Particle size distributions by laser scatter analysis were compared with other solids settlement performance indicators from trickling filters. Field and laboratory pilot plant data indicated smaller less flocculated solids from trickling filters than activated sludge or rotating biological contractors (RBC). Analysis of utility company treatment plants indicated settlement characteristics were linked to less consistent performance from the trickling filters compared to activated sludge. Experiments with synthetic sewage also demonstrated a link between fine influent solids and performance. The research found no simple association between residual COD to BOD ratio and type of bioreactor. Neither was it possible to establish a link between flow rate and solids characteristics in the effluents but more complex analysis including ambient temperature, flow and recycle rates was suggested.