Journal of Water Process Engineering 23 (2018) 230–238
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Effect of intermittent aeration cycles on EPS production and sludge characteristics in a field scale IFAS reactor
T
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Nitin Kumar Singha, , Siddhartha Pandeyb, Rana Pratap Singhc, Swati Dahiyab, Sneha Gautama, Absar Ahmad Kazmib a
Department of Environmental Science and Engineering, Marwadi Education Foundation's Group of Institutions, Rajkot, 360003, India Environmental Engineering Group, Dept. of Civil Engineering, Indian Institute of Technology, Roorkee, 247667, India c Dept. of Civil Engineering, Katihar College of Engineering, Katihar, 854106, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: IFAS Intermittent aeration Sludge characteristics Extracellular polymeric substances Energy saving
In the present study, an integrated fixed-film activated sludge (IFAS) bioreactor was subjected to dissimilar intermittent aeration (IA) cyclic operations, and its effects were investigated on extracellular polymeric substances (EPS) production, sludge characteristics, and specific power consumption. A total of three IA cycles (IA1, 150 min aeration on and 30 min off; IA2, 120 min aeration on and 60 min off; IA3, 90 min aeration on and 60 min off) were evaluated in the present IFAS reactor. Specific EPS production (mg/g of suspended solids) was found to be following the increasing trend with respect to the non-aeration to aeration time ratio, whereas, the sludge production followed the reverse trend. The amount of bound EPS was observed to be much higher (6–10 times) than soluble microbial product (SMP) in each intermittent aeration phase. During all the investigated IA cycles, the pilot was observed to be significantly affected in terms of biomass characteristics, which was also confirmed by increasing trends of sludge volume index (SVI) and filamentous index (FI) values. In-situ monitoring and measurement of reactor operation parameters such as pH, oxidation reduction potential (ORP), and dissolved oxygen (DO) was also done in all IA cycles. A maximum of 27.05% reduction in electrical energy was observed in highest non-aeration period cycle.
1. Introduction All the biological wastewater treatment systems (suspended and/or attached) produce a complex mixtures of high-molecular-weight substances i.e. polymers secreted by microorganisms, produced from cell disintegration, and adsorbed substrate from wastewaters [1], which are known as extracellular polymeric substances (EPS). These materials play a consequential role in enhanced settling of biomass, and helps them to form the microbial clusters in a wastewater treatment system. The prime components of EPS mixture include macromolecules such as carbohydrates and proteins, which exerts influence on bio-chemical characteristics of microbial consortium [2]. Furthermore, the two forms of EPS exist at outer surface of microbial cells which can be categorized as bound EPS and soluble EPS [3,4]. As the classification term implies, the bound EPS are closely attached with microbial cells, whereas soluble EPS are those compounds which are either loosely bound with cells or dissolved into the mother solutions [5]. In spite of significant research on EPS, many other factors play important role which could
influence the composition and production rate of EPS in environmental systems. This quantitative as well as qualitative variation in constituents of the extracted EPS may be attributed to various factors such as type of wastewater, biomass conditions, operational parameters, bioreactor type, and extraction method etc. [3]. Among the various important operational parameters of bioreactors, solid retention time, shear rate or aeration intensity, and aerobic or anaerobic conditions were found to be affecting the EPS production and composition as well. However, the results published in literature are slightly contradictory as some reported increasing trend while others observed the decreasing trend of EPS production with similar change in operating conditions [5,6–9]. To date, although ample amount of literature is published about EPS quantification methods but more studies are required to investigate their roles in the biological wastewater treatment systems under different operational conditions. Therefore, to gain confidence about the fate of EPS it is important to conduct more studies under stressed conditions to investigate the behaviour of biomass with respect to EPS production.
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Corresponding author. E-mail addresses:
[email protected] (N.K. Singh),
[email protected] (S. Pandey),
[email protected] (R.P. Singh),
[email protected] (S. Dahiya),
[email protected] (S. Gautam),
[email protected] (A.A. Kazmi). https://doi.org/10.1016/j.jwpe.2018.03.012 Received 25 December 2017; Received in revised form 13 March 2018; Accepted 20 March 2018 2214-7144/ © 2018 Elsevier Ltd. All rights reserved.
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used in this study is shown in Fig. 1.
In last two decades, various operational strategies such as intermittent aeration and variable dissolved oxygen mode have been investigated for biological wastewater treatment systems [10–13]. Among these operational stresses, intermittent aeration (IA) strategies, also called alternating anoxic/oxic process, have been proved very efficient in removal of the biodegradable components (organics, solids, and nutrients) of wastewaters in continuous as well as in batch reactors. During the times of 1980–1990, a good number of plants were built to operate under same operational conditions, and data on treatment performance and energy consumption were reported [14–17]. More advantages of alternating anoxic/oxic process are well documented in our recently published studies [12,13]. This type of operational strategies also gives a major reduction in electric energy consumption as well as in operational costs associated with aeration [18–23]. The integrated fixed-film activated sludge (IFAS) systems, which were introduced in 1994, emerged as one of the most encouraging alternative in decentralized context [24–26]. These systems have been proved efficient in developing as well as in developed countries for the municipal and/or industrial wastewater treatment [12,27–31]. To date, some studies have been reported the role and impact of various intermittent aeration strategies on organics and nutrient removal in natural and aerobic wastewater treatment systems [32–35]. However, the sludge characteristics of these systems are still unexplored, and very limited studies are available revealing the role of EPS, and nature of sludge produced in biological wastewater treatment systems [36]. Until now, very limited published literature is available reporting the effect of intermittent aeration strategies on the EPS quantity produced, and biomass characteristics in an IFAS and/or biofilm reactor [37]. Therefore, this study was carried out to investigate the effects of different IA cycles on the EPS production and sludge characteristics in an IFAS reactor.
2.2. Start-up and experimental methodology The start-up of pilot plant was done according to our previous study [12], and results of treatment performance analysis and changes in microbial diversity are presented in our recent studies [12,13]. The overall experimental campaign was divided into four phases: three intermittent aeration (IA) runs, and one continuous run. Each of the IA phase lasted for around three weeks with an interval of one week between two consecutive phases. The operational parameters for the pilot are presented in Table 2. The aeration was controlled by using an automated timer in the blower unit with fixed time periods of aeration and non-aeration.
2.3. Analysis and measurement During the entire experimental period, the influent, effluent, and sludge samples were placed in an icebox and brought to environmental engineering laboratory at IIT Roorkee, until analysis could be performed. Selected in-situ operational parameters and reactor conditions such as pH, dissolved oxygen (DO), HRT, waste activated sludge (WAS) rates, and return activated sludge (RAS) rates were monitored and measured on each sampling day, while oxidation reduction potential (ORP) was monitored in batch mode. The major treatment performance parameters such as 3-day biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), ammonia nitrogen (NH3-N), nitrate nitrogen (NO3-N), total nitrogen (TN), and total phosphorus (TP) were measured according to Standard Methods [66]. The mixed liquor suspended phase biom HRT, waste activated sludge (WAS) rates, and return activated sludge (RAS) rates were monitored and measured on each sampling day, while oxidation reduction potential (ORP) was monitored in batch mode. The major treatment performance parameters such as 3-day biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), ammonia nitrogen (NH3-N), nitrate nitrogen (NO3-N), total nitrogen (TN), and total phosphorus (TP) were measured according to Standard Methods [66]. The mixed liquor suspended phase biomass samples were collected from the IFAS reactor and characterized by mixed liquor suspended solids (MLSS) and sludge volume index (SVI) measurements by following the procedure as presented in Standard Methods [66]. The electrical energy consumption was calculated by considering the rating and running hours of blower unit.
2. Material and methods 2.1. Description of experimental set-up All the experiments were conducted on a pilot-scale fixed media based IFAS reactor (Table 1). The whole body of reactor was made up of stainless steel including fixed media holding frame. The fixed media curtains (a loop knitted polypropylene fabric in a rectangular geometry), placed within the aerobic zone of system, and mounted within a detachable frame assembly which can be simply lifted out from the aeration tank for maintenance or inspection, whenever required. Necessary aeration in the IFAS reactor was provided by using an automated blower supplying requisite air to the highly efficient membrane diffusers, installed at the bottom of the fixed media holding tank. The actual municipal wastewater was pumped from the sump well of the sewage pumping station, and settled activated sludge along with raw municipal wastewater flowed over a weir into the aeration chamber by using centrifugal pumps. The typical characteristics of wastewater fed to pilot were equivalent of medium strength sewage, and the values of main parameters are noted in the range of 400–600; 240–350; 200–360; 40–65; and 3–10 for COD, BOD, TSS, TN, and TP, respectively. The daily wastewater feeding rate was adjusted to yield a hydraulic retention time (HRT) of 11.1 h. The schematic diagram of experimental setup
2.4. EPS quantification In order to examine the physiochemical condition of the suspended biomass in IFAS reactor, the total concentration of EPS, termed as EPST, was measured in present study. The soluble (termed as soluble microbial products, SMP) and bound EPS concentrations were determined according to Zhang et al. [38], Di Bella et al. [39], Di Bella and Torregrossa [40]. The total EPS (both bound and soluble) concentration was calculated as the sum of the above two described fractions, according to the below equation:
Table 1 Technical design details of pilot plant.
EPST = EPSb + SMP = (EPSbp + EPSbc + (SMPp + SMPc)
Parameter
Unit
Value
Dimension of aerobic tank (L × W × H) Volume of aeration tank Volume of settling tank Media packing (by volume) No. of media pieces (curtains) Dimension of each curtain (L × W)
m m3 m3 % – m
3 × 2 × 3.34 20 4.2 0.5 64 2.7 × 0.96
Where EPSb = bound EPS; SMP = soluble microbial product. Here subscripts ‘p’ and ‘c’ indicate the content of proteins and carbohydrates in the EPSb and SMP, respectively. In order to measure the contents of proteins and carbohydrates in bound and soluble EPS, Lowry’s Folin method [41] and Anthrone’s method [42] were used, respectively.
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Fig. 1. Configuration of the pilot scale IFAS reactor used in this study.
∼8, ∼6, and ∼4 mg/L, respectively. The removal efficiency of total nitrogen varied in the range of 70–80% under all applied IA conditions. Overall, the experimental results showed that both nitrification and denitrification may successfully be carded out into present IFAS system. Developing anaerobic and aerobic conditions in IFAS reactors has been noticed by researchers recently. Therefore, TP removal efficiencies were examined in the present system, and removal efficiencies were found to be inversely proportional to the duration of non-aeration time in each IA phase. The average TP removal rate dropped from 80% to 43% when changing the aeration cycle from IA1 to IA3. Overall, the IFAS reactor was effective in removing BOD, COD, TSS, and TN, irrespective of cycle characteristics. More details about the treatment performance are documented in our previous published works [12,13].
Table 2 Operating conditions of IFAS reactor during intermittent operation. Intermittent aeration cyclea
Blower operation
αb
IA 1
2.5 h on 0.5 h off 2 h on 1 h off 1.5 h on 1 h off
0.16
IA 2 IA 3
0.33 0.4
a Operational conditions during IA operation (D.O., ∼3 mg/L; HRT, ∼11.1 h; RAS, ∼108 m3/d; WAS, ∼4.32 m3/d). b α = (Non-aeration/Total time).
2.5. Microscopic examination of sludge 3.2. EPS production and microscopic observations For microscopic analysis, the suspended biomass from the aeration tank of IFAS reactor was examined using an Olympus bright-field microscope (BH-2; Olympus Co. Ltd., Tokyo, Japan). Relative abundance of filamentous microorganisms in suspended biomass during different experimental runs was determined using the Filamentous Index (FI) method [43,44]. A scale range from 0 to 6 (from none to excessive) was used to determine the filamentous bacteria abundance, with values as follows: 0, none; 1, few; 2, some; 3, common; 4, very common; 5, abundant; and 6, excessive. Simplified Technique of Filamentous Count (STFC) was used for counting of filamentous bacteria in per mL of sludge samples [43,45,46].
A change in the bioreactor operational conditions such as shear rate, mixing and flow patterns, or aeration intensity could change the EPS content in activated sludge [47]. Some researchers also reported that the amount of carbohydrate content in extracted EPS from activated sludge increases with the increased air flow in some batch reactors such as sequential bioreactors, whereas the amount of protein substances remain almost unchanged at the same air intensities [48], which revealed that shear rate affects the EPS quantity produced and may trigger bacteria to excrete more carbohydrate substances. Moreover, Ramasamy and Zhang [49] also reported that however a sudden increase in shear rate may led to increased content of carbohydrates in EPS, but may returns to its original concentration level after a period of cultivation. The EPS from the reactor biomass could also be released by mechanic shear force and hydrodynamic conditions, which can lead to higher concentrations of soluble EPS [50,51]. Furthermore, the prevailing conditions of bioreactor such as aerobic or anaerobic can also influence the EPS production. It is also documented in literature that activated sludge flocs tend to break or solubilize in containing solutions under oxygen limiting environments. Such dissolution of microbial aggregates might be caused either by the hindrance in EPS production or by the hydrolysis process of EPS. Shin et al. [48] compared three bioreactors under different dissolved oxygen conditions in terms of EPS production and reported that production of carbohydrates in EPS increases with time at high dissolved oxygen levels, whereas insignificant changes were observed in protein content at the same conditions. Contrary, the similar concentrations of both carbohydrates and proteins were observed at a low dissolved oxygen levels. Fig. 2a reports the specific EPS content (total mg EPS per gram of SS) of suspended biomass in each experimental state, while Fig. 2b depicts the carbohydrate and protein concentration in SMP (which includes loosely bound and/or dissolved EPS) and bound EPS,
3. Results and discussion 3.1. Treatment performance under intermittent aeration conditions The treatment potential of the present IFAS system was also examined under studied IA conditions. The IFAS system was operated with F/M ratio of 0.38, 0.47, and 0.53, and sludge retention time as 4.6, 3.7, and 4.8 days in IA1, IA2, IA3 period, respectively. In spite of great variation in influent characteristics, the average COD values of effluent were observed as 33, 29, and 42 mg/L, achieving the mean removal efficiencies as 96, 97, and 96% in IA1, IA2 and IA3 period, respectively. The BOD removal efficiencies also follows the same trend, and reported in the range of 90–95%. These results indicate that non-aeration times of applied experimental conditions had an insignificant effect on overall removal efficiencies of organic matter. With regard to TSS concentrations, the average values were almost same and amounted as ∼15 mg/L under all IA conditions. Considering the nutrient parameters, the average ammonia concentrations in the effluent were noted below 1, 5, and 7 mg/L in the period IA1, IA2, and IA3, respectively, while the average nitrate concentrations in the effluent in the same periods were 232
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Fig. 2. Variation in a) total EPS contents and b) its constituents in different aeration cycles.
respectively. More specifically, the term SMP is first used by Namkung and Rittmann, and it is defined as soluble cellular components that are released from substrate metabolism and biomass decay. Results of our previous studies revealed that although the attached biomass/biofilms are capable to produce bit higher quantity of EPS than suspended biomass but as the biomass acclimatizes, the levels of EPS production by the suspended biomass also increases (Fig. 2a). These results suggest that suspended biomass may produce similar levels of EPS as attached biomass with the long run of IFAS system. This increasing pattern may be attributed to the release of attached microorganisms into mixed
liquor, also called as seeding effect of attached biomass [26]. Results of present investigations revealed that the EPST concentrations (e.g. the sum of EPSBound and SMP) in the present IFAS reactor were significantly higher than those observed under continuous aeration conditions. The percentage of bound EPS was much higher (6–10 times) than SMP in each experimental phase (even if sporadically measured). On the contrary, it is worth underlying that increased values of SMPs were observed in each consequent experimental state on reducing the duration of the aeration on time. A possible reason might be related to a stress condition on the biomass that anoxic conditions exerted on the biomass, 233
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time the FI remained between FI2–FI3. This observation is attested in Fig. 3(a–c). It was also observed that during intermittent aeration runs, most stalked ciliated disappeared and free-swimming small flagellates were found to be predominant. This caused the high SVI values during intermittent aeration period. However, free-swimming protozoa survived under the IA conditions. These results suggest that intermittent aeration conditions allow predators to survive, which contributed to the decrease in the sludge amount through their predation on the dispersed bacteria.
due to release of SMP by cell autolysis under anoxic conditions and loosely bound EPS (LBEPS) from attached biomass of reactor [52–54]. Differently, slightly less variation was observed in bound EPS as compared to SMP concentrations. However, further studies are required to confirm these hypotheses. Carbohydrate and protein mixtures are crucial components of EPS to control the microbial activity and destruction in bioreactor systems. Carbohydrate concentration was found to be in increasing order in bound EPS (21, 28, 36, and 49 mg/L) and SMP (18, 13, 22, and 34 mg/ L), while protein content in bound EPS was almost stable (varied between 176 and 191 mg/L) under all intermittent aeration conditions. On the other side, protein content of SMP was found to increasing and observed as 26, 32, 41, and 46 mg/L in continuous aeration, IA1, IA2 and IA3 runs, respectively. This might be due to increase in non-aeration period. Therefore, the longer durations of non-aeration time in IA cycles might enhance the denitrification activity but may have a significant impact on overall EPS production. These results were quite similar to previous studies [20] in which longer aeration off times were found to be responsible for exploded cell growth and consequently increased EPS concentrations.
3.4. Sludge production and characterization Fig. 4a shows the variation in MLSS and SVI values in the mixed liquor of IFAS reactor during various experimental schedules. Observing Fig. 4a it is worth noting that during the whole period, MLSS values were almost stable, concentrations varied between 1338–1452, 1355–1405, and 1280–1312 mg/L during IA1, IA2, and IA3, respectively. The averaged ratio of volatile to suspended solids was found to be fluctuating a little around a mean value of 0.73. This indicates that small amount of inorganic compounds and discernible inert are accumulated in the bioreactor during the whole experimental period. The sludge settleability was affected significantly by increasing the nonaeration period in intermittent aeration cycle and higher SVI values are achieved in run IA3 as compared to IA1 and IA2. However, in all the intermittent aeration runs, the SVI values were above the settleability threshold of 155 ml/g at continuous aeration mode. The increased SVI values, ranging between 186 and 203, 218–238, and 241–267 ml/g in IA run 1, 2, and 3, were observed for present IFAS system. These increased values of SVI were also evidenced by presence of thin filaments in suspended biomass. Another reason behind the increased SVI values may be more EPS production which may have a pessimistic effect on the settleability of microbial aggregates [56]. Generally, the EPS are negatively charged and high concentration of EPS increases the surface charge of microbes, which can result in an increased repulsive forces between microbial cells and a decrease in the settleability of microbial aggregates [57]. It is also important to mention here that whenever plant goes under intermittent aeration phase, the aeration basin acts as
3.3. Control of filamentous organisms The abundance of various types of filamentous microorganisms often leads to poor settling, and sometime promotes the bulking sludge in biological wastewater treatment systems. This phenomenon is mainly associated with low DO conditions, high F/M ratios, or septic sewage. The chances of septic sewage are possible in our study, as sewage was fed to pilot from a sewage pumping station. Moreover, the sludge settleability decreases (also evidenced by significant rise in the SVI) and the sludge appears light and fluffy. Similar, effects have been observed in this study. According to several researchers, alternating the aerobic/ anoxic periods in biological reactor enhance the reductive conditions, thus allowing the hydrolysis of the particulate matter and places the filamentous organism in system [55]. During the microscopic examination of suspended sludge, the concentrations of thin filaments were visually quantified using FI values. Over the whole experimental
Fig. 3. Micrographs of suspended sludge under intermittent aeration cycles. 234
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Fig. 4. a) Variation of SVI and MLSS, and b) Comparison of mean sludge production during continuous and intermittent operation mode.
3.5. ORP, DO, pH monitoring
sludge storage and due to the fact that the aeration had no mixers, hence whenever blowers are turned off, suspended phase biomass settled down within a short period of time. This cyclic phenomenon also affected adversely the sludge characteristics as reported in other similar systems [58]. The sludge production was also estimated in each phase and presented in Fig. 4b. It can be seen from this figure that reduced sludge production was observed in intermittent aeration mode with values noted as 8.56, 8.35, 7.39 and 5.79 kg/d during continuous aeration, IA1, IA2 and IA3, respectively. This reduction with aeration off time can be attributed to lesser biomass produced under anoxic conditions as compared to aerobic conditions. In general, length of non-aeration period determines the net sludge production during cyclic intermittent aeration operation of biological reactors. Furthermore, a reduced anoxic endogenous respiration rate may be responsible for the reduced sludge yield [59].
In biological wastewater treatment systems, pH, DO, and ORP charts can provide useful information about the biological process such as operational conditions (over-aeration and under-aeration), microorganism activity, and process inhibition [60,61]. To facilitate the process understanding, characteristic profiles of pH, DO, and ORP versus time in each intermittent cycle have been observed in the present IFAS system. (Fig. 5). In each graph, vertical line represents the start and end of non-aeration phase. It is possible to see from Fig. 5 that the DO concentration drops down almost to zero in each IA run. The peak (positive and negative) ORP values were observed as 25 (−32.7), 27 (−32.9), and 49 (−43.2) mV for IA1, IA2 and IA3, respectively. Compared with the negative ORP value of IA1 and IA2 run, the values for IA3 was slightly lower because the mixed liquor in aeration basin was much more in reduced state caused by decreased aeration time and increased non-aeration time. It can also be seen from the graph that although the positive peak values of ORP were high in IA3 but non-
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Fig. 5. Variation of pH, DO, and ORP during different intermittent aeration cycles in IFAS reactor.
aeration duration was lower than IA1 and IA2. That is why, significant ammonia removal could not be achieved. The appreciable fluctuation of ORP curve during non-aeration period and initial stage of aeration-on period was also encountered in IA 3. It may be also due to no mixing during the anoxic period in aeration basin and high concentration gradient of reduced substances between supernatant and settled sludge layers. With regard to pH change, not much variation was observed in each phase.
Table 3 Energy consumption data of reactor. Phase of operation
Total loada (KW-h/ d)
SPC (KW-h/ m3)
% Saving in energy
CA IA 1 IA 2 IA 3
107.808 95.808 83.808 79.008
2.495 2.21 1.94 1.82
– 11.42 22.24 27.05
a
Sewage Pump (0.746 Kw); RAS pump (0.746 Kw); Blower (3 Kw).
3.6. Electric energy consumption previous literature values [62,10]. In the intermittent aeration processes, alternation of aerobic and anoxic phases in the same bioreactor allows to obtain an effective decrease in the energy utilization [15,62]. Moreover, in a carbon-constrained future, intermittent aeration also has the potential of minimizing the operational costs associated with aeration and needs to be carefully evaluated [63–65]. Previous studies have also reported significant energy consumption varying in the range of 10–30% [18,19,23]. In the present study, the actual power requirement for the pilot was calculated by estimating the power required by the electro mechanical unit installed in pilot and the data of the performed cycles. A comparison of the total energy consumption during the continuous aeration and intermittent aeration periods (Table 3) showed that the energy savings achieved from IA schedules was 12, 24 and 28 kW per day for run IA 1, IA2 and IA3, respectively. The most significant savings was obtained by calculating the energy specific consumptions, and noted as 2.21, 1.94 and 1.82 KW/m3 for run IA1, IA2 and IA3, respectively. An average energy saving of 11.42% for IA1, 22.24% for IA 2 and 27.05% for IA3 was achieved which were in good agreement with
4. Conclusions The main conclusions of this study are as follows:
• EPS production by suspended biomass increased as the ratio of non-
•
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aeration to aeration time got longer, probably due to operational stress condition on the biomass and release of SMP from bound EPS during non-aeration periods. The content at 150/30 min, 120/ 60 min and 90/60 min aeration on/off time increased 1.32 times, 1.47 and 1.78 times respectively, as compared to values of continuous aeration mode operation. The total sludge production by pilot decreased up to 3.7, 13.6, and 32.3% in run 1, 2 and 3, respectively. The high SVI values confirmed that the settling characteristics disturbed during intermittent aeration and found to be proportional to ratio of non-aeration to aeration time. This fact was also evidenced by the increasing presence of filamentous microorganism.
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• Contemporary to the organics and nutrient removals, also propor-
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