Development of Sequencing Batch Reactor Performance For Nitrogen Wastewater Treatment

1Department of Civil Engineering, Faculty of Engineering, Naresuan University, Phitsanulok, Thailand 2Centre of Excellence for Innovation and Technology for Water Treatment, Naresuan University, Phitsanulok, Thailand 3Department of Science and Mathematics, Faculty of Agro-Industrial Technology, Rajamangala University of Technology Isan, Kalasin Campus, Kalasin, Thailand 4Department of Industrial Engineering, Faculty of Engineering, Naresuan University, Phitsanulok, Thailand


Introduction
Since nitrogen has become a key factor for water pollution from eutrophication and oxygen depletion, the stringent environmental regulations are carried out to decrease the nitrogen discharge. For example, the effluent nitrogen standards of 35 mg/L for household wastewater and that of 100 mg/L for industrial wastewater were reported in Thailand [1]. In general, the high nitrogen of 40-70 mg/L was found in the household and sewage wastewater, which mainly contain ammonium-nitrogen (NH 4 -N) [2,3]. Some industries such as dairy and tannery also generate the high nitrogen wastewater in the range of 50-500 mg NH 4 -N/L [4,5]. Moreover, the effluent from treatment system is one of significant sources for nitrogen wastewater discharge; the landfill leachate contained 250-600 mg NH 4 -N/L [6] and the anaerobic digestion effluent contained 710 mg NH 4 -N/L [7]. According to the World Health Organization (2004), the consumption of high nitrate-nitrogen (NO 3 -N), the oxidized form of nitrogen, causes for blue baby syndrome in infants, and the NH 4 -N contamination leads to unpleasant taste and smell of water. To maintain the good quality of water resource, the treatment technology is required to reduce the nitrogen contamination to be the acceptable level.
The common technology for nitrogen removal is biological nitrification and denitrification. The contaminated NH 4 -N is oxidized to NO 2 -N and continued to NO 3 -N under high oxygen condition (named nitrification process), then the NO 3 -N is reduced to N 2 releasing to the atmosphere under no oxygen condition (named denitrification process). The microorganisms involved in nitrification process have been reported; Nitrosomonas sp. and Nitrosococcus sp. for converting NH 4 -N to NO 2 -N [8,9], and Nitrobacter sp. and Nitrospira sp. for converting NO 2 -N to NO 3 -N [10,11]. In the meanwhile, several microorganisms were suggested to involve in denitrification process including Ochrobactrum anthropi, Pseudonomas stutzeri, Alcaligenes faecalis, and Pseudomonas stutzer [12][13][14]. Recently, various wastewater treatment systems including sequencing batch reactor (SBR), movingbed biofilm reactor and intermittently aerated membrane bioreactor [15][16][17] were proposed for achieving simultaneous nitrification and denitrification. Among of the above mentions, the SBR is a widely used system in plants, due to its cost-effectiveness and ease operation. The conceptual of SBR operation includes four steps of filling, reacting, settling, decanting and idling. However, the periods of each step and its condition (i.e., DO and pH) were various in previous studies. For example, Guo et al. operated the SBR containing a cycle of filling (instantaneous), reacting of 7.5 h, settling of 0.5 h, decanting (instantaneous) and idling of 4 h [18]. The hydraulic retention time (HRT) and DO value were 10 h and 0.5-1.0 mg/L respectively. The operating cycle was modified to enhance the nitrification and denitrification processes by including aerobic and anaerobic in the reacting period [19]. During the reacting period, there was air supply for 8 min and no air supply for 15 min, and so on, until completing the 6 h. The aim of this study was to evaluate the performance of SBR under a typical cycle for nitrogen wastewater treatment, and clarify the nitrogen removal mechanisms.
was the rate-limiting step in this reactor, although the excess oxygen of 5-6 mg/L was maintained.
Regarding the first cycle operation, the NH 4 -N concentration was dramatically decreased in the aerating period, while high NO 2 -N was generated (data not shown). The generated NO 2 -N was decreased immediately in the non-aerating period, and together with the reduction of total nitrogen and carbon concentrations. This phenomenon suggested that the nitrogen contaminant was removed by partial nitrification and denitrification. Due to the high DO of 5-6 mg/L in the aerating period, the lack of nitrite oxidizing microorganisms was the key reason for partial nitrification occurred in this reactor. However, the further study on microbial test is required to clarify the nitrogen removal mechanisms.
Since the acetate addition was controlled at the C/N ratio of 2, which was sufficient for simultaneous nitrification and denitrification [21,22], the ratio of carbon consumed and nitrogen removed (carbon consumption) was used as an indicator to define the reactor to 40 mg/L, while the low NO 2 -N and NO 3 -N of less than 1 mg/L was found in the influent. The fresh influent was prepared and immediately replaced with the 80% of water level in the reactor.

Reactor set-up and operation
The lab-scale 15-L SBR was set-up by adding 2 L of dense sludge taking from an aerobic wastewater treatment plant of Wangthong Hospital (Phitsanulok, Thailand) and 10 L of synthetic wastewater. Two spargers for air supply were set-up at the base of the reactor, and a stirrer was controlled at 200 rpm for circulating the water and sludge.
The typical operation was modified from the previous results by the authors [21]. The reactor was operated under 3 cycles of aerating of 3 h, non-aerating of 4 h and settling of 1 h. Filling and decanting were approximately 5 min at the first and last cycles (Figure 1). In the aeration, air was supplied at the flow rate of 0.5 L/min and the DO was around 5-6 mg/L. The DO was immediately dropped to 0.5 mg/L in the non-aeration, then approximately 50 mL of acetate solution was added in the first non-aeration to maintain the C/N ratio of 2 [21].

Analytical methods
The synthetic wastewater (influent) and treated water (effluent) were sampled for NH 4 -N, NO 2 -N and NO 3 -N analysis in accordance with the standard method [22]. The nitrogen removal efficiency was calculated, as present in Equation 1. The chemical oxygen demand (COD) in the effluent was determined using COD analyzer (AL200 COD Vario, Aqualytic). The mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) were measured after filtration and drying at 105°C [22]. Moreover, the pH and DO were frequently measured using pH meter (Eutech Instruments) and DO meter (CyberScan DO 110 Model).
To measure the NH 4 -N removal rates, the water samples were taken every 0.5 h from the reactor operating under continuously air supply, and the reduction of NH 4 -N referred to the NH 4 -N removal rate. Similarly, the reduction of total nitrogen including NH 4 -N, NO 2 -N and NO 3 -N in the reactor operating under no air supply and excess acetate was used to refer to the nitrogen removal rate.

Results and Discussion
The influent NH 4 -N fed to the reactor was started at 10 mg/L for being acclimatization. As shown in Figure 2, the nitrogen removal efficiency was relatively low of <10% in the beginning, and the efficiency was continuously increasing up to ∼ 36% in a week. The NH 4 -N was approximately 6.8 mg/L was found in the effluent, while no NO 2 -N and NO 3 -N was observed (Table 1). This present the low existence of microorganisms responsible for nitrogen removal in the initial sludge. The nitrogen removal efficiency was increasing to ∼ 50%, ∼ 64% and ∼ 82%, when the influent NH 4 -N was continuously increased to 20, 30 and 40 mg/L respectively. This revealed that the number of responsible microorganisms was increased by influent NH 4 -N concentrations. The significant evidence to confirm the increasing responsible microorganisms in the reactor was that the specific nitrogen removal rate continued to increase during operation, as summarized in Table  1. The value was gradually increased from 4.04 mg N/g MLVSS⋅h at NH 4 -N of 10 mg/L and reached to 4.2 mg N/g MLVSS⋅h at NH 4 -N of 40 mg/ L. The majority of nitrogen in the effluent was NH 4 -N (approximately 6-12 mg/L), while low values of NO 2 -N and NO 3 -N (of <2 mg/L) were remained. It can be note that the process of nitritation  performance by the typical SBR operation. However, the increasing NH 4 -N removal rate was higher than the increasing nitrogen removal rate. This caused the remaining of NO 2 -N and NO 3 -N in the effluent at higher concentrations.
The performance of SBR operating in this study was compared to previous studies which operated under different SBR cycles. From Table 2, it can be seen that the good performance of SBR operating under the typical cycle of aerating of 3 h, non-aerating of 4 h and settling of 1 h was obtained at the low carbon addition. Although the long HRT of 24 h was operated in this study, the HRT can be reduced to approximately 16 h (two cycles of SBR) with the efficiency of ∼ 80% (data not shown).

Conclusion
The SBR operating under three cycles of aerating of 3 h, non-performance and microorganisms' activity. At the low NH 4 -N of 10 mg/L, around 5.5 mg C was consumed to remove one gram of nitrogen. The carbon consumption was reduced to 4.0 and 3.1 mg C/mg N at the higher NH 4 -N concentrations. The effective carbon consumption of 2.4 mg C/mg N was found at the highest NH 4 -N of 40 mg/L, referring that the carbon was utilized efficiently for denitrification process and very low carbon was utilized by other competitive heterogeneous microorganisms.
In addition, the NH 4 -N and nitrogen removal rates at various influent NH 4 -N concentrations were present in Figure 3. At the low NH 4 -N of 10 mg/L, the removal rates for NH 4 -N was 3.2 mg/L⋅h and that for nitrogen was 3.5 mg/L⋅h. Both removal rates were continuously increasing up to 6.0 and 5.5 mg/L⋅h for NH 4 -N and nitrogen at the highest NH 4 -N of 40 mg/L. These revealed the enhancement of reactor    aerating of 4 h and settling of 1 h can remove nitrogen from the wastewater effectively. The best performance of 82% was found at the highest NH 4 -N of 40 mg/L. The average effluent NH 4 -N, NO 2 -N and NO 3 -N were 8.5, 1.8 and 1.7 mg/L respectively. The increase in active microorganisms for nitrification and denitrification enhanced the removal rates of NH 4 -N and nitrogen at the higher NH 4 -N concentrations. In addition, the carbon consumption and specific nitrogen removal rate were also more effective rather than a low NH 4 -N concentration.