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PubMed2006

Distillery wastewater treatment by the membrane-based nanofiltration and reverse osmosis processes.

A hybrid nanofiltration (NF) and reverse osmosis (RO) pilot plant was used to remove the color and contaminants of the distillery spent wash. The feasibility of the membranes for treating wastewater from the distillery industry by varying the feed pressure (0-70 bar) and feed concentration was tested on the separation performance of thin-film composite NF and RO membranes. Color removal by NF and a high rejection of 99.80% total dissolved solids (TDS), 99.90% of chemical oxygen demand (COD) and 99.99% of potassium was achieved from the RO runs, by retaining a significant flux as compared to pure water flux, which shows that membranes were not affected by fouling during wastewater run. The pollutant level in permeates were below the maximum contaminant level as per the guidelines of the World Health Organization and the Central Pollution Control Board specifications for effluent discharge (less than 1,000 ppm of TDS and 500 ppm of COD).

Coloring Agents↗

PubMed2001

Isolation of dissolved organic matter (DOM) from surface waters using reverse osmosis and its impact on the reactivity of DOM to formation and speciation of disinfection by-products.

Dissolved organic matter (DOM) from three low-hardness surface waters was isolated and concentrated using a reverse osmosis (RO) membrane system. The efficacy of the RO isolation method and its impact on the subsequent reactivity between DOM and chlorine were examined. DOM mass balances (quantified as dissolved organic carbon) ranged from 96.1 to 102.1% for the three waters tested, and DOM mass recoveries of 93.9 to 98.2% indicated successful isolation, minimal fractionation. and negligible loss of organic matter. RO isolates were diluted using distilled and deionized water in the laboratory to reconstitute the source waters. Both source water (collected at the time of isolation) and reconstituted source water samples were chlorinated. Formation of several disinfection by-products (DBPs: e.g., THMs, HAA9, HANs, and HKs) were measured. For all waters tested, DBP formation of source and corresponding reconstituted source water agreed within 95% confidence intervals. Therefore, RO isolation had no impact on the DOM reactivity of the three low-hardness surface waters tested in this study. In addition, the degree of bromine substitution, as expressed by the bromine incorporation factor, was calculated. Comparison of bromine incorporation factors for source and reconstituted source waters further indicated that, as with the total DBP formations, bromine speciation and the relative occurrence of individual species in THMs and HAA9 did not change as a result of the isolation. Overall, in terms of DBP formation, RO isolation appears to maintain the integrity and reactivity of DOM.

Disinfection↗

PubMed2002

A comparison of surface water natural organic matter in raw filtered water samples, XAD, and reverse osmosis isolates.

This research compared raw filtered waters (RFWs), XAD resin isolates (XAD-8 and XAD-4), and reverse osmosis (RO) isolates of several surface water samples from McDonalds Branch, a small freshwater fen in the New Jersey Pine Barrens (USA). RO and XAD-8 are two of the most common techniques used to isolate natural organic matter (NOM) for studies of composition and reactivity; therefore, it is important to understand how the isolates differ from bulk (unisolated) samples and from one another. Although, any comparison between the isolation methods needs to consider that XAD-8 is specifically designed to isolate the humic fraction, whereas RO concentrates a broad range of organic matter and is not specific to humics. The comparison included for all samples: weight average molecular weight (Mw), number average molecular weight (Mn), polydispersity (rho), absorbance at 280 nm normalized to moles C (epsilon280) (RFW and isolates); and for isolates only: elemental analysis, % carbon distribution by 13C NMR, and aqueous FTIR spectra. As expected, RO isolation gave higher yield of NOM than XAD-8, but also higher ash content, especially Si and S. Mw decreased in the order: RO > XAD-8 > RFW > XAD-4. The Mw differences of isolates compared with RFW may be due to selective isolation (fractionation), or possibly in the case of RO to condensation or coagulation during isolation. 13C NMR results were roughly similar for the two methods, but the XAD-8 isolate was slightly higher in 'aromatic' C and the RO isolate was slightly higher in heteroaliphatic and carbonyl C. Infrared spectra indicated a higher carboxyl content for the XAD-8 isolates and a higher ester:carboxyl ratio for the RO isolates. The spectroscopic data thus are consistent with selective isolation of more hydrophobic compounds by XAD-8, and also with potential ester hydrolysis during that process, although further study is needed to determine whether ester hydrolysis does indeed occur. Researchers choosing between XAD and RO isolation methods for NOM need to consider first the purpose of the isolation; i.e., whether humic fractionation is desirable. Beyond that, they should consider the C yield and ash content, as well as the potential for alteration of NOM by ester hydrolysis (XAD) or condensation/coagulation (RO). Furthermore, the RO and XAD methods produce different fractions or isolates so that researchers should be careful when comparing the compositions and reactivities of NOM samples isolated by these two different techniques.

Filtration↗

PubMed2003

Performance of a hybrid reverse osmosis-constructed wetland treatment system for brackish oil field produced water.

Using constructed wetlands, produced waters from oil fields (i.e., waters that have been in contact with oil in situ) can be treated to enhance water quality for irrigation purposes, or subsequent discharge to receiving aquatic systems. In produced water containing elevated levels of salt (i.e., brackish-produced waters), the ability to decrease the conductivity of the produced water may influence potential reuse of the water for irrigation purposes. The objectives of this research were (1) to determine the efficacy of a pilot-scale hybrid reverse osmosis (RO)-constructed wetland system for treatment and reuse of produced waters from an oil field, and (2) to assess the quality of treated water in terms of physicochemical characteristics and toxicity influencing reuse of the water for irrigation or other designated uses. Specifically, the performance of the hybrid treatment system was examined in terms of outflow water conductivity, total dissolved solids (TDS), and toxicity using Ceriodaphnia dubia and Pimephales promelas in 7-day static/renewal exposure tests. Prior to treatment, significant mortality was observed for C. dubia and P. promelas exposed for 7 days to 6.25% untreated produced water. Following treatment through the hybrid system, no significant mortality was observed in C. dubia or P. promelas exposed to 100% treated produced water when compared to the control organisms. The pilot-scale RO-constructed wetland system effectively decreased conductivity by 95% and TDS by 94% in the brackish produced water tested in this study. Following treatment, the produced water was suitable for irrigation or discharge to surface waters. Therefore, hybrid RO-constructed wetland treatment systems present a viable alternative for treatment and reuse of produced waters from oil fields.

Agriculture↗

PubMed2003

Comparing microfiltration-reverse osmosis and soil-aquifer treatment for indirect potable reuse of water.

Microfiltration (MF) followed by reverse osmosis (RO) and soil-aquifer treatment (SAT) are the two principal technologies considered for indirect potable reuse of wastewater. This study, conducted at the Northwest Water Reclamation Plant, Mesa (Arizona), evaluated MF/RO and SAT (>6 months residence time) treated tertiary effluent with respect to organics removal. Effluent organic matter was characterized as total organic carbon (TOC), by UV absorbance (UVA), solid-state carbon-13 nuclear magnetic resonance spectroscopy, and size exclusion chromatography. Several trace organic micropollutants, including EDTA, NTA, and alkylphenolethoxylate residues, were analyzed by GC/MS. The study revealed that final TOC concentrations of MF/RO and SAT are 0.3 and 1.0 mgl(-1), respectively. Based on the characterization techniques used, the character of bulk organics present in final SAT water resembles the character of natural organic matter present in drinking water. Depending on the molecular weight cut-off, RO membranes can efficiently reject high molecular weight organic matter (characterized as humic and fulvic acids). However, approximately 40-50 percent of the remaining TOC in permeates consists of low molecular weight acids and neutrals representing a molecular weight range of approximately 500Da and less. In the SAT treated effluent, EDTA and APECs were removed to approximately 4.3 and 0.54 microg/l, respectively, but were below the detection limit in the MF/RO treated effluent.

Carbon↗

PubMed1982

Water supply aluminium concentration, dialysis dementia, and effect of reverse-osmosis water treatment.

Dialysis dementia appeared in 18 of 258 patients treated by haemodialysis. All Cases developed in patients treated by home dialysis (150) and none in patients treated exclusively by hospital dialysis (108). Analyses of the domestic water supply for each month on dialysis showed that dementia occurred only in those whose water supply had a high aluminium concentration (greater than 80 micrograms/l). The significant exponential relation (p less than 0.01) between the mean aluminium concentration in the water used to prepare the dialysate and the time taken to death from dementia indicates that there is probably no safe aluminium concentration and that removal of aluminium from water is essential before haemodialysis, particularly in areas where alum is used in water treatment as a clarifying agent. Reverse osmosis treatment satisfactorily removes aluminium and many other substances from water. Its application had a beneficial effect on 7 of 9 patients previously exposed to dialysate prepared from water with a high aluminium content and prevented the appearance of dementia in 24 patients whose water was so treated from the start of haemodialysis.

Aluminum↗

PubMed2001

Rotating reverse osmosis: a dynamic model for flux and rejection.

Reverse osmosis (RO) is a compact process for the removal of ionic and organic pollutants from contaminated water. However, flux decline and rejection deterioration due to concentration polarization and membrane fouling hinders the application of RO technology. In this study, a rotating cylindrical RO membrane is theoretically investigated as a novel method to reduce polarization and fouling. A dynamic model based on RO membrane transport incorporating concentration polarization is used to predict the performance of rotating RO system. Operating parameters such as rotational speed and transmembrane pressure play an important role in determining the flux and rejection in rotating RO. For a given geometry, a rotational speed sufficient to generate Taylor vortices in the annulus is essential to maintain high flux as well as high rejection. The flux and rejection were calculated for wide range of operating pressures and rotational speeds.

Filtration↗

PubMed2001

Hybrid organic/inorganic reverse osmosis (RO) membrane for bactericidal anti-fouling. 1. Preparation and characterization of TiO2 nanoparticle self-assembled aromatic polyamide thin-film-composite (TFC) membrane.

Hybrid organic/inorganic reverse osmosis (RO) membranes composed of aromatic polyamide thin films underneath titanium dioxide (TiO2) nanosized particles have been fabricated by a self-assembly process, aiming at breakthrough of biofouling problems. First, positively charged particles of the colloidal TiO2 were synthesized by a sol-gel process, and the diameter of the resulting particles in acidic aqueous solution was estimated to be approximately 2 nm by analyzing the UV-visible absorption characteristics with a quantum mechanical model developed by Brus. Transmission electron microscopy (TEM) further confirmed the formation of the quantum-sized TiO2 particles (approximately 10 nm or less). The TiO2 particles appeared to exist in the crystallographic form of anatase as observed with the X-ray diffraction (XRD) pattern in comparison with those of commercial 100% rutile and commercial 70:30% anatase-to-rutile mixture. The hybrid thin-film-composite (TFC) aromatic polyamide membranes were prepared by self-assembly of the TiO2 nanoparticles on the polymer chains with COOH groups along the surface. They showed improved RO performance in which the water flux even increased, though slightly. Field-emission scanning electron microscopy (FESEM) exhibited the TiO2 nanoparticles well adsorbed onto the surface. X-ray photoelectron spectroscopy (XPS) demonstrated quantitatively that a considerable amount of the adsorbed particles were tightly self-assembled at the expense of the initial loss of those that were loosely bound, and became stabilized even after exposure to the various washing and harsh RO operating conditions. The antibacterial fouling potential of the TiO2 hybrid membrane was examined and verified by measuring the viable numbers and determining the survival ratios of the Escherichia coli (E. coli) as a model bacterium, both with and without UV light illumination. The photocatalytic bactericidal efficiency was remarkably higher for the TiO2 hybrid membrane under UV illumination, compared to that of the same membrane in darkness, as well as those for the neat membranes under either light condition.

Biofilms↗

PubMed2003

Removal of the natural hormone estrone from aqueous solutions using nanofiltration and reverse osmosis.

The ability of a variety of nanofiltration and reverse osmosis membranes to retain the natural hormone estrone are examined here as a function of solution conditions. While size exclusion dominates retention with the tighter membranes, both size exclusion and adsorptive effects appear to be instrumental in maintaining high retention on nanofiltration membranes that otherwise exhibit relatively low ion retentions. These adsorptive effects may be driven by hydrogen bonding between estrone and the membrane. Electrostatic attraction appears to aid retention with an apparent slight decrease in retention at high NaCl concentrations. Deprotonation of estrone leads to a significant decrease in retention, most likely as a result of the effect of strong electrostatic repulsive forces decreasing the proximity of the negatively charged estrone to the negatively charged membrane surface and thus lowering the potential for adsorptive retention. This deprotonation effect is absent for tight RO membranes. The results reported here indicate that while open nanofiltration membranes may be effective in retaining estrone under some conditions, the extent of retention may be very susceptible to maintenance of adsorptive capacity at the membrane surface and depend on solution chemistry.

Adsorption↗

PubMed2001

Structure-motion-performance relationship of flux-enhanced reverse osmosis (RO) membranes composed of aromatic polyamide thin films.

The present paper explores the role of dimethyl sulfoxide (DMSO) used as an additive to modify the morphological as well as the molecular nature of aromatic polyamide during the formation of thin-film-composite (TFC) membranes. In addition, it elucidates the mechanism of enhancing the reverse osmosis (RO) permeation of the resulting membranes in proportion to the addition of DMSO. Morphological studies by atomic force microscopy (AFM) observed that as the concentration of DMSO increased, the surface roughness and the surface area of the aromatic polyamide TFC membranes became higher and larger, compared to FT-30 membrane for which DMSO was not added during interfacial reaction. Such morphological changes were brought about from fluctuating interface through reducing the immiscibility between aqueous/organic phases by DMSO and provided more opportunities to have contact with water molecules on the surface, participating in the enhancement of the water permeability. Chemical composition studies by X-ray photoelectron spectroscopy (XPS) revealed that there was a considerable increase of the cross-linked amide linkages relative to the linear pendant carboxylic acid groups in the TFC membranes of more DMSO addition. The increase of such amide linkages as hydrogen bonding sites facilitated the diffusion of water molecules through the thin films and played a favorable role in elevating water flux without considerable loss of salt rejection. Relaxation and motion analyses by 1H solid-state nuclear magnetic resonance (NMR) spectroscopy also confirmed the XPS revelation on the basis of measurements of the spin-lattice relaxation time in the rotating frame, T1rho, and determination of the correlation time, tau(c), for the aromatic polyamides forming thin films. The trend of longer tau(c)'s with the increase of DMSO concentration reflected the thin-film aromatic polyamides of less locally mobile chains, accompanied by the higher degree of cross-linking and, hence, the greater number of amide groups. The combined results of AFM, XPS, and solid-state NMR provided a robust explanation for the mechanism of flux enhancement of the aromatic polyamide TFC membranes with the addition of DMSO, which would contribute to not only a fundamental understanding of the process but also an advanced designing of the so-called "tailor-fit" TFC membranes.

Dimethyl Sulfoxide↗

PubMed2003

Rejection efficiency of water quality parameters by reverse osmosis and nanofiltration membranes.

The objective of this study was to evaluate the effectiveness of reserve osmosis (RO) and nanofiltration (NF) membranes, under various solution chemistries, on water quality. The effects of organic carbon, divalent and monovalent cations, bacteria, and permeate drag on the rejection efficiencies of three different membranes were investigated through a series of laboratory bench-scale experiments. Quantitative models were successfully developed to predict the rejection of turbidity, divalent and monovalent cations, ultraviolet absorbance at 253.7 nm (UV254), and dissolved organic carbon (DOC) by membrane filtration. It was found that mechanical sieving (measured as molecular weight cutoff, MWCO) and electrostatic interactions were the most significant parameters since they were found to be important in nearly all models developed. For negatively charged membranes, under high ionic strength solution environments that repress electrostatic interaction between charged compounds and membranes, passage of compounds was mainly a function of size exclusion (i.e. MWCO). Further, of the feedwater parameters tested, bacteria concentration was observed to be the most significant influence on UV254, divalent cation and monovalent cation rejections. The developed models revealed that interactions between feedwater composition and membrane properties impacted the rejection efficiency of membranes as significantly as water composition and membrane properties individually.

Bacteria↗

PubMed2005

Desalination of mixed tannery effluent with membrane bioreactor and reverse osmosis treatment.

A limiting factor for the reuse and recycling of treated tannery wastewater for irrigation and other uses is the high salt content, which persists even after conventional treatment. Reverse osmosis (RO) membrane treatment has been shown to significantly reduce the salt contents of tannery effluents. However, the high organic content of tannery effluent leads to rapid scaling and biofouling of RO membranes with a consequent reduction in flux rates and performance. Membrane bioreactors (MBR) have been shown to be highly effective in the removal of organic pollutants and suspended solids from tannery effluent. This research investigated the use of a combined MBR and RO treatment process to treat tannery effluents to an acceptable level for irrigation purposes. The MBR was operated at 17-20 h retention time, at a F/M ratio of 0.52 kg COD x kg SS(-1) x day(-1) and a volumetric loading rate of 3.28 kg COD x m(-3) x day(-1). This treatment reduced the COD, BOD, and ammonia concentrations of the effluent by 90-100%. The MBR was shown to be an excellent pretreatment prior to RO technology, due to the high removal efficiency of organic compounds and suspended solids, with average concentrations of 344 mg x L(-1) COD and 20 mg x L(-1) BOD achieved in the permeate. RO treatment reduced the salt content of the MBR permeate by up to 97.1%. The results of the research demonstrated that the MBR system developed was appropriate for the treatment of tannery effluents and, in combination with the RO treatment, reduced the salt content to acceptable levels for irrigation. The MBR pretreatment reduced bio-fouling and scaling of subsequent RO treatment and improved the overall performance of the RO unit. It is believed that this is the first investigation of a combined MBR and RO treatment for tannery effluents. This research provided data for an outline design of a full-scale MBR and RO plant with a treatment capacity of 5000 m3 per day for mixed tannery effluents.

Bioreactors↗

PubMed2006

Relating organic fouling of reverse osmosis membranes to intermolecular adhesion forces.

Organic fouling of reverse osmosis (RO) membranes and its relation to foulant--foulant intermolecular adhesion forces has been investigated. Alginate and Suwannee River natural organic matter were used as model organic foulants. Atomic force microscopy was utilized to determine the adhesion force between bulk organic foulants and foulants deposited on the membrane surface under various solution chemistries. The measured adhesion force was related to the RO fouling rate determined from fouling experiments under solution chemistries similar to those used in the AFM measurements. A remarkable correlation was obtained between the measured adhesion force and the fouling rate under the solution chemistries investigated. Fouling was more severe at solution chemistries that resulted in larger adhesion forces, namely, lower pH, higher ionic strength, presence of calcium ions (but not magnesium ions), and higher mass ratio of alginate to Suwannee River natural organic matter. The significant adhesion force measured with alginate in the presence of calcium ions indicated the formation of a crossed-linked alginate gel layer during fouling through intermolecular bridging among alginate molecules.

Alginates↗

PubMed2006

Performance of forward (direct) osmosis process: membrane structure and transport phenomenon.

The performance of a forward (direct) osmosis (FO) process was investigated using a laboratory-scale unit to elucidate the effect of membrane structure and orientation on waterflux. Two types of RO membrane and a FO membrane were tested using ammonium bicarbonate, glucose, and fructose as the draw solution to extract water from a saline feed solution. The FO membrane was able to achieve higher water flux than the RO membranes under the same experimental conditions while maintaining high salt rejection of greater than 97%. Increasing operating temperature increased the water flux in FO process. To investigate the effect of membrane orientation on water flux, the FO membrane was tested normally (dense selective layer facing draw solution) and reversely (dense selective layer facing feed solution). Explanations on transport phenomenon in FO process were proposed which explain the observation that the FO membrane, when used in the normal orientation, performed better due to lesser internal concentration polarization. This study suggests that an ideal FO membrane should consist of a thin dense selective layer without any loose fabric support layer.

Membranes, Artificial↗

PubMed2006

Anthocyanin and flavonoid production from Perilla frutescens: pilot plant scale processing including cross-flow microfiltration and reverse osmosis.

Extraction and concentration at a pilot plant scale of anthocyanins and flavonoids from Perilla frutescens var. frutescens harvested in the Guangzhou area of China were investigated. The study of extraction efficiency using mineral acids and organic acids showed that 0.01 mol/L nitric acid was the most suitable to extract flavonoids from this slightly red leaf cultivar. The red extract contained 12 mg/L (as cyanidin equivalent) anthocyanins and other flavones. The multistep process included cross-flow microfiltration (CFM) with a ceramic type membrane, reverse osmosis (RO), and rotating evaporation (RE). The filtration fluxes were high and constant for CFM (150 L/h/m2 at 0.6 b) and for RO (22 L/h/m2 at 40 b). The red extract was concentrated 9.4 times by RO and then 5.4 times by RE. It contained 422 mg/L anthocyanins, representing 77% of the total extracted anthocyanin. The proportion of flavonoids was found unchanged during processing. The concentrated extract showed a pH of 2.7, and its free acidity was found to be 46% of the acidity added for extraction, because of the buffering capacity of the extract. At the concentration level reached, a crystallized deposit occurred and was identified as tartrate.

Anthocyanins↗

PubMed2003

Performance evaluation of reverse osmosis desalination plants for rural water supply in a developing country--a case study.

Performance evaluation of two reverse osmosis (RO) desalination plants (DSP) at villages: Melasirupodhu (30 m3 day(-1)) and Sikkal (50 m3 day(-1)) in Ramanathpuram district, Tamil Nadu (India) were studied so as to bring out the state-of-art of their operation and maintenance (O&M). Detailed information on plant design and engineering, water quality, plant personnel, and cost of O&M was collected for a period of three years after commissioning of the two plants. Feed water was brackish, the TDS varied in the range of 6500-8500 mg L(-1) at Melasirupodhu and 5300-7100 mg L(-1) at Sikkal villages. The product water quality was observed to be gradually deteriorating as the salt rejection by the membranes decreased with time. The salt rejection was 97-99% at the time of commissioning of the plants, and came down to 89-90% at the end of 3 years of operation. Product water TDS soon after installation of the plants was excellent and within desirable limits of BIS. After three years of operation, few parameters exceeded the desirable limits, however, they were found to be within permissible limits of BIS. The analyses of the data showed that both plants were operated only at 30-36% of the design capacity. Plant shut-down due to inadequate and erratic power supply, and plant break-down and inherent delay in repairs due to lack of adequate infrastructure were found to be the major causes for the low utilization of the plants. Consequently the recurring cost of product water production enhanced to Rs. 25.0/m3 at Melasirupodhu and Rs. 17.5 m(-3) at Sikkal, as against the estimated cost of Rs. 15.0/m3 and Rs. 11.0/m3, respectively, as per the design. Over the years, the energy consumption for the product water output increased reflecting higher operational pressures needed with the aging of the membranes.

Conservation of Energy Resources↗

PubMed2003

Kinetic model of osmosis through semipermeable and solute-permeable membranes.

The gas analogy of the van't Hoff equation for osmotic pressure deltapi = RT/V, where R is gas constant, T absolute temperature and V mole volume of water, remained unexplained for a century because of a few misconceptions: (1) Use of supported membranes prevented the recognition that osmotic forces exert no effect on the solid membrane. During osmotic flow frictional force of solvent within membrane channels equals osmotic kinetic force pi at the interface against the solution containing impermeant solute. (2) Retrograde diffusion of water is much less than osmotic flow even when dx in the gradient dc/dx approaches zero. (3) The gas analogy was thought to be accidental. Actually, the internal kinetic pressure is P = RT/V, because intermolecular forces cancel out at the liquid interface, just as within a gas. The kinetic osmotic pressure is the difference in solvent pressure across the interface: pi = RT/V-(RT/V)X1 = (RT/V)X2, where X1 and X2 are the mole fractions of water and impermeant solute, respectively. Integration gives pi = -(RT/V)lnX1, identical to the thermodynamic equation. This equation is correct up to 25 atmospheres, and up to 180 atmospheres by assuming that a sucrose molecule binds 4 and a glycerol molecule 2.5 water molecules. For solute-permeable membranes, the reflection coefficient sigma can be calculated by formulas proposed for ultrafiltration. Because the fraction (1-sigma) of solute concentration behaves as solvent, osmosis may well proceed against the chemical potential gradient for water. The analogy to an ideal gas applies because pi = -(RT/V)lnX1 is the small difference between enormous internal solvent pressures.

Diffusion↗

PubMed1999

A role for oxygen-induced osmosis in hyperbaric oxygen therapy.

The principles of gas-induced osmosis, demonstrated in the 1970s, have been applied to the very large steady-state gradients of O2 arising between arterial blood and hypoxic tissue during hyperbaric oxygen (HBO) therapy to produce a fluid 'pump' in the desired direction for resolving accompanying oedema. Thus, in soft-tissue injuries, an oxygen-induced fluid pump would break the vicious cycle between ischaemia, hypoxia and oedema at the point of oedema rather than hypoxia, as hitherto assumed. This osmotic mechanism enables the successes of HBO therapy in hypoxic disorders to be reconciled with early failures in such areas as hyperbaric radiotherapy, where substitution of O2 for N2 in inspired air was clearly not reflected at the tissue level. This argument also applies to the success of HBO in treating air embolism and decompression sickness over simple compression. The oxygen pump would seem to offer a more plausible explanation for the success of HBO therapy than theories based upon O2 delivery by the circulation, especially when considering cardiovascular reflexes to elevated PaO2 and the marginal increase in blood O2 content upon switching to HBO from normobaric oxygen breathing.

Animals↗