Sewerage - Nagar Nigam Raipur

Project Management Consultancy Services for Sewerage, Storm, Surface Water Drains and Lake
Protection Works For Raipur City
Detailed Design Report - Sewerage
Chapter 9: Design of Sewage Treatment Plant
9.2 Sewage Characteristics
Sewage comprises a mixture of various types of liquid wastes from residential; public and
industrial places. Sewage contains 99.9% of water and 0.1% are solids that pose threat as they
are offensive in nature, undergo changes by bio-degradation cause nuisance and pollution.
Understanding of the nature of physical, chemical and biological characteristics of sewage is
essential in planning, design and operation of treatment and disposal facilities and in the
engineering management of environmental quality. The typical characteristics of domestic
sewage as reported in various literature and recommended for use in design of treatment plants
is presented in Table 9.1.
The important sewage
characteristics of interest
during STP design is the
BOD5 value, which has a
direct bearing on the
sizing of various units.
Literature survey related
to various municipal STPs
under NRCD projects
reveal that the BOD5
considered in design is in
the range of 180 mg/l to
225 mg/l. Data published
by CPCB with respect of
influent sewage
characteristics for STPs in
Delhi reveals the same
trend (refer Annexure-3).
Sr.
No.
The CPHEEO manual recommends 90 grams solids and 45 grams BOD as per capita per day
contribution in sewage. If the CPHEEO standards are adopted, then for this project, the BOD5
concentration works out as 416 mg/l. However, due to dilution effect and other wastes joining
sewage, the BOD5 concentration rarely rises beyond 250 mg/l. The wastewater analysis of drains
carrying sewage was carried out in Raipur city by the Chhattisgarh Environment Conservation
Board (CGECB), Regional Office, Raipur. The analysis reveals composition of a typical wastewater
flowing in open drains, having less BOD and high COD, which means pollution is being carried in
to the receiving waters. The results are placed at Annexure-3. The sewage characteristics at this
stage, for detailed design of STPs are being adopted as per Table 9.2.
9.3 Sewage Treatment Methods
Table – 9.2: Sewage Characteristics Adopted For Design
No. Characteristics Value
1 pH 7.51
2 Appearance Blackish turbid
3 Odour Unpleasant
4 BOD 5 250 mg/l
5 COD 500 mg/l
6 Total solids 450 mg/l
Table – 9.1: Typical Domestic Sewage Characteristics
Characteristics Range
(gram/cap/day)
Typical
value
1 pH 7 – 10 7 – 10
2 Colour grey, light brown -
3 Odour Soapy, oily -
4 BOD 5 45 – 54 45
5 COD 1.60 – 1.90 BOD 5 1.60 BOD 5
6 Total solids 170 – 220 190
7 Suspended solids 70 – 145 100
8 Grease 10 – 30 10
9 Alkalinity 20 – 30 25
10 Chlorides 4 – 8 6
11 Total nitrogen 6 – 12 9
12 Total phosphorus 0.60 – 4.50 2.25
13 Micro-organisms (bacteria, 10
virus, coliforms, etc)
2 – 10 10 10 6
As mentioned above, sewage
treatment methods are broadly
classified into physical, chemical and
biological processes. The unit
operations included in each category is
given in Table 9.3 below.
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