Recycled-membrane biofilm reactor (MBFR). A sustainable biological alternative for microcystins removal

Resumo

In the last century, human beings have had a significant impact on continental water masses by causing eutrophication phenomena in many aquatic ecosystems. Global warming and excessive release of nutrients to water are considered the main factors triggering the mass proliferation of algae and cyanobacteria as harmful blooms. These blooms pose a serious risk to water quality given the numerous adverse effects that their appearance entails, such as the production of bad odours and flavours, loss of biodiversity, low dissolved oxygen concentration or toxicity. Specifically, the ability of some cyanobacteria species to produce microcystins (MC), highly toxic compounds for animals, including humans, has placed cyanobacteria blooms in the spotlight of authorities worldwide. For this reason, it is vital to develop strategies and technologies to control and eliminate these toxic compounds in water. To date, conventional water treatments have proven inappropriate for optimal MC elimination, and advanced water treatment technologies are necessary to ensure their removal. However these treatments, which are usually based on adsorption and chemical oxidation, also have their limitations, such as the high associated cost or harmful by-products are produced. Thus, for years, biological filtration has been suggested as a more economical and sustainable alternative. Yet despite the unquestionable potential of biological degradation for water treatment, current biological treatments are still less efficient than physico-chemical processes. This doctoral thesis aims to develop a new advanced biological system that allows microbial metabolic capacities to be enhanced to efficiently eliminate MC. In addition, the generation of a sustainable economic system is a priority in this work; therefore, recycled materials are used as supports for bacterial growth. In particular, reverse osmosis (RO) membranes are employed after their useful life in desalination plants. These membranes are of a composite type, similar to the membranes used by membrane biofilm reactors (MBfR), an emerging technology employed in wastewater treatment. Hence this innovative system, called the Recycled-Membrane Biofilm Reactor (R-MBfR), could address lack of efficient, sustainable and economical technologies to eliminate MC from drinking water. The development of the R-MBfR concept began by evaluating and selecting different bacterial genotypes for MC degradation. The mlr+ genotype possesses the MC degradation pathway encoded in the mlr gene cluster, while the mlr- genotype has a currently unknown degradation pathway. The results indicated that the genotype mlris slower than the mlr+ under all the tested conditions. In this way, of the different analysed strains, bacterial strain Sphingopyxis sp. IM-1, which has the group of mlr genes, was selected for its high efficiency to eliminate MC under different nutrient conditions. Two discarded desalination membranes, which had previously treated brackish water (BWd) and seawater (SWd), were characterised, and their suitability to generate unispecies biofilms with strain IM-1 was evaluated. An initial biofilm was developed in both the discarded membranes as a result of key characteristics for bacterial attachment, such as hydrophilicity and roughness. The previous fouling also operated as a membrane conditioner, which influenced the attraction of cells to the surface. After testing the capacity of both membranes for bacterial attachment, a proof-ofconcept was performed in a simulator cell of MBfRs. The thickness of the developed biofilm was greater and managed to eliminate 2,000-fold the maximum MC concentration allowed in drinking water (1 μg·L-1). This result, together with a preliminary economic analysis, suggested that the R-MBfR concept could be feasible and competitive on larger scales in comparison with other current physico-chemical treatments. The chlorine transformation of the discarded membranes was also evaluated to modify the surface characteristics for both bacterial attachment and gas permeability. The chlorination process also allows the elimination of previous membrane fouling which, despite being able to operate as a surface conditioner, would make it difficult to generate a standard recycling process for all membranes given their variability. The results showed that the transformation process could be beneficial to develop biofilms in certain membranes. In particular, chlorination in the BWt-NF membranes enhanced bacterial attachment without affecting MC degradation. Likewise, all the recycled desalination membranes allowed air to flow through them. So, composite-type membranes are of special interest as they offer optimal gas diffusion through the dense layer. Therefore, the use of these recycled composite membranes would offer a sustainable surface at a low cost, which would be optimal for aeration at low pressure without bubbles. Subsequently, the recycled membranes were also able to develop multispecies biofilms in the natural environment during cyanobacterial blooms. The trend of bacterial growth on membranes was similar, but with a lower biomass than that obtained with unispecies biofilms with strain IM-1. Likewise, the biofilms eliminated MC in days, while only h were required for them to be eliminated in the unispecies biofilm. These findings indicate that, although colonisation with MC-degrading bacteria from the natural environment is possible, more controlled conditions and bioaugmentation with high-performance strains would be necessary to improve the efficiencies obtained with multispecies biofilms. The high-throughput 16S rRNA gene sequencing analysis showed that the bacterial community (BCC) comprised high taxa diversity, with the Proteobacteria phylum being the dominant one in all the membranes. In addition, the different nutrition modes of the most abundant found orders suggest that R-MBfRs could eliminate not only MC, but also other undesired compounds. Indeed, most of the MC degrading bacteria described to date were found in the BCC, which shows the potential of recycled membranes to immobilise bacteria of great biotechnological interest. Finally, the diffusion of air through membranes stimulated the bacterial growth of strain IM-1 and biological MC degradation, which proves the potential of recycled membranes to work as MBfRs. Afterwards, the system was able to efficiently remove MC from natural surface water (SNW) and synthetic reclaimed wastewater (SRW). These water sources can be potentially used as irrigation water and could pose a risk to human and animal health given the possible accumulation of MC in edible plant parts. Therefore, it is concluded that the R-MBfR concept addresses lack of efficient sustainable technologies to treat water MC with different applications, and also contributes to circular economy by providing a new recycling alternative for all currently discarded RO modules.