arsenic is quickly adsorbed onto the surface of the ferric hydroxide particles. These arsenic
loaded ferric hydroxide particles are trapped on top of the fine sand layer. Most of the arsenic is
already adsorbed on to the ferric hydroxide, and almost all ferric hydroxide is trapped on the top
of fine sand layer, as a result, arsenic is effectively removed from the water. Figure illustrates the
arsenic removal mechanism arsenic removal mechanism.
Figure 7 : Arsenic Removal mechanism. Source: Ngai T, 2004.
During the process of arsenic removal, iron dissolved in water is also removed through co-
precipitation and filtration process.
2.9.4 Pathogen Removal
The pathogen in the Arsenic Biosand Filter can be removed by two mechanisms: physical-
chemical and biological.
2.9.4.1 Physical-Chemical Mechanism
When water passes through the sand bed, many foreign particles are trapped on the top, as the
particles are too large to pass through the bed. A tightly packed bed of the sand grains can detain
particles about 5% of the grain diameter. For instance, sand with a diameter of 0.1 mm will catch
particles that are 5 µm or larger. This is extensively larger than many particles to be removed
from surface water such as cysts (1-20µm). Viruses are much less than 1 µm, so must be removed
by other means, such as biological mechanisms.
34
, 2.9.4.2 Biological Mechanisms
This is the unseen process occurring in a sand bed, which refers its effectiveness in purifying
water. When water is passed through the sand bed, the particles it contains-large and small-
collide with individual grains of sand. Once a particle encounters a grain of sand, it stays
attached. Water that requires filtration usually contains various kinds of organic matter, including
living organisms. These particles and organisms accumulate in the uppermost layers of a sand
bed, since this is where most of the collision takes place, and eventually develop into a dense
biological population, which is known as the biological layer of biofilm. The biofilm consists of
threadlike algae, and many other organisms including plankton, protozoa and bacteria. The
biofilm needs stability, continuous water environment, diffuser level and basic nutrients, such as
organic matter and oxygen. Proper oxygen supply is critical to the biofilm's growth and
efficiency. The ABF and BSF are designed in such a way that there is always about 5 cm of
resting water above the fine sand layer. The 5 cm resting water level is the optimum heights as at
this height the biology surviving in the biofilm receives maximum oxygen. Moreover, this resting
water level serves a constant aquatic environment necessary for the organisms present in the layer
to survive. The water should not be allowed to flow freely or directly on to the sand as it disturbs
the biofilm, which may results incapability of pathogen removal. To protect the biofilm from this
disturbance, water should be always poured through the diffuser basin.
When microbiologically contaminated water is poured into the filter, predatory organisms present
in the biofilm layer will consume the incoming pathogens. In addition, the biological population
in the biofilm produces substances toxic to intestinal bacteria. Many studies and experiments
reveal that this process can be a significant cause of bacterial removal in slow sand filters..
Figure 8. Pathogens Removal Mechanism.
35
loaded ferric hydroxide particles are trapped on top of the fine sand layer. Most of the arsenic is
already adsorbed on to the ferric hydroxide, and almost all ferric hydroxide is trapped on the top
of fine sand layer, as a result, arsenic is effectively removed from the water. Figure illustrates the
arsenic removal mechanism arsenic removal mechanism.
Figure 7 : Arsenic Removal mechanism. Source: Ngai T, 2004.
During the process of arsenic removal, iron dissolved in water is also removed through co-
precipitation and filtration process.
2.9.4 Pathogen Removal
The pathogen in the Arsenic Biosand Filter can be removed by two mechanisms: physical-
chemical and biological.
2.9.4.1 Physical-Chemical Mechanism
When water passes through the sand bed, many foreign particles are trapped on the top, as the
particles are too large to pass through the bed. A tightly packed bed of the sand grains can detain
particles about 5% of the grain diameter. For instance, sand with a diameter of 0.1 mm will catch
particles that are 5 µm or larger. This is extensively larger than many particles to be removed
from surface water such as cysts (1-20µm). Viruses are much less than 1 µm, so must be removed
by other means, such as biological mechanisms.
34
, 2.9.4.2 Biological Mechanisms
This is the unseen process occurring in a sand bed, which refers its effectiveness in purifying
water. When water is passed through the sand bed, the particles it contains-large and small-
collide with individual grains of sand. Once a particle encounters a grain of sand, it stays
attached. Water that requires filtration usually contains various kinds of organic matter, including
living organisms. These particles and organisms accumulate in the uppermost layers of a sand
bed, since this is where most of the collision takes place, and eventually develop into a dense
biological population, which is known as the biological layer of biofilm. The biofilm consists of
threadlike algae, and many other organisms including plankton, protozoa and bacteria. The
biofilm needs stability, continuous water environment, diffuser level and basic nutrients, such as
organic matter and oxygen. Proper oxygen supply is critical to the biofilm's growth and
efficiency. The ABF and BSF are designed in such a way that there is always about 5 cm of
resting water above the fine sand layer. The 5 cm resting water level is the optimum heights as at
this height the biology surviving in the biofilm receives maximum oxygen. Moreover, this resting
water level serves a constant aquatic environment necessary for the organisms present in the layer
to survive. The water should not be allowed to flow freely or directly on to the sand as it disturbs
the biofilm, which may results incapability of pathogen removal. To protect the biofilm from this
disturbance, water should be always poured through the diffuser basin.
When microbiologically contaminated water is poured into the filter, predatory organisms present
in the biofilm layer will consume the incoming pathogens. In addition, the biological population
in the biofilm produces substances toxic to intestinal bacteria. Many studies and experiments
reveal that this process can be a significant cause of bacterial removal in slow sand filters..
Figure 8. Pathogens Removal Mechanism.
35
