'S' for Sewer Systems and Sewage Treatment
Sewer
Systems and Sewage Treatment
 An
understanding of the basic concepts of storm water and wastewater
collection and wastewater treatment is necessary in order to
help Surfrider activists address water pollution problems that
result in beach closures, adverse human health effects, and
a stressed aquatic environment.
The first important concept to understand is that in most locations,
there are two separate sewer systems a storm water sewer system
and a sanitary sewer collection and treatment system. These
systems have distinctly different purposes.
Storm Water Systems
Storm water systems, consisting of gutter drains, open storm
water drainage channels and underground storm sewers, are designed
to prevent flooding problems. There are typically no pumps in
a storm water drainage system. Water flows by gravity to a low
spot, generally the ocean. A basic design principal of storm
water systems is to get the rainwater and other urban runoff
away from residential, commercial, and industrial areas as fast
as possible, and discharge it to a creek, river, or directly
into the ocean. Civil engineers design these systems to be water
superhighways. The water receives no treatment before it enters
the receiving water body, which might be your favorite beach.
So any water running off the streets and gutters, off business
properties, off of your lawn and driveway, carrying with it
animal waste, pesticides, fertilizer, leaked automotive fluids,
brake lining residues, and general litter, will end up at the
beach.
During dry weather, any water flowing in a gutter or surface
drain should be looked at with suspicion. Before humans arrived
on the scene with our cities, associated infrastructure, and
miles and miles of pavement, there was no dry weather urban
runoff. Hence, there was no flow to flush pollutants down to
the beach. Even in wet weather, undeveloped land and wetlands
allowed infiltration and natural cleansing of runoff. Now, drainage
channels have become ugly pseudo rivers that nearly always have
water.
What can you do? The most important message for us is to simply
turn off the tap. Use as little water as possible. Don’t over-water
your lawn. Adjust your sprinklers to avoid watering the pavement
(it won’t grow). Commercial car washes generally contain and
recycle their water. If you do wash your own car, use as little
water as possible and try to divert it to non-paved areas. Fix
fluid leaks from your car. Pick up your pet waste. Don’t over-fertilize
your lawn and avoid the use of insecticides and herbicides (weed
killers). Use a broom instead of a hose. Encourage these practices
by your friends. Be on the lookout for water flowing down the
gutter and if you find the source, see if you can educate the
perpetrator to use more environmentally-friendly practices.
Sanitary Sewer Collection and Treatment Systems
Sewage collection and treatment systems, also sometimes referred
to as sanitary sewer systems, collect and treat wastewater flowing
from drains (toilets, sinks, showers, washing machines, and
dishwashers) inside your house and businesses. Like storm water
systems, they make use of gravity (as much as possible), to
convey wastewater to a treatment facility. Because of the gravity
flow concept, treatment plants are frequently located in low
areas next to rivers or the ocean. If wastewater does need to
be pumped, the water flows into a concrete pit in the ground
called a "lift station" or "pump station" where pumps are used
to transfer it through a "force main" to the treatment plant.
Sanitary sewers are always closed pipes, with vertical connections
to sewer manholes situated along the pipe. If there is a blockage
in the main sewer line (from grease, tree routes, or large objects
flushed down the sewer) raw (untreated) sewage will flow out
of the sewer manhole, down the street or nearby storm drain,
and then directly into the ocean. This situation is one of the
main causes of beach closures.
Once the sewage gets to the treatment plant, it is typically
treated using both physical and biological treatment methods.
The different stages of treatment are referred to as primary,
secondary, and tertiary treatment. Primary treatment consists
of separation of liquids from solids. Typically, the raw wastewater
first passes through some grates and screens to remove large
pieces of trash and debris. The wastewater then flows to large
basins called clarifiers where both floatable and settleable
solids are removed. Floatable solids are skimmed of the top
of the water and settleable solids are removed from the bottom
of the clarifier. The settleable solids are called primary treatment
sludge. The basic principle at work here is to use gravity and
density differences to allow water and solids to separate by
slowing the flow down. Sometimes chemicals such as ferric chloride
and/or anionic polymers are added to the clarifier to promote
the clumping together (agglomeration) and settling of fine particles.
This may be referred to as advanced primary treatment.
After primary treatment, the wastewater still contains substantial
concentrations of suspended organic waste solids, and both the
solids and the water contain very high concentrations of fecal
coliform and other types of bacteria. Further solids and bacteria
removal occurs during secondary treatment. Secondary treatment
utilizes special strains of aerobic bacteria (bacteria that
need oxygen to grow) to break down the organic waste left after
primary treatment. The two most common processes that use aerobic
bacteria are the trickling filter and activated sludge processes.
In a trickling filter, wastewater is sprayed over a bed of rocks
that are coated with a slimy layer of bacteria that eats organic
waste. The activated sludge process is more efficient but also
more complicated and energy intensive. Wastewater is mixed with
a bacteria-containing sludge and air and is then allowed to
settle in a secondary treatment clarifier. As in the trickling
filter, the bacteria eat a large percentage of the fine organic
waste. Wastewater overflows from the secondary treatment clarifier
to the outfall pipe or to tanks where it is stored for use as
reclaimed water.
Settled solids from the secondary treatment clarifier are combined
with solids from the primary clarifier and are sent to large,
closed tanks called sludge digesters. In the digesters, anaerobic
bacteria eat organic matter in the sludge and produce methane
gas, which can be used in heating or to generate power for the
treatment plant. The digested solids are then dewatered (water
is removed from the solids by filters) and the resulting biosolids
may then potentially be used as fertilizer or as a soil amendment.
Reclaimed water is usually wastewater that has gone through
primary and secondary treatment. It is often used for irrigation
of parks, golf courses, and general landscaping. It is not suitable
for drinking. The Federal Clean Water Act requires that all
wastewater treatment plants use primary and secondary treatment
before discharging their water. In some cases, however, sewer
agencies have been able to use a process known as a 301(h) waiver
to get permission to discharge their wastewater with less than
full secondary treatment. The Orange County Sanitation District
(OCSD) in Orange County, California is an example of an agency
that has received such a waiver. OCSD treats approximately half
of their wastewater using secondary treatment and the other
half using only advanced primary treatment. The two wastewater
streams are then combined and pumped out a 4-mile long outfall
pipe into the ocean. OCSD has (so far) been able to convince
the Regional Water Quality Control Board and the EPA that this
practice is not harming human health or the environment. Due,
in part, to recent beach closures in Huntington Beach, the continuation
of OCSD’s 301(h) waiver is coming under increasing scrutiny
and criticism.
Wastewater can also be treated to a higher standard than secondary
treatment. A number of processes, called tertiary treatment,
can be used to convert secondary treated wastewater into water
that is (theoretically) good enough to drink. The processes
used in tertiary treatment include reverse osmosis and microfiltration.
Both of these processes pump water through membranes to remove
very small particles, including bacteria and viruses. The water
may then be disinfected using ultraviolet light, ozone, or chlorine.
In theory, a municipal wastewater treatment plant could operate
with zero discharge. Secondary treated wastewater can be reused
for irrigation and other non-potable uses, while tertiary treatment
can be used to produce water for potable uses. Paradoxically,
although OCSD is still operating under and may be requesting
a continued 301(h) waiver, they are simultaneously moving ahead
with the design of a what may be the world’s most advanced water
treatment and recycling system. Called the Groundwater Replenishment
System (GWRS), this system would use microfiltration, reverse
osmosis and ultraviolet light disinfection to further treat
secondary treated wastewater. Up to 100 million gallons per
day (almost half of the raw wastewater sent to OCSD) will receive
this treatment and will then be pumped or infiltrated into the
local groundwater basin to recharge the drinking water aquifer.
Surfrider members should all push their local sewer agency to
work towards the goal of 100% recycle of their treated wastewater
with no ocean discharge.
See the Sewage Treatment Flow page.
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