Hygienic Characteristics of Underground Water

Subsoil water. It is better protected from pollution than water in open reservoirs, yet subsoil water in wells is frequently polluted.

Inter layer water is practically sterile, but it also has drawbacks: there are small supplies of such water, its extraction is expensive and it is frequently highly mineralized and highly thermal.

Atmospheric Water

This source is rather changeable, besides atmosphere is considerably polluted, so this water is polluted as well. As this water is low mineralized it causes health disorders.

Desalination of Seawater

It is a very expensive matter requiring a lot of electric power. As seawater is polluted, at its boiling toxic substances can form.

Hygienic Characteristics of Water Supply Systems

There are 2 systems of water supply:

v Centralized system — a city water pipe. Its structure includes:

Ø Basic constructions (water supply point, pump stations, sewage disposal plants),

Ø Distributive network (pipeline, internal water supply system).

v Decentralized system — small rural water pipes, wells. This system is worse in hygienicrespect as it does not have complex sewage disposal plants.

Zones of Sanitary Protection of Water Sources

· Zone of strict regimen. It includes water supply point, basic constructions of water pipe.

· Zone of restrictions — it is the whole or partial area of water reservoir. There is a restriction of locating objects of possible pollution of water sources.

· Zone of supervision. Supervision over epidemic conditions (cases of infectious diseases) in a big territory is carried out with the purpose of improvement of purification and disinfecting water at unfavourable epidemic conditions.

Methods of Improving Water Quality

They are divided into 3 basic groups:

· Methods of water purification — removal of mechanical impurities and improvement of organoleptic parameters of water (turbidity, colouring).

· Methods of water disinfecting — destruction of microflora in water.

· Special methods of improvement of water quality — desalination, dechlorination, fluorination, defluorination, deodorization, decontamination, deactivation of water.

Methods of Water Purification

Water purification is carried out by means of water precipitation and filtration through filters (slow and fast filters). For acceleration of purificationthe coagulation of water is applied, i.e. adding salts of Al or Fe, which results in formation of flakes with calcium or magnesium salts in water. Nowadays flocculants are used, e.g. polyacrylamide, etc.

Control of efficiency of water purification is carried out according to:

· organoleptic parameters — turbidity, colouring, smell, taste;

· oxidability of water.

Methods of Water Disinfection and their Hygienic Estimation

There are 2 groups of disinfection methods:

· physical

· chemical

Physical methods of disinfectinginclude the following:

· boiling has a favourable bactericidal effect, but it is a very expensive method. As a great amount of power is consumed, this method is applicable only in domestic conditions.

· UVRhas a 100 % effect, but it requires a great amount of power and purifies small volumes of water, as in clean water UV penetrates through 50 cm only, in muddy water — even less.

· gamma irradiation is seldom used, as it requires complex equipment, constitutes a threat of irradiation to personnel and induces water radioactivity.

· ultrasonic irradiation is a complex method affecting personnel.

Chemical methods of water disinfectionare as follows:

· ozonization — action of atomic oxygen produces a good bactericidal effect. The method requires great power consumption. It improves organoleptic properties of water. Full destruction of toxic substances in water takes place.

· oligodynamic effect of silver ions, for example, "sacred water" in churches. Ions of silver have a bactericidal effect. It is a very expensive method.

· chlorination of water is the most widespread method because of it is cheap.

Chlorination of Water

On entering chlorine into water the hydrolysis of chlorine takes place with formation of hydrochloric and hypochlorous (HOCl) acids dissociating into Н+ and OCl- ions producing a bactericidal effect.

The Scheme of Chlorination

90 % of chlorine are bound with various substances in water and inactivated (chlorine absorptivity), the residual or free chlorine is left. For sufficient bactericidal effect it should be in amount of 0.3-0.5 mg/l (if it is below, no bactericidal effect is produced, if higher — the change of water smell by more than 2 points occurs).

Chlorine absorptivity + residual chlorine = chlorine necessity for water. It is determined at experimental chlorination during practical classes.

Types of Water Chlorination

· According to chlorine necessity or chlorination by normal doses of chlorine under the control of content of residual chlorine amounting to 0.3-0.5 mg/l.

For improvement of bactericidal effect there are other kinds of chlorination:

· Superchlorination — application of big doses of chlorine exceeding the chlorine necessity of water. It is used for extremely polluted water, unknown waters by bacterial indications (field conditions), by epidemic indications. After that water requires dechlorination through activated coal, hyposulfite.

· Double chlorination — introduction of chlorine before and after water purification — is the increased time of chlorine action, but the formation of toxic chlorine-organic substances increases.

· Chlorination with pre-ammonation — introduction of chlorine and ammonia into water, thus chloramines are formed having a greater bactericidal effect, there is no "chemist's" smell as at usual chlorination when in water chlorphenols can be formed.

Disadvantages of Water Chlorination:

· Deterioration of organoleptic properties of water (smell),

· Disinfection is not always reliable (presence of hepatitis viruses, etc.),

· At water pollution during chlorination toxic chlorine-organic substances are formed, such as chloroform, tetrachlorethylene, etc, having a mutagenic and carcinogenic activity.

Problems of Water Pollution

The basic sources of reservoir pollution are:

· Natural — floods, ice drifts, rotting of biota, "flowering of water", seasonal fluctuations of number and dying off hydrobionts, etc.

· Anthropogenic — household sewage,industrial sewage, agricultural drainage, regulated stream of rivers (dams, water basins), water transport, port works, dumping waste products in reservoirs, atomic power stations and military nuclear grounds, recreational loading on reservoirs.