Design of sewage collection system for a subdivision
Special instructions:
Information provided is generally adequate; if any assumptions are made,
these should be clearly stated giving your reasons.
Design a sewer collection system incorporating flow rates, pipe sizes and pipe slopes
for the proposed residential development shown in the attached drawing. In your
design, locate suitable sewer alignments, manhole positions and other sewer
appurtenances. Clearly point out in the layout diagram which properties are served by
which section of the sewer.
The local approving authority accepts sewer alignments that service adjoining
properties, i.e. a common line that drains two adjacent properties. Contours indicating
the reduced levels are illustrated in the attached Figure. You can derive intermediate
levels by interpolation as appropriate. The development consists of residential
allotments, a proposed primary school for 400 pupils, a child care facility for 50
children, a church and local park. There are 3.5 equivalent person (EP) for each
residential allotment. The strength of waste discharged from Church and Local Park
can be assumed to have an average of 2.5 kg BOD/day and 1 kg BOD/day
respectively. BOD of an equivalent person can be considered to be 50 g/EP/day.
The sewer size must be capable of carrying the maximum flow at the appropriate
gradient and flowing not deeper than three-quarters pipe diameter. Inflow/Infiltration
(I/I) may be derived from lengths of contributing pipes or catchment areas whichever
is greater.
Use the following design data
ADWF, (L/c.d) 250
I/I, (L/s.ha) 0.8
I/I, (m3/d.km) 28
Primary school, EP/pupil 0.25
Child care, EP/child 0.25
Manning’s n 0.012
Use peaking factors shown in Fig. 6.6 of the study book
Wastewater flows from the development is to connect the proposed sewer main at
manhole M2. The manhole M2 also accepts upstream future flows from a catchment
consisting of residential area (3 hectares and 30 EP/ha) and light commercial area (4
ha and 50 EP/ha). Include calculations for a satisfactory sewer main and gradient
between M2 and M1. The limiting upstream invert level at manhole M1 for the
upstream flows is 44.5 m. Manhole M1 surface RL is 46.5 m. Surface levels at other
manholes may be assumed to be 50 mm or more below the nearest calculated reduced
levels.
Show clearly the steps taken in your design with accompanying detail working of
your calculations and present your design in a format as shown in Table 6.7 of the
study book. Marks will be allocated for documentation and clarity of design. Provide
relevant checks and comment about them.
Prepare and tabulate the design aid for partial flow as per the equations in study book
for interpolation of velocity and diameter for corresponding flows. Please provide
only two page excerpt of it in the submission to check for its accuracy. Any more than
two pages of this design aid will incur penalty of 5 marks.
