Floc that settles and later rises presents a different problem from floc that never settles. Gas can attach to or form within settled solids, increasing buoyancy and lifting a blanket or clumps toward the surface. Biological activity is one possible source in suitable wastewater; entrained air and other process conditions deserve investigation too. A stronger flocculant is not an automatic cure for gas-driven lift.
Record the sequence, not just the final surface
Observe when settlement starts, how long the bed remains stable and when lifting begins. A jar photographed only at the end can hide the fact that the initial settling performance was adequate. Compare the timing with clarifier residence and withdrawal behaviour, recognising that a bench container does not reproduce full-scale gas exchange or solids age.
Note bubbles, lifting clumps and the distribution of the event across the tank. Review feed temperature, upstream aeration, nitrate where biologically relevant and the history of retained sludge. These observations can support hypotheses for the process specialist; they do not establish the identity of an unknown gas. Do not use smell or a flame test, and follow site gas and access safety procedures.
Separate gas entry from gas generation
If bubbles appear immediately at the inlet, investigate upstream air entrainment, pump suction conditions or a pressure change that releases dissolved gas. If solids sit for an extended period before rising, review sludge residence and biological conditions. The distinction guides where to investigate, although more than one mechanism may operate at once.
Use jar testing for sedimentation to compare the initial separation step consistently. When consulting Xinqi Polymer, report both initial settling and later lift. A PAM product-family comparison should not replace investigation of the gas source or the solids-retention conditions.
Examine the withdrawal record
Check whether sludge withdrawal actually removes the accumulated material or merely draws a dilute route through the bed. Review valve function, pump availability and changes in withdrawal scheduling. An apparently regular pump-start log does not prove effective solids removal. Underflow concentration and the subsequent blanket response provide additional evidence.
Any change in withdrawal needs consideration of the downstream thickening or dewatering capacity. Moving more water out of a clarifier may reduce local retention while overloading another process. Agree a controlled adjustment with operations and track both ends of the transfer. Avoid making several upstream chemistry changes at the same time, which can make the cause impossible to distinguish.
Verify the mechanism before replacing the polymer
If earlier, effective solids removal or correction of air entry prevents lifting while the original polymer maintains clarity, the evidence supports an operational cause. If initial settling is also weak, a separate conditioning trial may still be warranted. Keep the two questions distinct in the report: can the floc settle, and can the plant remove it before later conditions make it buoyant?