Biological treatment and microbial composition of landfill leachate using a compost process in an airlift bioreactor

Abstract

Landfill leachate (LL) contains a large amount of toxic compounds and its treatment is currently a matter of concern. In this work, an adapted compost in different airlift bioreactors was used to depurate a complex, toxic and old landfill leachate without treatment (raw leachate) and after pretreatments (coagulation/flocculation; coagulation/flocculation and filtration; coagulation/flocculation, filtration and photo-Fenton process). After the complete pretreatments, a high removal of organic matter was achieved, with a global removal of chemical oxygen demand (COD) and humic acids of 72.4% and 83.4%, respectively; besides, copper and iron in the landfill leachate presented a total removal of 91.1% and 65.8%, respectively. On the other hand, after the contact of raw leachate with compost in the airlift, there was a decrease of COD, copper and iron, while for humic acids there was a slight increase. The same trend was observed when the leachate was submitted to the different pretreatments and subsequently subjected to the biologic process with compost. The respirometric analyses showed that the raw leachate was toxic, although this tendency decreased along the treatments, with inhibition values of 41.8%, 27.5%, 22.0% and 12.1% for raw and pretreated leachate (by coagulation/flocculation, filtration and photo-Fenton processes), respectively. Also, in order to improve our knowledge about the composition of the microbial assemblages in compost samples and their possible role in pollutants removal from LL, we analysed samples from both an adapted and a non-adapted compost before and after being in contact with the raw leachate by applying Illumina sequencing of the 16S rRNA gene. Bacteroidota, Proteobacteria, Firmicutes, Spirochaetota and Deinococcota were the most abundant phyla in all samples, being present in the 50 most abundant Amplified Sequence Variants (ASVs) of the study, which represented 56.5% of the total analysed sequences and were considered as our core community. Chryseolinea, Herbinix, Proteiniphilum, Pseudomonas and Sphaerochaeta seemed to be the most resilient genera when the compost was in contact to LL. Moreover, our results showed that the metabolisms related to chemoheterotrophy, fermentation and the nitrogen cycle were the most relevant in all samples. In general, the microbial community was able to adapt to adverse conditions and remove pollutants, as heavy metals.