The removal of excess of acidity or alkalinity, through the dosing of chemical reagents of opposite composition, is called "neutralization". Often this treatment is necessary because the industrial wastewater have very acidic or basic pH, while the biological oxidation needs a neutral or slightly alkaline pH. The neutralization takes place automatically with soda or acid, according to the characteristics of the wastewater, through dosing pumps, controlled by a pH-meter with an alarm signal. Sometimes, when the pH value of the inlet is higher than 11, it may be economically convenient to use boilers' smokes for the neutralization of wastewater. In the chimney's smokes, produced by the combustion of natural gas, naphtha and gas oil, there is a concentration of CO2, which is variable between 6 and 12%, depending on various factors, such as the type of used fuel and the excess air used for combustion of the same. A further advantage for the neutralization of alkali discharges is given by the quantity of sulfur content in diesel and naphtha, which, combining with condensate and with the air humidity, produces dilute sulfuric acid, that increases the neutralization capacity of the chimney's smoke.
The advantages, resulting from the use of the CO2 contained in the chimney flue instead of the use of sulfuric acid, are the following:
- Possibility of using CO2 at zero cost because it is already available in the smokes in excess of the needs
- Impossibility of super-acidify discharges over the limit value of 7,5 – 8, avoiding any failure to the oxidation section of the biological plant
- The concentration of SO4 or Cl- already present in the waste is not increased. This condition greatly increases the recovery percentage of water, downstream of the biological plant.
The smokes produced by combustion are sucked directly from the chimney, conveyed by piping and distributed in the storage tank by steel diffusers. To promote the thermal dissipation of the heat of the smokes, dispersants fins are arranged in the main pipe, or alternatively some vaporized water is sprayed, in order to cool the smokes.
After the neutralization of wastewater, made through the CO2, there is a residual quantity of soda to neutralize, approximately 1-2 gr/m³. In cases of boiler's block or maintenance of ejector pumps, a dosing pump of sulfuric acid starts running automatically, in order to balance the pH value. In the case of very alkaline wastewater, such as the discharges of the the mercerizing operations that have an approximate value of pH 13, it is possible to perform the neutralization not in the storage tank, but through the recirculation of the same in a neutralization tower. Here the water to be treated enters from the top, passes through the contact material placed into the tower and is invested in countercurrent by the cooled chimney smokes, introduced through a centrifugal electric fan, with appropriate flow. For discharges with pH 13, containing about 4 kg/m³ of NaOH, the recirculation and neutralization time is evaluated in about 45/60 minutes.
This technology is an integral part of the treatment process of a Biological plant.