What happens to sludge when the SVI is under 70?

The phenomenon of Sludge Volume Index (SVI) is crucial in evaluating the performance of a wastewater treatment system, particularly concerning the settleability of sludge. An SVI of under 70 typically indicates that the sludge is settling too rapidly. This rapid settling can lead to poor clarification in the settling tank, as the solids do not stay suspended long enough to adequately separate from the liquid effluent, potentially allowing some of the solids to re-enter the treated effluent.

This quick settling can be attributed to several factors, including high-density sludge that compacts more tightly and settles faster. While this may seem beneficial at first glance, the consequence is inefficient removal of suspended solids and a potential increase in the concentration of solids in the treated effluent, which does not meet quality standards for discharge.

In contrast to the other factors presented in the other choices, the scenario of sludge settling too rapidly fundamentally disrupts the intended functionality of the treatment process, leading to a compromise in effluent quality.