### Sewerage system design

```Sewerage system design
• Calculation waste water flow rate based on
population.
• Calculate BOD5 to indicates strength and
weakness of wastewater.
• Design criteria of sewer system based on UK
standard.
• Waste water treatment process
Types of sewer
Foul Sewerage(Sanitary)
• The main flow in a foul sewerage system is generally the
polluted water-borne waste from domestic buildings,
• Consider the connection from the house to the sewer, which is
known as a house connection or drain.
Drains
• To determine wastewater flows in building drainage.
• Following the guidance in BS EN 752 Drain and sewer systems
outside buildings, and BS EN 12056-2 Gravity drainage systems
inside buildings - Part 2: Sanitary pipework, layout and
calculation.
•For intermittent use a factor of 0.5 may be applied, so in a
total of N dwellings, the flow is then given by
Q [litres/second]
=
0.5 (3N)
•subject to a minimum design flow of 1.6 litres/second, and
with the depth of flow d  0.7 D to allow ventilation.
•Self cleansing conditions are usually ensured by simple
gradient rules, such as a 100 mm pipe at a slope of 1 in 40.
REFERENCES
• Building Regulations Approved Document H (reprinted 2010,
which follows a similar approach to that described above.
• Manufacturers’ organisations also publish useful information,
such as the Clay Pipe Development Association (CPDA)
booklet entitled The specification, design and construction of
drainage and sewerage systems using vitrified clay pipes.
Foul Sewers
•For sewerage systems, wastewater flows are usually
determined on a population basis, as described for the
UK in the following sections.
• Traditionally in UK, the average foul flow in a sewer is known as
the dry weather flow , DWF, and may be expressed
DWF = P.G + I + E
P = population
G = wastewater flow per person
I = infiltration
E = industrial effluent/sewage
*The average daily domestic flow per person (G) in UK is given as
ranging from 150 to 300 litres/day (BS EN 752:2008 p.97, Table
E.4).
Criteria for Population Equivalent (PE) Calculation (MS-1228)
Foul sewer design flows by the population method
•Foul sewers in UK are designed for up to 6 times of DWF (BS EN
752:2008, p.98, Table E.5). This is dependent on catchment size, and
For new housing areas in UK, the following is usually assumed:
3 people per dwelling
200 litres per person per day ( G )
10 % infiltration ( i.e. I = 10 % of P.G, so multiply P.G by 110/100 )
6 times peaking factor
So the peak flow per dwelling := 6xDWF =6( P.G + I + E)
= 6 [3 x 200 x 110/100]  4000 litres/day = 0.046 l/s
This is referred to as the design flow.
A lower peak of two times the average (2 DWF) is assumed to occur at least once daily
(see BS EN 752:2008 p.150, NA.11.2.3.1), and is used for minimum velocity considerations,
as described in the following section.
Self cleansing velocities for foul sewers
• BS EN 752 (2008) recommends a velocity of at least 0.7 m/s for pipes up
to 300 mm diameter (p.61, 9.6.3.1), but higher velocities may be needed
in larger pipes (p.150, NA.11.2.3.1). A flow of two times the average
domestic flow is used for this calculation.
• Sewers for Adoption (SfA) requires a velocity of 0.75 m/s at one-third of
the design flow. (One third of 6DWF equals 2 DWF, which is the flow at
which velocity should be checked).
• Where this velocity requirement cannot be met, then the self cleansing
criterion would be considered to be satisfied if one of the following
• a 150 mm nominal internal diameter gravity sewer having a gradient not
flatter than 1 in 150 where there are at least ten dwelling units
connected.
• a sewer or lateral drain with a nominal internal diameter of 100mm, or a
lateral drain serving ten or less properties, laid to a gradient not flatter
than 1:80 where there is at least one WC connected, or 1:40 if there is no
WC connected.
Pipe roughness
• The typical roughness value used for foul gravity sewer design is ks = 1.5
mm, for any pipe material.
• In time, pipes slime up and give a similar roughness, regardless of their
material.
From experimental work on clay and pvc pipes reproduced in CPDA (2001)
Other practical design criteria for foul sewerage
• The minimum pipe diameter for foul sewers in the UK was
traditionally 150 mm, but now 100 mm is allowed for ten properties
or less. 100 mm is the traditional size for individual house
connections.
• Manholes are required at changes in size, slope or direction;
• The maximum spacing is 100m, although this may be increased for
man entry sizes (greater than 900 mm diameter), but is often less
than this to accommodate branches and bends.
• At manholes where pipe sizes change it is usual to align the levels of
the pipe soffits(highest point of internal surface).
•



Slopes to be considered in design are:
the natural ground slope,
the slope to achieve or maintain minimum cover.
• A useful approximate rule is that a D (mm) diameter sewer laid at a
slope of 1 in D will give a velocity approximately equal to the self
cleansing value. This gradient rule is mentioned in BS EN 752:2008
p.61, 9.6.3.1.
e.g. 225 mm sewer with roughness k = 1.5 mm laid at 1 in 225 gives a
pipe full flow of 30 litres/s and pipe full velocity of 0.75 m/s.
• A pumping station may be needed to lift foul flows from a new
development into the existing sewer system, but is best avoided if
possible.
Waste water treatment
Waste water treatment
•Processes that combine physical, chemical and
biological methods.
•Wastewater treatment plants are usually classified
as providing primary, secondary or tertiary (or
advanced) treatment, depending on the purification
level to which they treat.
*BOD5 = amount of organic pollution (that can be oxidized biologically) in
a sample of water.
CHEMICAL CHARACTERISTIC
•Alkalinity: an indicator of wastewater’s capacity to neutralize
acids, alkalinity is measured in terms of bicarbonate, carbonate
and hydroxide alkalinity. Alkalinity is essential to hold the neutral
pH of the wastewater during biological treatment.
•Biochemical oxygen demand (BOD): an indicator of the amount
of biodegradable matter in the wastewater, normally BOD is
measured in a 5-day test conducted at 20°C (BOD5) and normally
ranges from 100 to 300mg/L
•Chemical oxygen demand (COD): an indication of the amount of
oxidizable matter present in the sample, the COD is normally in
the range of 200 to 500 mg/L. Industrial wastes present in the
wastewater can significantly increase this.
•Dissolved gasses: the specific gases and normal concentration
dissolved in wastewater are based on wastewater composition, and
under septic conditions may typically include oxygen in relatively
low concentration, carbon dioxide, and hydrogen sulfide.
•Nitrogen compounds: nitrogen’s type and amount vary from raw
wastewater to treated effluent, but nitrogen oxygen is mostly
found in untreated wastewater in the forms of organic nitrogen and
ammonia nitrogen (presence and levels determined by laboratory
testing).
•Total Kjeldahl nitrogen (TKN): the sum of these forms of nitrogen.
Normal wastewater contains 8 to 35 mg/L and ammonia nitrogen
ranging from 12 to 50 mg/L.
•pH: pH express wastewater’s acid condition. For proper treatment,
wastewater pH should generally range from 6.5 to 9.0.
•Phosphorus:
•in secondary treatment process, phosphorus must be present in at
least minimum quantities or the process won’t process.
•However, excessive phosphorus causes stream damage and
excessive algal growth.
•Phosphorus normally range 6 to 20 mg/L.
•Solids:
•most wastewater pollutants can be classified as solids, and
wastewater treatment is generally designed to either remove solids,
or convert them to more stable or removable forms.
• General practice classifies solids as setteleable, floatable, or
collodial.
•Total solids concentration in wastewater normally ranges from 350
to 1,200 mg/L.
Diagram of sewage treatment process cycle
•Remove 90-95% settleable solids 4060% total suspended solids, and 25 to
35% BOD
•Removes gravel, sand, silt and other
gritty materials
•Reduces odors, neutralizes acids,
reduces corrosion,reduces BOD5,
improves solids and grease removal,
subsequent processes.
Produces effluent/sewage with not
more than 20 mg/L BOD5 and 30
mg/L suspended solids
Removes pollutants, including
nitrogen, phosphorus, soluble COD
and heavy metals to meet discharges
or reuse criteria with respect to
specific parameters
Example of calculating average daily wastewater flow rate
Question: Table shows the information of daily wastewater flow rate and
BOD5 concentration for an urban area. Determine the average daily
wastewater flow rate and BOD5 concentration.
Utility
Domestic
Hospital
Restaurant
School with
cafe
School without
cafe
BOD5
(kg/day)
per capital
per capital
per capital
Average
Flowrate
(l/day)
625
600
30
1500 students each
per student
280
0.1
5400 student each
per student
60
0.03
Population Equivalent
(PE)
Unit
30 000
650 bed
250 customers per day
0.1
0.15
0.03
Solution
i. Calculate the average wastewater flow rate
Utility
PE x average daily flowrate
Domestic
Hospital
Restaurant
School with cafe
School without
cafe
Total wastewater
flow
30 000 x 625
650 x 600
250 x 30
1500 x 280
18,750,000
390,000
7500
420,000
5400 x 60
324,000
19,891,500
(litre/day)
ii. Calculate the average BOD5 concentration
Utility
Population
Equivalent
(PE)
BOD5
(kg/day)
PE x BOD5
Domestic
30 000
0.1
30000x0.1
3000
Hospital
650 bed
0.15
650x0.15
97.5
0.03
250x0.03
7.5
0.1
1500x0.1
150
0.03
5400x0.03
162
Restaurant
School with cafe
School without
cafe
250 customers
per day
1500 students
each
5400 student
each
3417
kg/day
Total BOD5
Average BOD5 concentration
=Total BOD5 /Total wastewater flow rate
Convert unit to
mg/L
3417
19,891,500
=0.000172
Kg/day
Liter/day
```