Program estimates membrane water treatment costs

By Irving Moch, Jr.

The US Bureau of Reclamation and I. Moch & Associates, Inc., sponsored by the American Membrane Technology Association, developed WTCost©, a CD ROM for estimating membrane water treatment costs. This disk, designed for use by end-users, consultants, managers and engineers, enables users to evaluate and compare water treatment processes employing reverse osmosis/nanofiltration (RO/NF), microfiltration/ultrafiltration (MF/UF), electrodialysis and ion exchange. The program is supplier neutral.

Water compositions are suggested for users without actual ion concentrations. Labour and supervision, membrane replacements, amortisation rates and tanks, piping and instrumentation are also included, thus, permitting calculation of plant capital requirements and operating and maintenance costs for all commonly employed aqueous separation processes.

The program has been found to be within 10% to 20% of bids for plants ranging in size from 50,000 to 100 million gallons per day (mgd). Required inputs for design parameters are water analysis and capacity. Dosing rates, element pricing, capital costs of equipment and their designs and other factors are supplied as default values that can be overridden when precise data are available.

The program helps answer an important seawater question asked by many managers and engineers: "What is the effect of changes in capacity and salinity on system capital and operating costs?" Such a study had not been undertaken before because of an inability to easily compare cases under equivalent conditions, but comparative evaluations can now be completed with WTCost.

For example, the WTCost program helped compare twelve cases in which plant capacities varied from one mgd to 25 mgd at two salinities (35,000 mg/l and 43,000 mg/l). Recoveries, energy prices, temperatures, feed water sources etc. were chosen that represent the conditions found where these salinities are generally present, i.e. California coast and the Middle East.

The WTCost methodology helped reveal that amortisation of the plant capital is a major constituent of operating cost. Its value, as a percent of the total water cost, increases from about 15% to 50% as the capacity rises from one mgd to 25 mgd. The importance of obtaining a good financing package thus becomes critical in assessing the viability of any project.

The evaluation of two five-mgd capital cases clearly shows how important it is to use wells as the feed source versus an open sea intake, provided that wells have the porosity to deliver the required feed flow with a reasonable number of production units. With a well, the fouling index is usually so low that very limited pretreatment or disinfection is required; thus the risks involved in designing and installing a facility and in the general engineering requirements are less with the more simplified well field plant than with the complicated open sea intake.

Salinity differences are reflected in desalting capital and operating costs. The 35,000-mg/l cases, seen in most of the world's oceans and seas, have somewhat less capital committed to membranes and pressure vessels despite reduced feed water temperatures. This advantage is more than offset in high salinity waters (43,000 mg/l) found in the Middle East by the low cost of energy in this area despite larger membrane replacement rates usually associated with high TDS waters. Capacity changes have a significant effect on costs even though RO/NF and MF/UF are modular constructions.

The membranes used in each technology, essentially, change linearly with alterations in plant size. These technologies, when coupled, constitute 50% to 60% of the capital even as they decrease price-wise in absolute terms with increasing capacity. Prices offered to an end-user by a contractor would be 10% to 15% higher than reflected in this study; the increase represents profitability. Lastly, specific project numbers, based on direct site conditions, do require competitive tenders by the buyer.

• Disinfection with chlorine, chloramine, ozone and ultra-violet irradiation
• De-chlorination with sodium bisulphite, sodium sulphite or sulphur dioxide
• pH adjustment with sulphuric or hydrochloric acid
• Coagulation/flocculation with alum, poly-aluminium chloride (PAC), ferric chloride, ferrous sulphate or lime/soda using up flow solids contact clarifiers
• Filtration enhancement with polyelectrolyte
• Filtration with granular activated carbon and other granular media
• Microfiltration and ultrafiltration as pretreatment to remove particulate and biological materials
• Antiscalant to prevent precipitation of sparingly soluble salts
• Reverse osmosis and nanofiltration for desalination
• Electrodialysis reversal and ion exchange as competitive processes
• Energy recovery for seawater desalting
• Intake and outfall infrastructures
• Post treatment of product water for disinfection and prevention of corrosion