Membrane Processes Introduction Membrane processes represent an important subset of filtration processes as there are very few pollutants found in water. The separation is based on the physical characteristics of the pollutants to be removed such as : size, diffusivity or affinity for specific contaminants.

The membrane processes with the greatest application to potable water treatment are reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF) RO membranes are principally used to remove dissolved salts from brackish or salt water. nanofiltration membranes are generally used to remove disinfection by-product precursors such as natural organic matter (NOM).

The larger pore size of microfiltration and ultrafiltration membranes means they are generally used to remove larger pollutants such as turbidity, pathogens and particles. No commercially successful membrane exists to remove uncharged inorganic molecules such as hydrogen sulphide and small uncharged organics.

Membrane separation overview

Porous membranes are generally rated according to the size of the material they should retain. Ultrafiltration membranes are more commonly rated in relation to a molecular weight cut-off in daltons (grams per mole). NF are generally rated in relation to multi-valent ions RO membranes are rated in relation to univalent ions.

Process science 1. Process definitions In membrane processes there are three possible streams. Dead-end and cross flow configurations

Diagram of cross-flow filtration

Dead-end Filtration

The performance of any membrane system is defined in terms of the hydraulic throughput per unit area or flux and the degree of conversion. The flux, or permeate velocity as it is sometimes known, has the SI unit m 3 m −2 s −1, or simply m s −1. Accumulation of material in dead-end and cross-flow filtration.

The conversion or recovery,, is the proportion of the feed that is passed through the membrane as permeate and the rejection, R, is the amount of the pollutant material that is retained by the membrane. Fig. 7.4 Membrane mass balance.

2. Fouling and its control During the operation of a membrane process the rejected material accumulates at the surface, or within the internal pores, of the membrane. This action is know as fouling. The driving force for the process is the pressure differential or transmembrane pressure (TMP), between the feed and permeate sides of the membrane.

Once a membrane is fouled it must be cleaned by either backflushing or chemical cleaning to reduce the TMP of the system. Impact of backwashing and chemical cleaning

The main groups of foulants are Suspended and colloidal matter Scalants Micro-organisms Organic matter Solids Solid foulants take the form of silts, colloidal clay, iron colloids and organic material.

Table 7.1 Common foulants and their control

The fouling propensity of water in relation to its solids content is most commonly determined by the fouling index or silt density index (SDI). The time taken to pass a given volume of water, normally 100 ml, is measured before (ti) and after (tf) a pre-determined operating period (tt), say 15 min.

Scalants Scalants are low solubility salts which precipitate onto the surface of the membrane as they increase in concentration due to their rejection. Scale formation is related to the solubility product of the given salt and represents the maximum value of the product of the molar concentrations of the two component ions of the salt.

The most common way to monitor the propensity for calcium carbonate scale formation is through the Langelier saturation index (LSI) which is defined as where pH is the measured value of the water and pHs is the calculated form from a thermodynamic standpoint. Effectively, the LSI measures the degree to which the water is either super-saturated or under-saturated with calcium carbonate.

Negative values indicate a corrosive water which will dissolve calcium carbonate off surfaces and a positive value indicates the potential for scale formation. Micro-organisms Micro-organisms are ubiquitous, can survive in very low nutrient environments and even utilise pre-treatment chemicals such as flocculants, scale inhibitors and even by-products formed after the addition of chlorine.

Consequently, biofilm formation is unavoidable and the main emphasis is towards control rather than prevention. Organics The key organic foulants are proteins, carbohydrates and natural organic matter. Two strategies are generally used: coagulation of the charged material followed by MF/UF, or direct filtration with NF membranes.

Organic foulants are most effectively cleaned by alkaline chemicals, primarily caustic soda. Configuration The configuration, geometry and mounting arrangement, are important in determining the overall process performance. Technology options Membrane technology can be classified in a number of ways such as 1.Approximate size of the removed species 2. The membrane material 3. Configuration

The optimum membrane configuration is one that has the following characteristics : 1.High membrane area to bulk volume ratio 2.Generates a high degree of turbulence as the water flows through it 3.A low energy expenditure per unit of permeate produced 4. A low cost per unit area 5.Easy to clean 6.Permits modularisation.

Applications Schematic of staging.