Summary Granular Filtration - 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...
1 Introduction at the outlet pipe (outlet-controlled).
Due to clogging, maximum resistance is reached,
In general, filtration is a process where water flows and the filter bed must be cleaned by backwashing.
through a permeable layer, either a membrane, During backwashing the filter bed is expanded, and
filter paper, a sieve, a porous medium or such. the accumulated suspended solids are removed.
In water treatment, granular filtration is a process The backwash water is drained through a central
where water flows through granular material (often trough to a waste receptacle. The backwash fre-
sand) while suspended solids (sand, clay, iron quency is repeated every few days.
and aluminum flocs) are retained, substances
are biochemically decomposed and pathogenic Rapid sand filters are present in nearly every water
microorganisms (bacteria, viruses, protozoa) are treatment plant.
removed. Surface water treatment uses the filters after floc
formation and removal to get rid of the remaining
The suspended solids slowly fill the pores, result- flocs and pathogens and to decompose ammo-
ing in an increase in hydraulic resistance. nium.
The suspended solids are removed by periodically In groundwater treatment, the filters are usually
cleaning the filter beds. This prevents the resis- placed after aeration to remove iron flocs, man-
tance from becoming too high or the break through ganese and ammonium. With softening, filters
of suspended solids. are often placed after pellet reactors to remove
Filters are also used for chemical and biological the ‘carry-over’.
reactions. This is mainly of importance for the
treatment of groundwater where the oxidation of
iron, manganese, ammonium and, in case of poor 2 Principle
gas stripping, methane takes place.
The removal of pathogenic microorganisms is of 2.1 Filtration mechanisms
importance for surface water treatment, and the When water flows through the filter bed, sus-
efficiency is approximately 90 to 99%. The removal pended and colloidal particles are retained by the
of pathogenic microorganisms occurs by decay filter material.
and retention on the (sand) grains. Particles that are larger than the pores in the filter
bed will remain on the bed (Figure 2). With rapid
The most common application of filtration is rapid filtration this does not occur often, because the
sand filtration (Figure 1). larger particles (iron or aluminum flocs) are already
Rapid sand filtration consists of a bed with a coarse removed in the preceding floc removal process
granular medium (0.8-1.2 mm) and supernatant (sedimentation or flotation).
water. The filtration velocities (between 5 and 20 If smaller filter material is used, the pores are also
m/h) are controlled by varying the supernatant smaller and the screening process results in the
water level (inlet-controlled) or by operating a valve so-called cake filtration. The cake will also retain
0,5 m backwash gutter
1,5 m v = 10 m/h
0,3 m
1,0 m
10 m 5m
Figure 1 - Principle of rapid sand filtration(side and front view respectively)
83
, filtration water treatment
grain diameter = 400 μm
lighter particles to diffusion. Due to these mecha-
nisms, the particle can switch to other trajectories
that flow nearer to a grain or can collide directly
with a grain, and it remains at the surface or on
the grain.
Al or Fe floc 10 μm
Bacillus 2 μm When the suspended and colloidal particles col-
pore diameter 62 μm
lide with the filter grains, attachment could take
place.
There are two types of forces that result in attrac-
tion and repulsion of the particles.
The VanderWaals forces ensure that two bodies
Asterionella 30 μm are attracted.
clay particle 20 μm Electrostatic forces can have an attracting or
repulsing effect, depending on the charge of the
Figure 2 - Principle of screening particles.
In general the filter material (sand) and the sus-
small particles, and treatment occurs mainly in the pended and colloidal particles have a negative
top layer of the filter. charge and repulsion takes place.
The disadvantage of cake filtration is that with
high concentrations of suspended and colloidal Attachment of the particles depends on the mag-
particles rapid clogging of the filter bed occurs. nitude of both opposing forces. If the particles
are destabilized by the addition of trivalent iron
During rapid filtration the removal of suspended or aluminum salts, attachment will be easier than
and colloidal particles usually occurs inside the without destabilization.
filter bed.
The clogging will thus be spread over the entire In addition to physical processes to remove sus-
height of the filter bed. pended and colloidal solids as described above,
The suspended and colloidal particles are trans- chemical and biological processes occur in the
ported to the filter material in different ways (Figure filter bed.
3). From groundwater, iron(II) and manganese must
Generally, the particle follows the trajectory of the be removed by oxidation. By adding oxygen in the
water that flows through the filter bed. This trajec- preceding aeration step, iron(II) will be transformed
tory follows the complicated pore structure of the into iron(III) and iron flocs will be formed. The iron
bed. When the trajectory curves, a heavy particle flocs are removed by the same mechanisms as
can be transported to the filter material due to in- described for the removal of suspended and col-
ertia. If the trajectory approaches the filter grains, loidal particles.
then particles can also be intercepted. Heavy
particles are especially subject to sedimentation, Manganese is transformed to manganese oxide in
Figure 3 - Transport of impurities towards the grain
84
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