Another objective was to determine the flow pattern of water inside Arsenic Biosand Filter.
Darcy’s law governs the flow rate of the filter. That is the filter flow rate is proportional to the
water level above the outlet pipe. The higher the water level, the higher the hydraulic head, which
leads to higher Darcy’s flux through the sand, which in turns means higher flow rate (Ngai.T,
2003.)
The imaginary line is drawn according to our hypothesis (Figure..). It is assumed that if the
volume of water in the basin is 100%, then the flow rate is maximum (100%) and if there is no
any water left in the basin (0%), the flow rate is also 0%
100%
80%
60%
Flow rate (%)
Imaginary Line
40%
20%
0%
1 2 3 4 5 6 7 8 9
Amount of water in the basin (%)
Figure 2: Imaginary line for the flow curves.
19
, CHAPTER 2 LITERATURE RIVIEW
2.1 Chemistry of Arsenic
Arsenic is P-Block, group IV element of the periodic table. It has an atomic number 33 and
atomic mass 74.91 with the five electrons in outer most shell. The oxidation state of Arsenic
compounds found in the environment is either III or V. The two-electron reduction of arsenate As
(V) to arsenite As (III) is favored in acidic solution, where as the reverse is true in basic solution.
Arsenic can exist in four valency states -3, 0, +3 and +5. Element arsenic is not soluble in water,
under moderately conditions, arsenite (+3) may be the dominant form, but arsenate (+5) is
generally the stable oxidation state in oxygenated environment.
Arsenic is stable in dry air, but tarnishes in moist air, giving first a bronze then black tarnish.
When heated in air it sublimes at 615?C and forms AS4O6 not AS4O10 but depending upon the
oxygen presents (Lee, 1994)
Table 2: Properties of arsenic
12
Atomic Weight ( C= 12.0000) 74.9216
Mp at 39.1 Mpa (38.6 atm), ?C 816
Bp, ?C 615, sublimes
Density at 26?C, Kg/m3 5778
Covalent radius 1.21?A
Ionization energy (Kg/mol) 947 (1st) 1950 (2nd) 2732 (3rd)
Latent heat of fusion, J/ (mol K) 2 27,740
Latent heat of sublimation, 31,974
Specific heat at 25?C, µm/(m ?C) 5.6
Electrical resistivity at 0?C, µ? cm 26
Magnetic susceptibility at 20?C, cgs -5.5*10-6
Bond type Covalent
Crystal system Hexagonal (rhombohedral)
Pauling’s electronegativity 2.0
Hardness, Mohr’s scale 3.5
(Source: Othmer, 2002; Lee, 1994; EHC224, 2002)
20
Darcy’s law governs the flow rate of the filter. That is the filter flow rate is proportional to the
water level above the outlet pipe. The higher the water level, the higher the hydraulic head, which
leads to higher Darcy’s flux through the sand, which in turns means higher flow rate (Ngai.T,
2003.)
The imaginary line is drawn according to our hypothesis (Figure..). It is assumed that if the
volume of water in the basin is 100%, then the flow rate is maximum (100%) and if there is no
any water left in the basin (0%), the flow rate is also 0%
100%
80%
60%
Flow rate (%)
Imaginary Line
40%
20%
0%
1 2 3 4 5 6 7 8 9
Amount of water in the basin (%)
Figure 2: Imaginary line for the flow curves.
19
, CHAPTER 2 LITERATURE RIVIEW
2.1 Chemistry of Arsenic
Arsenic is P-Block, group IV element of the periodic table. It has an atomic number 33 and
atomic mass 74.91 with the five electrons in outer most shell. The oxidation state of Arsenic
compounds found in the environment is either III or V. The two-electron reduction of arsenate As
(V) to arsenite As (III) is favored in acidic solution, where as the reverse is true in basic solution.
Arsenic can exist in four valency states -3, 0, +3 and +5. Element arsenic is not soluble in water,
under moderately conditions, arsenite (+3) may be the dominant form, but arsenate (+5) is
generally the stable oxidation state in oxygenated environment.
Arsenic is stable in dry air, but tarnishes in moist air, giving first a bronze then black tarnish.
When heated in air it sublimes at 615?C and forms AS4O6 not AS4O10 but depending upon the
oxygen presents (Lee, 1994)
Table 2: Properties of arsenic
12
Atomic Weight ( C= 12.0000) 74.9216
Mp at 39.1 Mpa (38.6 atm), ?C 816
Bp, ?C 615, sublimes
Density at 26?C, Kg/m3 5778
Covalent radius 1.21?A
Ionization energy (Kg/mol) 947 (1st) 1950 (2nd) 2732 (3rd)
Latent heat of fusion, J/ (mol K) 2 27,740
Latent heat of sublimation, 31,974
Specific heat at 25?C, µm/(m ?C) 5.6
Electrical resistivity at 0?C, µ? cm 26
Magnetic susceptibility at 20?C, cgs -5.5*10-6
Bond type Covalent
Crystal system Hexagonal (rhombohedral)
Pauling’s electronegativity 2.0
Hardness, Mohr’s scale 3.5
(Source: Othmer, 2002; Lee, 1994; EHC224, 2002)
20
