Biofilter systems for on-site greywater treatment: The effect of different filtration media and batch duration on the removal of selected organic and inorganic pollutants
This research evaluated the efficiency of greywater treatment in biofilters filled with different filter media composition such as biochar, woodchips, mycorrhiza, crushed bricks, and limestone chippings. The filter media additives were tested to determine their effect on the removal of selected organic pollutants (benzotriazole, caffeine, diclofenac, DEET, methylparaben, and an anionic surfactant) and inorganic pollutants (boron, copper, nickel, and zinc). Synthetic greywater (SGW) was introduced in five batches per week (weekdays) into biofilters to test the efficiency under two different batch durations (16 hours and 66 hours). The tested columns provided almost complete removal of all organic pollutants, with the prominent exception of benzotriazole. This compound was 99% removed only in the columns filled with biochar, while the other columns had much lower removal in the range of 38%-49%. The results indicated that biochar additive increased the efficiency of biofilters for the removal of organic compounds that are not readily biodegradable due to its high adsorption capacity. Anionic surfactant removal was high during the experiment for almost all additives due to biodegradation and negligibly by adsorption (with the exception of moderate adsorption by biochar). The studied metals were almost 99% removed within the biofilters. Boron removal in all studied types of biofilters was negligible. The results showed no significant effect of longer batch duration on the removal of the pollutants, except for slightly increased benzotriazole removal. The treatment of the SGW allowed reducing its toxicity (determined by the bioluminescence inhibition assay and seed germination test) to negligible or stimulatory levels. The proposed biofilters with biochar as an additive medium can be considered as an effective solution for the treatment of greywater containing organic micropollutants. This additive is expected to improve the resilience of the biofilters when the biotic processes could be hindered (cold weather conditions or presence of inhibitory compounds).  Analysis of the prokaryotic microbiome clearly showed a significant difference in the microbiome in the upper and lower part of the bioreactor, which also showed higher species richness. The upper part had a significantly higher abundance of myxobacteria (cellulose-degrading and predatory life style).In contrast, the lower part showed significantly lower richness and significantly higher abundance of fermentation bacteria from the Firmicutes phylum and bacterial and archaeal nitrifiers.