The Zeoflocc process besides its numerous advantages, has a significant drawback,
i.e. development of the bacteria layer on the surface of zeolites is a time
consuming process, which has a time demand of 5 - 7 days. This process however,
can be accelerated and the positive effect of zeolites on wastewater treatment
can be improved further with the addition of a zeolite additive modified with
cation active polyelectrolytes (CAP).
The theory of the new method is based on the following idea:
Natural zeolites contain easily removable Na+, K+, Ca2+ and Mg2+ ions. In aqueous media, as a result of dissociation, zeolites have negative lattice charge. Bacteria and bacteria flocks in the biological reactor also have a negative surface charge. Although particles of negative charges repulse each other, bacteria can adsorb on the zeolite surface. This can be explained by the presence of polyelectrolytes produced by the bacteria. The polyelectrolytes production, however, is a slow process, which is generally inhibited by the toxic components of wastewaters.
The CAPs organic compounds of high molecular weight have, after dissociation, a large number of positive charges. CAPs fixed on the zeolite surface, if the zeolite is modified properly, still have a number of free positive charges that facilitate the accumulation of bacteria on the zeolite surface. Consequently the formation of bacteria layer on zeolite surface accelerates regardless of the polyelectrolyte production of bacteria.
Characteristics of the link to be created between zeolite particles and CAP molecules depend on the form, the specific surface area, the Si/Al ratio, i.e., specific lattice charge of zeolites, the zeolite/CAP ratio, as well as the type and the length (molecular weight) of CAP used for modification. The zeolite-CAP formation rate is faster if natural zeolites (zeolites of Na+, K+, Ca2+ and Mg2+ form) are converted to an H+- or NH4+ - forms. With larger specific surfaces and lattice charge, there is a corresponding increase in the specific CAP concentration on zeolites modified with CAP. Thus, there is an optimal molecular weight range for modification; the shorter CAP molecules can adsorb only lower number of bacteria, but the longer ones can easily be remobilized from the zeolite surface. The zeolite: CAP ratio or more exactly the ratio of the equivalent weights (ew); zeoliteew : CAPew should be less than 1 and may have an optimal value. If this ratio is >1, i.e., the number of negative charges on the zeolite lattice is larger than the number of the positive charges associated with the CAP molecules, there will not be sufficient cationic character associated with the bacteria to attract. If the ratio is too small (<<1) the bond between zeolites and CAP will not be strong enough. To ensure high bond-strength between zeolite and CAP, CAP containing quaternary ammonium or amino groups should be used for modification. To create an effective zeolite-CAP bond resisting to the physical and chemical effects appearing in the biological reactor of wastewater treatment plants is one of the most critical milestone of the project.
To convert the natural zeolites to H+-forms treatment with aqueous HCl solution at room temperature will be applied. Conversion to NH4+ -form will be carried out with aqueous NH4Cl solution at room temperature. To convert the natural zeolite to H+ -form and increase its specific surface simultaneously, treatment with aqueous HCl at the temperature of boiling point will be applied. (On the effect of strong acids and high temperature not only the specific surface, but the Si/Al ratio will increase as well. Since the higher Si/Al ratio represents lower specific surface charge, which is disadvantageous, conditions of acidic treatment used for increasing the specific surface will also be optimized.)
To quantify the amount of CAP bounded to the zeolite surface, a total organic carbon (TOC) will be performed. First, the easily remobilizable part of the CAP will be removed by washing the modified zeolite through with TOC-free water. After this treatment zeolite will have only ²unwashable” CAP bounded chemically. The quantity of this CAP will be determined using a thermal TOC method. The bond-strength between zeolite and CAP will be determined by X-ray photoelectron spectroscopic (XPS) method.
Modified zeolites having chemically bound CAP will be tested further by respirometric method Experiments will be carried out with a continuous respirometer set to the operation parameters of an average Hungarian wastewater treatment plant (WTP). The effect of modified zeolite application on the decomposition rate of organic compounds and the quality of treated water will be determined. Based on the results, modified zeolites will be ranked. Respirometric examinations of the best modified zeolites will be continued in respirometers set to the operation parameters of the WTP where the bench-and full-scale experiments will be performed.
The modified zeolites showing the best laboratory results will be used in bench-scale and full-scale experiments at the WTPs selected.