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Wastewater Treatment:
Understanding the
Activated Sludge Process
The activated sludge process is used to treat
waste streams that are high in organics
and biodegradable compounds. Here are
some principles of design and operation.

Mark Sustarsic
Tetra Tech NUS

he activated sludge process (Figure 1) uses a mass of

microorganisms (usually bacteria) to aerobically treat
wastewater. Organic contaminants in the wastewater
provide the carbon and energy required to encourage microbial growth and reproduction; nitrogen and phosphorous are
sometimes added to promote this growth. The wastewater
is thoroughly mixed with air in an aeration tank, and the
organic matter is converted into microbial cell tissue and
carbon dioxide.
The microbial mass together with the wastewater is
referred to as a mixed liquor. After a specified time in the
aeration tank, the mixed liquor passes into a settling tank, or
clarifier, where the biomass, or sludge, settles by gravity from
the treated wastewater. This continuous process produces an
effluent that may be treated further or discharged directly to a
publicly owned treatment works (POTW).
The activated sludge process was developed in the early
1900s for the treatment of domestic wastewaters, and it has
since been adapted for removing biodegradable organics from
industrial wastewaters. Today, it is widely employed in the
treatment of both municipal and industrial wastewaters.

Applicability chemicals, concentrations, facilities

Activated sludge treatment is suitable for use at facilities
that have organic chemical waste. These may include municipal sewage-treatment plants, oil refineries, pulp and paper
mills, food processing plants, and chemical manufacturing
facilities.
When properly designed, the activated sludge process
provides good treatment for wastes containing biodegradable
compounds. It is generally used to treat biodegradable organic
wastewaters of moderate (101,000 ppm) to high (>1,000
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ppm) strength. Performance varies depending on the specific

chemicals present and their concentrations, as shown in Table
1. The results in Table 1 are based on laboratory-scale testing
and should be considered only as guidelines for developing
pilot-scale and full-scale studies.
Activated sludge treatment is not intended to remove metals or dissolved or suspended solids. A rather high concentration (>100 ppm) of organics is needed to maintain proper
operation. Low-strength industrial effluent and moderatestrength domestic wastewater may be combined for treatment
to achieve the required concentration. Wastewater containing
a wide variety of biodegradable organics can be treated by the
activated sludge process, as long as the concentration is in the
Table 1. The performance of activated sludge
treatment varies with the type of chemicals in
the waste being treated.
Chemical Type Treated

Typical Feed
Concentration, ppm

Typical
Removal

Phenolics (aromatic OH)

100 to 1,000

>99%

Polynuclear Aromatic
Hydrocarbons (PAHs)

0.01 to 1

>95%

Oil and Grease

10 to 100

40 to 92%

Total Organic Carbon

(TOC)

500 to 3,000

>90%

Ammonia (N)

10 to 1,000

>95%

Cyanide, Thiocyanide

1 to 1,000

40 to 98%

Metals
(arsenic, mercury, zinc)

0.5

Settles with
sludge

5 to 1,000

>99%

Benzene, Toluene, Xylene

(BTX)

hundreds of parts per million.

Many remediation sites require the treatment of groundwater, surface water, or landfill leachate with very low organic
concentrations, typically less than 10 ppm. This is not high
enough to sustain the biological mass needed, and such
streams are more appropriately treated by ozone, ultraviolet
light, trickling filters, and/or API separators. Some plants
combine groundwater, boiler blowdown and stormwater
runoff with process wastewater and feed the combined stream
to an activated sludge treatment facility.

erated by the aerators can be contained by anti-foam sprays.

The pH of the incoming wastewater or mixed liquor
must be steadily controlled for proper microorganism
growth. A neutral pH is maintained by adding a caustic or
acid, usually 25% by weight in solution. Supplemental nutrients in the form of nitrogen (urea) and phosphorus (phosphoric acid) are added on an as-needed basis, such as when
the raw wastewater is an insufficient source of required
nutrients.
In colder climates, supplemental heat is required to maintain an acceptable temperature range for the microorganisms.
Indirect heat is added to the base of the aeration basin via a
steam bayonet or a gas-fired heater. Some plants save energy
by first preheating the influent with a hot process stream
(e.g., distillation column bottoms) in a heat exchanger.
After spending a specified amount of time in the aeration
basin, water overflows into a clarifier, where the microbial
solids settle by gravity and are separated from the treated
water. The clarifier consists of a tank with a center feed well,
which promotes even dispersion. The settled sludge is raked
from the quiescent zone at the bottom of the clarifier with
mechanical scrapers. A portion of the biomass is recycled
back to the aeration basin to maintain the desired concentration of organisms in the basin. The remaining sludge is
pumped from the system, typically by a motor-driven centrifugal pump equipped with a flowmeter, for final disposal.
Floating materials that collect at the surface of the liquid in
the clarifier are collectively referred to as scum. A skimmer
constantly moves the scum toward a collection trough that
drops it into a receiver for later removal. Both the sludge
scraper and the scum skimmer are connected to a common

The process
Before it enters the aeration basin, raw wastewater (influent) is normally pretreated to remove easily settled solids
or other suspended matter, such as oil. Raising the pH will
slightly solidify iron and zinc (if present). A steam or air
stripper can remove ammonia, phenol, and any light ends,
while a filter removes heavy particulates. Because substantial changes in the incoming water characteristics will cause
a process upset, an equalization tank is sometimes included
in the pretreatment system to achieve the required design
inlet conditions, such as a certain flowrate or chemical
concentration.
The pretreated influent is pumped into the aeration tank
(or basin), where it is combined with the microorganisms.
In order to maintain the bacterial mass in the aeration basin,
aerators perform two important functions: they keep the liquid
and sludge agitated, and they promote the transfer of oxygen
into the wastewater. The aerators run continuously and are
strategically positioned to keep the mixed liquor in suspension
throughout and avoid the formation of dead zones. Foam gen-

Surface and/or
Deep Impellers

Raw Influent

Supplemental Nutrient
(N, P) Addition
(if required)

Recycle
Sludge

pH Control
(if required)
Pretreatment is Usually Required
to Remove Suspended Materials,
Oils, etc. by Primary Settling
and/or Other Physical Means

Slude Disposal
or Reuse

Final Settling
(Clarified)

Waste Sludge
Aeration
Tank

Supplemental Heat
(if required)

Oxygen Supply

Additional Treatment
(Thickening,
Dewatering, etc.)
(if required)

Treated Effluent to:

Additional
Treatment
(Chlorination,
Filtration, etc.)
Disposal
(POTW, Surface
Water, etc.)
Reuse

Disposal

S Figure 1. Conventional activated sludge systems process raw influent in the aeration tank before sending it to the clarifier for settling.

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Back to Basics

drive shaft powered by a small motor.

Clean water travels to the outside edge of the clarifier
and is discharged under a baffle and over a notched weir that
extends around the periphery. The concentration of total suspended solids (TSS) in the effluent discharge is low, 10100
ppm. This water may go to a POTW, be discharged as surface
water in accordance with a National Pollutant Discharge
Elimination System (NPDES) permit, or receive tertiary treatment such as by activated carbon adsorption.
Wasted sludge from the bottom of the clarifier is thickened, typically in a vacuum filter press, prior to its ultimate
disposal. The degree of wasting determines the solids retention time (SRT) of the system. At a low wasting rate, the
SRT would be relatively high i.e., the solids remain in the
system longer.

System design
Table 2 lists key design parameters that are determined by
lab tests and past experiences.
The first design methods were formulated based on the
retention time of the wastewater in the aeration basin. Generally, short hydraulic retention times (HRT) were chosen for
weak (dilute) wastewaters and long HRTs were chosen for
Table 2. Typical characteristics of wastewater
can be addressed in the design process
by examining these variables.
Wastewater
Characterization

Design Variables

Flowrate

Hydraulic Retention Rime (HRT)

Water Quality

Solids Retention Time (SRT)

Toxic or Inhibitory
Compounds

Recycle Ratio (r)

Optimization of HRT, SRT and r

strong (concentrated) wastewaters. HRT (measured in days)

is defined as the aeration tank volume (V, gal) divided by the
discharge rate (Q, gal/d):
HRT = V/Q

(1)

Typical HRTs range from 15 d for municipal waste to

110 d for industrial waste.
Around 1970, other design parameters were developed
using fundamental scientific concepts, such as mass balance
and microbial growth kinetics. Although design equations
ultimately serve as the primary basis for the design of an
activated sludge system, pilot plant and/or laboratory tests
are essential for developing the design criteria. The resulting
design reflects the reaction rates, settling rates, solids retention time, and oxygen requirements for a particular type of
wastewater. The rate equations somewhat resemble those
used by chemical engineers to describe reactor kinetics, suggesting that an aeration tank may be envisioned as a biological reactor.
The solids retention time is an important design factor and
provides a theoretical indication of how long the microorganisms remain in the aeration tank. Since these microorganisms
normally reproduce more quickly than they die, some activated sludge must be removed, or blown down, via a recycle
line. Blowdown, also called sludge wasting, should be carried
out daily to maintain the proper SRT. Without such wasting,
the mass of sludge could build up in the settling tank (clarifier) until solids overflow into the effluent.
The SRT (in days) is a function of the aeration tank volume
(V, gal), the sludge wasting rate (w, gal/d), and the concentrations of suspended solids in the mixed liquor in the aeration
tank (MLTSS, g/L) and in the recycle line (MLTSSr, g/L):

Oxygen Requirements
Bacterial Growth and Decay Factors
Sludge Settling Factors

SRT = (V MLTSS) / (w MLTSSr)

(2)

SRTs for municipal wastewater treatment range from

Table 3. When upsets occur, follow these suggested guidelines.

Problem

Probable Cause

Low or high pH in the aeration basin

Low dissolved O2 (DO) concentration in

the aeration tank

High total suspended solids (TSS)

concentration in POTW

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Remedy

Extremely high or low pH in influent

Add alkali or acid to the aeration tank

No biological activity

Reduce high concentrations of phenols and

oils, or add phosphorous or dissolved oxygen
to the aeration tank

Influent feed rate is too high or has slug of

organics

Reduce feed until DO concentration goes

above 1.0 mg/L

Concentration of total suspended solids in

the mixed liquor (MLTSS) is too high

Lower SRT by wasting more sludge

Presence of unwanted bacteria

Increase DO or reduce influent organics by

reducing feed flowrate

Clarifier is overloaded

Determine whether the recycle ratio and feed

rates are properly set

November 2009 CEP

5 to 20 d, whereas SRTs for industrial waste range from

20 to 1,000 d.
Another important design factor is the recycle ratio, r:
r = q/Q

(3)

where q is the recycled sludge flowrate (gal/d) and Q is the

influent wastewater flowrate (gal/d). Recycle ratios are generally 0.53.0 for both municipal and industrial wastewater
treatment plants.
HRT, SRT and r are interrelated and are the most important design variables. Combined with sludge settling characteristics, they determine the size of the aeration basin, sludge
settling tank, and related pumps. The recycle ratio also affects
the size of the clarifier.
Other important factors that determine the final design
include the bacterial growth coefficient (Y) and the decay
coefficient (b), each of which may be determined through
laboratory treatability testing. The bacterial growth coefficient is defined as the mass of bacteria produced per mass
of chemical consumed, and it has a value between 0.1 and
0.5. The b coefficient indicates the decay rate of the bacterial
organisms. Typical values range between 0.01 and 0.05 d1.
The decay rate does not have as much influence on wastewater treatment system design as the bacterial growth rate.

routinely checked for mixed liquor volatile suspended solids

(MLVSS, mg/L) to ensure that an adequate concentration of
organisms is present.

Final thoughts
It is important to understand that most of the design
considerations discussed here are site-specific. This requires
the collection of samples from the wastewater to be treated,
as well as performing lab analysis and onsite testing to
determine the relevant parameters for designing an activated
CEP
sludge system.
MARK SUSTARSIC is a senior process engineer at Tetra Tech NUS (Address:
Foster Plaza 7, 661 Andersen Dr., Pittsburgh, PA 15220; Phone: (412)
921-7090; Fax: (412) 921-4040); E-mail: mark.sustarsic@tetratech.
com), where he is involved in site investigation, project engineering,
detail design, construction supervision, system commissioning and
start-up, and customer training. His 33-yr career includes experience
with industrial wastewater treatment at steel, chemical, coal-based tar,
wood treatment and semiconductor facilities. He holds P.E. licenses
in six states and is a member of the National Council of Examiners
for Engineering and Surveying (NCEES), as well as Carnegie Mellons
Pittsburgh Clan and Eastern States Blast Furnace and Coke Oven Association. He earned BS degrees in chemistry and chemical engineering
from Carnegie Mellon Univ.

Operating conditions
Operation of an activated sludge process relies on
maintaining an acceptable environment for microbial activity
through routine monitoring and preventive maintenance, and
on the ability to survive any process upsets. The following
performance considerations may be relevant daily, weekly or
monthly.
Effluent. Limits on effluent water quality are determined
by the state or local environmental agency. The NPDES permit generally specifies the allowable discharge flowrate, pH,
total organic carbon (TOC, mg/L), and TSS (mg/L).
Influent. To ensure smooth operations and avoid upsets,
the plant should maintain the influent flowrate, temperature,
pH, and concentrations of TOC, TSS, oil and grease, and
phenol (which can cause microbial shock) within specified
ranges. Values that exceed design limits indicate that pretreatment modifications are necessary. For example, excess oil
and grease in the incoming water may indicate that the API
separator polymer-addition step needs to be modified.
Mixed liquor in the aeration tank. The aeration tank must
maintain the proper environment for microbial activity. The
pH should be between 6.5 and 7.5, the temperature between
15oC and 30oC, the dissolved oxygen (DO) concentration
above 2.0 mg/L, and nitrogen and phosphorous at the necessary levels. Aeration tanks have pH and DO monitors to automatically record these data. The aeration tank should also be
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