Summary Nanofiltration and Reverse Osmosis - Drinking Water Treatment 1, TU Delft
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Module
Water Treatment
Institution
Water Treatment
Drinking Water Treatment 1
The course gives the technological backgrounds of treatment processes applied for production of drinking water. The treatment processes are demonstrated with laboratory experiments. Study goals: Knowledge of technological basics and design parameters of drinking water tr...
,nanofiltration and reverse osmosis water treatment
Framework
This module examines nanofiltration and reverse osmosis.
Contents
This module has the following contents:
1. Introduction
2. Principle
2.1 (Reverse) osmosis
2.2 Fouling of membranes
2.3 Membrane configuration
2.4 Feed, permeate and concentrate
2.5 Cross-flow operation
3. Theory
3.1 Mass balance
3.2 Kinetics
3.3 Concentration polarization
4. Practice
4.1 Nanofiltration
4.1 Christmas tree configuration
4.2 Cleaning
4.3 Field installations
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, water treatment nanofiltration and reverse osmosis
1 Introduction into the water, a flow through the membrane from
the pure water to the water containing salts will
Reverse osmosis is one of the membrane filtration occur (Figure 1, left and middle). Nature tries to
processes. The process is used to remove salts equalize concentration differences.
and organic micropollutants from water.
Because reverse osmosis is able to remove very When pressure is applied on the side where the
small particles from water, fouling of the membrane salts are added, a new equilibrium will develope.
can easily occur. Reverse osmosis is therefore al- The extra pressure will result in a flow of water
ways preceded by a pre-treatment step to remove through the membrane, but the salts do not flow
particulate matter. This pre-treatment can be a through.
conventional pre-treatment (coagulation, floccula- This phenomenon is called reverse osmosis (Fig-
tion, sedimentation, filtration) or an ultrafiltration ure 1, right).
pre-treatment.
The driving force for reverse osmosis is the applied
In reverse osmosis almost all dissolved particles pressure minus the osmotic pressure.
present in water will be retained, so the produced The energy consumption of reverse osmosis is
flow (permeate) has a low mineral content. There- directly related to the salts concentration, since
fore, the permeate is sometimes conditioned (lime- a higher salt concentration has a higher osmotic
stone filtration or aeration) to correct the pH and pressure.
the aggressiveness of the permeate.
In nanofiltration almost all divalent ions are
retained; the monovalent ions are only partly 2.2 Fouling of membrane�
retained. The fouling of a reverse osmosis membrane is
almost inevitable.
Particulate matter will be retained and is an ideal
2 Principle nutrient for biomass, resulting in biofouling.
Another important fouling process is scaling, the
2.1 (Reverse) osmosis formation of salt precipitates.
Osmosis is a natural process of flow through a
semi-permeable membrane. When pure water of Both fouling processes (scaling and biofouling)
the same temperature is present on both sides of a should be avoided as much as possible to ef-
membrane and the pressure on both sides is also ficiently operate reverse osmosis.
equal, no water will flow through the membrane.
However, when the salt on one side is dissolved
osmotic reverse
pressure osmotic
$0 pressure
pure salt pure salt pure salt
water solution water solution water solution
Figure 1 - Principle of osmosis and reverse osmosis
191
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