Faecal sludge (FS) can be estimated using two primary methods:
a. Generation-Based Estimation: This method estimates the volume of sludge generated based on user numbers and accumulation rates. It is particularly useful for planning in areas without regular desludging data.

Formula for estimation is based on the following factors:
Faecal Sludge Generation (L/year) = No. of users × Accumulation rate (L/person/year)
Containment Type
Typical Range (L/person/year)

1. Type of containment: Septic tank (with soak pit)
   Typical Range (L/person/year): 30–40
   Source: https://forum.susana.org/214-vault-content-research-and-faecal-sludge-characteristics/22270-sludge-accumulation-rate-in-septic-tank?utm

2. Type of containment: Fully sealed septic tank
   Typical Range (L/person/year): 50–70 Source: https://www.frontiersin.org/journals/water/articles/10.3389/frwa.2023.1130081/full?utm

3. Type of containment: Pit latrine (lined)
   Typical Range (L/person/year): 40–60
   Source: https://link.springer.com/chapter/10.1007/978-981-33-4114-2_27?utm

The value accumulation value can be adopted based on the context adapted in the region, in Indian context, a report published by Centre for Science and Environment, suggest the accumulation rate varies between 70 to 150 litres/person/year. Therefore, any value falling between these upper and lower limits can be considered.

Example:
A 5-member household using a sealed septic tank:
5 (persons)× 60 (L/person/year)= 300 L/year
Over 5 years: 1,500 Litres

b. Collection-Based Estimation:
This method uses actual desludging records from vacuum trucks or manual operators. It measures how much FS is collected and transported, rather than what is theoretically produced.
Formula:
FS Collected (L/HH/year) = (Total Desludged volume (L))/[(Household Served x Years since last emptying)]
A vacuum truck removes 3,000 L from a household every 5 years:
FS Generation = 3000 / 5 = 600 L / year / household

For the construction of septic tanks followed by soak pits, the CPHEEO recommends adherence to BIS Code 2470 Part I and Part II. These codes take into account site-specific factors such as topography—including slope, drainage, gravity-based flow—and the required distance from water bodies. It is also advised to conduct a percolation test to assess soil stability and permeability.
However, in hilly and rocky terrains, alternate disposal methods are recommended when soak pits do not function effectively.
The land subsidence incident in Joshimath, Uttarakhand, in 2023 serves as a cautionary example. It highlighted how the widespread use of unlined soak pits and greywater discharge contributed to slope instability. According to the National Disaster Management Authority (NDMA), and as reported by Down to Earth, over 90% of households in Joshimath discharged greywater and effluent through soak pits. This practice exacerbated subsurface erosion in a town already situated on unstable morainic debris from historical landslides.
In such geologically sensitive areas, it is safer to adopt treatment systems followed by alternative disposal methods. Context-appropriate solutions—such as bio-digesters, compact DEWATS units, or reinvented toilets—should be considered based on site feasibility. These approaches also align with established guidelines such as the CPHEEO Manual – Part C (2013).

Wastewater generation at the household level can be estimated by calculating the total wastewater output, which is generally about 80% of the total daily water consumption, as per CPHEEO manual (CPHEEO, 2013).
For instance, in a standard Indian household of five persons using 135 litres per capita per day (lpcd), the total wastewater generated is:
• 5 × 135 × 0.8 = 540 litres per day.
This total wastewater is split into:
• Greywater (from bathing, washing, and kitchen use): usually 65–80% of total wastewater.
• Blackwater (from toilet use): makes up the remaining 20–35%.
Assuming a 75:25 greywater to blackwater ratio, the same household would generate approximately:
• 405 litres/day of greywater, and
• 135 litres/day of blackwater.
Additionally, per capita estimates for different conditions suggest:
• Greywater: 70–100 L/person/day
• Blackwater: 20–40 L/person/day
• (Source: IRC, 2020)
These values can vary depending on user behaviour, toilet technology (pour-flush, dry toilets, etc.), and water availability.