Screening is the first step in a wastewater treatment plant. The aim of the screening system is to retain, before the biological treatment plant, any solid having a dimension of 2 cm or more, which can disturb the good running of the plant. Once this stage is completed, the wastewater is discharged in an activation tank.
The screened wastewater is introduced inside the activation cell, where it is mixed with the activated sludge. The homogenization inside the bacterial environment and the oxygenation are obtained by means of positive displacement air blowers. The air is introduced into the tank through fine bubbles air diffusers, set in the best way to prevent dead spots in the aeration volume. In addition, for a proper denitrification process to occur in the activation tank, the aeration time should not exceed 13 hours per day, since the process takes place during an anoxic phase, when no free oxygen is available. The concentration of the sludge should be around 4 g/l; in case this parameter goes higher than this value, we should extract sludge from the clarifier.
The settling tank receives by gravity the bacterial suspensions (the settling rate should be around 0.6 m/h). Then progressively, the sludge leaves the liquid phase, due to a good retention period, and adapted surface load. Part of the sludge stuck in the bottom, the return activated sludge (R.A.S) is lifted, pumped, and taken back to the activation cells, in order to achieve the mineralization and avoid any anaerobic fermentation. The rest of the sludge (the wasted activated sludge (W.A.S)), is sent to the solid handling process. The suspended solids concentration of the recirculated and extracted sludge is about 8 g/l. The clarified water is taken by a crenellated weir, and evacuated by gravity towards the outlet. The retention time in the clarifier is approximately 3 hours, and the sludge volume index should be around 180 ml/g.
Nitrification- denitrification process
The nitrogen removal process consists of 3 steps:ammonification, nitrification and denitrification
Ammonification – Nitrification - Denitrification
The first step occurs in the sewer pipes before arriving to the station. The majority of nitrogen is converted from organic nitrogen to ammonium NH4+.
The conversion of ammonium to nitrate nitrogen is called nitrification; it is a two-step process. First, ammonium is converted to nitrite (NO2-). Then the nitrite is converted to nitrate (NO3-). In order to perform this step, the bacteria must have free oxygen to perform their works. The nitrification occurs only under aerobic conditions.
The conversion of nitrate (NO3-) to nitrogen gas (N2) is called denitrification. The process is performed under anoxic conditions.