Open trickling filter: an innovative, cheap and simple form of post-treatment of sanitary effluents from anaerobic reactors in small communities

We aimed to evaluate the performance and cost savings of an innovative design of a trickling filter (TF) for small population sizes, developed at the Federal University of Minas Gerais, Brazil referred to as an open trickling filter (OTF). The OTF had no side walls and no perforated bottom slab, and was applied for the post-treatment of sanitary sewage from an upflow anaerobic sludge blanket (UASB) reactor. The OTF had crushed-stone packing (3.5 m high) and was operated with an average surface hydraulic loading rate of 4.1 m³ m⁻² d⁻¹ and an average volumetric organic loading rate of 0.10 kg BOD m⁻³ d⁻¹ (biochemical oxygen demand). The average concentrations obtained at the OTF effluent were 48 mg TSS L⁻¹ (total suspended solids), 132 mg COD L⁻¹ (chemical oxygen demand), 51 mg BOD L⁻¹, 19 mg TKN L⁻¹ (total Kjeldahl nitrogen), 16 mg NH₄⁺-N L⁻¹ and 10 mg NO₃⁻-N L⁻¹, complying with local discharge standards. Analysis of the construction costs indicated savings of 74% compared to conventional TF. Based on the performance, compactness, simplicity and reduced capital costs, it is believed that the proposed OTF is a good alternative for small communities, especially in developing countries.