Wastewater Flow Rate: Chapter II: Wastewater Engineering CE 524
Wastewater Flow Rate: Chapter II: Wastewater Engineering CE 524
Wastewater Flow Rate: Chapter II: Wastewater Engineering CE 524
To develop a basis for properly assessing wastewater flow rates for a community, the
following subjects should be considered.
Definition of the various components that make up the wastewater flow rates.
Definition of the types of sewer.
Identifying the sources of the sewage.
Estimating the quantities of sewage from these sources.
Project population and land use up to the expected life or design period of the
planned sewer system.
Explain the variations of wastewater flow.
Distinguish the relationship of either peak flow or minimum flow to average
wastewater flow.
The components that make up the wastewater flow from a community depend on the type
of collection system used and may include the following:
Infiltration/Inflow (I/I) – I/I enters the collection system in a variety of ways. Some of
the most common sources of I/I are presented in Figure 2. Infiltration is defined as
storm water flows that enter the collection system by percolating through the soil
and then through defects in pipelines, manholes, and joints. Examples of defects
that allow infiltration into the collection system are cracked or broken pipes,
misaligned joints, deteriorated manholes, and root penetration. Inflow is definedas
storm water that enters the collection system via a direct connection to the system.
A few examples of inflow are downspout connections, foundation or yard drains,
leaky manhole covers, and illegal storm drain connections. The adverse effects of
I/I entering the collection system is that they increase both the flow volume and
peak flows in the system so that it is operating at or above its capacity. Excessive
I/I in the sanitary sewer collection system is the leading cause of sanitary sewer
overflows (SSOs).
Fig. 2 Typical Sources of Inflow and Infiltration Sanitary Sewer Master Plan
The percentage of wastewater components varies with local conditions and the time of the
year.
For areas served with sewers, wastewater flow rates are commonly determined from
existing records or by direct field measurements.
For new developments, wastewater flow rates are derived from an analysis of population
data and corresponding projected unit rates of water consumption or from estimates of
per capita wastewater flow rates from similar communities.
If field measurements of wastewater flow rates are not possible and actual wastewater
flow rate are not available, water supply records can often be used as an aid to estimate
wastewater flow rates.
Where water supply records are not available, useful data for various types of
establishments and water-using devices are provided for making estimates of wastewater
flow rates.
Sewer is a pipe or conduit carrying sewage. Sewers are usually not flow full (Gravity
Full). The full flowing sewers are called force main as the flow is under pressure.
Sewerage System
Separate system. In this system the sanitary sewage and storm water are carried
separately in two set of sewers.
Combined sewerage system. In this system the sewage and storm water are carried
combine in only one set of sewers to the waste water.
Partially separate sewerage system. This system is the compromise between separate
and combine system taking the advantages of both systems.
2. Brick Sewers
These types of sewer (Brick Sewers) are made at site and used for construction
large size sewer. Brick Sewers are very useful for construction of storm sewer or
combined sewer. Nowadays brick sewers are replaced by concrete sewer. Brick sewers
my get deformed and leakage may take place. A lot of labour work is required.
3. Cement Concrete
PCC - for dia upto 60 cm
Suitable for small storm drains. Not
durable.
RCC - for dia > 60 cm
They may be cast in situ or precast,
resistant to heavy loads, corrosion and high
pressure. These are very heavy and difficult to transport.
When the sewer line is subject to heavy external load e.g. under railway line,
foundation wall etc, below highways.
5. Steel Sewers
These types of sewer (steel sewers) are
Impervious, light, resistant to high pressure, flexible,
suitable when;
The sewage is carried under pressure
The sewage has to be carried across a river
under water
The sewer has to cross under a railway track
They are generally used for outfall and trunk
sewers
6. Plastic Sewers
Nowadays PVC sewers are used for
carrying sewage. Plastic sewers are resistant
to corrosion. Such types of sewer are light in
weight, smooth and can be bent easily. But
these types of sewer (Plastic sewers) are
having high co-efficient of thermal expansion
and cannot be used in very hot areas.
SOURCES OF SEWAGE
Large residential districts – wastewater flows developed based on land use areas
and anticipated population density (typically rates are based on wastewater flows
from nearby areas).
In all cases, should try to obtain local wastewater flows for a similar area.
2. Industrial Wastewater
Without internal reuse: 85-95% of the water used will probably become
wastewater.
Inflow – the water discharged into a sewer system including service connections from
such sources as roof downspouts; basement, yard, and area drains; manhole covers,
surface runoff, street wash water; etc.
Accurate estimation of sewage discharge is necessary for hydraulic design for sewers. Far
lower estimation than reality will soon lead to inadequate sewer size after commissioning of the
scheme or the sewers may not remain adequate for the entire design period. Similarly, very high
discharge estimated will lead to larger sewer size affecting economy of the sewerage scheme,
and the lower discharge actually flowing in the sewer may not meet the criteria of the self-
cleansing velocity and hence leading to deposition in the sewers.
Apart from accounted water supplied by water authority that will be converted to
wastewater, following quantities are considered while estimating the sewage quantity.
People using water supply from private wells, tube walls, etc., contribute to the
wastewater generation more than the water supplied by municipal authority.
Similarly, certain industries utilize their own source of water. Part of this water
after desired uses is converted into wastewater and ultimately discharged into
sewers. This quantity can be estimated by actual field observations.
This is additional quantity due to groundwater seepage into sewers through faulty
joints or cracks formed in the pipes. The quantity of the water depends upon the
height of the water table above the sewer invert level. If water table is well below
the sewer invert level, the infiltration can occur only after rain when water is
moving down through soil. The quantity of the water entering sewers depends
upon the permeability of the ground soil and it is very difficult to estimate. While
estimating the design discharge, following suggested discharge can be considered.
Suggested estimates for ground water infiltration for sewers laid below ground
water table (CPHEEO Manual, 1993).
Storm water drainage may also infiltrate into sewers. This inflow is difficult to
calculate. Generally, no extra provision is made for this quantity. This extra
quantity can be taken care of by extra empty space left at the top in the sewers,
which are designed for running ¾ full at maximum design discharge.
The water loss through leakage in water distribution system and house
connections, does not reach consumers and hence, not appear as sewage.
Certain amount of water is used for such purposes, which may not generate
sewage; e.g. boiler feed water, water sprinkled over the roads, streets, lawns, and
gardens, water consumed in industrial product water used in air coolers, etc.
POPULATION FORECASTING
Design of water supply and sanitation scheme is based on the projected population of a
particular city, estimated for the design period. Any underestimated value will make system
inadequate for the purpose intended; similarly overestimated value will make it costly. Changes
in the population of the city over the years occur, and the system should be designed taking into
account of the population at the end of the design period.
Objective: to project population and land use up to the expected life or design period of the
planned sewer system.
The present and past population record for the city can be obtained from the census
population records. After collecting these population figures, the population at the end of design
period is predicted using various methods as suitable for that city considering the growth pattern
followed by the city.
When planning and designing infrastructure for sewerage and water supply, the utility
has to forecast the future population. Things to consider are design period, land use, and service
life.
Design Period is the time period or duration that the capacity of the sewerage facility is
anticipated to be adequate to service its tributary area. Before commencing the facility
design, the design period must be determined.
Land Use helps define population densities and types of contributors to wastewater flows
within the tributary area. Zoning maps and field review of land use can be used to verify
the reasonableness of long range projections.
Service Life or Operational Life of a sewage facility should exceed the design period of
the facility, provided it is designed, constructed, and maintained properly.
Population Estimate
For the tributary area, the population estimate is the basis for computing the design flow.
It is customary to multiply the estimated population by the estimated per capita wastewater
contribution. Generally, population projections for land use planning have shorter projection
periods than are required for the design period for sewerage facilities.
Projections are conditional statements about the future. A Population Projection is:
An attempt to describe what is likely to happen under certain explicit assumptions about
the future as related to the immediate past.
A set of calculations, which show the future course of fertility, mortality and migration
depending on the assumptions used.
Residential
Density Population/Hectare
Single Family (greater than 10 frontage) 50 persons/hectare
Single Family (less than 10 frontage) 70 persons/hectare
Semi-detached 70 persons/hectare
Row dwelling 175 persons/hectare
Apartment 475 persons/hectare
Apartments
1
× 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠 (600 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠 𝑚𝑖𝑛𝑖𝑚𝑢𝑚)
3
1
× 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠 (900 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠 𝑚𝑖𝑛𝑖𝑚𝑢𝑚)
2
• Secondary Schools
2
× 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠 (1500 𝑠𝑡𝑢𝑑𝑒𝑛𝑡𝑠 𝑚𝑖𝑛𝑖𝑚𝑢𝑚)
3
• Hospitals
This method is suitable for large and old city with considerable development. If it is used
for small, average or comparatively new cities, it will give lower population estimate than actual
value. In this method the average increase in population per decade is calculated from the past
census reports. This increase is added to the present population to find out the population of the
next decade. Thus, it is assumed that the population is increasing at constant rate.
Pn = P + (n*i)
Example:
Population in year 1971 = 120
Average increase per decade = i = 8.57
In this method the percentage increase in population from decade to decade is assumed to
remain constant. Geometric mean increase is used to find out the future increment in population.
Since this method gives higher values and hence should be applied for a new industrial town at
the beginning of development for only few decades.
Pn = P*(1+ i)n
Example:
1981 = population*(1+i/100)𝑛
= 120*(1+10.66/100) = 132.8
1991 = population*(1+i/100)𝑛
= 120*(1+10.66/100)2 = 146.95
2001 = population*(1+i/100)𝑛
= 120*(1+10.66/100)3 = 162.60
2021 = population*(1+i/100)𝑛
= 120*(1+10.66/100)5 = 199.129
Example:
Population in year 1971 = 120
Average increase per decade = i = 11.57
1981 = population 1971 + 11.57 = 120 + 11.57 = 131.57
1991 = population 1981 + 11.57 = 131. 57 + 11.57 = 143.14
1994 = population 1991 + 11.57*3/10 = 143.14 + 3.47 = 146.61
2021 = population 1971 + 11.57*5 = 120 + 11.57*5 = 177.85
Ratio Method
𝑷𝟐 𝑷𝟏
= = 𝑲𝑹
𝑷𝟐𝑹 𝑷𝟏𝑹
Example:
Population at last census for the projected smaller region = 375
Population at last census for the projected larger region = 89,908
Projected Population in the larger region = 126,284
KR = 375/89,908 = 0.00417
P2 = 126,284*0.00417 = 527
pt = po 𝑒 𝑟𝑡
Where:
• Pt = population size at a time
• Po = population size at an earlier time
• 𝑒 = constant natural base (2.71878)
• 𝑟 = per capita rate of population change
• 𝑡 = time in years
𝑃𝑛
𝑎𝑛𝑡𝑖𝑙𝑜𝑔(log( ))−1
𝑃𝑜
r= 𝑡
Where:
Domestic Wastewater
RESIDENTIAL DISTRICTS
Basis for Average Flowrate Computation
• Population density, Pd and average per capita contribution of wastewater, q
Ave. Flow rate, Qa=(Pd)(A)(q)
• Percentage of the domestic water withdrawal rate
Ave. Wastewater Flow rate, Qw=(k)(Qa)
Where:
Qa= average domestic water withdrawal rate
K= percentage (range 60-85%), approximately 70%
COMMERCIAL DISTRICTS
• Use table
• Supervision
INFILTRATION/INFLOW
This is the total quantity of water from both infiltration and inflow without distinguishing the
source.
Factors affecting rate and quantity of infiltration/inflow:
Quality of sewer material
Workmanship
Type of soil
Groundwater condition
Topography
Length of pipes and conduits
Area serves
Age of sewers
Important in designing the different components of the wastewater collection, treatment, and
disposal systems.
affect the amount of water used (for example, the number of residents, bedrooms, customers,
students, patients, seats, or meals served).
Figure 3.
Wastewater
Treatment Plant
Flow Diagram
Whether a system serves a single home or an entire community, it must be able to handle
fluctuations in the quantity and quality of wastewater it receives to ensure proper treatment is
always provided. Systems that are inadequately designed or hydraulically overloaded may
fail to provide treatment and allow the release of pollutants to the environment. To design
systems that are both as safe and as cost-effective as possible, engineers must estimate the
average and maximum (peak) amount of flows generated by various sources.
The number, type, and efficiency of all water-using fixtures and appliances at the source is
factored into the estimate (for example, the number and amount of water normally used by
faucets, toilets, and washing machines), as is the number of possible users or units that can
Figure 4. A diagram of a wastewater system showing greywater feeding out of the bathroom
and laundry
Because extreme fluctuations in flow can occur during different times of the day and on
different days of the week, estimates are based on observations of the minimum and
maximum amounts of water used on an hourly, daily, weekly, and seasonal basis. The
possibility of instantaneous peak flow events that result from several or all water-using
appliances or fixtures being used at once also is considered.
Daily variation: usually, peak flow (maximum flow) occurs around lunchtime, while
minimum flow is during nighttime. According to studies, water use in many homes is
lowest from about midnight to 5 a.m., averaging less than one gallon per person per
hour, but then rises sharply in the morning around 6 am. to a little over 3 gallons per
person per hour. During the day, water use drops off moderately and rises again in the
early evening hours.
Weekly variation: Weekly peak flows may occur in some homes on weekends,
especially when all adults work during the week. The maximum day of the week is
usually the first day of the week (Sat) and the minimum is the last day of the week
(Fri).
Monthly variation: maximum month is usually during summer and minimum month
is during winter.
Peak flows at stores and other businesses typically occur during business hours and
during meal times at restaurants. Rental properties, resorts, and commercial
establishments in tourist areas may have extreme flow variations seasonally
Approximate equation:
fm = 0.051p0.199
where: p=population
Peaking factor, fp= ratio of peak flow to average flow
𝐐𝐩
fp = must be > 1
𝐐𝐚
𝐢𝐩 𝟓𝟒.𝟎
𝐟𝐩𝐢 =𝐢 = 𝟑𝟑.𝟓 = 𝟏. 𝟔𝟏
𝐚
∴𝐟𝐩𝐢 =1.60
DESIGN FLOWS
Variation in flow:
𝑸𝒑𝒆𝒂𝒌 𝟓. 𝟓
=
𝑸𝒂𝒗𝒆 𝑷
(𝟏𝟎𝟎𝟎)𝟎.𝟏𝟖
𝑸𝒎𝒊𝒏 𝑷 𝟎.𝟏𝟔
= 𝟎. 𝟐 ( )
𝑸𝒂𝒗𝒆 𝟏𝟎𝟎𝟎
Example Problem: You are required to estimate the peak and minimum sewage flows for a
town having an area of 2500 ha. The residential area is 60% of the total area, whereas
commercial and industrial areas are 30% and 10% of the total area, respectively. Of the
residential area, 40% are large lots, 55% small single-family lots and 5% multistory apartments.
The wastewater from the residential area is estimated to be 800 Lpcd. The sewage from
commercial and industrial areas is estimated to be 25000 L/ha/d and 40000 L/ha/d, respectively.
Density
Type Area(ha) Population Flow (m3 /s)
(persons/ha)
P= 252,975
Then,
𝑸𝒑𝒆𝒂𝒌 𝟓. 𝟓 𝟓. 𝟓
= = = 𝟐. 𝟎
𝑸𝒂𝒗𝒆 𝑷 𝟎.𝟏𝟖 (𝟐𝟓𝟐. 𝟗𝟕𝟓) 𝟎.𝟏𝟖
(𝟏𝟎𝟎𝟎)
𝑸𝒎𝒊𝒏 𝑷 𝟎.𝟏𝟔
= 𝟎. 𝟐 ( ) = 𝟎. 𝟐 (𝟐𝟓𝟐. 𝟗𝟕𝟓)𝟎.𝟏𝟔 = 𝟎. 𝟒𝟖
𝑸𝒂𝒗𝒆 𝟏𝟎𝟎𝟎
Hence,
Peak flow = Peak factor x wastewater + I/I = 2.0(2.68) + 0.03= 5.39 m3 /s
Minimum flow = 0.48(2.68) + 0.03 = 1.32 m3 /s
The common practice of using discharge figures found in reference books (e.g., Metcalf
and Eddie) tend to be gross averages, meaning half of the septic systems based on this averages
are over-designed and half are under-designed (Kaplan, 1988). Maximum and minimum flows
and instantaneous peak flow variations are necessary factors in properly sizing and designing
system components (Tchobanoglous and Burton, 1991). The system should be capable of
accepting and treating normal peak events without compromising performance (Tchobanoglous
and Burton, 1991).
Because peak flows can occur for a number of days, it is recommended that a peaking
factor of 2.5 be used for the design of downstream treatment processes of septic tanks (Crites and
Tchobanoglous, 1998). Table 2 provides peaking factors for wastewater flows from individual
residences, small commercial establishments, and small communities (Crites and
Tchobanoglous, 1998).
Table 2. Peaking Factors For Wastewater Flows From Individual Residences, Small Commercial
Establishments, And Small Communities
Small commercial
Individual residence Small community
Peaking factor establishment
Range Typical Range Typical Range Typical
Peak hour 4-10 4 6-10 4 3-6 4
Peak day 2-5 2.5 2-6 3.0 2-4 2.5
Peak week 1.25-4 2.0 2-6 2.5 1.5-3 1.75
Peak month 1.15-3 1.5 1.25-4 1.5 1.2-2 1.25
In many states it is quite common to use a flow allowance for design of 150 gpd per
bedroom, which in theory accounts for peak flow (Crites and Tchobanoglous, 1998). These same
authors recommend that a per capita design allowance, based on peak flow, be used for design.
Table 3 provides a comparison of design flows based on a per capita allowance times a peaking
factor versus design flows based on a per bedroom allowance (Crites and Tchobanoglous, 1998).
Table 3. Comparison Of Design Flows Based On A Per Capita Allowance Times A Peaking
Factor Versus Design Flow Based On A Per Bedroom Allowance
Design flow
Flowrates Design flow
based on per
Number of Number , Peaking based n peak,
bedroom
bedrooms of persons gal/capita. factor per capita
allowance,
d flow, gal//d
gal/d
DESIGN FLOW
Minimum Flowrate
• Flow should be designed to prevent suspended solid from deposition in the piping
system.
• The minimum velocity required to keep organic solids in suspension is 1.0 ft/sec
(.3 m/s).
• The minimum velocity required keeping silt and fine sand in suspension is 2.0
ft/sec (.6 ft/sec).
Design Flowrate
• Usually assumed to be the average daily flow at the end of the design period of
the system.
• The average daily flow is considered to be the average daily flow for a continue
12- months period.
Maximum Flowrate
• Is the peak hourly flow rate plus flow rate due to infiltration and inflow.
Occurring over a 24- hour period based on annual flow rate. The the average daily
flowrate is used in evaluating treatment plant capacity and in developing flowrate ratios used
in design.
Calculate on over a 24- hour period based on annual operating data. The maximum
daily flowrate is important particularly in the design of facilities involving retention time
such as equalization basins.
The peak sustained hourly flowrate occurring during a 24- hour period based on
annual operating date. Data on peak hourly flows are needed for the design of collection and
interceptor sewers, wastewater-pumping stations, wastewater flow meters, sedimentation
tanks and channels in the treatment plant.
The flow rate occurs over a 24-hour period based on annual operating data. Minimum
flow rates are important in the sizing of the conduits where solids deposition might occur at
low flow rates.
The minimum sustained hourly flow rate occurring over 24-hour period based on
annual operating data. Data on the minimum hourly flow rate are needed to determine
possible process effects and for sizing of wastewater flow-meters, particularly those that pace
chemical-feed systems. At some treatment facilities, such as those using trickling filters,
recirculation of effluent is required to sustain the process during low-flow period. For
wastewater pumping, minimum flow rates are important to ensure that the pumping systems
have adequate turndown to match the low flow rates.
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