Introduction to Collection Systems
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Introduction to
Collection
Systems
Sidney Innerebner, PhD, PE, CWP
Indigo Water Group
303-489-9226
Welcome to Introduction to Collection Systems. This course
will last about an hour and a half plus some time to answer
surveys and quizzes. We're going to talk about the pieces
and parts of collection systems, go through some design
criteria, and also some influent characteristics.
1
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Indigo Water Group, LLC has made attempts to verify the information
provided in this web-based training against respected reference materials
including Manual of Practice 11 by Water Environment Federation and the
Sacramento Manuals. Neither the Author nor the Publisher, Indigo Water
Group, assumes any responsibility for errors, omissions, or contrary
interpretation of the subject matter herein.
2
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4
Collection System Basics
Collection and transport of wastewater from each
home/building to the point where treatment occurs
Central Treatment Facility
Scalping Plants -
Source: iStockphoto.com
Let's start with the basics. The collection system exists so
that we can collect and transport wastewater from every
home or building in the service area down to the point
where treatment is going to occur. Most of the time, this is
going to be a centralized treatment facility. The centralized
treatment facility is going to be a large enough facility to
treat all of the flow generated in the entire service area. The
South Platte Water Renewal Partner's facility, for example,
can treat up to 50 mgd. The other option that we can do is
something called a scalping plant. And a scalping plant is a
little treatment plant that sits over a big interceptor and it
pulls water out of the sewer and it treats it right there on
site to be used for some immediate purpose like irrigating a
golf course or making reuse water for a factory.
5
MWRD Pump
Back Project
New $475 Million
Northern TP
Broke ground
August 17, 2011
Online in 2015 -
Pump back force
main
7 miles long
42 to 84 inch
diameter
$37 million
Source: June 2010 MWRD Northern Treatment Plant
Wastewater Utility Plan
Large, centralized treatment facilities is how wastewater is
typically treated. This is the new North treatment plant that
now serves the Denver Metro area. It was about a $475
million project that was built by Metro wastewater
reclamation district up near Brighton. This facility going to
collect and treat all of the additional flow that they don't
have room to treat down at the York and 58th Street
location. This is going to take water from Thornton and from
other people that are downstream of that existing plant.
What's unique about this project is that a lot of the people
that are sending wastewater down to the central treatment
plant have water rights that allow them to reuse the water.
What they're going to do is pump water all the way back
upstream about seven miles at a construction cost of $37
6
million. This will allow them to recapture that water for reuse.
This is going to be a really good way for some of the
contributers to augment their water supplies. However, it is
expensive to be pumping all that water all the way back.
6
Satellite Wastewater Management
Also called decentralized or distributed
Sewer Mining or Scalping
Undertaken by Utilities for a variety of
reasons -
Economics
Point of Service Reuse
Avoid expansion of centralized facility
Lack of potable water at desired location
So, the other option is to do decentralized treatment with
satellite facilities. This is sometimes called sewer mining, or
sometimes we call these scalping plants. Actually, I think
scalping plants with more technical term. I like sewer mining
sounds like we're going for the gold. As I said, these are little
tiny treatment plants. They can they sit over a big
interceptor to pull water, well sewage out. The solid
material continues to go downstream or may be removed
and then put back into the sewer. We're just treating for
whatever reuse purpose we have right there at that site or
location. This makes a lot of sense economically, because we
don't have to pump water back long distances like the seven
miles or so that the metro plant is going to do. Pumping it
uphill all that way isn’t desirable because pumping is
expensive and because pump stations fail. It allows us to
7
produce water right there where we need it. Sometimes, a
scalping plant can take the load off of a centralized facility like
that big North plant that's going to go in up in Brighton, and
that might help to avoid a future expansion ‐ just by taking off
some of the loads further up in the system. Sometimes a
scalping plant is put in because we don't have any potable
water where we need to do something with irrigation or
other kinds of reuse. All good reasons to do scalping or sewer
mining.
7
-
Taken by Indigo Water Group
I had the pleasure of visiting this reuse facility out in
Midway, California. It's actually pretty slick. This is the pump
station that sits over a huge interceptor, and I don't
remember how big it was, it was a ginormous interceptor.
What they're doing here is they're pumping water out of
sewer. They're screening and degritting it. You can see all of
the screenings collected back there in the dumpster. The
poor operators running this site were running around trying
to make sure their visitors – a big bus load of engineers –
had chairs and coffee and safety equipment. Looks like they
ran out of time to get a fresh dumpster. A little bit of
screenings are overflowing the dumpster. No biggie. What I
love about this pump station is the tile job. It’s kinda Mad
Hatter, don’t you think?
8-
Taken by Indigo Water Group
The wastewater is then sent on to be treated in a membrane
activated sludge process right there on site. This is a
photograph of part of the membrane system.
9-
Taken by Indigo Water Group
The reuse water is being used to irrigate a golf course and a
nature reserve and a few other things right there on site.
This building is the entire treatment plant that you're
looking at. Because it's a scalping plant, it’s fairly small and
able to blend into the neighborhood. If you weren’t in the
business, you probably wouldn’t realize that they're
producing reuse water in that building. Scalping plants are
getting to be more and more prevalent, especially in places
in the southwestern United States. That would be us, but
particularly in Mexico and Arizona because you just have to
reuse that water as many times as you can. There just isn't
enough of it.
They are pulling wastewater out of the sewer and putting
the waste solids from the treatment process back into the
10sewer. Screenings and grit, of course, go directly to the landfill
and don’t go back into the sewer. Because a lot of scalping
plants don't have any solids handling facilities, they just roll
the solids back in the sewer. That has the potential to create
some headaches for us from a collection standpoint because
there isn't as much water which means flow velocities are
decreasing and there's more solids which means the solid
materials would start accumulating the pipes. That equals
more frequent cleaning of the collection system for us. Still a
great and cost‐effective idea.
10Primary Categories of Collection
Systems
Sanitary Sewer
Storm Water Collection
Combined Sewers
-
Combined sewers
force treatment of
storm water and
construction of larger
treatment plants
More common in older Source: iStockphoto.com
Source: iStockphoto.com
systems
There are three main types of collection systems. We have the sanitary sewer,
which collects everything from toilets and kitchens and washing machines and
homes and office buildings and all that. This is sanitary waste. We have storm
sewers, which normally are completely separate from the sanitary, and collect water
that is running off the streets and other hard surfaces. There are also combined
sewers, where we're getting both types of flow collected in the same sewer:
sanitary wastewater and stormwater. Combined sewers are more prevalent on the
far west and far eastern coasts of the United States and in older cities. Fun fact:
storm sewers were built long before people figured out it was a good idea to put
sewage into pipes. Back in the day, everyone had a pit toilet or its equivalent in
their basement or they used chamber pots and just dumped them out into the
street. They had to wait for a good rain to wash it all away. Hence, the storm
sewers. As we figured out that sewage was associated with disease, storm sewers
were converted into sanitary sewers. However, they still received stormwater as
well.
The problem with a combined sewer is that it forces us to treat all of that storm
water. If you live in a place that gets a lot of rain, you will often see treatment plants
designed for both dry weather flow and for wet weather flow. The wet weather
flow takes up a lot of capacity and it makes us build larger treatment plants. A lot
of cities that originally had combined sewers are taking them out and trying to
separate those two systems by having a separate storm water system.
11Quiz
Click the Quiz button to edit this object
Do any of you have combined sewers?
-
Let’s do a quick poll. How many of your systems are partly
or entirely combined systems?
12Collection System Basics
Collection System Alternatives
Conventional Gravity Sewers
Septic Tank Effluent Gravity (STEG)
Septic Tank Effluent Pump
- (STEP)
Pressure Sewers with Grinder Pumps
Vacuum Sewers
Collection System Components
Pipes and Manholes
Lift Stations and Force Mains
Inverted Siphons
There is more than one way to build a collection system. The
system that most of us are most familiar with is the
conventional gravity sewer. That's what we're going to talk
mostly about today. You can also have STEG and STEP type
systems, which are septic tank‐based systems. With a STEG
or septic‐tank effluent gravity system, each home or
business has a septic tank that overflows into a gravity sewer
system instead of going to a leach field. STEP systems –
septic‐tank‐effluent‐pumped systems – are similar in that
every house and business has a septic tank, but now the
effluent is pumped into the collection system instead of
going by gravity.
13STEP and STEG systems are a nice way to convert a
community that already has septic tanks in place to a
centralized, mechanical treatment facility. Eldorado Springs,
CO put in a STEP‐STEG combination system, because every
house already had a septic tank. Some of the leach fields
were failing and they needed a treatment solution that
wouldn’t be too expensive or unsightly. Why do people visit
Eldorado Springs? So they can hike and rock climb in
beautiful El Dorado Canyon. The STEP/STEG system
combination allowed them to put in smaller diameter sewer
lines and a much smaller wastewater treatment plant. The
catch to the STEP and STEG systems is that the septic tanks
will still need to be pumped periodically. This was a
convenient way to kind of tie everybody into the system.
We can also have pressure sewers with grinder pumps, which
are kind of cool because they use small diameter sewer pipes
‐ about two inches. One other system is vacuum sewers,
which are good for collecting wastewater from low lying
areas, for example, say all the houses around a lake. Vacuum
sewers are mostly used for very small communities with less
than 100 connections.
Today, we're going to mostly focus on conventional gravity
sewers, and then we'll look at collection system components.
13Collection System Basics
Wastewater Characterization
Liquids
Solids
Gases -
Wastewater is mostly water, but also contains plenty of solid
material and dissolved gases. One of the most important
things is knowing how much flow to expect from different
parts of the service area. That allows us to determine what
pipe diameters we might need for normal operating
conditions. It’s also very useful to us because when flows
are abnormally high, we know that we should maybe be
looking for sources of inflow and infiltration, or did the
subdivision come online? Or do we have some illegal tie ins
that we should be concerned about? If the flow is
abnormally low, maybe we should be looking for exfiltration
issues. Being able to figure out how much flow to expect in
your service area is a useful thing to be able to do.
14Wastewater Characteristics
Expresses as gallons per capita day (gpcd)
generation rate
Generation rate of 70 – 110 gpcd typical
Lower in Colorado for many
- reasons
Water restrictions and conservation
Low water use appliances
Comparatively little I&I in areas
Generation rate depends on user
types in the service area
Source: iStockphoto.com
Most of the people in your service area are going to generate somewhere between
70 and 110 gallons per capita per day (gpcd). That's the gallons per person per day
of wastewater coming down the drain. That's not a huge amount of water, but it's a
great deal more water than people in some other countries use. Along the east
coast of the United States, well east of the Nebraska State line, where they get a lot
more rain, those wastewater flow rates can be quite a bit higher. They may be 120
or 150 gallons per person per day. Out here in Colorado, we tend to be on the low
end at 60 to 70 gallons per person per day.
Our wastewater generation rates are lower for several reasons. One of them is that
we don't have a lot of stormwater runoff or high groundwater. As a result, we don't
see very much inflow and infiltration in a lot of areas. We also have a lot of new
construction. Denver is relatively young city. New construction uses low water use
appliances and low water use fixtures. We’ve done an outstanding job of educating
our residents about conserving water. Do you guys remember back a few years ago,
more than a few years ago when the drought really first kicked in, back in 2001,
2002, Denver Water had people wearing sandwich boards walking around on the
16th Street Mall, trying to get people to conserve water. They would wear these big
orange sandwich board signs that had memorable slogans on them like: grass is
dumb, real men dry shave, and save water, shower with a friend. They also put up
billboard and sent out bill stuffer. They did all kinds of things to try to educate
people and make them think about how much water they were using. It worked!
As an industry, we've done a really good job. We've gotten water usage rates down
from 100 gallons per person per day down into the 60/70 gallons per person per
day range, which is great. Unless you’re in a utility trying to sell water because you
may not be able to sell enough water.
15Inflow
Direct rainwater runoff into the
sanitary sewer
Sources include
Below grade manholes -
Manholes with poorly fitting lids
Uncovered cleanouts
Storm sewer cross-connections
Basement sump pumps
incorrectly connected
Photo taken byby
Taken Indigo Water
Indigo Group
Water Group
Inflow is the primary cause of
overflows
I've mentioned inflow and infiltration already, let's define
them. Inflow is when we get direct rainwater runoff into the
sanitary sewer. It comes in through direct connections.
We're talking about things like uncovered clean outs, cross
connections with storm sewers, basement sump pumps
(they should not be connected to the sanitary sewer, they
should be connected to the stormwater system), below
grade manholes, manholes with missing lids, etcetera. These
are direct connections – actual openings into the system.
Inflow is a primary cause of sewer overflows in collection
systems; particularly in places where it rains a lot. Here in
Colorado, that's not our biggest problem.
16Infiltration
Water entering the collection system from a
variety of points including
Service connections
Defective pipes and -pipe joints
Defective connections
Leaky manholes
Photographs from City of Surrey, California
Infiltration is a little bit different. Infiltration comes in
through the cracks. Just think of the word infiltrating. It’s
associated with being a spy, being sneaky. So, when you're
getting infiltration, this is water that is sneaking in. It has
come into the collection system from areas that are not
normally considered openings. It’s coming in through gaps in
pipes and defective service connections. It’s coming in
through cracks and breaks in the pipes and though the sides
of leaky manholes. Almost all manholes leak to some extent
where the risers come together. There are lots of places
where infiltration can happen.
17Infiltration
-
Thank you to the Fremont Sanitation District.
Thank you to the operators down at Fremont Sanitation
District for supplying this excellent example of infiltration.
There is a small fountain of groundwater bubbling up
through a seam in this pipe. This is happening because the
groundwater level outside the pipe is just a little bit higher
than the bottom of the pipe. As the groundwater level rises,
the height of this fountain will also increase. It’s going to
match the height of the groundwater outside the pipe.
18Americans Like to Flush!
1.6
leaks Toilet
9.5 toilet
- Shower
18.5 Clothes Washer
faucets Dishwasher
10.9
Faucets
shower Leaks
11.6
1 clothes Other
15
Units are in gpcd.
Source: EPA /625/R-00/008 Onsite Wastewater Treatment Systems Manual
I thought this was kind of interesting when looked at the EPA
onsite wastewater treatment systems manual. If you're
interested in this kind of information, the EPA onsite
wastewater treatment systems manual is a great book to
have on your bookshelf. I’ve included a link to it under the
resources tab.
This is where our wastewater comes from in a typical
American household. This is how much water we're typically
using for different things during the day. What you can
conclude from this is that Americans like to flush. A typical
American is using about 18 and a half gallons of water a day
just to flush the toilet. A standard toilet uses about three
19and a half gallons per flush. There are some really low water
usage toilets out there now that get down to around a gallon,
a gallon and a half per flush. Some people do little tricks like,
putting a brick in the toilet’s water tank to reduce the amount
of water that it uses per flush.
19Americans Like to Flush!
1.6
leaks Toilet
9.5 toilet
- Shower
18.5 Clothes Washer
faucets Dishwasher
10.9
Faucets
shower Leaks
11.6
1 clothes Other
15
Units are in gpcd.
Source: EPA /625/R-00/008 Onsite Wastewater Treatment Systems Manual
The master bath at our house is an extremely low water use
toilet with a vacuum assist. It is extremely loud. If anyone
has to use it in the middle of the night, we enforce the “if it's
yellow, let it mellow, if it's brown, flush it down” rule.
Without it, someone would die of a heart attack during the
night.
Our next big use is washing clothes followed by showering.
You can reduce your water use there just by doing as few
loads as possible and waiting until you have full loads. Don’t
be my teenager who likes to wash one shirt or one pair of
jeans at a time. This is the same kid that likes to take hour
long showers. Perhaps installing that tankless water heater
20was a mistake? Faucets includes everything from washing
hands to dinner prep to washing dishes. You know what's
surprising? Look at how big that pieces of the pie chart is for
leaks. Almost 10% of wastewater generation is from leaky
toilets and leaky faucets. That's a lot of leaks just
contributing water to the system.
20Quiz
Click the Quiz button to edit this object
- per capita generation rates?
Why is it useful to know
21Light Commercial Flows
Facility Unit Range Typical
Airport Passenger 2-4 3
Apartment Person 40 – 80 50
Bar Customer 1–5 3
Employee 10 - 16 13
Hotel Guest - 40 – 60 50
Employee 8 - 13 10
Laundry Mat Machine 450 - 650 550
Office Employee 7 - 16 13
Restaurant Customer 8 - 10 9
Shopping Employee 7 – 13 10
Center Parking Space 1-3 2
Theatre Seat 2-4 3
Source: EPA /625/R-00/008 Onsite Wastewater Treatment Systems Manual
If you're trying to figure out how much water is going to get
generated in your service area, you also have to include
other things, not just the number of residents. If you have a
large shopping center for example, or a hotel or an
apartment or bars, all of these things, you must account for
those flows. Normally, we – and when I say we, I mean
design engineers and operators ‐‐ put all the light
commercial accounts into our per capita generation rate of
about 100 gallons per capita day. We just lump it in there.
We don't worry about it. That’s possible because most of the
time, the people who live in the service area also work and
shop in the service area. We have some movement of
people in and out of the service area, but it mostly comes
out in the wash. However, sometimes it doesn’t come out in
22the wash. For example, the Town of Morrison has about 400
residents. They are or were planning to build a huge
shopping center across the highway where you can see the
Soltera water tank from I‐470. Once that’s built, there's
going to be a lot more people coming into the service area to
shop. They won’t be accounted for in a generic, catch all per
capita generation rate calculation. The same is true for the
people who will be working in those shops, hotels, and
restaurants. Now, you have to start to account for those
things and the wastewater they are going to generate
separately.
This table from the EPA manual, and others like it, can help
you do that. Obviously, actual flow records or water usage
data would be better, but we don’t have that with a new
development. Instead, we turn to tables like this one for
suggested flow rates from EPA. Then, you can start to get a
handle on how much water you're actually expecting to come
down the pipe.
22Industrial and Commercial Users
Flow is usually estimated in terms of gallons per
day per acre
Thornton and Northglenn have used 583
gpd/acre -
Santa Monica, CA uses 13,600 gpd/acre
Highly industry dependent
Pulp and Paper Industry uses 16,000 gal per ton
product
Fruit Processing uses 200,000 to 800,000 gal/ton
Beef Processing uses 150 to 450 gal/animal
Source: www.p2pays.org/ref%5C01/0069206.pdf
With industrial and commercial users, we do things a little differently. If you know who your industry is, then
you can, of course figure that out from their typical generation rates, or if the factory knows how much water
they're going to be using, you can use that information. You can also estimate their wastewater flow rate
from their drinking water bill if they are already in place. That does assume that whatever they are using for
drinking water ends up down the drain and not in their product or on the lawn.
A lot of the time, what we're trying to figure out is, is the pipe out there in the collection system big enough
to accommodate the industrial user that wants to come in? Then to answer that question, you need to know
how much water they're going to use. If they haven't even built the factory yet, they might not have any
idea. If they have factories in other locations that are similar, you can use that information.
You can also look up different industries in various EPA publications to see how much water they might use
per amount of product produced. There are a few estimates shown here. If you're in a city, and you have
just an open field where there aren't any commercial users yet, you can estimate wastewater production in
gallons per day per acre.
In the Northglenn and Erie master plans, two municipalities that I’ve had the pleasure working on their
collection system models for them, we used 583 gallons per day per acre for light commercial properties.
That was based on generation rates in similarly zoned areas of each municipality. That let us guesstimate
how much wastewater would come out of those areas of the town when they were developed. Those areas
are slated to be commercial, but could be anything. This is one reason municipalities are constantly updating
their planning documents. The crystal ball doesn’t do very well if you get too many years into the future.
23Colorado Design Criteria
Policy WPC-DR-1, effective 09/12/2012
Where NO data is available, these flows
must be used
Commercial (Undefined) - – 1500 gpd/acre
Commercial (Defined) – Refer to Guidelines
on Individual Sewage Disposal Systems or
other accepted engineering references
Industrial (Undefined) – 2000 gpd/acre
Industrial (Defined) – from usage records
The State of Colorado has put together some design criteria
that they require you to use for planning if you don’t have
better data to use. They say that if the land is zoned for
commercial development and you have no idea what's going
to be there, they want you to use 1500 gallons per day per
acre and if it's going to be industrial, undefined, they want
you to use 2000 gallons per day per acre. Those flow rates
are for the maximum month average daily flow. That’s the
highest monthly average flow for a given year.
That's quite a lot of flow and higher than what I used in the
two master plans I mentioned on the previous slide. But it's
important to know what that number is. Because if you
24don't have an idea, then the pipe that you're putting in the
ground may not be big enough to actually accommodate
those flows, especially at peak hour flow.
24Typical Diurnal Flow Pattern
2.5
2
1.5
-
1
0.5
0
10:00 PM
11:00 PM
12:00 AM
10:00 AM
11:00 AM
12:00 AM
1:00 AM
2:00 AM
3:00 AM
4:00 AM
5:00 AM
6:00 AM
7:00 AM
8:00 AM
9:00 AM
1:00 PM
2:00 PM
3:00 PM
4:00 PM
5:00 PM
6:00 PM
7:00 PM
8:00 PM
9:00 PM
All the wastewater that comes into the collection system comes in a pretty
defined period of the day. If you look at most municipalities, the majority of
the population tends to get up six, seven o'clock in the morning, they have a
shower, they make breakfast, they do the dishes. A few hours later, all of the
water that they put down the drain of their house starts showing up at the
wastewater plant. We'll usually see two big peaks of flow during the day. The
first one occurs somewhere around 10, 11, 12 o'clock, that's the morning
water coming down and then we usually see another little bump right after
the dinner hour. So again, that’s people doing laundry after work or making
dinner or taking showers…. All that kind of stuff.
This has implications from an operations standpoint because if you are going
to do maintenance on the collection system, you want to try to do it at the
time of day when there isn't much flow and coincidentally not a lot of traffic.
It simply makes it easier to work. Performing maintenance during off hours
will minimize your bypass pumping if you have to take something offline or
repair a pipe. All municipalities have this sort of a diurnal curve. The shape of
the curve and the peak flow time of day, however, changes depending on the
size of the collection system and the number of people served.
25Magnitude of Variation is Affected
by Service Area
Population, Minimum Flow Maximum Flow
In thousands Peaking Factor Peaking Factor
1 0.2 3.9 – 5.5
5 0.26 –- 0.27 3.3 – 4.5
10 0.29 – 0.32 2.9 – 4.0
50 0.39 – 0.44 2.3 – 2.9
100 0.43 – 0.50 2.0 – 2.6
200 0.48 – 0.57 1.8 – 2.3
500 0.55 – 0.69 1.5 – 1.9
Source: Gravity Sanitary Sewer Design and Construction, MOP FD-5, WEF (1982)
The ratio of the average daily flow to the peak hour flow is going to change depending on how many people
live in the service area. It's actually not how many people. It's more related to the size of the collection
system and the kinds of things that might be going on in the service area. As the pipes get longer and longer,
we get more of what's called attenuation. Attenuation is how the water spreads out as it's moving through
the collection system.
You can test this yourself with a bucket of water and a slide. If I dump a bucket of water quickly on the top of
a slide, the water will race toward the bottom. However, it won’t all get there at the same time. Friction
between the water and the slide slows down the water that contacts the slide directly while the upper layers
of water move a little faster. The result is that the water spreads out as it moves down the slide. It doesn’t
all arrive at the bottom as a chunk of water. It more trickles down. The same thing happens in the collection
system.
As the service area gets larger, it’s also more likely to have people working different shifts. More people stay
home. There are more industries contributing to the collection system. That shifts some of the flow around
to different parts of the day.
In a small town, like the City of Victor, most of the residents get up around the same time of day, go to work
and send kids to school. They don’t have any big industries. As a result, most of the flow is generated at the
same time and it gets to the treatment plant by 9 o’clock in the morning. The peak hour flow is almost five
times greater than the average daily flow. Huge variation! At the same time, there isn’t much going on in
town after 9 or 10 o’clock at night, so the influent flow drops to almost zero.
Compare that to a really big municipality like the City of Thornton. I think they have around 140,000
residents. Their peak hour peaking factor is probably somewhere around 2.
26Estimating Flow Rate
Equivalent Dwelling Unit
Estimate population
Estimate number of taps
Determine flow per EDU or equivalent tap
-
Northglenn uses 2.54 persons = 1 EDU
(Census)
Design Peak Flow
Pipes and lift stations must handle peak hour
flow
Best way is to use historic data
Can guestimate peaking factor based on
population size
How do we actually go about figuring out how much water ought to be coming down the pipe at
us? Historic data and flow records are always the best sources of information, but they aren’t
always available. So, we look at maps and we look at zoning and we look at water use. Sometimes
we look at water billing records too. I'll show you an example on the next slide.
The first step is to look at the census data so we can figure out how many people we have in our
service area. An EDU is an equivalent dwelling unit, which is basically a house. If we know how
many houses we have and we know the population, we can figure out how many people, on
average, are going to be living in a single family home. We can also figure out how many people, on
average, live in an apartment building. Once we’ve mapped that information out, we can start to
calculate how much water we're going to expect in different parts of the collection system.
The collection system has to be designed to handle peak flow because if peak flow starts coming
down the pipe and the pipes aren’t big enough to handle it, we end up with surcharging and
sanitary sewer overflows. We don't want to deal with any of that. Then, we can go back to that
table that I showed you previously and look at the peaking factor based on population size or the
length or amount of pipe we have in the ground for the collection system. The State of Colorado
has a calculation in their design regulations for calculating peaking factors from population served.
27Source: Indigo Water Group collection system modeling project.
-
This is an example of a piece of a collection system model. I
hope that everybody's familiar with parcel maps and how
they look in GIS. We start by grouping parcels together that
ultimately discharge into the same manhole. All of these
parcels covered in yellow will send their flow to the mainline
at manhole CC‐17. Further south, the group of parcels
covered in orange will contribute their flows slightly
upstream in a different manhole. The large purple area is
zoned commercial, but doesn’t have any businesses there
yet. It’s contributions go in even further down the line. Why
does this matter? Well, we might be able to use a smaller
pipe diameter upstream and gradually make it larger
downstream as more flow is added to the line. That keeps
the cost down and it keeps our flow velocity in the desired
28range of about 2 ft per second. If the water goes too slow,
solids are going to settle out.
I know some of you guys are thinking, but I don't really do
this. This is the engineer's job. Well, that's partly true.
However, as you move up through the ranks or if you're
already the utility director, you are going to have to deal with
situations where a developer or an industrial user wants to
come in and connect to the existing system and you will have
to be able to figure out if the pipe that is already in the
ground can accommodate the additional flows they want to
send you. If the pipe can't accommodate the flows, if it is too
small, that business won’t be able to locate in that area
without upgrading the collection system. They might have to
go locate somewhere else.
28Quiz
Click the Quiz button to edit this object
-
How much water does a regular toilet use per flush?
29Conventional Gravity Sewer
Large pipe (8-inch min,
smaller service lines)
Manholes spaced 300-
-
500 feet
Uniform slope between
manholes
Source: iStockphoto.com
Let's talk about conventional gravity sewers. A conventional gravity sewer is what
most of us are familiar with. It uses fairly large diameter pipe. While service lines
can be as small as 4‐inch in diameter, we normally use minimum pipe diameter of
eight‐inches. Most of the time, an 8‐inch pipe is going to accommodate much more
flow than the subdivision it is running through is going to produce, we need to have
enough room in the pipe to pass larger objects; floaters and sinkers as it were.
Usually, we're going to have a manholes spaced at least one for every 300 to 500
feet of pipe. The reason that we've placed manholes that close together is because
most of our cleaning equipment can't go any further, at least not easily. Some of
the newer equipment can go up to 800 feet, which would be two manholes. I'm
also going to have a manhole any place that the pipe changes direction or any place
where we need to connect up to another sewer line. If the pipe has to change
direction, either horizontally – a change in slope – or because it's going side to side,
we need to have a manhole at that location. Pipes have flat ends, so if we try to put
two of them together in anything other than a straight line, we end up with a gap
somewhere. That gap is where I&I can enter and where exfiltration can occur when
the water table is lower than the pipe. What we're trying to achieve is an even
slope with no gaps all the way down to the treatment plant from the furthest
reaches of the collection system.
30Conventional Gravity Sewer
Self-Cleaning
Designed to transport
solids
- Minimum velocity >2 fps
during average daily
flow
Subject to Infiltration
and Inflow
Source: Indigo Water Group
A well designed collection system should be self‐cleaning.
Self‐cleaning sewers achieve a minimum flow velocity of two
feet per second. That minimum velocity of two feet per
second helps keep all of the solids that are in the
wastewater suspended and flowing on down the pipe. If we
get velocities less than two feet per second, then some of
the heavier solids will start to settle out in the pipe and then
we need to come along with a jet truck to clean it out.
In truth, very few sewers are completely self‐cleaning, which
is why we have to clean and inspect them regularly.
Gravity sewer systems are not watertight. As we saw from
the little fountain of groundwater coming up into the sewer
line on an earlier slide, groundwater can infiltrate easily.
Wastewater can leave through the same openings and
exfiltrate into the ground.
31Manholes
Located at changes in sewer size,
direction, or slope
Minimum every 300 to 500 feet
Provides access for -maintenance and
cleanout
Great opportunity for I&I
iStock photo
Manholes are our access points to the collection system.
This is where we can send in cleaning equipment or
cameras. We can use them as places to access the attached
sewer pipes when we need to make repairs or remove
blockages. We can install flow monitoring equipment within
them. Manholes are necessary, but also an asset that
requires ongoing maintenance and rehabilitation to keep
them in good condition.
32Manhole Construction
Built in place with brick
Cast in place concrete
Pre-cast concrete
- Synthetic materials
Source: Courtesy of DRC Construction Services in Sedalia, CO
This slide illustrates manhole construction. They can be
made from brick or fiberglass, but most manholes are made
from pre‐cast concrete. The manhole consists of a base with
multiple risers and a cone on top. The risers are the short
barrel sections. These are typically two foot high each and
fit together by nesting. It’s a bit like stacking Lego bricks.
The top of the manhole can have a concentric or eccentric
cone. Concentric cones come together in the middle of the
manhole just like a traffic cone. Eccentric cones come
together on one side so that one side of the cone is a
straight up continuation of the riser below it. Concentric
cones allow us to place cleaning and camera equipment in
from an angle. Eccentric cones, like the one shown here, are
easier to get down inside because the ladder rungs can
continue up into the cone.
33With precast barrel sections, we may have some ramneck or
other sealant that goes along the inside edge of the riser to
help it seal to the risers above and below it. The sealant
should prevent infiltration or at least minimize it. Ground
settling can upset that seal and then inflow will start coming
in where the risers meet.
Manholes often have metal or fiberglass steps for access. The
steps can degrade over time as can the concrete they are
attached too. Putting your full weight on one is often asking
for trouble because they can collapse beneath you. Some
municipalities no longer allow them to be installed. It’s much
safer to enter with a full harness and retrieval line.
Above the cone, there may be some adjustment rings. These
are used to adjust the height of the manhole a bit and bring it
up to grade or if I'm in an easement above grade a little bit.
Risers help minimize inflow and, where manholes are located
in easements, makes them easier to find. The top most piece
will be the manhole collar and cover.
33Brick Manhole
-
Manholes can also be constructed of brick. You probably
have a few of them in your service area. They can also be
square or rectangular instead of round. The biggest issue
with brick manholes is that the grout between the bricks
gets soft and crumbles over time. Inspections of brick
manholes includes looking for and replacing crumbling or
missing grout and missing bricks. These manholes tend to
have more infiltration than concrete or fiberglass manholes.
34-
Image from Leak Eliminators http://www.leakeliminators.com/manhole.html
The diagram on the right side of your screen shows an example of a brick
manhole on one side and a concrete manhole on the other side showing
some of the places where water can infiltrate. We get inflow through the
lid or under the lid. We get infiltration through the mortar, through
cracks, and through riser connections, if any.
Do you know how to figure out where the water is coming in in a brick
manhole when you don’t have obvious evidence like a wet surface? If
you pop the lid and shine a flashlight in there, you watch to see where
the bugs run to hide because they'll go into the openings where they can
get back to the dirt or they can make their little burrows. Follow the
cockroaches and you'll know where the grout is missing inside the
manhole and where you have deep cracks.
Brick and concrete manholes can be rehabilitated by lining them. There
are several different products on the market for this purpose.
35Cleanout
In addition to or in
place of manholes
Uses simple Y for
access to line
-
Generally on
service lines and
small diameter
sewer
Minimizes cold air
entry to sewer
Inflow source
Source: http://toledorotorooter.com/toledo-sewer-cleanout-installation
This is a clean out. A clean out can be used in addition to or
sometimes in place of manholes. It's just a simple Wye
connection. Usually these are used on service lines and
sometimes with small diameter sewers. A cleanout lets us
get access so we can get a pig in there or some other
cleaning device to remove a blockage inside of the pipe.
Every building service line should have a clean out on it.
Cleanouts can also be a source of inflow and infiltration
because the covers go missing. I don't know where they go,
but they tend to disappear. Maybe a trans‐dimensional
portal? The cleanout covers could be hanging out
somewhere with all the missing socks and pens. Missing
36cleanout covers are problematic because they can be a
source of inflow, especially if they are close to ground level.
In some mountain communities, they sometimes have
cleanouts in place of manholes. The idea is that because the
cleanout is smaller, it doesn't let as much cold air into the
collection system as a manhole potentially could. It may have
more to do with the fact that mountain communities often
don’t have a lot of topsoil so a cleanout is easier to install
than a larger manhole would be.
36Pipe Materials
Rigid Pipe Flexible Pipe
Asbestos-Cement Ductile Iron Pipe (DIP)
Pipe (ACP) Steel Pipe
Cast-Iron Pipe (CIP) Thermoplastic Pipe
Reinforced Concrete - Acrylonitrile Butadiene
Styrene
Pipe (RCP)
Polyethylene (PE)
Prestressed Concrete Polyvinyl Chloride (PVC)
Pressure Pipe (PCPP) Thermoset Plastic Pipe
Vitrified Clay Pipe Reinforced Plastic Mortar
(VCP) (RPM)
Reinforced Thermosetting
Resin (RTR)
What the collection system is made out of depends a lot on when it was built. Most
of what we use today is going to be thermoplastic pipe or thermoset plastic pipe.
There’s a lot of PVC and polyethylene out there in the ground. Older collection
systems tend to be built from vitrified clay pipe or brick.
Plastic pipe has a lot of great qualities. It’s light. It comes in long lengths and it’s
easy to install. It’s also easy to get watertight joints during installation. But, we
can’t use it everywhere because it isn’t strong enough for some applications. For
example, highway and railroad crossings and pressurized force mains. Then, you're
going to need something that can resist pressure and resist crushing forces. In
those situations, we might go with cast iron pipe as opposed to cement pipe or a
reinforced concrete pipe or ductile iron pipe. All of those materials have more
structural integrity than the plastic pipe and are less likely to break under pressure.
Plastic can crack in response to freeze and thaw cycles.
Clay pipe is probably about the worst when it comes to tree roots getting in and
cracking over long periods of time. It’s heavy and comes in shorter lengths than
plastic and steel pipe. However, clay pipe does have a phenomenal service life. I've
seen clay pipe that has been in the ground for well over 150 years and it is still
going strong. With a little slip lining and reinforcement, it will probably continue to
provide good service for many years to come.
37Quiz
Click the Quiz button to edit this object
What is the most prevalent type of pipe in your collection system
-
Just out of curiosity from the list below, what's the most
dominant type of pipe material in your collection systems?
38Pipe Material Selection
Gravity Line versus Pressure / Force Main
Sizes Available
Potential for Corrosion
Crush Strength -
Soil Type – expansive?
Freeze/Thaw Cycling
High Traffic?
Ease of Installation
Source: iStockphoto.com
Cost
What kind of pipe we're going to put in the ground depends
on what we're using it for. Do we need strength for crush
resistance? Are we concerned about corrosion potential?
Here in Colorado, we have a lot of high acid, high sulfide,
high chloride, salty soils. They can corrode iron and concrete
pipe from the outside. When we install these materials, we
may have to wrap it in plastic or coat it in some other way to
prevent that corrosion from happening, which is what makes
plastic really attractive because it's cheap, it's easy to install,
and it is very corrosion resistant. However, it doesn't have
very good strength to resist crushing. So, if it’s a high traffic
area, we might have to choose a different material besides
plastic. Last, but not least, cost is always a factor when
selecting pipe materials.
39Minimum Slopes
Minimum cover of 3.5 feet
Maintain desired velocity
> 2 fps at average flow for self cleaning
Maximum of 15 fps -
Can’t always be maintained
Crossing other utilities, streams, etc
Excessively Low/High velocities
High potential for dead spots, plugs, and
surcharging
Septic conditions
When we get to pipe in the ground, a couple of things: First,
we're trying to maintain a minimum slope along the length
of the pipe that will give us a flow velocity of greater than
two feet per second at peak hour flow and preferably at the
average daily flow. Practically speaking, this is almost
impossible to achieve in subdivisions and with your smaller
diameter sewer lines. There just isn’t enough flow in some
of those pipes to achieve our desired velocity. That’s
because we have to have a minimum diameter of about
eight inches, just to keep everything, including solids,
flowing downstream. That eight‐inch pipe helps prevent
blockages, but also results in slower velocities and lower
water depths in the pipe that can’t carry solids. We're never
going to get enough flow from a small subdivision to reach
those minimum desired flow velocities most of the time.
40Often, the pipes have little more than a trickle of flow in
them.
Velocities greater than two feet per second are needed to
keep things, solids, moving along. If material settles out in
the pipe, and we need to re‐suspend it, then I need to get the
velocity up to about four feet per second. That's a tough thing
to do especially with just sanitary sewer flows.
I also don't want the water to go too too fast. I want to keep
the velocity under 10 or 15 feet per second, the maximum
depends on your municipality’s particular standards, but 15
fps would be the absolute top velocity we want to see in
gravity lines and force mains. When the water gets going
faster than that, the grit in the wastewater can scour out
pipes and wear holes in them. We can also get a separation
in a gravity line of the water and the solids. Then, the solids
stay against the bottom of the pipe and may roll along while
the water skates over the top at a much higher velocity. Pipes
with very high velocities can, as odd as it seems, have a lot of
solids settle out in them.
With really low velocities, we get dead spots, plugging, and
surcharging. Sometimes we get bellies across a section of
pipe, not because of the low velocity, but because of poor
construction or soil washing away. Bellies are tough to keep
clean, especially if we always have low velocity water. When
we talk about cleaning collection systems later on in this
series, we'll talk about how you can tell that you've got a belly
40or a sag in a line just by cleaning it even when you don't send
a camera of a line. The big clue is the foul smell in the
downstream manhole caused by moving that septic material.
40Minimum Slope Maintains Desired
Flow Velocity
Minimum Slope
Sewer Diameter (feet/100 feet)
8-inch 0.40
-
10-inch 0.28
12-inch 0.22
14-inch 0.17
16-inch 0.14
18-inch 0.12
24-inch 0.08
To achieve our target minimum velocity, we are going to put
pipes in the ground with a minimum slope. You can see from
the chart that the minimum slope gets flatter and flatter
with the bigger diameter pipe. There's more flow in a bigger
diameter pipe, so it's easier to keep things in suspension.
With a smaller diameter pipe, we need more of that slope to
kind of help move things along inside the pipe. In places in
the country that have relatively flat topography, Kansas, for
instance, collection systems can struggle to maintain
minimum slopes on pipes. Often, they will just run a long
pipeline with the absolute minimum slope down for as long
as they possibly can. They might start off with a pipe at
three and a half feet underground, below grade, and then
they'll just keep running that minimum slope as far as they
can until they're 25 or 30 feet below grade. Then, they need
41a pump station to pump the wastewater back up to three and
a half feet of depth where it will be dumped into the next
gravity line. A community that is very flat in topography can
have many, many lift stations out in the service area. We are
fortunate here that we have topography working for us. It’s
easy to get water to run downhill when you have hills.
Sometimes topography works against us by dividing areas
that might have flowed to the same gravity line if it weren’t
for a big hill or ridge dividing them.
I can’t always get these minimum desired slopes. Sometimes,
other utilities get in the way and we have to build over or
under them. Sometimes, we have to cross streams or gullies.
Sometimes the topography is really, really steep or really,
really flat. We’re forced to have an excessively steep slope on
our pipe and or excessively flat slope, which gives us really
high velocities or really low velocities.
41Typical Pipe Profile
Ground
Level
-
Manhole
Source: Indigo Water Group project – collection system model.
This is a pretty typical type profile. You can see the gray bars
running perpendicular to the pipeline. Those are the
individual manholes. You can see how they're spaced. Every
place I need to have a change in the slope of the pipe or the
diameter of the pipe, I'm getting another manhole in there.
That’s why some of the manholes are closer together and
some are further apart. The green line at the top is ground
level. You can see that the ground level is going up and
down, but I'm still trying to maintain that minimum slope. A
gravity sewer line does not follow the above ground
topography. A force main can, but not a gravity line. There
are a few places where, all of a sudden, I have a much
steeper slope, or a little bit flatter slope and that just has to
do with where other sewer lines are connecting into the
system.
42Finding Velocity
Velocity can be estimated with this equation:
Velocity = Flow
Area
-
Only really works for full pipes
Math to find area of a partially full pipe is difficult
Let’s take a minute here and talk about a couple of
hydraulics terms in collection systems. When you take a
certification exam, we are asked to calculate the velocity
inside a pipe, but we always assume that the pipe is running
100% full. That shouldn’t be happening in a gravity sewer
line, but it does happen in a forcemain.
The basic equation for finding velocity is flow divided by
area. When the pipe is running 100% full or 50% full, it’s
easy to calculate the area, but it becomes a lot harder if the
pipe is any other percentage full. That requires some special
trigonometry to figure that out.
43Finding Velocity
Mannings Equation
adds more variables
and gets closer to
actual velocities in -
sewers
V = 1.486 R S
2/3 1/2
n
Slope
R = Area
Perimeter
The simple formula for velocity that we just looked at is
really too simple because the velocity of the water in the
pipe doesn't just depend on the cross‐sectional area of the
pipe. It also depends on the slope of the pipe. The steeper
the slope, the faster the water is going to go. Another factor
is the roughness of the pipe. In Manning’s equation, pipe
roughness is shown an “n”. In Hazen‐Williams equation,
which is another equation for finding velocities in pipes, pipe
roughness is shown as a “C”. The C‐factor or n‐factor tell us
how rough the inside pipe surface is. The rougher the pipe,
the more friction we get, and the slower the velocity.
It also depends on something called the hydraulic radius.
They hydraulic radius helps us figure out how much friction
there is between the moving water and the wall of the pipe.
44The hydraulic radius is calculated by taking the cross‐sectional
area of the pipe that is full of water and dividing it by the
wetted perimeter. The wetted perimeter is basically where
the water is touching the inside of the pipe. This is important
because the water rubs on the inside of the pipe. The greater
the area that the water is coming into contact with, the
greater the amount of friction that is generated. More
friction results in more head loss, which decreases the
velocity in the pipe.
44Finding Velocity
Mannings Equation
adds more variables
and gets closer to
actual velocities in -
sewers
V = 1.486 R S
2/3 1/2
n
Slope
R = Area
Perimeter
Manning's equation is a bit more complicated than the
previous formula. Before, we only looked at the flow and
the area. That works okay for full pipes. Now, we’re taking
into account the slope, the pipe roughness, and the
hydraulic radius. That gets us much closer to finding the
actual or true velocity inside the sewer. Manning’s equation
(or Hazen‐William’s equation) gets us a lot closer but even
these equations can’t perfectly predict velocity under field
conditions. Why not? Well, because the installed pipe slope
might not exactly match what we think it is AND because the
roughness of the pipe starts to change as it ages and as
material accumulates in the pipe.
45Floats and Dyes
If you really need to know velocity,
measure it!
Meters
Floats -
Dye
Take the average travel time
Drop dye in upstream manhole
Measure time for dye to first appear
Measure time at dye disappearance
Use average time
If you really need to know the velocity out there in the
collection system, you need to go out there and measure it.
You can measure flow velocity with meters and floats but
what most people use to measure velocity is dye. If you
have ever done any dye testing, what you do is you go out
there, you get two manholes, somebody stands at the
upstream manhole, somebody stands at the downstream
manhole and you synchronize your stopwatches. The person
at the upstream manhole drops the dye into the manhole
and starts their stopwatch. The person at the downstream
manhole, looks in there waiting for the dye to show up.
Then, he or she checks the time when the die first appears
and they check the time when it goes away. We need the
start and stop times because the flow through the pipe is
not going to be all at once. Think back to the bucket of
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