Julius Glater

The search for a chlorine-resistant reverse osmosis membrane

Abstract Reverse osmosis membranes processing natural and waste waters are often exposed to low concentrations of chlorine in feed water. This biocide is chemically aggressive toward most commercial high performance membrane polymers. Chemical attack by chlorine ultimately results in membrane failure as measured by enhanced passage of both salt and water. Membrane failure is due to certain structural changes within the polymer in response to chlorine exposure. These changes in polyamide type membranes result from chlorine attack on amide nitrogen and aromatic rings. The resulting substitution products may cause deformation in the polymer chain or cleavage at amide linkages. The exact chemical mechanism of chlorine-polymer interaction and subsequent membrane failure is not, as yet, clearly understood. A review of published work on membrane-chlorine interaction will be presented here. Experimental evidence supporting various models for membrane failure will also be documented. In addition, certain common structural features known to enhance chlorine resistance of polymeric membranes are identified. It is anticipated that this paper will stimulate research efforts toward development of polymeric reverse osmosis membranes with high levels of chlorine resistance.

REVERSE OSMOSIS MEMBRANE SENSITIVITY TO OZONE AND HALOGEN DISINFECTANTS

SUMMARY The sensitivity of commercial reverse osmosis (RO) membranes toward halogen and ozone disinfectants has been measured at carefully controlled concentration and pH levels. Membrane sensitivity varies with polymer type, disinfectant chemical, and solution pH. Aromatic polyamide membranes are damaged by halogen addition to aromatic rings within the polymer. This process follows predictable reaction kinetics. Polymer viscosity changes with increasing membrane damage have also been followed. Results of this study will be useful in planning disinfection strategies for RO units in the field.

The early history of reverse osmosis membrane development

Abstract The birth of pressure driven membrane desalination took place nearly 100 years ago. Early developments in this technology remain shrouded in some mystery, especially with respect to application of the osmotic phenomenon to desalination. Existing literature is also somewhat confused regarding first usage of the term “reverse osmosis”. A comprehensive review of early literature is presented in this paper in an effort to resolve some of these interesting questions. The unique feature of this review, as contrasted to several others, is inclusion of previously unpublished reports and experimental data. These documents, dating back to 1950, will reveal surprising insights into the origin of present-day technology now known as reverse osmosis.

Low-pressure RO membrane desalination of agricultural drainage water

Agricultural drainage water is a complex mixture of dissolved and suspended chemical species and may contain a wide variety of microorganisms. The application of membrane systems for desalination of agricultural drainage (AD) water requires careful consideration of feed water quality, suitable membrane selection and operating conditions. In order to evaluate the potential applicability of low-pressure reverse osmosis (RO) to the treatment of AD water, a diagnostic approach to membrane selection and process evaluation was undertaken in support of a pilot field study in the California San Joaquin Valley. Five candidate membranes were evaluated in a diagnostic laboratory membrane system which provided an initial selection based on salt rejection and product water flux performance for model salt solutions of univalent and divalent cations. Biofouling potential of the selected membranes was also evaluated using two standards strains of bacteria. Preliminary pilot plant performance, based on the selected membranes, was encouraging and has provided the basis for long-term pilot plant testing at higher recoveries to assess the impact of fluctuating AD water feed composition.

Alkaline scale formation in boiling sea water brines

Abstract A laboratory method for the study of alkaline scale is described. Evaporator conditions are easily simulated in experiments of relatively short duration (5 hours). Reproducible results have been achieved by chemical analysis for total scale which consisted of mixtures of calcium carbonate and magnesium hydroxide. Experiments with natural sea water show the amount and composition of alkaline scale to be a function of temperature, brine concentration, bicarbonate ion concentration, and flow conditions through the evaporator. The transition between calcium carbonate and magnesium hydroxide was shown to be influenced by factors other than temperature. A new mechanism for alkaline scale formation is proposed in this paper. The first step involves a unimolecular breakdown of bicarbonate ion to form hydroxide ion. This concept is in disagreement with the generally accepted mechanism described in the literature.

The effects of small halocarbons on RO membrane performance

Abstract The effects of small halocarbons, CHCl3, CHBr3 and CCl4, at 50 mg/l concentrations on the performance of three typical RO membranes were examined. Cellulose acetate, polyamide and advanced composite membranes were used in this study. The flux, total dissolved solids (TDS) rejection, halocarbon rejection, partition coefficient and void volume tests were conducted to evaluate the effects of halocarbon addition. In general, the halocarbons were poorly rejected by all three membranes. This was accompanied by increased rates of flux decline and increased TDS rejection over controls without halocarbons. Partition tests revealed the advanced composite membrane adsorbed all three test halocarbons much more strongly than either cellulose acetate or polyamide membranes. The strong affinity for halocarbons must be considered when using the advanced composite membrane for various waste water applications.

Calcium sulfate hemihydrate scaling threshold enhancement by magnesium ion augmentation

Abstract Efficient high temperature distillation of sea water cannot be realized without development of scale control systems for calcium sulfate. This paper describes a new concept of scaling threshold enhancement based on complexing a portion of dissolved sulfate ion in the form of stable MgSOo ion pairs. Complexing may be accomplished by augmentation of feed water with magnesium ion recovered and recycled from distilling plant blowdown. This “tying up” of sulfate ion results in higher solubility levels of calcium sulfate. Magnesium ion augmentation at levels up to three times ambient results in markedly improved resistance to hemihydrate scaling in natural sea water. Significantly higher levels of operating temperature and concentration factor may be achieved by this technology.

Changes in water and salt transport during hydrolysis of cellulose acetate reverse osmosis membranes

Abstract Cellulose acetate membranes undergo hydrolytic decomposition which is accelerated at very high or low pH levels. Hydrolysis results in the loss of acetyl groups leading to a sharp decline in membrane performance. Minimum hydrolysis rates occur in the pH range of 4–6. The two parameter transport equation (Merten et al.) gives salt and water permeability coefficients (A and B) which depend on membrane acetyl content. In this paper we have shown both coefficients, calculated from performance changes, to increase exponentially with time. Rate constants calculated from these changes show changes in salt permeability to be an order of magnitude greater than changes in water permeability. The observed discrepancy in salt and water transport kinetics suggests differing mechanisms for these two processes. Evidently, salt-water coupling during transport increases as membrane hydrolysis progresses. Thus, the two parameter transport equations do not apply since they neglect the salt-water coupling phenomenon. This paper discusses the use of other membrane transport models to account for changes in membrane performance as hydrolysis occurs. The relation between acetyl content and hydrolysis rates is also discussed.