114
5.2.2.2 Sewage Farming
Sewage farms use sewage for farming for crop growth. Even though farming is
practiced without any treatment, it is advisable to treat sewage to some extent to
reduce pathogens, odour and avoid vector nuisance. In the absence of pre-treatment,
the crops may get contaminated with pathogens and farm may become breeding
ground of vectors and odour, besides contaminating ground water.
5.2.2.3 Screening
Screening (Fig. 5.5) is the first unit operation used in WWTPs for the removal of
course objects like leaves, cigarette butts, rags, paper, plastics, etc., to prevent clogging/damage downstream pining, equipment and appurtenances.
5.2.2.4 Grit Removal
Grit is conventionally defined as particles greater than 0.21 mm with a specific gravity more than 2.65. Grit removal is achieved by grit chamber prior to sedimentation
tank. Figure 5.6 shows the view of empty grit chamber.
Mathematically
HRT = V Q
/
Where
V = volume of treatment unit/operation
Q = volumetric flow rate
All treatment units/operations will have optimum HRT. Higher HRT will
not guarantee better performance due to various reasons linked to the concerned treatment unit/operation.
In theory, each water molecule of substance entering the treatment unit/
operation will stay in the unit/operation for a period of HRT. But in practice,
the treatment units/operation will have some dead zones (where molecules
will be relatively stagnant and stay for a period more than HRT) and short
circuits (where molecules will pass out of the reactor comparatively earlier
than HRT).
A good design, construction, erection, commissioning and operation
should avoid dead zones and short cuts.
Box 5.1 (continued)
5 Water and Wastewater Treatment
5.2.2.2 Sewage Farming
Sewage farms use sewage for farming for crop growth. Even though farming is
practiced without any treatment, it is advisable to treat sewage to some extent to
reduce pathogens, odour and avoid vector nuisance. In the absence of pre-treatment,
the crops may get contaminated with pathogens and farm may become breeding
ground of vectors and odour, besides contaminating ground water.
5.2.2.3 Screening
Screening (Fig. 5.5) is the first unit operation used in WWTPs for the removal of
course objects like leaves, cigarette butts, rags, paper, plastics, etc., to prevent clogging/damage downstream pining, equipment and appurtenances.
5.2.2.4 Grit Removal
Grit is conventionally defined as particles greater than 0.21 mm with a specific gravity more than 2.65. Grit removal is achieved by grit chamber prior to sedimentation
tank. Figure 5.6 shows the view of empty grit chamber.
Mathematically
HRT = V Q
/
Where
V = volume of treatment unit/operation
Q = volumetric flow rate
All treatment units/operations will have optimum HRT. Higher HRT will
not guarantee better performance due to various reasons linked to the concerned treatment unit/operation.
In theory, each water molecule of substance entering the treatment unit/
operation will stay in the unit/operation for a period of HRT. But in practice,
the treatment units/operation will have some dead zones (where molecules
will be relatively stagnant and stay for a period more than HRT) and short
circuits (where molecules will pass out of the reactor comparatively earlier
than HRT).
A good design, construction, erection, commissioning and operation
should avoid dead zones and short cuts.
Box 5.1 (continued)
5 Water and Wastewater Treatment