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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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View the document The septic tank
View the document Operation and maintenance
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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