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oxic process for organic and nutrient removal
Chinese Journal of Chemical Engineering 24 (2016) 394–403
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Chinese Journal of Chemical Engineering journal homepage: www.elsevier.com/locate/CJChE
Biotechnology and Bioengineering
Performance evaluation of a modified step-feed anaerobic/anoxic/oxic process for organic and nutrient removal A.R. Majdi Nasab, S.M. Soleymani, M. Nosrati ⁎, S.M. Mousavi Biotechnology Group, Faculty of Chemical Engineering, Tarbiat Modares University, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 4 July 2014 Received in revised form 26 September 2014 Accepted 20 November 2014 Available online 29 October 2015 Keywords: Biological nutrient removal Step-feed bioreactor Nitrification Denitrification Phosphorus removal
a b s t r a c t A pilot scale modified step-feed process was improved to increase nutrient (N and P) and organic removal operations from municipal wastewater. It combined the step-feed process and a method named “University of Cape Town (UCT)”. The effect of nutrient ratios and inflow distribution ratios were studied. The highest uptake efficiency of 95% for chemical oxygen demand (COD) has been achieved at the inflow distribution ratio of 40/35/25. However, maximum removal efficiency obtained for total nitrogen (TN) and phosphorus at 93% and 78%, respectively. The average mixed liquor suspended solids (MLSS) was 5500 mg·L− 1. In addition, convenient values for dissolved oxygen (DO) concentration, and pH were obtained throughout different stages. The proposed system was identified to be an appropriate enhanced biological nutrient removal process for wastewater treatment plants owing to relatively high nutrient removal, sturdy sludge settle ability and COD removal. © 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
1. Introduction More and more progressive wastewater treatment technologies are needed to protect the limited water sources owing to increasing the eutrophication and urbanization of receiving waters, especially slow mobile rivers and lakes [1]. Organic and surplus nutrients in wastewater, especially nitrogen and phosphorus, must be widely removed prior to discharging into natural bodies of water [2,3]. In comparison with physical-chemical procedures, biological methods are recommended to treat municipal wastewater in terms of simultaneous removal of phosphorus and nitrogen, as well as cost and energy savings [4–6]. However, usual biological nutrient removal (BNR) processes have often become inoperative in simultaneously maximizing nutrient and organic removal, as heterotrophic bacteria, nitrifying autotrophic bacteria and phosphorus accumulating organisms (PAOs) compete with each other for growth and maintenance under the same operational circumstances [7–9]. To date, several biological processes related to activated sludge have been generally adopted in wastewater treatment plants (WWTP), such as anaerobic/anoxic/oxic (A/A/O), pre-denitrification (A/O) and the “University of Cape Town” (UCT) method [2,10]. However, modifications in such biological nutrient removal activated sludge (BNRAS) systems have demonstrated that step-feeding is a considerable process in removing internal recycling and sludge recirculation, as well as to optimize organic carbon for denitrification [11,12]. These ⁎ Corresponding author. E-mail address: [email protected] (M. Nosrati).
processes display relatively good nutrient removal efficiencies in treating wastewater with adequate carbon sources or high chemical oxygen demand to total nitrogen ratio (COD/TN) [13,14]. However, phosphorus and nitrogen removal efficiency would be deteriorated if influent COD concentrations are inadequate for releasing phosphorus or denitrification [15–17]. The modified UCT process is qualified to sustain a fully anaerobic environment due to increasing the internal recycle subsystem to minimize the inhibitory influence of nitrate to the reaction of anaerobic phosphorous released by PAOs [9,18]. The process is also beneficial to the attainment of anoxic denitrifying phosphorus removal; thereby enhanced phosphorus removal could be achieved with a significant devaluation in the total necessity of organic substrates [19,20]. A modified step-feed process, which combines the benefits of stepfeed A/O process and UCT process together, is proposed for increasing both phosphorus and nitrogen removal efficiency in this study. This process consists of an anaerobic selector zone followed by three uniform pairs of anoxic and aerobic zones with the elimination of a preliminary sedimentation tank. As performance and operation of the modified step-feed process in wastewater treatment plants, including nutrients removal, are much more complicated compared to those of conventional treatments, this study contributes to a) investigating the sufficiency of modified step-feed treatment in COD, nitrogen and phosphorous removal through given volume ratios of anaerobic (1.35), anoxic (1) and aerobic (1.50) compartments and, b) finding quantitative relations between inflow distribution ratios in anoxic zones and COD and nitrogen contents in the outlet stream.
http://dx.doi.org/10.1016/j.cjche.2015.10.010 1004-9541/© 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
A.R. Majdi Nasab et al. / Chinese Journal of Chemical Engineering 24 (2016) 394–403
2. Materials and Methods 2.1. The pilot-plant step-feeding system A lab-scaled step-feed reactor consisting of seven segments in sequence – each made of colorless Plexiglass – was used in this study as illustrated in Fig. 1. The reactor was constructed with a working volume of 22.32 L comprising an anaerobic selector (3.42 L) followed by a series of three uniform pairs of aerobic regions (3.78 L) and anoxic regions (2.52 L). The volume ratio of anaerobic/anoxic/oxic was 1.35:1:1.5. An air compressor was utilized to supply the required oxygen which provided sufficient gas–liquid mixing in all aerobic regions, whereas appropriate liquid mixing was obtained in anaerobic and anoxic regions by using four mixers in anaerobic and anoxic zones, respectively. The influent flow was divided into three streams prior to entering the anaerobic region, the second anoxic region and the third anoxic region. Those streams that fed into the beginning (anaerobic zone) remain in the reactor for longer and consequently have more time to achieve better treatment (COD removal); whereas, those
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streams that enter in the middle, or at the last anoxic zone, help denitrification through carbon source supply (nitrate removal). According to this fact, three distribution ratios including 60/25/15, 40/35/25 and 25/35/40 were examined in order to investigate their effect on phosphorous, nitrogen, and COD removal in the outlet stream. In order to achieve stable performance, feeding to the reactor was carried out for one week after alternating each of the inflow distribution ratios. Therefore, steady state conditions were obtained after one week feeding the reactor by the new inflow distribution ratio. All the inflow rates were regulated by altering the rotational speeds of the peristaltic pumps. Two used influent flow rates, 1.44 L·h−1 and 1.80 L·h− 1, resulted in hydraulic retention times (HRT) of 16 h and 12 h, respectively. The sludge was recycled from the clarifier to the first anoxic region in order to decrease the concentration of nitrate that existed in the returned sludge. In the aerobic regions, air was conveyed by bubble diffusers installed at the bottom of the reactor resulting in appropriate mixing in the aerobic zones. The temperature was maintained at room temperature (24–28 °C). The resulting mixed liquor
1. anaerobic zone
10. rotor
2. first anoxic zone
11. thermometer
3. first aerobic zone
12. DO probe
4. second anoxic zone
13. air diffuser
5. second aerobic zone
14. air compressor
6. third anoxic zone
15. clarifier
7. third aerobic zone
16. recycle pump
8. wastewater reservoir
17. controller
9. flow meter
18. internal recycle
Fig. 1. Schematic representation of the experimental set up.
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suspended solids (MLSS) was in the range of 5000–5500 mg·L−1, and mixed liquor volatile suspended solids (MLVSS) concentration was 3200 mg·L−1. In the aerobic regions, DO values were maintained between 2 and 2.5 mg·L−1, while DO values were maintained at about 0.25 mg·L−1 and 0.5 mg·L−1 for anaerobic and anoxic regions, respectively. The sludge recycle ratio was held constant at 60%. The experimental runs comprised total inflow rates, inflow distribution ratios 3− and HRTs, as well as COD, BOD, NH+ 4 -N and PO4 -P loading rates, are summarized in Table 1.
60/25/15 in both COD concentrations of 300 and 500 mg·L− 1. Therefore, a remarkable variation was observed in COD and NH+ 4 concentrations through one to nine runs while the influent COD varied from 500 to 300 mg·L− 1 (Figs. 2(a) and 6(a)). Furthermore, most of the influent COD concentration was consumed in the anoxic or anaerobic regions. This was considered as an advantageous condition for nitrification due to the observed residual COD inhibitory effect elimination. The effluent COD concentration increased while total influent flow rose from 1.44 L·h− 1 to 1.80 L·h− 1 in all influent distribution ratios, and this event was more visible in the inflow distribution ratio of 25/35/40 (Fig. 6(b)). As shown in Fig. 2(b), NH+ 4 concentration fluctuated between 2.24 and 9.20 mg·L− 1 during three different distribution ratios, while total influent flow was 1.80 L·h− 1. The average NH+ 4 removal rate was 85%, with a maximum of 95%. The result was in accordance with the research by Ge et al. [6], who remarked that the ratio for the first stage of the modified step-feed process over 50% would lead to negative effects on nitrification and nitrogen removal. In addition, exceeding the influent may bring about supplementary carbon sources being oxidized in the following oxic zones if it is not fully consumed in the anaerobic and anoxic segments. Therewith, reduction in the existing carbon sources available in the second and third anoxic zones resulted in an enhancement in effluent TN owing to a lack of necessary organics required for heterotrophic denitrifying nitrogen removal. Moreover, there was a relatively positive relationship between inflow to anaerobic, second and third anoxic segments and anaerobic phosphorus release. Anaerobic phosphorus release could be negatively impressed by the availability of excess nitrate and insufficient influent organic compounds. Thus, another progressive strategy was obtained by incorporating the step-feed process and the modified UCT biological nutrient removal method together to prevent the detrimental effects of nitrate existence on phosphorus release. In the modified step-feed type, a part of the excess nitrate was reduced to nitrogen gas by the initial movement of sludge returned mixed liquor between the clarifier unit and the first anoxic segment. Moreover, the minor current of the mentioned mixed liquors with less nitrate concentrations was recycled to the anaerobic segment, at which point slightly more anaerobic conditions were prepared due to phosphorus release. As a result, the enhancing phosphorus release and removal efficiency were obtained with less nitrate concentrations in the anaerobic region as had been expected. The PO3− removal efficiency 4 ranged significantly between 39% and 79% with an average value of 62% during the whole experimental period (Fig. 5(a)). A recent research remarked the magnitude of desirable operating conditions and influent concentrations for the processes coupled with successful phosphorus removal [22]. As a result, higher influent ratios to the anaerobic and anoxic regions represented that more carbon sources would be accessible in anaerobic and anoxic segments where high carbon sources for denitrification and anaerobic phosphorus release have been prepared. Generally, higher
2.2. Seed sludge and wastewater characteristics The seed sludge was taken from the aerobic region of the full scale reactor of the Tehran South WWTP, where biological nitrogen and chemical phosphorus removal were obtained, respectively. The reactor in Tehran South WWTP (450000 m3·d−1) is a Carrousel biological nutrient removal and had been under operation for about four years. After a one-month cultivation period of the activated sludge and its adaptation to the synthetic wastewater, a permanent denitrifying operation was obtained and the process performance was stabilized. Two kinds of synthetic wastewater with the concentration and characteristics of real domestic wastewater were fed to the reactor: first, a normal wastewater with total COD of 300 mg·L−1 and, second, a thickened wastewater with total COD of 500 mg·L−1. 2.3. Analytical methods The pH and temperature were found using WTW level-2 pH meters (WTW Company, Germany). Further, DO values were detected by WTW, pH/oxi340i meters with fully equipped DO sensors and calibra3– – tion facilities. COD, BOD, NH+ 4 , PO4 , NO3, TN, MLSS and volatile MLSS concentrations were analyzed according to standard methods [21]. COD solutions have been heated by a special stainless steel heater B75hls (WEL Company, Japan) for the analyses of COD concentration in each experimental period. 3. Results and Discussion 3.1. Effect of different inflow distribution ratios on the performance of nutrients removal Figs. 2–6 display the influent and effluent concentrations coupling – 3– with removal performances of NH+ 4 , NO3, TN, PO4 and COD during nine operational runs, respectively. The influent distribution ratio had a significant effect on the rate of COD and NH + 4 removal (Figs. 2(a) and 6(a)). It is obvious that by entering the third part of the influent distribution ratio to the last anoxic zone, nitrifiers became considerably active, so total nitrification was obtained during the whole experimental period in the influent distribution ratio of
Table 1 Operational process conditions, sludge loading rates (F/M), influent loading rates for organic, phosphorus and nitrogenous substrates in each run Run number
1 2 3 4 5 6 7 8 9
Q F/L·h−1
1.44 1.44 1.44 1.44 1.44 1.44 1.80 1.80 1.80
inflow distribution ratios AN/DN2/DN3/%
60/25/15 40/35/25 25/35/40 60/25/15 40/35/25 25/35/40 60/25/15 40/35/25 25/35/40
Loading/g·d−1
HRT/h
COD
BOD
NH+ 4 -N
PO3− 4 -P
10.36 10.36 10.36 17.28 17.28 17.28 21.6 21.6 21.6
6.21 6.21 6.21 10.36 10.36 10.36 12.96 12.96 12.96
0.518 0.518 0.518 0.864 0.864 0.864 1.08 1.08 1.08
0.103 0.103 0.103 0.172 0.172 0.172 0.216 0.216 0.216
16 16 16 16 16 12 12 12 12
Total influent/mg·L−1
Sludge loading rate (F/M)/BOD kg·(MLVSS kg)−1·d−1
COD
BOD
ANA
DN1
DN2
300 300 300 500 500 500 500 500 500
180 180 180 305 305 305 305 305 305
1.73 1.15 0.73 2.91 3.20 2.23 3.64 4.00 2.78
1.92 2.70 2.70 1.94 4.50 3.71 2.42 5.62 4.63
1.34 2.22 3.56 1.21 4.50 5.94 1.51 5.62 7.42
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Fig. 2. Effluent ammonium concentration and removal efficiency regarding (a) total influent COD concentration, and (b) total inflow rate.
influent ratios to the anaerobic segment would be advantageous to phosphorus uptake, whereas a ratio higher than a certain level results in negative effects on nitrogen elimination [6]. As a consequence, the optimum inflow distribution ratio would be desired to improve nutrient removals. The optimum inflow ratios should be regulated to make more COD to be consumed for phosphorus release during the anaerobic region and denitrification zones. Consequently, this event leads to a reduction in common intrusion of residual COD to nitrification and attempts to decrease the aeration consumption in aerobic regions. Larrea et al. [4] reported that the optimal influent distribution ratio to the three inlet regions of the reactor was in the range of 40/40/20 and 33/33/34, which frequently concerns effluent requirements and wastewater characteristics. Funamizu et al. [23] declared that the selection of the step-feed ratio alone was not enough to enhance nitrogen removal due to low denitrification rates in anoxic segments. Table 2 has classified and compared several previous researches that have been conducted on a variety of step-feed processes with different influent distribution ratios and configurations.
Fig. 4(a) and (b) illustrates the trend of total nitrogen (TN) concentration and removal efficiency. In this study, total nitrogen is the sum− mation of effluent NH+ 4 -N and NO3 -N concentrations during each run. The average effluent TN removal efficiency was 76.16% with the average effluent TN concentration of 4.34 mg·L−1 during all experiments. As the results indicated, a high TN removal rate leads to a high phosphorus removal rate. Table 3 has been prepared to better understand the figures. According to Fig. 4(b), TN removal efficiency decreased as the inflow to the last anoxic segment grew, both in total inflow of 1.44 and 1.8 L·h−1. Consequently, this event has occurred due to the availability of high carbon sources in pre-anoxic zones (particularly the last anoxic region), resulting in weak nitrification in the last aerobic segment and high effluent ammonium concentration, which led to high effluent TN. Most likely, low HRT may be another reason for the mentioned event due to the high level of influent in the last pre-anoxic zone in the last influent distribution ratio (25/35/40). Accordingly, more anoxic denitrification rates would be obtained at limited HRTs, which had clear a positive correlation with TN removal [24,25].
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Fig. 3. Effluent nitrate concentration regarding (a) total influent COD concentration, and (b) total inflow rate.
3.2. Organic and nutrient removal procedures 3.2.1. COD removal COD concentration and removal efficiency in different stages and under different operational situations are illustrated in Fig. 6. Results indicated that the influent COD distribution had a significant effect on denitrification capability and anaerobic phosphorus release. The residual COD trended to increase from 64 mg·L− 1 in the inflow rate of 1.44 L·h − 1 to 87 mg·L − 1 in the inflow rate of 1.8 L·h − 1 in the ratio of 25/35/40 according to Fig. 6(b). This event was established for two other ratios, although it was significantly observed during the ratio of 25/35/40 due to low HRT and a high ratio to the last pre-anoxic zone. Consequently, it caused different anaerobic phosphorus release capabilities and removal performances. Moreover, the COD utilization capacity for phosphorus and nitrogen removal varied with changing the inflow distribution ratios. Although in the inflow distribution ratio of 60/25/15 HRT was the maximum amount, the highest effluent COD removal efficiency was observed during the ratio of 40/35/25 (Fig. 6(a)). In addition, this maximum organic removal efficiency was simultaneously coupled with maximum nitrate removal efficiency due to the high anoxic denitrification rate. Accordingly, the results displayed that enhancement in COD removal efficiency directly related to the improvement in nitrate removal efficiency. In other words the more denitrification takes place, the more organics are eliminated owing to denitrifying phosphorus and nitrate removal. The performance of COD removal failed in the ratio of 25/35/40 owing to low HRT. Additionally, as expected the effluent COD concentration increased while the wastewater
inflow rate and concentration enhanced. This enhancement in effluent COD was more visible in the ratio of 25/35/40, which demonstrated that in two other distribution ratios the process was more capable to adapt itself to the wastewater inflow rate and concentration enhancement. 3.2.2. Phosphorus removal Fig. 5 illustrates effluent PO 34 − concentration and removal efficiency during different experimental periods under different operational conditions. Results displayed that anaerobic HRT had a distinct effect on anaerobic phosphorus release, based on observations during the experimental period. As expected, PAOs was capable of eliminating the degradable organic substrates to produce polyhydroxyalkanoates (PHA) with the reserved polyphosphate (poly-P) utilized as an energy source [26]. As a result, it could be concluded that the aerobic/anoxic phosphorus removal directly relies on the mechanisms of phosphorus release under anaerobic conditions in the first stage. In addition, sustaining an appropriate microorganism population in the anaerobic segment should not be neglected. In order to enrich the convenient population, more inflow organic concentrations are required. Wang et al. [27] reported that the highest anaerobic phosphorus release was 350% and 497% of the influent total phosphorus (TP) concentration, and achieved significantly high TP uptake. Maximum phosphorus removal efficiency was observed in the inflow distribution ratio of 60/25/15, which prepared the required organics according to Fig. 5(a). Therewith, significant phosphorus uptake was obtained in anoxic zones in the mentioned ratio, which might be
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399
Fig. 4. Effluent total nitrogen concentration and removal efficiency regarding (a) total influent COD concentration, and (b) total inflow rate.
attributed to denitrifying phosphorus removal. Afterwards, the majority of PO34 − was further eliminated under aerobic conditions in the oxic 1 segment, while the minor amount of PO34 − was removed in the oxic 2 and oxic 3 segments, respectively. Wachtmeister et al. [28] remarked that virtually 32.6%–39.5% of DNPAOs were responsible for PAOs imputed to favorable phosphorus uptake capacity under appropriate distribution ratios. Once in anaerobic zones sufficient accessible organic substrates were confected for PAOs releasing phosphorus and producing storable carbon source PHA, the anoxic/oxic phosphorus elimination process and denitrification would be obtained in the subsequent anoxic/oxic regions, as Kapagiannidis et al. [29] reported in their recent research. However, convenient phosphorus release would run against an adverse effect by the availability of nitrate in the recycled sludge stream, resulting in a significant weakness in the optimal anaerobic conditions required for PAOs to release phosphorus. Therefore, the returned sludge should be recycled to the first anoxic zone to reduce the amount of accessible nitrate, primarily, prior to entering the anaerobic zone. 3.2.3. Nitrogen removal − The alteration of NH+ 4 , NO3 and TN along the experimental period under different operational conditions is illustrated in Figs. 2, 3, and 4,
respectively. As results indicated, less influent NH+ 4 -N loads, adversely decayed nitrification qualification in oxic 1 and oxic 2. Meanwhile, the results confirmed that more NH+ 4 uptake was observed in the preanoxic regions, attributed to the dissolved oxygen transportation from aerobic segments resulting in partial oxidation of available carbon sources which were not conductive to TN elimination. Probably, this could be a logical reason for comparing the restricted TN uptake with 83% TN removal efficiency demonstrated in Vaiopoulou and Aivasidas (2008) in comparison with Ge et al. (2010) who demonstrated TN removal efficiency of 88%. Nitrification was fully accomplished during all steady-state experiments except in the low HRT runs. The average removal efficiency of NH+ 4 and TN were 82% and 78.6%, respectively, during the whole experimental period. Moreover, a possible advantageous parameter to desirable nitrogen removal in the step-feed process was the occurrence of simultaneous nitrification and denitrification (SND). To be more informed about the significance of the SND in TN removal, a summarized discussion should be held in order to make the subject more explicit. Generally, the total mass of nitrogen available in the influent entering the treatment process could be utilized according to the following fractions: (1) cell growth and synthesis attributed to the waste activated sludge; (2) proportion of nitrate to nitrite and TN in effluent; and
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Fig. 5. Effluent phosphate concentration and removal efficiency regarding (a) total influent COD concentration, and (b) total inflow rate.
(3) N2 gas released through denitrification process along anoxic or, probably, aerobic regions [30]. The anoxic denitrification sufficiencies play eminent roles in TN elimination, owing to the fact that the possibility of reducing NO− 2 -N and NO− 3 -N to nitrogen gas and other volatile nitrogen oxides solely exists in anoxic zones as Zhu et al. [2] and Fue et al. [31] reported in their recent studies. It could be concluded that as the inflow ratio to the anaerobic zone is enhanced, the required HRT for nitrification was established, whereas by enhancing the inflow ratio to the last anoxic region the HRT became lower and ammonium was left in the reactor prior to being converted into nitrate. Moreover, the total inflow rate enhancement had no distinct effect on effluent ammonium, illustrating that the bioreactor had the ability to eliminate more values of ammonium if total inflow was divided between the different stages, appropriately. If ammonium removal efficiency improvement is desired in the optimum inflow distribution ratio, alkalinity and dissolved oxygen concentration should be increased. In fact, the nitrification rate improves by increasing pH up to virtually eight, resulting in significant ammonium removal efficiency. Additionally, more dissolved oxygen concentrations and also more fluid mixing through aerobic segments provide more supplied oxygen
inside the sludge flocs, resulting in ammonium concentration even below 1 mg·L−1 in the effluent. According to Fig. 3, maximum nitrate removal efficiencies were obtained in the inflow ratio of 60/25/15 in comparison with the two other ratios. Low HRTs led to higher effluent nitrate concentration. In fact, nitrification required more retention time; however, by enhancing the total inflow rate this goal was inaccessible (Fig. 3(b)). 3.3. COD variations along different aerobic stages Variation of COD in all aerobic regions under different operational conditions was investigated in this study as shown in Fig. 7. As results indicated, COD removal mostly occurred in N1 which was similar in all inflow ratios; however, the differences in COD removal performance via aerobic zones among different inflow ratios were started upon entering the following aerobic zones after N1 . Maximum COD removal performance could be interpreted according to significant COD utilization via anaerobic phosphorus release and anoxic denitrification (in the first anoxic zone). Moreover, in the inflow ratio of 25/35/40, the major ratio of feed was entered into the last anoxic zone, resulting in low COD removal qualification in
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401
Fig. 6. Effluent COD concentration and removal efficiency regarding (a) total influent COD concentration, and (b) total inflow rate.
Table 2 Comparisons of configurations, operational parameters and performance of the step-feed process Item Configuration Primary sedimentation Anaerobic unit Inflow/L · d−1 Bioreactor volume/L Biological operation mode Country Operational parameters Inflow type Inflow distribution ratio/% Sludge recycle ratio/% Stages DO/mg·L−1 SRT/d HRT/d Removal efficiency/% COD NH+ 4 -N TN TP Effluent quality/mg·L−1 COD NH+ 4 -N TN TP
Viaopoulou and Aivasidis [9]
Wang et al. [27]
Ge et al. [6]
Liang et al. [32]
Wang et al. [33]
Cao et al. [34]
This study
No Yes 96 44 Activated sludge
No Yes 960 320 Activated sludge
No Yes 1020 340 Activated sludge
No No 100 42 Biofilm
No Yes 185 67 Activated sludge
No Yes 22.32 33.6 & 43.2 Activated sludge
Greece
China
China
China
No No 60 20 Activated sludge and fluidized bed China
China
Iran
Real 60/25/15 100 3 N2.0 10 8.9
Real 45/35/20 60 3 2.2–4.2 10 8
Real 40/30/30 75 3 1.5–2.0 10 8
Real 42/33/25 0 3 2.0–3.5 80 10
Synthetic 33/33/17/17 50 4 1.0–1.5 No date 8
Real 20/35/35/10 75 4 1.0–1.5 10–15 8.7
Synthetic 60/25/15 60 3 0.25–2.55 No date 13.8
94 95 83 93
86.6 98 73.61 89.83
89 99 88 93
80 96 No data No data
88.2 95.7 86.4 No data
78.9 98.31 70.24 86.11
95 95 93 78
30 1.7 6.9 0.2
35.74 1.22 16.31 0.63
32.7 0.14 6.61 0.43
b50 b8 No data No data
b50 b8 b10 No data
33.05 0.58 9.26 0.46
26 1.7 1.8 1.8
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Table 3 A guideline for making the figures more obvious Figure
Remark
Influent COD/mg·L−1
Q /L·h−1
Influent COD/mg·L−1
Q /L·h−1
Effluent concentration and removal efficiency of …
2-A 2-B 3-A 3-B 4-A 4-B 5-A 5-B 6-A 6-B
Same flow rate Same COD Same flow rate Same COD Same flow rate Same COD Same flow rate Same COD Same flow rate Same COD
300 500 300 500 300 500 300 500 300 500
1.44 1.44 1.44 1.44 1.44 1.44 1.44 1.44 1.44 1.44
500 500 500 500 500 500 500 500 500 500
1.44 1.80 1.44 1.80 1.44 1.80 1.44 1.80 1.44 1.80
NH+ 4 NH+ 4 – NO3 – NO3 TN TN PO3– 4 PO43– COD COD
Note: All experiments performed in three inflow distribution ratios.
Fig. 7. COD values during various aerobic stages under different operational conditions: INF — influent wastewater; N1 — the first oxic zone; N2 — the second oxic zone; N3 — the third oxic zone.
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N3 output with the mean concentration value of 88 mg·L− 1 in all total influent COD values, according to Fig. 7. The mean COD values throughout aerobic zones during the three inflow ratios were 433.3, 131.1, 104.43 and 55.55 mg·L−1 for influent wastewater, N1, N2 and N3, respectively. According to the results, optimum inflow ratio for favorite COD removal with high efficiency was 40/35/25, which is exactly visible in Fig. 6 as well. However, total COD concentrations and removal efficiencies during the aerobic regions significantly relied on DO concentrations, which were completely discussed in the previous section. 4. Conclusions Compared to other BNR processes, the modified step-feed process was found to be an attractive choice for urban wastewater treatment. Maximum organics and NO − 3 removal were achieved in the ratio of 40/35/25 with the optimum COD removal efficiency of 95%, whereas maximum PO34 −-P, TN and NH + 4 removal were obtained in the ratio of 60/25/15 with the optimum efficiency of 78%, 93% and 95%, respectively. pH attained convenient values, which were suitable for P-release, denitrification and nitrification during the experimental period. According to the optimum operational conditions, the process was considerably advantageous for both anaerobic P-release and anoxic/aerobic P-uptake, and TN removal efficiency could be significantly improved via the nitrification and denitrification processes. Acknowledgments The authors would like to acknowledge Mahab Ghodss Company for their extensive cooperation. References [1] Eddy Metcalf, Wastewater engineering: Treatment and reuse, McGraw Hill, New York City, USA, 2003. [2] G.B. Zhu, Y.Z. Peng, B.K. Li, J.H. Guo, Q. Yang, S.Y. Wang, Biological removal of nitrogen from wastewater, Rev. Environ. Contam. Toxicol. 192 (2008) 159–195. [3] H.M. Zhang, X.L. Wang, J.N. Xiao, F.L. Yang, J. Zhang, Enhanced biological nutrients removal using MUCT-MBR, Bioresour. Technol. 100 (2009) 1048–1054. [4] L. Larrea, A. Larrea, E. Ayesa, J.C. Rodrigo, M.D. Lopez-Carrasco, J.A. Cortacans, Development and vertification of design and operation criteria for the step feed process with nitrogen removal, Water Sci. Technol. 43 (2001) 261–268. [5] G.H. Cao, Y. Huang, Y. Pan, The problems and the solutions of conventional biological nitrogen and phosphorus processes, Technol. Water Treat. 25 (2009) 102–106. [6] S.J. Ge, Y.Z. Peng, S.Y. Wang, J.H. Guo, B. Ma, L. Zhang, X. Cao, Enhanced nutrients removal in a modified step feeding process treating municipal wastewater with different inflow distribution ratios and nutrients ratios, Bioresour. Technol. 101 (2010) 9012–9019. [7] J.A. Baeza, D. Gabriel, J. Lafuente, Improving the nitrogen removal efficiency of an A2/O based WWTP by using an on-line Knowledge Based Expert System, Water Res. 36 (2002) 2109–2123. [8] E. Viopoulou, P. Melidis, A. Aivasidis, An activated sludge treatment plant for integrated removal of carbon, nitrogen and phosphorus, Desalination 211 (2007) 192–199. [9] E. Viopoulou, A. Aivasidis, A modified UCT method for biological nutrients removal: Configuration and performance, Chemosphere 72 (2008) 1062–1068.
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Contents lists available at ScienceDirect
Chinese Journal of Chemical Engineering journal homepage: www.elsevier.com/locate/CJChE
Biotechnology and Bioengineering
Performance evaluation of a modified step-feed anaerobic/anoxic/oxic process for organic and nutrient removal A.R. Majdi Nasab, S.M. Soleymani, M. Nosrati ⁎, S.M. Mousavi Biotechnology Group, Faculty of Chemical Engineering, Tarbiat Modares University, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 4 July 2014 Received in revised form 26 September 2014 Accepted 20 November 2014 Available online 29 October 2015 Keywords: Biological nutrient removal Step-feed bioreactor Nitrification Denitrification Phosphorus removal
a b s t r a c t A pilot scale modified step-feed process was improved to increase nutrient (N and P) and organic removal operations from municipal wastewater. It combined the step-feed process and a method named “University of Cape Town (UCT)”. The effect of nutrient ratios and inflow distribution ratios were studied. The highest uptake efficiency of 95% for chemical oxygen demand (COD) has been achieved at the inflow distribution ratio of 40/35/25. However, maximum removal efficiency obtained for total nitrogen (TN) and phosphorus at 93% and 78%, respectively. The average mixed liquor suspended solids (MLSS) was 5500 mg·L− 1. In addition, convenient values for dissolved oxygen (DO) concentration, and pH were obtained throughout different stages. The proposed system was identified to be an appropriate enhanced biological nutrient removal process for wastewater treatment plants owing to relatively high nutrient removal, sturdy sludge settle ability and COD removal. © 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
1. Introduction More and more progressive wastewater treatment technologies are needed to protect the limited water sources owing to increasing the eutrophication and urbanization of receiving waters, especially slow mobile rivers and lakes [1]. Organic and surplus nutrients in wastewater, especially nitrogen and phosphorus, must be widely removed prior to discharging into natural bodies of water [2,3]. In comparison with physical-chemical procedures, biological methods are recommended to treat municipal wastewater in terms of simultaneous removal of phosphorus and nitrogen, as well as cost and energy savings [4–6]. However, usual biological nutrient removal (BNR) processes have often become inoperative in simultaneously maximizing nutrient and organic removal, as heterotrophic bacteria, nitrifying autotrophic bacteria and phosphorus accumulating organisms (PAOs) compete with each other for growth and maintenance under the same operational circumstances [7–9]. To date, several biological processes related to activated sludge have been generally adopted in wastewater treatment plants (WWTP), such as anaerobic/anoxic/oxic (A/A/O), pre-denitrification (A/O) and the “University of Cape Town” (UCT) method [2,10]. However, modifications in such biological nutrient removal activated sludge (BNRAS) systems have demonstrated that step-feeding is a considerable process in removing internal recycling and sludge recirculation, as well as to optimize organic carbon for denitrification [11,12]. These ⁎ Corresponding author. E-mail address: [email protected] (M. Nosrati).
processes display relatively good nutrient removal efficiencies in treating wastewater with adequate carbon sources or high chemical oxygen demand to total nitrogen ratio (COD/TN) [13,14]. However, phosphorus and nitrogen removal efficiency would be deteriorated if influent COD concentrations are inadequate for releasing phosphorus or denitrification [15–17]. The modified UCT process is qualified to sustain a fully anaerobic environment due to increasing the internal recycle subsystem to minimize the inhibitory influence of nitrate to the reaction of anaerobic phosphorous released by PAOs [9,18]. The process is also beneficial to the attainment of anoxic denitrifying phosphorus removal; thereby enhanced phosphorus removal could be achieved with a significant devaluation in the total necessity of organic substrates [19,20]. A modified step-feed process, which combines the benefits of stepfeed A/O process and UCT process together, is proposed for increasing both phosphorus and nitrogen removal efficiency in this study. This process consists of an anaerobic selector zone followed by three uniform pairs of anoxic and aerobic zones with the elimination of a preliminary sedimentation tank. As performance and operation of the modified step-feed process in wastewater treatment plants, including nutrients removal, are much more complicated compared to those of conventional treatments, this study contributes to a) investigating the sufficiency of modified step-feed treatment in COD, nitrogen and phosphorous removal through given volume ratios of anaerobic (1.35), anoxic (1) and aerobic (1.50) compartments and, b) finding quantitative relations between inflow distribution ratios in anoxic zones and COD and nitrogen contents in the outlet stream.
http://dx.doi.org/10.1016/j.cjche.2015.10.010 1004-9541/© 2015 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
A.R. Majdi Nasab et al. / Chinese Journal of Chemical Engineering 24 (2016) 394–403
2. Materials and Methods 2.1. The pilot-plant step-feeding system A lab-scaled step-feed reactor consisting of seven segments in sequence – each made of colorless Plexiglass – was used in this study as illustrated in Fig. 1. The reactor was constructed with a working volume of 22.32 L comprising an anaerobic selector (3.42 L) followed by a series of three uniform pairs of aerobic regions (3.78 L) and anoxic regions (2.52 L). The volume ratio of anaerobic/anoxic/oxic was 1.35:1:1.5. An air compressor was utilized to supply the required oxygen which provided sufficient gas–liquid mixing in all aerobic regions, whereas appropriate liquid mixing was obtained in anaerobic and anoxic regions by using four mixers in anaerobic and anoxic zones, respectively. The influent flow was divided into three streams prior to entering the anaerobic region, the second anoxic region and the third anoxic region. Those streams that fed into the beginning (anaerobic zone) remain in the reactor for longer and consequently have more time to achieve better treatment (COD removal); whereas, those
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streams that enter in the middle, or at the last anoxic zone, help denitrification through carbon source supply (nitrate removal). According to this fact, three distribution ratios including 60/25/15, 40/35/25 and 25/35/40 were examined in order to investigate their effect on phosphorous, nitrogen, and COD removal in the outlet stream. In order to achieve stable performance, feeding to the reactor was carried out for one week after alternating each of the inflow distribution ratios. Therefore, steady state conditions were obtained after one week feeding the reactor by the new inflow distribution ratio. All the inflow rates were regulated by altering the rotational speeds of the peristaltic pumps. Two used influent flow rates, 1.44 L·h−1 and 1.80 L·h− 1, resulted in hydraulic retention times (HRT) of 16 h and 12 h, respectively. The sludge was recycled from the clarifier to the first anoxic region in order to decrease the concentration of nitrate that existed in the returned sludge. In the aerobic regions, air was conveyed by bubble diffusers installed at the bottom of the reactor resulting in appropriate mixing in the aerobic zones. The temperature was maintained at room temperature (24–28 °C). The resulting mixed liquor
1. anaerobic zone
10. rotor
2. first anoxic zone
11. thermometer
3. first aerobic zone
12. DO probe
4. second anoxic zone
13. air diffuser
5. second aerobic zone
14. air compressor
6. third anoxic zone
15. clarifier
7. third aerobic zone
16. recycle pump
8. wastewater reservoir
17. controller
9. flow meter
18. internal recycle
Fig. 1. Schematic representation of the experimental set up.
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suspended solids (MLSS) was in the range of 5000–5500 mg·L−1, and mixed liquor volatile suspended solids (MLVSS) concentration was 3200 mg·L−1. In the aerobic regions, DO values were maintained between 2 and 2.5 mg·L−1, while DO values were maintained at about 0.25 mg·L−1 and 0.5 mg·L−1 for anaerobic and anoxic regions, respectively. The sludge recycle ratio was held constant at 60%. The experimental runs comprised total inflow rates, inflow distribution ratios 3− and HRTs, as well as COD, BOD, NH+ 4 -N and PO4 -P loading rates, are summarized in Table 1.
60/25/15 in both COD concentrations of 300 and 500 mg·L− 1. Therefore, a remarkable variation was observed in COD and NH+ 4 concentrations through one to nine runs while the influent COD varied from 500 to 300 mg·L− 1 (Figs. 2(a) and 6(a)). Furthermore, most of the influent COD concentration was consumed in the anoxic or anaerobic regions. This was considered as an advantageous condition for nitrification due to the observed residual COD inhibitory effect elimination. The effluent COD concentration increased while total influent flow rose from 1.44 L·h− 1 to 1.80 L·h− 1 in all influent distribution ratios, and this event was more visible in the inflow distribution ratio of 25/35/40 (Fig. 6(b)). As shown in Fig. 2(b), NH+ 4 concentration fluctuated between 2.24 and 9.20 mg·L− 1 during three different distribution ratios, while total influent flow was 1.80 L·h− 1. The average NH+ 4 removal rate was 85%, with a maximum of 95%. The result was in accordance with the research by Ge et al. [6], who remarked that the ratio for the first stage of the modified step-feed process over 50% would lead to negative effects on nitrification and nitrogen removal. In addition, exceeding the influent may bring about supplementary carbon sources being oxidized in the following oxic zones if it is not fully consumed in the anaerobic and anoxic segments. Therewith, reduction in the existing carbon sources available in the second and third anoxic zones resulted in an enhancement in effluent TN owing to a lack of necessary organics required for heterotrophic denitrifying nitrogen removal. Moreover, there was a relatively positive relationship between inflow to anaerobic, second and third anoxic segments and anaerobic phosphorus release. Anaerobic phosphorus release could be negatively impressed by the availability of excess nitrate and insufficient influent organic compounds. Thus, another progressive strategy was obtained by incorporating the step-feed process and the modified UCT biological nutrient removal method together to prevent the detrimental effects of nitrate existence on phosphorus release. In the modified step-feed type, a part of the excess nitrate was reduced to nitrogen gas by the initial movement of sludge returned mixed liquor between the clarifier unit and the first anoxic segment. Moreover, the minor current of the mentioned mixed liquors with less nitrate concentrations was recycled to the anaerobic segment, at which point slightly more anaerobic conditions were prepared due to phosphorus release. As a result, the enhancing phosphorus release and removal efficiency were obtained with less nitrate concentrations in the anaerobic region as had been expected. The PO3− removal efficiency 4 ranged significantly between 39% and 79% with an average value of 62% during the whole experimental period (Fig. 5(a)). A recent research remarked the magnitude of desirable operating conditions and influent concentrations for the processes coupled with successful phosphorus removal [22]. As a result, higher influent ratios to the anaerobic and anoxic regions represented that more carbon sources would be accessible in anaerobic and anoxic segments where high carbon sources for denitrification and anaerobic phosphorus release have been prepared. Generally, higher
2.2. Seed sludge and wastewater characteristics The seed sludge was taken from the aerobic region of the full scale reactor of the Tehran South WWTP, where biological nitrogen and chemical phosphorus removal were obtained, respectively. The reactor in Tehran South WWTP (450000 m3·d−1) is a Carrousel biological nutrient removal and had been under operation for about four years. After a one-month cultivation period of the activated sludge and its adaptation to the synthetic wastewater, a permanent denitrifying operation was obtained and the process performance was stabilized. Two kinds of synthetic wastewater with the concentration and characteristics of real domestic wastewater were fed to the reactor: first, a normal wastewater with total COD of 300 mg·L−1 and, second, a thickened wastewater with total COD of 500 mg·L−1. 2.3. Analytical methods The pH and temperature were found using WTW level-2 pH meters (WTW Company, Germany). Further, DO values were detected by WTW, pH/oxi340i meters with fully equipped DO sensors and calibra3– – tion facilities. COD, BOD, NH+ 4 , PO4 , NO3, TN, MLSS and volatile MLSS concentrations were analyzed according to standard methods [21]. COD solutions have been heated by a special stainless steel heater B75hls (WEL Company, Japan) for the analyses of COD concentration in each experimental period. 3. Results and Discussion 3.1. Effect of different inflow distribution ratios on the performance of nutrients removal Figs. 2–6 display the influent and effluent concentrations coupling – 3– with removal performances of NH+ 4 , NO3, TN, PO4 and COD during nine operational runs, respectively. The influent distribution ratio had a significant effect on the rate of COD and NH + 4 removal (Figs. 2(a) and 6(a)). It is obvious that by entering the third part of the influent distribution ratio to the last anoxic zone, nitrifiers became considerably active, so total nitrification was obtained during the whole experimental period in the influent distribution ratio of
Table 1 Operational process conditions, sludge loading rates (F/M), influent loading rates for organic, phosphorus and nitrogenous substrates in each run Run number
1 2 3 4 5 6 7 8 9
Q F/L·h−1
1.44 1.44 1.44 1.44 1.44 1.44 1.80 1.80 1.80
inflow distribution ratios AN/DN2/DN3/%
60/25/15 40/35/25 25/35/40 60/25/15 40/35/25 25/35/40 60/25/15 40/35/25 25/35/40
Loading/g·d−1
HRT/h
COD
BOD
NH+ 4 -N
PO3− 4 -P
10.36 10.36 10.36 17.28 17.28 17.28 21.6 21.6 21.6
6.21 6.21 6.21 10.36 10.36 10.36 12.96 12.96 12.96
0.518 0.518 0.518 0.864 0.864 0.864 1.08 1.08 1.08
0.103 0.103 0.103 0.172 0.172 0.172 0.216 0.216 0.216
16 16 16 16 16 12 12 12 12
Total influent/mg·L−1
Sludge loading rate (F/M)/BOD kg·(MLVSS kg)−1·d−1
COD
BOD
ANA
DN1
DN2
300 300 300 500 500 500 500 500 500
180 180 180 305 305 305 305 305 305
1.73 1.15 0.73 2.91 3.20 2.23 3.64 4.00 2.78
1.92 2.70 2.70 1.94 4.50 3.71 2.42 5.62 4.63
1.34 2.22 3.56 1.21 4.50 5.94 1.51 5.62 7.42
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Fig. 2. Effluent ammonium concentration and removal efficiency regarding (a) total influent COD concentration, and (b) total inflow rate.
influent ratios to the anaerobic segment would be advantageous to phosphorus uptake, whereas a ratio higher than a certain level results in negative effects on nitrogen elimination [6]. As a consequence, the optimum inflow distribution ratio would be desired to improve nutrient removals. The optimum inflow ratios should be regulated to make more COD to be consumed for phosphorus release during the anaerobic region and denitrification zones. Consequently, this event leads to a reduction in common intrusion of residual COD to nitrification and attempts to decrease the aeration consumption in aerobic regions. Larrea et al. [4] reported that the optimal influent distribution ratio to the three inlet regions of the reactor was in the range of 40/40/20 and 33/33/34, which frequently concerns effluent requirements and wastewater characteristics. Funamizu et al. [23] declared that the selection of the step-feed ratio alone was not enough to enhance nitrogen removal due to low denitrification rates in anoxic segments. Table 2 has classified and compared several previous researches that have been conducted on a variety of step-feed processes with different influent distribution ratios and configurations.
Fig. 4(a) and (b) illustrates the trend of total nitrogen (TN) concentration and removal efficiency. In this study, total nitrogen is the sum− mation of effluent NH+ 4 -N and NO3 -N concentrations during each run. The average effluent TN removal efficiency was 76.16% with the average effluent TN concentration of 4.34 mg·L−1 during all experiments. As the results indicated, a high TN removal rate leads to a high phosphorus removal rate. Table 3 has been prepared to better understand the figures. According to Fig. 4(b), TN removal efficiency decreased as the inflow to the last anoxic segment grew, both in total inflow of 1.44 and 1.8 L·h−1. Consequently, this event has occurred due to the availability of high carbon sources in pre-anoxic zones (particularly the last anoxic region), resulting in weak nitrification in the last aerobic segment and high effluent ammonium concentration, which led to high effluent TN. Most likely, low HRT may be another reason for the mentioned event due to the high level of influent in the last pre-anoxic zone in the last influent distribution ratio (25/35/40). Accordingly, more anoxic denitrification rates would be obtained at limited HRTs, which had clear a positive correlation with TN removal [24,25].
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Fig. 3. Effluent nitrate concentration regarding (a) total influent COD concentration, and (b) total inflow rate.
3.2. Organic and nutrient removal procedures 3.2.1. COD removal COD concentration and removal efficiency in different stages and under different operational situations are illustrated in Fig. 6. Results indicated that the influent COD distribution had a significant effect on denitrification capability and anaerobic phosphorus release. The residual COD trended to increase from 64 mg·L− 1 in the inflow rate of 1.44 L·h − 1 to 87 mg·L − 1 in the inflow rate of 1.8 L·h − 1 in the ratio of 25/35/40 according to Fig. 6(b). This event was established for two other ratios, although it was significantly observed during the ratio of 25/35/40 due to low HRT and a high ratio to the last pre-anoxic zone. Consequently, it caused different anaerobic phosphorus release capabilities and removal performances. Moreover, the COD utilization capacity for phosphorus and nitrogen removal varied with changing the inflow distribution ratios. Although in the inflow distribution ratio of 60/25/15 HRT was the maximum amount, the highest effluent COD removal efficiency was observed during the ratio of 40/35/25 (Fig. 6(a)). In addition, this maximum organic removal efficiency was simultaneously coupled with maximum nitrate removal efficiency due to the high anoxic denitrification rate. Accordingly, the results displayed that enhancement in COD removal efficiency directly related to the improvement in nitrate removal efficiency. In other words the more denitrification takes place, the more organics are eliminated owing to denitrifying phosphorus and nitrate removal. The performance of COD removal failed in the ratio of 25/35/40 owing to low HRT. Additionally, as expected the effluent COD concentration increased while the wastewater
inflow rate and concentration enhanced. This enhancement in effluent COD was more visible in the ratio of 25/35/40, which demonstrated that in two other distribution ratios the process was more capable to adapt itself to the wastewater inflow rate and concentration enhancement. 3.2.2. Phosphorus removal Fig. 5 illustrates effluent PO 34 − concentration and removal efficiency during different experimental periods under different operational conditions. Results displayed that anaerobic HRT had a distinct effect on anaerobic phosphorus release, based on observations during the experimental period. As expected, PAOs was capable of eliminating the degradable organic substrates to produce polyhydroxyalkanoates (PHA) with the reserved polyphosphate (poly-P) utilized as an energy source [26]. As a result, it could be concluded that the aerobic/anoxic phosphorus removal directly relies on the mechanisms of phosphorus release under anaerobic conditions in the first stage. In addition, sustaining an appropriate microorganism population in the anaerobic segment should not be neglected. In order to enrich the convenient population, more inflow organic concentrations are required. Wang et al. [27] reported that the highest anaerobic phosphorus release was 350% and 497% of the influent total phosphorus (TP) concentration, and achieved significantly high TP uptake. Maximum phosphorus removal efficiency was observed in the inflow distribution ratio of 60/25/15, which prepared the required organics according to Fig. 5(a). Therewith, significant phosphorus uptake was obtained in anoxic zones in the mentioned ratio, which might be
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399
Fig. 4. Effluent total nitrogen concentration and removal efficiency regarding (a) total influent COD concentration, and (b) total inflow rate.
attributed to denitrifying phosphorus removal. Afterwards, the majority of PO34 − was further eliminated under aerobic conditions in the oxic 1 segment, while the minor amount of PO34 − was removed in the oxic 2 and oxic 3 segments, respectively. Wachtmeister et al. [28] remarked that virtually 32.6%–39.5% of DNPAOs were responsible for PAOs imputed to favorable phosphorus uptake capacity under appropriate distribution ratios. Once in anaerobic zones sufficient accessible organic substrates were confected for PAOs releasing phosphorus and producing storable carbon source PHA, the anoxic/oxic phosphorus elimination process and denitrification would be obtained in the subsequent anoxic/oxic regions, as Kapagiannidis et al. [29] reported in their recent research. However, convenient phosphorus release would run against an adverse effect by the availability of nitrate in the recycled sludge stream, resulting in a significant weakness in the optimal anaerobic conditions required for PAOs to release phosphorus. Therefore, the returned sludge should be recycled to the first anoxic zone to reduce the amount of accessible nitrate, primarily, prior to entering the anaerobic zone. 3.2.3. Nitrogen removal − The alteration of NH+ 4 , NO3 and TN along the experimental period under different operational conditions is illustrated in Figs. 2, 3, and 4,
respectively. As results indicated, less influent NH+ 4 -N loads, adversely decayed nitrification qualification in oxic 1 and oxic 2. Meanwhile, the results confirmed that more NH+ 4 uptake was observed in the preanoxic regions, attributed to the dissolved oxygen transportation from aerobic segments resulting in partial oxidation of available carbon sources which were not conductive to TN elimination. Probably, this could be a logical reason for comparing the restricted TN uptake with 83% TN removal efficiency demonstrated in Vaiopoulou and Aivasidas (2008) in comparison with Ge et al. (2010) who demonstrated TN removal efficiency of 88%. Nitrification was fully accomplished during all steady-state experiments except in the low HRT runs. The average removal efficiency of NH+ 4 and TN were 82% and 78.6%, respectively, during the whole experimental period. Moreover, a possible advantageous parameter to desirable nitrogen removal in the step-feed process was the occurrence of simultaneous nitrification and denitrification (SND). To be more informed about the significance of the SND in TN removal, a summarized discussion should be held in order to make the subject more explicit. Generally, the total mass of nitrogen available in the influent entering the treatment process could be utilized according to the following fractions: (1) cell growth and synthesis attributed to the waste activated sludge; (2) proportion of nitrate to nitrite and TN in effluent; and
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Fig. 5. Effluent phosphate concentration and removal efficiency regarding (a) total influent COD concentration, and (b) total inflow rate.
(3) N2 gas released through denitrification process along anoxic or, probably, aerobic regions [30]. The anoxic denitrification sufficiencies play eminent roles in TN elimination, owing to the fact that the possibility of reducing NO− 2 -N and NO− 3 -N to nitrogen gas and other volatile nitrogen oxides solely exists in anoxic zones as Zhu et al. [2] and Fue et al. [31] reported in their recent studies. It could be concluded that as the inflow ratio to the anaerobic zone is enhanced, the required HRT for nitrification was established, whereas by enhancing the inflow ratio to the last anoxic region the HRT became lower and ammonium was left in the reactor prior to being converted into nitrate. Moreover, the total inflow rate enhancement had no distinct effect on effluent ammonium, illustrating that the bioreactor had the ability to eliminate more values of ammonium if total inflow was divided between the different stages, appropriately. If ammonium removal efficiency improvement is desired in the optimum inflow distribution ratio, alkalinity and dissolved oxygen concentration should be increased. In fact, the nitrification rate improves by increasing pH up to virtually eight, resulting in significant ammonium removal efficiency. Additionally, more dissolved oxygen concentrations and also more fluid mixing through aerobic segments provide more supplied oxygen
inside the sludge flocs, resulting in ammonium concentration even below 1 mg·L−1 in the effluent. According to Fig. 3, maximum nitrate removal efficiencies were obtained in the inflow ratio of 60/25/15 in comparison with the two other ratios. Low HRTs led to higher effluent nitrate concentration. In fact, nitrification required more retention time; however, by enhancing the total inflow rate this goal was inaccessible (Fig. 3(b)). 3.3. COD variations along different aerobic stages Variation of COD in all aerobic regions under different operational conditions was investigated in this study as shown in Fig. 7. As results indicated, COD removal mostly occurred in N1 which was similar in all inflow ratios; however, the differences in COD removal performance via aerobic zones among different inflow ratios were started upon entering the following aerobic zones after N1 . Maximum COD removal performance could be interpreted according to significant COD utilization via anaerobic phosphorus release and anoxic denitrification (in the first anoxic zone). Moreover, in the inflow ratio of 25/35/40, the major ratio of feed was entered into the last anoxic zone, resulting in low COD removal qualification in
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401
Fig. 6. Effluent COD concentration and removal efficiency regarding (a) total influent COD concentration, and (b) total inflow rate.
Table 2 Comparisons of configurations, operational parameters and performance of the step-feed process Item Configuration Primary sedimentation Anaerobic unit Inflow/L · d−1 Bioreactor volume/L Biological operation mode Country Operational parameters Inflow type Inflow distribution ratio/% Sludge recycle ratio/% Stages DO/mg·L−1 SRT/d HRT/d Removal efficiency/% COD NH+ 4 -N TN TP Effluent quality/mg·L−1 COD NH+ 4 -N TN TP
Viaopoulou and Aivasidis [9]
Wang et al. [27]
Ge et al. [6]
Liang et al. [32]
Wang et al. [33]
Cao et al. [34]
This study
No Yes 96 44 Activated sludge
No Yes 960 320 Activated sludge
No Yes 1020 340 Activated sludge
No No 100 42 Biofilm
No Yes 185 67 Activated sludge
No Yes 22.32 33.6 & 43.2 Activated sludge
Greece
China
China
China
No No 60 20 Activated sludge and fluidized bed China
China
Iran
Real 60/25/15 100 3 N2.0 10 8.9
Real 45/35/20 60 3 2.2–4.2 10 8
Real 40/30/30 75 3 1.5–2.0 10 8
Real 42/33/25 0 3 2.0–3.5 80 10
Synthetic 33/33/17/17 50 4 1.0–1.5 No date 8
Real 20/35/35/10 75 4 1.0–1.5 10–15 8.7
Synthetic 60/25/15 60 3 0.25–2.55 No date 13.8
94 95 83 93
86.6 98 73.61 89.83
89 99 88 93
80 96 No data No data
88.2 95.7 86.4 No data
78.9 98.31 70.24 86.11
95 95 93 78
30 1.7 6.9 0.2
35.74 1.22 16.31 0.63
32.7 0.14 6.61 0.43
b50 b8 No data No data
b50 b8 b10 No data
33.05 0.58 9.26 0.46
26 1.7 1.8 1.8
402
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Table 3 A guideline for making the figures more obvious Figure
Remark
Influent COD/mg·L−1
Q /L·h−1
Influent COD/mg·L−1
Q /L·h−1
Effluent concentration and removal efficiency of …
2-A 2-B 3-A 3-B 4-A 4-B 5-A 5-B 6-A 6-B
Same flow rate Same COD Same flow rate Same COD Same flow rate Same COD Same flow rate Same COD Same flow rate Same COD
300 500 300 500 300 500 300 500 300 500
1.44 1.44 1.44 1.44 1.44 1.44 1.44 1.44 1.44 1.44
500 500 500 500 500 500 500 500 500 500
1.44 1.80 1.44 1.80 1.44 1.80 1.44 1.80 1.44 1.80
NH+ 4 NH+ 4 – NO3 – NO3 TN TN PO3– 4 PO43– COD COD
Note: All experiments performed in three inflow distribution ratios.
Fig. 7. COD values during various aerobic stages under different operational conditions: INF — influent wastewater; N1 — the first oxic zone; N2 — the second oxic zone; N3 — the third oxic zone.
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N3 output with the mean concentration value of 88 mg·L− 1 in all total influent COD values, according to Fig. 7. The mean COD values throughout aerobic zones during the three inflow ratios were 433.3, 131.1, 104.43 and 55.55 mg·L−1 for influent wastewater, N1, N2 and N3, respectively. According to the results, optimum inflow ratio for favorite COD removal with high efficiency was 40/35/25, which is exactly visible in Fig. 6 as well. However, total COD concentrations and removal efficiencies during the aerobic regions significantly relied on DO concentrations, which were completely discussed in the previous section. 4. Conclusions Compared to other BNR processes, the modified step-feed process was found to be an attractive choice for urban wastewater treatment. Maximum organics and NO − 3 removal were achieved in the ratio of 40/35/25 with the optimum COD removal efficiency of 95%, whereas maximum PO34 −-P, TN and NH + 4 removal were obtained in the ratio of 60/25/15 with the optimum efficiency of 78%, 93% and 95%, respectively. pH attained convenient values, which were suitable for P-release, denitrification and nitrification during the experimental period. According to the optimum operational conditions, the process was considerably advantageous for both anaerobic P-release and anoxic/aerobic P-uptake, and TN removal efficiency could be significantly improved via the nitrification and denitrification processes. Acknowledgments The authors would like to acknowledge Mahab Ghodss Company for their extensive cooperation. References [1] Eddy Metcalf, Wastewater engineering: Treatment and reuse, McGraw Hill, New York City, USA, 2003. [2] G.B. Zhu, Y.Z. Peng, B.K. Li, J.H. Guo, Q. Yang, S.Y. Wang, Biological removal of nitrogen from wastewater, Rev. Environ. Contam. Toxicol. 192 (2008) 159–195. [3] H.M. Zhang, X.L. Wang, J.N. Xiao, F.L. Yang, J. Zhang, Enhanced biological nutrients removal using MUCT-MBR, Bioresour. Technol. 100 (2009) 1048–1054. [4] L. Larrea, A. Larrea, E. Ayesa, J.C. Rodrigo, M.D. Lopez-Carrasco, J.A. Cortacans, Development and vertification of design and operation criteria for the step feed process with nitrogen removal, Water Sci. Technol. 43 (2001) 261–268. [5] G.H. Cao, Y. Huang, Y. Pan, The problems and the solutions of conventional biological nitrogen and phosphorus processes, Technol. Water Treat. 25 (2009) 102–106. [6] S.J. Ge, Y.Z. Peng, S.Y. Wang, J.H. Guo, B. Ma, L. Zhang, X. Cao, Enhanced nutrients removal in a modified step feeding process treating municipal wastewater with different inflow distribution ratios and nutrients ratios, Bioresour. Technol. 101 (2010) 9012–9019. [7] J.A. Baeza, D. Gabriel, J. Lafuente, Improving the nitrogen removal efficiency of an A2/O based WWTP by using an on-line Knowledge Based Expert System, Water Res. 36 (2002) 2109–2123. [8] E. Viopoulou, P. Melidis, A. Aivasidis, An activated sludge treatment plant for integrated removal of carbon, nitrogen and phosphorus, Desalination 211 (2007) 192–199. [9] E. Viopoulou, A. Aivasidis, A modified UCT method for biological nutrients removal: Configuration and performance, Chemosphere 72 (2008) 1062–1068.
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