Water Purification and Disposal of Sewage.

(Part 6, Environmental Hygiene, VPE)

Topic: Water Purification methods for community water supplies. Disposal of Sewage and Disposal of Farm Waste.

Water Purification and Disposal of Sewage

Water Purification 

The method employed depends upon quantum of water to be purified. 

Large Scale Purification 

The aim is to ensure the absence of pathogenic organisms, other factors like relative hardness in the water. 

The treatment received by public supplies at the present time may comprise one or all of the following processes. 

• Storage 
• Filtration with or without the aid of coagulants. 
• Chemical sterilization 

Small Scale Purification 

• Boiling 
• Chlorination 
• Potassium Permanganate

Storage 

• The storage of water in large open reservoir is practiced principally as a water conservation project and it also fulfils an important function as a water purifier. 
• Sedimentation of suspended matter results in the removal of turbidity and sedimentation also carries down with it an appreciable quantity of organic contaminants. 
• Exposure to sunlight and the oxygenation of the water by respiring algae effect the removal of many pathogenic and other organisms. 
• Although algal growth is confined to the surface layer, the oxygen introduced into the water through its agency diffuses downward, raising the oxygen content of the middle layer.
• The bottom of the reservoir is frequently devoid of available oxygen and is the site of anaerobic decomposition of organic materials. 
• To avoid the removal of the tainted water from the bottom of the reservoir, and the filter blocking algae from the surface, means are usually provided for drawing off water from the reservoir at points between these layers.

Filtration

• Filtration is normally carried out by allowing the water flowing from the reservoir to percolate through sand, and two types of sand filters are recognized slow sand filters and rapid sand filters. 
• The slow sand filters when first put into use act simply as strainers removing solid matter in suspension without retaining bacteria but after a time the sand grains become coated with a film of organisms which are effective in biologically purifying the water as it passes through the filter. 
• Slow sand filters which have reached this condition are said to have “ripened” and such filters require careful supervision to maintain them in this ripened condition out of doors . 
• Slow sand filters function efficiently , but as they only pass about 120 litres per square metre per hour they suffer from the obvious drawback that they need to be very extensive to cope with a large volume of water.
• In rapid sand filters the water drawn from the reservoir is forced through the filter under pressure, either by means of a pump or by maintaining a head of water above the filter. 
• Suspended particles are removed but biological purification does not takes place. Purification is effected in this case by introducing a coagulant before filtration or by sterilizing with chlorine afterwards. 
• Coagulants in the form of freshly precipitated hydroxides of many metals possess the property of adsorbing considerable quantities of organic matter and this property is made use of in removing organic contaminants from water, the metallic hydroxide in common use is aluminium hydroxide. 
• In practice the aluminium hydroxide precipitate is formed in the water by adding to it either aluminium sulphate or sodium aluminate. In both cases the aluminium salt is hydrolysed to aluminium hydroxide which settles rapidly, dragging with it at the same time the organic matter dissolved or suspended in the water. 
• Bacterial contamination is largely removal by this procedure and the removal of filter blocking particles permits of a more rapid and efficient filtration subsequently. 
• Impure water when it has filtered is improved is appearance , colour, taste and odour. Its organic matter is reduced and its bacterial content may be reduced 99 per cent. 
• The microorganisms that are pathogenic to man are more easily killed in the film than are the harmless ones and coliform bacilli which are always found in sewage are taken as the index of the efficiency of the filtration because if they are absent form filtered water it may rightly be presumed that pathogenic organisms are absent. 
• If water containing much suspended solid matter or muddy water is habitually drunk by animals it leaves an even increasing deposit of silica, mica in the alimentary tract. 
• This is a cause of colic, constipation and in the case of mica which forms a coating on the mucous membrane, malnutrition. Water of this nature can be roughly filtered by passing it through a gravel and sand bed or in case of emergency, through coarse canvas.

Chemical Sterilization 

• Its aim is to destruction of all non sporulating pathogenic organisms, particularly those of intestinal origin, which may be present in it. 
• Sterilization of water, whether on a large or small scale by the use of chemicals should always be preceded by some form of filtration so as to remove the grosser particles of organic matter. 
• The chemical substance used for sterilization must not impart a disagreeable taste to the water nor make it harmful to animals or people, while at the same time it must possess sufficient germicidal power to effect its purpose. 
• The sterilizing agent most frequently used for the treatment of potable waters are chlorine and ozone. Chlorination is more generally applicable and is cheaper than ozonisation, although the latter process possesses certain advantage over the former. 
• In both processes the essential feature is the introduction into the water of the sterilizing agent in such concentration as to leave a small residuum of free agent after a contact time between agent and water of sufficient duration to ensure destruction of pathogenic bacteria. 
• Efficient sterilization can only be carried out in water free from suspended organic matter, since bacteria embedded in particles of organic material may not be reached by the sterilizing agent . 
• Also organic matter reduces the efficiency of most chemical sterilizing or disinfecting agents, the concentration of sterilizing agent available for the destruction of bacteria being reduced rapidly to negligible proportions. 

Small Scale Purification 

Boiling

Except for domestic purposes or for the sterilization of very limited amounts of water, boiling is impracticable in most circumstances, as for instance in the supply of water for a dairy form. 

Boiling is however of great value when there is reason to believe that the filters of a public supply have become temporarily faulty or when owing to heavy flooding, surface water has gained entrance to wells or springs. When such an accident as the letter occurs, warning is usually given by the turbid appearance of the water. 

Chlorination 

Is the best method for the routine treatment of small supplies of water, the chlorine being readily obtainable in the form of chloride of lime (bleaching powder). The latter should be added to the water in such quantity that free chlorine will be available to the extent of at least one part per million of water. This may be done by first preparing a solution of chloride of lime consisting of 30 gm of chloride of lime to 250ml water, which will suffice to treat 2000 gallons of a water not containing an excessive amount of organic matter. After chlorination the water should be allowed to stand in an open tank for four hours before being used. 

In the case of waters which contain much organic matter or are heavily contaminated by bacteria, super chlorination followed by de-chlorination should be the method adopted . After standing for not less than 30 minutes, small quantities of this water may be dechlorinated by adding sodium thiosulphate, which removes the taste of chlorine 

Potassium permanganate 

Potassium permanganate very rapidly decomposes in the presence of organic matter, yielding nascent oxygen. It is feeble disinfectant in water, but has a specific action on the vibrio cholera owing to the lethal effect of oxygen on this particular organism. A better reaction is obtained by the addition to the water of some dilute acid . 

This treatment is not used on a large scale but is sometimes adopted for the disinfection of wells and water tanks. The amount of permanganate added must obviously vary with the degree of pollution. 

The treatment of wells is effected by adding to each gallon of water 3.9 gms of permanganate with 12 ml of strong hydrochloric acid, leaving for twenty four hours; pumping until the water is colourless , removing dead aquatic fauna.

Chlorination

• Chlorination is essentially the addition of “free” chlorine to water; it may be introduced as chlorine gas, as sodium hypochlorite, or as bleaching powder. 
• Chlorine gas is used in large scale practice while sodium hypochlorite and bleaching powder find use in small scale sterilizers. The two recognized procedures for the treatment of water by free chlorine are normal chlorination and super chlorination. 
• In normal chlorination the concentration of chlorine is brought to such a level that a residuum is left after a contact time between chlorine and water of at least half an hour; this contact time is extended as along as practicable. 
• It is arranged for the residuum of chlorine to be so small that it imparts no detectable smell or taste to the water , so that the water is available for use without further treatment. 
• Difficulties occasionally arise where the water before treatment contains substances that react with chlorine to yield products possessing a powerful and objectionable taste. 
• Traces of phenol compounds for example , even in concentrations as low as 1 part in 2000 millions, react with chlorine to produce chlorphenols which impart a very objectionable and persistent iodoform like taste and smell to the water. 
• These difficulties may sometimes be obviated by super chlorination which results in the complete destruction by oxidation of the substances concerned, but the safest procedure appears to be removal of such compounds before chlorination by filtering the water through charcoal. 
• Super chlorination is adopted where the contact time between water and sterilizing agent is necessarily reduced (where storage tanks are not available or their use is inconvenient. In this process chlorine is introduced into the water to a concentration about ten times that used in normal chlorination. 
• Rapid and complete sterilization is effected in a few minutes but the treated water contains a very high residuum of the free chlorine and must be dechlorinated. 
• Sulphur dioxide (in large scale practice) and sodium thiosulphate ( in small scale practice) are the dechlorinating agents in general use. 
• The advantages of super chlorination lie in the very short contact time required for sterilization and also in the fact that as the high concentration of chlorine tends to oxidize organic matter completely there is less tendency for taints of the chlorphenol type to develop. 
• Chlorination is essentially the addition of “free” chlorine to water; it may be introduced as chlorine gas, as sodium hypochlorite, or as bleaching powder. 
• Chlorine gas is used in large scale practice while sodium hypochlorite and bleaching powder find use in small scale sterilizers. The two recognized procedures for the treatment of water by free chlorine are normal chlorination and super chlorination. 
• In normal chlorination the concentration of chlorine is brought to such a level that a residuum is left after a contact time between chlorine and water of at least half an hour; this contact time is extended as along as practicable. 
• It is arranged for the residuum of chlorine to be so small that it imparts no detectable smell or taste to the water , so that the water is available for use without further treatment. 
• Difficulties occasionally arise where the water before treatment contains substances that react with chlorine to yield products possessing a powerful and objectionable taste. 
• Traces of phenol compounds for example , even in concentrations as low as 1 part in 2000 millions, react with chlorine to produce chlorphenols which impart a very objectionable and persistent iodoform like taste and smell to the water. 
• These difficulties may sometimes be obviated by super chlorination which results in the complete destruction by oxidation of the substances concerned, but the safest procedure appears to be removal of such compounds before chlorination by filtering the water through charcoal. 
• Super chlorination is adopted where the contact time between water and sterilizing agent is necessarily reduced (where storage tanks are not available or their use is inconvenient. In this process chlorine is introduced into the water to a concentration about ten times that used in normal chlorination. 
• Rapid and complete sterilization is effected in a few minutes but the treated water contains a very high residuum of the free chlorine and must be dechlorinated. 
• Sulphur dioxide (in large scale practice) and sodium thiosulphate ( in small scale practice) are the dechlorinating agents in general use. 
• The advantages of super chlorination lie in the very short contact time required for sterilization and also in the fact that as the high concentration of chlorine tends to oxidize organic matter completely there is less tendency for taints of the chlorphenol type to develop.

Chloramine 

A combination of chlorine and ammonia is now widely used for the sterilization of water. 

It is less affected by the presence of organic matter and its sterilizing action is more prolonged than that of chlorine. 

Its only advantage over chlorine is that it does not to the same extent give raise to iodoform and chlorinous tastes in the water. 

Its bactericidal effect is however, very much slower than that of chlorine and long contact is therefore necessary which in many circumstances is a decided drawback to the use of chloramine.

Ozonisation 

Ozonisation is practiced chiefly for the sterilization of swimming bath water.

 

It is generally preferable to chlorination for this purpose as no objectionable taste or smell is imparted to the water. 

It is of interest to note that the efficient sterilization by ozone is due in part to the presence of impurities in the form of oxides of nitrogen which are more toxic to bacteria than ozone itself . 

The wider use of ozonisation is prevented by the high initial cost of the ozone production plant and the high consumption of electric current by the plant in use.

Disposal of Sewage 

Sewage consists of a mixture of solid and liquid human excreta, waste water from dwelling houses and usually road water and rain washings, it may also contain a greater or lesser amount of trade wastes. The disposal of sewage in a manner which will be free from nuisance and danger to public health through contamination of water and food supplies is a matter of great importance in connection with food control and inspection. 

• Sewage and animal health 
• Conservancy Methods 
• Composition and Strength of sewage 
• Disposal of water carried sewage by dilution 
• Purification treatment of water carried sewage 
• Classification of sewage treatment 
• Preliminary treatment 
• Artificial biological treatment 
• Bio aeration or Activated sludge process 
• Chemical Sterilization of Sewage Effluent 
• Small scale sewage purification and disposal 

Sewage and Animal Health 

• It has frequently been alleged that the drinking water polluted by domestic sewage has given rise to symptoms of poisoning in cattle and other animals. 
• It has been affirmed that cattle may be rendered ill by drinking water containing sewage. In this connection it is important to make it clear that there are two kinds of sewage namely household or domestic sewage and industrial sewage. The latter consists of waste products from factories and its composition will naturally vary with the industry concerned, but if it contains some poison whether chemical or bacteriological it is liable to cause harmful effects on stock. Domestic sewage is also a product of variable composition and consists of water containing human faeces and urine as well as kitchen waste. 
• It is especially in connection with the effluent from works engaged in the disposal of domestic sewage that so called sewage poisoning of animals and particularly cattle has been alleged. It has been stated that the symptoms of so called sewage pollution in cattle resemble in some respects those of johne‟s disease , and there can be no doubt that streams contaminated with faeces from cattle with Johne‟ s disease play a large part in the spread of infection. Intestinal parasites such as nematodes are also quite commonly transmitted in a similar manner, and in countries where the human population carries the cestode T. saginata, cattle grazed on the sewage farms become invariably infected with cysticercus bovis from the eggs which have been passed in the human feaces. There is possibility that bacteria pathogenic to cattle may be also conveyed by water. 
• In general human sewage does not injure cattle, since cattle and other stock are commonly kept on sewage farms where they eat grass wet with sewage and even drink sewage with no apparent harm. 
• On the other hand there is plenty of evidence that some industrial effluents are definitely poisonous for animals. Thus cyanide in toxic amounts have been found in the drinking water of animals on several occasions, which is being discharged into drinking water in the effluent from factories. 
• The method of sewage disposal may be divided broadly into the conservancy and water carriage systems. 

Methods of Sewage Disposal

Conservancy Methods 

It is now practically confined to rural areas where the water supply is inadequate for the installation of a water carriage system. The simplest method of disposing of human excreta is burial in the soil. Since proper disposal is of some importance owing to the risk of flies carrying pathogenic bacteria such as typhoid bacilli from infected excreta to the milk. 

The human excreta should be buried in shallow trenches and covered with top soil without chemicals since disinfectants and antiseptics merely retard putrefaction. The faeces decompose without causing any harm. The place chosen for the ultimate deposition should be distant from the house and not near a well. 

Water Carriage System 

In this system the private drains from dwelling houses and other buildings empty into public sewers, which convey the sewage to a purification works where it is treated so as to render the resulting effluent suitable for ultimate disposal into the sea or into rivers and streams.

Composition and Strength of Sewage 

Sewage is a mixture of saline matter in solution and nitrogenous and carbonaeous organic matter in solution and suspension together with a certain amount of grit and mineral matter. 

The strength of an ordinary domestic sewage depends on the amount of water used per head of population. If there are industrial wastes the strength will usually be increased. 

These are of course great variations daily, seasonal and those due to storm water. Industrial wastes such as those from breweries, gas works and dairies etc introduce additional complications. 

Chemical analyses of sewage usually indicted in parts of 100,000 

1. The solids in suspension and in solution 
2. organic nitrogen, ammonical and albuminoid 
3. Chlorides and 
4. Quantity of oxygen required for oxidation of the organic matter to produce a non putrefactive effluent.

In the sewage settling solids will be about two third of the total. About three fourths of the total suspended solids would be organic. The non settling solids are in a very fine state of division or in colloid form. The total suspended matter appears to be very small in the sewage.

Discharge into the Sea and the River

Sea 

• This is a way of disposal of sewage from coastal towns. The following points are taken into consideration. 
• The state of the tides, action of discharging rivers and influence of prevailing winds, tending to carry the sewage to the foreshore the possible deleterious effects of the sewage on boating and bathing. The pollution of the beach by the deposit of solids, the possible injury to fishing and the possible infection of shell fish by sewage contaminated with pathogenic organisms such as typhoid bacilli. 
• The sewage is either discharged in its crude state into the sea , or after it has been screened and treated in a settling tank, but unless the position of the outlet and the tidal flow are particularly favourable, further preliminary treatment is usually carried out. In some cases tanks are used to store the sewage during the raising tide and to enable discharge to take place during the ebb tide only. As a method of purifying sewage screening is not very effective, particularly if the sewage has travelled through several kilometers of pipes, because by the time that it has arrived at the screen it is already in a state of emulsion, the solids having become disintegrated but not reduced while passing through the pipes.

Rivers 

• Where the stream is large enough to give a dilution of over 500 volumes the crude sewage may be discharged into the stream.

Purification Treatment of Water Carried Sewage 

The object of a sewage purification works are 

• To separate the solids and suspended matter, which are for the most part putrifiable from the fluid portion. 
• To purify the fluid portion, so that it can be ultimately discharged into a river or stream with safety and without causing a nuisance and To dispose of the more solid portion of the sewage which becomes settled out to form sludge. 
• The treatment of sewage depends on whether it is generated from residential or industrial area. Sewage from a purely residential town with a good water supply is comparatively easy to treat that from a manufacturing town with a less generous water supply and with the inclusion of industrial wastes presents for more difficulty.
• The volume of sewage is usually stated in gallons per head of population per day, this may very from about 10 gallons up to 100 gallons or even more, depending on the water supply available and the habits of the community under consideration. A common figure for daily dry weather flow is 30-40 gallons per head. 
• The sewerage systems may be separate or combined in the first mentioned separate sewers are provided for sewage and for rain water. In the combined system rain water and sewage are carried in one sewer. The latter method is the more common and where it is adopted the quantity of sewage to be dealt with is of course greatly increased in wet weather.

Classification of Sewage Treatment

Preliminary treatment 

• Arrangement for dealing with storm water 
• Screening 
• Detritus or Grit tanks 
• Sedimentation or Precipitation tanks 
• Disposal of sludge 
• Treatment of tank liquor

Artificial Biological Treatment

• Percolating or trickling filters
• Contact Beds
• Humus Tanks

1) Preliminary Treatment :

Storm water 

The quantum of storm of upto three times the dry weather flow should be dealt with in the works as ordinary sewage. Anything over this amount should be diverted to special storm tanks. 

Screening

This is necessary to remove gross solids such as lumps of faeces or paper etc. The process may consist of coarse or fine screening. In the former method the sewage is passed through fixed screens made up of vertical iron bars which are 1 cm to 5 cm apart. These screens may remove as much as 10 percent of the total suspended matter in the sewage. Fine screens are of various types, the opening being from 0.5 to 1 cm in wide. 

Detritus or Grit tanks 

These are intended to remove the heavier inorganic matter such as road grit which is easily settled. 

Sedimentation or Precipitation tanks 

• These are tanks usually built of concrete, which may be worked with or without the addition of chemical precipitants, and in which finer solids in the sewage are allowed to settle by sedimentation the resultant precipitate or sludge is pumped out daily or weekly as may be found necessary in practice. 
• Tanks may be operated on the fill and draw method or on the continuous method. In the former the top liquid is drawn off leaving the sludge. Continuous flow tanks may be designed for horizontal flow or vertical flow. 
• Horizontal flow tanks are usually long in relation to width. Where the bottom is flat the sludge may be removed after the removal of the supernatant liquid. The capacity of a sedimentation tank is usually fixed to give a period of detention of 6 to 8 hours in the tank. 
• Chemical precipitation of the suspended matter is adopted where sewage is very concentrated or where trade wastes have to be removed. Lime or lime and ferrous sulphate or sulphates of aluminum are used as precipitants. The amount of sludge produced by the aid of precipitants is greater than with simple sedimentation. 

Disposal of sludge

• One of the most troublesome parts of sewage treatment is the disposal of sludge including screenings and detritus. Screenings and detritus are always very offensive and are usually dealt with by shallow burial The screenings may also be burnt. The sludge may also be discharged into the open sea .Where sea disposal is not possible, disposal on land is resorted to either direct or after partial dehydration. Where direct disposal is adopted the sludge is pumped on the land and after drying is ploughed in. 
• Sludge has considerable manurial value but before it can handle the water content must be reduced. This may be done on sludge filter beds , where the sludge is spread on comparatively thin layers of porous material, such as ashes well under drained. In some cities the sludge may be pressed into cake and afterwards converted into manure. 
• Sludge may be digested in deep tanks during which process a considerable amount of gas is given off, consisting chiefly of methane with a small proportion of Co2 and H2. These gases are collected and utilised for power generation. Digested sludge contains of course considerably less organic material than the original and is non-putrefactive. It may be used for raising the level of low lying areas of land without causing any offence. In some cases the digested sludge is used as a base in the manufacture of artificial manure. 

Septic tanks 

If sewage were held sufficiently long in a tank the greater proportion of the organic matter would pass into solution and little or sludge remain such a tank would then become a septic tank. This is due to putrefaction of organic matter under anaerobic condition. Septic tanks constitute a suitable means of sewage treatment for single houses or for all communities provided that not too much inorganic solids are carried in the sewage.

2) Land or Artificial Biological Treatment :

Percolating or trickling filters 

Trickling filters are really aerating beds, not filters they consist essentially of circular beds of broken stone of various sizes on which sewage is distributed. 

The broken stone becomes coated after a short period with a gelatinous covering which in the presence of oxygen forms a nidus for aerobic bacteria which is then said to be ripened or activated. 

Distribution of the tank liquor is obtained by means of a revolving sprinkler driven by a constant head of water or by the intermittent discharge of a siphon. 

Contact beds 

It comprise tanks filled with broken stone the object being to permit digestion of organic material by aerobic bacteria. 

The tank is filled and the liquor allowed to remain in contact with the material in the bed for a sufficient period to allow purification to take place, thereafter the tank is emptied, and in the process air is drawn into the interstices of the stone, thus providing the oxygen necessary for the bacteria. 

The cycle of operation occupies eight to twelve hours, the resting period being about half this to keep the beds in condition. 

Humus tanks 

The effluent from percolating filters and contact beds often contain a good deal of fine organic matter termed humus. 

It is necessary to allow this to settle before discharging the final effluent into a stream. This may be done by holding the filter effluent in tanks.

Bio Aeration or Activated Sludge Process 

In this method screened sewage free from detritus is purified in tanks largely by aerobic bacterial action. 

The bacteria are attached to the particles of ripened or activated sludge which is mixed with the incoming sewage and mixed liquid is kept in continuous motion during its passage through aeration tanks in order to prevent settlement of the sludge, secondly to bring the organic matter in contact with activated sludge and thirdly to supply the bacteria with oxygen.

Chemical Sterilization of Sewage Effluent

Pathogenic bacteria, typhoid and paratyphoid bacilli have frequently been isolated from sewage and sewage effluents. 

To make the effluent innocuous disinfectants such as pure chlorine, bleaching powder and chloros (containing 10% available chlorine) added. 

In the case of a ordinary sewage, it has been found that these would have to be added in the proportion of atleast 10-15 parts of available chlorine per million parts of sewage, in addition a long period of contact would have to be allowed.

Small Scale Sewage Purification and Disposal

Where no public sewer is available, other methods such as the impervious cesspool or a septic tank installation must be adopted for the disposal of sewage. 

Cesspool 

• Cesspools of two kinds may be met with pervious and impervious. 
• The pervious cesspool is a chamber built of concrete or brick or stone rendered in cement and surrounded if necessary by puddled clay, having the bottom open so that the liquid percolates into the soil, this type of cesspool is thus a soak-away pit and it is obvious that to be effective it must be situated in a sufficiently porous sub-soil. 
• In the impervious cesspool the wall and the bottom is made of impervious material through which nothing can be percolate. 
• In both types of cesspool decomposition of the sewage which is affected by bacteria is incomplete so that there is a continuous accumulation of partially decomposed evil smelling material which has to be removed periodically. 
• During the process of decomposition objectionable gases including large volumes of hydrogen sulphide are given off. 
• In the pervious cesspool comparatively little material accumulates; with the impervious type frequent emptying is necessary. 
• In the case of singe houses or farms it is a common practice to  provide an overflow drain, the liquid being disposed of by irrigation over land. When no overflow is provided the whole of the liquid requires to be pumped out. 
• The cesspool is not placed in proximity to a well or other water supply 

Septic tank Installation 

An installation suitable for the purification and disposal of sewage from isolated building usually comprises 

• a single from of septic tank designed to retain the sewage for a period of time sufficient to permit anaerobic digestion of its organic solids and to allow insoluble matter to settle out as sludge and 
• a percolating filter or a contact bed in which the effluent from the septic tank is purified by aerobic bacterial action. The final effluent should be fit for discharge into a stream or for disposal by subsoil irrigation without any potential risk arising. Rain and other surface water should not be allowed access to a septic tank. The sludge needs removal only at long intervals extending in some cases to as long as two years.

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