Wastewater Treatment Principles
Sewage is the wastewater released by residences,
businesses and industries in a community. It is 99.94 percent water, with
only 0.06 percent of the wastewater dissolved and suspended solid material.
The cloudiness of sewage is caused by suspended particles which in untreated
sewage ranges from 100 to 350 mg/l. A measure of the strength of the wastewater
is biochemical oxygen demand, or BOD5. The BOD5 measures
the amount of oxygen microorganisms require in five days to break down
sewage. Untreated sewage has a BOD5 ranging from 100 mg/l to
300 mg/l. Pathogens or disease-causing organisms are present in sewage.
Coliform bacteria are used as an indicator of disease-causing organisms.
Sewage also contains nutrients (such as ammonia and phosphorus), minerals,
and metals. Ammonia can range from 12 to 50 mg/l and phosphorus can range
from 6 to 20 mg/l in untreated sewage.
Sewage treatment is a multi-stage process
to renovate wastewater before it reenters a body of water, is applied to
the land or is reused. The goal is to reduce or remove organic matter,
solids, nutrients, disease-causing organisms and other pollutants from
wastewater. Each receiving body of water has limits to the amount of pollutants
it can receive without degradation. Therefore, each sewage treatment plant
must hold a permit listing the allowable levels of BOD5, suspended
solids, coliform bacteria and other pollutants. The discharge permits are
called NPDES permits which stands for the National Pollutant Discharge
Elimination System.
Preliminary Treatment
Preliminary treatment to screen out, grind
up, or separate debris is the first step in wastewater treatment. Sticks,
rags, large food particles, sand, gravel, toys, etc., are removed at this
stage to protect the pumping and other equipment in the treatment plant.
Treatment equipment such as bar screens, comminutors (a large version of
a garbage disposal), and grit chambers are used as the wastewater first
enters a treatment plant. The collected debris is usually disposed of in
a landfill.
Primary Treatment
Primary treatment is the second step in
treatment and separates suspended solids and greases from wastewater. Waste-water
is held in a quiet tank for several hours allowing the particles to settle
to the bottom and the greases to float to the top. The solids drawn off
the bottom and skimmed off the top receive further treatment as sludge.
The clarified wastewater flows on to the next stage of wastewater treatment.
Clarifiers and septic tanks are usually used to provide primary treatment.
Secondary Treatment
Secondary treatment is a biological treatment
process to remove dissolved organic matter from wastewater. Sewage microorganisms
are cultivated and added to the wastewater. The microorganisms absorb organic
matter from sewage as their food supply. Three approaches are used to accomplish
secondary treatment; fixed film, suspended film and lagoon systems.
Fixed Film Systems
Fixed film systems grow microorganisms
on substrates such as rocks, sand or plastic. The wastewater is spread
over the substrate, allowing the wastewater to flow past the film of microorganisms
fixed to the substrate. As organic matter and nutrients are absorbed from
the wastewater, the film of microorganisms grows and thickens. Trickling
filters, rotating biological contactors, and sand filters are examples
of fixed film systems.
Animation
of Trickling Filters (Java)
Suspended Film Systems
Suspended film systems stir and suspend
microorganisms in wastewater. As the microorganisms absorb organic matter
and nutrients from the wastewater they grow in size and number. After the
microorganisms have been suspended in the wastewater for several hours,
they are settled out as a sludge. Some of the sludge is pumped back into
the incoming wastewater to provide "seed" microorganisms. The remainder
is wasted and sent on to a sludge treatment process. Activated sludge,
extended aeration, oxidation ditch, and sequential batch reactor systems
are all examples of suspended film systems.
Animations
of Activated Sludge systems (Java)
Lagoon Systems
Lagoon systems are shallow basins which
hold the waste-water for several months to allow for the natural degradation
of sewage. These systems take advantage of natural aeration and microorganisms
in the wastewater to renovate sewage.
Various
lagoon layouts (Java)
Final Treatment
Final treatment focuses on removal of
disease-causing organisms from wastewater. Treated wastewater can be disinfected
by adding chlorine or by using ultraviolet light. High levels of chlorine
may be harmful to aquatic life in receiving streams. Treatment systems
often add a chlorine-neutralizing chemical to the treated wastewater before
stream discharge.
Advanced Treatment
Advanced treatment is necessary in some
treatment systems to remove nutrients from wastewater. Chemicals are sometimes
added during the treatment process to help settle out or strip out phosphorus
or nitrogen. Some examples of nutrient removal systems include coagulant
addition for phosphorus removal and air stripping for ammonia removal.
Sludges
Sludges are generated through the sewage
treatment process. Primary sludges, material that settles out during primary
treatment, often have a strong odor and require treatment prior to disposal.
Secondary sludges are the extra microorganisms from the biological treatment
processes. The goals of sludge treatment are to stabilize the sludge and
reduce odors, remove some of the water and reduce volume, decompose some
of the organic matter and reduce volume, kill disease causing organisms
and disinfect the sludge.
Untreated sludges are about 97 percent
water. Settling the sludge and decanting off the separated liquid removes
some of the water and reduces the sludge volume. Settling can result in
a sludge with about 96 to 92 percent water. More water can be removed from
sludge by using sand drying beds, vacuum filters, filter presses, and centrifuges
resulting in sludges with between 80 to 50 percent water. This dried sludge
is called a sludge cake. Aerobic and anaerobic digestion are used to decompose
organic matter to reduce volume. Digestion also stabilizes the sludge to
reduce odors. Caustic chemicals can be added to sludge or it may be heat
treated to kill disease-causing organisms. Following treatment, liquid
and cake sludges are usually spread on fields, returning organic matter
and nutrients to the soil.
More about digestion of sludges
Wastewater treatment processes require
careful management to ensure the protection of the water body that receives
the discharge. Trained and certified treatment plant operators measure
and monitor the incoming sewage, the treatment process and the final effluent.
Portions of this page were taken
from:
"Wastewater Treatment Principles"
by Karen Mancl
Ohio State University
Food, Agricultural and Biological
Engineering
http://www.ag.ohio-state.edu/~ohioline/aex-fact/0768.html
Main Page
Page design: Renay Jacob and Emily
Cordaro, RPI, Fall 2000