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Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20

Simultaneous removal of nutrients from milking parlor wastewater using an AO2 sequencing batch reactor (SBR) system a

b

Xiao Wu & Jun Zhu a

Southern Research and Outreach Center, University of Minnesota, Waseca, Minnesota, USA

b

Click for updates

Department of Biological and Agricultural Engineering, University of Arkansas, Fayetteville, Arkansas, USA Published online: 27 Feb 2015.

To cite this article: Xiao Wu & Jun Zhu (2015) Simultaneous removal of nutrients from milking parlor wastewater using an AO2 sequencing batch reactor (SBR) system, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 50:4, 396-405, DOI: 10.1080/10934529.2015.987543 To link to this article: http://dx.doi.org/10.1080/10934529.2015.987543

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Journal of Environmental Science and Health, Part A (2015) 50, 396–405 Copyright © Taylor & Francis Group, LLC ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934529.2015.987543

Simultaneous removal of nutrients from milking parlor wastewater using an AO2 sequencing batch reactor (SBR) system XIAO WU1 and JUN ZHU2 1

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2

Southern Research and Outreach Center, University of Minnesota, Waseca, Minnesota, USA Department of Biological and Agricultural Engineering, University of Arkansas, Fayetteville, Arkansas, USA

The feasibility of using a lab-scale, anaerobic-aerobic-anoxic-aerobic sequencing batch reactor ((AO)2 SBR) to simultaneously remove biological organics, nitrogen and phosphorus from dairy milking parlor wastewater was investigated in this study. Three hydraulic retention times (HRT D 2.1, 2.7, and 3.5 days) and three mixing-to-process time ratios (TM/TP D 0.43, 0.57, and 0.68) were evaluated as two controlling factors using a 3 £ 3 experimental design to determine the optimal combination. Results showed that the HRT of 2.7 days with TM/TP D 0.57 was the best to achieve simultaneous nutrients removal for the influent with initial soluble chemical oxygen demand (SCOD) of about 2000 mg L¡1 (only 0.55 mg L¡1 NH4-N, < 0.1 mg L¡1 nitrate, and 0.14 mg L¡1 soluble phosphorus in the effluent). Good correlations between pH and ORP, and ORP and DO, were also obtained with correlation coefficients all higher than or equal to 0.975. These relationships could be used to develop real-time control strategies to optimize the duration of each operating phase in the (AO)2 SBR system to save energy and enhance treatment efficiency. Keywords: Milking parlor wastewater, nutrients removal SBR system.

Introduction Milking parlor wastewater is rich in nutrients, such as protein, fat, and lactose, originating from wasted milk characterized by high chemical oxygen demand (COD), which contributes to the organic matter, phosphorus, and nitrogen in the wastewater. According to literature data, almost all the nitrogen in the milking wastewater comes from milk proteins when urine and manure are not involved.[1] Similarly, the esterified phosphate group, an important bond for milk caseins that are conjugated proteins and represent about 80% of milk proteins, is found to be the largest phosphorus contributor, followed by another two phosphorus sources, i.e., the phosphoric acid and the detergent used for rinsing the milking pipelines.[2, 3] In the past, discharging milking parlor wastewater to a stream, a road ditch leading to a stream, or field drain tile leading to a stream was largely ignored because of the small volume with a lower level of pollution potential for the environment. Address correspondence to Jun Zhu, Department of Biological and Agricultural Engineering, University of Arkansas, Fayetteville, AR 72701, USA; E-mail: [email protected] Received August 27, 2014. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lesa.

However, it has become increasingly clear that these destinations are unacceptable locations today for discharging a large amount of high strength wastewater, which could result in significant degradation of water quality. An alternative way of handling milking parlor wastewater is to store it along with dairy manure slurry, but the relatively large volume of wastewater dilutes the slurry, thus increasing the demand for manure storage and leading to increased costs in distribution and land application, which will significantly drive up the production costs for many dairy producers. Therefore, to support the dairy industry (especially those small- to mid-sized farmers) to comply with the increasingly stringent environmental regulations for pollution control while still maintaining continued growth, effective technologies have to be developed and implemented to remove nutrients from milking parlor wastewater before discharge. In recent years, the sequencing batch reactor technology (SBR) has found itself being increasingly researched to treat animal wastewaters.[4–6] Many authors reported that organics, nitrogen, and phosphorus removal from animal, or other, wastewaters could be simultaneously achieved in a single tank of an SBR system when operating conditions were properly controlled to include anaerobic, anoxic and aerobic phases according to a timed cycle.[6–11] Meanwhile, a number of researchers[9, 12]

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SBR used to remove nutrients from milking parlor wastewater claimed that the nutrient removal processes had affinities among each other since both N and P removal processes required the presence of COD, which was often the limiting substrate in the incoming wastewater. Therefore, appropriately allocating time to anaerobic, anoxic, and aerobic phases in an SBR cycle that is realized by altering the length of aeration time to maximize the available COD for N and P removal is one of the major factors to improve the SBR performance. Artan and Tasli[13] concluded that the aeration time fraction was the most important operational parameter that influenced the nutrient removal efficiency, and they also found that when mixing to process time ratio (Tm:Tp) exceeded 0.65, sludge settleability began to deteriorate due possibly to prolonged anaerobisis. Yang and Wang[14] and Ndegwa et al.[15] stated that the aeration time mode (airon: air-off) of 3:3 was better than 5:1 in terms of effluent quality for nitrogen for treating swine wastewater. This again verifies that, to achieve simultaneous nutrients removal, the aeration pattern of anaerobic-aerobic-anoxicaerobic ((AO)2) is indeed an important parameter in an SBR operation that deserves further research. Hydraulic retention time (HRT), which determines the nutrients loading rates to the biomass in an SBR, has been investigated extensively in SBR technology for treating different wastewaters.[16–18] However, research related to the roles that HRT plays in SBR treatment of dairy milking parlor wastewater is still very limited. Previous researchers reported that the HRT for SBRs to treat swine wastewater could be as short as 2 days without significantly losing treatment efficiencies.[6, 19, 20] Since HRT is pivotal to an SBR system particularly for simultaneous N and P removal, more information is needed to understand the effect of HRT on milking parlor wastewater treatment using the SBR technology. In a nutshell, information in the current literature about the application of SRB technology to the treatment of dairy milking parlor wastewater is rare. Given the success

Table 1. Main properties of the influent wastewater fed into the SBR. Parameters and Unit

Levels or Ranges

TS (mg/L) TVS (mg/L) TSS (mg/L) TCOD (mg/L) SCOD (mg/L) BOD5 (mg/L) TKN (mg N/L) NH4C-N (mg N/L) NO3¡-N (mg N/L) NO2¡-N (mg N/L) TP (mg P/L) DP (mg P/L)

1960–3660 1040–2360 360–1160 3500–4300 1800–2200 2000–2400 148.3–213.4 14.0–138.0 0 0 40.2–80.4 35.0–80.0

397

of using SRB in treating other wastewater streams, it is not unreasonable to research the feasibility and potential of using this technology in a dairy milking parlor wastewater treatment. Therefore, the objectives of this study are to (1) understand the way in which the nutrients of milking parlor wastewater are simultaneously removed in an (AO)2 SBR system; (2) determine the influence of operating conditions (HRT and the ratio of mixing and process time, Tm/Tp) on the efficiency of the nutrients removals; and (3) determine the changes in pH, ORP, and DO and their relationships during SBR operation using real-time monitoring data.

Materials and methods Source of milking wastewater The milking parlor wastewater in this study was collected from a dairy farm in Waseca County, Minnesota where the wastewater from the milk operation flowed through a floor drain by gravity to an outside storage tank, from which liquid samples were collected. Immediately after collection, the liquid samples were refrigerated at 4 C to minimize microbiological activity, and only transferred into a holding tank and slowly mixed for several hours before use. If needed, the COD level of the wastewater was adjusted by dilution with tap water or addition of milk to accommodate different concentrations for the SBR experiment, and the characteristics of the influent were summarized in Table 1. Seed sludge The seed sludge was obtained from the Waseca Advanced Municipal Wastewater Treatment Plant located in Waseca, Minnesota. To increase the active biomass content before use in the SBR, the sludge was incubated under room temperature for 3 days using a nutrient solution composed of glucose, ammonium chloride (NH4Cl), and potassium phosphate (K2HPO4), with the C:N:P ratio of 100:5:1, which was verified by preliminary trials to be effective for microbial growth. SBR configuration and operation The main body of the lab-scale SBR used in this study was fabricated from a transparent PVC cylinder, 19.0 cm in diameter, with a total volume of 12.8 L and a working volume of 8 L. The SBR was equipped with an operating system that could perform influent feeding/effluent discharging, aeration, mixing, and online monitoring and data acquisition. The SBR running cycle included FILL, REACT, SETTLE, DECANT, and IDLE following a preset time cycle that could repeat continuously at room temperature (20 § 1 C). During all FILL and REACT

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398 phases, a mechanical mixer (catalog#: A-50002-10, ColeParmer Company, Vernon Hills, IL, USA) with a labmade paddle (15.2 cm £ 11.1 cm) was running at 25 rpm to keep the reactor content under suspended conditions. An air compressor provided air into the SBR through five air diffusing stones placed evenly at the bottom of the reactor to achieve the aerobic REACT condition. Filling and decanting events were accomplished using two peristaltic pumps (catalog#: 900-0857, Barnant Company, Vernon Hills, IL, USA). The sludge retention time (SRT) was maintained by removing biomass from the reactor at the end of the REACT phase and before the SETTLE phase of the second cycle. During the decanting periods, supernatant was drawn until a predefined minimum water level in the reactor was reached. The reactor temperature, pH, and oxidation-reduction potential (ORP) were monitored in real time using different probes (catalog numbers: 107L, CSIM11, and CSIM11-ORP, for temperature, pH, and ORP probes, respectively, Campbell Scientific Inc., North Logan, UT, USA). The configuration of the whole system was shown in Figure 1. The SBR was operated using the data acquisition and control software (Campsci PC400, Campbell Scientific, Inc.), which was able to repeat a predefined cycle regime in a time sequence by controlling the on-and-off of the filling/drawing pumps, the mixing devices, and the air compressor; and to connect to the monitoring module that acquired and stored on-line temperature, pH, and ORP data that could be transferred later into simple text files in the computer for further processing.

Fig. 1. Schematic of the Sequencing Batch Reactor (SBR) system.

Wu and Zhu Experimental design After sludge inoculation, the SBR went through a start-up period of one month during which the adjusted milking parlor wastewater was used as influent with the organic loading (BOD) level increasing gradually from 500 mg L¡1 to 2000 mg L¡1. The initial mixed liquor suspended solids (MLSS) level was around 2000 mg/L, and no sludge was wasted in the first month, resulting in about 6000 mg L¡1 MLSS at the end of the start-up period when the stable reactor behavior was observed based on a stable COD level (40–50 mg L¡1) in the effluent. A 3 £ 3 factorial design leading to nine operational conditions were adopted by changing the hydraulic retention time (HRT) and the ratio of anaerobic/anoxic mixing to total process times (TM/TP), excluding the settle and draw time, with each at three levels (HRT D 3.5, 2.7 and 2.1 days; TM/TP D 0.43, 0.57, and 0.68). In each cycle, there were two filling events at the beginning of each anoxic phase; therefore, in the period of 3.5 days of HRT, clarified supernatant of 750 mL was withdrawn from the reactor at the end of the settling stage, and fresh wastewater of 550 mL during the first filling stage and 200 mL during the second filling were pumped into the reactor. In a same way, for HRT of 2.7 days, a total of 1000 mL was pumped into and drawn from the reactor during a cycle, with 750 mL and 250 mL, respectively, in the first and second filling. The HRT of 2.1 days was realized by adding and removing 1250 mL (950 mL and 300 mL for the two feedings) of liquor during each cycle. The reason for two filling events in each cycle at the beginning of the two anoxic REACT phases was to provide a better distribution of the carbon source between the first and second A/O phase, with the influent being pumped into the reactor at a high velocity to achieve instantaneous feeding, leading to a ratio of filling time to cycle time less than 1/100. Three operating regimes with air-on time of 4 h, 3 h, and 2 h out of a total cycle of 8 h resulted in three mixing to process time ratios (TM/TP) of 0.43, 0.57, and 0.68, respectively (shown in Fig. 2). The nine test combinations of HRT and TM/TP were performed randomly, and the experiment parameters for each run were presented in Table 2. Since filling events were occurring under anoxic mixing condition, anoxic REACT began simultaneously due to constantly mixing the influent with the microorganisms in the reactor. The following aerobic REACT period was able to achieve nitrification and phosphorus removal. The last aerobic REACT phase before SETTLE was aimed to achieve effective phosphorus uptake by the biomass. The entire REACT phase employed in this study was carried out in the order of anaerobic react, aerobic react, anoxic react, and aerobic react. One hour was allocated to the SETTLE, DECANT, and IDLE phase during a cycle, resulting in the total REACT time being 7 h.

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SBR used to remove nutrients from milking parlor wastewater

399

Fig. 2. Time location in a cycle for three regimes operated in SBR.

Sampling and analysis The break-in period is the time needed for the SBR to enter a steady state after changing any operating variables to achieve a new condition, which is defined as consistent nutrient reduction rates, constant mixed liquor suspended solids concentrations (biomass), and constant BOD and COD removal efficiencies from batch to batch effluent. In this study, stable effluent COD value (variation less than 5% in a 1-week period) was taken as a major indicator for the steady state due to the simplicity of COD measurement. During the break-in period, two samples of 200 mL each from influent and effluent were collected in the last cycle of every 2 days and analyzed in order to determine steady state status. Under each experimental condition, systematic collection and analysis of samples began after the reactor was operated for one prevailing HRT in the steady state. During a typical cycle for each condition, 30mL samples from the SBR were taken roughly every 30 min during the 8-h cycle to investigate the dynamic changes in nutrients and biosolids distributions. Liquid samples were analyzed for chemical oxygen demand (COD), five day biochemical oxygen demand (BOD5), ammonium nitrogen (NH4-N), nitrate (NO3-N), Table 2. Schedules of the factorial experiments. Exp. Run 1 2 3 4 5 6 7 8 9

Condition

HRT (d)

TM/TP

T2 £ R3 T1 £ R2 T1 £ R3 T3 £ R1 T2 £ R2 T3 £ R3 T1 £ R1 T3 £ R2 T2 £ R1

2.7 3.5 3.5 2.0 2.7 2.0 3.5 2.0 2.7

0.68 0.57 0.68 0.43 0.57 0.68 0.43 0.57 0.43

Note: T D HRT, R D regime of TM/TP.

nitrite (NO2-N), and dissolved orthophosphate (DP) following standard methods (APHA, 1998). A DR/3000 spectrophotometer (Hach Company, 1993) was used for colorimetric analysis.

Results and discussions Comparison of nutrient removal under different operating conditions Nine operating conditions were experimented in this study by varying HRT with TM/TP. The results of COD, ammonium nitrogen (NH4-N), and dissolved phosphorus (DP) at the beginning and the end of a typical 8-h cycle under steady state for each operating condition were presented in Table 3. For HRT D 3.5 days (small loading rate), the initial COD:DP ratio was about 16 on average, and all three treatment regimes in terms of TM/TP achieved similar but good results in removing COD, NH4-N, and DP, indicating that aerobic react time could be reduced to save energy without losing treatment effectiveness. With the loading rate increased as the HRT was reduced to 2.7 days, the average COD: DP ratio was reduced to 9, and the removal of COD was observed to decline slightly as opposed to those for HRT D 3.5 days. However, improved reductions of NH4-N were obtained except for Regime 3 for which a significantly higher NH4-N concentration in the effluent was detected. For Regime 3, the TM/TP ratio was 0.68 (the shortest aerobic react time), leading to insufficient nitrification to convert NH4-N to nitrate, which could be the reason for the relatively high concentration of NH4-N in the effluent. Continuing to reduce HRT to 2 days appeared to have more impact on DP than on COD and NH4-N because only the DP concentrations in the effluent under all three regimes demonstrated significant increases (exceeding

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Wu and Zhu

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10 mg L¡1), with the other two fluctuating to some degree. The reason for unsuccessful phosphorus removal could certainly be the lack of a sufficient carbon source, indicated by a low COD:DP ratio of less than 5.5, and the obviously lower COD levels in the effluents. This observation confirms the past reports that the COD:DP ratio is a key influential factor for phosphorus removal in SBR systems,[21] regardless of the substrates treated. According to the experiment results from this study, a combination of 2.7 days HRT and a 0.57 TM/TP was determined to be the optimum operating scheme for treating milking wastewater with a SCOD level of around 2000 mg L. Therefore, in the subsequent sections, this operating scheme was used for discussions on the SBR performance.

COD and phosphorus removal To simplify the presentation, the typical treatment characteristics of the (AO)2 SBR process was represented by the COD and phosphorus profiles in an operating cycle for Run 5 (the middle running regime), which was shown in Figure 3. Please note that the initial soluble COD actually measured (about 124 mg L) within 2 min of the quick fill phase was lower than the calculated COD (about 200 mg L¡1) for the first feeding. The difference could be attributed to substrate adsorption or absorption and storage of biomass.[22] During the first anaerobic phase, the reactor environment forced phosphorus accumulating organisms (PAOs) to use energy, i.e., adenosine tri-phosphate (ATP), to obtain readily biodegradable organic carbon substrates and store them as polyhydroxyalkanoates (PHAs),[9] with the release of phosphorus into the liquid, which was indicated by the rapid increase in dissolved phosphorus (DP), with the maximum release rate of phosphorus by the PAOs being 3.35 mg P/g MLSS/h for the first half hour. In parallel, a rapid COD reduction was observed during the first 30 min after feeding because the readily biodegradable substrates were utilized at extremely high rates as a result of the high rate of phosphorus release. It was also found that a large part of the removed COD was used as a carbon source for phosphorus release during the cycle because when the phosphorus release rate became reduced

Fig. 3. COD and phosphate profiles during Run 5.

later in the first anaerobic phase, a corresponding increase in COD level was observed due possibly to the accumulation of unused soluble COD from the solubilization of insoluble COD in the system. In the following aeration phase, COD was reduced from 77 mg L¡1 to 50 mg L¡1 by oxidation, and phosphate was reduced from 22.12 mg L¡1 to basically zero, with the maximum phosphate uptake rate of about 3.93 mg P (g MLSS) ¡1 h¡1 in the first 45 min (Fig. 3), indicating the achievement of the enhanced biological phosphorus removal. When the second feeding was executed, the COD obtained from the influent was again consumed as the carbon source for phosphorus release (and for denitrification as well) but at a much lower rate, and it declined from 69 mg L¡1 to 54 mg L¡1 during the second anaerobic/ anoxic phase, which was 2 h and 45 min. In a similar pattern, the second phosphorus release was at a relatively slow rate as compared to that in the first anaerobic phase, probably because of the smaller carbon source provided by the second feeding. In addition, in the second aerobic phase, COD was not reduced, which could be attributed to the lack of biodegradable organic matter (measured by soluble COD) that was completely consumed by PAOs in taking up phosphorus in the liquid.[23] Since the phosphorus level was fairly close to 0 mg L¡1 (0.14 mg L¡1) at the end of treatment, it could be concluded that there was no need to add additional carbon sources when treating

Table 3. Results of COD, NH4-N, and DP in SBR under different conditions. HRT D 3.5 days

Reg. 1 Reg. 2 Reg. 3

B E B E B E

HRT D 2.7 days

HRT D 2.1 days

COD

NH4-N

DP

COD

NH4-N

DP

COD

NH4-N

DP

102 44 113 51 103 50

3.13 1.02 6.11 0.55 2.59 0.77

6.50 0.24 4.72 0.18 8.33 0.14

132 77 124 54 121 59

3.16 0.32 6.34 0.55 8.38 2.20

14.38 0.66 10.97 0.14 15.52 0.75

145 48 158 45 150 33

8.50 0.89 11.11 0.77 11.79 0.32

27.26 10.78 23.85 10.97 30.86 14.76

B- concentration at the beginning of a cycle, E- concentration at the end of a cycle, Reg.- Regime, Unit of all the values is mg/L).

SBR used to remove nutrients from milking parlor wastewater milking parlor wastewater by SBR under the selected operating condition in this study.

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Nitrogen removal Figure 4 presented information on a successful simultaneous removal of ammonium nitrogen, nitrate, and nitrite during a typical cycle with a combination of HRT of 2.7 days and TM/TP ratio of 0.57. Rapid filling at the very beginning of the cycle contributed to a prompt increase of NH4-N concentration (to 6.3 mg L¡1) in the mixed liquor when very little NOX-N was detected. After that, a slight increase in NH4-N concentration from 6.3 mg L¡1 to 7.4 mg L¡1 was observed during the anaerobic period, implying a low anaerobic hydrolysis rate. With the aerated phase being introduced, the NH4-N was converted to nitrite/nitrate by microorganisms with 6.5 mg L¡1 removed within the first 1.5 h of aeration, but only 2.2 mg L¡1 oxidized nitrogen (nitrate and nitrite) was measured at the end of the first aeration phase. This observation implied that during the aerobic phase, since the DO level was low (below 0.7 mg L¡1), there likely existed a simultaneous denitrification process that converted nitrate into nitrogen gas which left the reactor liquid. This phenomenon was consistent to the results reported by other workers,[24] and was termed as simultaneous nitrification and denitrification (SND). During the following anoxic/anaerobic phase, the NH4-N level increased with the second filling by 1.7 mg L immediately and then gradually by another 0.8 mg L¡1 due to the hydrolysis of organic nitrogen.[25] After that, the NH4-N level stayed at about 3 mg L¡1 during the entire phase probably due to the termination of hydrolysis. In the meantime, denitrification was completed within this phase as the nitrite/nitrate produced during the aerobic period was denitrified to nitrogen gas that emitted from the liquid with little oxidized nitrogen left at the end.

Fig. 4. Nitrogen profiles for Run 5.

401

The aerobic reaction pattern repeated for the second aerobic phase, but at a much lower level as a result of the smaller influent volume. The SND process converted the majority of NH4-N into nitrogen gas with only 0.2 mg L¡1 nitrate left in the liquid at the conclusion of this stage, indicating that about 2.8 mg L¡1 of nitrogen was eliminated by SND. Little change in NH4-N but slight denitrification was observed during the anoxic settle and draw phase in which the activated sludge settled quickly and the supernatant was clear and ready to decant. The (AO)2 process in this study successfully removed almost all the input nitrogen, with the effluent having 0.55 mg L¡1 of NH4-N and less than 0.1 mg L¡1 of nitrate in it.

Influence of aeration time allocation on nutrients removal in the SBR The anaerobic mixing and aerating periods in the SBR system refer to the mass fractions as do the volume fractions in continuous flow systems. Incomplete nitrification usually occurs during short aeration-time cycles, while incomplete denitrification happens during long aeration-time cycles. Processes of phosphate release during the mixing period and phosphorus uptake during the aerobic period are also found to be influenced by the time fractions.[13] Therefore, the optimal time fraction desired in this study is the highest mixing to total reaction time ratio for energy saving, while still assuring successful nitrification and phosphorus uptake. In Figure 5, the failure of complete phosphorus removal under the HRT of 2.1 days could be attributed to the lack of carbon,[23] and lack of sufficient aeration time could be another factor as well. As seen in Figure 5, the uptake of phosphorus in all regimes was not complete, with Regime 3 apparently lower than Regime 1 and 2, probably because it had a shorter aeration phase, only 2 h and 15 min total in the 8-hour cycle. In contrast, under HRT of 2.7 days (Fig. 6), all three regimes achieved complete phosphorus uptake during a typical cycle, indicating that the unsuccessful

Fig. 5. Dissolved phosphorus change processes during different time regimes at HRT D 2.1 days.

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402

Fig. 6. Dissolved phosphorus change processes during different time regimes at HRT D 2.7 days.

phosphorus removal happened for all three time regimes at HRT D 2.1 days were most likely caused by the inadequate release of phosphorus associated with a lack of sufficient carbon. Therefore, it can be inferred that a proper HRT, which can promote phosphorus release by providing more COD, is more critical than the time allocations for anaerobic/anoxic mixing and aeration in phosphorus and COD removals. This is supported by the fact that for the HRT of 2.7 days, even the largest TM/TP of 0.68 worked well to have achieved virtually complete dissolved phosphorus removal (Fig. 6). It appears that the length of the anoxic phase did not contribute much to the nitrogen removal for the wastewater in this study because denitrification was usually finished at the beginning of the anaerobic phase, and the nitrate to be denitrified was very limited due to the low nitrogen level. The impact of different mixing and process time regimes (TM/TP D 0.43 for Regime 1, TM/TP D 0.57 for Regime 2, and TM/TP D 0.68 for Regime 3) on nitrogen removal is presented in Figure 7, showing the change of ammonium nitrogen in 8-h cycles at a 2.7-day HRT. During the first aerobic phase for Regime 1 and 2, the NH4CN level was reduced to near zero, indicating sufficient aeration time for nitrification, while for Regime 3 which had a short aerobic phase and high initial NH4C-N concentration in the reactor, there was still 2.5 mg L¡1 left at the end of the aerobic phase, and the NH4-N concentration in the effluent was also high compared to the other two regimes, mainly due to insufficient aerobic react time. Hence TM/TP ratio in Regime 3 was considered too high for HRT D 2.7 days, and Regime 2 with TM/TP of 0.57 was found to be the optimal among the three, which could reduce energy consumption by 12.5% as compared to Regime 1 without losing nitrogen removal efficiency. Again, the combination of HRT D 2.7 days and TM/TP D 0.57 was proved to be the optimal operating condition for the ((AO)2) SBR system for simultaneously removing N and P from the particular milking parlor wastewater examined in this study.

Wu and Zhu

Fig. 7. Change of ammonium nitrogen in cycles with different regimes at HRT of 2.7 days.

Characteristics of ORP and pH profile during a cycle in SBR Figure 8 presents the typical profiles of on-line monitoring values of oxidation-reduction potential (ORP), pH, dissolved oxygen (DO) in a steady state cycle of (AO)2 SBR in this study. Although these observations are somewhat similar to the observations reported by many previous workers using SBR technology to treat either synthetic or swine wastewater,[6, 26, 27] some features of the ORP and pH curves that were not observed in others’ work were obtained from this research. First, the ORP and pH curves showed almost exactly the same rise-and-fall pattern, clearly implying that there existed a good correlation between these two parameters. This finding was not reported by past researchers,[6, 9] which could be due to the different substrates used between previous research

Fig. 8. Redox potential (ORP), pH and DO profiles during a typical cycle in the anaerobic-aerobic-anoxic-aerobic SBR.

403

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SBR used to remove nutrients from milking parlor wastewater (synthetic and swine wastewater) and this project (milking parlor wastewater). However, it could also be caused by inaccurate on-line recording of pH changes over the course of reaction. For instance, in one of the previous reports,[9] it was found that pH increased significantly during the anoxic phase, when it was supposed to be either flat or slightly decline under that condition.[28] Second, a sharp drop of pH was observed at the beginning of the cycle mainly due to the feeding of influent with pH around 7 which was lower than that in the SBR. The pH decreased continuously but at a slower pace because of phosphorus release, and was supposed to stop when the phosphorus release ended.[9] However, this was not observed in this cycle, indicating that phosphorus release might not be completed at the end of the anaerobic phase (implying that the anaerobic phase could be a little longer to achieve better phosphorus removal results).[6, 9, 29] Third, the ORP curve displayed a sharp decrease at the beginning of the anaerobic phase due to influent feeding that reduced the redox potential by increasing the ratio of reduced materials to oxidized materials in the reactor. The sudden increase of pH occurred at the beginning of aerobic phase was mainly due to the stripping of CO2 out of the system.[30] This finding might shed light on the selection of a more appropriate air distribution method or speed to reduce its impact on the liquid pH. Finally, the DO showed a gradual increase when air pump was turned on at both aeration phases, and the DO curves rose and fell also in tandem with both the ORP and pH curves, indicating that there was a correlation between DO and ORP and pH during the aerobic phase, which was different than the results obtained by Lee et al.,[9] and could provide useful information in SBR automation using real-time pH or ORP data, but not both.

of HC. The term RT/nF is a constant when the reaction is carried out at constant temperature, and the term [HC] could be represented by pH. Based on this, Eq. 2 can further evolve into Eq. 3, where “a” is a constant and “b” is the slope of the regression line developed by the ORP vs. pH values. Up to this point, a linear relationship between pH and ORP can be obtained. E D E0 ¡ . ¡ RT=nF/ln[H C ]b

(2)

ORP D a ¡ b £ pH

(3)

To verify Eq. 3, the results of ORP vs. pH during the two anaerobic/anoxic phases after feeding in a cycle were shown in Figure 9 (a and b). Good linear correlations were observed for both anoxic phases with the correlation coefficients of 0.975 and 0.995, respectively, which confirmed the validity of Eq. 3 developed above. Interestingly, such relationships for ORP vs. pH were not observed based on data collected for the two aerobic phases, which showed non-linear correlations (data not shown). Therefore, it can be concluded that the linear relationship

Development of relationship between pH and ORP over the anoxic phases in a cycle Due to the close relationship observed between pH and ORP, it would be interesting to further examine their correlations. According to general chemistry, any chemical reactions can be described using Eq. 1, where A and B are the reactants, and C and D are the products; a, b, c, and d are constants aA C bB ! cC C dD

(1)

At the beginning of each anoxic phase without aeration, the influent with a pH value lower than in the SBR mixed liquor was pumped into the reactor, and hydrolysis occurred in the reactants, including HC generation, leading to ORP changes. As such, Eq. 1 could be written as Eq. 2, where the difference of ORP value between time t (E) and time zero (E0) is correlated with the molar concentration

Fig. 9. Relationship between pH and ORP values in a cycle, (a) in the first non-aerated phase; (b) in the second non-aerated phase.

404 between pH and ORP values exists only during the hydrolysis phase without aeration. This finding has not been reported by past researchers.

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Development of relationship between ORP and DO during aerobic phases One last thing that is interesting to investigate is the relationship between ORP and dissolved oxygen (DO) during the aerobic phase because the importance of DO level in determining ORP in activated sludge during aerobic phases was highly recognized.[31] Heduit and Thevenot[32] stated that the platinum electrode potential at equilibrium (Eh) and the DO concentration would obey a form: Eh D a x log [O2] C b, at pH D 7, and the constants, a and b, mainly depended on the sludge loading, the aeration conditions and the sludge concentration. Following this line, in this study the ORP and logarithm of DO values in the two aerobic phases were plotted to verify the linear relationship. The results were shown in Figure 10. It can be seen that the ORP correlated well with log [O2] in a linear manner in both aerobic phases, with high Rs of 0.9931 and 0.9849, respectively. Also noticed is that the

Wu and Zhu two lines almost passed the origin, with very small intercepts. Since the slope, a, was influenced by the sludge loading under steady aeration condition and stable sludge concentration during the same cycle, it thus could be inferred that the constant, a, would increase with increasing influent loading rate, which was supported by the fact that a much larger loading in the first anaerobic phase than in the second was used (about three times) in this study. This information may help develop real-time control of aeration device to provide correct, but sufficient, aeration to the SBR system according to the loading rate change without wasting energy.

Conclusions An SBR system with two anaerobic/anoxic-aerobic alternation (AO)2 processes was established that could successfully treat milking parlor wastewater with simultaneous nutrients removal. The possibility of removing almost 100% of the phosphorus and the majority of input nitrogen (0.14 mg L¡1 DP, 0.5 mg L¡1 NH4-N, and < 0.1 mg L¡1 nitrate in the effluent) in milking parlor wastewater by the SBR system studied was demonstrated without addition of any chemical reagents. It was also observed that HRT had a major impact on the initial SCOD:DP ratio in a cycle, which was critical for the phosphorus release process under the anaerobic phase and determined the phosphorus uptake rate under the aerobic phase. For an influent with 2000 mg L¡1 SCOD and an initial SCOD:DP ratio of 5.5, the optimal HRT among the three tested was found to be 2.7 days, which lead to nearly complete phosphorus removal in this study. The time allocation for anaerobic/ aerobic phases was important for nitrification, but was not significantly influencing the phosphorus removal process. Good correlations between pH and ORP, and ORP and DO, were also obtained with correlation coefficients all higher than or equal to 0.975. These relationships could be used to develop real-time control strategies to optimize the duration of each operating phase in the (AO)2 SBR system to save energy and enhance treatment efficiency.

References

Fig. 10. Relationship between log [O2] and ORP values in a cycle, (a) during the first aerobic phase; (b) during the second aerobic phase.

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Simultaneous removal of nutrients from milking parlor wastewater using an AO2 sequencing batch reactor (SBR) system.

The feasibility of using a lab-scale, anaerobic-aerobic-anoxic-aerobic sequencing batch reactor ((AO)2 SBR) to simultaneously remove biological organi...
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