Formation of crystals on membrane surfaces

The ions start clustering and form proto-nuclei, then start ordering themselves which are regularly shaped. This is the reversible stage but the tendency of reversibility is less than the previous stage. The final stage is the irreversible growth of crystals. Formation of crystals on the membrane surface depends on the solubility products and the concentration factor.

Antiscalants

Antiscalants

•Are used for membrane filtration to clean the membrane surface.

•Effectively controls calcium sulfate, barium

sulfate, strontium sulfate and calcium fluoride.

•Effectively controls silica (preferably).

Good tolerance to aluminum, iron and manganese oxides.

fig 1. Sales on the membrane surface. (image source)

Most common antiscalants ,

Organophosphonate  

Polyaspartic acid 

Basic concepts of membrane filtration : Rejection and Fouling

Rejection: How much solutes are rejected by the membrane is defined as the solute rejection of a membrane.

Rejection coefficient of a specific salt (NaCl, MgCl, CaCO₃) can be calculated using below equation

Salt rejection coefficient R;

where, Cji0 and Cji respectively, are the salt concentrations on the feed and permeate sides of the membrane

Fouling: Fouling is the precipitation of unwanted materials on the membrane, during the membrane filtration. Fouling is categorized based on the material which is available in the feed waters; i.e.

Scaling (inorganic fouling)........compounds CaCO₃, MgCO₃

Organic fouling..........................humic substances

Colloidal fouling........................colloids such as silica substance

Bio fouling.................................the growth of bacteria on the membrane surface.

Effects of fouling;

Nano filtration membranes

Basic aspects of NF

Nano filtration (NF) membrane comes under the categorization of RO membrane. RO membrane can be categorized into three groups according to their performances; i.e. salt rejection, permeate flux/permeability and operation pressure. The categorization is as below,

1. Seawater and brackish water desalination membranes operated with 0.5–5 wt% salt solutions at pressures of 10–60 bar

2. Low-pressure nanofiltration membranes operated with 200–5000 ppm salt solutions at pressures of 5–10 bar.

3. Hyperfiltration membranes used to separate solutes from organic solvent solutions.

Further, they are recognized as loose reverse osmosis, low-pressure reverse osmosis, or more commonly, nanofiltration membranes.