Sunday, May 9, 2010

When people think of sand filtration, they automatically relate to municipal water
treatment facilities. In general that’s the arena that this classical filter has ruled, but
it most certainly has found applications in pure industrial settings, oftentimes for
niche applications where suspended solids and organic matter persist in process
waters. Very common applications that I have seen in Eastern Europe have been
part of cooling and process water treatment plants, particularly for large cooling
tower applications in refineries and coke chemical plants, where biological growth
problems can adversely impact on heat exchange equipment. Also, in many former
Soviet block republics, municipal water treatment plants were almost always part
of large industrial complexes, so that both the communities and plant water
treatment requirements were met by a single operation. This leads to a very distinct
set of problems that we in the U.S. and parts of Western Europe don’t face,
because of the separation of operations, and one which is way beyond the scope of
this volume.
Sand filtration is almost never applied as the primary treatment method. Most often
it is a pretreatment or final stage, but sometimes intermediate stage of water
treatment, and is most often used along with other filtration technologies, carbon
adsorption, sedimentation and clarification, disinfection, and biological methods.
The term sand filtration is somewhat misleading and stems from older municipal
wastewater treatment methods. While there is a class of filtration equipment that
relies principally on sand as the filter media, it is more common to employ multiple
media in filtration methods and equipment, with sand being the predominant media.
In this regard, the terms sand filtration and granular media filtration are considered
interchangeable in our discussions. The design, operation and maintenance of these
systems are very straightforward, and indeed may be viewed as the least complex
or simplest filtration practices that exist. It is a very old technology and much of WATER AND WASTEWATER TREA”MJ3h’T TECHNOLOGIES
look at this technology and then try some Questionsfor Thinking and Discussing.
Remember to refer to the Glossary at the end of the book if you run across any
terms that are unfamiliar to you.
Before getting into the subject of sand filtration, we should first attempt to put the
technologies of municipal wastewater treatment into some perspective. Wastewater
treatment plants can be divided into two major types: biological and
physicalkhemical. Biological plants are more commonly used to treat domestic or
combined domestic and industrial wastewater from a municipality. They use
basically the same processes that would occur naturally in the receiving water, but
give them a place to happen under controlled con&tions, so that the cleansing
reactions are completed before the water is discharged into the environment.
Physical/chemical plants are more often used to treat industrial wastewaters
directly, because they often contain pollutants which cannot be removed efficiently
by microorganisms-- although industries that deal with biodegradable materials,
such as food processing, dairies, breweries, and even paper, plastics and
petrochemicals, may use biological treatment. Biological plants generally use some
physical and chemical processes also.
A physical process usually treats suspended, rather than dissolved pollutants. It may
be a passive process, such as simply allowing suspended pollutants to settle out or
float to the top naturally-- depending on whether they are more or less dense than
water. Or the process may be aided mechanically, such as by gently stirring the
water to cause more small particles to bump into each other and stick together,
forming larger particles which will settle or rise faster-- a process known as
flocculation. Chemical flocculants may also be added to produce larger particles.
To aid flotation processes, dissolved air under pressure may be added to cause the
formation of tiny bubbles which will attach to particles.
Filtration through a medium such as sand as a final treatment stage can result in a
very clear water. In contrast -- ultrafiltration, nanofiltration, and reverse osmosis
(RO) are processes which force water through membranes and can remove colloidal
material (very fine, electrically charged particles, which will not settle) and even
some dissolved matter. Absorption (adsorption, techcally) on activated charcoal
is a physical process which can remove dissolved chemicals. Air or steam stripping
can be used to remove pollutants that are gasses or low-boiling liquids from water,
and the vapors which are removed in this way are also often passed through beds
of activated charcoal to prevent air pollution. These last processes are used mostly
in industrial treatment plants, though activated charcoal is common in municipal
plants, as well, for odor control.
Examples of chemical treatment processes, in an industrial environment, would be:
1. converting a dissolved metal into a solid, settleable form by precipitation
with an alkaline material like sodium or calcium hydroxide. Dissolved iron
downloaded from http://www.moitruongxanh.info
or aluminum salts or organic coagulant aids like polyelectrolytes can be
added to help flocculate and settle (or float) the precipitated metal.
converting highly toxic cyanides used in mining and metal finishing
industries into harmless carbon dioxide and nitrogen by oxidizing them
with chlorine.
destroying organic chemicals by oxidizing them using ozone or hydrogen
peroxide, either alone or in combination with catalysts (chemicals which
speed up reactions) and/or ultraviolet light.
In municipal treatment plants, chemical treatment-- in the form of aluminum or iron
salts-- is often used for removal of phosphorus by precipitation. Chlorine or ozone
(or ultraviolet light) may be used for disinfection, that is, killing harmful
microorganisms before the final discharge of the wastewater. Sulfur dioxide or
sulfite solutions can be used to neutralize (reduce) excess chlorine, which is toxic
to aquatic life. Chemical coagulants are also used extensively in sludge treatment
to thicken the solids and promote the removal of water. A conventional treatment
plant is comprised of a series of individual unit processes, with the output (or
effluent) of one process becoming the input (influent) of the next process. The first
stages will usually be made up of physical processes that take out easily removable
pollutants. After this, the remaining pollutants are generally treated further by
biological or chemical processes. These may convert dissolved or colloidal
impurities into a solid or gaseous form, so that they can be removed physically, or
convert them into dissolved materials which remain in the water, but are not
considered as undesirable as the original pollutants. The solids (residuals or
sludges) which result from these processes form a side stream which also has to be
treated for disposal.
Common processes found at a municipal treatment plant include:
Preliminary treatment to remove large or hard solids that might clog or damage
other equipment. These might include grinders (comminuters), bar screens, and grit
channels. The first chops up rags and trash; the second simply catches large
objects, which can be raked off; the third allows heavier materials, like sand and
stones, to settle out, so that they will not cause abrasive wear on downstream
equipment. Grit channels also remove larger food particles (i. e., garbage).
Primary settling basins, where the water flows slowly for up to a few hours, to
allow organic suspended matter to settle out or float to the surface. Most of this
material has a density not much different from that of water, so it needs to be given
enough time to separate. Settling tanks can be rectangular or circular. In either
type, the tank needs to be designed with some type of scrapers at the bottom to
collect the settled sludge and direct it to a pit from which it can be pumped for
further treatment-- and skimmers at the surface, to collect the material that floats
to the top (which is given the rather inglorious name of "scum".) The diagram
below shows the operation of a typical primary settling tank
downloaded from http://www.moitruongxanh.info
Secondary treatment, usually biological, tries to remove the remaining dissolved
or colloidal organic matter. Generally, the biodegradation of the pollutants is
allowed to take place in a location where plenty of air can be supplied to the
microorganisms. This promotes formation of the less offensive, oxidized products.
Engineers try to design the capacity of the treatment units so that enough of the
impurities will be removed to prevent significant oxygen demand in the receiving
water after discharge.
There are two major types of biological treatment processes: attached growth and
suspended growth.
In an attached growth process, the microorganisms grow on a surface, such as rock
or plastic. Examples include open trickling filters, where the water is distributed
over rocks and trickles down to underdrains, with air being supplied through vent
pipes; enclosed biotowers, which are similar, but more llkely to use shaped, plastic
media instead of rocks; and so-called rotating biological contacters, or RBC's,
which consist of large, partially submerged discs which rotate continuously, so that
the microorganisms growing on the disc's surface are repeatedly being exposed
alternately to the wastewater and to the air. The most common type of suspended
growth process is the so-called activated sludge system. This type of system
consists of two parts, an aeration tank and a settling tank, or clarijier. The aeration
tank contains a "sludge" which is what could be best described as a "mixed
microbial culture", containing mostly bacteria, as well as protozoa, fungi, algae,
etc. This sludge is constantly mixed and aerated either by compressed air bubblers
located along the bottom, or by mechanical aerators on the surface. The wastewater
to be treated enters the tank and mixes with the culture, which uses the organic
compounds for growth-- producing more microorganisms-- and for respiration,
which results mostly in the formation of carbon dioxide and water. The process can
also be set up to provide biological removal of the nutrients nitrogen and
phosphorus. Refer to Figure 1 for a simplified process flow sheet.
recycle t pump
Figure 1. Simplified process flow sheet of activated sludge process. downloaded from http://www.moitruongxanh.info
After sufficient aeration time to reach the required level of treatment, the sludge is
carried by the flow into the settling tank, or clarifier, which is often of the circular
design. (An important condition for the success of this process is the formation of
a type of culture which will flocculate naturally, producing a settling sludge and a
reasonably clear upper, or supernatant layer. If the sludge does not behave this
way, a lot of solids will be remain in the water leaving the clarifier, and the quality
of the effluent wastewater will be poor.) The sludge collected at the bottom of the
clarifier is then recycled to the aeration tank to consume more organic material. The
term "activated" sludge is used, because by the time the sludge is returned to the
aeration tank, the microorganisms have been in an environment depleted of "food"
for some time, and are in a "hungry", or activated condition, eager to get busy
biodegrading some more wastes. Since the amount of microorganisms, or biomass,
increases as a result of this process, some must be removed on a regular basis for
further treatment and disposal, adding to the solids produced in primary treatment.
The type of activated sludge system that has just been described is a continuous
flow process. There is a variation in which the entire activated sludge process take
place in a single tank, but at different times. Steps include filling, aerating, settling,
drawing off supernatant, etc. A system ldce this, called a sequencing batch reactor,
can provide more flexibility and control over the treatment, including nutrient
removal, and is amenable to computer control.)
Nutrient removal refers to the treatment of the wastewater to take out nitrogen or
phosphorus, which can cause nuisance growth of algae or weeds in the receiving
water. Nitrogen is found in domestic wastewater mostly in the form of ammonia
and organic nitrogen. These can be converted to nitrate nitrogen by bacteria, if the
plant is designed to provide enough oxygen and a long enough "sludge age" to
develop these slow-growing types of organisms. The nitrate whch is produced may
be discharged; it is still usable as a plant nutrient, but it is much less toxic than
ammonia. If more complete removal of nitrogen is required, a biological process
can be set up which reduces the nitrate to nitrogen gas (and some nitrous oxide).
There are also physicalkhemical processes
which can remove nitrogen, especially
ammonia; they are not as economical for
domestic wastewater, but might be suited for
an industrial location where no other biological
processes are in use. (These methods include
alkaline air stripping, ion exchange, and
"breakpoint" chlorination.)
Phosphorous removal is most commonly
done by chemical precipitation with iron or
aluminum compounds, such as ferric chloride
or alum (aluminum sulfate). The solids which
are produced can be settled along with other
sludges, depending on where in the treatment
train the process takes place. "Lime", or
~~ ~
The chemical formula for
limestone is CaCO, and
upon burning forms
calcium oxide (CaO),
which is known as burnt
lime. Calcium oxide, when
mixed with water, forms
calcium hydroxide
(Ca(OH)3. Calcium
hydroxide is used to treat
water as a coagulation aid
along with aluminum
sulfate. downloaded from http://www.moitruongxanh.info
calcium hydroxide, also works, but makes the water very alkaline, which has to be
corrected, and produces more sludge. There is also a biological process for
phosphorus removal, which depends on designing an activated sludge system in
such a way as to promote the development of certain types of bacteria which have
the ability to accumulate excess phosphorus within their cells. These methods
mady convert dissolved phosphorus into particulate form. For treatment plants
which are required to discharge only very low concentrations of total phosphorus,
it is common to have a sand filter as a final stage, to remove most of the suspended
solids which may contain phosphorus.
Disinfection, usually the final process before discharge, is the destruction of
harmful (pathogenic) microorganisms, i.e. disease-causing germs. The object is not
to kill every living microorganism in the water-- which would be sterilization-- but
to reduce the number of harmful ones to levels appropriate for the intended use of
the receiving water.
The most commonly used disinfectant is chlorine, which can be supplied in the
form of a liquefied gas which has to be dissolved in water, or in the form of an
alkaline solution called sodium hypochlorite, which is the same compound as
Either slow or rapidfiltration
(depends on size ofplantholume of
water considerations).
Rapid-sand filters force water ,
through a 0.45-lm layer of sand
(d=0.4-1.2mm) and work faster,
needing a smaller area. They need
frequent back-washing.
Slo w-sand filters (d=O. 15-0.35mm)
require a much larger area but
reduce bacteriological and viral
levels to a greater degree. The top 1
inch must be periodically scraped
off and the filter occasionally back-
common household chlorine
bleach. Chlorine is quite
effective against most
bacteria, but a rather high
dose is needed to kill viruses,
protozoa, and other forms of
pathogen. Chlorine has
several problems associated
with its.use, among them 1)
that it reacts with organic
matter to form toxic and
carcinogenic chlorinated
organics, such as chloroform,
2) chlorine is very toxic to
aquatic organisms in the
receiving water-- the USEPA
recommends no more than
0.011 parts per million
(mg/L) and 3) it is hazardous
to store and handle.
Hypochlorite is safer, but still
produces problems 1 and 2.
Problem 2 can be dealt with by adding sulfur dioxide (liquefied gas) or sodium
sulfite or bisulfite (solutions) to neutralize the chlorine. The products are nearly
harmless chloride and sulfate ions. This may also help somewhat with problem 1.
A more powerful disinfectant is ozone, an unstable form of oxygen containing three
atoms per molecule, rather than the two found in the ordinary oxygen gas which
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makes up about 21 % of the atmosphere. Ozone is too unstable to store, and has to
be made as it is used. It is produced by passing an electrical discharge through air,
which is then bubbled through the water. While chlorine can be dosed at a high
enough concentration so that some of it remains in the water for a considerable
time, ozone is consumed very rapidly and leaves no residual. It may also produce
some chemical byproducts, but probably not as harmful as those produced by
chlorine. The other commonly used method of disinfection is ultraviolet light. The
water is passed through banks of cylindrical, quartz-jacketed fluorescent bulbs.
Anything which can absorb the light, such as fouling or scale formation on the
bulbs' surfaces, or suspended matter in the water, can interfere with the
effectiveness of the disinfection. Some dissolved materials, such as iron and some
organic compounds, can also absorb some of the light. Ultraviolet disinfection is
becoming more popular because of the increasing complications associated with the
use of chlorine.
Sludge from primary settling basins, called primary or "raw" sludge, is a noxious,
smelly, gray-black, viscous liquid or semi-solid. It contains very high
concentrations of bacteria and other microorganisms, many of them pathogenic, as
well as large amounts of biodegradable organic material. Because of the high
concentrations, any dissolved oxygen will be consumed rapidly, and the odorous
and toxic products of anaerobic biodegradation (putrefaction) will be produced. The
greasy floatable skimmings from primary treatment are another portion of this
putrescible solid waste stream. In
addition to the primary sludge,
wastewater plants with secondary
treatment will produce a "secondary
sludge", consisting largely of
microorganisms which have grown
as a result of consuming the organic
wastes. While not quite so
objectionable, due to the
biodegradation which has already
taken place, it is still very high in
pathogens and contains much
material which will decay and
produce odors if not treated further.
Ultimately, the sludge must all be
disposed of. The way in which this
is done depends on the quality of
the sludge, and determines how it
needs to be treated. The most
desirable final fate for these solids
would be for beneficial use in
agriculture, since the material has
organic matter to act as a soil
G Disinfection - water completely
free of suspended sediment, is
treated with a powerful oxidizing
agent usually chlorine, chlorine and
ammonia (chloramine), or ozone. A
residual disinfectant is le# in the
water to prevent reinfection.
Chlorine can form harmful
byproducts and has suspected links
to stomach cancer and miscarriages.
GpH adjustment - so that treated
water leaves the phnt in the desired
range of 6.5 to 8.5pH units. downloaded from http://www.moitruongxanh.info
conditioner, as well as a some fertilizer value. This requires the highest quality
"biosolids", free of contamination with toxic metals or industrial organic
compounds, and low in pathogens. At a somewhat lower quality, it can be used for
similar purposes on non-agricultural land and for land reclamation (e.g.. strip
mines). Poorer quality sludge can be disposed of by landfilling or incineration.
One commonly used method of sludge treatment, called digestion, is biological.
Since the material is loaded with bacteria and organic matter; why not let the
bacteria eat the biodegradable material? Digestion can be either aerobic or
anaerobic. Aerobic digestion requires supplying oxygen to the sludge; it is similar
to the activated sludge process, except no external "food" is provided. In anaerobic
digestion, the sludge is fed into an air-free vessel; the digestion produces a gas
which is mostly a mixture of methane and carbon dioxide. The gas has a fuel value,
and can be burned to provide heat to the digester tank and even to run electric
generators. Some localities have compressed the gas and used it to power vehicles.
Digestion can reduce the amount of organic matter by about 30 to 70 percent,
greatly decrease the number of pathogens, and produce a liquid with an inoffensive,
"earthy" odor. This makes the sludge safer to dispose of on land, since the odor
does not attract as many scavenging pests, such as flies, rodents, gulls, etc., which
spread pathogens from the disposal site to other areas-- and there are fewer
pathogens to be spread.
A liquid sludge, which might contain 3 to 6% dry weight of solids, can be
dewatered to form a drier sludge cake of maybe 15 to 25 percent solids, which can
be hauled as a solid rather than having to be handled as a liquid. Equipment used
to dewater sludge includes centrifuges, vacuum filters, and belt presses or plate-
and-frame presses. Chemical coagulants are commonly added to help form larger
aggregates of solids and release the water. Further processes such as composting
and heat drying can produce a drier product with lower pathogen levels. Another
approach involves treatment with lime (calcium oxide), which kills pathogens due
r3 Heavy metal removal: most treatment
plants do not have special stages for
metals but rely on oxygenation,
coagulation and ion exchange infilters
to remove them. Where metals persist,
additional treatment would be needed.
r3 Troublesome organics: Activated
carbon filters are required where
soluble organic constituents are present
because many will pass straight through
standard plants, e.g. pesticides, phenols,
MTBE and so forth.
to its highly alkaline nature as
well as the heat that is generated
as it reacts with the water in the
sludge; this also results in a drier
product. A final disposal method
which eliminates all of the
pathogens and greatly reduces the
volume of the sludge is
incineration. This is not
considered a beneficial use,
however, and is becoming less
popular due to public concerns
over air emissions.
Sludges from physical-chemical
treatment of industrial waste
streams containing heavy metals downloaded from http://www.moitruongxanh.info
and non-biodegradable toxic organic compounds often must be handled as
hazardous wastes. Some of these will end up in hazardous waste landfills, or may
be chemically treated for detoxification-- or even for recovery of some components
for recycling. Recalcitrant organic compounds can be destroyed by carefully
controlled high-temperature incineration, or by other innovative processes, such as
high-temperature hydrogen reduction.
Granular media filtration is used for treating aqueous waste streams. The filter
media consists of a bed of granular particles (typically sand or sand with anthracite
or coal). The anthracite has adsorptive characteristics and hence can be beneficial
in removing some biological and chemical contaminants in the wastewater. This
material may also be substituted for activated charcoal.
The bed is contained within a basin and is supported by an underdrain system which
allows the filtered liquid to be drawn off while retaining the filter media in place.
As water containing suspended solids passes through the bed of filter medium, the
particles become trapped on top of, and within, the bed. The filtration rate is
reduced at a constant pressure unless an increase in the amount of pressure is
applied to force the wastewater through the filter bed. In order to prevent plugging
of the upper surface and uppermost depth of the bed, the filter is backflushed at
high velocity to dislodge the filtered particles. The backwash water contains high
concentrations of solids and is
sent to further treatment steps
within the watsewater treatment
The filter application is typically
applied to handling streams
containing less than 100 to 200
mg/Liter suspended solids,
depending on the required
effluent level. Increased-
suspended solids loading reduces
the need for frequent
backwashing. The suspended
solids concentration of the
filtered liquid depends on the
particle size distribution, but
typically, granular media filters
are capable of producing a
One of the reasons why it is important to
remove suspended solids in water is that
the particles can act as a source of food
and housing for bacteria. Not only does
this make microbiological control much
harder but, high bacteria levels increase
the fouling of distribution lines and
especially heat transfer equipment that
receive processed waters (for example, in
one’s household hot water heater). The
removal of suspended contaminants
enables chemical treatments to be at
their primary jobs of scale and corrosion
prevention and microbial control.

One of the reasons why it is important to
remove suspended solids in water is that
the particles can act as a source of food
and housing for bacteria. Not only does
this make microbiological control much
harder but, high bacteria levels increase
the fouling of distribution lines and
especially heat transfer equipment that
receive processed waters (for example, in
one’s household hot water heater). The
removal of suspended contaminants
enables chemical treatments to be at
their primary jobs of scale and corrosion
prevention and microbial control. downloaded from http://www.moitruongxanh.info
filtered liquid with a suspended solids concentration as low as 1 to as high as 10
mg/Liter. Large flow variations will affect the effluent’s quality.
Granular media filters are usually preceded by sedimentation in order to reduce the
suspended solids load on the filter. Granular media filtration can also be installed
ahead of biological or activated carbon treatment units to reduce the suspended
solids load and in the case of activated carbon to minimize plugging of the carbon
columns. Granular media filtration is only marginally effective in treating colloidal
size particles in suspensions. Usually these particles can be made larger by
flocculation although this will reduce run lengths. In cases where it is not possible
to flocculate such particles (as in the case of many oil/water emulsions), other
techniques such as ultrafiltration must be considered. Figure 2 illustrates a common
sand filter that most people are familiar with in swimming pool applications. Such
systems rely on very fine sand media that can typically remove suspended particles
about 0.5 pm in size. Filtration is an effective means of removing low levels of
solids from wastes provided the solids concentration does not vary greatly and the
filter is backwashed at appropriate intervals during the filtration cycle. The
operation can be easily integrated with other treatment steps, and further, is well
suited to mobile treatment systems as well as on-site or fixed installations. In short,
sand filtration technologies, although simple, are quite versatile in meeting
treatment challenges.
Figure 2. A simple sand filtration unit. downloaded from http://www.moitruongxanh.info
A typical multi-media sand filtration unit is shown in Figure 3. In this configuration
a coarse layer of media is used to reduce the contaminant loading to the final layer.
This allows multimedia filters to use finer media. Such units generally remove
suspended solids down to about 15 pm, and they require large volumes of water to
properly remove contaminant that is trapped deep within the bed. Often
manufactures of these types of systems claim 90 % removal of 0.5 pm particles and
larger. This can be a misleading statement as quite often only about 5 % of the 0.5
pm particles will be removed. Grouping the 0.5 pm particles with much larger
particles allows the claim to be met by removing a few large volume particles from
the tower sump, even though the vast majority of fine particles remain to foul heat
exchange equipment.
A typical physical-chemical treatment system incorporates three "dual" medial (sand
anthracite) filters connected in parallel in its treatment train. The major maintenance
consideration with granular medial filtration is the handling of the backwash. The
backwash will generally contain a high concentration of contaminants and require
subsequent treatment.
Figure 3. Mulimedia sand filter. downloaded from http://www.moitruongxanh.info
In this application, the operations of precipitation and flocculation play important
roles. Precipitation is a physiochemical process whereby some, or all, of a
substance in solution is transformed into a solid phase. It is based on alteration of
the chemical equilibrium relationships affecting the solubility of inorganic species.
Removal of metals as hydroxides and sulfides is the most common precipitation
application in wastewater treatment. Lime or sodium sulfide is added to the
wastewater in a rapid mixing tank along with flocculating agents. The wastewater
flows to a flocculation chamber in which adequate mixing and retention time is
provided for agglomeration of precipitate particles. Agglomerated particles are then
separated from the liquid phase by settling in a sedimentation chamber, and/or by
other physical processes such as filtration.
Precipitation is often applied to the removal of most metals from wastewater
including zinc, cadmium, chromium, copper, fluoride, lead, manganese, and
mercury. Also, certain anionic species can be removed by precipitation, such as
phosphate, sulfate, and fluoride. Note that in some cases, organic compounds may
form organometallic complexes with metals, which could lnhibit precipitation.
Cyanide and other ions in the wastewater may also complex with metals, making
treatment by precipitation less efficient. A cutaway view of a rapid sand filter that
is most often used in a municipal treatment plant is illustrated in Figure 4. The
design features of this filter have been relied upon for more than 60 years in
municipal applications.
Figure 4. Cutaway view of a rapid sand filter. downloaded from http://www.moitruongxanh.info
Fecal coliform
Biochemical Oxygen
Total Suspended
Demand (BOD)
Solids (TSS)
Percent Pollutant Percent
Removal Removal
76 Total Organic 48
Carbon (TOC)
70 Total Nitrogen (TN) 21
70 Iron, Lead, Zinc 45
A typical sand filter system consists of two or three chambers or basins. The first
is the sedimentation chamber, which removes floatables and heavy sediments. The
second is the filtration chamber, which removes additional pollutants by filtering
the runoff through a sand bed. The third is the discharge chamber. The treated
filtrate normally is then discharged through an underdrain system either to a storm
drainage system or directly to surface waters. Sand filters are able to achieve high
removal efficiencies for sediment, biochemical oxygen demand (BOD), and fecal
coliform bacteria. Total metal removal, however, is moderate, and nutrient removal
is often low. Figure 5 illustrates one type of configuration. Typically, sand filters
begin to experience clogging problems within 3 to 5 years. Accumulated trash,
paper, debris should be removed every six months or as needed. Corrective
maintenance of the filtration chamber includes removal and replacement of the top
layers of sand and gravel as they become clogged. Table 1 provides some typical
removal efficiencies for specific pollutants.
GraIed Cover Solid Coni
&ale (Fabric Wrappd
Over Entire Grate Opening)
Figure 5. Example of a sand filter configuration.
Table 1. Typical removal efficiencies. downloaded from http://www.moitruongxanh.info
We will be examining these subjects in a little more detail in the next chapter. But
for now, we should cover some of the basics because of their importance to sand
filtration. The process offlocculation is applicable to aqueous waste streams where
particles must be agglomerated into
larger more settleable particles prior to
sedimentation or other types of
treatment. Highly viscous waste streams
will inhibit the settling of solids. In
addition to being used to treat waste
streams, precipitation can also be used as
an in situ process to treat aqueous wastes
in surface impoundments. In an in-situ
application, lime and flocculants are
added directly to the lagoon, and mixing,
flocculation, and sedimentation are
allowed to occur within the lagoon.
Precipitation and flocculation can be
integrated into more complex treatment systems. The performance and reliability
of these processes depends greatly on the variability of the composition of the waste
being treated. Chemical addition must be determined using laboratory tests and
must be adjusted with compositional changes of the waste being treated or poor
performance will result.
Precipitation is nonselective in that compounds other than those targeted may be
removed. Both precipitation and flocculation are nondestructive and generate a
large volume of sludge which must be disposed of. Coagulation, flocculation,
sedimentation, and filtration, are typically followed by chlorination in municipal
wastewater treatment processes.
Coagulation involves the addition of chemicals to alter the physical state of
dissolved and suspended solids. This facilitates their removal by sedimentation and
filtration. The most common primary coagulants are alum ferric sulfate and ferric
chloride. Additional chemicals that may be added to enhance coagulation include
activate silica, a complex silicate made from sodium silicate, and charged organic
molecules called polyelectrolytes, which include large-molecular-weight
polyacrylamides, dimethyl-diallylammonium chloride, polyamines, and starch.
These chemicals ensure the aggregation of the suspended solids during the next
treatment step-flocculation. Sometimes polyclectrolytes (usually polyacrylamides)
are also added after flocculation and sedimentation as an aid to the filtration step.
Coagulation may also remove dissolved organic and inorganic compounds. The
hydrolyzing metal salts may react with the organic matter to form a precipitate, or
they may form aluminum hydroxide or ferric hydroxide floc particles on which the
organic molecules adsorb. The organic substances are then removed by
sedimentation and filtration, or filtration alone if direct filtration or inline filtration
is used. Adsorption and precipitation also removes inorganic substances.
The process of sedimentation involves the separation from water, by gravitational
settling of suspended particles that are heavier than water. The resulting effluent is
then subject to rapid filtration to separate out solids that are still suspended in the
water. Rapid filters typically consist of 24 to 36 inches of 0.5 to 1-mm diameter
sand and/or anthracite. Particles are removed as water is filtered through the media
at rates of 1 to 6 gallons/minute/square foot. Rapid filtration is effective in
removing most particles that remain after sedimentation. The substances that are
removed by coagulation, sedimentation, and filtration accumulate in sludge which
must be properly disposed of.
Coagulation, flocculation, sedimentation, and filtration will remove many
contaminants. Perhaps most important is the reduction of turbidity. This treatment
yields water of good clarity and enhances disinfection efficiency. If particles are not
removed, they harbor bacteria and make fmal disinfection more difficult.


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