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close this book Water purification, distribution and sewage disposal for Peace Corps volunteers
close this folder Section 9: Water carried sewage systems construction and maintenance
View the document Overview:
View the document The septic tank
View the document Operation and maintenance
View the document Lesson plans

Overview:

The construction of the various water carried sewage systems is covered in this section. The trainees are instructed in the testing of sites for the water and sewage disposal and in the operation and maintenance of these systems. Again the health standards to be improved must be stressed so that the design and execution of the project will achieve these results.

SECTION 9: WATER-CARRIED SEWAGE SYSTEMS

CONSTRUCTION AND MAINTENANCE

OBJECTIVE: Construct and maintain cesspools, seepage pits, and septic tanks.

TASKS:

1. Determine absorption area requirements.

2. Locate site of construction.

3. Set up a construction program.

4. Set up a maintenance program that identifies:

a. How often the system must be cleaned.

b. The number of personnel needed to clean the system.

c. The equipment needed for maintenance.

FUNCTIONAL SKILLS:

1. Identify the components of various types of septic tanks, cesspools, and seepage pits.

2. Identify the logical sequence of operations involved in building septic tanks, cesspools, and seepage pits.

3. Conduct percolation tests.

4. Draw a map of the proposed system and the location of its components.

5. Identify the maintenance requirements of various types of water-carried sewage systems.

6. Identify the skills a person would need to maintain various types of water-carried sewage systems.

TERMINAL PERFORMANCE TESTS:

1. Design, locate and construct a water-carried sewage system that satisfies the requirements of Section Village Privy Design Criteria.

2. Prepare a manual on the maintenance of septic tanks, cesspools, end seepage pits.

LOCATION AND SELECTION OF WATER-CARRIED DISPOSAL FACILITIES

SPECIFICATIONS

There are minimum distances that different types of disposal facilities should be placed in relation to water sources and dwellings. These distances will increase in direct proportion to the porosity of the soil.

TABLE 12: DISTANCES FROM VARIOUS SOURCES TO DISPOSAL FACILITIES

Component

Septic Tank

Leaching Field Seepage Pit and Cesspool

Building Sewer

Privy

 

Feet

Feet

Feet

Feet

Well or suction line

50

50*

(a)

50*

Water supply line (pressure)

(b)

(b)

(b)

(b)

Property line

10

10

-

30

Dwelling

5

20

-

30

Surface water supplies or tributaries, including? open and subsurface drains

50*

50*

50*

 

Watercourses, including streams, ponds, open and subsurface drains

10

25

   

Edge of fill

 

25

   

 

Table I. Sanitary Facilities Location Requirements

* 50 feet is a minimum acceptable distance.

(a) 10 feet if constructed of durable corrosion resistant material with watertight joints, or 50 feet if any other type of pipe is used.

(b) Disposal facilities should be installed as far as possible from water supply lines. Where sewer lines cross water supply lines, both pipes should be constructed of durable corrosion resistant materials with watertight joints.

SUITABILITY OF THE SOIL

Along with the specifications mentioned above, the location and implementation of water-carried sewage depends on the suitability of the soil. The first step in the design of subsurface sewage disposal systems is to determine whether the soil is suitable for the absorption of septic tank effluent and, if so, how much area is required. The soil must have an acceptable percolation rate*, without interference from ground water or impervious strata below the level of the absorption system. In general, two conditions must be met:

1. The percolation time should be within the range of those specified in Table

2. The maximum seasonal elevation of the ground water table should be at least 4-feet below the bottom of the trench or seepage pi.. Rock formations or other impervious strata should be at a depth greater than 4-feet below the bottom of a cesspool or seepage pit. Unless these conditions can be satisfied, the site is unsuitable for a conventional subsurface sewage disposal system.

TABLE 13: REQUIRED ABSORPTION AREA FOR GIVEN PERCOLATION RATES

Percolation rate (time required for water to fall one inch, in minutes)

Required absorption area in sq. ft. per standard trench and seepage beds

1 or less

70

2

85

3

100

4

115

5

125

10

165

15

190

30

250

45

300

60

330

 

* A percolation test is a test to determine the rate of flow of water through the interstices or pores of a soil.

The soil should be considered unsuitable for seepage pits if the percolation rate is over thirty and unsuitable for any subsurface disposal system if this rate is over 60.

PERCOLATION TESTS

Subsurface explorations are necessary to determine subsurface formations in a given area. An auger with an extension handle, is often used for making the investigation. Wells and well defiles, logs can also be used to obtain information on ground water and subsurface conditions. In some areas, subsoil strata vary widely in short distances, and borings must be made at the site of the system. If the subsoil appears suitable, as Judged by other characteristics described in section 2 below, percolation tests should be made at points and elevations selected as typical of the area in which the disposal field will be located.

The percolation tests help to determine the acceptability of the site and establish the design size of the subsurface disposal system. The length of time required for percolation tests will vary in different types of soil. The safest method is to make tests in holes which have been kept filled with water for at least 4 hours, preferably overnight. This is particularly desirable If the tests are to be made by an inexperienced person, and in some soils it is necessary even if the individual has had considerable experience ( as in soils which swell upon wetting). Percolation rates should be figured on the basis of the test data obtained after the soil has had opportunity to become wetted or saturated and has had opportunity to swell for at least 24 hours. Enough tests should be made in separate holes to assure that the results are valid.

The Procedure for a Percolation Test

This percolation test incorporates the principles cited above. Its use is particularly recommended when knowledge of soil types and structure is limited.

A. Number and location of tests.

Six or more tests shall be made in separate test holes spaced uniformly over the proposed absorption field site.

8. Type of Test Hole

Dig or bore a hole, with horizontal dimensions of from 4 to 12 inches and vertical sides to the depth of the proposed absorption trench. In order to save time, labor, and volume of water required per test, the holes can be bored with a 4 loch auger.

C. Preparation of Test Hole

Carefully scratch the bottom and sides of the hole with a knife blade or sharp-pointed instrument, in order to remove any smeared soil surfaces and to provide a natural soil interface into which water may percolate. Remove all loose material from the hole. Add 2 inches of coarse sand or fine gravel to protect the bottom from scouring and sediment.

D. Saturation and Swelling of the Soil

It is important to distinguish between saturation and swelling. Saturation means that the void spaces between soil particles are full of water. This can be accomplished in a short period of time. Swelling is caused by instruction of water into the individual soil particle. This is a slow process, especially in clay-type soil, and is the reason for requiring a prolonged soaking period.

In the conduct of the test, carefully fill the hole with clear water to a minimum depth of 12 inches over the gravel. In most soils, it is necessary to refill the hole by supplying a surplus reservoir of water, possibly by means of an automatic syphon, to keep water in the hole for at least 4 hours and preferably overnight.

Determine the percolation rate 24 hours after water is first added to the hole. This procedure is to insure that the soil is given ample opportunity to swell and to approach the condition it will be in during the wettest season of the year. Thus, the test will give comparable results in the same soil. whether made in a dry or in a wet season. In sandy soils containing little or no clay. the swelling procedure is not essential. and the test may be made as described under item E 3 after the water from one filling of the hole has completely seeped away.

E. Percolation-rate measurement

With the exception of sandy soils. percolation-rate measurements shall be made on the day following the procedure described under item D above.

1. If the water remains in the test hole after the overnight swelling period, adjust the depth to approximately 6 inches over the gravel. From a fixed reference point, measure the drop in water level over a 30 minute period. This drop is used to calculate the percolation rate.

2. If no water remains in the hole after the overnight swelling period. add clear water to bring the depth of water in the hole to approximately 6 inches over the gravel. From a fixed reference point. measure the drop in water level at approximately 30 minute intervals for 4 hours, refilling 6 inches over the gravel necessary. The drop that occurs during the final 30 minute period is used to calculate the percolation rate. The drops during prior periods provide information for possible modifications of the procedure to suit local circumstances.

3. In sandy soils (or other soils in which the first 6 inches of water seeps easy in less than 30 minutes, after the overnight swelling period). the time interval between measurements shall be taken as 10 minutes and the test run for one hour. The drop that occurs during the final 10 minutes is used to calculate the percolation rate.


Fig. 87 Methods of making percolation tests

Guide For Estimating Soil Absorption Potential

A percolation test is the only known means for obtaining a quantitative appraisal of soil absorption capacity. However, observation and evaluation of soil characteristics provide useful clues to the relative capacity of a soil to absorb liquid. Most suitable and unsuitable soils can be identified without additional testing. When determined and evaluated by trained or experienced soil sceentists or soil engineers, soil characteristics may permit further categorizing of suitable soils. This has been done for some areas of the country and described in the soils reports mentioned below.

Soil Maps

The capacity of a soil to absorb and transmit water is an important problem in agriculare, particularly in relation to irrigation, dr drainage, and other land management practices. Through studies in these fields, a variety of aids have been developed for Judging the absorption of water transmission properties of soils, which could be helpful in the sewage field. Considerable information has been accumulated by agricultural authorities on the relative absorption capacities of specific soils in many areas of the United States. Much of this information is included in Soil Survey Reports and Maps published by the United States Department of Agriculture in cooperation with the various State agricultural colleges. The general suitability of specific soils for effluent disposal may often be interpreted from these reports and maps.

Clues to Absorption Capacity

Considerable information about relative absorption capacities of soils may also be obtained by a close visual inspection of the soil. The value of such an inspection depends upon some knowledge of the pertinent soil properties. The main properties indicative of absorption capacity are soil texture, structure, color, depth or thickness of permeable strata, and swelling characteristics.

Texture

Soil texture, the relative proportion of sand, silt, and clay, is the most common clue to water absorption capacity. The size and distribution of particles govern the size and distribution of pores which, in turn, govern the absorption capacity. The larger the soil particles, the larger are the pores and the faster is the rate of absorption.

Texture can best be Judged by the feel. The lighter or sandier soils have a gritty feel when rubbed between the thumb and fore-finger; silty type soils have a "floury" feel and, when wetted, have no cohesion: heavier, clay type soils are dense and hard when dry, and have a slick greasy feel when wetted.

The use of texture as a clue to absorption qualities has its limitations; it is primarily reliable in the sandier soils. In the heavier type soils, including sandy soils containing appreciable amounts of silt or clay, one must look for additional clues, such as structure and soil color, as indicators of absorption capacity.

Structure

Soil structure is characterized by the aggregation or grouping together of textural particles, forming secondary particles of larger size. Such secondary particles then tend to govern the size and distribution of pores and, in turn, the absorption properties. Structure can easily be recognized by the manner in which a clod or lump breaks apart. If a soil has structure, a clod will break with very little force, along well defined cleavage planes, into uniformly sized and shaped units. If a soil has no structure, a clod will require more force to break apart and will do so along irregular surfaces, with no uniformity in size and shape of particles.

In general, there are four fundamental structure types, named according to the shape of the aggregate particles: platy, prism-like, block-like, and spheroidal. A soil without structure is generally referred to as massive. Spheroidal structure tends to provide the most favorable absorption properties, and platy structure, the least. Although other factors, such as size and stability of aggregates to water, also influence the absorption capacity, recognition of the type of structure is probably sufficient for a general appraisal.

Color

One of the most Important practical clues to water absorption is soil color. Most soils contain some iron compounds. This iron, like iron in a tool or piece of machinery, if alternately exposed to air and to water, oxidizes end takes on a reddish-brown to yellow oxidized color, it indicates that there has been free alternate movement of air and water in and through the soil. Such a soil has desirable absorption characteristics. At the other extreme are soils of a dull gray or mottled coloring, indicating lack of oxidizing conditions or very restricted movement of air and water. These soils have poor absorption characteristics.

Depth or Thickness of Permeable Strata

The quantity of water that may be absorbed is proportional to the thickness or volume of the absorbent stratum, when all other conditions are alike. In a soil having a foot or more of permeable material above tight clay, absorption capacity is far greater than that of the same kind of material lying within 3 inches of tight clay. When examining soils or studying soil descriptions, the depth and thickness, therefore, are important criteria of absorption capacity.

Swelling Characteristics

Most, but not elf, clays swell upon the addition of moisture. There are many clays (in the tropics, in particular) that do not swell appreciably. There are also some soils in the United States which do not swell noticeably. On the other hand, some soils have a very high percentage of swelling, and these in particular must be suspect. relative swelling of different soils is indicated by relative shrinkage when dry, as shown by the numbers and sizes of cracks that form. Those that shrink appreciably when dry are soils that may give trouble in a tile field when they are wet.

Information obtained through inspection or from soil maps and reports can be of particular value in preliminary appraisal of soils for sewage disposal. for instance, in many cases, unsuitable soils may be immediately ruses out on the basis of such information; in other cases, selection of the bast of several sites may be made on the basis of the inspection. Absorption capacity information obtained in this manner is relative. For quantitative information upon which to base specific design, we still must depend on some direct measurement, such as a water absorption rate as measured by a percolation test.