In onsite sanitation planning, what is the typical rate of accumulation of digested sludge per person per year to be used for preliminary sizing (at ambient Indian conditions)?

Introduction / Context:
Accurate estimates of sludge accumulation are essential for sizing septic tanks, sludge drying beds, and desludging schedules. Underestimating accumulation causes premature tank failure and overflows; overestimating wastes capital and land.

Given Data / Assumptions:

• Domestic wastewater with typical per-capita wastewater generation.
• Digested (stabilized) sludge volume, not raw sludge.
• Indian climate with warm temperatures aiding digestion.

Concept / Approach:
Standard design handbooks recommend using a per-capita annual digested sludge volume for preliminary sizing. For warm climates, a typical planning value is around 0.02 m³/person·year (that is, 20 litres/person·year). This incorporates solids reduction during anaerobic digestion and excludes excessive scum or grit allowances, which are handled separately.

Step-by-Step Solution:
Identify recommended planning value for digested sludge accumulation.Convert cubic metres to litres: 0.02 m³ = 20 L.Select the option matching 20 litres/person·year.

Verification / Alternative check:
Cross-check with typical design ranges (15–30 L/person·year). The central tendency of 20 L/person·year is widely used for first-cut design and then refined with local data.

Why Other Options Are Wrong:

• 10 or 15 L: May under-size tanks, risking frequent desludging.
• 25 or 30 L: Conservative; may be used in colder climates or high solids loads but not the common baseline.

Common Pitfalls:
Confusing raw sludge volume with digested sludge; ignoring allowances for scum storage and grit; not adjusting desludging interval assumptions, which changes required storage volume.

Final Answer:
20 litres