The technologies applied in the treatment of water are expanding each year. We cannot discuss every method here. Libraries are well stocked with technically detailed books. This chapter will serve to provide you with a literary competence of the major or more common technologies in use today. Some of these will be expanded on in later chapters as they apply to the "point of use" treatment industry. We will begin by walking you through one of the most commonly misunderstood treatment systems in place today; a typical municipal treatment and distribution system.
This section is meant to describe the operation of older conventional treatment systems. These systems are designed to cope with the contaminants described in the previous chapter with the exception of excessive mineral levels and chemical contaminants.
New systems are now in place in selected major cities due to the dramatic levels of chemical pollutants present in their source water supplies but these systems are still rare. These are generally describes as "specialized" systems and will only be touched on briefly.
The term "raw public water supply" refers to waters that are used as the intake source for public and industrial use. These can be either surface or ground water. The majority of municipal users obtain their water from a piped water distribution system, most of which include some form of treatment between the raw water supply and the end user.
Guidelines have been developed that recommend limits for physical, chemical, radiological and microbiological characteristics of drinking water. These are not law in Canada. Drinking water supplies that contain substances in concentrations greater than those limits are capable of producing deleterious health effects or are aesthetically objectionable.
Conventional water treatment is considered to be processes that are commonly used to condition various surface and ground water supplies. By referring to the diagram we will walk through the process.
Water is drawn from a raw supply into the first stage. The water is reduced in depth and flocculation is introduced. This will cause particles and bacteria to bind together thus becoming heavy enough to settle. These tanks are generally exposed to sun light to take advantage of Ultraviolet rays for the control of bacteria and microbes. As the water will now enter some form of closed distribution, the first stages of chlorination must begin. This ensures that bacteria does not gain a foothold in the closed piping system.
In the next stage the water passes through sand filtration beds of various densities. These beds will further reduce the levels of silt, bacteria, turbidity and colour as the physical contaminants are trapped in the bed. These beds are continuously cleaned or replaced to maintain the required flow rate.
Disinfection is enhanced by the addition of Ammonia, which in combination with chlorine, produces a stronger killing agent that will boil off rapidly thereby reducing the accumulation factor.
A mass reservoir is shown, but not all systems use them. It is dependent on the water source or the demand cycle of the community. As the water is pumped to and through the community, chlorination is maintained by introducing the chemical at strategically placed pumping/injection stations.
In newer systems [but still of conventional design], the bacteria levels are electronically monitored and the chlorination level is constantly being adjusted. In older systems it is mechanically set depending on the time of the year. In the prairies for example, during sub zero winter weather conditions, bacteria levels are very low and the chlorine levels are set accordingly. In spring and summer the levels are set so high that the smell is noticeable while showering or just running the kitchen tap. These levels are well in excess of government guidelines, but this threat is deemed to be lessor that which would be caused by the bacteria.
Specialized water treatment is that which is used for removal of chemical contaminants, the reduction of iron and other heavy metals, or the control of the PH level by the removal or introduction of base minerals. These systems are expensive to operate and are only applied in extreme areas. Some of these technologies will be discussed as they do apply on a smaller scale for use in domestic water treatment.
Before we can begin to discuss the specific make-up of water treatment products you must be familiar with the terminology surrounding them. The following is a condensed glossary.
ADSORPTION:
The process in which matter adheres to the surface of an adsorbent.
An adsorbent is a material, usually solid, capable of holding gases, liquids
and/or suspended matter at its surface and in exposed pores. Granular activated
carbon [GAC] and KDF are common adsorbates used in water treatment to remove
a host of chemical contaminants [organics], certain inorganics, odours
and tastes. This will be discussed in depth later, but for simplicity just
think of these media as "chemical magnets".
CAUTION EXCHANGE SOFTENING:
Removes calcium and magnesium, two minerals which seriously
impair water's cleaning capabilities and which can deposit a damaging scale
in boilers, heaters, plumbing fixtures and appliances. This process is
employed in commercial and domestic point of entry systems.
CHEMICAL TREATMENT:
A process often used for controlling corrosion which is the
destructive disintegration of a metal by electrochemical means. Usually
involves the feeding of polyphosphates and/or silicates with a chemical
feed pump.
DE-IONIZATION:
A process to provide water which is virtually free of minerals
[less than 1 mg/l] required in high pressure boilers, metal plating, electronics
manufacture, jet engine operations, pharmaceuticals, ice-making and so
forth.
FILTRATION:
This is the physical removal of particulates down to a specified
size. This can range from very coarse filters, like the ones you would
use on a swimming pool, to very fine units [<10 microns] for pre-filters
in various water treatment devices, but is not to be confused with sub-micron
filtration.
DISINFECTION:
There are a variety of methods for disinfecting water at the
point-of-use. To "disinfect" is to remove or kill the living
organisms [bacteria, cysts, viruses etc.]. Disinfection methods may include:
BROMINATION: A disinfection method where bromine is released from impregnated resin cartridges; now used in marine and offshore applications. Organisms are killed.
CHLORINATION: A chlorinator [chemical feed pump] feeds chlorine gas or solutions of its compounds, such as hypo-chlorides, into a water supply in proportion to the flow of water. Organisms are killed.
IODINATION: Disinfection method recommended for transient population and for temporary disinfection. Iodine is added in droplet or capsule form into small quantities of water. Organisms are killed.
OZONATION: Ozone, an unstable form of oxygen and a strong oxidizing agent, is being used as a disinfectant for water. Organisms are killed.
ULTRAVIOLET LIGHT: A method of disinfection which employs ultraviolet light to destroy water borne bacteria. Sometimes used in conjunction with other disinfection methods to maintain bacterial quality during storage. Most organisms are killed, but cysts are impervious.
SUB-MICRON DEPTH FILTRATION: water disinfection by the physical removal of bacteria and cysts by sub-micron filtration. Water is forced through a porous element of defined micron rating. Filters defined as .1 to .45 microns are considered, bacteriologically "sterile". Filters rated between .45 and 1 micron are considered bacteriologically "safe" and filters greater than 1 micron are not considered in terms of disinfection. Some common filtering media which meets the "sterile" guideline are Doulton's line of ceramic candles and a man-made nylon screen [originally developed for pharmaceutical use] called "#66 nylon". Both are extremely effective on organisms and cysts, but minimally efficient on viruses. The Doulton ceramic elements have the advantage of being cleanable and reusable.
SILVER BACTERIOSTASIS: A method which inhibits bacteriological growth through oligo-dynamic means. A device that is "Bacteriostatic" means that the levels of bacteria contained within it will not increase during periods when it is not in use. The device itself will not become a breeding ground for organisms. One of the most common inorganics which can provide this is function is silver. This will be expanded on later.
DISTILLATION:
The process in which water is converted into its vapour state
by heating and then cooled. The vapour condenses into a collection point
and is stored for use. This is a duplicate of Mother Natures process. The
water is left virtually free of contaminants.
NEUTRALIZATION:
The addition of either an acid or an alkaline to produce a neutral
solution.
OXIDATION:
The action of an oxidant for precipitating and removing water
impurities such as iron, manganese, hydrogen sulphide and many organics.
RESIN BASED ION EXCHANGE:
Specially formulated resins which interact with minerals and
heavy metals to allow their collection or containment.
REVERSE OSMOSIS:
Used to remove substantially all suspended or dissolved matter
from water. A process in which pressure is used to force water through
a semipermeable membrane which will slowly transmit the water but reject
most dissolved ions. Soluble contaminants [chemicals] are not rejected
by the membrane. Membranes are not one size fits
all. Simply stated, a chlorine based city supply requires a different
membrane than a rural "hard" water well based supply. The chemical
chlorine will destroy the hard water designed membrane. When asked; "How
does it work?" An easily understandable answer is to actually
ask another question. "When you blow up balloons
for a party, why do they deflate after a day or so?". "Did the
air escape because you tied a poor knot?" The answer is that
the air inside the balloon passed through the "membrane" of the
balloon. The "gas" in this case passed from a high pressure area
to a low pressure area. This process will continue until eventually the
pressures on either side of the balloon wall are equal.
In most cases combinations of the technologies described are used to solve water quality problems. For example, chlorination may be used for disinfection and the precipitation of iron or hydrogen sulphide, which then is removed by physical filtration. GAC and KDF [adsorbates] are used to reduce low molecular weight organic compounds. Asbestos fibres, fine silt and turbidity can be reduced to undetectable levels through micro filtration. Lead, cadmium and similar heavy metals can be reduced through resin based ion exchange. We will take a closer look at these functions in chapter 6.
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