Pit size
When calculating the dimensions of a hole for a pit latrine, three conditions must be satisfied.
1. The pit should have sufficient storage capacity for all the sludge that will accumulate during its operational life or before its planned emptying.2. At the end of the pit's operational life there should still be sufficient space left for the contents to be covered with a sufficient depth of soil to prevent surface contamination with pathogenic organisms (soil seal depth).
3. There should be sufficient wall area available at all times to enable any liquid in the pit to infiltrate the surrounding soil.
Storage volume
The storage volume required to accommodate the sludge that accumulates in the pit during its operational life can be calculated from:
V = N × P × R
where 
V = the effective volume of the pit (m^{3}) 

N = the effective life of the pit (years) 

P = the average number of people who use the pit each day 

R = the estimated sludge accumulation rate for a single person (m^{3} per year). 
Once the effective volume of the pit has been calculated, the plan area is decided. This should be based on local preference, ground conditions and construction materials, and is generally circular or rectangular in shape. Note that only the area inside the lining is utilized for sludge accumulation, not the excavated area.
Having determined the plan shape and area, the depth of pit required for sludge accumulation is calculated as follows:
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Soil seal depth
This is usually taken as 0.5 m. In the case of double pit latrines it is the depth to the bottom of the inlet drain.
Infiltration area
In communities where people use water for anal cleaning or bathe in the toilet, a considerable amount of water may enter the pit. If it is assumed that the soil pores below the sludge surface are blocked, then additional wall area must be allowed for infiltration of the liquids above the sludge.
The infiltration area cannot include the soil seal depth since the top 0.5 m of a pit has a fully sealed lining.
Assuming that all the liquid entering the pit lies on top of the sludge, then the liquid depth will rise until the area of contact between liquid and soil is large enough to permit infiltration of the daily intake of liquid.
Pit depth
The total depth of the pit is calculated as follows:
Pit depth = sludge depth + infiltration depth + soil seal depth
Example 8.1
A family of six intends to dig a pit latrine with an operational life of 20 years. The family uses newspaper and corncobs for anal cleaning, and sullage is disposed of separately.
Sludge volume
V = N × P × R
The values of N and P are given (20 years and 6 people) but the sludge accumulation rate (R) is not. In the absence of local information the rate given in Chapter 5 can be used. The accumulation rate cannot be determined without some knowledge of the depth to the water table. Assuming this is greater than the likely pit depth, an accumulation rate of 90 l/year is used (see Table 5.3).
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If it is found that the pit does enter the groundwater, then the calculation should be done again using the appropriate sludge accumulation rate (60 l/year, from Table 5.3).
Plan area
The pit will be rectangular, with internal dimensions of 1.2 m by 2.0 m. Thus the depth required for sludge is:
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Infiltration area
Since solid objects are used for anal cleaning and sullage is disposed of elsewhere, there will be very little liquid to infiltrate. Accordingly the infiltration area can be ignored.
Soil seal depth
Assumed to be 0.5 m. Therefore the designed pit depth is:
4.5 + 0.5 m = 5 m.
This is very deep and consideration could be given to increasing the plan area or reducing the life of the pit.
Example 8.2
A family of six intends to construct a pit latrine to last 20 years. The family uses water for anal cleaning and intends to use the toilet as a bathing area. The ground is mainly a fine sand with a water table 3 m below the surface.
Sludge volume
Using the figures given in Table 5.3, the sludge accumulation rate will be 60 l/year above the water table and 40 l/year below. First assume that the pit will be mainly above the water table. If it is found that it enters into the groundwater by more than 1.0 m then the volume can be recalculated.
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Sludge depth
If the pit is to be circular, with an inside diameter of 1.3 m, the sludge depth will be:
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A pit of these dimensions would mean that most of the sludge would collect below the water table. Therefore the volume should be recalculated using a sludge accumulation rate of 40 l/year.
V = 6 × 20 × 0.04 = 4.8 m^{3}
Therefore the new sludge depth will be:
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Infiltration rate
The infiltration capacity of a fine sandy soil is about 33 l/m2 per day (see Table 5.4). Assuming the volume of water entering the pit each day is 200 l then the infiltration area required will be:
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Therefore liquid will build up in the pit until a contact area of 6.1 m^{2} is achieved.
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Assuming a soil seal depth of 0.5 m, the total depth required for the pit is:
3.62 + 1.49 + 0.5 = 5.61 m
This is a slight underestimate of the required depth because some of the sludge will accumulate above the groundwater level. Bearing in mind the inaccuracy of the basic design data, however, it is not necessary to carry out a more accurate calculation.
Example 8.3
An offset pourflush doublepit latrine is to be constructed for a family of six who use water for anal cleaning. The groundwater table is within 0.5 m of the surface during the rainy season and the soil is a sandy silt.
Sludge volume
As for the previous examples:
V = N × P × R
In a large pit the value of R would be taken as 40 l/year (see Table 5.3) but as this is a double pit, full consolidation of the sludge is unlikely to have taken place within the time taken to fill the pit (generally 2 years). Therefore a higher sludge accumulation rate (such as 60 l/year) should be used.
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Sludge depth
If each pit is 1.2 m wide and 1.2 m long, the sludge depth will be:
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Infiltration depth
An offset pourflush toilet uses about 3 l of water per flush. Assuming 20 flushes per day the total liquid inflow will be:
3 × 20 = 60 litres
If 6 l of urine enter the pit each day, the total daily inflow of liquid will be 66 l. The infiltration rate for sandy silt is about 25 l/m^{2} per day (see Table 5.4); therefore the infiltration area required is:
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The perimeter length of each pit is 1.2 × 4 = 4.8 m, therefore the liquid depth will be:
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Pit depth
The pit depth is the sum of the component depths, i.e.:
depth to bottom of inlet pipe 
0.2 m 
liquid depth 
0.5 m 
sludge thickness 
0.5 m 
Total depth of each pit below ground level 
1.2 m 