Opening Unserviced Lots to Building by Use of Septic Tank

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Opening Unserviced Lots to Building by Use of Septic Tank

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_{NATIONAL RESEARCH COUNCIL}

CANADA

DIVISION O F BUILDING RESEARCH

HOUSING NOTE NO. 27

ORIEMIIMG MMSlERVAClElP LOTS TO RILIIAAIPIIMG

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MSlE OF SEPTIC TAMK

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W. B. WATSON

REPRINTED FROM

CANADIAN BUILDER, VOL. XVI, NO.

5 MAY 1966, P.

42-45

OTTAWA,

JULY 1966

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Opening unserviced lots

to

building

T A B L E I L E N G T H O F T I L E R E Q U I R E D T i m e "t" ( m i n ) 50 6 0 o v e r 60 L e n g t h of t i l e / p e r s o n in 1 8 - i n . wide a b s o r p t i o n t r e n c h " 1 5 ( f t )

I

.?, -a* M i n i m u m t o t a l l e n g t h 150 ft

T A B L E 111 DIMENSIONS F O R LOCATING A DISPOSAL B E D D i s t a n c e to D i s p o s a l B e d

I

T A B L E I1 C A P A C I T Y O F SIPHON CHAMBERS

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S h a l l o w w e l l + 1 0 0 ( f t ) ( m i n i m u m )

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L e n g t h of T i l e in 1 8 - i n . Wide A b s o r p t i o n T r e n c h ( f t ) I

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50 ( D r i l l e d w e l l Siphon C h a m b e r C a p a c i t y ( C U f t ) B u i l d i n g

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There are a substantial number of building lots in suburban areas where people wish to build before public sewage systems have been provided. In addition, there are many lots where servicing will probably not be provided because of their location or the nature of the terrain. These include not only rural and cottage sites but also many choice lots for custom homes, where the problem of sewage treatment is often compounded by the presence of an extra bathroom, auto- matic washers, dishwashers and gar- bage disposal devices.

The septic tank has been widely used in the past in such locations. When properly installed with adequate terrain conditions it can be a very acceptable solution for disposal of wastes from isolated houses. The system can also create serious hazards unless its limitations are understood and its use properly controlled. Dis- ease-causing bacteria are not destroy- ed in the septic tank; consequently the liquid effluent will dangerously contaminate any water into which it is discharged. Seepage through the ground may remove the bacteria by the filtering action of the soil or may slow down the flow so that the bacteria will die out before the effluent reaches a source of water. Special problems may arise, however, where the subsoil consists of coarse gravel or fissured rock such as limestone or where relatively impermeable soils such as clay prevent proper absorption. The effectiveness of this system of handling sewage depends to a

considerable extent on the nature of the terrain and the amount of soil available.

Population density is another im- portant factor. Large rural lots or farms where residences are widely separated are usually suitable for septic systems, but when these are enveloped by the suburb of an ex- panding city and reliance must be placed on septic tanks in an area of increasing density, problems can arise. In the suburbs of some cities this increase in population density and subsequent increase in waste water production has been so pronounced

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y

use

of

septic tank

where septic tanks are used that the absorption capacity of the soil is exceeded and the unpurified effluent contaminates the ground water or even flows in open roadbed ditches. This pollution may spread the many water-carried diseases such as infec- tious hepatitis and enteric infections, constituting a very real danger to public health. This concern has led the various Provincial health departments to place restrictions on the use of septic tanks in some areas. The position of the health authorities in this matter must be appreciated. They know the local conditions and from their experience can often predict the problems that will eventually result from unrestricted usage.

The economic effect of pollution is also of importance, because it may appreciably lower the market value of the properties concerned. As a result, mortgage money is becoming increasingly difficult to obtain for properties where municipal services are not provided. CMHC, for ex- ample, since January 1961 has accepted septic tanks for houses built

under the National Housing Act only: (1) in rural areas;

(2) on leftover lots in subdivisions previously approved for septic tank use;

(3) where the municipality provides

evidence that piped services will be available within a reasonable period of time; and

(4) in certain exceptional circum- stances where septic tanks are the only means of servicing the land and written recommendations for their use are provided by the Municipal and Provincial authorities.

Despite its limitations the septic tank is still the principal method of sewage disposal in unserviced areas. It is of prime importance, if it is to be used safely, that the system be properly designed. This note discusses the essential features of septic tank design and performance. Consider- ation is also given to other disposal methods including a small aeration type plant that has shown considerable promise in early trials and may offer a practical alternative to septic tanks

By W. B. WATSON

Research Officer, Building Services Section, Division of Building Re- search, NRC, Ottawa.

for individual sewage treatment in the future.

Septic Tanks

In a properly designed septic tank the solids settle to the bottom where they are broken down by bacteriologi- cal action and their volume reduced by partial conversion to liquids and gases. The remainder stays in the tank, forming the sludge that must be cleaned out occasionally. In a poorly designed tank the sludge flows out to the disposal bed, and this gives rise to the erroneous assumption that all of the solids are converted to liquids and gases.

Even with a properly designed tank the discharge or effluent has a high content of organic matter and micro- organisms that may include disease- causing pathogens. It is the purpose of the tile bed to spread the effluent over a large enough area of soil to purify it by filtration, bacterial action and aeration within the soil. There should be at least 4 ft of soil above bedrock or the water table to enable the bed to function properly. Rocky lots that have little or no soil over-

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burden are unsuitable locations for septic tanks.

One of the principal functions of a septic tank is to act as a sedimentation tank. The velocity of flow is reduced over a long enough period to allow solids to settle to the bottom. The time required for this to take place depends on their size and density, as well as _{the velocity of flow;}

heavy solids will drop out almost immediately, but smaller particles may take several hours. The solids removal efficiency of the septic tank depends on the pattern of flow, its velocity, and the total retention time, and these factors are determined by the design of the baffles, the cross- sectional area and the total capacity of the tank. A two-compartment tank is usually more effective than a single one, because of the additional baf- fling provided by the separating wall. Baffles should also be provided to prevent floating scum, which accumu- lates at the top of the tank, from flowing out with the effluent. It is usual to provide a free space about 12

in. deep above the sewage for the gases that are produced by the action in the tank. These are allowed to escape through a vent on the tank or on the outlet pipe adjacent to the tank.

Design Of A Septic Tank System

Once it has been determined that there is sufficient soil and that the water table is not too high, the next step is to find the capacity of the soil to absorb effluent. Some evidence of this can be determined by the perco- location test. To conduct this test, at least four holes are dug or drilled with a post hole auger to the depth of the absorption trenches. These are kept filled with water for at least 4 hours until the soil is saturated, and then the time (in minutes) for the water to drop 1 in. is recorded. This is the time "t" given in Table I and in the absence of specific Provincial requirements it can be used to deter- mine the length of tile required for each person in the household.

Some standard details of a tile bed are shown in Figure 1. A distribution box should be used to distribute the effluent evenly over the bed and the tile laid in gravel absorption trenches at least 18 in. wide. The trenches should be spaced at least 6 ft apart and have a maximum length of 60 ft. In some cases the length may be

extended to 100 ft, provided a siphon is used in the septic tank to distribute the effluent over the entire run.

Typical construction details of a septic tank are shown in Figure 2. They are usually of concrete; steel is subject to corrosion unless adequately protected. Recently there have been interesting developments using glass fibre reinforced plastics. In sizing the tank the Table in Figure 2 may be used as a guide, subject to the over- riding requirements of the Provincial health authorities.

Siphons are not always required by the regulations, but their use will improve the performance of the disposal bed. A siphon automatically discharges the correct volume of effluent to fill all of the tile approxi- mately two-thirds full, thus spreading it over the entire bed. Without a siphon the effluent will trickle almost continuously and use only the portion of the tile nearest the tank. The siphon, siphon chamber and tile bed must be matched in size to function properly. A 3-in. diameter siphon is adequate for a household tank, and the capacity of the siphon chamber varies according to the length of tile used. The volume of the siphon chamber for various lengths of tile is given in Table 11.

Disposal beds should be sited cer-

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tain distances from wells, lot lines and buildings. Provincial requirements vary, hut as a minimum the dimensions given in Table 111 should he followed.

Maintenance And Operation

A properly constructed septic tank requires little maintenance other than periodic removal of accumulated sludge. This may be as often as once a year, but generally at least every three years, depending on the rate of accumulation.

T o clarify some points of possible confusion it should be noted that it is not necessary to seed a new tank, as there are sufficient organisms in the incoming sewage. Neither laundry wastes nor household cleaners and solvents in reasonable amounts are considered harmful, provided the tank is of recommended size, and it is no longer considered necessary to install a grease trap for kitchen wastes. Large quantities of sterilizing agents will, of course, arrest the growth of organisms necessary for the proper operation of the tank. Freezing in winter is not usually a problem with septic tank systems and the risk can he further reduced by allowing snow to accumulate over the disposal area. A tank associated with a summer cottage or a residence left vacant for the winter should be pumped out in the autumn, however, to prevent dam- age from freezing of the contents. Alternative Disposal Methods

There are few alternative methods of disposal that warrant consideration. Household wastes arc sometimes disposed of separately whcre water is scarce or waste disposal is particularly difficult. The sink and laundry wastes are run on or into the ground and various kinds of water-saving toilets may he used including the chemical and incinerator types. These do not provide the amenities of the conven- tional water closet, but they may he an improvement over more primitive methods.

Of greater interest are the develop- mental studies on small aerobic trcat- ment units for use as a possible alternative to the septic tank. The principle of sewage treatment by acration has long been used in municipal plants, and some attempts have been made in the past to produce units small enough for individual household use. These have met with only limited success, hut recent developments in the field supported by CMHC are showing more promise.

Aerobic treatment units work on a principle entirely different from that of the septic tank. Oxygen is supplied to the sewage by an air compressor to oxidize the organic material and to encourage and maintain a population of aerobic (air using) bacteria that are more effective in purifying the sewage than the anaerobic organisms found in a septic tank. Under good conditions they can be made to destroy a large proportion of the organic matter present.

The system consists of two parts, the aeration tank (or tanks) and a sedimentation chamber (Figure 3). Sewage from the house enters the aeration tank, where it is aerated continuously by the compressor, and then flows over into the sedimentation chamber to be clarified before it goes to the disposal bed. Sludge accumulated in the sedimentation tank is automatically returned to the aeration tank at predetermined intervals and the continuous aeration eventually oxidizes all the organic materials. The higher quality effluent from these units does not have the same pollution potential as the septic tank effluent, and may be disposed of in smaller tile beds. The possibility of using these systems has been under study for several years by the Ontario Research Foundation for Central Mortgage and Housing Corporation. Another part of the ORF-CMHC Research Program is directed towards the development of a totally enclosed recirculating unit. In this system only the toilet is connected to the treatment unit, and the effluent is recirculated

and used as the flushing liquid. Such a system, with various modifications, was installed in the NHBA Mark I11 and Mark IV research houses at Rockcliffe, as well as in a school at Cape Dorset, N.W.T. for trial. Results so far are satisfactory. The process may he of particular value for isolated Arctic regions and in areas where water is scarce. The laundry and other wastes are not treated in this system and must he disposed of elsewhere.

Regulation For Private Sewage Disposal Plants

The control of private sewage disposal systems comes under the juris- diction of the Provincial health departments, and regulations pertaining to design and construction are en- forced by the local medical health officers. These authorities issue per- mits for construction of private systems that must conform to their regulations. In areas where there is no municipal organization, advice and assistance may be obtained from the Provincial Departments of Health.

The comments given in this paper are no way intended to conflict with the recommendations or regulations of these authorities, and wherever such conflict exists their regulations must naturally govern. There is a need, however, for information on individual sewage disposal systems and it is hoped that the contents of this paper will supplement the information available from various other sources and provide an appreciation of what a septic tank will or will not do.

C o m p r e s s e d a i r

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