Post-treatment of secondary wastewater treatment plant effluent using a two-stage fluidized bed bioreactor system
© Safari et al.; licensee BioMed Central Ltd. 2013
Received: 17 December 2012
Accepted: 9 June 2013
Published: 17 June 2013
The aim of this study was to investigate the performance of a two-stage fluidized bed reactor (FBR) system for the post-treatment of secondary wastewater treatment plant effluents (Shahrak Gharb, Tehran, Iran). The proposed treatment scheme was evaluated using pilot-scale reactors (106-L of capacity) filled with PVC as the fluidized bed (first stage) and gravel for the filtration purpose (second stage). Aluminum sulfate (30 mg/L) and chlorine (1 mg/L) were used for the coagulation and disinfection of the effluent, respectively. To monitor the performance of the FBR system, variation of several parameters (biochemical oxygen demand (BOD5), chemical oxygen demand (COD), turbidity, total phosphorous, total coliform and fecal coliform) were monitored in the effluent wastewater samples. The results showed that the proposed system could effectively reduce BOD5 and COD below 1.95 and 4.06 mg/L, respectively. Turbidity of the effluent could be achieved below 0.75 NTU, which was lower than those reported for the disinfection purpose. The total phosphorus was reduced to 0.52 mg/L, which was near the present phosphorous standard for the prevention of eutrophication process. Depending on both microorganism concentration and applied surface loading rates (5–10 m/h), about 35 to 75% and 67 to 97% of coliform were removed without and with the chlorine addition, respectively. Findings of this study clearly confirmed the efficiency of the FBR system for the post-treatment of the secondary wastewater treatment plant effluents without any solid problem during the chlorination.
Application of fluidized bed reactors (FBR) in wastewater treatment has received much attention in the world today due to their high efficiency, and low capital and operating costs. This technology is also gaining popularity as a result of increasingly stringent discharge standards and increased water reclamation demand. In recent years, FBR technology has also been conducted as an effective method to treat various types of high-strength wastewaters such as corn steep liquor , distillery effluent , synthetic sago wastewater , high-sulfate wastewater  and so on.
In general, effluents from secondary wastewater treatment plants are further treated in tertiary plants for reuse. In tertiary treatment plants, various methods such as membrane processes, advanced oxidation process, adsorption, filtration and others can be used for purification or post-treatment of wastewater effluents [5, 6]. The properties of wastewater effluents can directly influence the filtration rate and selection of the appropriate filter media. In water treatment plants, filter media are often selected as sand matter and fixed in a tank . In filtration of secondary wastewater treatment plant effluents for the future reuse, the filter media sizes are often lighter and greater for prevention from clothing, as well as for providing higher operation time. However, in that case, the filter media may not be able to properly treat the effluents due to greater filter media size . If the filter media is chosen as fine as possible, the clothing and lower operation time will become the main problems. To overcome these problems, a proper alternative can be the treatment of effluents before passing through the filter media . Considering the above-mentioned facts, it is noteworthy that FBR technology can be used as an effective method for the complete biological treatment of the secondary wastewater effluents before the filtration process.
To the best of the authors' knowledge, there are no systematic papers in the literature specifically devoted to a study regarding the application of a two-stage FBR system with a filtration column for treatment of secondary effluents. Therefore, clarification of the place of the present subject in the scheme of secondary wastewater treatment can be considered as a specific field of investigation to compare results with the above-mentioned studies. For that reason, the aim of the present work was to investigate the performance of a two-stage fluidized bed bioreactor system, filled with PVC as the fluidized bed (first stage) and gravel for the filtration purpose (second stage), for the post-treatment of the secondary wastewater treatment plant effluent.
Materials and methods
Characteristics of secondary wastewater treatment plant effluents
5-day Biochemical Oxygen Demand (BOD5, mg/L)
Chemical Oxygen Demand (COD, mg/L)
Total Kjeldahl Nitrogen (TKN, mg/L)
Total Phosphorus (TP, mg/L)
Total Solids (TS, mg/L)
Electrical Conductivity (EC, μmoss/cm)
Total Coliform (MPN/100 mL)
22 × 103
175 × 103
Fecal Coliform (MPN/100 mL)
14 × 103
105 × 103
Preparation of bacterial culture
Multiple tube fermentation method was used for determination of total coliform and also fecal coliform. The Lactose broth, Brilliant Green and EC broth (Merck, Germany) were used for the preparation of the microbial culture. Microbial culture was prepared according to the procedures described in the Standard Methods .
On the basis of the preliminary jar test results, aluminum sulfate (30 mg/L) was added into secondary effluents for the coagulation purpose. Chlorine (1 mg/L) was added for the disinfection of the effluents. The wastewater samples were mixed with a vertical stirrer (Lovibond, USA) to obtain a uniform environment in feeding material. The FBR system was operated in a continuous mode feeding by pumping of the fresh feed into the reactors. The bioreactors were maintained at respective temperatures for about 10 days to allow temperature equilibration and the growth of microorganisms. After this period, the performance of the present FBR system was investigated in a temperature-controlled environment (35°C) by collecting wastewater samples from the effluent of the second bed at predetermined intervals of 2 h for the further microbial and physicochemical analyses.
Pressure drop and washing of beds
The pressure drop was determined as a measure of decrease in flow rate and removal efficiency. The height of the water column above of both columns was maintained as 2 cm to ensure continuous flow rate and to avoid channeling of the wastewater. When the height of the water column reached to about 2.2 cm, the columns were removed and backwashed with the distilled water. Moreover, when the pollutant concentration in bioreactor effluents exceeded the influent concentration, the bioreactor was cleaned by backwashing. In the present work, both methods were applied to investigate the pressure drop problem. The second column was backwashed once per two days of operation to prevent pressure drop. During backwashing of the second bed, the effluent from the first column was re-circulated towards the inlet of the first bed to maintain the microbial activity. For the present case, it was observed that most of the sludge was captured at the internal parts of the second bed.
Results and discussion
Removal of BOD5 and COD
The concentration of biological oxygen demand (BOD5) in wastewater treatment plant effluents can significantly influence the dissolved oxygen rate in receiving water bodies. The daily standard for BOD5 in secondary wastewater effluents is limited to be maintained at or below 30 mg/L by authorized organization [7, 13]. In addition, the concentration of BOD5 in clean water or clean rivers is restricted to as low as 2 mg/L . Therefore, any treatment methods must reduce the BOD5 concentration to below 2 mg/L.
Figure 2b illustrates the removal of COD at different surface loading rates. Likewise, higher removal efficiency for COD was observed at 7.5 m/h of surface loading rate and initial COD concentration of 34 mg/L. At this condition, about 90.05% of COD was removed and the effluent concentration was reduced to about 3.23 mg/L. For the initial COD concentration of 38 mg/L and at 7.5 m/h of surface loading rate, the effluent concentration of COD was reached to be 4.06 mg/L. Furthermore, the lower removal efficiency was observed at 10 m/h hydraulic rate. Based on the relationship between BOD5 and COD [7, 9], the present system can reduce the influent COD concentration to below available standards at initial COD concentration of 34 to 38 mg/L and at 7.5 m/h of hydraulic rate.
In the tertiary treatment (coagulation–flocculation–disinfection for irrigation reuse) of a secondary wastewater treatment plant effluent, removal percentages for BOD5 and COD (initial BOD5 and COD concentrations were 11.6 (± 3.1) mg/L and 38.8 (± 6.3) mg/L, respectively) was obtained to be 46% and 39%, respectively. In the study, the authors reported that the final BOD5 and COD concentrations were reached to 6.3 (± 2.4) mg/L and 25.4 (± 4.8) mg/L, respectively . In another work , slow sand filtration system was conducted for the post-treatment of up-flow anaerobic sludge blanket (UASB) reactor effluent (average BOD5 and COD was about 50 and 120 mg/L, respectively). The study concluded that removal percentages for BOD5 and COD were achieved to be 43% and 34% in during the first few hours (36 h) and reached to be about 85% and 79% after 7 days operation, respectively. Therefore, after 7 days operation, the BOD5 and COD concentrations in slow sand filter effluent were observed to be 7.5 and 25.2 mg/L, respectively . With respect to slow sand filtration and coagulation–flocculation–disinfection process, results obtained from the present work was remarkable since the final concentrations of BOD5 and COD were 1.67 and 3.23 mg/L, respectively, which were much lower than those works.
Removal of turbidity
Considering the higher percentage removal of turbidity (89.67%) and lower turbidity in the effluent, it can be concluded that the results obtained in the present work was better than the values reported in post treatment of UASB reactor effluent by slow sand filtration . In that work, the average turbidity was 56.5 NTU and the maximum removal percentage was reported to be 91.60%. Based on this removal efficiency, the turbidity in effluent was reached to an average of 2.9 NTU. In that work, the sand depth and effective size was 54 cm and 0.43 mm, respectively. As compared to that work, however higher initial turbidity, the present system containing a fluidized bed with a filter medium with 50 cm gravel depths and 2 mm effective size, seem to be more effective and reliable for the turbidity removal. In another work conducted for the tertiary treatment of municipal sewage via slow sand filtration, higher removal efficiency for turbidity was reported to be about 88% for the sand with an effective size of 0.23 mm and a sand depth of 84 cm . When compared with slow sand filtration [15, 19], the present results showed better removal percentage for turbidity. In another work for tertiary treatment of a secondary effluent by the coupling of coagulation–flocculation–disinfection, the initial turbidity was reported to be 6.9 (± 4.3) NTU and after the treatment reached to 1.2 (± 0.4) NTU, which is near to the present results . The overall results clearly demonstrate that only slow sand filtration is not only enough to removal of turbidity and so needs more treatment process such as coagulation or two-stage bed filtration like the present system.
Removal of total phosphorous
Phosphate exists primarily in the ionized form in the environment and naturally found in some rocks and soils. It is necessary for plant growth as a macronutrient of most of biological being and categorized as one of the main limiting nutrients of organisms living in water resource. The main role of phosphorus in environment is well known as eutrophication in surface water in its excessive limit .
Coliform removal without chlorination at different organism and hydraulic rate
Water borne diseases are one of the main cases of human illness. Discharge of wastewater treatment plant effluents containing any type of microorganism can cause pollution of water supply resource . On the other hand, improperly designed and operated water treatment plants cannot effectively remove pathogenic organisms from drinking water. This problem can result in nuisance consequences in developing counties where there is no significant water disinfection facilities [18, 23, 24]. World Health Organization  estimated that 1.1 billion people over the world cannot access to safe water. Consumption of contaminated water can cause many severe diseases such as typhoid fever, hepatitis A and E, polio and cholera . Many microorganisms, especially coliform group, can cause various types of waterborne diseases. Therefore, inactivation of pathogenic organisms by appropriate methods is necessary for protection of human health from drinking of contaminated water. Originally, the chlorine and its intermediates are common chemicals that are added into the water to inactivation of pathogenic microorganisms [6, 25]. Newly, many researchers indicated that addition of chlorine for disinfection of water can produce many by-products that are carcinogen for human health. Removal of these compounds by an efficient method itself is a serious subject in the field of water disinfection . For that reason, any effective methods such as advanced oxidation processes [26, 27] that can be capable to remove pathogenic microorganisms without production of by-products would be valuable. It can be seen from the literature that, the commonly used methods for the microorganism removal are chlorination, ozonation, UV irradiation and other oxidation methods . However, to the best of our knowledge, there is little work for removal of fecal and total coliform with fluidized bed accompanying with a filtration unit. For that reason, in the present work, the efficiency of FBR system with and without chlorination was also investigated for removal of total and fecal coliform organism.
At surface loading rate of 7.5 m/h, removal efficiency was increased about linearly for fecal total coliform from 52 to 60% for increases in total coliform concentration from 27×103 to 140×103 (MPN), respectively. For these values, the effluent total coliform concentrations were 13×103 to 56×103 (MPN), respectively. For the fecal coliform, removal efficiency increased from 44 to 73% with increases in coliform concentration from 16×103 to 105×103 (MPN), respectively. The effluent fecal coliform concentration was 9×103 (MPN) at 44% removal efficiency and 28×103 (MPN) at 73% removal efficiency. Result indicated that either total coliform or fecal coliform at those values of removal efficiency did not meet the standard levels for environmental discharges. Consequently, increasing of surface loading rate from 5 to 7.5 m/h led to decreases in coliform removal efficiencies.
At surface loading rate of 10 m/h, total and fecal coliform removal efficiencies decreased with the increase in coliform concentration. With the increase in total coliform concentration from 33×103 to 110×103 (MPN), removal efficiency decreased from 64 to 45% respectively. On the other hand, fecal coliform removal efficiency was decreased from 53 to 35% with the increase in fecal coliform concentration from 17×103 to 60×103 (MPN), respectively. The concentration of total coliform in filter effluents was 12×103 (MPN) at 64% removal efficiency and 60×103 (MPN) at 45% removal efficiency. Similarly, fecal coliform concentration was 8×103 (MPN) at 53% removal efficiency and 39×103 (MPN) at 35% removal efficiency.
Results demonstrated that total and fecal coliform could be removed effectively in surface loading rate of 5 and 7.5 m/h, with increasing in organism concentration. Decreases in coliform removal efficiency at 10 m/h surface rate may be attributed to the removal of biofilm layer from the fluidized bed and also due to disorienting of the filter bed at high hydraulic loading conditions. Therefore, it can be concluded that both surface rate and microorganism concentration will affect the removal efficiency. In overall, the microorganism concentration as coliform from wastewater plant effluent and influent to the FBR system was in the range of 15–175×103 (MPN), and the present system removed about 30% to 83% of microorganism depending on the microorganism type and the surface loading rate. However, it is noted that the discharge standard for secondary wastewater as coliform organism has been reported to be below 200–400 MPN/100 mL . The removal percentage of present system was considerable, but the FBR could not meet the compliance with the effluent discharge standards for the coliform organism without chlorination. Therefore, a proper disinfection will be required when treating secondary wastewater with the FBR system.
Coliform removal with chlorination
Comparison of different FBR typologies on treatment of various types of wastewaters
Reactor type and dimensions
Initial concentrations and loading rates
Reference and region
(i.e. COD, BOD 5, TS removals, etc.)
Secondary wastewater treatment plant effluents
Two-stage FBR, V = 106 L, H = 1.5 m, D = 0.30 m
PVC material and gravel
COD = 32–40 mg/L
pH = 7.2-7.5
90.5%, 91%, 89.7%, 89.1% and 98% of COD, BOD5, turbidity, TP and coliform removals, respectively
Present study, Iran
BOD5 = 16.9-21.5 mg/L
T = 35°C
vf = 5, 7.5 and 10 m/h
TS = 556–704.6 mg/L
Turbidity = 6.2-7.5 NTU
Synthetic starch wastewater
Anaerobic tapered FBR, V = 7.8 L
Granular activated carbon (GAC)
OLR = 1.0-85.44 kg COD/(m3.day)
pH = 6.8-7.2
92% of COD removal
Parthiban et al. , India
HRT = 1.97-26.74 h
COD = 1100–7000 mg/L
BOD5 = 690–5960 mg/L
GAC-FBR, H = 4.9 m, D = 0.51 m
TNT = 3.5-56.2 mg/L
pH = 6.8-7
Effluent TNT = <0.03 – 2.8 mg/L
Maloney et al. , USA
HRT = 125–375 min
T = 67–106.5°F
Real textile wastewater
Anaerobic FBR, V = 4 L, H = 73 cm, D = 5.2 cm
OLR = 1–5 kg COD/(m3.day) COD = 1030–6000 mg/L
HRT = 24 h
82%, 94% and 59% of COD, BOD5 and color removals, respectively
Sen and Demirer , Turkey
T = 35 (±2)°C
vf = 19 m/h
Diesel fuel (DF)-contaminated wastewater
Three-phase FBR, V = 200L, H = 3 m, D = 0.17 m
Lava rock particles
DF = 50–700 mg/L COD = 547–4025 mg/L
pH = 6.7-7.8
>99.9%, 96.2%, 99.9% and 47.8% of DF, COD, TS and turbidity removals, respectively
Lohi et al. , Canada
HRT = 4 h
T = 20 (±5)°C
vf = 0.3 cm/s
Anaerobic inverse FBR, V = 1.9 L, H = 1.37 m, D = 4.48 cm
pH = 7
>90% of COD removal
Alvarado-Lassman et al. , Mexico
particles and polyethylene material
OLR = 70 kg COD/(m3.day)
T = 35°C
vf = 6 m/h
BOD5 = 1375 mg/L
Anaerobic FBR, V = 3.75 L, H = 750 mm, D = 80 cm
OLR = 1.5-8.4 kg COD/(m3.day) COD = 810–4200 mg/L
pH = 7.8
98%, 95% and 65% of COD, BOD5 and color removals, respectively
Haroun and Idris , Malaysia
HRT = 4–12 h T = 35°C
High-strength distillery wastewater
Anaerobic FBR, V = 5.9 L, H = 74 cm, D = 6.5 cm
OLR = 3–20 kg COD/(m3.day)
pH = 6.7-7.6
>80% of COD removal
Fernandez et al. , Chile
T = 30 (±2)°C
As seen from the recent literature, there are no systematic papers specifically devoted to a study regarding the application of a two-stage FBR system with a filtration column for the post-treatment treatment of secondary wastewater treatment plant effluents. For this reason, the present study aimed at fulfilling the gap in this field by focusing upon the treatment performance of FBR technology on real secondary effluents. Moreover, most of studies were conducted at laboratory-scale; however, the applicability of the FBR system was specifically investigated at pilot-scale in the present work. Besides conventional waster parameters, the efficiency of a two-stage FBR system was also investigated as a specific objective for inactivation of pathogenic organisms (i.e. total and fecal coliform organism) with and without chlorination. Based on the above-mentioned facts, the novelty of the present study is highlighted with comparison of experimental results from the previous publications.
The study revealed that a two-stage pilot-scale FBR system for secondary wastewater treatment was technically feasible in terms of BOD5, COD, turbidity, TP, total coliform and fecal coliform. Depending on both microorganism concentration and applied surface loading rates, the FBR system could meet the available standards with removal percentages above 90%. The FBR–chlorination system could be used as a promising post-treatment process to improve the quality of the final discharge without any increase in TS concentration during the chlorination.
The authors would like to thank the Alborz University of Medical Sciences for supporting of this work.
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