Enhancing the settleability of trickling-filter effluent solids by flocculation

Influence of physical flocculation on the settleability of particles in effluents of full-scale trickling filters was investigated using a jar-test apparatus. Three sets of samples were collected from each of two different trickling-filtration plants in Riyadh-Saudi Arabia, and flocculated for 0 to 30 minutes followed by quiescent settling for 30 and 60 minutes consecutively. The supernatants were analyzed for turbidity, suspended solids (SS), and chemical oxygen demand (COD). Statistical analysis of data revealed that flocculation was effective in increasing the rate of settling and removal of particulate and organic matter. Flocculation for periods of 10 to 30 minutes increased the 30-min settling removal efficiency of turbidity, SS, and COD from about 67%, 35-52%, and 22-38% to 74-81%, 53-74%, and 36-52% on the average, respectively. The major decrease in supernatant levels of turbidity, SS, and COD typically occurred within 10 to 15 minutes of flocculation. Increasing the settling time from 30 to 60 minutes for a given flocculation time did not improve the clarity of the supernatant significantly, indicating that with flocculation greater removal can be obtained with lower clarification detention time. Results have also revealed that solids in effluents of lightly trickling filters are more amenable to flocculation than those of highly loaded systems.