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8/10/2019 Notes of Environment and Ecology for 2nd Sessional (1)
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SAMPLING
Some tests require the measurement to be conducted at the site because
the process of obtaining a sample may change the measurement. For
example, to measure the dissolved oxygen in a stream or lake, either the
measurement should be conducted at the site or the sample must beextracted with great care to ensure that there has been no loss or addition
of oxygen as the sample is exposed to the air. Similarly, it is better to
measure pH at the site if you are sampling water that is poorly buffered
from pH changes (see discussion on alkalinity).
Most tests may be performed on a water sample taken from the stream.
The process, by which the sample is obtained, however, may greatly
influence the result. The three basic types of samples are grab samples,
composite samples, and flow-weighted composite samples.
Dissolved Oxygen (DO):
Oxygen dissolved in water is vital for aquatic life. The optimum
value for dissolved oxygen in a good quality water is 4-8mg/L. It is
consumed by oxidation of organic matter/ reducing agent etc. present in
water.Water which has DO value less than 4 mg/L is termed as polluted
and is unfit for human or aquatic animal consumptions.
Chemical Oxygen Demand (COD):
It is an index of the organic content of water, since the most
common substance oxidized by the dissolved oxygen in water is organic
matter, which has a biological origin, such as dead plants etc. The COD
of a water sample is determined by the chemical oxidation of the organic
matter by K2Cr
2O
7in 50% H
2SO
4. This method includes other reducible
inorganic species that may be present in water such as,., and hence thismethod does not truly reflect the organic content in water. However
since this method is rapid, it is widely used. - 2 NO2- 2 3 S O 2- S
Biological Oxygen Demand (BOD):
The capacity of the organic matter in the sample of natural water to
consume oxygen is called its BOD. It is determined experimentally by
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determining the dissolved oxygen (DO) at the beginning and at the end
of a 5-day period in a sealed sample. The BOD gives the measure of
oxygen utilized or consumed in the period as a result of oxidation of
dissolved organic matter present in the water sample.
Threshold limit value (TLV):
This value indicates the permissible level of a toxic pollutant in
atmosphere to which a healthy industrial worker can be exposed during
an eight-hour day without any adverse effect. TLV of a pollutant is
found by experimentation on animals, medical knowledge and
experience and environmental studies.
Now let us discuss the various segments of our environment in detailone by one. In the first instance let us discuss about the atmosphere and
atmospheric chemistry.
Measurement of total organic carbon (TOC):
Both BOD and COD give indications of the oxidisability and oxygen
demand of water samples. Neither, however, measures the total organic
content of water. When this is required a determination of total organic
carbon (TOC) is made. This is done by quantitatively oxidising all theorganic matter in the sample to carbon-di-oxide soon after acidification
to remove interference from carbonates or bicarbonates. Oxidation in a
gas stream passing through a heated tube or wet oxidation with
potassium peroxodisulphate have both been used. The latter is less
convenient, but more sensitive and can be used at low levels below 1
mgdm-3
. The carbondioxide produced is measured either by
conductivity after absorption in solution or by catalytic conversion to
methane which is then passed to a flame ionisation detector as used ingas chromatography.
The TOC test can be performed in a relatively short period of time (few
minutes) compared to BOD and COD measurements and, hence offers a
valuable supplement to BOD and COD estimations.
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TURBIDITYWater that is not clear but is dirty, in the sense that light transmission
is inhibited, is known as turbid water. Many materials can causeturbidity, including clays and other tiny inorganic particles, algae, and
organic matter. In the drinking water treatment process, turbidity is of
great importance, partly because turbid water is aesthetically
displeasing, and also because the presence of tiny colloidal particles
makes it more difficult to remove or inactivate pathogenic organisms.
Turbidity is measured using a turbidimeter. Turbidimeters are
photometers that measure the intensity of scattered light. Opaque
particles scatter light, so scattered light measured at right angles to abeam of incident light is proportional to the turbidity. Formazin polymer
is currently used as the primary standard for calibrating turbidimeters,
and the results are reported as nephelometric turbidity units (NTU).
COLOR,TASTE, AND ODOR
Color, taste, and odor are important measurements for determining
drinking water quality. Along with turbidity, color, taste, and odor are
important from the standpoint of aesthetics. If water looks colored,
smells bad, or tastes swampy, people will instinctively avoid using it,
even though it might be perfectly safe from the public health aspect.
Color, taste, and odor problems in drinking water are often caused by
organic substances such as algae or humic compounds, or by dissolved
compounds such as iron. Color can be measured visually by comparison
with potassium chloroplatinate standards or by scanning at different
spectrophotometric wavelengths. Turbidity interferes with color
determinations, so the samples are filtered or centrifuged to removesuspended material. Odor is measured by successive dilutions of the
sample with odor free water until the odor is no longer detectable.
(Odor-free water is prepared by passing distilled, deionized water
through an activated charcoal filter.) This test is obviously subjective
and depends entirely on the olfactory senses of the tester. Panels of
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testers are used to compensate for variations in individual perceptions of
odor.
Taste is evaluated using three methods: the flavor threshold test (FIT),
the flavor rating assessment (FRA), and the flavor profile analysis
(FPA). For the FIT, water samples are diluted with increasing amountsof reference water until a panel of taste testers concludes that there is no
perceptible flavor. In the FRA, a panel of testers is asked to rate the
flavor from very favorable to very unfavorable. The oldest, and most
useful, of the taste tests is the FPA, which measures both taste and odor
of a water sample in comparison to taste and odor reference standards.
The intensity of specific tastes and odors are described on a 12-point,
ranging from no taste or odor (0) to taste or odor (12).
pHThe pH of a solution is a measure of hydrogen (H+)io n concentration,
which is, in turn, a measure of acidity. Pure water dissociates slightly
into equal concentrations of hydrogen and hydroxyl (OH-) ions:
The measurement of pH is now almost universally done using electronic
pH meters. A typical pH meter consists of a potentiometer, a glass
electrode and a reference electrode (or a single, combination
electrode), and a temperature-compensating device. The glass electrode
is sensitive to H+ activity and converts the signal to electric current,
which can be read as electrode potential (mV) or pH.
The pH of an effluent or water sample is important in almost all phases
of drinking water and wastewater treatment. In water treatment as well
as in disinfection and corrosion control, pH is important in ensuring
proper chemical treatment. Aquatic organisms are sensitive to pH
changes, as well as to the actual pH of the water. Few aquatic organisms
tolerate waters with a pH less than 4 or greater than 10. Acid mine
drainage, unregulated acids or bases in industrial effluents, oratmospheric acid deposition may alter the pH of a water body
substantially and have detrimental effects on aquatic life.
pH, Acidity and Alkalinity:
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The pH of a water sample measures its hydrogen ion concentration and
indicates whether the sample is acidic, neutral or basic. The pH may be
measured accurately using a pH meter. The pH meter must be calibrated
before making pH measurements. For calibration standard buffers of pH
4.00, 7.00 and 10.00 are used.
Table.2 pH ranges for
environmental waters
Type of water
pH range
Soft water 5.3-7.4
Hard water 7.6-8.8
Sea water 8.2-9.2
Water affected by acidic
pollutants
2.2-4.8
pH of water in equilibrium
with atmosphere
5.6
It should be noted that the unpolluted rain water is slightly acidic due to
the presence of dissolved carbon dioxide (pH=5.6). The range of pH
values of hard and soft water samples are also shown in Table.2.
H2O CO2(gas) H2O.CO2(solution) H HCO3 2H CO3Hard water is
slightly alkaline. The hardness is due to the presence ofpolyvalent metal
ion, e.g. Calcium and magnesium arising from dissolution ofminerals.
For instance, the dissolution of limestone involves the following
equilibria:
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In general, hard waters originate in areas where top soil is thick
and lime stone formations are present. Soft waters originate in areas
where the top soil is thin and lime stone formations are sparse or absent.
Analysis is normally performed by complexometric titration using
the disodium salt of ethylene diamine tetra acetic acid (EDTA).
Advanced Waste Water Treatment
Different waste water treatment processes
Removal Of Suspended SolidsMicrostraining
Coagulation and flocculation
Filtration
Removal of dissolved solids
Ion exchange
Reverse osmosis
Electrodialysis
Removal of nitrogen
Phosphate removal (chemical treatment)
Phosphate removal (biological treatment)Removal of dissolved organic compounds
Adsorption
Sludge treatment and disposal
The effluent from a typical secondary treatment plant still contains
20-40 mg/L BOD which may be objectionable in some streams.
Suspended solids, in addition to contributing to BOD, may settle on thestream bed and inhibit certain forms of aquatic life. The BOD if
discharged into a stream with low flow can cause damage to aquatic life
by reducing the dissolved oxygen content. In addition the secondary
effluent contains significant amounts of plant nutrients and dissolved
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solids. If the waste water is of industrial origin, it may also containtraces of organic chemicals, heavy metals and other contaminants.
Different methods are used in advanced waste treatment to satisfy any of
the several specific goals, which include the removal of (1) suspendedsolids (2) BOD (3) plant nutrients (4) dissolved solids and (5) toxic
substances. These methods may be introduced at any stage of the total
treatment process as in the case of industrial waterways or may be used
for complete removal of pollutants after secondary treatment.
Removal of Suspended Solids:
This treatment implies the removal of those materials that have been
carried over from a secondary treatment settler. Many methods wereproposed of which two methods were commonly used. The two methods
are microstaining and chemical coagulation followed by settling and
mixed media filtration.
Microstraining:
It is a special type of filtration procedure which makes use of
filters oven from stainless steel wires with opening only 60-70 m
across to remove very small particles. High flow rates and low backpressures are normally achieved.
Coagulation and flocculation:
Key Difference:Coagulation means to curdle; it basically refers to a chemical
process in which the destabilization of non-settleable particles takes place. These
particles form clumps with the help of a coagulant. On the other hand, flocculation
means to form flocs. It can be described as a physical or a mechanical process in
which the coagulated clumps or flocs are joined together to form masses as a
cloud and then a precipitate.
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In coagulation, the forces responsible for keeping the particles apart after
they contact, are reduced. Flocculation brings the de-established
colloidal particles together and they form large aggregates. Both these
techniques are employed in the treatment of water. Thus, we canunderstand the process of coagulation and flocculation with the help of
examples derived from water treatment techniques.
Water may contain colloidal solids like non-settleable organic matter,
clay particles, bacteria, plankton, small particles of decayed plant
material, etc. Thus, coagulation and flocculation techniques are
employed for separating these impurities from water.
Coagulation is achieved by neutralizing the particles and thus, therepelling force between the particles is greatly reduced. After employing
the flocculation process, the coagulated particles form a large
agglomeration, which is also known as floc.
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Coagulation can be easily achieved with the help of a coagulant. In raw
water, inorganic salts of aluminum or iron can be added, these salts
neutralizes the charge on the particles that are responsible for the raw
water turbidity. These salts also hydrolyze to form insoluble precipitatesentrapping the particles. Some of the common inorganic coagulants are
aluminum sulphate, alum, ferric sulfate and aluminum chloride.
In flocculation, these agglomerations of destabilized particles take the
form of large particles. This can also be achieved by adding high
molecular weight, water soluble organic polymers. Due to these
polymers, the size of the floc increases and then the particles settle
down. It is very important to gently mix the flocculating agent at a slowspeed so that small flocs can easily agglomerate into large particles, and
finally settle down.
For water treatment, coagulation is generally followed by flocculation.
Thus, in terms of water treatment, both can be differentiated easily.
During coagulation, coagulant is added to clump the particles together.
On the other hand, during flocculation, the solution is mixed gently, so
that the small clumps formed during coagulation, gather together and
form larger clumps. These large clumps easily settle down and thus can
be separated.The object of coagulation is to alter these particles in such a
way as to allow them to adhere to each other. Most colloids of interest in
water treatment remain suspended in solution because they have a net
negative surface charge that causes the particles to repel each other. The
intended action of the coagulant is to neutralise that charge, allowing the
particles to come together to form larger particles that can be more
easily removed from the raw water.
The usual coagulant is alum [Al2(SO
4)
2 18H
2O ], though FeCl
3,
FeSO4
and other coagulants, such as polyelectrolytes, can be used. Alum
when added to water, the aluminium in this salt hydrolyses by reactions
that consume alkalinity in the water such as:
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The gelatinous hydroxide thus formed carries suspended material with it
as it settles. In addition, however, it is likely that positively chargedhydroxyl-bridged dimers such as
and higher polymers are formed which interact specifically with
colloidal particles, bringing about coagulation. Metal ions in coagulants
also react with virus proteins and destroy upto 99% of the virus in water.
Anhydrous ion (III) sulphate can also act as effective coagulant similar
to aluminium sulfate. An advantage with iron (III) sulfate it that it works
over a wide range of pH.
Reverse osmosis:In the reverse osmosis process, demineralisation water is produced by
forcing water through semipermeable membranes at high pressure. In
ordinary osmosis, if a vessel is divided by a semipermeable membrane
(one that is permeable to water but not the dissolved material), and one
compartment is filled with water and other with concentrated salt
solution, water diffused through the membrane towards the compartment
containing salt solution until the difference in water levels on the twosides of the membrane creates a sufficient pressure to counteract the
original water flow. The difference in levels represents the osmotic
pressure of the solution (fig.1a).
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The process can be reversed by applying sufficient pressure to the
concentrated solution to overcome the osmotic pressure force the net
flow of water through the membrane towards the dilute phase. The
solute concentration (impurity) builds up on one side of the membrane
while relatively pure water passes through the membrane. In order to
obtain adequate solvent (water) flux through the membrane, pressures of
the order of 4000 to 7000 kN/m2
are required. Fig.1b represents the
principle of operation of the reverse osmosis unit.
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- -
Electrodialysis:
Electrodialysis uses ion-selective membranes and an electricalpotential difference to separate anions and cations in solution.
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In the past electrodialysis was most often used for purifying
brackish water, but it is now finding a role in hazardous waste treatment.
Metal salts from plating rinses are sometimes removed in this way.
Fig.2 shows a simple dialysis cell in which waste water may be
deionised. As shown in the figure two types of membranes (anionic and
cationic) are arranged alternatively to form many compartments betweenthe electrodes placed at the two ends. When the voltage is applied across
the cell containing mineralised water, the anions migrate to the positive
electrode and the cations migrate to the negative electrode. This causes
solution in alternate compartments to become more concentrated while
that in the remaining becomes more dilute. The electric power
requirement is proportional to the number of ions removed from the
water.
In the electrodialysis process, organic molecules are not removed andthey can collect on and clog the membranes. Another disadvantage of
this method is that it still leaves concentrated waste water to be disposed
of by some appropriate scheme. The process does not require any
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chemical additives and has low energy requirements and as such it can
be an economically feasible means of demineralisation.
Disinfection:
Disinfection, using chemical and physical methods is the final step in
drinking water purification. The finished water is disinfected often with
chlorine. It kills the remaining microorganisms in the water, some of
which will be pathogenic. It is a very efficient oxidising, bleaching anddisinfecting agent. In water chlorine reacts as follows:
The hypochlorous acid (HOCl) is the prime disinfecting agent. Its
dissociation in pH dependent yielding less effective hypochlorite ions
(OCl-
) at higher pH values:
Together, HOCl and OCl-
are called the free available chlorine.
A principal advantage of chlorination over other forms of disinfection is
that a chlorine residual is created that can protect the treated water after
leaving the treatment plant. This is guard against possible contamination
that might occur in water distribution system. To increase the lifetime of
the residual, some systems add ammonia to the treated water, forming
chloramines.
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Chloramines, although they are less effective as oxidants than
HOCl, are more persistent. Residual chlorine that exists as chloramine is
referred to as combined available chlorine.
Chlorine may have adverse secondary effects. It has the potential
to combine with trace amounts of organic substances to form
trihalomethanes (THMs) such as the carcinogen chloroform. Some
studies have shown an association between bladder and rectal cancer andconsumption of chlorinated drinking water. One approach to reducing
THMs is to remove more of the organics before any chlorination takesplace, which can be accomplished by adsorption on activated carbon.
The problem faced with the formation of THMs has spurred
interest in alternatives to chlorination as the preferred method of
disinfection. Alternative disinfectants include chlorine dioxide andozone. Chlorine dioxide (ClO
2) is a potent bactericide and viricide and it
does not form a residual capable of protecting water in the distributionsystem. However, there is concern for certain toxic chlorate and chlorite
substances that it may create, and it is a very costly method of
disinfection. Ozonation involves the passage of ozone (O3) through
water.
Ozone is a very powerful disinfectant that is even more effective against
cysts and viruses than chlorine, and it has the added advantage of having
no taste or odour problems. Unfortunately, the disinfective power ofozone is limited by its relatively low solubility in water.
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Trickling Filter Process for waste water treatment
In the biological-film system also known as trickling filters, the waste
water is brought into contact with a mixed microbial population in the
form of a film of slime attached to the surface of a solid support medium
whereas in the activated sludge system the waste water is brought in
contact with a diverse group of microorganisms in the form of a
flocculants suspension in an aerated tank. In both cases the organic
matter is metabolized to more stable inorganic forms. The most popular
means of treating domestic sewage has been the biological film system
because of its ease of operation. However the activated sludge process
can be more reliably be handled when handling large volumes of waste
water, and a high degree of treatment is achieved.
Conventional trickling filters normally consist of a rock bed, 1 to 3
meters in depth, with enough opening between the rocks to allow air to
circulate easily. The influent is sprinkled over a bed of packing which is
coatedwith a biological slime. As the liquid trickles over the packing, oxygen
and the dissolved organic matter diffuse into the film to be metabolized
by the microorganisms in the slime layer. End products such as CO2, ,
etc., diffuse back, out of the film and appear in the filter effluent. Milk
processing, paper mills and pharmaceuticals wastes are among those
treated by trickling filters. Like all biological units trickling filters are
affected by temperature; therefore cold weather slows down thebiological activity in the filter.
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Range for Drinking water:
NO. PARAM
ETERS
UNITS DRINKING
WATER
WHO Standard
Expe
ental
Value
(Rang
HDL MPL
1 Temperature
0C --- ----- 22-30
2 Turbidity NTU 5 10 18-47
3 pH value - 6.5 to 8.5 No
relaxatio
8.4- 8
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n
4 Total
hardness
(asCaCO3)
mg/l 300 600 122-2
5 Iron mg/l 0.3 1.0 0.2-0.
6 Chloride
s
mg/l 250 1000 7-26
7 Dissolve
d Solids
mg/l 500 2000 256-5
8 Calcium mg/l 75 200 28-48
9 Sulphate mg/l 200 400 50-91
10 Nitrate mg/l 50 No
relaxatio
n
0-1.77
11 Fluoride mg/l 1.0 1.5 0-0.4
12 Total
Alkalinity
mg/l 200 600 13-24
13 Magnesi
um
mg/l 30 150 9.23-
26.24
14 Oxygen
Observed from
KMnO4
at 370Cin 3 hrs.
mg/l 3.0 No
relaxation
2.4-7.
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15 Suspend
ed Solids
mg/l 20 150 70-28
Electrostatic Precipitator:
An electrostatic precipitator (ESP) is a particulate collection device that
removes particles from a flowing gaseous stream (such as air) using the
force of an induced electrostatic charge.
ESP can be operated at high temperature and pressures, and its power
requirement is low.
For these reasons the electrostatic precipitation is often the preferred
method of collection where high efficiency is required with smallparticles.
Steps in Electrostatic Precipitation
1.Generation of Electric field high voltage Direct current 20-80kv.
2.Generation of electric charges
3.Transfer of electric charge to a dust particle.
4.Movement of the charge dust particle in an electric field to the
collection electrodes.
5.
Adhesion of the charge dust particle to the surface of the collectionelectrode.
6.Dislodging of dust layer from collection electrode
7.Collection of dust layer in a hopper
8.
Removal of the dust from the hopper.
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PRINCIPLE OF ESP
Principle of ESP has four distinct phases as follows:
(I) Ionization or corona generation: When the potential difference
between the wire and electrode increases, a voltage is reached where an
electrical breakdown of the gas occurs near the wire. This electrical
break down or ion discharge is known as corona formation and therebygas is transformed from insulating to conducting state.
Two types of corona discharge can be generated which are:
(a) Negative corona: In negative corona, discharge electrode is of
negative polarity and the process of electron generation occurs at narrow
region
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(b) Positive corona: When positive voltage is applied to discharge
electrodes in the same way as negative corona, large number of free
electron and positive ions are generated. Or large number of positive
ions produced move towards collecting electrode and thus transfer
charge to dust particles upon collision.Negative coronas are more commonly used in industrial application,
while for cleaning air in inhabited space positive coronas are used. Due
to ozone generation in negative corona itsapplication for air cleaning in
inhabited area is avoided.
(II) Charging of Particles: Particle charging takes place in region
between the boundary of corona glow and the collection electrode,
where particles are subjected to the rain of negative ions from the corona
process. Mainly two mechanisms are responsible for particle charging.Each mechanism becomes significant according to particle size ranges.
For particles having diameter greater than 1m, field charging is
dominant force; and for particle size less than 0.2m diffusion charging
predominates.
(III) Migration and precipitation of particle:
(IV) Removal of deposited dust: Once collected, particle can be
removed by coalescing and draining, in the case of liquid aerosols and
by periodic impact or rapping, in case of solid material. In case of solid
material, a sufficiently thick layer of dust must be collected so that it
falls into the hopper or bin in coherent masses to prevent excessive re-
entrainment of the material into the gas system
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STANDARD PROCESSES FOR MANAGING MUNICIPAL WASTE
[7]
Incineration: Energy is stored in chemical form in all MSW materialsthat contain organic compounds i.e. which can be used to generate
electricity and steam. It is being done by a few major hospital for managing
clinical wastes.
Composting: The natural organic components of MSW (Food and plant
wastes, paper, etc.) can be composted aerobically to carbon dioxide, water,
and a compost product that can be used as soil conditioner. Anaerobic
digestion or fermentation produces methane, alcohol and a compost
product. Recovery/recycling: Recovered paper, plastic, metal, and glass can be re-
used. 18. In the absence of formalized waste segregation practices,
recycling has emerged only as an informal sector using outdated
technology, which causes serious health problems to wastepickers [9].
Land filling: MSW materials that cannot be subjected to any of the above
three method, plus any residuals from these processes (e.g. ash from
combustion) must be disposed in properly designed landfills.
Almost all categories of waste may be disposed to better managed landfills
directly. However, those types of wastes which will destroy the
microbiological degradation processes within the landfill are unwelcome
i.e. the non-biodegradable wastes. Management of these could include:
incineration, recycling and reusing
Sanitary Landfill:
Basic requirementsAs a minimum, four basic conditions should be met by any site design and
operation before it can be regarded as a sanitary landfill:
Full or partial hydrogeological isolation: if a site cannot be located
on land which naturally contains leachate security, additional lining
materials should be brought to the site to reduce leakage from the
base of the site (leachate) and help reduce contamination o
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groundwater and surrounding soil. If a liner - soil or synthetic - is
provided without a system of leachate collection, all leachate will
eventually reach the surrounding environment. Leachate collection
and treatment must be stressed as a basic requirement.
Formal engineering preparations: designs should be developed fromlocal geological and hydrogeological investigations. A waste disposal
plan and a final restoration plan should also be developed.
Permanent control: trained staff should be based at the landfill to
supervise site preparation and construction, the depositing of waste
and the regular operation and maintenance.
Planned waste emplacement and covering: waste should be spread
in layers and compacted. A small working area which is covered daily
helps make the waste less accessible to pests and vermin
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