Michael J. Semmens

Mass transfer in various hollow fiber geometries

Abstract Mass transfer coefficients in commercial modules, including blood oxygenators, agree with literature correlations at high flows but are smaller at low flows. The smaller values at low flows probably result from channelling in the hollow fiber bundle. For the special case of flow within the fibers, the slight polydispersity of the hollow fibers causing this channelling can be used to predict deviation from the Leveque limit. These deviations can not be predicted from extensions to the Leveque analysis, or the analysis by Graetz. For the special case of flow outside the fibers, the mass transfer coefficients in commercial modules of various geometries are surprisingly similar, and fall below those of carefully handmade modules. These results can be used to develop still better membrane module designs.

Use of sealed end hollow fibers for bubbleless membrane aeration: experimental studies

Abstract A novel microporous hollow fiber membrane module was evaluated for the oxygenation of water. Pure oxygen is maintained inside a bundle of sealed-end polypropylene fibers at a pressure below the bubble point. The water to be aerated is pumped over the outside of the fibers. The high oxygen concentration gradient encourages the oxygen to diffuse across the porous membrane wall, and to dissolve directly into the water without the formation of bubbles. The membrane is hydrophobic and the pores small enough that they stay dry and gas filled so that the transport across the membrane is by gaseous diffusion. The membrane itself therefore provides little resistance to transfer, and the mass transfer rate is controlled by diffusion across the liquid film on the outside of the fibers. Modules of varying size were tested and transfer correlations were developed for module design. The process gave 100% oxygen transfer efficiency at a reasonable power input. For water quality management and waste treatment, oxygenation by membranes appears to offer many advantages over conventional aeration devices.

COD and nitrogen removal by biofilms growing on gas permeable membranes

A bioreactor was constructed and used to treat a synthetic wastewater containing ammonium acetate and trace nutrients for about 190 days. The reactor was aerated by means of bundles of gas-permeable hollow-fiber membranes that were installed in the reactor. The membranes provided a specific surface area of 422 m(2)/m(3) and the external surface of the membranes rapidly became covered in an active biofilm. The membrane bundles were agitated by an internal gas recycle. The gas bubbles in the water encouraged fiber-fiber contact and were intended to control biofilm growth. Chemical oxygen demand (COD) removals in excess of 95% were achieved in a 6h nominal detention time. Nitrification developed rapidly and complete oxidation of the influent ammonium was evident within 20 days. Even though the reactor was equipped with a large membrane surface area, the oxygen was consumed within the biofilm growing on the membrane surface. As a result, the external dissolved oxygen (DO) dropped to zero and the reactor was able to support essentially complete denitrification. After about 3 months of operation the reactor showed excellent removals of both COD and inorganic nitrogen but the performance could not be sustained. Excess biofilm accumulation eventually contributed to a deterioration in process performance. This study demonstrates that while membrane aeration can provide simultaneous BOD and N removal in the same reactor, the membrane modules/bioreactor must be designed to allow for the development of thick biofilms. In addition, options for controlling the biofilm thickness need to be investigated.

The influence of pretreatment on the capacity and selectivity of clinoptilolite for metal ions

Abstract Many investigators studying the selectivity of the natural zeolite clinoptilolite for metal ions have noted inconsistencies in measured exchange capacities and selectivity data that appear to be related to how the zeolite was treated prior to testing. In this study the cation composition of a clinoptilolite from the Ash Meadows deposit was characterized after samples were exposed to varying amounts and concentrations of sodium chloride solution. Potassium and calcium ions were strongly held by clinoptilolite in preference to sodium and extensive exposure to high sodium concentrations was required to displace these ions from the zeolitic matrix. Since these ions are so difficult to displace from the zeolite it is apparent that the sites occupied by potassium and calcium are not readily able to participate in ion exchange. Studies are presented which demonstrate the influence of zeolite cation composition, and calcium concentration in solution, on the ion exchange performance of clinoptilolite for the removal of certain heavy metals. It is concluded that the zeolite conditioning procedure has an important influence on zeolite performance and it should be determined by the treatment or study objectives.

An evaluation of pretreated natural zeolites for ammonium removal

Abstract Clinoptilolite has been widely studied for ammonium removal in the past 2 yr. However, many investigators have reported variations in the measured capacities of samples of clinoptilolite. These studies and the factors believed to influence measured zeolite capacity are reviewed. In addition no studies to evaluate other natural zeolites for ammonium removal have been reported. In this study samples of clinoptilolite, erionite, mordenite and phillipsite provided by the Anaconda Company were evaluated for ammonium removal from wastewaters. In addition, samples of clinoptilolite were pretreated in various ways to determine whether an improvement in ammonium removal performance could be realized. Total exchange capacities, capacities for ammonium removal from a synthetic waste, packed bed densities and crushing strengths were measured. Phillipsite was found to have almost twice the weight capacity for ammonium removal from synthetic waste compared to that of clinoptilolite. The volumetric capacity was 26% better than that of clinoptilolite. However, the phillipsite sample was extremely friable and it could not be used for water treatment unless it was strengthened with a binder. Pretreatment of clinoptilolite with NaOH, HNO 3 and steam did little to improve the zeolites performance. However, heat pretreatment (600°C for 1 h) improved the zeolites selectivity for ammonium significantly. Ammonium removal capacities were increased by approximately 17% for heat treated zeolite samples although the total exchange capacity of the zeolite was reduced somewhat.

Effect of protein, polysaccharide, and oxygen concentration profiles on biofilm cohesiveness

ABSTRACT It is important to control biofilm cohesiveness to optimize process performance. In this study, a membrane-aerated biofilm reactor inoculated with activated sludge was used to grow mixed-culture biofilms of different ages and thicknesses. The cohesions, or cohesive energy levels per unit volume of biofilm, based on a reproducible method using atomic force microscopy (F. Ahimou, M. J. Semmens, P. J. Novak, and G. Haugstad, Appl. Environ. Microbiol. 73:2897-2904, 2007), were determined at different locations within the depths of the biofilms. In addition, the protein and polysaccharide concentrations within the biofilm depths, as well as the dissolved oxygen (DO) concentration profiles within the biofilms, were measured. It was found that biofilm cohesion increased with depth but not with age. Level of biofilm cohesive energy per unit volume was strongly correlated with biofilm polysaccharide concentration, which increased with depth in the membrane-aerated biofilm. In a 12-day-old biofilm, DO also increased with depth and may therefore be linked to polysaccharide production. In contrast, protein concentration was relatively constant within the biofilm and did not appear to influence cohesion.

A novel in situ technology for the treatment of nitrate contaminated groundwater.

A novel in situ membrane technology was developed to remove nitrate (NO3-) from groundwater. Membrane-fed hydrogen gas (H2) was used as an electron donor to stimulate denitrification. A flow-through reactor fit with six hollow-fiber membranes (surface area = 93 cm2) was designed to simulate groundwater flowing through an aquifer with a velocity of 0.3 m/day. This membrane technology supported excellent NO3- and nitrite (NO2-) removal once H2 and carbon limitations were corrected. The membrane module achieved a maximum H2 flux of 1.79 x 10(-2) mg H2/m2 s, which was sufficient to completely remove 16.4 mg/L NO3(-)-N from a synthetic groundwater with no NO2- accumulation. In addition, this model in situ treatment process produced a high quality water containing <0.5 mg/L total organic carbon.

Stratification of Activity and Bacterial Community Structure in Biofilms Grown on Membranes Transferring Oxygen

ABSTRACT Previous studies have shown that membrane-aerated biofilm (MAB) reactors can simultaneously remove carbonaceous and nitrogenous pollutants from wastewater in a single reactor. Oxygen is provided to MABs through gas-permeable membranes such that the region nearest the membrane is rich in oxygen but low in organic carbon, whereas the outer region of the biofilm is void of oxygen but rich in organic carbon. In this study, MABs were grown under similar conditions but at two different fluid velocities (2 and 14 cm s−1) across the biofilm. MABs were analyzed for changes in biomass density, respiratory activity, and bacterial community structure as functions of biofilm depth. Biomass density was generally highest near the membrane and declined with distance from the membrane. Respiratory activity exhibited a hump-shaped profile, with the highest activity occurring in the middle of the biofilm. Community analysis by PCR cloning and PCR-denaturing gradient gel electrophoresis of 16S rRNA genes demonstrated substantial stratification of the community structure across the biofilm. Population profiles were also generated by competitive quantitative PCR of gene fragments specific for ammonia-oxidizing bacteria (AOB) (amoA) and denitrifying bacteria (nirK and nirS). At a flow velocity of 14 cm s−1, AOB were found only near the membrane, whereas denitrifying bacteria proliferated in the anoxic outer regions of the biofilm. In contrast, at a flow velocity of 2 cm s−1, AOB were either not detected or detected at a concentration near the detection limit. This study suggests that, under the appropriate conditions, both AOB and denitrifying bacteria can coexist within an MAB.

HOLLOW FIBRE BIOREACTOR FOR WASTEWATER TREATMENT USING BUBBLELESS MEMBRANE AERATION

A laboratory scale hollow fibre membrane bioreactor which used bubbleless membrane aeration was tested over 5 months for its ability to treat synthetic sewage. A biofilm was grown on the surface of 280 μ diameter, gas permeable, sealed-end, polypropylene based fibres. The fibre lumens were pressurized with pure oxygen and synthetic sewage was pumped over the surface of the biofilm. At the highest volumetric COD (chemical oxygen demand) loading of 8.94 kg/m3/d and a hydraulic retention time (HRT) of 36 min, 86% COD removal was achieved. A daily backwash was necessary to prevent channelling. The oxygen transfer efficiency was 100%.

Biofilm cohesiveness measurement using a novel atomic force microscopy methodology

ABSTRACT Biofilms can be undesirable, as in those covering medical implants, and beneficial, such as when they are used for waste treatment. Because cohesive strength is a primary factor affecting the balance between growth and detachment, its quantification is essential in understanding, predicting, and modeling biofilm development. In this study, we developed a novel atomic force microscopy (AFM) method for reproducibly measuring, in situ, the cohesive energy levels of moist 1-day biofilms. The biofilm was grown from an undefined mixed culture taken from activated sludge. The volume of biofilm displaced and the corresponding frictional energy dissipated were determined as a function of biofilm depth, resulting in the calculation of the cohesive energy. Our results showed that cohesive energy increased with biofilm depth, from 0.10 ± 0.07 nJ/μm3 to 2.05 ± 0.62 nJ/μm3. This observation was reproducible, with four different biofilms showing the same behavior. Cohesive energy also increased from 0.10 ± 0.07 nJ/μm3 to 1.98 ± 0.34 nJ/μm3 when calcium (10 mM) was added to the reactor during biofilm cultivation. These results agree with previous reports on calcium increasing the cohesiveness of biofilms. This AFM-based technique can be performed with available off-the-shelf instrumentation. It could therefore be widely used to examine biofilm cohesion under a variety of conditions.