Header
Review Article
Analysis of a Vertical Bioreactor for Denitrification and Biological Phosphorus Removal from Wastewater
Maryam Reza1* and Manuel Alvarez-Cuenca2
1Laboratory of Water Treatment Technologies, Chemical Engineering Department, Ryerson University, Canada
2Chemical Engineering Department, Ryerson University, Canada

ABSTRACT
Current environmental regulations regarding the nutrient discharge limits are becoming increasingly strict in Canada and other industrialized countries. As a result, there are strong social and economic needs for the development of nutrient removal technologies that are compact, cost effective and highly efficient. The present article explains in detail the know-how to design, operate and evaluate an advanced nutrient removal bioreactor called Compact Upright Bioreactor for the Elimination of Nutrients (CUBEN). CUBEN has been designed and tested in a pilot-scale unit. It has a unique configuration which occupies smaller foot print and has higher nutrient removal efficiency and lower pumping costs compared to conventional technologies. This new bioreactor requires fewer pumps due to its vertical alignment in which water flows by gravity from one stage to the next. CUBEN consists of four consecutive stages including Deaeration, Anoxic, Anaerobic and Aerobic where dissolved oxygen (DO) removal, denitrification and phosphorus removal processes take place respectively. The bioreactor exhibits a superior performance removing 100% of DO and 98% of nitrate. The results obtained for the biological phosphorus removal (BPR) reached the target value of 0.5 mg/L in the effluent after nine months of continuous operation. The BPR process efficiency of CUBEN has been found to be over 95%.
Keywords
Nutrient removal process; Vacuum deaeration; Anoxic; Anaerobic; Aerobic; Denitrification; Biological phosphorus removal; PAOs; PHAs

INTRODUCTION
The applications, management and processing of wastewater have experienced an extraordinary transformation in the last two decades. Methods and terminology once the domain of disciplines like Chemical Engineering or, Microbiology are incorporated in the academic programs of water/wastewater treatment. What used to be management and disposal of waste is being replaced by processing of a resource to obtain value added products. In this context, the configuration or topology of a conventional wastewater treatment plant is being replaced by that typical of a chemical engineering processing plant. These reactors offer a much smaller construction surface, exhibit a greater operational flexibility than conventional horizontal basins and deliver equal or superior process performance.
There are many different commercially available, nutrient removal technologies in the wastewater industry, trying to meet the stringent limits of nutrient discharges. Most commercial and experimental biological nutrient removal (BNR) plants consist of horizontal, rectangular cross section bioreactors. These technologies have various drawbacks which limit their operation including:
· Large construction area
· Control complexity
· Excessive sludge recycles
· High capital costs
· Undesirable sludge production
· Long residence time
· Provision of excessive carbon source requirement
· Moderate pumping
This article presents a biological nutrient removal reactor,with unique configuration. The Compact Upright Bioreactor for the Elimination of Nutrients (CUBEN) consists of four consecutive stages each removing specific constituents from the wastewater. CUBEN has a smaller footprint due to its vertical configuration than any of the existing commercial horizontal BNR reactors. The enlargement of this unit is simple since it can be expanded vertically by adding more Aerobic, Anaerobic or Anoxic stages. The biological processes in CUBEN are based on well recognized nutrient removal processes such as separate denitrification for nitrate removal and Anaerobic/Aerobic (A/O) process for biological phosphorus removal (BPR). The design of a vacuum Deaeration stage, located up stream of denitrification and BPR processes, is another unique feature of CUBEN. Vacuum Deaeration not only provides anaerobic condition by removing dissolved oxygen (DO) from wastewater, it also removes toxic or inhibitory compounds (NO, N2O, N2H2, etc. ) produced through inherent biological/biochemical reactions in wastewater. The results presented herein show that DO can be virtually 0 mg/L in the Anoxic and Anaerobic stages, the elimination of nitrates in the Anoxic stage exceeds 98 % and the concentration of phosphorous in the effluent can be reduced to less than 1 mg/L without the addition of any salt or chemical.
MATERIALS AND METHODS
CUBEN [1] has four consecutive stages. First, wastewater is pumped to the vacuum Deaeration stage where DO and other volatile compounds are being removed. Vacuum Deaeration can work in either batch or continuous mode. Under the batch mode water can flow by gravity to the next stage. However, in a continuous mode, wastewater needs to be pumped out from the vacuum Deaeration stage. Due to the vertical alignment of CUBEN, water/wastewater flows by gravity from one stage to another (Anoxic-Anaerobic-Aerobic). Figure 1 is a block diagram showing the arrangement of the stages.
Figure 1 Block Diagram of the Compact Upright Bioreactor for the Elimination of Nutrients (CUBEN).



Figure 1

Block Diagram of the Compact Upright Bioreactor for the Elimination of Nutrients (CUBEN).

×
CUBEN design basis
Table 1 shows the feed flowrate and concentration of the constituents of the wastewater used in the design and operation of CUBEN. The CUBEN's influent contains nitrate and phosphorus concentrations which represent a wastewater that has undergone secondary treatment. The wastewater flowrate of 120 (L/day) is considered as the basis for the design of this unit.
Table 1 CUBEN Bioreactor Design Basis.

Table 1

CUBEN Bioreactor Design Basis.

Parameters

Design Influent Criteria

Design Effluent Criteria

Flowrate (L/day)

120

120

COD (mg/L)

100-300

<10

NO3 (mg/L)

25

< 0.5

Total Phosphorus (mg/L)

10-30

<1

Dissolved Oxygen (mg/L)

4-6

2.5-3.5

×
Synthetic wastewater of the following composition was used as the feeding solution: KNO3 (4.109g), KH2 PO4 (5.535g), Na2HPO4.H2O (5.614g), Na2HPO4 (5.776g), Acetic Acid (10 ml),  Butyric Acid (10 ml), Propanoic Acid (10 ml), and Methanol (5ml). Minerals: NaHCO3 (34.7 g),   KCl (4.5g),  CaCl2.H2O (1.512g),  MgSO4.7H2O (1.512),  FeCl3 (1.5g /L).
Parameters that highly influence the biological processes in wastewater have been considered in the designing of the CUBEN. These are:
· Hydraulic Residence Time (HRT)
· Carbon Source
· Sludge Residence Time (SRT)
· pH of the wastewater
· Temperature
· Dissolved Oxygen Concentration
Hydraulic Residence Time (HRT): For CUBEN is calculated to be approximately 14 hours from the time wastewater enters the Deaeration stage until it leaves the Aerobic stage. Each stage in CUBEN has different HRT associated with the contact time required for microorganisms to perform their specified tasks.
Carbon Source: In wastewater, under anaerobic condition, Phosphorus Accumulating Organisms (PAOs) have the capability to take up acetates and convert them into interacellular carbon polymers called poly-hydroxyalkanoates (PHAs). Normal heterotrophic bacteria under anaerobic conditions ferment complex volatile fatty acids (VFAs) into acetate. The ability of PAOs to take up acetate anaerobically creates a competitive advantage over normal heterotrophic microorganisms. VFAs are limited resources in biological phosphorus removal systems and their use by PAOs must be maximized to optimize the phosphorus removal process. The biological phosphorus removal is a hypersensitive process and the quantity and quality of the organic carbon mixture added to the anaerobic phase directly affect the phosphorus removal efficiency. There were some instances in which phosphorus removal in a wastewater treatment plant or a bench scale experiment favored the growth of PAO's competitors called Glycogen Accumulating Organisms (GAOs). These organisms are also able to remove VFAs under anaerobic conditions; therefore, they compete with PAOs for the same substrate and thereby diminish the removal of phosphorus by PAOs [2]. Other types of VFA important in BNR process are propionic and butyric acids which are abundant in many prefermentation processes. In previous studies propionic acid has shown to be a more favorable carbon source than acetic acid [3]. Other studies have also shown that the maximum rates of anaerobic acetate uptake and phosphorus release can be achieved with optimum concentrations of acetic, butyric and propanoic acids mixture [4]. There are different ratios of mgCOD/mgP suggested by some authors to be added in the anaerobic stage. For instance, some suggest a ratio of COD to phosphorus concentration of 15:1, 35:1 or greater such as 45:1 (mgCOD/mgP) [5] and 50:1 mgCOD/mgP [6] is required to achieve an effluent phosphorus concentration of 1.0 mg/L or less. As a result, the amount of COD (VFA or carbon source) is a parameter difficult to predict therefore, the present research protocol started the tests with 300 mg/ L of COD or 30:1 (mgCOD/mgP) added in the synthetic wastewater. The COD concentration was varied during the commissioning of the unit in order to find the optimum value.
Sludge Residence Time (SRT):is one of the important factors in biological phosphorus removal. Indeed, this is due to the nature of PAOs responsible for BPR process. PAOs under long SRTs can break down the cellular polyphosphates and release ortho-phosphates into the environment. Therefore, adequate SRT and proper handling and recycling of the sludge dominated by PAOs are essential. The simulation results of various phosphorus removal processes such as University of Cape Town (UCT) have shown that phosphorus removal efficiency reaches maximum at short SRT between 3 to 5 days. If the sludge age is shorter than this range the low sludge concentration causes incomplete conversion of biodegradable material into volatile fatty acids (VFAs) which reduces the availability of substrate for PAOs. In terms of denitrification process in the Anoxic stage, SRT of 7 day (approximately one week) can result in high nitrate removal [7]. The SRT of the Anoxic stage in CUBEN is higher than conventional denitrification processes. This is due to the use of packing and formation of denitrifying biofilm which produces less sludge and consequently high SRT in this stage.
pH: The effect of pH on the stoichiometry and kinetics of acetate uptake by PAOs is an important element in a successful BNR process. In the Anaerobic stage, the amount of phosphorus released per acetate taken up is linearly dependent on the pH due to the additional energy requirements for acetate transport at higher pH [8]. Also, low pH results in the production of GAOs which are PAOs competitors in BPR process. GAOs are able to uptake acetate in the anaerobic stage and store PHAs within their cells. However, they cannot uptake phosphorus in the subsequent Aerobic stage which results in the deterioration of the BPR process [9]. The optimum pH for both denitrification and BPR in the CUBEN is in the range of 6.5 and 8.0.
Temperature: The effect of temperature on nutrient removal and specially BPR is not well understood. Past studies have shown that phosphorus release and/or phosphorus uptake can increase with increasing temperature from 5oC to 30oC. Phosphate release rates were observed to decline at temperatures higher than 35oC and at temperatures higher than 45oC, no phosphate release or uptake were observed. These results indicate that PAO population decays and consequently phosphorus removal deteriorates at that temperature [7]. In addition, lower temperature can shift the microbial community from GAOs, PAOs competitors, to purely PAO population [10]. In BPR process, low temperatures (5oC or less) can decrease the rate of phosphorus removal by negatively influencing the biochemical processes such as phosphorus release/uptake, acetate uptake, PHA synthesis and utilization. A successful BPR process is achievable at lower temperature only through increasing the SRT of the process. Low temperature decreases the kinetics of the process therefore high sludge age in cold weather results in better management and utilization of the PAOs. The temperature for CUBEN is in the range of 18-25oC (room temperature). Temperatures in this range do not adversely affect the denitrification or the phosphorus removal process.
Dissolved Oxygen (DO): In past experiments, DO concentration has shown significant effect on the PAO-GAO competition hence, the BPR efficiency. It has been frequently observed that oxygen concentrations of approximately 2.5-3.0 mg/L in the aerobic zone can favor the growth of PAOs. On the other hand, very high DO concentrations (i. e. 4.5-5.0 mg/L) have deteriorated the BPR [3]. Therefore, in the Aerobic stage of the CUBEN, aeration was monitored and controlled using air flowmeter and DO sensors to maintain the DO concentration within the range 2.5-3.0 mg/L. Also, the presence of DO in the Anoxic and Anaerobic stages has a negative influence on both denitrification and BPR. CUBEN has an excellent DO removal ability due to vacuum operation. Therefore, Anoxic and Anaerobic stages were continuously monitored to maintain the DO concentration of less than 0.1 mg/L. Table 2 exhibits the parameters and process conditions in all stages of CUBEN.
The deaeration stage
The removal of DO from the wastewater entering CUBEN is a critical step for the subsequent nitrate and phosphorus removal that take place in the Anoxic, Anaerobic and Aerobic stages. In medium and large scale plants, it is very difficult to consistently and reliably remove and control the DO. The removal of DO from water can be achieved either physically or chemically. Chemical methods are not used due to either the undesirable effects of scavengers such as sulfite or increased sludge content from the chemical addition to the water. Physical methods of oxygen removal from water include thermal degassing, vacuum degassing and nitrogen stripping. Among the above physical methods, vacuum degassing (Deaeration) and nitrogen stripping are relatively fast and simple. Vacuum Deaeration has been applied successfully to remove DO in a three-phase fluidized bed [11]. Nitrogen stripping is more cost effective in small installations compared to the vacuum stripping which requires a greater initial capital cost. However, in the long term vacuum stripping has shown to be more economic due to lower maintenance and lower consumable costs [12]. Vacuum stripping is a method used in the Deaeration stage of CUBEN for effective and fast removal of DO from wastewater. The performance of the Deaeration stage in CUBEN is very important because the performance of subsequent stages depends on it. The lower the oxygen concentration in the effluent leaving the Deaeration stage, the better is the efficiency of the Anoxic, Anaerobic and Aerobic stages. Wastewater first enters into the bioreactor through a water distributor (misting nozzle) connected to the inlet pipe at the top of the column. The small droplets are spread on the packing section of the Deaeration stage. The type of packing used in this stage is plastic hollow spherical packing called Tri-Packs® which are commercially available. Tri-Packs® have 2.54 cm diameter and are made of polypropylene. The packing provides proper liquid distribution and more surface area for DO removal. A plastic rack with large openings is used to hold the packing 30cm above the Deaeration reservoir. The volume of Deaeration stage is 0.0615 m3 given the HRT of 6 hours, 0.3 m of column diameter, 0.3m of the packing height and 0.15m of the distributor zone [1].
Table 2 Design Parameters.

Table 2

Design Parameters.

Sequence of Stages

Type of Nutrients

Nutrients to be removed

Influent Concentration (mg/l)

Effluent Concentration (mg/l)

Carbon Source Addition

Deaeration
Stage

DO
NO3
TP


DO


4-6


0

Acetic + Propionic + Butyric Acids

Anoxic
Stage

NO3
TP


NO3


25


0.5


Methanol

Anaerobic
Stage


TP


TP


8-12


16-24

Acetic + Propionic + Butyric acids

Aerobic
Stage


TP-DO


TP


16-24


> 1


NONE

×
The anoxic stage
The Anoxic stage in CUBEN provides the necessary conditions for the denitrification process. These conditions are summarized as follows [13]:
. Presence of facultative bacteria which use both oxygen and nitrate (NO3-) to oxidize the organic matter. It has been established experimentally that activated sludge generated under aerobic conditions will use NO3 - immediately after entering an anoxic phase.
. The presence of DO in the Anoxic stage inhibits the development of denitrification. In general, it has been observed that DO concentrations of more than 0.2-0.5 mg/L reduces the rate of denitrification significantly.
. The presence of an electron donor or biodegradable organic matter is essential for the reduction of NO3 -. Methanol is frequently used as a carbon source for denitrification.
NO3- concentration in the influent to the Anoxic stage of the CUBEN is approximately 25 mg/L which resembles the effluent concentration of a secondary treatment level in a wastewater treatment plant. NO3- concentration in the effluent from the Anoxic stage can be as low as 0.5 mg/L. The presence of high NO3- concentration in the Anoxic stage effluent can reduce the reliability of phosphorus removal process in the following stages.The Anaerobic stage must be protected against NO3 - and DO so that PAOs can carry out the BPR process. Bacteria can use NO3 -and oxidize part of the VFAs which must be utilized by PAOs.The presence of NO3- therefore can reduce the fraction of PAOs in the system thus the phosphorus removal capacity. Based on the data obtained from BNR plants, the hydraulic residence time for successful denitrification normally requires 1.8-2 hr to be completed [14].
The anaerobic stage
In the Anaerobic stage, under proper conditions and sufficient VFA, PAOs break down their internal polyphosphate into orthophosphate molecules. As well, they convert Adenosine Triphosphate (ATP) to Adenosine Diphosphate (ADP) and Adenosine Monophosphate. The breakage of these bonds releases high amounts of energy required by PAO cells to uptake VFAs and convert them interacellulary into polymer compound such as PHAs. Under aerobic conditions, PAOs uptake phosphorus from the wastewater for the reconstruction of cell structure, for growth and reproduction [15]. In a BPR process, the optimal phosphorus removal occurs when the HRT of the Anaerobic stage is sufficiently large to allow efficient fermentation of VFAs from biodegradable organic matter and subsequent uptake of acetate. The HRT of two hours has shown excellent results in BPR process [15].
The aerobic stage
Biological phosphorus removal is accomplished by creating conditions favorable for the growth of PAOs. As it was discussed previously, the Anaerobic stage provides selective advantage for the PAOs to dominate the heterotrophic bacterial community. Due to the lack of oxygen and nitrate in this zone, PAOs cannot oxidize the organic matter and it accumulates interacellularly as carbon polymer (PHAs). When PAOs arrive into the Aerobic stage, these carbon polymers are oxidized providing the energy source to take up phosphorus from the wastewater. Then PAOs use a small portion of this phosphorus to build up their internal cell structure, to grow and reproduce. The remaining phosphorus is accumulated in the form of polyphosphate inside their cells. The enrichment of the biomass with PAOs provides the biological mechanism by which phosphorus is removed from the wastewater. The Aerobic stage in CUBEN must provide sufficient oxygen transfer to the PAOs. Compressed air is injected into this stage through a fine-bubble air diffuser located near the bottom of the Aerobic stage. Oxygen is transferred from the rising air bubbles into the bulk solution and used by PAOs. The injected air also provides continuous mixing of this zone. Another important parameter in regards to PAOs is the decay rate. The decay and growth rates of PAOs are significantly slower than that of normal heterotrophic bacteria. The decay and growth rates of PAO have been experimentally found to be 0.04/day and 0.04/h respectively [16]. Therefore, it is expected to achieve a stable and efficient phosphorus removal process after a continuous long term operation. Practical experience suggest that acclimatization of biological nutrient removal process requires at least 40 to 100 days to reach stable and good phosphorus removal yields [17]. Figure 2 illustrates the flow diagram of CUBEN in combination with a membrane filtration unit.
Figure 2 Process Flow Diagram of a Pilot-Scale CUBEN.



Figure 2

Process Flow Diagram of a Pilot-Scale CUBEN.

×
As shown in Figure 2, the wastewater enters from the top of the column into the Deaeration stage or vacuum stage where DO is rapidly removed from the bulk liquid. Then, the effluent from the Deaeration stage enters the Anoxic stage which is located under the vacuum Deaeration stage. In the Anoxic stage, nitrate concentration is reduced and converted to free nitrogen. A carbon source such as methanol must be added in the Anoxic stage to provide energy for the growth of denitrifying bacteria and enhancement of the denitrification. Then, wastewater flows by gravity to the subsequent anaerobic stage where DO concentration is below 0.1 mg/L and NO3 - concentration is less than 0.5 mg/L. In the Anaerobic stage, PAOs uptake VFAs and accumulate them in their cells in the form of PHAs. As PAOs take up VFAs and store PHAs inside their cells, they also release phosphorus into the water. Therefore, the phosphorus concentration in water highly increases in this stage. Then, PAOs enriched with PHAs enter the Aerobic stage where they oxidize the cellular PHAs as a source of energy and uptake both the phosphorus already present in the influent to the bioreactor as well as the amount released by the PAOs in the Anaerobic stage. The effluent from the Aerobic stage of CUBEN enters a membrane filtration unit or a secondary clarifier. The collected sludge (membrane's retentate) contains high concentration of PAOs enriched with cellular polyphosphates and the membrane's filtrate contains very low concentration of phosphorus less than 0.5 mg/L. A portion of the collected sludge (Approximately 80%) is recycled back into the anaerobic stage to be reused in the BPR process. Sludge recycling is an important requirement for successful biological phosphorus removal process. Sludge contacting PAOs can highly improve the phosphorus removal efficiency and reduces the COD concentration in the final effluent.
RESULTS AND DISCUSSION
Dissolved oxygen (DO) concentration results
DO concentrations in all four stages of CUBEN were recorded every minute to ensure that oxygen is removed in the Deaeration stage and is well supplied in the Aerobic stage. DO concentrations were recorded throughout the experimental operation. DO concentrations were logged using four oxygen sensors connected to a Data Acquisition System. Table 3 represents the average DO concentrations in all four stages of CUBEN from May to November 2011.
Figure 3 Nitrate Concentration Profile in CUBEN.



Figure 3

Nitrate Concentration Profile in CUBEN.

×
It was very difficult to adjust the DO concentration to remain within the range of 2.5-3.5 mg/L due to the large size of the installed air diffuser. Oxygen transfer in the Aerobic stage was influenced by many variables such as high concentration of soluble and particulate contaminants as well as biological species. Also, oxygen transfer was highly affected by the biochemical reactions that took place in this stage. In addition, parameters like SRT, HRT, organic loading rate and biomass recirculation affected oxygen transfer in the Aerobic stage. The DO concentrations in other stages of CUBEN including Deaeration, Anoxic and Anaerobic stages were satisfactory. This is due to the excellent performance of the vacuum stage in removing DO from the inlet synthetic wastewater
Table 3 Dissolved Oxygen Concentration (May-November 2011).

Table 3

Dissolved Oxygen Concentration (May-November 2011).

Dates
Stages

7/5/11

9/6/11

9/7/11

12/8/11

5/9/11

8/10/11

8/11/11

20/11/11

Deaeration
(DO Concentration) mg/L


0.78


0.28


0.15


0.07


0.05


0.05


0.05


0.05

Anoxic
(DO Concentration) mg/L


0.10


1.6


0.06


0.06


0.06


0.07


0.07


0.07

Anaerobic
(DO Concentration) mg/L


1.5


2.9


0.07


0.07


0.07


0.07


0.07


0.07

Aerobic
(DO Concentration) mg/L


5.3


8.4


5.5


3.2


2.2


3.5


3.0


2.8

×
Denitrification results
The influent concentration of NO3-was maintained between 24-25 mg/L throughout the experimental period. The denitrification process in the Anoxic stage began approximately three days after the start-up of the unit. The Anoxic stage was inoculated with fresh sludge collected from Conestoga Meat Packers Ltd. , Wastewater Treatment Plant. The effluent NO3-concentration at the beginning was about 4-5 mg/L which showed almost 80% removals. After one week from the start-up date, denitrification efficiency reached 98-100% removal. The denitrifiers responsible for the denitrification process showed a remarkable adaptability to the new environment caused by synthetic wastewater, carbon source, temperature and pH. To maintain the high nitrate removal efficiency of the unit, pure Methanol (about 5-10ml) was added directly to the Anoxic stage. Another important factor in the high denitrification rate was the inclusion of packing in this stage. The presence of Hydroxyl-Pac media in the Anoxic stage resulted in denitrification via biofilm formation. The attached growth offered several advantages over the suspended growth denitrification. The following factors associated with the development of biofilm were key elements in the successful denitrification in the Anoxic stage of CUBEN.
. Protection against washout of slow growing bacteria under high HRT
. Attached microbial community on the surface of the packing has interspecies interaction that is beneficial for the   individual denitrifying bacteria
. The presence of packing in the Anoxic stage provides higher surface area and consequently increases the   concentration of the denitrifiers in this stage
. The biofilm formation of denitrifiers on the surface of the packing reduces their flow to the Anaerobic stage thus   avoiding the interference of the denitrifiers in the BPR process
. It provides an extremely cost-effective retrofit solution for future expansion of the unit
· The existence of a high-density population on fixed film bacteria reduces the sludge generation
. Table 4 summarizes the average nitrate concentration in water passing through different stages of CUBEN   starting in the feed tank, Anoxic, Anaerobic and Aerobic stages.
As it can be seen from Table 4, denitrification took place not only in the Anoxic stage but also in later stages of CUBEN (Anaerobic and Aerobic) which resulted in the overall nitrate removal of 98-100%. This proves the presence of denitrifiers along with the presence of PAOs in both Anaerobic and Aerobic stages. These results reveal three outcomes:
. Integration of both denitrifying bacteria and phosphorus accumulating organisms (PAO) in the same   environment (anaerobic and aerobic phases)
. Presence of denitrifying PAOs (DPAOs) in the Anaerobic and Aerobic stages responsible for both denitrification   and phosphorus removal processes
. Integration of both DPAOs and normal PAOs in the two later stages
DPAOs are capable of using NO3 - as electron acceptor when there is no oxygen available. In CUBEN, DO is completely removed from the solution in the Deaeration stage and small concentration of NO3 - is present in the Anoxic stage effluent entering the Anaerobic stage. The denitrification in the Anaerobic stage might have been due to the presence of DPAOs. Figure 3 illustrates the nitrate removal profile throughout various stages of CUBEN for different dates. The inlet concentration of nitrate was varied to investigate the effect of the inlet nitrate concentration on the performance of denitrifying bacteria. As expected initially, high inlet nitrate concentration decreased the efficiency of the unit. However, denitrifiers quickly adapted to this high concentration and after few days the unit performance experienced a remarkable improvement. During the last months of the experimental period, the inlet nitrate concentration in the feed was fixed at 25 mg/L and denitrification process continued to take place efficiently.
Table 4 Nitrate Concentrations Profile in CUBEN.

Table 4

Nitrate Concentrations Profile in CUBEN.

NO3 - Concentrations
(mg/L)

7/5/11

9/6/11

9/7/11

12/8/11

5/9/11

8/10/11

8/11/11

20/11/11

8/12/11


Feed


25


25


24


26


25


25


24


25


24


Anoxic


22


7


14


3.4


2.7


1.2


0


0


0


Anaerobic


7.6


9.8


9.4


2


1.7


4.2


0.7


1.7


0.6


Aerobic


5.4


0.3


2.8


2


0.2


3.9


0


0.3


0.1


% Removal


78


99


88


92


99


84


100


99


99

×
Phosphorus removal results
Biological phosphorus removal in the Anaerobic and Aerobic stages of CUBEN took place several months after the inoculation of the bioreactor in April 2011. The phosphorus removal development was much slower than the denitrification since the microbial growth rate of PAOs is much slower than that of denitrifiers. The enrichment of PAOs after the bioreactor's inoculation was a long process. As it was mentioned earlier, biological phosphorus removal process and in particular PAOs responsible for phosphorus removal are highly sensitive to experimental conditions and require long term operation to reach steady state. Many of the environmental parameters such as the inlet phosphorus concentration, pH and DO concentrations have been varied during the commissioning period to enrich the PAOs and improve the overall phosphorus removal. Initially, the inlet phosphorus concentration was increased from 10 mg/L up to 30 mg/L to determine the optimum inlet phosphorus concentration relative to the concentration of PAOs. The phosphorus concentration increased in the Anaerobic stage as expected. The increase in phosphorus concentration in this stage confirmed the presence of PAOs as they release phosphorous to their surrounding water under anaerobic condition while taking up acetate from water. The Aerobic stage in CUBEN offered proper environmental conditions such as sufficient DO, neutral pH and complete mixing. The BPR results reached the target value of 0.5 mg/L in the effluent after nine months of operation. As it can be observed from Figure 4, the overall phosphorus removal efficiency was low until mid September 2011. Ultimately, the phosphorus removal efficiency increased to over 95% in the last months of continuous operation (October and November 2011). To optimize the operating performance of CUBEN, many parameters were changed, for instance the COD concentration, type of COD mixture (various ratios of propionic, butyric and acetic acid), frequency of inoculation (from twice to three times per week). Table 5 shows the concentration of total phosphorus (TP) in influent and effluent as well as TP removal efficiency from August to November 2011.
Figure 4 Phosphorus Concentration in CUBEN.



Figure 4

Phosphorus Concentration in CUBEN.

×
Figure 5 shows the TP removal efficiency of CUBEN since biological phosphorus removal was initiated inside CUBEN. This graph shows that TP removal efficiency in CUBEN increased to above 95%.
Interactions between phosphorus and nitrate removal processes
Based on the results obtained from the Anoxic, Anaerobic and Aerobic stages, there was a close relationship between denitrification and biological phosphorus removal processes. First of all, denitrification is not limited to the anoxic conditions provided in the second stage of the CUBEN. Further, denitrification occurred in the Anaerobic as well as Aerobic stage and resulted in high nitrate removal efficiency. Denitrification similarly happened in the last two stages along with the phosphorus removal process [1]. This correlation can be either due to the presence of denitrifying PAOs or solely denitrifying bacteria (non-PAOs) which coexist with PAOs in the same environment. The similarity between the two species (normal heterotrophic bacteria and PAOs) depends on many factors including inlet nitrate and phosphorus concentrations, sequence of reactor stages and biomass transport between the Anoxic, Anaerobic and Aerobic stages [18]. Another reason for possible interaction between phosphorus and nitrate removal processes is the slow phosphorus removal development and consequently delays in reaching the steady state TP removal in CUBEN. The presence of denitrifying PAOs in the Anoxic and Anaerobic stages can slow down the overall phosphorus removal process.
Table 5 Phosphorus Concentration in CUBEN.

Table 5

Phosphorus Concentration in CUBEN.

Date

Influent TP Concentration (mg/L)

Effluent TP Concentration(mg/L)

% Removal

08/21/2011

31.8

30.3

5%

08/28/2011

30

27.9

7%

09/27/2011

31.2

29

7%

09/28/2011

31

20.8

33%

09/29/2011

31

19.2

38%

10/10/2011

31

24.8

20%

10/13/2011

23.5

16.7

29%

10/14/2011

25.1

14.7

41%

10/28/2011

25

16.8

33%

10/29/2011

25

12.3

51%

11/06/2011

15

5.8

61%

11/08/2011

15

4.6

69%

11/11/2011

15

2.3

85%

11/14/2011

15

1.5

90%

11/15/2011

15

1

93%

11/20/2011

15

0.5

97%

11/22/2011

15

0.5

97%

×
Figure 5 Biological Phosphorus Removal Efficiency in CUBEN.



Figure 5

Biological Phosphorus Removal Efficiency in CUBEN.

×
CONCLUSIONS
This article presents the operation and performance analysis of a bioreactor for the removal nutrient from wastewater. Compact Upright Bioreactor for the Elimination of Nutrients (CUBEN) is cost effective and innovative in both structure and performance. It has a smaller foot-print, lower pumping costs, and higher removal efficiency than existing conventional systems. CUBEN is easy to modify, expand and retrofit for the development of other biological processes [19]. CUBEN consists of four stages:
. The Deaeration stage where physical removal of DO takes place under vacuum. The DO concentration in the effluent of this stage was less than 0.1 mg/L.
. Anoxic stage where the anoxic conditions (high nitrate and no DO concentration) promote the enrichment of denitrifying bacteria to accomplish denitrification. The effluent concentration of nitrate from this stage approached less that 0.5 mg/L. The denitrification results in CUBEN showed a removal efficiency of 98-100%.
. The Anaerobic stage where PAOs are used to uptake acetates from water/wastewater and form PHAs interacellularly. In this stage, PAOs release orthophosphates into the surrounding liquid as a result of breakages of internal polyphosphate bonds to obtain energy, and
. In the Aerobic stage (final stage) PAOs enriched with PHAs are exposed to oxygen concentration of 2.5-3.5 mg/L. In this stage PAOs utilize reserved PHAs for cellular growth, reconstruction and reproduction. They also have the unique capability to uptake orthophosphates from the water/wastewater and form intracellular polyphosphates. Thus, removing phosphorus from the liquid phase. The results obtained for the BPR process reached the target value of 0.5 mg/L in the effluent after nine months of operation. During the first month of the unit operation, the overall phosphorus removal efficiency was as high as 60%. Ultimately, the phosphorus removal efficiency increased to over 95% in the last months of continuous operation.
ACKNOWLEDGMENT
We would like to extent our appreciation to the late Dr. Stalin Boctor, Dean of Faculty of Engineering, Architecture and Science (FEAS) at Ryerson University for his financial support.

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Cite this article: Reza M, Alvarez-Cuenca M (2013) Analysis of a Vertical Bioreactor for Denitrification and Biological Phosphorus Removal from Wastewater. Chem Eng Process Tech 1: 1006.
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