Coastal Pollution Control

Marine Pollution may arise from various sources such as (i) domestic sewage (it) Industrial waste (iii) dumping of waste materials (iv) sewage and rubbish discharge from ships and from handling cargo (v) exploration and exploitation of sea bed and ocean floor (vi) accidental pollution by oil and other toxic substances due to accidents of cargo ships (vii) various other routes from land.

In India sewage from coastal towns and cities is discharged into sea as a temporary measure till such time inland arrangements for sewage treatment and disposal are made. Madras city alone is pumping 30 mgd.

sewage to sea.

In view of unsatisfactory nature of disposal arrangements made and health hazards involved thereby in the existing sea outfalls, there is a general feeling against sea disposal of sewage and treatment and disposal on land is preferred even for coastal towns in India.

Sewage disposal on land would involve more extensive area and as a result disposal site will be far away from town involving extensive length of pumping main or outfall. The health hazards involved by extensive sewage farming are of water pollution and mosquito nuisance. Sea food grown in polluted waters may constitute a serious health hazard. In many parts of the world, the use of industrial and agricultural chemicals has been increasing at an alarming rate and this has also resulted in a widespread contamination of environment including the sea. Almost all the sea food is found to contain residues of toxics. There are at present very few data in the overall area of marine toxicology, as it relates to pollution. Future monitoring system should take into consideration all the various toxicological aspects relating to marine pollution. New systems of automated sample analysers may be required coupled with long range sensor

By N. T. Desai,

SUPERINTENDING ENGINEER

PUBLIC HEALTH CIRCLE, AHMEDABAD.

system and a rapid communication and data retrival programme on an international scale.

Waste Treatment for Marine
Disposal

Two essential steps have to be taken before proceeding with the design of a purification scheme.

1. Data has to be collected on the source of pollution.

biological analysis, of waste water, time series of analysis frequency analysis of occurance and study of future conditions.

This will require physical, chemical and

2. Data on receiving water prior to the discharge of treated effluent.

The subsequent step is to define the problem, which involves the treatmet technique and formulation of models for prediction of changes imposed upon water quality by the discharge.

Design is usually based on experience and trial and error technique.

A reasonable preliminary design is first chosen based on experience. The cost of establishment and maintenance is worked out. Based on predicted parameters for effluent and receiving water a prediction for changes to the water quality. The resulting water quality is then compared with the requirements. If these require ments are not fulfilled, changes have to be made to the design.

Waste water purification is the treatment in which the material is subjected to 'Unit Operation' and 'Unit processes'. In 'Unit operations' the material is treated without changing property. Following operations are involved.

Water Treatment For Small Communities

The commonest method of filtration of public water supply is to pass it through a layer of sand. It has been found from past experience that by passing water through sand, the suspended and collodial matter is practically removed, the chemical characteristics are changed and the number of bacteria is materially reduced. These phenomena are explained on the basis of 4 actions:

1. Mechanical straining.

2. Sedimentation and absorption. 3. Biological metabolism.

4. Electrolytic changes.

The ability of the water to be filtered, or its filterability, has been explained quantitatively as the volume of water obtained per unit of head loss when passed at a standard rate through a unit area of a standard filter. This is known as the filterability index. The higher this index the greater the volume of water that will pass between the cleaning of the filter.

There are two types of granular water filters the slow sand filter and the rapid sand filter. They differ hydraulically and structurally:

1. because they are intended to be operated at relatively slow rates (two to 4 gal./hr./sq. ft.) or relatively fast rates (100 gal./hr./sq. ft).

2. because penetration of suspended matter and its consequential removal during the cleaning operations are purposely confined to surface layer in the "slow sand filter" while the full depth of sand of rapid filters is intended to contribute to purification and hence also to require cleaning.

The slow sand filter therefore requires a very large area for getting the required quantity of water as compared to the rapid filter. However, while the latter requires various types of mechanical equipment for its proper working and also particular arrangements for cleaning of the filters the

scraped out and a new sand layer is put on top. In the rapid sand filter the same sand can be used for many years without replacement as the cleaning of the sand is effected almost daily.

Before World War II all the mechanical equipment for rapid sand filters was being imported from foreign countries and there were only a few specialised firms in the market to provide such filtration plants.

During the war it became very difficult to obtain such equipment from foreign countries and the defence departments were in urgent need of providing some type of water treatment plant for their camps, hospitals etc. with completely indigenous materials. The Chief Engineer, southern Command, therefore, approached the then Bombay State to design a simple type of Public Health Engineering Department of water treatment plant for them which could be quickly constructed using indigenous equipment. After some experimentation the Department designed a new type of filtration plant which was a hybrid of the slow sand filter and the rapid sand filter, the aim being to take the good qualities of

the rapid sand filter and at the same time avoid the use of expensive mechanical equipment which was not readily available. With this type of filter, a rate of 25 to 30 gal/hr/sq. ft of filter area was easily obtai

ned if the head of water on the filter bed was increased slightly. It was also considered desirable that the filter beds should not be kept open to the sun but should be kept covered to avoid fromation of algae. On the basis of the above observations a filtration plant plant was designed and was named the Semi-Rapid filter. This filter was designed with a filtration rate of 30 gal/hr/sq. ft. and many filters of this type were constructed for various military camps in Bombay State during the war years. Some of these are still in operation.

Based on the observations and data obtained on the working of such filtration plants the Public Health Engineering Department also constructed a few such plants for small communities. One was put up at Alandi Pilgrim centre and this plant has been working for the last twenty years and has been found to be working satisfactorily and economically.

Clarification of Water :-In all such water treatment plants proper pre-treatment of water is essential if the filter is to work satisfactorily. At no time the turbidity of the water going into the filter bed should be more than 30 ppm. The waters of the rivers of Maharashtra State are for nearly 8 months quite clear but during the monsoon months the turbidity rises as high as 1000 ppm and clarification becomes an essential part for such treatment. The department therefore designed a simple flocculator and clarifier arrangement with a minimum of equipment. The flocculator consists of wooden or steel paddles on a shaft with a reduction gear, driven by a 1.5 HP motor or a small diesel engine.

Clarifier-This is a simple circular tank with a low overflow rate and a serrated bottom. For collecting the effluent a weir is provided over a portion of the periphery. There is absolutely no equipment required. Due to the serrated bottom the sludge is easily removed by hydrostatic head. With a properly regulated slum dose and regular

sludge removal the required water with low turbidity can be easily obtained.

Filters-The filters consist of plain masonry tanks. The underdrainage system comprises of Asbestos pipe channel with a central collecting pipe. The sand provided is coarse sand of 27" thick layers supported on graded gravel of a total depth of 21". The sand provided is local sand but it must be clean, round and properly graded. The size of sand provided is that passing through 8 meshes to an inch and restrained on 16 meshes to an inch. Due to the coarse sand the filter clogs very slowly and it is found to work for more than one month. After this it is thrown out of operation and the upper layer of sand is manually removed washed and replaced. This operation takes about three days. It is therefore necessary to provide one stand by filter. The experience gained shows that it is not necessary to provide any expensive sand washing arrangement.

A design for supplying water at the rate of 15,000 gal/hr, embodying all the above features is enclosed for ready reference. The writer is confident that if such a filter is constructed for a small community using river, canal or tank water, the water can be economically treated.

Other advantages of the semi-rapid filter are that such plants are within easily construction controlable limits and the costs are not so great as for the slow sand filter and considerably below those of the rapid sand filter. The staff required to operate such a filtration plant can be easily and readily obtained from the community and no specially trained staff is necessary.

The chemicals required for treatment of the water is only aluminium sulphate which is readily available in this country.

The filtered water obtained from such a

treatment plant will be of very low turbidity and will safely meet the necessary standards.

The dose of chlorine or T. C. I. will always be very small.

For construction of such a plant ordinary local staff can be depended upon and there is no need for the services of specialists.