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WATER QUALITY/WATER POLLUTION – LWR 215/413
Hand out 1
WASTEWATER TREATMENT
Wastewater can be treated to remove physical, chemical and organic pollutants.
Simple settling may be sufficient, e.g. a well designed canal intake or a system of silt
traps can remove sufficient sediment and silt to avoid a water supply from choking
up.
Primary or physical treatment
This involves screening out or settling of sediments
Secondary treatment
It involves the activity of micro-organisms to break down wastes using the techniques
of slow filtration, activated sludge treatment etc. It requires more expensive facilities,
space and time. The secondary treatment reduces the BOD risk and reduces the
number of disease-carrying organisms in the effluent bur does not remove all bacteria
and viruses, toxic chemicals or all nutrients. Eutrophication may still take place after
secondary treatment.
Tertiary treatment
Tertiary treatment involves the use of chemicals, activated carbon or electrochemical
techniques to neutralize chemical pollutants and kill pathogenic organisms.
Land disposal
Sewage effluent or agro-industrial waste is applied directly around crops, i.e. nonfood crops like cotton or food crops that are heat-treated before consumption. It is a
cheap and viable way of using WW and reducing the risk of eutrophication of streams
and other water bodies. Crop growth is favoured due to the nutrient content.
However, crops may be contaminated with pathogenic organisms, heavy metals, high
levels of nitrates, salts and even antibiotic, drug and hormone contaminants. There
may also be a drift of pathogens and other contaminants on the wind or into the
ground water thus harming people and livestock.
Physical Unit Operations
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Those operations used for the treatment of WW in which change is bought
about by means of or through application of physical forces
Form the basis of most process flow diagrams
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Most commonly used unit operations include: Screening, Comminution, Flow
equalization, Mixing, Sedimentation, Accelerated gravity settling, Floatation,
Filtration, Gas transfer, Volatilisation and gas stripping
1. Flow metering
Consists of two elements, a sensor or detector and a converter device. The
sensor is exposed to or affected by the flow and the converter is the device
used to translate the signal or reading from the sensor into a flow reading.
Types of flow metering devices
For open channels: flumes, weir plate esp. the Parshall flume.
For closed conduits: (a) insertion of an obstruction to create a predictable
headloss or pressure difference e.g. flow tubes, orifices, pitot tubes, rotameters
and venturi tubes (b) measurement of the effect of the moving fluid, e.g.
momentum change, sonic wave transmittance, magnetic field shift; e.g.
magnetic, target, ultrasonic, vortex measurement devices (c) measurement of
incremental units of fluid volume e.g. tubing and propeller meters
Selection criteria for metering devices
The main factors to consider are type of application, proper sizing, fluid
composition, accuracy, headloss, installation requirements, operating
environment and ease of maintenance. However accuracy and repeatability
are critical, esp. where the readings from the metering device are to be used
for process control.
2. Screening
Is the first unit operation encountered in WW treatment plants. A screen is a
device with openings, generally of uniform size, which is used to retain coarse
solids found in WW. Examples: bar racks, screens. Bar racks are used to
screen for coarse materials and is normally employed during pre-treatment.
Screens, can be fixed or rotary and are used to screen for coarse, medium and
fine materials. They are also used for pre-treatment, primary treatment,
removal of residual and secondary suspended solids. A rotary disk screens for
medium and fine materials and is used during primary treatment, secondary
treatment with settling tank and removal of residual secondary suspended
solids.
3. Flow equalization
It is the damping of flow rate variations so that a constant or nearly constant
flow-rate is achieved. It is used to overcome the operational problems caused
by flow rates variations, to improve the performance of the downstream
processes and to reduce the size and cost of downstream treatment facilities.
4. Mixing
Mixing is an important unit operation in many phases of WW treatment
including (a) mixing one substance completely with one another (b) mixing of
liquid suspensions (c) the blending of miscible liquids (d) flocculation and (e)
heat transfer. Example: mixing WW with chemicals e.g. chlorine is mixed
with the effluent from secondary tanks.
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5. Sedimentation
It is the separation from water by gravitational settling of suspended particles
that are heavier than water. It is used for grit removal, particulate-matter
removal in primary settling basins, biological-floc removal in the activatedsludge settling basin and chemical-floc removal in the chemical coagulation
process. The primary purpose is to produce a clarified effluent but it is also
necessary to produce sludge with a solids concentration that can be easily
handled and treated. It is therefore, desirable to produce both a clarified
effluent and a concentrated sludge.
Types of sedimentation
(a) Discrete particle: Sedimentation of particles in a suspension of low solids
concentration. Particles settle as individual entities and there is no
significant interaction with neighbouring particles. Removes grit and sand
particles from WW.
(b) Flocculant: A dilute suspension of particles that coalesce or flocculate
during the sedimentation operation. By coalescing, the particles increase
in mass and settle at a faster rate. Removes a portion of SS in untreated
WW in primary settling facilities. Also removes chemical-floc in settling
tanks.
(c) Hindered (zone): Suspension of intermediate concentration in which interparticle forces are sufficient to hinder the settling neighbouring particles.
The particles tend to remain in fixed positions with respect to each other
and the mass of particles settles as a unit. A solids-liquid interface
develops at the top of a settling mass. Occurs in secondary settling
facilities used in conjunction with biological treatment facilities
(d) Compression: Settling in which the particles are of such concentration that
a structure is formed and further settling can occur only by compression of
the structure. Compression takes place from the weight of particles, which
are constantly being added to the structure by sedimentation from the
supernatant liquid.
During sedimentation it is common to have more than one type of settling
occurring at a given time and it is possible to have all four occurring
simultaneously.
6. Accelerated Gravity Separation
A number of devices that take advantage of gravitational and centrifugal
forces and induced velocities have been developed for the removal of grit from
WW. Example: Teacup separator.
7. Flotation
It is a unit operation used to separate solid and liquid particles from a liquid
phase. Separation of brought about by introducing fine gas, usually air,
bubbles into the liquid phase. Bubbles attach to the particulate matter and the
buoyant force of the combined particle and gas bubbles is great enough to
cause the particle to rise to the surface. The rising particles with a lower
density than the liquid can also be facilitated, e.g. oil suspension in water.
Used primarily to remove suspended matter and to concentrated biological
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sludges. The advantage of flotation over sedimentation is that very small or
light particles that settle slowly can be removed more completely and in a
shorter time. Once particles have been floated to the surface, they can be
collected by the skimming operation.
8. Filtration
Used extensively for achieving supplemental removals of SS from WW
effluents of biological and chemical treatment processes. It is also used to
remove chemically precipitated P. The complete filtration operation is
comprised of two phases filtration and cleaning or regeneration
(backwashing). There are two types of filtration, (a) Semi-continuous
filtration, where the filtering and cleaning phases occur sequentially and (b)
Continuous filtration, where filtering and cleaning phases occur
simultaneously.
9. Gas transfer
Is the process by which gas is transferred from one phase to another, usually
from the gaseous to the liquid phase. It is a vital part of a number of WW
treatment processes, e.g. the functioning of aerobic processes, such as
activated sludge, biological filtration and aerobic digestion, depends on the
availability of sufficient quantities of oxygen.
10. Volatilization and gas stripping
The release of VOCs from WW surfaces to the atmosphere is termed
volatilisation. VOCs are released because they partition between gas and
water phase until equilibrium is reached. Mass transfer (movement) of a
constituent between these two phases is a function of the constituent
concentration in each phase relative to the equilibrium concentration.
Therefore, the transfer of a constituent between phases is greatest when the
concentration in one of the phases is far from equilibrium. Because the
concentration of VOCs in the atmosphere is extremely low, the transfer of
VOCs usually occurs from WW to the atmosphere. Gas stripping of VOCs
occurs when a gas (usually air) is temporarily entrained in WW or is
introduced purposefully to achieve a treatment objective. When a gas is
introduced into a WW, VOCs are transferred from the WW to the gas. Gas
stripping is most effective when contaminated WW is exposed to contaminant
free air. In WW treatment, air stripping occurs most commonly in aerated grit
chambers, aerated biological treatment processes and in aerated transfer
channels.
Chemical Unit Processes
These are processes in which change is brought about by means of or through
chemical reaction. It is usually used in conjunction with the physical unit
operations and biological unit processes to meet treatment objectives.
1.
Chemical precipitation
Chemical precipitation involves the addition of chemicals to alter the physical
state of dissolved and SS and to facilitate their by sedimentation. The most
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common precipitants are Alum, Ferric chloride, Ferric sulphate, Ferous
sulphate and lime. Alum is added in WW containing Ca and MG bicarbonate
alkalinity.
2.
Adsorption
It is the process of collecting soluble substances that are in solution on a
suitable interface. The interface can be between a liquid and a gas, or a solid
or another liquid. Activated carbon is the most commonly used material. It is
prepared by first making a char from materials such as almond, coconut and
walnut hulls, other woods and coal. The char is produced by heating the
material red, to drive off the hydrocarbons, with an insufficient supply of air to
sustain combustion. The char particle is then activated by exposure to an
oxidizing gas as a high temperature. The gas develops a porous structure in
the char and thus creates a large internal surface area. It can be used as a
powder, with a diameter of less than 200 mesh or as granular with a diameter
of > 0.1 mm. The adsorption process takes place in 3 steps:
(a) Macro-transport: It is the movement of organic material through the water
to the liquid-solid interface by advection and diffusion.
(b) Micro-transport: It involves the diffusion of the organic material through
the macro-pore system of the granular activated carbon (GAC) to the
adsorption in the micro-pores and submicro-pores of the GAC granule.
(c) Adsorption: Occurs on the surface of the granule and in the macro-pores
and meso-pores but the surface area of these parts of the GAC are so small
compared with the surface area of the micro-pores and submicro-pores that
the amount of material adsorbed is usually negligible.
3 Disinfection:
It is the selective destruction of disease-causing organisms (bacteria, viruses
and amoebic cysts). All the organisms are not destroyed during the process.
This differentiates disinfection from sterilization, which is the destruction of
all organisms. Disinfection is most commonly accomplished in different
ways.
(a) Chemical agents: Chlorine and its compounds, bromine, iodine, ozone,
phenol and phenolic compounds, alcohols, heavy metals and related
compounds, dyes, soaps and synthetic detergents, quarternary ammonium
compounds, hydrogen peroxide and various alkalies and acids. Highly
acidic or alkaline water can be used to destroy pathogenic bacteria because
water with a pH > 11and < 3 is relatively toxic to most bacteria.
(b) Physical agents: The physical disinfectants that can be used are heat and
light. Heating water to the boiling point will destroy the major diseaseproducing nonspore-forming bacteria. Heat is commonly used in the
beverage and dairy industry, but it is not a feasible means of disinfecting
large quantities of WW because of high costs. Sunlight is also a good
disinfectant, i.e. ultraviolet radiation is used to sterilize small quantities of
water.
(c) Mechanical means: By the use of coarse screens, fine screens, grit
chambers, plain sedimentation, chemical sedimentation, trickling filters,
activated sludge, chlorination of treated WW.
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(d) Radiation: The major types are electromagnetic, acoustic and particle.
Gamma rays are emitted from radio-isotopes such as Cobalt 60. Radiation
is effective in disinfecting (sterilizing) water and WW.
Mechanisms of Disinfectants
Four mechanisms explain the action of disinfectants;
(a) Damage the cell wall: results in cell lysis and death, e.g. penicillin
(b) Alteration of cell permeability: the selective permeability of the membrane
is altered and vital nutrients e.g. N and P are allowed to escape (phenolic
compounds and detergents)
(c) Alteration of the colloidal nature of the protoplasm: e.g. heat, radiation
and highly acidic or alkaline agents. Heat will coagulate the cell protein
and acids and bases denature the proteins producing a lethal effect.
(d) Inhibition of enzyme activity: e.g. oxidizing agents like chlorine can alter
the chemical arrangement of enzymes and deactivate the enzyme
Factors affecting disinfection agents are as follows: contact time,
concentration and type of chemical agent, intensity and nature of physical
agent, temperature, number of organisms, types of organisms, nature of
suspending liquid.
Biological Unit Processes
The objectives of the biological treatment of WW are to coagulate and remove
nonsettleable colloidal solids and to stabilize the organic matter. For domestic
WW, the major objective is to reduce the organic content and in many cases
the nutrients such as N and P. Sometimes for the removal of trace organic
compounds that may be toxic. For industrial WW the major objective is to
remove or reduce the concentration of organic and inorganic compounds.
Because many of these compounds are toxic to micro-organisms, pretreatment may be required. The removal of carbonaceous BOD, the
coagulation of nonsettleable colloidal solids and the stabilization of organic
matter are accomplished biologically using a variety of micro-organisms,
principally bacteria. The important micro-organisms in biological treatment
are bacteria, fungi, protozoa and rotifers and algae. Regardless of the type of
waste, the biological treatment processes consists of controlling the
environment required for the optimum growth of the micro-organisms
involved.
The major biological treatment processes used for WW treatment are as
follows:
(a) Aerobic processes: (Suspended growth, attached growth, combined
suspended and attached growth processes)
(b) Anoxic processes: (Suspended growth and attached growth)
(c) Anaerobic processes: (Suspended growth and attached growth)
(d) Combined aerobic, anoxic and anaerobic processes: (Suspended growth
and combined suspended and attached growth
(e) Pond processes:
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The natural treatment systems
In the natural environment, physical, chemical and biological processes occur
when water, soil plants, micro-organisms and the atmosphere interact.
Natural treatment systems are designed to take advantage of these processes to
provide WW treatment. The processes involved in the natural systems include
many of those used in mechanical or in-plant treatment systems, e.g.
sedimentation, filtration, gas transfer, adsorption, ion exchange, chemical
precipitation, chemical oxidation and reduction and biological conversion and
degradation, plus others unique to natural systems such as photosynthesis,
photo-oxidation and plant uptake. The processes occur at natural rates. The
natural treatment systems include:
(a) The soil-based or land treatment systems: slow rate, rapid infiltration and
overland flow
(b) The aquatic-based systems: constructed and natural wetlands and aquatic
plant treatment systems
Slow rate method
Slow rate treatment involves the application of WW to vegetated land to
provide treatment and to meet the growth needs of the vegetation. The applied
water is consumed through evapotranspiration or percolates vertically and
horizontally through the soil profile. Any surface run-off is usually collected
and reapplies to the system. The percolate enters the underlying ground water
but in some cases it may be intercepted by natural surface waters.
Rapid infiltration method
WW that has received some pre-application treatment is applied on an
intermittent schedule usually to shallow infiltration or spreading basins.
Vegetation is usually not provided in the infiltration basins.
Overland flow
Pretreated water is is distributed across the upper portions of carefully graded,
vegetated slopes and allowed to flow over the slope surfaces to run-off
collection ditches at the bottom of the slopes.
Wetlands
These are inundated land areas. The vegetation provides surfaces for the
attachment of bacteria films, aids in the filtration and adsorption of WW
constituents, transfers oxygen into the water column and controls the growth
of algae by restricting the penetration of sunlight. Both natural and
constructed wetlands may be used.
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