131–142. Hao, O.J. and Huang, J. (1996). Alternating aerobic-anaerobic process for nitrogen removal : process evaluation. Wat. Environ. Res., 68(1), 83–93. Jenkins, C.J. and Mavinic, D.S. (1989). Anoxic-aerobic digestion of waste activated sludge: Part II – supernatant characteristics, ORP monitoring results and overall rating system. Environ. Tech. Lett., 10, 371–381. Kaballo, H.P., Zhao, Y. and Wilderer, P.A. (1995). Elimination of p-Chlorpophenol in bio?lm reactor – a comparative study of continuous ?ow and sequenced batch operation. Wat. Sci. Tech., 31(1), 51–60. Nielsen, M.K. and Onnerth, T.B. (1996). Strategies for handling of on-line information for optimising nutrient removal. Wat. Sci. Tech., 33(1), 211–222. Ohashi, A., Viraj de Silva, D.G., Mobarry, B., Manem, J.A., Stahl, D.A. and Rittmenn, B.E. (1995). In?uent of substrate C/N ratio on the sturcture of multi-species bio?lms consisting of nitri?ers and heterotrophs. Wat. Sci. Tech., 32(8), 75–84. Peddie, C.C., Mavinic, D.S. and Jenkins, C.J. (1990). Use of ORP for monitoring and control of aerobic sludge digestion. J. Environ. Enging, 116(3), 461–471. Plisson-saune, S., Capdeville, B., Mauret, M., Deguin, A. and Baptiste, P. (1996). Real-time control of nitrogen removal using three ORP bending-points: signi?cation, control strategy and results. Wat. Sci. Tech., 33(1), 275–280. van Benthum, W.A.J., van Loosdrecht, M.C.M., Tijhuis, L. and Heijnen, J.J. (1995). Solids retention time in heterotrophic and nitrifying bio?lms in a bio?lm airlift suspension reactor. Wat. Sci. Tech., 32(8), 53–60. Yu, R.F., Liaw, S.L., Chang, C.N., Lu, H.J. and Cheng, W.Y. (1997). Monitoring and control using on-line ORP on the continuous-?ow activated sludge batch reactor system. Wat. Sci. Tech., 35(1), 57–66.

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