Optimizing biomembrane reactor systems for water reclamation and reuse applications

Membrane separations promise to yield substantial environmental and economic benefits leading to enhanced global competitiveness by significantly reducing energy consumption, increasing industrial productivity, lowering waste generation, and addressing global water shortage problems. Membrane technologies face several scientific and technological challenges that must be overcome before witnessing widespread use in environmental, industrial and commercial applications. Environmental applications include wastewater treatment, water reclamation and reuse, water treatment, water purification, and water desalination. The specific challenges include membrane fouling and permeate flux decline, poor rejection or selectivity, and large energy footprint. The research presented here was intended to address most of these critical issues. ❧ An important aspect of this study was the examination of water reclamation processes using combination of ultrafiltration membranes and activated carbon adsorption, and to develop novel, high-performance nanofiltration membranes superior to existing commercial membranes. Firstly, flat-sheet ultrafiltration membranes were tested to evaluate the fundamental performance criteria including permeate fluxes and total organic carbon (TOC) removals in water reclamation applications using real wastewaters. Secondly, ultrafiltration membranes were employed in continuous flow hybrid membrane bioreactor (MBR) systems for treating wastewaters after secondary treatment. Thirdly, the novel membranes were synthesized using impregnation of graphene oxide (GO) nanoparticles in polymeric matrices. These membranes were intended to be superior to existing commercial membranes for water reclamation and reuse as well as other uses regarding various criteria: aqueous transport and permeability properties, anti-fouling potential and fouling resistance, rejection and separation characteristics, cleanability and flux recovery, chemical tolerance, mechanical strength, and overall durability. These membranes were tested in batch systems to evaluate their feasibility for the above applications. The finished water quality was intended to meet the necessary treatment standards for water reclamation and reuse applications regarding chemical and biological purity. Different types of cleaning agents such as caustic solution (NaOH), surfactant (Triton X-100), and biological enzyme (RID-X) were evaluated for foulants removal and permeate flux recovery. Lastly, modeling approaches were employed to predict permeate fluxes and TOC removals for MBR systems in various configurations. ❧ The study further included laboratory-scale flat-sheet plate-and-frame membrane filtration tests for investigating the permeate flux patterns and the TOC removals with secondary clarifier effluent obtained from Los Angeles County. As the TOC removals were not satisfactory with the ultrafiltration membrane itself, additional processes were required such as powder activated carbon (PAC) adsorption, microbial degradation (microorganisms including E.coli), and oxidation (ozone, and peroxone). In all these case, the permeate fluxes were also evaluated. Bench-scale studies were conducted to determine parameters used for prediction of permeate flux and TOC concentration using mathematical models. ❧ A micro-scale hollow fiber ultrafiltration membrane bench setup was designed and tested to evaluate membrane performances regarding permeate fluxes and TOC removals. The micro-scale tests were intended to provide a guideline for the design of hollow fiber membrane modules used in a mini-pilot-scale system. The mini-pilot-scale system represented a continuous flow hybrid MBR process using a hollow-fiber ultrafiltration membrane module, and it was used to assess the feasibility of water reclamation applications using permeate fluxes and TOC removals as criteria. The uniqueness of the MBR unit was the special design for controlling membrane fouling and permeate flux decline, combining powdered activated carbon (PAC) sorption and fluid management techniques. The membrane module was operated in “outside-in” dead-end fluid-dynamic regime, and equipped with structural features to promote local vortex and turbulence for fouling control. The mini pilot-scale MBR studies evaluated permeate flux decline patterns, membrane fouling, and organic rejection as TOC and UV254 (ultraviolet absorbance at 254 nm wavelength). The feed and effluent streams were also analyzed to a limited extent for biochemical oxygen demand (BOD), chemical oxygen demand (COD), biomass, and other relevant water quality parameters. ❧ A transport model using the resistances of various layers constituting concentration polarization and gel layer besides adsorbent and biofilm layer was employed for predicting the permeate fluxes and TOC removals for flat-sheet and hollow-fiber membrane configurations. The model considered membrane surface fouling, internal pore fouling, and membrane rejection in its formulation as well. The necessary model parameters were obtained from bench-scale membrane filtration tests. ❧ This investigation involved a feasibility study of the laboratory-scale MBR process for the purification of potable water sources. Reliable predictions of process performance were obtained regarding organic removal efficiency reflected by the effluent concentration profiles as functions of time. Such predictions were made on the basis of easily determined laboratory experiments, pilot-plant scale studies would be minimized and considerable savings in cost and time can be achieved. This objective was achieved by developing and employing a mathematical modeling approach. Simultaneously, a modeling protocol was observed for the process design and upscaling using dimensional analysis and similitude (although it was not the focus of this research). ❧ The model for performance prediction of organic removals (TOC removals) in the mini-pilot-scale MBR system incorporated the following phenomenological aspects including adsorption mass transfer resistance, adsorption equilibrium, biological reaction due to suspended microorganisms in bulk solution, and biological reaction within biofilms. In this regard adsorption equilibrium studies was performed to evaluate the adsorption equilibria for dissolved organic matter (DOM) using total organic carbon (TOC) as a surrogate parameter. Adsorption rate studies were conducted in batch reactors for determining the adsorption kinetics and the associated mass-transfer parameters. Batch biokinetic studies were undertaken for the estimation of biological parameters pertaining to TOC removal using an indigenous population of microorganisms. Laboratory scale MBR experiments were performed to evaluate organic removals (TOC removals) and membrane permeate fluxes as functions of operating time for a variety of process conditions. The experimental determination of TOC removal efficiencies in MBR systems provided the necessary feedback for MBR model verification and refinement. This adsorption and biodegradation model provided excellent predictions of TOC removals in MBR systems under a variety of operating scenarios including the following: (i) using PAC adsorbent alone (PAC at 40 mg L⁻¹); (ii) using microorganisms alone (E.coli at 10⁸ CFU per 100 mL), and (iii) combination; and (iii) using PAC adsorbent with microorganisms (PAC at 40 mg L⁻¹ and E.coli at 10⁸ CFU per 100 mL. The TOC removals were the highest for the combination of PAC and E.coli demonstrating the synergistic effects of adsorption with microbial degradation for organic removal. The model was employed under these conditions by suppressing the effects of adsorption or biodegradation wherever necessary. The model predictions were in good agreement with the experimental results for all the three scenarios. Model sensitivity analyses was performed with respect to various reactor flow, biological and adsorption parameters for the following reasons: (i) to obtain a priori estimates of the accuracy required for the determination of each parameter, and (ii) to predict/simulate the behavioral patterns of process dynamics under a variety of process and operating conditions. ❧ Limited work was directed at development and production of novel high-performance membranes for water reclamation and other applications. This research in an ongoing collaborative effort between the groups of Professor Massoud Pirbazari and Professor Thieo Hogen-Esch with their expertise in the areas of membrane processes and polymer science, respectively. The initial work included the development of polymer synthesis protocols with appropriate reaction schemes, free-radical processes, syntheses conditions such as reaction times, curing procedures, and quantitatively controlled incorporation of graphene oxide (GO) into the polymers. Superior membranes were fabricated by adjusting these conditions. The membranes used in the series of preliminary tests were prepared by interfacial polymerization by sequential addition of MPD and TMC on a commercial polyether sulfone (PES) ultrafiltration membrane base with a nominal pore size of 4–10 nm and molecular weight cutoff off (MWCO) of 10,000 Daltons. The monomers used in the preparation of polyamide membrane were m-phenylene diamine (MPD) and 1,3,5-benzene tricarbonyl chloride or trimesoyl chloride (TMC). Another set of membranes were cast using these monomers MPD and TMC, but with the addition of camphor sulfonic acid (CSA) and triethanol amine (TEA) to make the membranes material more solvophilic in nature, and to observe their hydrophilicity, aqueous transport and rejection characteristics. These novel membranes were synthesized by the impregnation of GO nanoparticles into polymeric matrices at different concentrations and various conditions. The fundamental idea was to enhance the aqueous transport, fouling resistance, rejection characteristics, chemical cleanability, and mechanical durability of the membranes to make them far superior to existing commercially available membranes such as the nanofiltration NF90 membranes in performance levels. The preliminary results were promising in so far as manifesting increased aqueous permeability and permeate fluxes by the impregnation of GO nanoparticles. The future work would involve polymer and nanomaterial combinations to introduce fine levels of tunability of membrane characteristics including pore sizes, pore-size distributions, charge effects, rejection characteristics and fouling resistance besides chemical tolerance. The novel membranes would possibly be superior to existing commercial membranes for nanofiltration applications in the realm of wastewater treatment, water reclamation and reuse. It is strongly believed that these concepts could be extrapolated to produce superior microfiltration, ultrafiltration and reverse osmosis membranes as well.