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Explaining
the Activated Sludge Process
It
is to everyone’s advantage for a community to be able to treat its
wastewater in the most economical way. The activated sludge process has
the advantage of producing a high quality effluent for a reasonable operating
and maintenance costs.
The activated sludge process uses microorganisms to feed on organic contaminants
in wastewater, producing a high-quality effluent. The basic principle
behind all activated sludge processes is that as microorganisms grow,
they form particles that clump together. These particles (floc) are allowed
to settle to the bottom of the tank, leaving a relatively clear liquid
free of organic material and suspended solids.
Described simply, screened wastewater is mixed with varying amounts of
recycled liquid containing a high proportion of organisms taken from a
secondary clarifying tank, and it becomes a product called mixed liquor.
This mixture is stirred and injected with large quantities of air, to
provide oxygen and keep solids in suspension. After a period of time,
mixed liquor flows to a clarifier where it is allowed to settle. A portion
of the bacteria is removed as it settles, and the partially cleaned water
flows on for further treatment. The resulting settled solids, the activated
sludge, are returned to the first tank to begin the process again.
Initially developed in England in the early 1900s, the activated sludge
process did not become widespread in the U.S. until the 1940s. Today a
number of variations of the basic process have been developed. This issue
of Pipeline includes descriptions of three of the most common variations:
Extended aeration, sequencing batch reactors, and oxidation ditches. A
glossary of terms can be found here.
The activated sludge plant is the most popular biological treatment process
for larger installations or small package plants being used today. These
plants are capable of producing a high quality effluent for the price.
Other advantages of the activated sludge process are the low construction
cost and the relatively small land requirement.
The activated sludge process is widely used by large cities and communities
where large volumes of wastewater must be highly treated economically.
Activated sludge process plants are good choices too for isolated facilities,
such as hospitals or hotels, cluster situations, subdivisions, and small
communities.
The process
A basic activated sludge process consists of several interrelated components:
• An aeration tank where the biological reactions occur
• An aeration source that provides oxygen and mixing
• A tank, known as the clarifier, where the solids settle and are
separated from treated wastewater
• A means of collecting the solids either to return them to the aeration
tank, (return activated sludge RAS), or to remove them from the
process (waste activated sludge WAS).
Aerobic bacteria thrive as they travel through the aeration tank. They
multiply rapidly with sufficient food and oxygen. By the time the waste
reaches the end of the tank (between four to eight hours), the bacteria
has used most of the organic matter to produce new cells.
The organisms settle to the bottom of the clarifier tank, separating from
the clearer water. This sludge is pumped back to the aeration tank where
it is mixed with the incoming wastewater or removed from the system as
excess, a process called wasting. The relatively clear liquid above the
sludge, the supernatant, is sent on for further treatment as required.
See Figure of typical
activated sludge process.
Sludge characteristics
By analyzing the different characteristics of the activated sludge or
the sludge quality, plant operators are able to monitor how effective
the treatment plant’s process is. Efficient operation is ensured
by keeping accurate, up-to-date records; routinely evaluating operating
and laboratory data; and troubleshooting, to solve problems before they
become serious. A wide range of laboratory and visual and physical test
methods are recommended. Principally, these include floc and settleability
performance using a jar test, microscopic identification of the predominant
types of bacteria, and analysis of various chemical parameters.
The treatment environment directly affects microorganisms. Changes in
food, dissolved oxygen, temperature, pH, total dissolved solids, sludge
age, presence of toxins, and other factors create a dynamic environment
for the treatment organisms. The operator can change the environment (the
process) to encourage or discourage the growth of specific microorganisms.
See the problem effect
table.
Food (organic loading) regulates microorganism numbers, diversity, and
species unless other factors limit it. It is important to maintain the
proper ratio of food to microorganisms (F:M) to ensure optimum operation.
Activated sludge consists of a mixed community of microorganisms, approximately
95 percent bacteria and 5 percent higher organisms (protozoa, rotifers,
and higher forms of invertebrates). Particular ones are considered indicator
microorganisms that can be observed using inexpensive microscopes. Significant
numbers of a particular species can indicate the condition of the process.
The most predominant microorganisms are aerobic bacteria, but there are
also substantial populations of fungi and protozoa. Rotifers and nematodes
are most frequently found in systems with long aeration periods.
Amoeboid forms, the flagellates, and the ciliates are the most common
protozoans in a working sludge. Amoeboids predominate in ‘young’
sludges, such as at plant start-up or after an upset, such as a shock
load (when a stronger than usual batch of influent comes into the plant).
Typically, little or no sludge forms at this time.
Flagellates are free-swimmers and predominate in light mixed liquors during
high food to microorganism conditions. Their presence usually indicates
poor effluent quality.
Free-swimming ciliates predominate as the F:M ratio decreases. Stalked
ciliates predominate when there is an abundance of bacteria. Effluent
and sludge quality are typically best when these types of microorganisms
predominate.
Filamentous bacteria can cause the sludge not to settle properly, a condition
called bulking, which causes clouds of billowing sludge rather than settling.
These bacteria flourish when the excess sludge is not removed at the proper
rate. Filamen-tous sludge bulking is a common problem at small, extended
aeration treatment plants.
Developing and maintaining good floc structure is critical for optimum
system performance. A multiple jar test is a procedure used to evaluate
the effectiveness of coagulants, optimum dosage for coagulation, concentration
of the coagulant aid and the most effective order in which to add various
chemicals. It consists of a multiple stirring apparatus with a variable-speed
drive. Samples are held in one- or two-liter jars or beakers.
The activated sludge samples are mixed and agitated for varying lengths
of time, and then allowed to settle. The nature and settling characteristics
of the floc are noted, as well as the clarity of the supernatant.
Chemical testing reveals sludge conditions and can warn of impending process
problems. Compliance with the plant’s National Pollutant Discharge
Elimination System (NPDES) permit requires specific chemical analyses.
Alkalinity, solids (total, suspended and dissolved), biological oxygen
demand, chemical oxygen demand, nitrogen and phosphorus are some of the
parameters that plant operators must monitor.
Variations of the Activated Sludge Technology
Package plants are pre-manufactured treatment facilities used to treat
wastewater. Usually designed to treat flows between 10,000 and 250,000
gallons per day, these are good choices for areas with a limited number
of people and small wastewater flows. These plants are options for small
communities or in such isolated locations as trailer parks, highway rest
areas, hospitals and prisons. Some of the most common types of package
plants use biological aeration processes: extended aeration, sequencing
batch reactors and oxidation ditches.
Extended aeration
The extended aeration process holds wastewater in an aeration tank for
18 hours or more and the organic wastes are removed under aerobic conditions.
Air may be supplied by mechanical or diffused aeration. Mixing is by aeration
or mechanical means.
This process operates at a high solids retention time (low F:M), resulting
in a condition where nitrification may occur. The microorganisms compete
for the remaining food. This highly competitive situation results in a
highly treated effluent with low solids production.
The wastewater is screened to remove large suspended or floating solids
before entering the aeration chamber, where it is mixed, and oxygen is
added. The solids settle out and are returned to the aeration chamber
to mix with incoming wastewater. The clarified wastewater flows to a collection
channel before being diverted to the disinfection system.
This is the process many package plants that schools, housing developments,
and small communities use. Due to the light food to microorganism loading,
extended aeration plants are considered one of the most stable wastewater
treatment processes.
The extended aeration process can accept periodic (intermittent) loadings
without upsetting the system. Extended aeration does not produce as much
waste sludge as other processes; however, wasting still is necessary to
maintain proper control of the process.
Sequencing batch reactors
The sequencing batch reactor (SBR) is considered a fill-and-draw activated
sludge system. The processes of equalization, aeration, and clarification
are all achieved in the same tank, unlike a conventional activated sludge
system, in which the same processes are accomplished in separate tanks.
Wastewater is added to the tank, treated to remove undesirable components,
and then discharged.
SBR systems consist of five common steps carried out in sequence: (1)
fill, (2) react (aeration), (3) settle (sedimentation/clarification),
(4) draw (the effluent is decanted), and (5) idle. Sludge wasting usually
occurs during the settling phase. The SBR acts as an equalization basin
when filling with wastewater, enabling the system to tolerate peak flows
or loads.
After passing through a screen to remove grit, the effluent enters a partially
filled reactor. Once the reactor is full, it performs like a conventional
activated sludge system without a continuous influent or effluent flow.
Aeration and mixing are discontinued after the biological reactions are
complete, the solids are allowed to settle, and the treated effluent (supernatant)
is removed. Excess solids are removed at any time during the cycle. See
Figure of SBR.
SBRs are typically used where flowrates are five million gallons per day
or less. Due to their relatively small footprints, they are useful in
areas where available land is limited. In addition, it is easy to modify
cycles within the system for nutrient removal if necessary. SBRs are also
cost effective if treatment beyond biological treatment, such as filtration,
is required. SBRs also offer a potential capital cost savings by eliminating
the need for clarifiers.
SBRs require a sophisticated level of maintenance due to the timing units
and controls. Depending upon the downstream processes, it may be necessary
to equalize effluent after leaving the SBR.
Oxidation ditches
The oxidation ditch is an extremely effective variation of the activated
sludge process, consisting of a ring or oval shaped channel equipped with
mechanical aeration devices, such as brush rotors or disc aerators. See
oxidation ditch Figure.
Oxidation ditches typically operate in an extended aeration mode with
long solids retention times (SRTs). Solids are maintained in suspension
as the mixed liquor circulates around the ditch.
Preliminary treatment involves bar screens and grit removal. Secondary
sedimentation tanks are used for most applications. Tertiary filters may
be required after clarification and disinfection. Re-aeration may be necessary
prior to final discharge.
Oxidation ditch process plants can be designed to achieve specific objectives
including nitrification, denitrification, and/or biological phosphorus
removal. And due to the constant water level and continuous discharge,
oxidation ditch technology is very reliable and does not cause an effluent
surge common to other biological processes, such as SBRs.
Oxidation ditches are more energy efficient than other similar processes,
so this technology can be a better choice for small communities and isolated
institutions over conventional treatment plants. But oxidation ditches
require a larger land area which sometimes limits their use in areas where
land costs are high.
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