(176e) BF-MBR: Pros and Cons of the Potential for New Reactor Deigns and Operation

INTRODUCTION In recent years development and commercialization of membrane bioreactors (MBR) for advanced treatment of wastewater has seen an exponential growth. MBRs are commonly understood as the combination of membrane filtration and biological treatment using activated sludge (AS), where the membrane primarily serves to replace the clarifier in the wastewater treatment system. Market growths have been in excess of 10% per year and have essentially been driven by the municipal sector, in particular by the construction of the larger plants with installed capacities above 5.000 m3/d. The MBR technology is now a cost competitive option for municipal and industrial wastewater treatment, where the applications require exceptional specifications such as enhanced water quality (for bathing water, water reuse), reduced footprint or where the technology is used to upgrade existing plants.

Although much R&D has been conducted, key bottlenecks of the technology are still being investigated; gaining a better fundamental understanding of membrane fouling and fouling control, reducing the energy demand and consumption, and reducing allover costs through improved operations and increase in system lifetime. Recent research has therefore looked at alternative integrated membrane based wastewater treatment options which potentially can resolve some the challenges found in AS-MBR systems.

Coupling biofilm reactors with membrane filtration as biofilm membrane bioreactors (BF-MBR) is an interesting alternative technology to AS-MBRs. Biofilm technology for wastewater treatment can provide a substantially lower suspended solids environment for membrane filtration compared to activated sludge processes. Potential benefits are; less membrane clogging / sludging problems, lower fouling potentials, ease of membrane cleaning, reduced energy consumption for air-scouring. These characteristics of the BF-MBR also represent an opportunity to design and operate the membrane unit for enhanced solids removal and thus improved membrane fouling mitigation and control as well potentials for using less energy. A tool in achieving this is implementing new membrane module/reactor designs.

The aim of this paper is to highlight recent developments in BF-MBR studies. Alternative and novel biofilm reactor designs have been investigated. Different biofilm reactor combinations and concepts have been tested to investigate the effect of the biodegradation stage on the performance of the membrane reactor. Different membrane modules have been tested in these studies (i.e. flat sheet, capillary / tubular membranes) with varying reactor geometries and designs, and a broad range of operating conditions (i.e. fluxes, backwashing settings, air-scouring rates etc.).

METHODS Lab-scale pilot plant trials have been conducted coupling a biofilm reactor based on the moving-bed-biofilm reactor process. The membrane filtration unit was designed as an externally submerged configuration where modifications to the membrane reactor were investigated. A GE Zenon membrane module was installed in the different membrane reactor configurations investigated. Simple models based on classical mass balances of MLSS have been developed and tested to predict the behaviour of the reactor designs.

Three different process configurations and designs of BF-MBR systems have also been investigated. Both side-stream module configuration and external-submerged module configurations were tested. The three schemes investigated include: 1) a double-deck aerated biofilm reactor with external flat-sheet membrane filtration unit (membranes supplied by A3 Water Solutions GmbH, Germany); 2) an aerobic moving biofilm-anoxic biofilm MBR with energy recovery from aeration and side-stream tubular membrane filtration unit (Puron membranes supplied by Koch Membrane Systems, Germany); 3) a two-stage aerobic moving biofilm-anaerobic activated sludge MBR with a side-stream tubular membrane filtration unit (membranes supplied by Polymem, France) . All experiments were done with municipal wastewater from a combined sewer system which was pretreated using a small gravity settler and then pumped from an overflow the respective pilot plants. The pilot plants were equipped with National Instruments / LabVEIW data acquisition units and online measurements using various sensors, i.e. temperature, pressure, flow etc. All the pilot plants have been operated with varying conditions, i.e. flux, specific aeration demand, organic loads etc. Standard water quality sampling and analyses have been conducted, including more detailed studies of the suspended solids and particle size distribution etc. Membrane filtration performance has been assessed by evaluating overall fouling rates as a function of operating conditions.

RESULTS AND DISCUSSION The impact of the membrane reactor design was investigated by studying three membrane reactor designs: 1) a completely mixed reactor (CM-MR), 2) membrane reactor with integrated sludge pocket (SP-MR), and 3) a membrane reactor with a modified sludge pocket (MSP-MR). The biofilm reactor was operated with a 4 HRT. Suspended solids (SS) in the effluent from the biofilm reactor varied between 90 ? 150 mg L-1, while COD and FCOD were between 120-242 and 25.9-43.1 mg O2 L-1 respectively. The membrane reactor was operated in a 5 minute cycle with production flux 35 LMH, backwash flux 42 LMH and recovery 96%. Continuous air scouring of the membrane was employed for all tests with specific aeration demand (SADm) set on approximately 1 Nm3airm-2membrane areah-1.

Steady state concentrations of MLSS around the membrane area for the 3 designs were ~3900, ~1000, ~400 mg/L, giving fouling rates within production cycle of 20, 6, and 3×10-5 bar sec-1, respectively. Lower concentrations of MLSS and COD around the membranes as a function of the modified rector designs results in a better membrane performance. Reduction in MLSS is not directly proportional to a reduction of fouling rates (i.e. dTMP/dt). The characteristics of suspended matter around the membrane play an important role in membrane fouling, however, a reduction in these foulants by an enhanced membrane reactor design is a significant contribution to controlling and minimizing fouling of the membrane. Soluble matter (i.e. FCOD) was reduced by ~ 40% in MSP-MR compared to CM-MR. Particle size distribution analyzed in membrane reactors showed that the differential number percentage of submicron particles around the membrane in the reactors with a sludge pocket design (SP-MR and MSP-MR) could be reduced by ~ 8 % and ~ 10 % respectively, compared to a completely mixed design membrane reactor (CM-MR). Modification of the membrane reactor in the BF-MBR process was demonstrated to be beneficial and relatively easily done by introducing an integrated flocculation zone with a sedimentation zone beneath membrane unit.

One of the objectives of the alternative process configurations was to maximize and improve solids removal from the overall process. With suspended solids in the inflow to the membrane reactor of less than 45 mg/L the concentration around the membrane during operation may only accumulate to around 150 mg/L. Combined with integrated sludge management strategies a further reduction in the volume of excess sludge removal from the process can also be achieved. For example, in the second configuration, the sludge traps in both bioreactor and membrane tank provided the concentrated sludge with concentration of approximately 15 g/L. For the third configuration, the solid concentration of the sludge waste removed from the anaerobic tank is approximately 26 g/L which is significantly higher than the aerobic sludge trap. This also allowed the membrane reactors to be operated with less aeration demand while achieving lower fouling rates.

CONCLUSIONS High quality effluents are achieved independent of the operating conditions, though the performance of the membrane filtration unit is affected by the operating conditions of the system. In general, the BF-MBR shows that stable operation of the membrane unit with low fouling rates, overall less energy demands, and sustainable operation can be achieved. By coupling a biofilm process with membrane separation there is a potential to maximize the biodegradation stage as well as the membrane filtration stage.

Modification of the membrane reactor in a BF-MBR process is beneficial for establishing stable operation, where the introduction of an integrated flocculation zone with a sedimentation zone beneath membrane unit is one strategy. Such a modified rector design provides a significantly lower concentration of MLSS and COD around the membranes and thus a better overall membrane performance. The proposed simplified model and equations for calculating steady state values of MLSS inside the membrane reactor have been verified and a refinement of the model by determining adequate expressions for kS will be investigated with the aim of developing a design tool for improved membrane reactor designs for the BF-MBR process. The potentials of biofilm-MBR processes are that new and flexible process configurations possible, they offer an alternative strategy for solids control and management which results in lower suspended solids load on the membrane. Furthermore, the operating conditions achieved may provide enhanced membrane performance / less fouling and overall lower energy requirements. Although there is room for optimization and improvement of the treatment schemes, the BF- MBR configurations with alternative membrane reactor unit designs investigated in these studies are found to be competitive treatment schemes for treating municipal wastewater.