By: Linda Dudley
Foulant sampling from membrane inlet
Reverse osmosis (RO) systems producing large volumes of quality product water for industrial and municipal can run without significant performance problems for many years; however operational problems can cause systems to run below optimum performance levels. Observing specific guidelines can help to achieve optimised performance and to reduce fouling risks and their effects. These guidelines, if followed, will bring about significant economical benefits and increase overall plant lifetime.
Good monitoring and data collection
Good plant design is essential but many system failures could be prevented by closer plant monitoring once installed. The major factors that affect membrane performance are:
• condition of the raw water supply;
• effectiveness of pretreatment procedures;
• system operating parameters;
• degree of plant maintenance and continuous monitoring;
• responsiveness of plant operators to significant performance changes;
• and the rate and degree of fouling.
Regular monitoring should be routine with data taken and plotted either electronically or by hand. Key parameters include pressure drop, output, pH, temperature, conductivity, and TOC. New advancements in scale inhibition technology have made traced products available that monitor and optimise treatment chemicals dosed on-line.
A crucial measure for ensuring trouble-free operation is regular monitoring of feed water condition throughout the pretreatment system and inspection of associated equipment and pipe-work. Analysing samples of feed water at each treatment stage enables a good assessment of plant conditions.
Plant monitoring
Pretreatment systems may fail during the lifetime of a plant for many reasons. It should be assumed that failures would happen, so design features and monitoring procedures should detect these occurrences early to minimise their effects.
What factors could cause plant failure?
• overdosed pretreatment flocculants;
• residual chlorine levels in feed stream;
• poor choice of cartridge filter;
• poor choice of scale inhibitor or incorrect/ failed antiscalant or acid dosing;
• high iron (Fe) or organic loading;
• colloidal breakthrough or sand or carbon fines;
• and high microbiological counts.
These conditions should be prevented because they contribute to severe membrane fouling, which is the most common reason for performance problems. The effects are often reduced membrane productivity, poor salt rejection characteristics and increasing differential pressure across membranes.
Common potential foulants
The fouling potential of a water source is determined by feed-water quality. It can be assessed by a chemical analysis of the micro-foulants including, iron and metal oxides/hydroxides, silt, colloids, bacteria and organic matter and by the Silt Density Index (SDI) measurement. An SDI <5 is recommended for spiral wound membranes and an SDI <3 for hollow fine fibre elements for minimal fouling risk.
Calcium carbonate creates the greatest fouling risk. Water analysis and operational parameters can be used to predict the scaling potential within an RO system. The Langelier Saturation Index (LSI) and Stiff & Davis (S&DI) are used for brackish water and seawater, respectively. The pH, calcium and bicarbonate (alkalinity) in feedwater in addition to plant operating data must be determined first in order to estimate calcium carbonate scale potential. Expected seasonal variations of feed quality must be considered. Using this information in a computer program enables recommendations to be made on the type of scale inhibitor needed and its dose level.
Flow instrumentation
One of two main types of scale inhibitors is based on a range of organic compounds. These polymers function as crystal distortion agents, but at a higher molecular weight they also exhibit dispersancy properties. The second type includes phosphonates, which are widely used in water treatment formulations as scale and corrosion inhibitors and iron sequestrants. They act as "super-threshold" agents in membrane systems that can stabilise a wide range of supersaturated salt solutions (up to +3.0 LSI). This allows engineers to design RO systems with maximum recovery rates. The majority of brackish water systems now dose proprietary scale inhibitors in preference to acid dosing. Severe scaling in systems is a rare occurrence.
Microbiological growth prevention
Careful inspection of pipe-work, dosing tanks and cartridge filters can identify the presence of biofilm slime requiring sanitisation procedures to prevent continued widespread microbiological growth.
Cleaning recommendations to remove biofilms are specific to each individual plant. Biomass composition, membrane material and the degree of fouling determine the type of cleaning chemicals used and their frequency of use. Cleaning procedures, typically three- or four-stage cleaning schedules that incorporate alkaline surfactant cleaners and a non-oxidising biocide, have been developed to remove biofouling. Cleaning is recommended when pressure differentials or membrane flow rates change by 15% of the design specification. Cleaning solutions are most effective when circulated at elevated temperatures, preferably 30oC with alternating periods of soaking and recirculation.
SEM of biomass of surface of a fouled membrane.
Several proprietary non-oxidising biocides approved for use are membrane-compatible, easily deactivated for discharge and have good biocidal properties. Many of these compounds can be dosed intermittently at low dose rates in non-potable applications, and are a cost-effective means of maintaining a clean membrane surface. Biocides are applied as off-line cleaners in potable applications.
Many credible RO equipment suppliers (OEMs) fabricate well-designed RO units with appropriate monitoring and control features. Combined with good maintenance and operational practices, on-site trouble-free RO systems are achievable.
Author's note
Linda Dudley is the market development manager for PermaCare- Membrane Separations Group, Ondeo Nalco Limited, UK. For more information on PermaCare and PermaTreat TRASAR technologies contact your local Ondeo Nalco representative through the company website: www.permacare.com
Guidelines
• Monitoring and data normalisation is required on a regular basis.
• Good training of plant operators is needed.
• Regular analysis of raw water quality should be conducted.
7bull; Scale inhibitors dosed for high LSI brines will allow maximised recovery rates.
• Good cleaning and biogrowth control procedures are vital particularly when operating systems in high ambient temperatures.
• Site specific procedures must be developed for monitoring, data evaluation, maintenance and cleaning practices.
Autopsy identifies RO system problems in power plant
A 2,500 m3/day reverse osmosis plant that produces cooling water for a power station experienced continual output loss, which required membrane cleaning every two weeks, increasing to every four days in warm weather. PermaCare technicians performed an autopsy and analysis on a fouled membrane from the plant and identified bacteria and iron as the two major culprits. These findings led to improvements in operation and maintenance.
The twin unit RO plant is of a two-stage design using thin film composite spiral wound membranes operating at 20 bar and at 75% recovery. The pretreatment system consists of ozonation, dissolved air flotation (DAF), lime soda softening, dual media filtration and cartridge filters. The chemicals used in the pre-treatment system are sodium hypochlorite, ferric chloride, lime and soda, polyelectrolyte, sulphuric acid and sodium bisulphite. To control bacteria, the free chlorine level is maintained at 1.5 ppm throughout the latter stages of the pretreatment system up until the 1.5 µ cartridge filter where the sodium bisulphite is added.
The antiscalant sodium hexametaphosphate (SHMP).dosed into the feedwater later changed to a polyacrylic acid product due to high bacterial levels found in the SHMP supply tank. The biological slime seen in the SHMP dosing tank confirmed the need to regularly monitor and inspect all pretreatment stages to eliminate any source of microbiological growth.
PermaCare recommended several steps to reduce fouling:
• optimise pretreatment plant, in particular, the ferric chloride dosing;
• improve cleaning procedures - acid-based cleaner to remove traces of inorganic scale; alkaline surfactant cleaner to break down organics together with a chelating agent to aid removal of organic debris.
• a phosphonate-based antiscalant to prevent scale formation and sequestering small quantities of excess iron;
• a non-oxidising on-line biocide treatment to be dosed intermittently to sanitise the system and prevent bacterial growth.
These recommendations were implemented and resulted in decreased downtime.
