Comprehensive studies of membrane autopsy results show that 60% of failures tested in the Genesys laboratory are caused by poor pre-treatment design and/or poor operation. Alternative cost-effective autopsy techniques allow us to optimise pre-treatment dosage and design specific membrane cleaning programmes based on accurate deposit identification.
Membrane Fouling Issues
Membrane autopsy has been widely accepted as the standard tool for evaluating and identifying deposits on membrane surfaces. The cost of the extensive analytical procedures involved in a membrane autopsy coupled with the need for element replacement can make this technique prohibitive for small to medium sized systems. Typically, the autopsy option is used when system performance is already dramatically affected and operational costs are increasing. This has driven our research for complementary and/or alternative laboratory techniques.
Poor feed water quality causes membrane problems – irreversible fouling, flux loss and an increase in salt passage. These issues will increase membrane cleaning frequency and system downtime leading to higher water production costs. In more than 200 autopsies at the Genesys Laboratories 60% of membrane damage cases can be linked, directly or indirectly, to problems or inefficiencies in pre-treatment processes.
Particulate matter present in reverse osmosis (RO) feed water due to pre-treatment issues can be inorganic in nature such as alumino-silicates (clays), metallic oxides, or organic matter such as humic acids and polysaccharides. Micro organisms such as bacteria, fungi, moulds or protozoa, invariably present as a biofilm also have a role to play in membrane fouling. To prevent these membrane deposits from occurring, improvements in pre-treatment design or operation were specifically recommended in each autopsy case for each type of deposit.
Reducing Membrane Fouling
Optical Particle Counting Technology
Particle counters are proving to be an essential tool for improving RO pre-treatment design and system operation, not only when evaluating coagulant and flocculants performance, but also when evaluating sand filter and cartridge filter performance.
Optimising Pre-Treatment
Particle size is one of the most important parameters when selecting or designing a filtration system and yet in many cases is seldom considered. Turbidity has commonly been used as a reference parameter, but turbidity is affected by factors such as size, shape, and colour and particle refraction index. The turbidity of RO feed water tends to be significantly lower than that of wastewater, alternative procedures to coagulation or flocculation assays have therefore been developed to study coagulant and flocculants behavior and evaluate their efficiency.
Particle counters help to determine the best product and the optimum dosage for improving the performance of existing pre-treatment systems. The objective of coagulation is to aggregate particulate matter to form flocs of sufficient size to guarantee retention by the filtration system. Comparison of particle size distribution under different experimental conditions is a valuable tool to aid decision making. Dose rate data can then be obtained in the laboratory simply using different compounds, testing variable dose rates and pH conditions.
Deposit Identification
SEM – EDAX of SDI & Cartridge Filters
The SEM-EDAX technique is used to study deposits on membrane surfaces, cartridge and security filters and even SDI papers. The information gained is invaluable when proposing preventative and corrective actions in pre-treatment operation. In many cases the examination of cartridge filters and SDI papers avoids the need for sacrificing membrane elements to autopsy.
Membrane Cleaning Using Particulate Matter Characterisation by SEM-EDX
A decrease in normalized product flow and an increase in differential pressure or permeate conductivity are symptoms of membrane fouling. Colloidal fouling dramatically affects the product flow rate particularly of membrane elements in the first position. Although SDI measurement has been used for many years to determine the fouling potential of RO feed water, the SDI measurement cannot provide any scientific information about the nature and type of fouling expected.
When fouling occurs, the nature of the foulant must be identified if an effective cleaning program is to be suggested. An alternative procedure to membrane autopsy is to evaluate colloidal matter retained in an SDI filter paper. RO feed water is passed through a 0.45µm paper membrane filter, then the filter surface is analyzed by SEM-EDAX and the nature of the colloidal matter is characterized.
Deposits from the surface of the SDI papers can then be analysed using energy dispersive X-ray analysis which identifies the elemental composition of the deposit. The results of analysis of these SDI papers allow our technicians to recommend specific cleaning protocols. An effective cleaning programme minimizes operational costs and helps to increase membrane life.
Summary
While full membrane autopsy techniques remain financially viable for the majority of RO systems, deposit analysis of the surface of cartridge filters and SDI papers provides a cost effective alternative for troubleshooting smaller size RO plant.
In addition to optimising the treatment programme results can also be used to identify correct membrane cleaning techniques helping to reduce operational costs and downtime.
Genesys membrane autopsy results show that particle counting techniques can be an important method for optimising RO pre-treatment systems and determining technically correct chemical treatment programmes.
When used correctly these methods provide vital scientific evidence in troubleshooting RO systems and also as part of a programme to optimise operation.