INTRODUCTION and constructed for proper treatment of wastewater

INTRODUCTION

 

The science and engineering of
wastewater treatment has progressed tremendously over the last four or five
decades. As knowledge and understanding of the relationship between waterborne
pathogens and public health has increased, so has the impetus for innovation of
new technologies for treatment of wastewater. In the last century, population
growth and industrialization have resulted in significant degradation of the
environment. Disposal of untreated wastes and wastewater on land or in streams
and rivers is no longer an option. Newer regulations are aimed at protecting
the environment as well as public health. Wastewater engineering has come a
long way from the time when city residents had to place night soil (fecal
waste) in buckets along the streets, and workers collected the waste and
delivered it to rural areas for disposal on agricultural lands. With the
invention of the flush toilet, night soil was transformed into wastewater. It
was not feasible to transport these large liquid volumes for land disposal. So
cities began to use natural drainage systems and storm sewers to transport the
wastewater to streams and rivers, where it was discharged without any
treatment. The common notion was, “the
solution to pollution is dilution.” However, with increasing urbanization,
the self-purification capacity of the receiving waters was exceeded, causing
degradation of the water bodies and the environment. In the late 1800s and
early 1900s, various treatment processes were applied to wastewater. By the
1920s, treatment plants were designed and constructed for proper treatment of
wastewater prior to disposal. With newer and more stringent regulations,
existing processes are modified and innovative technologies are introduced to
achieve enhanced removal of pollutants.

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The objectives of wastewater treatment are to reduce

(1) the level of solids,

(2) the level of biodegradable organic matter,

(3) the level of pathogens,  

(4) the level of toxic compounds in the wastewater, to meet
regulatory limits that are protective of public health and the environment.

SOURCES OF WASTEWATER

The following are common sources
or types of wastewater:

•       
Domestic
or municipal wastewater: this includes wastewater discharged from residences,
institutions such as schools and hospitals, and commercial facilities such as
restaurants, shopping malls, etc.

•       
 Industrial wastewater: wastewater discharged
from industrial processes, e.g. pharmaceutical industry, poultry processing.

•       
 Infiltration and inflow: this includes water
that eventually enters the sewer from foundation drains, leaking pipes,
submerged manholes, and groundwater infiltration, among others.

•       
Storm
water: rainfall runoff and snow melt.

Municipal
wastewater is usually collected in sanitary sewers and transported to the
wastewater treatment plant. Storm water may be collected in separate sewer
lines called storm sewers. In some cities, especially older cities, storm water
is collected in the same sewer line as the domestic wastewater. This type of
system is called a combined sewer system. Each system has advantages and
disadvantages. Industrial wastewater may be treated on-site, or pretreated and
then discharged to sanitary sewers, after appropriate removal of pollutants.

WASTEWATER CONSTITUENTS:

The
major constituents of municipal wastewater are suspended solids, organic
matter, and pathogens. Nutrients such as nitrogen and phosphorus can cause
problems when present in high concentrations. In recent years, the presence of
EDCs (endocrine disrupting compounds) has been recognized as an area of
concern. Industrial wastewater can contain the above-mentioned contaminants, as
well as heavy metals, toxic compounds, and refractory organics. Stormwater may
contain petroleum compounds, silt, and pesticides when it includes urban runoff
and agricultural runoff. Suspended solids consist of inert matter such as rags,
silt, and paper, as well as food waste and human waste. Biodegradable organic
matter is composed of 40% to 60% proteins, 25% to 50% carbohydrates, and about
10% lipids. Proteins are mainly amino acids and contain nitrogen. Carbohydrates
are sugars, starches, and cellulose. Lipids include fats, oils, and grease. All
of these exert an oxygen demand. The constituents of industrial wastewaters
vary widely depending on the type of industry and the processes used in
manufacturing the product.

 

WASTEWATER TREATMENT METHODS

Wastewater
can be treated using any or a combination of the following types of treatment
methods, depending on the nature of pollutants and the level of desired removal.

1.   
Physical
treatment

Physical
treatment involves the removal of pollutants from the wastewater by simple
physical forces, e.g. sedimentation, screening, filtration. Physical treatment
processes are used mainly for removal of suspended solids.

2.   
Chemical
treatment

Chemical
treatment involves the addition of chemicals to achieve conversion or
destruction of contaminants through chemical reactions, e.g.
coagulation–flocculation for solids removal, disinfection for pathogen
destruction, chemical precipitation for phosphorus removal.

3.   
Biological
treatment

Biological
treatment involves the conversion or destruction of contaminants with the help
of microorganisms. In municipal wastewater treatment plants, microorganisms
indigenous to wastewaters are used in biological treatment operations. Examples
of biological treatment include activated sludge process, membrane bioreactor,
trickling filter, and others. The primary purpose of biological treatment is to
remove and reduce the biodegradable organic matter from wastewater to an
acceptable level according to regulatory limits. Biological treatment is also
used to remove nutrients such as nitrogen and phosphorus from wastewater.

 

 

 

LEVELS OF WASTEWATER TREATMENT

A wastewater treatment system is a combination of unit operations
and unit processes designed to reduce contaminants to an acceptable level. The
term unit operation refers to processes that use physical treatment methods.
The term unit processes refers to processes that use biological and/or chemical
treatment methods. Unit operations and processes may be grouped together to
provide the following levels of treatment

                  
I.           
Preliminary
Treatment

Preliminary treatment involves the physical removal of pollutant
substances such as rags, twigs, etc. that can cause operational problems in
pumps, treatment processes, and other appurtenances. Examples of preliminary
treatment are screens for removal of large debris, comminutor for grinding
large particles, grit chamber for removal of inert suspended solids, and
flotation for removal of oils and grease.

                  
II.           
Primary
treatment

 Primary treatment involves the physical
removal of a portion of the suspended solids from wastewater, usually by
sedimentation. Primary clarifiers are used for this purpose. Primary clarifier
effluent contains significant amounts of BOD and requires further treatment.
Primary treatment often includes preliminary as well as primary treatment
operations.

             
III.           
Enhanced Primary
Treatment

Enhanced primary treatment
involves the use of chemical treatment to obtain additional solids removal in a
sedimentation process. Chemical coagulants are used to promote coagulation and
flocculation of solids in a sedimentation tank, resulting in enhanced suspended
solids removal.

         
IV.       
Conventional Secondary Treatment

Conventional secondary treatment
involves biological treatment for degradation of organic matter and solids
reduction. Efficiency is measured mainly in terms of BOD5 and suspended solids
removal. Treatment is carried out in a biological reactor followed by a
sedimentation tank or secondary clarifier.

               
V.           
Secondary
Treatment With Nutrient Removal

When removal of nutrients ,such as
nitrogen or phosphorus, is required, it may be combined with the secondary
treatment for BOD removal. Additional reactors may be required to achieve
nitrogen removal through the nitrification–denitrification process. A
combination of chemical and biological treatment can be used.

         
VI.       
Tertiary Treatment

Tertiary treatment includes
treatment processes used after the secondary, e.g. granular media filtration
used for removal of residual suspended solids, and disinfection for pathogen
reduction. Additional treatment for nutrient removal is also included in
tertiary treatment.

       
VII.       
Advanced treatment

Advanced treatment processes are
used when additional removal of wastewater constituents is desired due to
toxicity of certain compounds, or for potential water reuse applications.
Examples include activated carbon adsorption for removal of volatile organic
compounds, ion exchange for removal of specific ions, etc.

 

RESIDUALS
AND BIOSOLIDS MANAGEMENT

Each of the treatment processes
described above generates a certain amount of waste solids. The waste generated
is semisolid in nature and is termed sludge. The waste generated from
preliminary treatment includes grit and screenings. These waste residuals are
low in organic content and are disposed of in landfills. The sludge generated
from primary and secondary clarifiers has a significant amount of organic
matter and requires further treatment and processing prior to disposal. The
term biosolids is used to denote treated sludge. The cost of treatment of
sludge and disposal of biosolids can be equivalent to 40% to 50% of the total
cost of wastewater treatment.

The main objectives of sludge
treatment are

(a) to reduce the organic content,

(b) to reduce the liquid fraction,

(c) to reduce the pathogen
content.

If the sludge contains heavy
metals or other toxic compounds, local or state regulations may require
additional treatment depending on the final disposal of the biosolids produced.
The liquid fraction is reduced by a number of processes. These include gravity
thickening, dissolved air flotation, centrifugation, belt filter press, etc.
Organic content and pathogen reduction is achieved by processes that include
anaerobic digestion, aerobic digestion, air drying, heat drying, thermophilic
digestion, composting, lime stabilization, pasteurization, etc. A combination
of these processes may be used depending on the quality of biosolids desired.
Over the last four decades, most of the research has focused on treatment of
wastewater, while treatment of sludge has lagged behind. The traditional method
of biosolids disposal in landfills is still used extensively. Land application
of biosolids is practiced in some areas. In recent years, the concept of
beneficial reuse of biosolids as a soil conditioner and fertilizer on
agricultural lands has gained importance, both from the viewpoint of green
engineering and necessity. As a result, we have seen increased research on
biosolids for the purpose of further reducing pathogens for safe reuse of the
product.

 

TYPES OF BIOLOGICAL TREATMENT PROCESSES

There are two main types of
wastewater treatment processes:

1.
Suspended growth process—The
microorganisms are kept in suspension in a biological reactor by suitable
mixing devices. The process can be aerobic or anaerobic. Examples of suspended
growth processes include activated sludge process, sequencing batch reactor,
ponds and lagoons, digesters, etc.

2.
Attached growth process—The microorganisms
responsible for bioconversion attach themselves onto an inert medium inside the
reactor, where they grow and form a layer called biofilm. The wastewater
flowing through the reactor comes in contact with the biofilm, where conversion
and removal of organic matter takes place. The inert medium is usually rock,
gravel, slag, or synthetic media. The process can be operated aerobically or
anaerobically. Examples are trickling filters, biotowers, and rotating
biological contactors (RBCs).

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