Application of Hydrodynamic Cavitation for the Treatment of Natural Waters from High-Molecular-Weight Organic Compounds and Ammonium Nitrogen

Abstract The groundwaters of unprotected aquifers are influenced by both natural and anthropogenic pollutants, among which compounds of heavy metal ions, ammonium nitrogen, humic complexes, and phenols are predominant. Traditional technologies (aeration/filtration, coagulation/settling/filtration) can not provide efficient purification from them. The use of strong oxidizers (O3, H2O2, KMnO4, NOCl, Cl2) as reagents increases the cost of technology and complicates operation. One of the ways to solve this problem is to use the method of hydrodynamic cavitation (HDC). The objective of this study was to investigate the effect of hydrodynamic cavitation on the oxidation kinetics of ammonium nitrogen, humic complexes, and phenols and to develop the mechanisms for the destruction and removal of these substances from aqueous solutions. The medium to be studied was a simulant prepared on the basis of tap water with the addition of a concentrated solution containing a balanced potassium–nitrogen–phosphorus fertilizer, which imparted the water with color and contamination with dissolved humic complexes (up to 3.0 mg/dm3) and ammonium nitrogen (up to 1.5 mg/dm3). Phenol (CAS no. 108-95-2) was used to simulate water contamination with phenols (up to 1.0 mg/dm3). As a result of performed studies, it has been established that the dissolved oxygen (DO) concentration in the treated water grows (up to 3 mg/dm3), when hydrodynamic cavitation (hereinafter, HDC) is applied (without air access), and the combination of HDC with coagulation makes it possible to remove all the aforesaid concentrations of contaminants. Complex organic complexes were subject to destruction−conversion to colloids with destabilization by an electrolyte solution. Ammonium nitrogen was extracted by conversion into gaseous N2. Based on the results of comparative single- and multistage HDC studies, it has established that the use of multistage HDC increases the efficiency of treatment to 21% at the same consumption of electric power. Using the contemporary cavitation theory, some mechanisms for the destruction and oxidation of humic complexes, ammonium nitrogen, and phenols were proposed.