Environmental aspects of the implementation of project results

A natural zeolite-based additive is used in the ZEOCAP technology to increase wastewater treatment efficiency.

Cation active polymers (CAP) containing quaternary ammonium groups are bound electrostatically to the external surface of zeolite crystals embedded in rock (comprising the “natural zeolite”). The “free” ammonium groups of CAP attach positively charged sites of bacteria active in biological degradation. The ion exchange capacity of the zeolite is thus, by a large degree, not exhausted. Most of cationic sites are occupied by H⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, or NH₄⁺, depending on the zeolite pre-treatment. These latter cations can selectively be exchanged for transition or heavy metal cations eventually present in the sewage when the pH of the wastewater is above 3.5. At lower pH values, protons occupy cationic sites of zeolites instead of the metal cations. If the pH is higher than 6, as usual in the case of municipal wastewaters, most of metal cations are in form of oxide-hydroxide precipitate, and the rest can be removed by the zeolite additive. When the excess sludge containing the zeolite and heavy metals mostly in form of oxides-hydroxides (eventually in form of organic chelates) is further treated (dewatered, stabilized, composted) the pH may decrease resulting in dissolution of heavy metals which are then fixed in the cationic sites of the zeolite.

Heavy metal cations occupying a small fraction of cationic sites of the zeolite are strongly bound, e.g., these harmful cations are retarded under conditions of plant cultivation. Excess sludge of the ZEOCAP technology, after composting, can thus be safely and economically utilized as fertilizer. This method of sludge disposal as an environmentally sound method has been used in a number of successful experiments around the world. Without fixation of the toxic cations, i.e., in zeolite-free sludge, they are mobile and so are taken up by plants.

The amount of zeolite in excess sludge is about 8 wt % in the dry substance. The zeolite additive contains 5·10^-2 wt % CAP, the remobilization of which is slow. Since the treatment (dewatering) of surplus sludge containing zeolites requires 10 percent less quantity of CAP, the CAP content of the ZEOCAP sludge is smaller and, consequently, less harmful than sludge of conventional wastewater treatment technology.

In summary: The by-product of the excess sludge of the ZEOCAP technology results in essentially a lower environmental loading with contaminants than that produced with conventional wastewater treatment.