Evaluation of ultrafiltration as an alternative to dissolved air flotation for the post-treatment of activated sludge effluent — pilot study in the Brasília norte wastewater treatment plant

ABSTRACT

Improvement in the effluent quality of the Brasília Norte and Brasília Sul wastewater treatment plants became necessary after their effluent receiving body, Paranoá Lake, started being used as source of drinking water for the population of Brasília (Federal District, Brazil). This is a comparative study between the tertiary treatment with ultrafiltration membrane (pilot scale) and the existing dissolved air flotation of Brasília Norte wastewater treatment plant. Both ultrafiltration pilot plant and dissolved air flotation were fed with the same matrix, the effluent of the activated sludge stage of Brasília Norte wastewater treatment plants, after coagulation. Seven operational conditions were evaluated at ultrafiltration pilot plant, with variations in flow rate, time of operation between cleaning processes, and the duration of backwashing. The operational behavior and the quality of the produced effluent were monitored. The increase in permeate flux (from 40.2 to 50.6 and 61.0 L.m⁻².h⁻¹) and in the time of operation between cleaning processes (from 25 to 38 and 50 min), as well as the reduction of backwashing duration (from 80 to 60 and 30s), resulted in a more intense evolution of transmembrane pressure. Ultrafiltration was more efficient than dissolved air flotation in the removal of turbidity, total suspended solids, chemical oxygen demand, and total phosphorous, with average gain in removal of 9.4, 13.0, 8.5, and 12.8%, respectively. It is worth mentioning the removal of coliforms by ultrafiltration, which was, on average, 3.4 log higher than that by dissolved air flotation. The use of ultrafiltration as an alternative to dissolved air flotation has the advantage of producing better quality effluent, particularly regarding microbiological aspects, providing lower quality fluctuation; however, treatment and destination of the ultrafiltration waste must be considered.

Keywords:
ultrafiltration; activated sludge; tertiary effluent; coagulation; dissolved air flotation; wastewater treatment plant