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Potable water treatment
Potable water treatment

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4.9.3 Nanofiltration

Nano is a prefix that means 10−9, i.e. very, very small. You may have come across nanotechnology. Nanofiltration is similar to reverse osmosis and employs membranes that are capable of physical sieving and diffusion-controlled transport. Nanofiltration systems operate at much lower pressures than reverse osmosis systems, but yield higher flow rates of permeate. The quality of the permeate is not as good as with reverse osmosis, with particles in the size range 0.0005–0.005 μm being removed. It is used for the removal of colour, humic acids, and trace organics such as pesticide residues, from drinking water.

Self-assessment question

Which of the following is true?

A There are three important zones in a sedimentation tank, namely, the inlet zone, the settling zone and the outlet zone.

B Flocculent particles in general will reach the base of a settlement tank ahead of discrete grit particles.

C For a surface loading of 30 m3 m−2 d−1, in a sedimentation tank with floor dimensions 14 m × 50 m, the maximum flow rate allowed is 2.4 m3 s−1.

D It is the positive charge carried by colloidal particles and micro-organisms that prevents them from aggregating and settling.

E An ion exchange resin to remove nitrate from water can be represented as NaR where R is the complex resin base.

F Free available chlorine is a disinfection agent and is present in water as HOCl, hypochlorous acid.

G Both granular and powdered activated carbon can be regenerated.

Answer

A False. There are four zones. The missing zone is the sludge zone.

B False. Discrete particles settle ahead of flocculent particles.

C False. The maximum flow rate allowed is 0.24 m3 s−3. This is because

Therefore

D False. The colloids and the micro-organisms carry a negative charge.

E False. The ion exchange resin will be represented by R*Cl where R* is the complex resin base. The equation for its action will be

F True.

G False. Only a granular activated carbon can be regenerated.