Wastewater treatment by anaerobic filter and sand filter: Hydraulic loading rates for removing organic matter, phosphorus, pathogens and nitrogen in tropical countries

This study evaluated a wastewater treatment system combining anaerobic filters and sand filters, focusing on the suitability of such system to remove solids, organic matter, phosphorus, and pathogens, as well as the nitrification efficiency of the sand filters employed. The anaerobic filters were filled with shells of coconuts of the species Cocos nucifera, while the sand filters included 0.75-meter deep sand beds,exposed to hydraulic loading rates ranging from 100 to 800 L m−2 day−1. As a result, we could ascertain that the system has the capacity to receive rates above those suggested as the maximum threshold by a Brazilian standard, namely ABNT (1997), and the U.S. standard (USEPA, 1999). A minimum removal of 95% of COD and BOD was found, regardless of the hydraulic loading rates used. The only parameter evaluated that showed a significant difference in the means found was nitrogen. At a hydraulic loading rate of 700 L m−2 day−1, the concentration of NNH4+surpassed that of nitrate, pointing to the loss of nitrification efficiency. On account of this, this compound should be considered the reference parameter to ascertain the hydraulic loading threshold connected to the anaerobic effluents flown into sand filters.