ENVR 30, P. Chau © 2007
Chapter 7
S
Seew
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Worldwide, over a billion people lack safe drinking water. Sewage, or in the technical term wastewater, is a
major source of pollution in aquatic ecosystems. Discharge of inadequately treated sewage has led to pollution
problems not only in the developing countries, but also in Europe (e.g. the Mediterranean Sea and the North
Sea) and the United States. This chapter explains the processes of wastewater treatment and some water
purification technologies.
Oxygen-consuming wastes lead to high BOD, one of
the indicators of polluted wastewater.
In natural aquatic ecosystems, dissolved oxygen is required to support fish and other
organisms. When a polluted stream contains an elevated amount of (biodegradable) organic
matter, it will lead to a population explosion of aerobic bacteria. These decomposers consume
oxygen for respiration and their growth can deplete the dissolved oxygen and suffocate the
fish population. After oxygen is depleted, anaerobic bacteria take over and produce toxic and
foul-smelling chemicals such as hydrogen sulfide (H2S, rotten-egg smell), ammonia, and
methane (CH4, swamp gas).
The amount of oxygen-consuming wastes (i.e., organic matter) in water is measured by
the biological oxygen demand (BOD). It is also commonly called biochemical oxygen
demand. BOD is defined as the amount of dissolved oxygen required (by the aerobic
decomposers) to break down the organic materials. A high BOD means that the water is badly
polluted with oxygen-demanding organic waste.
Other than BOD, wastewater is also characterized by its bacterial count, suspended and
dissolved solids content, turbidity, color, odor, pH, inorganic compounds (e.g. nitrate,
phosphate, chloride, heavy metals like copper, lead, and chromium) and synthetic organic
compounds (e.g. pesticides).
Figure 1. Schematic diagram of primary and secondary sewage treatment. [Botkin & Keller, Fig. 10-20]
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The general goal of wastewater treatment is
to lower the BOD and remove the pathogens.
Untreated sewage is a public health hazard because sick humans and animals can discharge
pathogenic (disease-causing) bacteria. Along river communities, the use of water is
nonconsumptive. That is, the water is returned to the river for communities downstream. Thus,
in many parts of the U.S., the single most important criterion used in classifying water as
“polluted” is the count of coliform bacteria—related to the colon and found in fecal matters.
So in wastewater treatment, in addition to removing degradable organic materials, BOD, and
other suspended solids (to prevent sediment formation), we also need disinfection. To avoid
eutrophication, we also need to remove inorganic nutrients. Sulfates are removed to prevent
anaerobic bacteria from converting it to hydrogen sulfide.
Preliminary and primary treatments remove the
large debris and easily settled solids.
Treatment plants are designed to remove pollutants in stages: (1) suspended solids, (2)
dissolved biodegradable organic material, and (3) inorganic nutrients. These stages are
commonly referred to as primary, secondary and tertiary treatments. The regulated targets are
the removal of greater than 90% of suspended solids, greater than 90% of BOD, and 80% of
phosphate.
To prevent sewage spills, the municipal collection system is also very important. In older
cities or less developed towns, domestic wastewater pipes and storm drains are interconnected
or combined into the same collection lines. Under heavy rainfall, the sewer lines and the
treatment facility can become overloaded and untreated sewage will overflow into natural
waters. Upgrading the collection system is very costly.
Preliminary treatment removes large debris with a bar screen with about 1 inch spacing.
A scraper later removes the debris from the bars. The wastewater continues onto a settling
tank (grid chamber). The flow of water is roughly 1 foot per second, sufficient to settle solids
(grit) such as sand and coffee grounds. Afterward, the wastewater flows through a shredder to
make sure no solid particles are larger than 1/4 inch. The solids is called the sludge.
The primary treatment follows. The wastewater flow is slowed to 1 to 2 inches per
minute in another sedimentation tank. This stage provides a sufficiently quiescent condition
for 1 to 2 hours such that about 50% of the suspended solids settle out as raw sludge, which
will further be treated later. This accounts for the removal of 40-50% of the biodegradable
pollution (BOD) in the wastewater. Grease and oily material (called skimmings) are skimmed
off the top.
At this point, the treatment is quite simple and “low-tech,” but we have removed a good
amount of the solids and organic materials at minimal cost. Further treatment is required for
the rest of the pollutants.
Secondary treatment uses biological
processes to reduce the BOD.
The major objective of secondary treatment is to reduce the BOD by removing dissolved
organic matter. While it is possible to use chemical processes, the almost universally accepted
method is a biological treatment which imitates the natural detritus feeding community. A
mixture of aerobic decomposers and detritus feeders is used, and they are referred to as the
microbes for short. They are either freely suspended or attached to a solid surface. They feed
on the biodegradable organic matter, including coliform and pathogenic bacteria, and require
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a high level of dissolved oxygen to grow. (In a way, we are providing the oxygen in the
treatment facility to remove the oxygen-demanding wastes so they won't do the same thing in
the natural environment.) After the treatment, 90% or more of the total organic matter can be
removed.
Biological secondary treatment system consists of what are called aerated or activated
sludge tanks. Microbes are mixed thoroughly with the wastewater in vigorously aerated tanks
(that is with air pumped in) where biodegradation occurs. The wastewater enters another
settling tank (secondary clarifier) in which the activated sludge settles out. It is called
“activated” because it contains the microbes. Some of the sludge is recycled back to the
aeration tank to replenish the microbes. In the secondary clarifier, the particles settle out
mostly by flocculation or coagulation. This treatment method can handle a high volume of
wastewater.
Some other secondary systems that may work well in small municipals are:
• Lagoons (or marshland). They require large areas and thus only work for smaller towns.
• Trickling filters. Tanks (about 6 to 8 feet deep, often dug into the ground) filled with solid
rocks on which the microbes are attached. Wastewater is sprayed on top and trickles down
and over the attached microbes.
• Rotating biological contactor. Large drums or disks with a rough surface on which
microbes attached are half submerged in a tank of wastewater. The drum rotates slowly and
the microbes are thus alternately exposed to air and the wastewater.
Biological treatment is more expensive than a simple primary stage because it is energy
intensive to run the aeration (air) pumps, and it generates large volumes of biological sludge
which must be treated and disposed of.
At the end of the secondary treatment, only mostly inorganic nutrients (phosphates and
nitrates) should be left. Before discharging it into the environment, the wastewater is
generally disinfected by adding chlorine or ozone to kill any pathogenic organisms that may
still be present.
The treated wastewater can be discharged into rivers, streams, lakes, and oceans. In some
communities, the wastewater is used to recharge aquifers. When water seeps through soil,
bacteria are usually filtered out in about 50 feet or less.
Advanced treatment further reduces nitrates,
phosphates and trace organic compounds.
There are circumstances when we need to go beyond secondary treatment. Examples are
freshwater communities where eutrophication may be a problem. Advanced (or tertiary)
treatment must be used to remove the nutrients. In arid regions, we may want to reclaim the
water for reuse in agriculture, landscaping, and groundwater recharge.
Most advanced treatments are based on physical-chemical methods. A common method
to remove nutrients and inorganic compounds is chemical coagulation. To remove
phosphates, an inexpensive strategy is to add inorganic chemical coagulants such as ferric
chloride, sodium aluminate, aluminum sulfate, or calcium oxide. There are other chemical
methods to remove nitrogen compounds. Biological microbial processes can also be used to
remove phosphates and nitrates.
To remove trace organic compounds, we can use adsorption by activated carbon. Finally,
membrane technologies such as ultrafiltration and reverse osmosis can be used to purify
water to the extent that it is potable. (How else you think astronauts live in the space shuttle
and space station?)
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Sludge treatment processes the raw sludge
and activated sludge.
The raw sludge from primary treatment and activated sludge from secondary treatment are
usually treated further by anaerobic microbial digestion. The sludge is put into an airtight
tank heated to 35 °C where the organic material is first converted to organic acids by acidforming bacteria and subsequently to methane and carbon dioxide by methane-forming
bacteria. The methane generated, called “biogas,” is burned to drive gas turbines. The turbines
generate electricity to run the wastewater treatment plant.
Afterward, the treated sludge must still be disposed of. The most common method is to
bury it in sanitary landfills, but the treated sludge could be used as fertilizer or soil conditioner
in landscaping (but not agriculture for human consumption).
Another sludge treatment method is composting. Raw sludge is filtered, mixed with wood
chips to improve aeration, and piled in windrows, which are elongation piles lined up like
sand berms, and often in the direction of wind. A fan draws additional air through the
compost pile where aerobic decomposers and detritus feeders break down the organic material
and produce a humus-like material.
Alternatives to a treatment plant
An increasing number of communities are making use of treated nutrient-rich wastewater
(reclaimed water) in irrigation, watering lawns, parks, and golf courses. Some communities
simply use the water to recharge underground water. The nutrient-rich water can also be used
in aquaculture (e.g. fish and shellfish farming). As mentioned in the section on secondary
treatment, artificial or restored wetlands (e.g. Arcata, California) can be used as a treatment
plant in small towns. On the domestic scale, one can use septic tanks or reverse osmosis
systems.
Adsorption by activated carbon can remove
low level organic contaminants.
Carbon adsorption is effective in removing low level organic contaminants from either waste
water or air. This is the same technology used in gas masks and small household water
purification devices.
In simple terms, adsorption is a process where a molecule is adhered to a solid surface.
The adsorbed molecule is named the adsorbate. The solid surface is the adsorbent. The
universal choice of an adsorbent is activated carbon. One way to make it is to heat, for
example, coconut shells in the absence of oxygen such that it would be charred and form a
very porous granular solid with an extremely high surface area.
The now so-called activated carbon is put in canisters or large containers. As
contaminated water flows through the activated carbon, organic compounds are removed by
adsorption. The same set up can be used to remove organic compounds in contaminated air.
The activated carbon can be regenerated by using steam to drive off the adsorbed chemical
species.
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Membrane technologies are based on semi-permeable
membranes that exclude larger molecules.
The use of polymer membranes in water treatment is a well established, reliable technology.
The biotechnology and microelectronic industries use membrane filtration systems to make
ultra-pure water that they need. Closer to home, water vending machines outside a grocery
store and household tap water purifying systems work on the same principles. Using the same
technologies to remove contaminants, where economics is justified, is becoming more
common in the environmental field.
Membrane separation is based on selective permeation (Fig. 2). The membrane is porous,
made of polymers with well-define pores. By choosing a membrane with a certain pore size,
molecules and particles larger than the pores are excluded. Only water and molecules smaller
than the pores can diffuse through the membrane. For this reason, the membranes are termed
semi-permeable.
The range of pore size determines the actual application and specific terminology (Fig. 3).
Microfiltration applies to membranes with pores roughly 0.1 to 1 m and is intended to
remove larger particles. Membranes with pores of 0.2 m are used to remove bacteria. To
remove viruses and protein molecules, we need ultrafiltration membranes with finer pores.
Dialysis, as used in kidney dialysis machines, also uses membranes in this range. The term
nanofiltration may be used for membranes in between microfiltration and ultrafiltration.
Reverse osmosis (RO) membranes have pores less than 0.001 m in size and can remove
molecules as small as salt (sodium chloride).
Water and
smaller
particles
Larger
particles
Semi-permeable
membrane
Figure 2. Principle of membrane purification. A semi-permeable membrane with tiny
pores excludes larger particles such as bacteria and viruses. Only water and smaller
molecules can pass through the pores. In reverse osmosis, the pores are so tiny that
even salt molecules are rejected and only water molecules can pass through.
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Figure
3.
Different
membrane
separation
processes based on the
pore
size
of
the
membrane (lower panel).
The upper panel is a
rough estimate of the
sizes
of
different
substances.
In physics, reverse osmosis is different from other membrane technologies. A classic
application of reverse osmosis is desalination of seawater (Fig. 4). If unimpeded,
thermodynamics driving force will cause water to flow into the salt solution compartment. As
the name suggests, in reverse osmosis, we apply pressure in the solution compartment to force
water flow back toward the water compartment. The result is that we have more clean water,
while the solution becomes more concentrated in salt.
In reality, there are not too many commercial desalination plants. The problem is the high
pressure (up to 50 atmospheres) that we need for desalination, and such high pressure
translates to expensive energy cost. Nevertheless, concentrations of contaminants in
wastewater are usually low enough that we may operate reverse osmosis processes around 10
atmospheres. This lower pressure range allows this technology to be economically feasible in
the treatment of wastewater.
Figure 4. Illustration of
reverse osmosis. When
we apply pressure higher
than the osmotic pressure
of a solution, we can force
water to flow into the pure
water compartment.