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by Dr. Vincent J. Emmanuel

Septic tanks are still a popular method of disposal of domestic wastes in non-sewered semi-urban and rural areas. Most people are not conversant with the scientific aspects of this system of disposal, and rely on the fact that it is a method in common use.

Any mis-information about sewage treatment in septic tanks, coming from seemingly reliable sources, can have serious consequences with bona fide users who venture out on the basis of incorrect information.

In recent times pre-cast plastic septic tanks have been offered for sale, supported by incorrect and totally misleading statements about the biological treatment process utilised in a 'septic tank'. It is in the interests of the reading public that these misconceptions and misdirection, whether mischievous or bona fide, are corrected to ensure that aggressive marketing interests do not displace the truth.

The statements of questionable veracity are reproduced below, partly to focus attention on them, before demonstrating their fallacies.

1. "....." is the new, clean, safe and environmentally friendly method of toilet waste disposal.

2. . . . new sewerage waste recycling system recently introduced to the market...

3. And relieves architects, contractors and householders of... health risks, and environmental pollution concerns that have been caused by old-style septic tanks.

4. . . . system that recycles toilet waste effluent and releases odourless water that is clean enough to be channelled into the public drainage system or water the garden.

5. Since the tanks are completely leak proof, wells can be dug at very close proximity.

6. The use of "...." tanks will prevent water-borne diseases and other health hazards caused by the contamination of ground water.

7. This natural process effectively purifies the effluent to such a high level that the water released from the "...." tank can be safely connected directly into the public drainage system, or re-used for watering the garden! Further filtration could even provide recycled drinking water.

Advertising

The source of the advertising material calls this a "revolutionary new... Sewerage Waste Recycling (SWR) tank", thus revealing unfamiliarity with the terms 'sewerage' and 'recycling' as they are used in environmental engineering practice. For the information of the reader, the term 'sewerage' is used to describe the system of pipes that conveys wastes to a treatment plant.

The waste is called sewage and any treatment process applied to the (liquid) domestic waste is known as sewage treatment.

As a prelude to analysing the marketing superlatives in the newspaper article, an understanding of the biological process applied in a septic tank, and its limitations, would give the reader a more meaningful insight. To start with, there is nothing new with the biological process used in a septic tank - the system was first used in the United States as far back as 1883.

Karl Imhoff and Gordon M. Fair, two world authorities in environmental engineering, whose names are linked with the development of the science of environmental engineering, described septic tanks as "plain sedimentation basin in which the deposited sludge is held sufficiently long to undergo partial and possibly complete digestion. This primitive form of digestion tanks is no loner used in municipal treatment works, because septic action cannot be confined to the sludge proper, but reaches up into the flowing sewage. As a consequence, the effluent from septic tanks generally possesses a high immediate B.O.D., smells of hydrogen sulphide and contains gas-lifted solids in suspension. Subsequent biological treatment becomes more difficult. In properly proportioned small treatment works and in residential and industrial sewage disposal, however, septic tanks still serve an useful purpose. Here their bad features are offset in part by the advantage that they are easy to build and need but little attention."

A much later WHO Monograph on "Excreta Disposal for Rural Areas" states: The effluent "is still offensive in character; on standing it yields little sediment but has a characteristic putrid odour. In addition, this effluent is potentially dangerous to health, as it may contain pathogenic bacteria, cysts, and worm eggs which have passed unharmed through the tank during the relatively short detention period." This WHO Monograph provides a lucid explanation of the manner in which bacterial and chemical soil pollution moves in the ground, to enter the water bearing stratum, polluting well-water sources, and the physical factors that govern it.

Option

In considering the use of Septic tanks as an option for disposal of their domestic wastes, most people overlook the fact that the 'septic' tank consists of two components, the septic tank proper where biological treatment of the sewage waste takes place and the soak-away system which serves as the means of disposal of the effluent into the soil. Of these two components, Environmental Engineers will agree that the latter is the more critical. In fact, very little can go wrong with a septic tank provided some basic conditions are observed, but the design of an efficient leaching system is much more complex. When a septic tank fails, more often than not, it is because the soak-away system has failed, causing the septic tank to "back up" with water, and so interfering with the entire system, from the flushing cistern down to the septic tank. In fact it would be true to say that the septic tank is only as good as its soak-away system.

It is significant that the Joint Committee on Rural Sanitation set up by the Public Health Service of USA, in 1947 (4), which made the most comprehensive study of Septic tank design and efficiency ever undertaken, termed this as the "septic tank - soil absorption system", indicating quite clearly that the septic tank alone was not the complete disposal system. In fact, the literature shows (3) that the disposal field provides the "secondary" treatment, where nitrifying (aerobic) bacterial in the soil convert some of the dissolved organic matter (in the effluent) to stable in organic compounds.

One of the objectives of the Joint Committee was to establish whether the numerous modifications and innovations to the Septic tank, claiming improved performance, were factual, and if so, to evaluate the benefits. Having examined a host of modified designs over a period of 6 years, the Committee reported that the "improved designs" had only marginal impact on the quality of the effluent. The Report (two volumes) was published in early 1954, and was discussed at Harvard Graduate School of Engineering by no less an expert than Prof. G. M. Fair, when the writer had the privilege of participating as a graduate student.

Domestic

The Septic tank was fist developed for purely domestic applications, as the handling of night soil is a very unpleasant and in-sanitary task. The advantage of the Septic tank (on a domestic scale) is that it requires attention only once in several years, while being an acceptably sanitary way of dealing with human wastes. It is also pertinent that it was originally used in farm houses, where the toilets were located on sizeable parcels of land. This method of sewage treatment requires that the effluent be soaked or "leached" into the soil, and Local Authorities generally require that the soak-away system should be around 20 meters from any water source. The reason for this will be evident from the explanation provided in the WHO Monograph on the movement of pollution through the soil.

With urbanisation, land became scarce and it became impractical to provide a distance of 20 meters from the leaching field to the well, if indeed a well was used as the source of domestic water. The use of septic tanks to serve houses on small blocks of land became increasingly difficult. However, with the availability of piped water from the city supply, the danger of drinking polluted ground water ceased to be a health hazard. Septic tanks are therefore the preferred choice for the disposal of domestic sewage, in urban areas which are not sewered, but served by a town piped water supply.

Needless to say, if the septic tank is not cleaned out over an extended period of time and is allowed to fill up with settled sewage solids, the entire system will fail, as the septic tank can no longer store solids, thus forcing the sewage to pass the soakage system. The solids entering the soakage system, will clog the pores of the media in the percolation system.

Two types of bacteria are responsible for the breaking down of organic matter (sewage), the Anaerobic bacteria and the Aerobic bacteria. Anaerobic bacteria can live and proliferate only in the absence of air. Aerobic bacteria, on the other hand, require a plentiful supply of oxygen for their growth and action. A septic tank which excludes air, uses anaerobic bacteria, solely, for the reduction/digestion of the sewage matter.

The Anaerobic bacteria use most of the sewage solids as food and convert a small part of the solid organic matter into liquid. In consuming the larger part of the sewage solids as food, they cause a substantial reduction in the volume of the sewage solids in the holding tank (septic tank) and given time, they will "digest" the organic solids, converting them into stable in organic compounds. This explains how a considerable reduction in volume of sewage solids occurs over time, and why a septic tank requires to be cleaned out only once every few years. It has to be appreciated that anaerobic bacteria do not have any action on the dissolved organic matter.

Organic

By converting some of the solid organic matter into solution, these bacteria further reduce the solid organic matter, but increase the content of dissolved organic matter. Thus the effluent contains more dissolved organic matter than it originally did, and is therefore putrescible. This means that the effluent can decompose further, producing unpleasant odours, and can pollute the ground water, as explained earlier. It is significant that only aerobic bacteria can extract dissolved organic matter from the liquid.

The clarification provided above establishes two facts:

1. The septic tank is a "primary" treatment process, in which the solid organic matter is separated from the liquid, and held in the tank for anaerobic bacterial action. The bacteria use the organic matter as food, thus causing a substantial reduction in volume. Some solid organic matter is converted into liquid. The liquid passes into a leaching or soakage system for absorption by the soil.

2. As much liquid as enters the septic tank passes out of the tank into the leaching system, as a continuous process. This is why a septic tank is known as a "continuous-flow sedimentation tank". It is essential that the soakage system should be able to readily absorb the effluent from the septic tank. Any flooding of the soakage system will seriously affect the operation of the septic tank, causing failure of the septic tank, even though the septic tank itself is otherwise in "working" condition.

Failure of the soil absorption component of the system causes the soil absorption system/soakage system/leaching field to fill up with effluent. When this happens the septic tank fills up above the level of the inlet pipe, resulting in an air-lock between the septic tank and a flushing device such as a water closet. Flushing of the toilet then results in very slow discharge of the toilet bowl, and the failure of the sewage solids to be siphoned out of the bowl.

The pre-cast plastic septic tank incorporates a coarse sand filter through which the effluent has to pass before it exists the septic tank. This sand filter will reduce the suspended solids content of the effluent during early operation, but will soon choke with humus and become inoperative, resulting in failure of the system.

Environmental engineers routinely carry out a field "percolation factor" test to determine the rate of percolation of the soil, for the purpose of deciding on the leaching field area and other design elements required for the disposal of the daily discharge.

The documented evidence shows that:

a) the effluent from the septic tank has, ideally, to be soaked into the soil almost as fast as it enters the soakage system, and

b) the effluent from the septic tank is far from safe, and cannot, therefore, be discharged into any open drain system.

The septic tank is unsuitable for the disposal of sewage from large communities such as hotels, housing developments or resident schools, because:

i) the effluent from the septic tank is far below the quality stipulated by the Central Environmental Authority for discharge into surface water courses, and

ii) the volume of effluent produced is so large that it is impractical to dispose of the effluent by soakage into the ground. Alternatively, the leaching field will take up a great deal of land.

The pamphlet issued by the manufacturer claims that the effluent from their septic tank could be discharged into the 'public drainage system'. The figure shown in the pamphlet is quite intriguing, as the reader may interpret this to mean the roadside drain, while the manufacturer can argue (if contested) that the figure shows a public sewer! Obviously, if a public sewer is available, there will be no need for a septic tank! It is a matter for conjecture whether the quibble was intentional.

Authority

No regulatory authority (local authority) in the world will allow for the affluent from a septic tank to be disposed of into a public drain or any surface water course. Any claim that "you can even dispense (the effluent) into the drainage system or use it for garden watering" is misleading erroneous and mischievous, as no doubt, the Central Environmental Authority or the Colombo Municipal Authority will verify. It is significant that the Central Environmental Authority will not countenance even sewage that has been put through 'full' treatment, producing an effluent with a Biochemical Oxygen demand of 30 mg/l and a Suspended Solid content of 50 mg/l to be used as garden water, without disinfection by chlorination. Septic tank effluent falls far, far short of this standard!

The claim that the septic tank will not contaminate and/or will 'prevent water-borne diseases' is irrelevant and insupportable, as it is not the septic tank that pollutes the soil or contaminates drinking water, but the soakage system or leaching field. The claim that this system is 'deal for low-lying areas' is equally baseless, as the problem in low-lying areas is not the septic tank, but the soakage system.

The introduction of a pre-cast septic tank (provided it embodies the minimum design features) is welcome, as it saves the builder some labour and expense. In fact, pre-cast septic tanks of cement concrete have been in use for more than two decades. A pre-cast septic tank of structurally suitable plastic is an improvement on the concrete septic tank, as it is easier to handle and is free from acid attack (a common problem with cement tanks). The plastic septic tank could have stood on its own merits, instead of seeking the dubious support of fictitious claims born of marketing exuberance. A local authority which is called upon to approve a septic tank for a residential unit in a non-sewered urban or semi-urban area which is served by a city water supply, will generally approve the system, without concerning itself with the efficiency of the soakage system. They will not, however, approve a system which shows that the effluent is discharged into a public drain.

Engineers and architects engaged in the building industry welcome the introduction of improved and innovative materials, devices and appurtenances. While they would be cautious with untried materials and devices, they would be suspicious of extravagant claims, some of which may be in conflict with known facts. Marketing exuberance should be tempered with fact. Such flights of fancy can turn out to be counter-productive, at the very best.

(References: (1) Karl Imhoff, Consulting Engineer, Essen, Germany. (2) Gordon Maskew Fair, professor of Sanitary Engineering, Harvard Graduate School of engineering, Cambridge, Mass, USA, joint authors of 'Sewage Treatment', John Wiley & Sons Inc., New York. (3) Excreta Disposal for Rural Areas and Small Communities, E. G. Wagner & J. N. Lanoix, World Health Organisation, Geneva (1958). (4) Studies on Household Sewage Disposal Systems, Federal Security Agency, Public Health Service, Health Service, Environmental Health Center, Cincinnati, Ohio.)

(The writer was Chief Public Health Engineer, in the Ministry of Health, till 1964, when he joined WHO as a WHO Consultant, serving in that capacity in Nepal, India and Indonesia over a period of 15 years. He is the Chairman of the Sri Lanka branch of the Chartered Institute of Water and Environmental Management. He is a Life Fellow of the Institution of Civil Engineers, London, the Chartered Institute of Water and Environmental Management and the Sri Lanka Institute of Engineers. He is the author of the "Handbook of Applied Sanitary Engineering for Architects and Building Services Engineers.")

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