The document discusses wastewater treatment processes. It explains that preliminary treatment removes solids but has little effect on pathogens. Primary treatment uses sedimentation to remove suspended solids, reducing downstream pathogen loads, but does not remove most pathogens. Secondary treatment uses physical separation and biological processes to remove dissolved and suspended organic compounds. It differentiates between equalization, which controls flow rates, and neutralization, which is a chemical reaction between acids and bases. Finally, it discusses design criteria for screens, which remove debris, and grit chambers, which remove grit and have specific dimensions for weirs and flow areas.

1. Environmental Science (Assignment III)
---ARGHYADEEP SAHA
BSc (Hons.) Geography (SEM-II)
Roll No: - UG/05/BSGEO/2019/017
1
11-06-2020 16:21:05
1. Explain Preliminary, Primary and Secondary Treatment.
Answer: - Preliminary treatment is used to remove screenings and grit that enters a wastewater
treatment plant from a sewered system. Preliminary treatment will have little effect on pathogens
in the liquid waste stream. Primary treatment (also called primary sedimentation) is a sanitation
technology that removes suspended solids and floating organic material (called scum) to reduce
the suspended solids load for subsequent treatment processes. The removal of pathogens during
primary treatment is not high; therefore, downstream treatment will require further pathogen
removal technologies to meet discharge or reuse guidelines. It is not expected that discrete
pathogens and indicator organisms are removed by settling during conventional primary
treatment
Primary treatment removes material that will either float or readily settle out by gravity. It
includes the physical processes of screening, comminution, grit removal, and sedimentation.
Screens are made of long, closely spaced, narrow metal bars. They block floating debris such as
wood, rags, and other bulky objects that could clog pipes or pumps. In modern plants the screens
are cleaned mechanically, and the material is promptly disposed of by burial on the plant
grounds. A comminutor may be used to grind and shred debris that passes through the screens.
The shredded material is removed later by sedimentation or flotation processes.
Secondary treatment is treatment processes for wastewater (or sewage) to achieve a certain
degree of effluent quality by using a sewage treatment plant with physical phase separation to
remove settle able solids and a biological process to remove dissolved and suspended organic
compounds.

2. Environmental Science (Assignment III)
---ARGHYADEEP SAHA
BSc (Hons.) Geography (SEM-II)
Roll No: - UG/05/BSGEO/2019/017
2
11-06-2020 16:21:05
2. Differentiate between Equalization and Neutralization citing suitable examples for each.
Answer: -
Equalization Neutralization
 Flow equalization is the process of
controlling hydraulic velocity, or flow
rate, through a wastewater treatment
system. The equalization of flow
prevents short term, high volumes of
incoming flow, called surges, from
forcing solids and organic material out of
the treatment process.
 Chemical reaction, according to the
Arrhenius theory of acids and bases, in
which a water solution of acid is mixed
with a water solution of base to form a
salt and water; this reaction is complete
only if the resulting solution has neither
acidic nor basic properties. Such a
solution is called a neutral solution.
Complete neutralization can take place
when a strong acid, such as hydrochloric
acid, HCl, is mixed with a strong base,
such as sodium hydroxide, NaOH. Strong
acids and strong bases completely break
up, or dissociate, into their constituent
ions when they dissolve in water.
 Storage is considered a necessary control
alternative for wet weather flows because
of the high volume and variability
associated with stormwater and
combined sewer overflows. Some of the
most critical problems currently facing
wastewater management agencies are the
control of Infiltration/Inflow (I/I)-
induced sanitary sewer overflows (SSOs)
and treatment facilities, including
hydraulic overloading and disruptions of
biological and other plant processes.
 In the case of hydrochloric acid,
hydrogen ions, H+, and chloride ions, Cl-
, are formed. In the case of sodium
hydroxide, sodium ions, Na+, and
hydroxide ions, OH-, are formed. The
hydrogen and hydroxide ions readily
unite to form water. If the number of
hydrogen ions in the hydrochloric acid
solution is equal to the number of
hydroxide ions in the sodium hydroxide
solution, complete neutralization occurs
when the two solutions are mixed. The
resulting solution contains sodium ions
and chloride ions that unite when the
water evaporates to form sodium
chloride, common table salt.

3. Environmental Science (Assignment III)
---ARGHYADEEP SAHA
BSc (Hons.) Geography (SEM-II)
Roll No: - UG/05/BSGEO/2019/017
3
11-06-2020 16:21:05
3. Discuss the design criteria for screens and grit chambers.
Answer : - The submerged area of the surface of the screen, including bars and opening should be
about 200% of the cross sectional area of the incoming sewer for separate system, and 300% for
the combined system. the spacing between the bars could be between 6 mm and 20 mm. from 25
mm to 75 mm.
The dimension of the weir is usually assumed between 25 mm and 50 mm, with a suggested value
of 35 mm. The proportional flow weir should be set 100 to 300 mm above the bottom of grit
chamber to provide for grit storage or for operation of mechanical devices for cleaning of grit
chamber.