Use of natural zeolite at different doses and dosage procedures in batch and continuous anaerobic digestion of synthetic and swine wastes

The aim of the present work was to study the behavior of conventional digesters working while adding natural zeolite to the anaerobic treatment of swine wastes. High nitrogen concentrations (3g/l) were also applied when synthetic media was used as feed in anaerobic reactors operating in batch mode. Three sets of experiments were carried out. The first experiment was carried out in batch mode, in order to find the most appropriate zeolite doses for the digester operation. One gram per litre of zeolite was found to be the dose that produced the best reactor performance. The second experiment was carried out using three reactors operating with swine wastes at three different hydraulic retention times (HRT): 10, 20 and 30 days. From this experiment, an HRT of 20 days was selected for the following continuous experiments. For the third experiment, the digesters operated in four different ways using synthetic waste and a zeolite dose of 1g/l, with different procedures for zeolite addition: (1) not adding natural zeolite; (2) adding zeolite at the start-up of anaerobic digestion; (3) adding natural zeolite at the start-up of the anaerobic digestion and again when the steady-state was reached; (4) adding natural zeolite on a daily basis. Finally, from these results, a fourth experiment was carried out using swine waste and the same zeolite doses and digesters, operating: (1) without adding natural zeolite; (2) adding natural zeolite at the start-up of the digestion; (3) adding natural zeolite on a daily basis. It was found that the application of natural zeolite allowed for a 17–20% increase in organic matter removal with respect to the experiments carried out where natural zeolite was not added. A daily methane production increase of 11.1–30.8% with respect to the reactors working where no natural zeolite was added. The daily addition of zeolite with the influent fed to the digesters was found to be the most appropriate procedure for dosage of this material on the basis of TCOD removal efficiencies and methane productions achieved in the process. It was also demonstrated that when zeolite was added, the digester did not need more frequent cleaning.