These waters are not normally considered as suitable for drinking supplies, but undoubtedly as demand for water increases all available sources will have to be examined.. It should, how
Trang 1INTRODUCTION
Water, of course, is used for many purposes associated with
human activity In its natural state it occurs in and on the
ground in subsurface and surface reservoirs The quality and
reliability of a source of water will vary considerably, both
in time and space This means that characteristics (chemical,
physical, and biological) will differ greatly depending upon
the location and type of source It also means that a given
source may vary over the seasons of the year
Thus, in the selection of a water source, consideration is
usually given to the use to which the water will ultimately be
put so as to minimize the cost of treatment Simultaneously
consideration must be given to the reliability of the source
to provide an accurate and constant source of supply It will
be seen later in this section that a groundwater supply may
enjoy the benefit of requiring little or no treatment, while
a surface supply such as a river, pond or lake may require
considerable and perhaps seasonally varying treatment
However, a surface supply is visible and therefore more
reli-able whereas a groundwater supply may just disappear with
no warning or notice
In certain areas, freshwater is so scarce that the source
must be accepted and choices are not available The history
of water treatment dates back to the early Egyptian
civiliza-tions where the murky waters of the Nile River were held in
large open basins to allow the mud to settle out The earliest
archeological records of a piped water supply and
waste-water disposal system date back some five thousand years to
Nippur of Sumaria. 1 In the Nippur ruins there exists an arched
drain with an extensive system of drainage from palaces and
residences to convey wastes to the outskirts of the city Water
was drawn through a similar system from wells and cisterns
The earliest records of water treatment appear in the
Sanskrit medical lore and Egyptian wall inscriptions. 2
Writings from about 2000 BC describe how to purify “foul
water” by boiling in copper vessels, exposing to sunlight,
filtering through charcoal and cooling in an earthenware
vessel There is little concerning water treatment in the
Old Testament, but Elisha under instruction from the Lord
“healed” certain waters so that “there shall not be from
thence any more death or barren land.” This “healing” was
accomplished when Elisha “went forth unto the spring of the
waters and cast salt in there …” It is not clear if this “salt”
was a fertilizer to help grow crops or if it was some chemical
to render the water safe
Settling was first introduced as a modification of decant-ing apparatus used for water or wine This apparatus was pictured on the walls of the tombs of Amenhotep II and Rameses II in the 15th and 13th Centuries BC An engineer-ing report on water supply was written by the then water commissioner for Rome in AD 98 He described an aqueduct with a settling basin
In 1627 the experiments of Sir Francis Bacon were pub-lished just after his death, and were the first to describe coag-ulation as well as sedimentation and filtration as a means
of treating drinking water The first filtered supply of water for an entire town was built in Paisley, Scotland in 1804
Starting with a carted supply, a piped distribution system was added in 1807. 2
However it was not until 1854, in London, that it was demonstrated that certain diseases could be transmitted by water Dr John Snow suggested that a cholera outbreak in a certain area in London resulted directly from the use of the Broad Street pump, and was in fact the source of infection in the parish of St James Dr Snow recommended that the use
of the pump should be discontinued and the vestrymen of the parish agreeing, the disease subsequently abated in that area
The discovery was all the more incredible as the germ theory of disease, defined by Pasteur and subsequently postu-lated by Koch, had not at that time been clarified Subsequently disinfection of water by addition of chlorine was introduced
on a municipal scale This step, together with an adequate and sanitary distribution system, probably did more to reduce the deaths due to typhoid and cholera and any other single item
In 1854, cholera claimed a mortality of 10,675 people in London, England In 1910, the death rate from typhoid fever
in the City of Toronto, Canada, was 40.8 per 100,000 By
1931 it had fallen to 0.5 per 100,000 These improvements all related to the extensive water purification and steriliza-tion techniques which are being introduced to municipal water treatment systems during that period. 3
In general, the treatment processes of water can be sub-divided into three groups: physical, chemical, and biological processes The biological processes are generally reserved for waters grossly contaminated with organic (putrescible) carbon such as sewage or industrial waste waters These waters are not normally considered as suitable for drinking
supplies, but undoubtedly as demand for water increases all
available sources will have to be examined However, for the present purposes we will consider that the biological stabiliza-tion of originally polluted waters will be dealt with under the
Trang 2section on wastewater treatment It should, however, be
real-ized that there is a very fine line between treated wastewater
discharged into a water body and the use of that water body
as a source for drinking water and the treatment of the
waste-water before discharge Clearly in those areas where wastes
are still not treated prior to release, a water treatment plant is
essentially dealing with the treatment of diluted wastewater
We must therefore determine the significance of water
quality before we examine the types of treatment necessary
to achieve this quality Water quality very much depends
upon the use for which the water was intended For example,
industrial boiler feed water requires a very low hardness
because the hardness tends to deposit on the pipes in the
boiler system and reduces the efficiency of the heat transfer
However, if the hardness of the boiler feed water is zero, the
water tends to be very corrosive and this of course is also
very undesirable for a boiler system
If the water is to be used for a brewery or a distillery, a
number of other chemical parameters are important If the
water is to be used for cooling then clearly the temperature
is one of the most important parameters
In the past the methods for setting standards for water
supplies was very much a hit and miss affair and relied pretty
well upon the philosophy of “If no one complains, all is well.”
Clearly, that is not a very satisfactory criterion There are a
number of drinking water standards or objectives published
by various nations of the world, such as the World Health
Organization International and European Drinking Water
Standards (1963 and 1961), the US Public Health Service
Drinking Water Standards (1962) and Objectives (1968) These
standards are established on the principle that water in a public
water supply system must be treated to the degree which is
suitable for the highest and best use The highest and best use
for water of course is human consumption This can frequently
be argued as a rather unnecessary quality when one considers
that much water which is processed in a municipal plant is used
for watering lawns, washing cars and windows However, the
difficulty in ensuring that a second-class, perhaps unsafe water
supply is not used as a potable supply is extremely difficult It
will be found that very few cities have a dual water supply
rep-resenting a drinking water system and a non-potable system
A few large cities, particularly when they are adjacent
to large standing bodies of water, occasionally have a fire
water supply system where the water is taken untreated from
the lake or river and pumped under high pressure through a
system connected only to fire hydrants and sprinklers
Thus, assuming that natural water requires some kind
of treatment in order to achieve certain predetermined
stan-dards, and the process of treating these waters can be
subdi-vided into physical and chemical processes, the remainder of
this section will deal with the physical and chemical
meth-ods of treating water for municipal or industrial use
WATER SOURCES
The magnitude of the problem of supplying water to the major
cities of the world is in fact a huge engineering problem
According to a US Department of Commerce estimate, the cities of the United States in 1955 with a total population
of 110 million produced and distributed 17 billion gallons
of water daily to their domestic, commercial, and industrial consumers Of this, 12.88 billion gallons were from surface water sources which usually, it will be seen, require more elaborate treatment, whereas the remaining 4.12 billion gal-lons came from groundwater sources—only a small propor-tion of which would require treatment. 4 The most voluminous source of water is the oceans It is estimated that they contain about 1060 trillion acre-feet. 5 Clearly this water is of little value as a potable source, but it certainly remains the main reservoir in the hydrologic cycle
step in the purification of ocean water, and this requires the full energy of the sun in order to accomplish Precipitation, percolation, and runoff are all parts of the cycle of water which is without a beginning or an ending Of the water which falls upon the earth, part of it directly runs off to the nearest stream or lake, and part of it infiltrates down to the groundwater table and percolates through the groundwater, also into a stream or lake Transpiration takes place through the leaves of green plants, and evaporation takes place from the groundwater, where it surfaces through swamps, lakes
or rivers, and of course from the ocean Of the water that soaks into the ground, part of it is retained in the capillary voids near the surface Thus it can be said that the poten-tial sources of water for society consist of wells, which are drilled or dug down to the groundwater table and withdraw water from that level; springs, which are natural outcrop-pings of groundwater table through rocks or ground; rivers, where the groundwater table has naturally broken through the ground and flown in a certain direction sufficiently to gouge out a channel for the water to flow in; lakes, where large bodies of water gather usually somewhere along a river system; and finally the ocean, if not other sources are avail-able and the ocean is close by The benefit derived from the costly treatment required to desalinate the ocean under these circumstances is outweighed by the necessity of having a fresh water source at any cost
There are new water sources which exist deep in the earth’s crust These sources are rarely considered, due to the high salt and sulphur content which is frequently found in them
The recycling of used water of course is a further source which may be tapped directly It can be seen from the hydro-logic cycle that all water is being continually reused, but the direct recycling of municipal treated sewage into the potable treatment plant is being considered in some water-scarce areas
Some of the advantages and disadvantages which might
be listed for the various sources of water are as follows:
1) Wells provide usually an extremely pure source
of potable water Rarely is any treatment required
of this water, certainly before it is safe to drink, although certain industrial uses may require the removal of some of the soluble salts such
It can be seen from Figure 1 that evaporation is the first
Trang 3as hardness One of the major disadvantages of wells is that they cannot be observed and there-fore must be considered as somewhat unreliable
Frequently it has been experienced in the coun-try, if a drought has persisted for a few days or a few weeks (depending on the environment) and the well has been pumped unusually hard, that the well will run dry There is never very much warn-ing of this kind of occurrence and therefore for a municipal supply it has the distinct disadvantage
of being considered somewhat unreliable
2) Springs are similarly unreliable, and have a
fur-ther disadvantage in that they require a rafur-ther elaborate engineering system to capture them and concentrate them into one manageable system
Also, springs require rather a large protected area
to ensure that man does not pollute this environ-ment, thereby rendering the springwater unsafe
3) Rivers tend also to be a little unreliable, although
they do have the advantage that they can be observed and to some extent controlled through dams and other waterflow structures Thus it can
be seen, if the water level is falling, that a munici-pality may wish to impose water use restrictions
to conserve water until such time as further aug-mentation of the supply is received through the hydrologic cycle One of the major problems with
a river source is that there is a considerable varia-tion in the quality of the water During the high
flow flood period, there is frequently a consider-able amount of silt and organic material which
is washed off the ground, whereas at other times
of year the water may be relatively clean and require remarkably little treatment prior to distri-bution This of course means that water treatment facilities must be installed to deal with the worst possible condition, and at other times of the year
it may not in fact be necessary and therefore the equipment lies idle
4) Lakes and manmade reservoirs, due to the nature
of flow through them, have a certain stability both from the point of view of quantity and qual-ity Undoubtedly, water coming from a lake or a reservoir would require far more elaborate treat-ment than would water from a well However, the extreme reliability and the predictability of supply may well outweigh the considerations of cost of treatment This of course is subject to an economic feasibility study
5) Oceans A good deal of attention is currently being focused on the desalination of ocean water, and some attention will be paid to this subse-quently in this section It should, however, be remembered that the ocean is only, economi-cally available to these communities which are immediately adjacent to the ocean This leaves a very large area of hinterland in most continents which does not have access to the sea Thus the
Precipitation
Surf ace r unoff
Infiltr ation
Percolation
Snow
Ground w ater tab
le (G.W T) Spring
Lake Swamp
River
Ocean
G.W.T
Ground water
From land and water surfaces
=Surface runoff and ground-water runoff
Runoff or stream flow
FIGURE 1 Hydrologic cycle (Fair and Geyer, Water Supply and Wastewater Disposal).
Trang 4desalting of sea water as a major water source has
a restricted application to small islands and those coastal stretches of countries where fresh water reserves are either not available or not reliable
6) Recycled water A considerable amount of research
has been undertaken in the United States and else-where for the renovation of treated wastewater for the purposes of returning it directly into the potable supply Some rather complex chemical and physi-cal processes are required to make this a satisfactory process, and the details of many of these processes will be described subsequently in the next section
of this chapter
PHYSICAL TREATMENT
The items of treatment described under this section will be
only those which alter the physical properties of the water
or represent a unit process which is physical in nature All
of the processes described may be used individually,
collec-tively or in any combination, in order to accomplish a
prede-termined water quality
Screens
Whatever the source of water, it is necessary to insert some
kind of screen in the system in order to prevent the passage
of solids into the subsequent steps of water treatment If
the source of water is simply a well, the screens tend to be
simply designed to prevent the admission of sand from the
water-bearing strata into the pumping system Where water
supply is drawn from rivers or lakes, the intakes usually have
to be screened and built of corrosion-resistant materials in
order to prevent the admission of fish or logs or any other
undesirable solids into the system Intake screens are usually
provided with openings approximately equal to one and
one-half to two times the area of the intake pipe The purpose of
this is to ensure that the velocity through the screens is
suf-ficiently low to prevent jamming of the screens On occasion
other screens are required as a backup system within the water treatment plant
In some locations where it is found that seasonally algal blooms become a nuisance, a new type of screening known
as microstraining has been introduced Microstrainers are a very fine weave of stainless steel wire with apertures suffi-ciently small to prevent the passage of the microscopic algae which is normally found in an algal bloom Such a screen-ing system is normally only required on a seasonal basis and in certain locations where these problems are prevalent
Microstraining is conducted at such a very small diameter orifice that it is sometimes considered to be a part of a filtra-tion process
Coagulation
Although the basis of coagulation is in fact chemical treatment and will be discussed in the next section, the coagulation process itself (sometimes referred to as flocculation) is accomplished by a physical process involving the gentle agi-tation of the fluid which allows the small suspended particles
to collide and agglomerate into heavier particles or flocs and settle out Flocculation or coagulation is the principle used in the removal of turbidity from water It will be shown subse-quently that colloidal or very finely divided material will not settle very rapidly Various processes have been employed
to accomplish flocculation Some of these are; diffused air, baffles, transverse or parallel shaft mixers, vertical turbine mixers, to mention but a few
The most common type of flocculator used today is the paddle type, the other methods having shown some disad-vantage such as being too severe for the fragile floc, or being too inflexible, or being too costly to operate Horizontally mounted paddles, either located transverse or parallel to the floor, consist of a shaft with a number of protruding arms
on which are mounted various blades The shaft rotates at a very slow rate of 60 to 100 rpm, causing a very gentle agita-tion which results in the flocculaagita-tion of the particles The time required for the flocculation process is very carefully controlled and strongly related to the dosage of chemical which is used The chemicals used and the chemistry of this process will be described later
Prior to the flocculation step which has just been described, occurs a flash mixing step when the chemicals are added and mixed very rapidly at high speed to get uni-form distribution of the chemical in the stream A variety of devices are used for this rapid mixing operation; frequently one of the most common includes the low lift pumps which are usually located adjacent to the intake where the water is lifted up into the treatment plant Here of course the chemi-cals must be pumped into the pump casing at a higher pres-sure than the pump is producing, and the mixing takes place
in the casing of the pump
Other devices frequently used are venturi flumes, air jets, paddles, turbines, propellers, the latter being one or the most favored and most widely used of the rapid mixing devices
It usually is composed of a vertical shaft driven by a motor
CASING
WET WELL SCREEN
INTAKE SCREEN
(I) WELL SCREEN (II) LAKE OR RIVER SCREEN
FIGURE 2
Trang 5on which one or more propeller blades are mounted Baffles
are frequently used to reduce the vortexing about the
propel-ler shaft Vortexing hinders the mixing operation Detention
periods are usually of the order of one to five minutes,
usu-ally at the lower end of this range Considerable study has
been done on the baffling arrangement in a flash mixing
unit, and a variety of arrangements have been shown to be
successful
Sedimentation
Sedimentation or settling may be accomplished by a variety
of means and mechanisms, depending on the material which
is to be settled from the liquid
Discrete settling This type of sedimentation is primarily
concerned with the settling out of non-flocculent discrete
particles in a fairly dilute system The primary feature of
this type of settling is that the particles do not flocculate and
therefore their settling velocity and particle size remain the
same throughout the period of settling It will be seen later
that this is quite different from other forms of settling
The particles in discrete settling will accelerate until the fluid/drag reaches equilibrium with the driving force acting
on the particle In other words, the resistance of the water
is equal to the accelerating force of gravity of the particle
When this velocity is reached, it will not increase This is known as the terminal settling velocity, and it is normally achieved quite rapidly The loading rate which is used fre-quently for the design of a settling tank is known as the over-flow rate and may be expressed in cubic feet per square foot per day based on the area It can be seen that cubic feet per square foot per day is in fact the same as feet per day, or in fact a simple velocity This velocity is defined as the set-tling velocity of the particles which are removed in this ideal basin if they enter at the surface
Overflow rates or surface loadings of 150 gallons per day per square foot of tank surface are not unusual where the settling and sand, silt or clay are being accomplished by plain sedimentation
Flocculent settling The primary difference between this
type of settling and the previous one is that in a flocculent system the larger particles subsiding at a slightly higher rate
CHEMICAL FEED
HIGH SPEED PROPELLER
PADDLE
ROTATION
FIGURE 3
OUTLET
IN LET
SLUDGE HOPPER
SLUDGE COLLECTOR
MIXING ZONE INLETRAW
WATER
(Accelator by Infilco)
CIRCULAR COMBINATION SETTLING FLOCCULATOR
LONGITUDINAL SETTLING
TANK
FIGURE 4
Trang 6Flocculent settling path
Discrete settling
path inlet
Zone settling
Combined Settling Pattern
outlet
FIGURE 5
Biological Growth Filtration Phenomena FIGURE 6
will overtake and coalesce with smaller particles to form
even larger particles, which in turn increase the overall
set-tling rate Clearly, the greater the liquid depth, the greater
will be the opportunity for this type of contact There is no
mathematical relationship which can be used to determine
the general effect of flocculation on sedimentation, and
empirical data is still required by studying individual
labora-tory cases As a result, in flocculent settling the removal of
suspended matter depends not only on the clarification rate
but also on the depth
This is one of the significant differences between
non-flocculent and non-flocculent settling
Zone settling The previous two types of settling described
have one property in common, and that is that they both
deal with dilute suspensions Zone settling, on the other
hand, deals with very concentrated suspensions where it is
assumed that one particle will in fact interfere with the
set-tling rate of another particle It is clear that in the type of
dis-crete settling, where the particles are somewhat non-reactive
and usually quite dense such as sand, the difference between
dilute suspensions and concentrated or hindered suspensions
is less apparent, so the zone settling phenomenon is usually
considered for the flocculating materials When the particles
reach the vicinity of the bottom of the settling tank, a more
concentrated suspension zone will be formed and the settling
particles will tend to act in concert and reduce the overall
rate of subsidence
It can clearly be seen that in a water treatment plant,
particularly if coagulation is applied to remove turbidity, all
three types of settling will occur and any settling tank which
is designed must take into account all three types (Figure 5)
Filtration
As described earlier, it has been found even in the early
Egyptian days that passing water through sand resulted in a
reduction in suspended and colloidal matter, and resulted in a
further clarification of the water Water which is on occasion
extremely turbid should, of course, first of all be treated by
some coagulation or settling or combination of both However,
water which is normally not too turbid may be directly applied
to filters or water which has previously been treated by sedi-mentation and/or coagulation may also be applied to filters to provide the final polishing and the production of clear, aes-thetically acceptable water
The filtration process actually consists of three phenom-ena occurring simultaneously (Figure 6)
Settling takes place in the small settling basins which are provided between the particles Screening takes place where particles which are larger than the interstices will
be retained simply physically because they cannot pass through And finally, a biological action takes place through bacterial growth which may occur on the particles of the filter which may occur on the particles of the filter which grow at the expense of the soluble organic carbon passing through in the water This latter phenomenon is not a very satisfactory way of removing organic carbon, because it does tend to plug up the filter fairly rapidly and reduce its effectiveness
Filters have been developed through the ages through a series of steps which are mainly related to their operating characteristics or the material which is used as a filtering medium
Slow sand filter The slow sand filter is, as it suggests,
a process whereby water is allowed to pass very slowly through the system at rates of 2.5 to 7.5 million gallons per acre per day
Although this type of filter has been used traditionally and has been very effective in the past, it has certain operat-ing disadvantages in that it cannot readily be cleaned While some of these filters are still in use in some parts of the Orient, in Europe and North America, where labor tends to
be more costly, other types of filters have been developed
When the difference in water level between the outlet and the water over the filter becomes too great, the filter is taken out of service and the top inch or two of sand is removed from the bed and may or may not be replaced with fresh sand
Rapid sand filter A far more popular and common process
for the filtration of water is the rapid sand filter Instead
of sitting on a sand bed of approximately three feet, as is the case in the slow sand filter, the bed is twelve to thirty inches thick and supported on a layer of gravel or other coarse grain, heavy material six to eighteen inches thick
Filtration rates on the rapid sand filter are of the order of three to four gallons per square foot per minute Occasionally before the filter is put back into operation (Figure 7)
Trang 7a plant is designed to operate at two gallons per square foot
per minute, but provided for an overload when necessary
(Figure 8)
The cleaning of the rapid sand filter, instead of
throw-ing the filter out of service, is accomplished by simply
backwashing This is accomplished by passing clean water
backwards through the filter at a high velocity This velocity
should not be greater than the terminal settling velocity of
the smallest particle of sand which is in the filter which is
not to be washed over the side Through this mechanism the
sand bed is expanded and the sand is lifted and floated while
the particles rub mechanically against one another and wash
off the foreign material The dirty water is washed away in
drains After this has been conducted for a few moments, the
filter is allowed to go back into service and the head loss is
now smaller so the rate of flow through the filter is increased
once more
Pressure filters Whereas the rapid sand filter is indeed a
grav-ity filter, a pressure filter is somewhat the same type of system
only pressure is applied to the water to pass it through the
filter The most common household unit nowadays would be
the swimming pool filter, where the water is pumped vertically
through the sand and the filter, and when the head loss through
the filter becomes excessive as registered on the pressure
gauge, the operator will reverse the flow through the filter, accomplishing the backwash described above (Figure 9)
Diatomaceous earth filter Diatomaceous earth is the silicious
residue of the bodies of diatoms which were deposited in past geological ages and now form extensive beds where they are mined The earth is processed and ground, and the silica par-ticles are extremely irregularly shaped and thus provide a very good porous coating The diatomaceous earth filter was devel-oped by the army for field use to remove certain chlorine-resistant organisms responsible for dysentery
BACKWASH WATER OUT
RAY WATER IN
SAND
GRAVEL
FILTERED WATER OUT BACKWASH
WATER
FIGURE 9
WASH WATER TROUGHS
EXPANDED SAND GRAVEL BACKWASH WATER FILTERED EFFLUENT
GRAVEL SAND
RAPID SANDFILTER
FIGURE 8
FILTER CAKE
WASTE FOR WASH WATER
PRECOAT POT
FILTERED WATER
BODY FEEDER
RAW WATER FEEDER
DIATOMACEOUS EARTH FILTER FIGURE 10
HEAD LOSS
CLARIFIED
WATER
OUTLET
SAND
5 FEET
3 FEET
SLOW SAND FILTER FIGURE 7
Trang 8The filter medium is supported on a fine metal screen
or a porous material There are three steps in the filtration
cycle There are three steps in the filtration cycle First of all,
the deposit of a pre-coat, which is a thin layer of diatomite
deposited on the filter element The second step is the actual
filtration and the body feed addition The reason why body
feed is continually added to the filter is to reduce the amount
of clogging that occurs at the surface This also permits
sig-nificantly longer filter runs The third step, when the
pres-sure drops or the filtration rate reaches such a low very thin
film over or under the source of irradiation Commercial
equipment is currently being developed for the individual
water supply of the small household or institution, and is
gaining some acceptance in some quarters The irradiation
of water by ultra-violet light of suitable wave-lengths for a
proper period of time will kill bacteria, spores, molds, and
viruses and in fact all microorganisms The bactericidal
wave-lengths extend from about 2000 to 2950 Å (angstrom
units) with a maximum effect around 2540 Å
CHEMICAL TREATMENT
The unit operations of chemical coagulation, precipitation,
ion exchange and stabilization all produce change in the
chemical quality of the water Some of these are aimed at the
removal of the suspended and colloidal substances, others
are aimed at the removal of dissolved substances Finally, some chemicals are simply added for their own sake, but these will not be discussed in this section
To understand some of the basic chemistry of the treat-ment processes, it is first of all essential to understand a phenomenon known as chemical equilibrium and reaction velocities An analogy might be considered as the physical equilibrium between ice and water
Add Heat Remove heat
If the ice-water system is maintained at 0°C, then molecules
of water are transferred from the solid to the liquid state and back again at the same rate The addition of heat or the removal of heat from the system will result in the equilib-rium moving in one direction or the other The same princi-ples might be applied to what is known as ionic equilibrium, which, like molecular equilibria, are subject to a shift under given stresses
As an example, we might consider pure water
H 2 O U H⫹ ⫹ OH⫺ Certain stresses will give rise to an increase in hydrogen ion concentration (H⫹ ) The expression of this shift is a reduction
in pH, whereas an increase in the OH⫺ concentration brings about an increase of pH One of the most important equilib-ria which exists in natural waters is the relationship between carbon dioxide and carbonate ion, which is shown in the fol-lowing four equilibrium expressions
CO 2 (gas) U CO 2 (solution) (1)
CO 2 (solution) ⫹ H 2 O U H 2 CO 3 (2)
H 2 CO 2 U H⫹ ⫹ HCO 3
HCO 3 ⫺ U H⫹ ⫹ CO 3
FIGURE 11
Filter
Small particles Medium density
Low density Large particles
Bacteriacidal
Infra Red
ELECTROMAGNETIC SPECTRUM
Light
FIGURE 12
Trang 9The equilibrium of the first of these equations is purely
physical, since the solubility of gas and water is determined
by the pressure of that gas and the temperature and a number
of other physical parameters
Coagulation
The principle function of chemical coagulation is known as
destabilization, aggregation, and binding together of
col-loids Alum, or aluminum sulphate, (Al 2 (SO 4 ) 3 · 18H 2 O) is
one of the most common coagulants which may be added
to a water system Such a coagulant possesses tiny positive
charges and therefore has the ability to link together with
negatively charged color or turbidity particles by mutual
coagulation Alum also reacts with the natural alkalinity
(carbonate- bicarbonate system) of the water to produce a
precipitate which is usually thought to be aluminum
hydrox-ide If the reaction takes place with natural alkalinity, it may
be expressed as follows:
Al 2 (SO 4 ) 3 · X H 2 O 3Ca(HCO 32 ) → 2Al(OH) 3 ⫹ 3CaSO 4
⫹ X H 2 O ⫹6CO 2
In the event that there is insufficient natural alkalinity for
this to occur, then calcium oxide (lime) may be added to
create the same effect Because this system is very poorly
understood, the optimum dosage required in practice has to
be done by trial and error through a series of tests known as
jar tests
In these jar tests, the flash mixing and flocculation steps
described previously are stimulated at various
concentra-tions of alum and the clarification which takes place and the
reduction of turbidity and the rate at which the floc settles
are all observed in order to determine the optimum dosage
of coagulant If too much coagulant is added, then the
col-loidal system which is primarily negatively charged will
become supersaturated by the aluminum system which is
primarily positively charged and the suspension will become
restabilized and this can be observed by conducting jar tests
over a wide range of concentrations of coagulant
The reason why alum is so generally used is that it is
highly effective over a wide pH range in waters of vastly
different chemical make-up Other materials such as ferrous
sulphate are occasionally used to increase the settling rate of
plankton and thus increase the time of the filter run, making
the filter process more efficient
Precipitation
There are two important processes which are associated with precipitation in the treatment of water One is the reduction
of hardness (calcium and magnesium) and the other is the reduction of iron and manganese
Water Softening The lime-soda-ash process involves the
addition of Ca(OH) 2 and Na 2 CO 3 to water The reactions which occur are as follows:
Ca(HCO )3 2 Ca(OH)2 2CaCO 2H O
Lime
3 + 2
(1) Mg(HCO )3 2⫹Ca(OH)2→MgCO3⫹CaCO3 ⫹2H O2 (2)
MgCO3⫹Ca(OH)2 →Mg(OH)2⫹CaCO3 (3)
Soda Ash
(4)
In this reaction it can be seen that the lime is added to precipitate the carbonate hardness, while the soda ash provides the car-bonate ion to precipitate the non-carcar-bonate hardness
Precipitation of Iron and Manganese Normally, iron and
manganese are only highly soluble if they are in their ferrous (Fe 2⫹ ) and manganous (Mn 2⫹ ) forms Normally, these two metals will only occur in this form if there is an absence of dissolved oxygen However, on occasions when the water is particularly acid, such as might occur in mine drainage areas, the metals may remain in solution even though a very high dis-solved oxygen is present Under these circumstances, aeration
is frequently sufficient to drive off the surplus carbon diox-ide, increase the pH and bring about a natural precipitation
of these materials in their ferric and manganic form In order
to catalyze or accelerate this reaction, the water is frequently caused to trickle over coke or crushed stone, or to flow upward through some contact material This allows deposits of iron and manganese to accumulate on the surfaces and catalyze the further precipitation of ferric and manganic oxides
If the pH of the system is forced to values higher than 7.1, the positively charged ferric hydroxide particles may be
Drive Motor
Control Stirrer
FIGURE 13 Jar test equipment—coagulant dosage varied in each jar to deter-mine optimum concentration.
Trang 10adsorbed on the negatively charged calcium carbonate
par-ticles and a stable colloidal suspension may result Iron and
manganese are objectionable constituents of water supplies
because they impart a brown colour to laundry goods and
frequently will stain household plumbing fittings
Precipitation of iron and manganese can also be
satisfac-torily accomplished by using the lime-soda-ash process as
described above for softening
Ion Exchange
Ion exchange units are most frequently used for softening
waters, but are also used by certain industries for the production
of de-ionized water This is quite common in the brewery
industry, where an attempt is made to strip the water down
to its most pure constituents so that water in one part of the
world is similar to water in other parts of the world Following
de-ionization, breweries and often distilleries will
reconsti-tute the water so that the water used for the production of a
certain type of beer will be the same all over the continent
and not have the variations which were characteristic of beers
when native waters were used for their production
The chemistry of the ion exchange process is shown below,
where a cation resin which will exchange the sodium (Na⫹ )
for the calcium and magnesium (Ca 2⫹ , Mg 2⫹ ) When the resin
is saturated with calcium and magnesium, a regeneration is
required such as is used in household water softening units,
when a very strong brine solution is forced back through the
resin and in turn displaces the calcium and magnesium into the
backwash line and restores the sodium on the resin for further
softening
Softening Na Ca
Mg
SO
2
3 2
4
2
⎡
⎣
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
(1)
R
2NaHSO
Na SO 2NaCl
4
⫹
⎡
⎣
⎢
⎢
⎢
Regeneration Ca
Ca
Mg Cl
⎤
⎦
(2)
Desalination
Although the principles of desalination were fully known in
Julius Caesar’s time, the energy requirements of this process
are presently so high that these will be usually considered as
a last resort after all other water sources have been explored
Water quality is frequently referred to as fresh, brackish, sea
water or brine Fresh water normally contains less than 1000
mg/liter of dissolved salts, while brackish water ranges from
1000–35,000 mg/liter of dissolved salts Sea water contains 35,000 mg/liter of dissolved salts, whereas brine contains very much more from salt water by a semi-permeable membrane, the fresh water will tend to flow into the salt water to equal-ize the concentration of salts on both sides of the membrane
Bearing in mind that the membrane will not allow the salts to pass back, it is clear that a certain pressure which is known
as the osmotic pressure is forcing the fresh water through
to the brine side of the membrane If a force greater than this osmotic pressure is applied on the sea water side, then fresh water will flow backwards through the semi-permeable membrane at a rate proportional to the incremental pressure over the osmotic pressure In practice, quite high pressures are required in order to get a useful volume of water to pass through the membrane—such pressures as 40–100 kg per square centimeter This has been shown to work for waters
of fairly high dissolved solids, but the structural properties of the membranes must be fairly well developed and of course the membranes must be very well supported Membranes used for this type of process are frequently cellulose acetate
or some derivatives thereof The power requirement for this process is considerably less than electrodialysis, but it is a
S.W.
14 IN 18 HEAT EX.
10
19 11
STEAM
SOLUTION HEAT EX.
VENT 27
CONC WATER SEA OUT FRESH WATER OUT
FIGURE 14 Conventional multistage flash evaporation – MSF evaporating and cooling of hot feed brine (vertical arrows down) on left side at succes-sively lower pressures after heating
to highest temperature in prime heater (PH) at top; vapors (horizontal ar-rows) from MSF, passing to preheat the sea water by condensation-heating
on right side; fresh water condensate passing stagewise from top to dis-charge at bottom; additional sea water coolant (dotted line) rejecting heat in lower stages, withdrawing of vapors from prime heater to be condensed in half-stage (dashed lines) increasing the production of fresh water.