The study covers different investigations related to the upgrading of a post-treatment system for the effluent from a upflow anaerobic sludge blanket reactor. The original post-treatment scheme comprised three ponds in series and a small coarse rock filter inserted in the last pond. Upgrading involved reducing the pond depths, applying baffles in the second pond and converting all of the third pond into a rock filter (three decreasing grain sizes). The system was conceived for 250 population equivalents, occupied an area of only 1.5 m2/inhabitant and aimed at very good removals of all major wastewater constituents. Overall final effluent concentrations for Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD) and Total Suspended Solids (TSS) were very good, and lower than those from the previous treatment line. Hydraulic flow patterns in both ponds showed daily thermal stratification and destratification periods. The conversion of the third pond into a rock filter increased the removal capacity of TSS (algae), but could potentially result in clogging, a phenomenon that was evaluated by a geophysical method called Georadar. The influence of accumulated sludge on treatment performance was tested before starting the operation of the upgraded system by operating the two ponds in parallel (one with sludge accumulated after 11 years of operation and the other without sludge). In this test, ammonia removal was virtually the same in both ponds.

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