Improvement of the quality of sewage sludge compost by adding natural clinoptilolite

Zeolites are naturally occurring minerals characterized by high cation exchange and adsorption capacity and, thus, are used for metal uptake from sewage sludge subjected to composting process. In the present paper, changes in the physicochemical characteristics of sewage sludge samples after composting were recorded and analysed. The composting process was carried out for 120 days in pilot bioreactors, which were filled with 20% w/w clinoptilolite and sludge. The experimental results indicated differences in the metal content of the initial sludge and essential changes in pH, %Corg, % organic matter (OM), %N values and heavy metals content. Furthermore, the phytotoxicity tests that were performed showed that sludge samples affected seed germination and root elongation, but no phytotoxic effects were observed. According to the experimental results, it is concluded that the use of natural clinoptilolite leads to the improvement of the quality of the composted sewage sludge.

In conclusion, the series of the tests performed in this work shows that: C The addition of 20% zeolite in compost mixtures affects the compost physicochemical characteristics. C The concentration of metal cations in composted sludge was decreased when mineral was added during the process. C The addition of zeolite (2–5 mm) contributes to better aeration for the composting process, as well as to the better maturation of the final product (pH=7.6, C/N=18, OM=15.7%). C Through the composting process the break-up of OM increases the presence of metals in the exchangeable form and helps natural minerals, such as zeolite, to adsorb them. This behavior helps the immobilization of metals. C Water extractable metals were lower in the sludge–zeolite compost in comparison to rawsludge compost. It seems that zeolite can be used for isolation and minimization of metal mobility. C The phytotoxicity tests performed with the composted samples showed that no phytotoxic effects were observed and, in particular, the addition of zeolite slightly improved the quality of the composted sludge. Concluding, on the basis of the overall results, it is shown that the use of the natural zeolite leads to a better compost product, a fact that allows its safe further disposal or application for agricultural or other purposes.