Sludge Accumulation, Short-Circuiting and Hydraulic Distribution
Sludge accumulates, and nothing tells you
Solids settle in a lagoon and digest anaerobically on the bottom, but digestion does not keep
up with deposition. Sludge therefore accumulates year after year. Because it is under water and
out of sight, a lagoon can lose a large fraction of its volume with no change in appearance
and no alarm of any kind.
The consequences follow directly from loading and detention:
- Less volume means less detention time, so treatment falls.
- Solids build unevenly, deepest around the inlet, which distorts the flow path and can
short-circuit the cell.
- Loss of depth reduces freeboard, which matters for berm protection and storage.
- A thick benthic layer is a large reservoir of oxygen demand and sulfide, which is released on
turnover.
Measuring sludge depth
You ca