Post-treatment of municipal sewer in shallow polishing ponds

This work aims to investigate the influence of polishing ponds (PP) depth on the post-treatment of sanitary sewers. Two treatment systems were designed and monitored. One system was an upflow anaerobic sludge blanket (UASB) rector and intermittent flow sand filter; followed by a PP of 0.20 m depth with low superficial application rate (24.9 kgBOD5.ha-1.day-1), high sunlight incidence (597 Wm-2) and enough biological carbon dioxide used to raise the average pH to 9.6, ensure an average removal of 81% orthophosphate and concentration of Escherichia coli lower than 103UFC/100 ml, which are the meeting requirements for unrestricted irrigation. The other system was a UASB followed by two PPs operated in parallel, at 0.20 and 0.60 m depths. The 0.20 m PP depth removed 80% of total Kjeldahl nitrogen, 53% of total phosphorus and 44% of orthophosphate. The 0.60 m pond depth showed low nutrient removal and a poor E. coli removal efficiency, 98.33% equivalent to 2 logs units.  The ponds were fed by continuous systems. Key words: Nutrient removal, polishing ponds, disinfection, sunlight.