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After reading this week’s Lecture; answer the following:1. Have you ever experienced a sewer backup in your home?2. What things do you think you can do to prevent sewer backups in your home?And, of course, feel free to share your thoughts about the lecture/article.
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Wastewater
Wastewater Collection System
M
e le d
eall hi k m ch ab
he e he a e ha c me
f hei fa ce
comes from. In a future lecture we will discuss the sources of water that serve the residents
and businesses of Los Angeles. This week, I want to talk about the wastewater that leaves
these properties. Fewer people think about where their wastewater goes once they flush their
toilet and/or use their kitchen sink, bathroom sinks, and showers!
Each
e ha a sewer lateral ha c
ec hei h me
b i e
he ci -owned
main in the street. This sewer lateral i
all made
f Vitrified Clay Pipe (VCP). The
city sewer main varie i i e b i m
e ide ial a ea he a e VCP i e Wh a e
sewer pipes usually made out of clay pipe? The primary reason is because clay is inert and
d e
eac i h m
f he chemical hich c ld e d
i he e e i e chl i e bleach
from homes, acids/bases from businesses, etc.). Some local communities are installing newer,
m e ad a ced la ic fibe gla
i e i e ide ial a ea beca e m h me d
discharge chemicals in high enough concentrations to react with these newer materials.
However, it is important to keep in mind that over 99% of sewer pipes in the United States
today are made out of clay. And, in older cities, it is not uncommon to see VCP sewer pipes last
over 100 years!
The city sewer main transports the e age f m each
e
la e al a l cal a e a e
ea me
la
M
malle ci ie c
ac i h egi al a e
ea hei c
me
sewage. For example, the City of Los Angeles operates 4 regional wastewater treatment plants
that are used by the following entities: the City of San Fernando, the City of Glendale, the City
of La Canada Flintridge, unincorporated County areas (i.e., La Crescenta), the City of Los
Angeles, the City of Burbank, the City of Santa Monica, the City of Culver City, state properties
(i.e., UCLA, CSUN, community colleges), and numerous federal properties. The primary reason
each city does not build its own treatment plant is simply cost. For example, up in the City of
Davis (near Sacramento), UC Davis is the only UC Campus that owns and operates its own
a e a e ea me
la
M UC Da i fficial i h he did
hei
ea me
plant due to the cost and environmental compliance headaches caused by operating your own
treatment plant.
Wastewater Collection System Design Criteria
In order to efficiently collect and transport wastewater to the local or regional wastewater
treatment plant, sewer systems must be properly designed. For example, if you live in
Minnesota, you would not want to build a shallow sewer system that would freeze in the
middle f i e O if
li e i a e ci
ld
a
b ild a
e e mai
that will be at capacity in 10 years after the area is developed! Believe it or not, there is quite a
bit of engineering that goes into properly designing and building a sewer system. I will focus on
just a few design criteria for our discussion:
1. Proper slope
Obviously, we want to make sure the sewage flows by gravity if at all possible. So, we want to
minimize the number of sewage pump stations because they are very costly to build and
operate. Electrical pumps are very expensive to run in some cities with large pump stations
monthly electric bills can approach or exceed $100,000! Yes, imagine paying over $1 million a
ea j
m e age a highe ele a i
clea l he e a e be e e f hi m e
e eciall i
da ec
mic e i me
he e ci ie a e
ggli g j
a
f
bankruptcy. Of course, there are times when the need for a m a i ca be a ided
the topography of some parts of a city might just be lower than the surrounding area.
In general, sewer pipes should be constructed to allow for a minimum cleansing velocity of 2ft
per second. If the water in a sewer pipe travels too slow then the chance of debris building up
and causing a blockage is higher. Therefore, whenever possible, sewers should be built to allow
for a minimum cleansing velocity of 2 ft. ec d Of c
e he e e d e
ha e al a
be flowing at this speed (i.e., 2 am), but at least once per day (at peak hours) the water should
travel at this speed to carry solids/debris to the treatment plant. Also, if the slope is too
shallow and the water travels too slowly, the likelihood of Hydrogen Sulfide forming is
increased and this can cause odor complaints from local residents and/or businesses (rotten
egg smell). Hydrogen sulfide is naturally created as a byproduct of bacterial decomposition of
wastewater however, if sewers are improperly designed the levels of hydrogen sulfide may
become a problem.
This brings me to a related point: the health and safety of sewer workers. It is extremely
important for sewer workers to ALWAYS test the atmosphere inside a sewer manhole and/or
sewer pipe prior to entering. As stated above, hydrogen sulfide (HS) is a naturally occurring gas
in sewers so if a worker fails to test the level of HS prior to making an entry, death may result.
This is exactly what happened in the City of Burbank in the 1980s. A sewer worker entered a
manhole without testing the atmosphere and collapsed after breathing in HS gas. His coworker
k a dee b ea h ab e he ma h le a d he e i
e c e him Needle
a he
was quickly overtaken by the HS gas also. Please click
he eb i e li k de hi eek
Moodle page to read about a person who died after breathing HS under a kitchen sink. The
b
m li e i e all eed be c g i a
f he da ge f e e ga
2. Proper capacity
Clearly, the construction of a new sewer main is a costly undertaking. Therefore, public sewer
agencies need to make sure that the sewer line they are installing (or replacing) will have
adequate capacity for the future. I often tell regional elected officials that the only saving grace
cities have in terms of sewer capacity is the significant drop in per capita wastewater use we
have seen over the years. The bottom line is this: contrary to popular opinion, our engineering
forefathers did not have the wisdom to install large diameter sewer lines with future
population growth in mind. The fact that most cities have 70 -100 year old sewer lines that are
still capable of handling sewer flows is simply due to the advent of low flush toilets and water
saving shower heads and appliances. The sad truth is that if Americans were still using the
same amount of water (per capita) as they did in the past, the majority of sewer lines would be
undersized and we would have sewage flowing on our city streets. This, in fact, is exactly what
we see in a lot of cities when it rains!
In the City where I work, we design new sewers to be flowing at 50% at full buildout. What do
e mea b f ll b ild
I i a i i he f
e he e e i gle a ailable a cel i he
city is developed t i f ll
e ial F e am le le
a he e i a aca l i he
downtown area. Today this lot is discharging zero gallons of sewage into our sewer system.
H e e af e l ki g a he Ci
Ge e al Pla e lea ha hi l i i a
i g area that
allows six story buildings. So, when we go to design our new sewer we assume that this lot will
be developed into a six story residential building and we assume the maximum amount of
sewage will be discharged even though this might not ever happen. So, when we identify every
parcel discharging into our new sewer we assume that eventually every parcel will be fully
developed and we take these increased discharge volumes into account when we size our new
sewer line. Then, once we know what the maximum amount of sewage discharged will be, we
then oversize our sewer by 50%! Why do we ensure that in the future our sewer line will only
be flowing 50%? Because we want to ensure that we have enough capacity during extremely
large rainstorms. Rainwate al a fi d i
a i
ci e e
ei he h gh ickh le i
the manholes or illegal connections to the sewer system (i.e., sinks illegally installed in
backyards next to bbqs, etc.). In fact, I have personally seen maximum peak dry weather flows
more than double during some rainstorms! Since the majority of the cost of installing a new
sewer line is digging the trench and associated costs, it really makes sense to oversize the sewer
li e Rece l e i alled a e
sewer line at a cost of $5.2 million dollars. It would have
cost us $4.7 million d lla
i all a
i e A mi g ha hi e e e li e ill la
a
d
ea
h
ld e i all he
i e
make
e e igh
Sanitary Sewer Overflows (SSOs)
In the 1980s and 1990s the EPA did some studies and learned that SSOs were a significant
problem across the United States, especially in older cities. The EPA estimates approximately
40,000 SSOs every year. This is more than likely a severe underestimate because reporting is
nonexistent in some states. So what causes SSOs:
1. Wet-Weather Capacity Issues:
A) Inflow: Illegal connections, manhole covers, etc.
B) Infiltration: In some parts of the country, high groundwater tables lead to water flowing into
sewer lines through cracks/joints/etc.
I fl a d I fil a i a e b h k
a I a d I I I a d ca be a h ge
the groundwater depth and the amount of rainfall in the area.
blem de e di g
2. Roots in city mains:
Many cities planted city-owned trees between the sidewalk and the street over the years. The
roots of these trees oftentimes make their way into sewer laterals and city mains. The majority
of SSOs in my city are caused by roots they are a severe problem in older cities.
3. Grease in city mains:
Fats, oils, and grease (FOG) are a problem in virtually every sewer system in the United States.
Grease discharged into kitchen sinks is usually mixed with hot water and as this water cools
downstream the grease coagulates and sticks to the sides of sewer mains. Over time, grease
blockages form and cause SSOs. Because of this, most cities have instituted a FOG Ordinance
which requires restaurants to install grease interceptors to capture grease onsite, before
entering the city main. Of course, private homes and multi-family buildings continue to be
significant sources of grease and despite public education campaigns, sewer lines in high
density housing areas need to be cleaned more frequently. Educational pamphlets encourage
homeowners to dispose of left over grease into trash cans and not the kitchen sink. More
importantly, garbage disposals should hardly, if ever, be used. Most people think if they dump
their leftover chili into the sink and turn on their garbage disposal the grease magically
di a ea
he i eali all i d e i b eak i
a d all i
e i he ki che i k
hich
just leads to grease blockages downstream in the city main. If I were king for a day, I would ban
garbage disposals across the United States period. All food/leftovers should be thrown in the
trash.
4. Over-development without corresponding larger diameter sewer pipes:
As I stated above, my city has been very diligent about ensuring proper sewer capacity as sewer
lines are replaced and/or areas of the city develop. However, the sad truth is that most cities
have failed to keep up with development, in terms of sewer capacity. The EPA has estimated
that the nation is facing a huge problem in terms of sewer infrastructure. The estimated cost of
replacing broken or over-ca aci e e li e i i he e
f billi
f d lla
a de e
year the problem gets worse. Of course, the nation is facing similar aging infrastructure
problems with water mains, bridges, and gas mains. The basic problem is that cities (and
private companies) have simply not properly maintained their infrastructure, nor have they put
away adequate amounts of money to fund these repairs/replacements. Over the next 10 to 20
years the United States is facing severe problems if these pipes/bridges are not repaired and/or
replaced.
Why are SSOs a big deal?
1. Property Damage
Sometimes, depending on the elevation of manhole covers in the street vs drains inside a
residence/business, SSOs backup into properties. Imagine being home on Sunday and watching
your favorite television show at 8pm with your loved ones and slowly seeing water rise
h gh
li i g
m a d e i e fi fl
B he ime
eali e he Ci
e e mai
is backing up i
e
a e alki g i
f e age Thi e ac ce a i j
happened a few weeks ago in my city the damage to this property was quite extensive. The
carpets, flooring, and lower parts of the drywall all had to be removed and the areas contacted
by sewage had to be properly sanitized. As you can imagine, this can be quite costly and the
psychological damage done to inhabitants, knowing that their house was contaminated with
human feces, can be equally damaging.
2. Human Health
In addition to property and psychological damage, human health can obviously be negatively
impacted by SSOs. Mold can grow in areas that made contact with the sewage. In addition,
just walking through the sewage can lead to infections if open sores/cuts were present on the
skin where sewage made contact.
More importantly, if the SSO reached receiving waters where people swim (i.e., the Pacific
Ocean) then people can get ear, eye, and gastrointestinal infections. A CDC study in 1997-1998
found 1,387 cases of enteric illnesses associated with sewage contamination in lakes, ponds,
and rivers. Eating contaminated fish and/or shellfish can also cause human illness as a result of
SSOs.
3. Wildlife/Ecosystems
Of course, sewage contamination in receiving waters affects wildlife, aquatic systems, and
associated ecosystems. While it is hard to put a monetary value on these resources, most
e le ill ag ee ha e d
a e age c ami a i g
a e a – period.
4. Monetary losses to tourism
Just one SSO last year in Los Angeles County during the summer was estimated to cause over $1
million in lost revenue for beach businesses! The simple truth is that when beaches have to be
closed for several days there is a significant economic loss to local businesses. And, this loss can
extend for long periods of time just because of the negative publicity that sewage infested
waters bring. It is not uncommon to see beach attendance suffer for weeks after a significant
sewer overflow, long after the waters are certified as safe.
This week we focused on sewage collection and how sewage gets transported to a wastewater
treatment plant. Next week we will talk about what happens to the sewage once it arrives at
he a e a e ea me
la
Plea e d
lose sleep waiting for next weeks lecture!
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