DIFFERENCE BETWEEN SURFACE AND DEPTH FILTRATION

• The size of particles retained is slightly higher than the mean pore size of the medium.

• Mechanical strength of the filter medium is less, unless it is made up of stainless steel

• It has low capacity • The size of particles retained is more

predictable. • Equipment is expensive.

Ex. Cellulose membrane filter

• The size of particles retained is much smaller than the mean pore size of the medium.

• Mechanical strength of filter medium is high.

• It has high capacity. • The size of particles retained is

less predictable • Equipment is cheaper.

o Ex. Ceramic filter and sintered filter

Woven materials, perforated sheets, granular materials, porous solid, membrane filter

are used as filter medium

MEMBRANE FILTRATION Membranes are thin porous sheet of materials enable to separate contaminants from

water under certain driving force across the membranes. It is employed in both

drinking water as well as in waste water treatment to remove microorganisms,

particulate materials, micropollutants, salts, natural organic materials etc.

The general membrane processes for water treatment are microfiltration (MF),

ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) and all are pressure

driven process.

MICROFILTRATION Microfiltration is defined as a separation process using membranes with a pore size of

approximately 0.1 to 10 micron, and operating at applied pressure of approximately 1-

6 bar. Microfiltration remove large particles including sand, silt, clays, cysts, algae, and

some bacterial species. MF is not an absolute barrier to viruses. However, when used

in combination with disinfection, MF appears to control these microorganisms in water.

Another application for the technology is for removal of natural synthetic organic matter

to reduce fouling potential of membranes. In its normal operation, MF removes little or

no organic matter; however, when pretreatment is applied, increased removal of

organic material can occur. MF can be used as a pretreatment to RO or NF to reduce

fouling nature of membrane filter. Both RO and NF have been traditionally employed

to desalt or remove hardness from groundwater.

ULTRAFILTRATION Ultrafiltration membranes has a pore size of approximately 0.01 to 0.1 microns, and

operating at applied pressure of approximately 1-10 bar. UF can remove all

microbiological species removed by MF (partial removal of bacteria), as well as some

viruses (but not an absolute barrier to viruses) and humic materials. Disinfection can

provide a second barrier to contamination and is therefore recommended. However,

fouling can cause difficulties in membrane technology for water treatment.

NANOFILTRATION Nanofiltration is a process of filtration where the membranes have a nominal pore size

of approximately 0.001 microns and operating at higher pressure (20-40 bar) than that

of MF or UF. These systems can remove virtually all cysts, bacteria, viruses, and

humic materials. NF can also removes hardness from water (divalent ions) which

accounts for NF membranes sometimes being called “softening membranes.” Hard

water treated by NF will need pre-treatment to avoid precipitation of hardness ions on

the membrane. However, more energy is required for NF than MF or UF. Nanofiltration

membranes are relatively new, and sometimes called as loose reverse osmosis (RO)

membranes. They are porous membranes but their pores lie the in between that of UF

and RO.

Application of Nanofiltration

• Decoloring of effluents and removal of spent mineral acids used to scavenge

organics and heavy metal impurities.

• Heavy metals are rejection, purification of acid and base.

• Dye effluent enrich with dye, salts and some acids can be effectively separated

and concentrated. It is commonly used in common effluent treatment plants.

REVERSE OSMOSIS Reverse osmosis can effectively remove nearly all inorganic contaminants from water.

RO can also effectively remove natural organic substances, monovalent ions,

pesticides, cysts, bacteria and viruses. RO is particularly effective when used in series

with multiple units. Disinfection is also recommended to ensure the safety of water.

Advantages of Reverse Osmosis

• Removes nearly all contaminant ions and most dissolved non-ions,

• Relatively insensitive to flow and total dissolved solids (TDS level and suitable

for small systems with a high degree of seasonal fluctuation in water demand.

• RO operates immediately, without any minimum break-in period,

• Low effluent concentration possible,

• Bacteria and particles are also removed

• Operational simplicity and automation allow for less operator attention and

make RO suitable for small system applications.

Limitations of Reverse Osmosis Membranes:

• High capital and operating cost.

• Managing the wastewater (brine solution) is a potential problem.

• High level of pretreatment is required in some cases.

• Membranes are prone to fouling and produces 40% waste water of the feed.

DIALYSIS Dialysis pertains to the transport of a solute across a membrane by diffusion resulting

from a concentration difference. Since concentration difference, is the sole driving

force, the difference should be large and the membrane should be thin to reduce the

diffusion path. The process is slow compared with pressure driven membrane

processes. Unlike UF or RO where solvent passes through the membrane, it is the

solute that passes through the membrane. Separation in dialysis is governed by the

small pores and diffusion, and therefore small molecules diffuse faster than large ones.

The membrane pores must be very small to prevent convective transport of the

solution resulting from a small pressure difference across the membrane. In certain

pharmaceutical and animal cell culture applications that require addition and removal

of small molecular weight products simultaneously while retaining high molecular

weight solutes, sterile dialysis membrane modules have possible applications.

HEMODIALYSIS

Dialysis has the largest market segment of all the membrane processes. The biggest

market application of dialysis is in artificial kidneys for hemodialysis to treat patients

suffering renal failure/ kidney failure, to remove biotoxic metabolites from the blood

stream. Hemodialysis is governed by the molecular mass (size) of the uremic toxins

to be eliminated relative to the mass of serum proteins. The process is described as

follows: “Blood is drawn from the patient and passed through the lumen of the hollow

fibres, while water is passed through the shell side of the unit. Since water is loaded

with salts, it has the same osmotic pressure as blood cells as there would be in a

pressure-driven process. Urea diffuses from the blood into the water, and is removed

quite selectively but is mixed again with salts. The urea loaded salt solution is disposed

of as waste. Since the process is very gentle, there is less damage to blood cells than

there would be in a pressure-driven process. The human kidney processes about 1000

l of aqueous solution every week. A typical patient suffering from chronic kidney failure

requires 150 treatments per year.

Hemodialysis units are usually hollow-fibre devices with a membrane area of 0.5-

1.5 m2. The classical membrane material is regenerated cellulose, closest to the

natural material. Other membranes include polyether sulphone (PES) and

polysulphone (PS), which are made somewhat hydrophilic by blending with PVP, a

necessary requirement to address the problems of biocompatibility and fouling by

proteins. The membranes are asymmetric with a narrow pore size distribution and the

pore diameter less than 10 nm.

DIFFUSION DIALYSIS

Diffusion dialysis (DD) is an ion-exchange membrane separation process driven by a

concentration gradient across the membrane, i.e., ion transport is driven by the

concentration gradient with Donnan criteria of co-ion rejection and preservation of

electrical neutrality. Since it is a spontaneous process, DD results in an increase in

entropy and a decrease in Gibbs free energy; hence it is thermodynamically

favourable.

A DD unit consists of a multi-cell stack containing anion-exchange (AEX) membranes

or cation-exchange (CEX) membranes. The membrane thickness is 100– 500

micrometer. Typically, the membranes are polystyrene cross-linked with divinyl

benzene containing anion or cation-exchange groups. For example, in the case of acid

recovery, protons and anions penetrate the AEX membranes while the salt cations are

rejected. The net result is the more than 90% removal of acids from a mixture of salts.

The energy consumption in DD is very low as compared to other membrane

processes. It is a simple, economical, and energy efficient process wherein the feed

and permeate are pumped counter-currently as in most dialysis types of processes.

No external driving force such as pressure difference or electrical potential difference

is required. The only power required is by the metering pumps on the stack outlet side.

Since DD relies on concentration difference and there is no external force driving the

separation, the process is very slow, systems have low capacities and require large

membrane areas for separation. However, this is not a drawback for specific

applications; a 6-stack DD system with a capacity of 6 m3/day is sufficient for

regenerating a 24 m3/day deionised water ion-exchange system.

The process is used extensively in the metals industry for recovering acids and metals.

Hydrofluoric acid and nitric acid are used as etching agents for stainless steel. In order

to recover the acid, DD is used since the protons pass through the membrane but Fe3+

ions cannot. Other applications include recovery of acids from ion-exchange

regeneration processes and in metal refining. It is also applied for the separation and

recovery of acid/alkali waste solutions and purifying/separating alkalis. It is used to

remove electrolytes from colloidal suspensions to render the latter more stable, e.g.,

it is used to recover NaOH from certain industrial wastes that are contaminated with

organic substances. The Na+ and OH- ions pass through the permeable membrane

cell walls into the surrounding water, which is next evaporated to recover sodium

hydroxide while the organic waste in the cells is disposed of.

ELECTRODIALYSIS

Electrodialysis (ED) is an electrochemical process used to separate charged particles

from an aqueous solution or from other neutral solutes. A stack of membranes is used,

half of them passing positively charged particles and rejecting negatively charged

ones; the other half doing the opposite. An electrical potential is imposed across the

membranes, and a solution with charged particles is pumped through the system.

Positively charged particles migrate toward the negative electrode, but are stopped by

a positive-particle-rejecting membrane. Negatively charged particles migrate in the

opposite direction with similar results. Both types migrate in opposite directions out of

one set of cells and collect in the remaining cells. The result is a concentrated solution

of both positively and negatively charged particles in every other cell and a low

concentration (the product) in the remaining cells

The major applications for ED are in concentration of electrolyte solutions or in the

diluting or de-ionising of solutions. The latter application has over the years been the

dominant application in the desalination of brackish water. Electrodialysis is also used

extensively for desalting and concentrating seawater in salt production. In principle the

technique has many potential applications in the removal or recovery of ionic species.