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close this folder Section 9: Water carried sewage systems construction and maintenance
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Section 9: Water carried sewage systems construction and maintenance

 

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.

 

The septic tank

THE FUNCTION OF SEPTIC TANKS

When raw sewage enters the septic tank, it should quiescent for a period of one to three days depending on the tank capacity. During this period the heavier solids, including grease end fats, remain in the tank end form the scum over the water surface, while the rest is carried away by effluent into the final disposal system.

The solids which are retained in a septic tank undergo anaerobic decomposition through the activity of bacteria and fungi. The significant result of this process is a considerable reduction in the volume of sludge. which allows the tank to operate for periods of one to four years or more, depending on circumstances, before it needs to be cleaned. This decomposition involves not only the sludge, but also the dissolved and colloidal organic contents of the sewage.

In this manner the turbidity of the effluent is significantly reduced so that it may be morn readily percolated into the subsoil of the ground. Thus, the most important function of a septic tank is to provide protection for the absorption ability of the subsoil

The three functions that take place in the tank, then, are:

Removal of Solids

Clogging of soil with tank effluent varies directly with the amount of suspended solids in the liquid. As sewage from a building sewer enters a septic tank, its rate of flow is reduced so that larger solids sink to the bottom or rise to the surface. These solids are retained in the tank, and the clarified effluent is discharged.

Biological Treatment

Solids and liquid in the tank are subjected to decomposition by bacterial and natural processes. Bacteria present are of a variety called anaerobic which thrive in the absence of free oxygen. This decomposition or treatment of sewage under anaerobic conditions is teemed "septic". hence the name of the tank. Sewage which has been subjected to such treatment causes less clogging than untreated sewage containing the same amount of suspended solids.

Sludge and Scum Storage

Sludge is an accumulation of solids at the bottom of the tank, while scum is a partially submerged mat of floating solids that my for. at the surface of the fluid in the tank. Sludge, and scum to a lesser degree will be digested end compacted Into a smaller volume. However, non matter how efficient the process is a residual of inert solid material will l remain. Space must be provided in the tank to store this residue during the interval between cleaning; otherwise, sludge and scum will eventually be scoured from the tank and may clog the disposal field.

DESIGN

The design of the septic tank should promote and facilitate the separation and digestion of the sewage solids and provide for periodic inspection and occasional physical removal of accumulated sludge and scum.

The average daily of sewage depends on the average water consumption in the area under consideration. In rural areas and small communities the water consumption per person is likely to be lower than in municipalities.. As e result. sewage flows of less than 26 US gal. per person per day may be expected in most rural areas of the world. However, experience indicates that such low figures cannot be used for the design of small septic tanks, which should be provided with ample capacity since such tanks are seldom cleaned before trouble develops. It is therefore important that their capacity be ample to permit reasonably long periods of trouble-free service end to prevent frequent and progressive damage to the effluent absorption systems due to discharge of sludge by the tanks. For this reason the capacity. of residential. single-chambered. septic tanks should not be less than 500 US gal. below water-level.

The liquid capacities of the septic tanks described in Tables 14 and 15 are based on a sewage contribution of:

50 US gal. per person daily in dwellings;

25 US gal. per person daily in cmaps;

17 US gal. per person daily in day schools.

TABLE 14: REQUIRED CAPACITIES FOR SEPTIC TANKS SERVING INDIVIDUAL DWELLINGS

Maximum number of person served

Nominal liquid capacity of tank (US gal.)

Recommended dimensions

 

width

length

liquid depth

total depth

 

ft

In

ft

In

ft

In

ft

In

4

500

3

0

6

0

4

0

5

0

6

600

3

0

7

0

4

0

5

0

8

750

3

6

7

6

4

0

5

0

10

900

3

6

8

6

4

6

5

6

12

1100

4

0

8

6

4

6

5

6

14

1300

4

0

10

0

4

6

5

6

16

1500

4

0

10

0

4

6

5

6

 

Liquid capacity based on number of persons served indwelling. The volume based on total depth air space above liquid level

The capacities indicated in Table 14 should in most countries provide sufficient sludge-storage space for a period of two years or more, and an additional volume equal to the sewage flow for 24 hours.

TABLE 15: REQUIRED CAPACITIES FOR SEPTIC TANKS SERVING CAMP AND DAY SCHOOLS

Maximum number of person served

Nominal liquid capacity of tank (US gal.)

Recommended dimensions

 

width

length

liquid depth

total depth

 

ft

In

ft

In

ft

In

ft

In

40

60

1000

4

0

8

6

4

0

5

0

80

120

2000

5

0

11

0

5

0

6

3

120

180

3000

6

0

13

6

5

0

6

3

160

240

4000

6

0

18

0

5

0

6

3

200

300

5000

7

6

18

0

5

0

6

6

240

380

6000

8

0

20

0

5

0

6

6

280

420

7000

9

6

20

0

5

6

7

0

320

480

8000

8

6

23

0

5

6

7

0

Note: Tanks with capacities in excess of 8000 gallons should be designed for the specific requirements involved; however. In such cases the necessary for a more complete type of treatment should receive consideration

The capacities shown in Table 15 are based on e 24-hour flow of sewage without allowance for sludge-storage space. since it is expected that septic tanks serving camps and schools will receive regular inspection and maintenance, including more frequent cleaning than those for residences.

In the case of public institutions, such as rural hotels and hospitals, and groups of houses, such as housing projects, the figures given in Tables may not apply. It will first be necessary to secure the advice of a competent engineer whose duty it will be to determine the probable daily water consumption end sewage flow, both of which are likely to be much higher then the figures cited above. Most recent information indicates that:

1. For flows between 500 gal. and 1500 gal. per day, the capacity of the septic tank should be equal to at least 1 1/2 days, sewage flow.

2. For flows between 1500 gal. end 10,000 gel. per day, the minimum effective tank capacity should be 1125 gal. plus 75% of the dally sewage flow, or:

V = 1125 + 0.75 Q, where

V is the net volume of the tank in gallons, and Q is the daily sewage flow, also In gallons.

Tanks may be of either single- or multi-compartment design. The single compartment tank is satisfactory for a wide range of conditions and is simpler and less expensive to build and maintain. A two-compartment tank, with the first compartment equal to one-half to two-thirds of the total volume, provides an opportunity for removing more solids, which may be valuable under tight soil conditions. The compartments may be sections of one continuous shell separated by partitions, or separate units connected in series. Each compartment should be vented and provided with inlet end outlet fittings and access facilities for inspection and cleaning.

Whether e tank is rectangular, round, or oval has little effect on its performance, provided it has the necessary capacity and other features. Rectangular tanks are usually built with the length two to three times the width. It is recommended, however, that the smallest horizontal dimension be at least 2 feet and that the liquid depth be between 30 and 60 inches. These dimensions should be observed in single compartment tanks. About 12 inches (or about one-fourth the liquid depth) is required above the flow line to allow space for scum accumulation and free passage of gases for venting.

Tank performance is affected by the type and arrangement of the inlet and outlet fittings. The inlet invert (flow line) should be at least 1 inch - preferably 3 inches - higher then the-outlet 1 invert to prevent backwater and stranding of solids in the house sewer. Use either tees or straight pipe and baffles, arranged as shown in fig. 88. Provide a vertical clearance of at least an inch for venting purposes between the tops of the fittings and the under side of the tank roof. Submerged entry in a downward direction tends to confine entrance disturbance and helps mix the incoming sewage with the more biologically active sewage and sludge already in the tank. The inlet tee or baffle should extend to at least 6 inches below the surface of the liquid, but not deeper than the outlet device. Depth of submergence of the outlet tee or baffle is a critical factor in the performance of the system. If too shallow, scum can pass out of the tank with the effluent. If too deep, sludge can scour out. In either case the particles of solids in the effluent can lead to early clogging of the soil in the absorption area. The ideal depth for the outlet is at a point of balance between the scum and sludge accumulations. This point has been found to be at a depth below the flow line of about 35 to 40 percent of the total liquid depth.

Siphons and dosing chambers are not necessary in ordinary farm installations. They are useful, however, in large installations where the combination of sewage volume and tight soil conditions calls for more than 500 linear feet of disposal tile in the absorption field. The siphon and chamber serve to accumulate a near-continuous, small flow of effluent and provide an intermittent discharge of a larger volume to the absorption field. This loads the field more uniformly and allows some time for rest and aeration between discharges. The frequency and volume of the discharge are controlled by the sizes of the siphon and the chamber. A 3- or 4-inch siphon is adequate. Capacity of the dosing chamber (volume of single discharge) should be about two-thirds the interior volume of the disposal tile. Installation should be inaccordance with the manufacturer's instructions.


Fig. 88 Longitudinal section of single-compartment Concrete Septic Tank

Construction Methods For Septic Tanks

Two construction methods for septic tanks have been developed by the Agricultural Engineering Department of the South Dakota State College Agricultural Experiment Station, Brookings, South Dakota.

The methods use readily available building materials. One method employs concrete silo staves, and the tank is built in the form of a vertical cylinder. The other uses standard concrete blocks for a rectangular tank.


Fig. 89 Concrete Silo Stave Turin

Concrete Silo Stave Tank

The construction of both tanks issimple and sizes can be adjusted to the needs of the family. One step in the building process has to be kept in mind as important. Both silo staves and concrete blocks are of relatively porous concrete, therefore the danger of ground water pollution is present unless careful waterproofing is provided.

This tank is in the form of a vertical cylinder 6 feet in inside diameter and 5 feet in depth, with a capacity below the outlet of 850 gallons (Fig. 89 ). It is suitable for a family of eight.

Materials:

12 6-inch concrete silo staves

14 24-inch concrete silo staves

34 30-inch concrete silo staves

9 sacks cement

1 cubic yard of sand

1 cubic yard of gravel

3 pieces of l/2-inch round steel rod, 13 feet 8-inches long for hoops

3 pieces of l/2-inch round steel rod, 10 feet 8-inches long for hoops

6 steel silo lugs, 12 nuts

120 feet (45 pounds) of 3/8-inch knobbed reinforcing rod

10 pounds or 1 gallon of waterproofing material

2 sewer tile tees, 4-inches in diameter

Excavation

The excavation should be 7 1/2 feet in diameter, with a depth of about 7 feet, depending on the depth at which the sewer from the house will enter. Dig the sides vertical and level the floor before pouring concrete.

Floor

The floor is poured in two courses. The first course, 4 inches thick, is of concrete mixed 1 part cement, 2 1/2 parts sand, 3 1/2 parts gravel. The first course of the floor should cover the whole bottom of the excavation. The concrete should be we 1 worked and carefully leveled to provide a firm, smooth base for placing the staves. Covering the floor with paper or a tarpaulin will make it easier to keep it clean while working on the walls. The pouring of the second course is postponed until the walls are fully constructed, so that the second layer ties floor and wall closely together.

Walls

The walls are made of concrete silo staves 2 1/2 inches thick, 10 inches wide, in lengths of 30, 24, and 6 inches.

First mark a circle of 3-foot radius on the floor to serve as a guide in placing the staves. The staves are set with the inner edge just touching this mark, with 24-tech and 30-tech staves alternating. When this first tier of staves has been completed, a hoop is placed around the outside, 6 inches above the floor, and tightened. A tier of 30-tech staves is now placed upon the top of the 24-tech staves except where the inlet and outlet are to be where 24-inch staves should be used. A second hoop is placed 3 inches above the top of the 24-inch staves in the first tier and tighten the second hoop. Fill in the remaining spaces with 6-inch staves, leaving openings 12 inches high for the inlet and outlet fittings. Place the top hoop Just below these openings, and tighten.

Inlet and Outlet Fittings

Cut forms to fit around sewer tile tees and place in position in the openings. The outlet tee should be placed at the bottom of the 12-loch opening left for it, the inlet tee 2 inches above the bottom of the opening. Fill in the spaces around the tees with a rather dry mortar, tamping it carefully to make a watertight joint around the tee.

Plastering the walls

Apply a 3/8-tech coat of plaster of 1 part cement, 3 parts sand, and 1/4 part "Cem-mix" after thoroughly wetting the staves. Smooth the plaster as much as possible.

Finishing The Floor

Make sure the floor is perfectly clean, dampen it to obtain a good bond, and pour a finish course of l part of cement to 3 parts of sand mortar l inch thick. Smooth and level tints carefully, being sure to obtain a good joint with the plaster on the walls. Allow to cure for seven days or more.

Waterproofing

Apply two coats of a waterproofing material according to instructions on the package. Waterproofing is essential in order to prevent seepage through the porous staves.

Cover

The cover is made of reinforced concrete slabs, 4 1/2 inches thick and 8 inches wide, of varying lengths as shown in Fig. 90.


Fig. 90 Cover slab for septic tank

Mixture for slabs

Mix 1 part cement, 2 1/2 parts of sand, and 3 1/2 parts of gravel or crushed stone to a smooth consistency in order to get a good bond between the concrete and the reinforcing rod. Each slab is reinforced with two 3/8-inch knobbed steel rods, spaced l Inch from the bottom and 2 inches from the sides. The rods should be placed at both ends of the slab.

These slabs may be made in forms of 2-inch by 6-inch lumber, placed on asphalt paper on any flat surface. Keep the slabs moist and allow to cure for at least three days before moving them.


Fig. 91 Concrete Block Tank

Concrete Block Tank

This tank is rectangular in shape, 7 feet 4 inches long, 2 feet 8 inches wide, and 4 feet 8 inches deep (inside measurements) with a fluid capacity of 550 gallons (Fig. 91.

Materials

119 standard concrete blocks (8 inches by 8 inches by 16 inches

15 sacks of cement

1 1/2 cubic yards of sand

3 3/4 cubic yards of gravel

90 ft. of 3/8-tech reinforced rod

2 4-inch sewer tile tees

110 pounds or l gallon of waterproofing material

Excavation

The excavation should be made 9 feet 4 inches long, 4 feet 8 inches wide, and about 6 feet 9 inches deep, depending on the level at which the sewer will enter. Dig the sides vertical, and level he bottom before pouring.

Floor

The floor is poured in the same way as for the silo stave tank.

Walls

The walls are built of standard concrete block laid up with mortar consisting of 1 part cement, 3 parts sand, and 1/4 part lime or "Cam-mix." The corners should be kept square and plumb by use of a straight edge or level.

To add strength to the walls, be sure the joints are staggered between adjacent courses, and fill the cores of the blocks with concrete (1:2 1/2: 4 mix). Cut openings in the block inlet and outlet fittings.

Plastering and Waterproofing

Follow the instructions given for plastering and waterproofing the silo stave tank. Both silo staves and concrete blocks are Of relatively porous concrete which will allow the passage of liquids and contaminating material. Proper waterproofing is essential to reduce the danger of ground water pollution.

Cover

Make precast slabs 4 feet long, 12 inches wide, and 4 l/2 inches thick, using two 3/8 inch reinforcing rods. Follow the instructions given for the silo stave tank. Eight slabs will be required.

Size of tanks

Both tanks may be made larger if required (see Table 16 below). Add more staves to the silo stave tank to increase the size. A larger excavation and longer rods wilt be required. The concrete block tank may be enlarged by using one more block in each course at the ends, resulting in a width of 4 feet inside, and a capacity of 845 gallons. A tank this size would be large enough for a family of eight, or a smaller family that has methods of construction would be the same, but more materials would be required, and the size of excavation, length of cover slabs, etc., would be increased.

Tanks should not be made smaller than described. In the case of the stave tank, little saving would result, whereas the 550 gallon concrete block tank is little larger than the recommended minimum of 500 gallons.

TABLE 16: CAPACITIES. DIMENSIONS AND MATERIALS FOR SEPTIC TANKS

BUILT OF CONCRETE SILO STAVES

Number of person

Liquid Capacity Gallons

Diameter (Inside)

Liquid Depth

Total Depth

(Inside)

Number of Staves Required

6''/24''/30'

Red Required

8 or less

850

6'0''

4'

5'

12

14

34

3-13'8'', 3-10'8''

10

1050

6'10''

4'

5'

13

15

37

3-13'8'', 3-2'-4''

14

1220

7'5''

4'

5'

14

16

40

3-13'8'', 3-14'-0''

BUILT OF CONCRETE BLOCK

Number of person

Liquid Capacity Gallons

Liquid

(Inside)

With

(Inside)

Liquid Depth

Total Depth

(Inside)

Red Required

4 or less

550

7'4''

2'8''

3'9''

4'8''

110

6

680

7'4''

3'4''

3'9''

4'8''

126

8

810

7'4''

4'0''

3'9''

4'8''

133

10

950

7'4''

4'0''

4'5''

5'4''

152

12

1150

8'8''

4'0''

4'5''

5'4''

168

Note: A somewhat greater quantity of sand, cement, gravel and waterproofing material for tanks larger than those described in the teat

Note: The foregoing material was digested from New Construction Methods for Septic Tanks and Cisterns, by T.R.C. Pokeby, Circular 99, March 1953, Agricultural Engineering Dept. , Agricultural Experiment Station, South Dakota State College. Brookings, South Dakota

 

Operation and maintenance

A newly built septic tank should be filled with water up to the outlet level and then seeded with several buckets of ripe sludge. Although most design recommendations call for dislodging about every two years, it is suggested that private installations be examined at least once a year and septic tanks serving public institutions be examined every six months. The inspection should be directed towards the determination of:

(a) the distance from the bottom of scum to bottom of outlet* (scum clear space)

(* The scum-clear space should not be less than 3 in. and the total depth of scum and sludge accumulations should not he more than 20 in.)

(b) the depth of accumulation of sludge over tank bottom.

Sludge may be bailed out by means of a long-handled, dipper-type bucket, or pumped out by a specially equipped cesspool-emptying vehicle. It is import tent to recall that the scum and sludge removed from ordinary septic tanks will normally contain some portion which is still offensive and dangerous to health. It is. therefore, wise to compost these materials before using them as a crop fertilizer.

THE DISPOSAL OF EFFLUENT

In rural areas and small communities, the choice of methods available for treating and disposing of the effluent is usually limited to dilution, seepage pits, subsurface irrigation, filter trenches, sand filters, or trickling filters. Here the discussion will be confined to subsurface irrigation systems and seepage pits.

THE EFFLUENT SEWER

The effluent sewer conveys the effluent from the septic tank to the absorption or disposal area and may be constructed of the same materials and in the same manner as the house sewer. Joints should be tight and root-proof. A 4-inch line to a slope of 1/8 or 1/4 inch per foot is recommended.

SUBSURFACE IRRIGATION SYSTEMS

Disposal Lines

The effluent is discharged to the soil through a system of open-jointed or perforated disposal tile or pipelines laid in absorption trenches or beds having a total bottom area as determined from table . Dividing this bottom area by the effective absorption area in, square feet per lineal foot, from table gives the total length required, in feet. Lateral seepage is neglected

Proper design and careful workmanship are important to successful operation of the system. Arrangement of the lines varies with the absorption area required and the topography of the available terrain.

Four-inch open-jointed agricultural tile or perforated drain pipe is customarily used. Individual lines should not exceed 100 feet in length and should be laid on a flat grade, never sloping more than 6 inches per 100 feet. All lines should be about the same length.

The preferred depth for an absorption trench (allowing for a gravel bed) is from 24 to 30 inches. However, depths from 18 to 36 inches may be used if it is necessary to clear high ground-water, maintain grade, allow for an extra deep gravel bed, or to meet some other special condition. If it is necessary to go deeper than 36 inches, the deeper portions should be confined to short stretches totaling only a small percentage of the field as a whole. As previously stated, the trench bottom should be at least 4 feet above the highest. seasonal ground-water level, the top of any rock formation, or impervious stratum.

Trench width should be from 18 to 24 inches, although widths up to 36 inches may be used in the deeper trenches. Wider trenches call for wider spacing between trenches, as indicated in table

TABLE 17: ABSORPTION TRENCH AREA AND SPACING

Trench width (inches)

Effective absorption area

Minimum clear distance between trenches

 

Square feet per lineal foot of trench

Feet

18

1.5

6.0

24

2.0

6.5

30

2.5

7.0

36

3.0

7.5

The tile or pipe should be laid in a bed of clean gravel, crushed or broken stone, or similar material. The gravel bed should extend from at least 6 inches below the bottom of the line to at least 2 inches above the top. The bed material may range in size from l/2 to 2 l/2 inches. Cinders, broken shells, slag, and similar materials are not recommended because they are usually too fine and may cause clogging. About l/8 to l/4 inch Joint space should be allowed between sections if agricultural tile is used. The upper half of this joint space should be covered with tar paper or similar material to keep out fine material from above. A cover of untreated building paper, straw, hay, pine needles, or similar pervious material should be placed over the bed material to keep out particles of the earth backfill. Impervious material should not be used for this covering as it would interfere with the action of the trench.


Fig. 92 Closed or continuous tile system arrangement for level ground


Fig. 93 Serial distribution system arrangement for sloping ground.

If it is necessary to locate a disposal line within reach of the roots of trees or shrubs, deepen the gravel bed in the affected area by about 12 to 18 inches, keeping the line itself level. This provides extra space between the moist trench bottom and the line and may keep the roots from entering the line,

Exercise care during construction to preserve the natural absorptive quality of the soil. Protect the trench from silt and debris while open. Avoid unnecessary walking in the trench. Place gravel or stone carefully and tamp backfill lightly with a hand tamper. Do not machine-tamp and do not use a hydraulic backfill. Overfill the trench about 4 to 6 inches to allow for settling.

Closed or Continuous System

In flat locations, where the slope of the ground surface does not exceed 6 inches in any direction within the area of the absorption field, the disposal lines may be arranged in a closed or continuous system as shown in figure 92 In this system, open-jointed tile or perforated pipe is used throughout the field. It is laid on a flat grade and the entire trench length is counted in the effective absorption area. Because of the flat grade and interconnecting lines, the effluent will distribute satisfactorily without a distribution box.

Serial Distribution System

Serial distribution of effluent is recommended for practically all situations where soil conditions permit subsurface absorption and where the slope of the ground surface exceeds 6 inches in any direction within the confines of the absorption field. Excessively steep slopes that are subject to erosion should be avoided. In the serial distribution system, the individual trenches of the absorption field are arranged so that each trench is forced to pond to the full depth of the gravel fill before the effluent flows into the succeeding trench. (See Fig. 93 )

Advantages of this system are: (l) It minimizes the importance of variable absorption rates in different parts of the field by forcing each trench to absorb effluent until its ultimate capacity is utilized; (2) it causes each trench in the system to be used to full capacity before failure occurs; and (3) it eliminates the cost of a distribution box and the runs of tight-jointed pipe from the box to the absorption trenches.

The following design and construction features should be observed for satisfactory operation of this system:

1. Individual trench bottoms and disposal lines should be level, following contours to minimize variation in trench depth.

2. A minimum of 12 inches of earth should cover the gravel fill in the trenchers.

3. A minimum of 6 feet of undisturbed Berth should be allowed between adjacent trenches, and between the septic tank and the nearest trench.

4. Overflow lines should connect the trenches in such a manner that a trench will be filled with effluent to the depth of the gravel before the effluent flows to the next lower trench. This may be done as shown in figure , by having the invert of the overflow line at the top of the gravel fill.

5. The overflow lines should be 4-inch diameter tight-jointed sewers, connecting directly to the distribution lines in the trenches. The trench for an overflow line, at the point where it leaves an absorption trench, should be dug no deeper than the top of the gravel fill in the absorption trench.

6. The outlet (overflow) from a given absorption trench should be as far as practical from the inlet to that trench in order to prevent short-circuiting of the effluent.

7. The invert of the first overflow line should be at least 4 inches lower than the invert of the septic tank outlet.

8. All other features should match those for subsurface absorption fields generally.

Distribution Box

Experience has shown that distribution boxes and similar devices seldom achieve the uniform distribution of effluent that is expected of them. Effluent distribution by the continuous or serial distribution systems gives as good results or better, and generally at less cost.

If a distribution box is used, the following essential design features should be observed:

1. All outlets must be set at exactly the same level-about 4 to 5 inches above the bottom is recommended. This gives space for carryover sludge to accumulate and be detected by inspection. It also serves in lieu of a baffle to prevent short-circuiting and thus aid in obtaining equal distribution of the effluent.

2. A separate outlet is needed for each line of tile; adjacent outlets should be separated by at least a full pipe diameter.

3. The inlet should be about 2 inches higher than the outlets.

4. A watertight, removable cover should be provided for access.

If a box is to serve an absorption field in which it is desired to "work" and "rest" certain lines alternately or in rotation, because of tight soil conditions or other reason, facilities should be provided in the box for opening and closing the corresponding outlets. Also, if there is prospect of future need for more 'lees from the box, additional outlets may be provided at the time of construction and fitted with plugs that can be readily removed when the need develops. More than one box may be used if the ground slope warrants.

Fig. 94 illustrates a distribution box such as used on farms in the U. S.


Fig. 94 Typical Distribution Box

 

 

Lesson plans

WATER-CARRIED SEWAGE SYSTEMS

CONSTRUCTION AND MAINTENANCE

LESSON NO. 1

LESSON OBJECTIVE: Outline the factors that must be considered in designing a water-carried sewage systems.

TOPIC

INSTRUCTIONAL PROCEDURE

SUPPLEMENTAL MATERIALS/ RELATED READING

System Location

Outline the factors that must be considered ; in the location of water-carried sewage systems.

 

The Percolation Test

Demonstrate digging a test hole with a hand held auger.

 
 

Supervise students in digging test holes for a percolation test.

 
 

Conduct a percolation test.

 

Selection of Disposal System

Review the factors that must be considered in determining the typo of facility to be implemented.

 

WATER-CARRIED SEWAGE SYSTEMS

CONSTRUCTION AND MAINTENANCE

LESSON NO. 2

LESSON OBJECTIVE: Set up and carry out a program to construct a water-carried sewage system.

TOPIC

INSTRUCTIONAL PROCEDURE

SUPPLEMENTAL MATERIALS/ RELATED READING

The Components of Water-Carried

Disposal Systems

Discuss the functions of the fundamental components of the seepage pit, cesspool and septic tank

Diagrams of these systems

WHO Monograph Series #39

Chapter 3.

 

Illustrate the and components for these systems

Manual of Septic Tank Practice, p. 9-38.

 

Outline the be of construction for these system

Constructing a Concrete (Technical Digest Survey)

System Selection

Assemble students to select the most feasible locations of these systems in a rural or village community.

 

Construction Techniques

Outline construction methods for septic tanks.

 
 

Set up a program (allocate construction tasks to the students) for the construction of a septic tank.

 
 

Supervise students in the construction of this septic tank.

 
     

 

WATER-CARRIED SEWAGE SYSTEMS

CONSTRUCTION AND MAINTENANCE

LESSON NO 3

LESSON OBJECTIVE: Define the need for and the methods of establishing a maintenance program for this system.

TOPIC

INSTRUCTIONAL PROCEDURE

SUPPLEMENTAL MATERIALS/ RELATED READING

Rules for Efficient Operation and Maintenance of Septic Tank Systems

Discuss the need for septic tank inspection.

 
 

Recall how often inspection should be carried out and what it should establish.

 
 

Discuss how to maintain a septic tank.

 
 

Visit a site where a septic tank is being desludged.

 

The Need for Simplifying Technical Materials

Discuss the difficulty of finding skilled labor in developing countries.

WHO Monograph Series #4

Chapter 9

 

Outline how to simplify instructional materials.