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Water And Wastewater Treatment
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Water and wastewater Treatment Regarding the subject
Activated Sludge Process
Filtration
By Yousef Mehnatisorkhabi
Faculty of Technology
Environmental Engineering
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Table of Contents
Introduction.................................................................................................................................................... 3
Wastewater .................................................................................................................................................... 3
Classification................................................................................................................................................... 3
Activated Sludge Process ................................................................................................................................ 4
Bacteria and Microorganisms ......................................................................................................................... 4
Grit chamber and mixing flow ........................................................................................................................ 5
Settlement tank and filtration ........................................................................................................................ 5
Conclusion ...................................................................................................................................................... 6
References ...................................................................................................................................................... 7
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Introduction
ith the present rate of population growth, worlds population will rise up to 9 billion by 2000, which will cause the
severe shortage of fresh water in near future. Unfortunately, 97% of the worlds water is salt water; two thirds of the
remaining 3% are frozen while the remaining 1% of the worlds water supply is not evenly distributed, and the lack of
available water in such case is a huge problematic issue for developing countries. Finland is rich in surface waters, with a total of
187 888 lakes and ponds larger than 500 square meters, and rivers totaling 25 000 kilometers in length. Almost a tenth of the
countrys land area is covered by water. Finlands lakes contain only 235 cubic kilometers of water.Finland has achieved
satisfactory result in treating the water and wastewater by settling quantitative national water protection in specific timetable.
Finlands shallow lakes are easily contaminated by pollution. Even relatively low concentrations of excess nutrients, acidic
deposition or other harmful contaminants can easily disrupt their sensitive aquatic ecosystems.
Wastewater
Water treatment in total is a process that makes water more purify which may be usable for different reason.Wastewater is
thecommon daily used water and in order to be used again it must be treated carefully before it is released into another body of
water, so that it does not cause further pollution of water sources or even water-borne diseases. Wastewater comes from a
variety of sources. Everything that you flush down your toilet or rinse down the drain is wastewater. Rainwater and runoff, along
with various pollutants, go down street gutters and eventually end up at a wastewater treatment facility are accounted as
wastewater. Wastewater can also come from agricultural and industrial sources. Some wastewaters are more difficult to treat
than others; for example, industrial wastewater can be difficult to treat, whereas domestic wastewater is relatively easy to
treat.Waste water treatment can involve physical, chemical or biological processes or combinations of these processes depending
on the required outflow standards. It is to everyones advantage for a community to be able to treat its wastewater in the most
economical way.
Classification The classification in 2008 was carried out to meet the regulations
under the EU Water Framework Directive and related national
legislation. During the meeting that supervisors and directives of
Environmental ministry of Finland had, A target has been set that all
surface waters should have a good or excellent ecological status by
2015, and conditions in waters already classed as good or excellent
should not deteriorate.
W
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Ecological Status Rivers Lakes Coastal Waters
High, good 52% 87% 36%
Moderate, poor or bad 48% 13% 64%
The chemical state of surface waters is classified on the basis of environmental quality norms defined for 42 harmful or hazardous
substance and substance groups. The norms, which refer to annual average concentrations of the substances in aquatic
environments, were included in Government Decree 1022/2006 on Substances Dangerous and Harmful to the Aquatic
Environment. Some of the norms applied in evaluating the chemical state of water bodies have not yet been fully enacted in
official legislation, but they still serve as useful guidelines in the classification procedure.
Activated Sludge Process
The activated sludge process has the advantage of producing a high quality effluent for a reasonable operating and maintenance
costs. The microorganism are used in the activated sludge process 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. The advantages, disadvantages and the applications of this process are summarized in the following table
Technology Applications Advantages Disadvantages Activated Sludge Low concentration
Organics Some inorganics
Removal of dissolved constituents Low maintenance Destruction process Relatively safe Low capital costs Relatively easy to operate
Volatile emissions Waste sludge disposal High energy costs Susceptible to shock and toxins Susceptible to seasonal changes
Bacteria and Microorganisms
The particles (floc), in the activated sludge, are allowed to settle to the bottom of the tank, leaving a relatively clear liquid free of
organic material and suspended solids. 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 the settlement
is allowable. A portion of the bacteria is removed as it settles,
and the partially cleaned water flows on for further
treatment.Activated sludge contains a mixed community of
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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 populationsof fungi and protozoa.Rotifers and nematodes aremost frequently
found insystems with long aerationperiods.
Amoeboid forms, the flagellates, and the ciliates are the most common protozoan in a working sludge. Amoeboid predominate in
young sludge, 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. Filamentous sludge bulking is a
common problem at small, extended aeration treatment
plants.
Grit chamber and mixing flow
Generally two types of mixing regimes are of major interest
in activated sludge process: plug flow and complete mixing.
In the first one, the regime is characterized by orderly flow of
mixed liquor through the aeration tank with no element of
mixed liquor overtaking or mixing with any other element.
There may be lateral mixing of mixed liquor but there must be no mixing along the path of flow.
Grit chambers that usually use after screening part are of two types: mechanically cleaned and manually cleaned. In mechanically
cleaned grit chamber, scraper blades collect the grit settled on the floor of the grit chamber. Several mechanisms occur to elevate
the grit such as bucket elevators, jet pump and air lift. The grit washing mechanisms are also of several designs most of which are
agitation devices using either water or air to produce washing action. Manually cleaned grit chambers should be cleaned at least
once a week. The simplest method of cleaning is by means of shovel.
In complete mixing, all the contents of aeration tank are well mixed and uniform throughout. Thus, at steady state, the effluent
from the aeration tank has the same composition as the aeration tank contents.The type of mixing regime is very important as it
affects (1) oxygen transfer requirements in the aeration tank, (2) susceptibility of biomass to shock loads, (3) local environmental
conditions in the aeration tank, and (4) the kinetics governing the treatment process.Waste water is introduced into an aerated
tank ofmicro-organisms which are collectively referredto as activated sludge or mixed liquor.
Settlement tank and filtration Submerged diffused or surface mechanical aeration or combination are used to achieve the aeration that maintain the activated
sludge in suspension. Following a period of contactbetween the waste water and the activated sludge,the outflow is separated
from the sludge in asecondary settlement tank.The purpose of a secondary settlement tank is to: remove suspended solids; and
return settled sludge to the aeration tank.The sludge from the secondary settling tank following a trickling filter consists of
partially decomposed organic matter. It is usually dark brown and flocculent, more homogeneous in appearance, and has less odor
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than raw sludge. The excess sludge picked from the activated sludge process is also partially decomposed, is golden brown and
flocculent, and has a specific earthy odor. Both sludge, with further decomposition can become septic and cause offensive
odors.Disposal to digesters is very common, particularly with completely mixed units. The usage of some different filters happen
in the dewatering part as it is a part of activated sludge process. Dewatering requires exact control to avoid media plugging. In
dewatering part, different methods of filtration use are: Vacuum filtration, Pressure filtration, centrifugation and filter press.Filter
presses operate applying very high pressures to the cake (from 5 to 15 bars and sometimes even more).The vacuum filter for
dewatering sludge is a drum over which is laid the filtering medium consisting of a cloth of cotton, wool, nylon, dynel, fiber glass
or plastic, or a stainless steel mesh, or a double layer of stainless steel coil springs Skimming, however, could be added to a
vacuum filter after a sludge precoat has been formed.Pressure filtration is a process similar to vacuum filtration where sludge
solids are separated from the liquid.Basically, centrifuges separate solids from the liquid through sedimentation and centrifugal
force. Vacuum filter dewatering normally requires prior mixing with other more easily drained materials.High rate activated sludge
processes are generallyapplied to strong industrial wastes. Another different kind of filtering is the membrane separation that use
usually domestically and in industrial treatment which reduces the BOD in effluents.
Thickness Stabilization Conditioning Dewatering Disposal
Gravity Flotation
Centrifugation
Anaerobic digestion Aerobic digestion
Thermal Chemical
Vacuum filtration Pressure filtration
Centrifugation Drying beds
Composting
Sludge lagoons Incineration
Landfill For achieving the best flow quality as thought objectivesof the treatment system depends on thesettleability of the mixed liquor.
Whilesettlement of solids is prevented from happeningin the aeration tank by the action of the aerationequipment, the secondary
settlement tank isdesigned to promote settlement.Many techniques are employed to find out thereturn sludge rate. The control
strategies arebased upon either maintaining a target MLSS (Mixed Liquor Suspended Solid)concentration in the aeration tank or a
targetsludge blanket level in the secondary settlementtank. The more regular control techniques are:
Observation of the sludge blanket height;
The sludge settleability test;
maintaining a solids balance across the aeration tank; and
maintaining a solids balance across thesecondary settlement tank.
During the consumption of organic load along a plug flow tank, the oxygen demand decreases. To avoid making excessive oxygen
not needed, tapered aeration is used to modulate the air supply to meet the individual oxygen requirements of each section. At a
stable temperature and pressure, oxygen has a constant saturation concentration in water. Using air to oxygenate water,
saturation concentration may not be reached because only 21 percentage of oxygen exist in the air. If 100% oxygen is used, the
available oxygen is increased five-fold thus increasing the gradient and decreasing the gap between the saturation concentration
and the actual concentration. The force driving the oxygen into solution will be increased by an increased gradient. This increased
transport across the gas/liquid interface helps satisfy the elevated oxygen demands of micro-organisms under high loading rates.
Conclusion Many of thewell-designed activated sludge plants have a small layer of light chocolate colored foam covering up to 30% of the
aeration tank. However, if the foam passes the limit, the operation of the plant will be affected.Excessive build-up of foam can
result inwalkways and plant equipment being covered byfoam during windy conditions. As well ascreating an unsightly
appearance in the plant thiscan lead to odors and windblown foam. In the activated sludge process the microorganisms are
dispersed throughout the water phase. The microorganisms are doing the treatment and therefore all precautions factors must be
taken into account to assure a desirable environment for their life cycle.
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References
Huisman, L. & Wood, W.E. (1974) Slow Sand Filtration, World Health Organisation: Geneva.
Stevenson, D.G. (1994) The Specification of Filtering Materials for Rapid Gravity Filtration, Institution of Water and Environmental Management, Vol.8, October 1994.
Engineering School, UCD (1972). Course Notes - Water Quality Management. Department of Civil Engineering, University College: Dublin.
C.-H. Xing, X,-H. Wen, Y. Qian, E. Tardieu (2001) Microfiltration-membrane-coupled bioreactor for urban wastewater reclamation, Desalination, 141.p.67-73.
Finnish Environment Institute (SYKE), VesienLaatu GB [Online] www.enviroment.fi/waterquality [Accessed: 25th October 2014]
Government decree (2006) urban waste water treatment. [Online] Available from: http://www.finlex.fi/fi/laki/kaannokset/2006/en20060888.pdf. [Accessed: 30th October 2014].
