Description

Given the incoming average flow, the number of settling tanks and the geometric characteristics of each one, verifies if it fits the Brazilian standards for domestic sewage treatment plants.

Number	Purpose
1	when preceding a biological filtration process
2	when preceding an activated sludge process
3	when the process is a process for chemically assisted primary decantation - CEPT process

Parameters established by NBR 12209:2011

• 6.3.1 The sizing flowrate of the primary clarifier should be the maximum hourly influent flowrate to the unit, except in the case of item (c) of 6.5.1.9.

• 6.3.2 The surface overflow rate should be compatible with the desired removal efficiency and not exceed:

  • 60 m³/(m².d) when preceding a biological filtration process;
  • 90 m³/(m².d) when preceding an activated sludge process;
  • 90 m³/(m².d) when the process is chemically enhanced primary sedimentation (CEPT process).

• 6.3.3 Wastewater treatment plants with a design flow rate greater than 250 L/s must have more than one primary clarifier.

• 6.3.4 The hydraulic detention time for the average flow rate should be less than 3 hours and, for the maximum flow rate, greater than 1 hour.

• 6.3.5 The flow rate for the maximum flow through the outlet weir must not exceed 500 m³/(m.d) per weir.

• 6.3.6 The sludge removal piping must have a minimum diameter of 150 mm; the sludge transport piping in free-flow conduits must have a minimum slope of 3%; sludge removal from the bottom should preferably be done in a way that allows observation and control of the removed sludge.