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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
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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