Quantificação dos mecanismos que afetam o pH em lagoas de polimento.

Abstract

The rise of pH in polishing ponds can be predicted quantitatively from the variations of alkalinity and acidity. Alkalinity and acidity in turn are affected by several processes simultaneously: (1) CO2 desorption, (2) biological CO2 removal, (3) NH3 desorption (4) CaCC*3 precipitation. In this thesis experimental investigations were carried out to determine the rates of the processes that affect alkalinity and acidity. CO2 and NII3 desorption can be described by Fick's law. Batch pond experiments were carried out in pond models to determine desorption rates and the corresponding desorption constants for CO2 and NH3. The rate of biological C02 removal was equated to the rate of oxygen production and this rate was determined by using a respirometer that enabled semi continuous determination of the oxygen production rate. Calcium carbonate precipitation was not observed in the range of observed pH values (up to 9,5). Both theory and experiment indicate that biological CO2 removal is the most important process to effect the rise of pH in polishing ponds above a value of 9, which is necessary to obtain a significant removal rate of the nutrients. The model developed to predict pH, alkalinity and acidity changes was tested in several batch ponds with different depths. It was shows that there was a good correlation between the experimental changes and the changes predicted by the model. Factors that tend to accelerate the rate of biological C02 removal (or oxygen production) are: (1) intense sunshine, (2) high transparency of the liquid phase, (3) a low organic material concentration and (4) a shallow depth. If the pond depth and the rate of photosynthesis (i.e. the rate of oxygen production) is known, it is possible to predict the rate of change of pH and consequently of the ammonium concentration. Thus the time to effect a certain required ammonium removal efficiency can be estimated.