Robert Y. Ning

ARSENIC REMOVAL BY REVERSE OSMOSIS

Abstract Arsenic is widely distributed in nature in air, water and soil. Acute and chronic arsenic exposure via drinking water has been reported in many countries, especially Argentina, Bangladesh, India, Mexico, Mongolia, Thailand and Taiwan, where a large proportion of ground water is contaminated with arsenic at levels from 100 to over 2,000 micrograms per liter (ppb). Public health standards of maximum of 50 ppb have been adopted by the US and World Health Organization in the 1970s and the 80s. Carcinogenicity and genotoxicity led to the WHO recommendation of 10 ppb maximum level in 1993, followed by the US adoption of the same in 2001, with the US estimate that 5% of all US community water systems will have to take corrective actions to lower the current levels of arsenic in their drinking water. In high arsenic areas of the world, the need for better water treatment and resulting economic impact would be even greater. In this article, we briefly review the geochemistry, natural distribution, regulation, anthropogenic sources and removal mechanisms of arsenic, pointing especially to the promise of reverse osmosis (RO) as a practical means of purification. We conclude that arsenic in the commonly high oxidation states of (V) is very effectively removed by RO. With further attention to the removal of the weakly acidic arsenic (III) species in waters by the operation of RO at sufficiently high pHs made possible by the newer antiscalants, practical processes can be developed with RO to remove all major species of arsenic from water. Further studies are needed in the characterization of the arsenic species being treated and in the design of the RO process to match the demands.

Discussion of silica speciation, fouling, control and maximum reduction

Abstract In the production of ultrapure water for the power and microelectronics industries, multiple pass reverse osmosis (RO) process is commonly the major step in the reduction of dissolved and suspended matter before polishing by ion exchange and other methods to attain the high purity requirements. With the diverse location of power plants and microelectronic manufacturing facilities around the world, silica concentrations in source waters can range between 1 and 60 ppm (mg/L) to even 300 ppm in some volcanic regions. High pressure steam generators and fine microelectronic structures now require water containing less than 1 ppb (ug/L) concentrations of silica. In designing purification processes, silica has presented issues not only as formidable challenges in many locations as RO membrane foulants, as well as a contaminant requiring efficient removal. Analyses of RO membrane foulants and correlation with water chemistry in the course of trouble-shooting numerous RO processes continues to offer us opportunities to understand silica chemistry, the patterns of silica fouling and methods by which we can chemically control the RO process. Such understanding is applicable to the operation of ion-exchange resin beds as well. In this paper we review the speciation of silica in feedwaters, and chemical approaches to controlling fouling and maximizing silica reduction. Silica and silicates are addressed in the three categories of reactive soluble, non-reactive soluble (colloidal, not filterable) and non-reactive insoluble (particulate, filterable) forms. A brief review of geochemistry, the chemical and biochemical dissolution and deposition of silica and silicates in nature is provided for insights and understanding of natural processes that can be applied to the task of process design and control in silica removal from water.

Reverse osmosis process chemistry relevant to the Gulf

The cost of seawater desalination by reverse osmosis technology (RO) continues to drop with time. This portends accelerating adoption of RO around the world in the 21st century. Since chemicals are used in the operation of RO systems as coagulants, antifoulants and cleaners, continuing and increasing the adoption of the RO process in the Gulf necessitates conservation and efficient utilization of these chemicals. This is possible through understanding and optimization of the RO process chemistry. In this paper we survey the chemical factors that affect the operational characteristics of RO systems. The factors include the chemistry of the feedwater, membrane fouling mechanisms, composition of foulants found in membrane elements from many plants around the world and examples of antifoulants and their application. Current approaches towards the development of antifoulants to prevent fouling and membrane cleaners for plant maintenance are discussed in this context. The chemistry presented is part of a conceptual framework in which process optimization and chemical usage conservation can progress well into the next century.

Complete elimination of acid injection in reverse osmosis plants

Abstract Antiscalants with broad activity spectra are available today. When properly chosen, a single antiscalant can efficiently and simultaneously control calcium carbonate, calcium sulfate, strontium sulfate, barium sulfate and calcium fluoride scales as well as inorganic foulants resulting from iron, aluminum and reactive silica present in any given water or wastewater. The effectiveness of many antiscalants towards controlling calcium carbonate scaling in reverse osmosis (RO) plants has allowed us, in the past five years, to successfully help eliminate the continuous injection of acid in all RO systems operating with polyamide membranes. A minority of systems with cellulose acetate membranes however still require acidification due to hydrolytic sensitivity of cellulose acetate towards feedwater pH higher than 6. The chemistry behind the main reason for acidification — the prevention of scaling by calcium carbonate — is reviewed. The mechanisms of calcium carbonate scaling and its avoidance by acidification or with antiscalants are discussed. Major seawater and brackish water RO plants around the world are designed and operated with the continuous injection of concentrated sulfuric or hydrochloric acid for scale control, sometimes simultaneously with an antiscalant. We present a case study in southeastern USA of a 5 million gallons permeate per day plant as a typical example among many for the conversion of major municipal RO plants. By suitable selection of an antiscalant, acid elimination was successfully attained with a simultaneous reduction in the antiscalant dosage, in this case to 2–3 mg/1 in the feedwater. While process optimization continues in the plant, current chemical cost savings due to the deletion of 93% sulfuric acid and the associated caustic soda neutralizer alone amounts to 67% of the annual cost of all chemicals used. This does not include savings through benefits accrued in equipment, operation, maintenance and safety in the plant.

Standardized Data and Trending for RO Plant Operators

Long-term reliable membrane system performance can only be maintained by collection, analysis and evaluation of system data, allowing for effective actions whenever needed. Procedures for data collection, etc., have been devised and are the foundation of easy to use programs. And they are available at no cost to system operators. These spreadsheet programs convert system data into standardized data that can be trended. However, adoption and use of these programs has been low among personnel actually running membrane systems. Consequentially, in the aggregate, membrane systems are not being run as efficiently as is practically possible. Many tasks that confront membrane system operators require knowledge that is available only by trending of standardized system data. Such analysis provides a rational basis for overall management of system resources of labor, membranes, chemicals and equipment. These tasks include determining when to clean membrane, changing out membranes, trouble shooting, running pilot tests, and optimizing day to day system performance. This chapter reviews the common tasks operators carry out and gives examples of using standardized system data as a tool.