ABSTRACT
A combination of membrane processes was applied to treat the digestate produced after the anaerobic treatment of pig manure in a biogas plant, aiming towards the recovery of nutrients and effective water treatment for potential reuse. Initially, coarse filtration (sieving and microfiltration) was used to remove particles larger than 1 µm, followed by ultrafiltration, to reduce the suspended solids concentrations below 1 g/L. Subsequently, selective electrodialysis is employed to recover the main nutrient ions, primarily ammonium and potassium. The ion-depleted digestate is then fed to a reverse osmosis unit, where clean water was recovered, yielding a by-product (concentrate) stream enriched in phosphates and organics content. The presence of antibiotics and the concentrations of heavy metals were monitored during all treatment stages to assess their behavior/removal in the various membrane processes. The results indicate that almost 51% of the digestate could be recovered as water free from ions and antibiotics, suitable for reuse in the biogas plant for process needs and irrigation purposes. The selective electrodialysis process can recover 51% of initial NH4+ content (corresponding to 96% of the electrodialysis feed), while the remainder largely ended up in the ultrafiltration concentrate. A similar behavior was observed for the case of K+, while approximately 68% of the phosphates content was retained by the coarse filtration process, with another 24% remaining in the ultrafiltration concentrate and the remaining 8% in the reverse osmosis concentrate. Most of the antibiotics and heavy metals were retained by the coarse and ultrafiltration steps, with smaller amounts detected in the reverse osmosis concentrate.
ABSTRACT
A series of technologies have been employed in pilot-scale to process digestate, i.e. the byproduct remaining after the anaerobic digestion of agricultural and other wastes, with the aim of recovering nutrients and reducing the load of solids and organics from it, hence improving the quality of digestate for potential subsequent reuse. In this case the digestate originated from a mixture of dairy and animal wastes and a small amount of agricultural wastes. It was processed by the application of several treatments, applied in series, i.e. microfiltration, ultrafiltration, reverse osmosis, selective electrodialysis and combined UV/ozonation. The initially applied membrane filtration methods (micro- and ultra-filtration) removed most of the suspended solids and macromolecules with a combined efficiency of more than 80%, while the reverse osmosis (at the end) removed almost all the remaining solutes (85-100%), producing sufficiently clarified water, appropriate for potential reuse. In the selective electrodialysis unit over 95% of ammonium and potassium were recovered from the feed, along with 55% of the phosphates. Of the latter, 75% was retrieved in the form of struvite.
