A Baffled Membrane Bioreactor─ New Mbr for Efficient Nitrogen Removal

Submerged membrane bioreactors (MBRs) have been gaining in popularity in various types of wastewater treatment. One drawback of submerged MBRs is the difficulty in removing nitrogen because intensive aeration is usually carried out in the tank and the MBRs must therefore be operated under aerobic condition. In this paper, feasibility of a baffled MBR (BMBR), treating municipal wastewater particularly in terms of nitrogen removal, was examined on the basis of pilotscale experiments. Simultaneous nitrification/denitrification in a single reactor was possible by inserting baffles into a single submerged MBR equipped with polyvinylidenfluoride flat sheet membranes as long as the wastewater was fed in the appropriate way. When operating conditions of the BMBR were appropriately set, the nitrogen removal efficiency exceeded over 95% without adding any external organic carbon. Total nitrogen concentration in the treated water could be reduced to about 1 mg L with the hydraulic retention time of 5.3 h. Very efficient and stable removal of organic carbon and phosphorus was also found. Average concentration of total organic carbon and total phosphorus in the treated water was 3.6 and 0.17 mg L, respectively. *Corresponding author Email: [email protected] INTRODUCTION As an efficient technology for municipal wastewater treatment, membrane bioreactors (MBRs) have gained significant popularity in the past decade. MBRs, in which biomass is strictly separated by a membrane, offer several advantages over the conventional activated sludge process, including a high biomass concentration, reduced footprint, low sludge production, and better performance quality [1]. MBRs can be generally classified into two categories: recirculated MBRs and submerged MBRs. Recently, submerged type of MBRs have been preferred since energy consumption can be significantly reduced [2]. Generally, intensive aeration is carried out in a submerged MBR to supply oxygen to microorganisms and clean the membrane. As a result of the intensive aeration, one weak point in the use of submerged MBRs becomes obvious: poor removal of nitrogen. Regarding elimination of NH4 + (i.e., nitrification), submerged MBRs generally show good performances due to dense populations of nitrifiers and the aerobic condition provided by the intensive aeration. However, anoxic condition that is indispensable for the promotion of biological denirification cannot be created under such a high intensity of aeration. One possible approach to overcome this drawback is to install an additional anoxic reactor in which denitrification occurs followed by the aerobic MBR [3-5]. Mixed liquor is circulated between the two reactors at a fixed ratio. With this configuration, nitrogen removal in submerged MBRs can be possible. Removal of up to 80% of total nitrogen (TN) in municipal wastewater has been reported. This type of MBR, however, apparently impairs the advantages of the submerged MBRs such as small footprint or ease of operation. To address the problems stated above, insertion of baffles into the membrane chamber was proposed by the authors. Although this approach proposed herein is rather simple, in combination with an appropriate way of the feed water addition, it can significantly improve the performance of submerged MBRs by promoting simultaneous nitrification/denitrification. Preliminary experiments [6] demonstrated the proposed baffled MBR (BMBR) certainly worked well. This paper will describe the results obtained in pilotscale experiments with a real wastewater and show the feasibilities of the proposed reactor. 436 J. Environ. Eng. Manage., 16(6), 435-439 (2006) MATERIALS AND METHODS 1. Concept of the BMBR Figure 1 shows the concept of the BMBR. Air diffuser is placed inside the inserted baffles. In the operation of the BMBR, membrane filtration is carried out in the constant flow rate mode and the flow rate of the raw wastewater feed is larger than that of filtration. When the water level is higher than the top of the baffles (Fig. 1a), the entire reactor is vigorously mixed by the aeration and kept aerobic. Addition of the raw wastewater is stopped when the water level reaches the set highest level. Then, the water level goes down due to membrane filtration and is eventually lowered below the top of the inserted baffles. From this point, the reactor is separated by the baffles and discriminated into two zones. The outside of the baffles should become anoxic due to oxygen consumption by biomass while the inside of the baffles should be kept aerobic due to the aeration (Fig. 1b). When the water level reaches the set lowest level, addition of the raw water is restarted and the water level is allowed to rise. The addition of raw wastewater must be done to the outside of the baffles so that organic carbon contained in the raw wastewater can be utilized for denitrification. Eventually, the water level exceeds the top of the inserted baffles and the outer zone should become aerobic again due to vigorous mixing provided by the aeration (Fig. 1a). Thus, in the operation of the BMBR, aerobic and anoxic conditions are alternatively created in the outer zone at a constant interval and therefore the improvement in nitrogen removal is expected in comparison to normal submerged MBRs, which are principally aerobic. 2. Experimental Apparatus Continuous operation of a pilot-scale BMBR was conducted at an existing municipal wastewater treatment facility (Soseigawa Wastewater Treatment Plant, Sapporo, Japan). Temperature of the raw wastewater sometimes decreased below 10 °C during winter season, making biological treatment (especially nitrification) difficult. The BMBR used in this study equipped 6.0 m of flat-sheet type of micro-filtration membranes (Toray, Japan). Nominal pore size and material of the membrane were 0.1 μm and polyvinylidenfluoride, respectively. Effective volume of the BMBR was 500 L, in which the ratio of the outer zone to the inner zone was approximately 2.2 when the water level was at the top of the inserted baffles. Aeration rate was fixed at 110 L min. As a result, dissolved oxygen concentration inside the baffles was always maintained above 5 mg L. The authors have been proposing “the hybrid MBR” which is composed of pre-coagulation/sedimentation and a MBR [7]. By carrying out the pre-treatment, enhanced Fig. 1. Concept of the baffled membrane bioreactor