Environmental Engineering Reference
In-Depth Information
In recent years, UV radiation has been initialized to produce cost-effective cheap and
clean water. Commercially produced drinking water based on solar water disinfection
(“SODIS”) is produced by the Swiss at EAWAG (Swiss Federal Institute for Aquatic Science
and Technology; www.sodis.ch). This water is made by exposing the contaminated water
to solar radiation, using solar energy to destroy pathogenic microorganisms. The principle
is based on the bimodal effect of UVA light (λ = 320-400 nm) and heat, i.e., the synergistic
effect. Polyethylene terephthalate (PET) water bottles are exposed to sunlight to obtain a
temperature of 55°C. The bottles are placed on corrugated iron sheets or aluminum sheets.
The water bottles used are either transparent PET bottles or half-blackened plastic bottles.
Under normal conditions, the disinfection eficiency is about 99.9%. The dosage contains
energy of 555 Wh/m 2 in UV-A in the range of 350-450 nm, corresponding to 6 h of expo-
sure in typical temperate latitude sunshine [14].
Late entrants to the water disinfection technologies are glass beads and crumb rubber
technologies [15-17]. Glass beads are manufactured from 100% virgin glass. They con-
tain 66-73% (wt%) SiO 2 , 13-14.5 wt% Na 2 O, 8-9.2 wt% CaO, 3.2 wt% (max) Al 2 O 3 , and 4.2
wt% (max) MgO. They provide very high effective pore space owing to their spherical
shape. A very smooth surface leads to deceleration of lead and manganese contamination.
A large pore space allows optimal regeneration. They are chemically inert and need less
water (20%) for backwashing. Photospheres are TiO 2 -coated hollow spheres. They are low-
density composite buoyant material. The buoyancy of glass beads allows photocatalytic
application in open water systems, and it is a less expensive photocatalytic technology
than construction of photocatalytic reactors. The principle of photocatalysis on which they
work has been described above. The buoyancy of spheres allows reuse without a signii-
cant material loss. Titanium can be coated by sol-gel techniques.
The choice of granular media is very important. Studies on clogging phenomena were
performed on zeolite, river sand, glass beads, and scoria. The overall treatment eficiency
shown by glass beads was 0.93 compared with 0.68 and 0.01 for scoria (volcanic rock) and
river sand, respectively. The treatment performance was measured as the difference in total
suspended solid concentration (TSSC) between the inlow and outlow ilters and equals
Inflow (TSSC)Outflow (TSS
C)
×
100
Treatmentefficiency
=
Inflow (TSSC)
It was suggested that the zeolite ilters with unrestricted low had an iniltration rate
of 62,000 mm/h compared with ilters with restrictive low, which had iniltration rate of
16,500 mm/year. The low low rate caused increased contact time within the ilter media
[15]. The effect of porosity of grains, reduction in size, and packing of grains on iniltration
rate is not conclusively understood [16].
Crumb rubber has shown promising results in the iltration of water. Crumb rubber is a
term usually applied to recycled rubber from automotive and truck scrap tires. It is in the
form of small broken pieces or powder in different meshes. Microscopic studies have shown
that the micro hairs that protrude from the surface attract the solids present in the inluent.
Junk rubber tires provide a breeding ground for vectors of diseases like dengue. The use of
recycled crumb rubber is a green technology for water treatment. It has provided in-depth
iltration and provides longer iltration time and high iltration rate [17]. Because of the elas-
ticity and compression caused by the weight of crumb rubber and a high hydraulic head
loss in the media, it has the smallest pore size at the bottom and the largest at the top. This
provides a higher iltration rate because of the favorable porosity gradient. It provides a new
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