Waste water treatment by multi-stage biofilm processes: Results of the VESITURVA project

Municipal and industrial waste waters in Finland are treated before their release into the environment. New legislation also requires that waste waters from all households with running water are treated before release, whereas the methods for treatment may vary. In the Tekes-Symbio project VESITURVA research groups at the University of Helsinki, VTT, Tampere University of Technology and Lahti University of Applied Science, in collaboration with companies in the field and municipal stakeholders, pooled their resources in an effort to study and improve waste water treatment. In the case of household waste waters, minimum removal requirements exist only for the bulk components, organic matter (BOD, COD), nitrogen, and phosphorus. While we also monitored the removal of these components in VESITURVA, the main focus was on micropollutants (pollutants that exist in waste water in ng per litre to µg per litre concentrations, for example hormone disruptors, farmaceuticals, musks, components of personal care products etc.) - how they behave and how their removal can be improved. In VESITURVA we tested waste water treatment methods that are based on biofilms colonising the surfaces of matrix materials using multi-phase water treatment systems, where the water passes through two or three reactors. The model substances for the micropollutant removal process were Bisphenol A (BPA), a component used in the plastics industry, and the commonly used polycyclic musk compound HHCB. Two different types of reactors were used for studying the effect of biofilm activity: Rotating Bed Bioreactors (RBBR), where the biofilm develops on plastic beads with a large surface area, and Fixed Bed Bioreactors (FBBR), where wood chips were used as the support. In both cases a continuous or semicontinuous flow of waste water passed through the reactors. In the RBBR set-up, municipal waste water was led through a three-phase treatment process, while in the FBBR artificial grey water was treated in a two-phase process. In both processes, the removal of the bulk components was good, and substantial reductions were also achieved in the case of the model micropollutants. Multi-stage biofilm reactors seemed to be efficient at removing BPA and musk HHCB from waste water. Major parts of the micropollutants were already removed in the first reactors but the model of sequential reactors enhanced the removal of both BPA and HHCB. Diversity of bacterial communities decreased as a function of time suggesting that the bacterial communities in the reactors became specialized over time. The same bacterial groups were dominant in all sequential reactors, but differences were observed at genus taxonomic level. Also, the microbial diversity was similar to that seen earlier studies of waste water treatment microbiology. Carrier material (polyethylene, wood chips) affected the biofilm community profile in FBBR. The performance of FBBR in grey water purification was evaluated in field conditions when the reactor was installed in a detached house. The nitrogen removal efficacy of the reactor was very good and the maximum nitrogen removal efficiency in the system was 84%. Nitrogen removal in grey water treatment system was verified by an evaluation of the abundance of denitrifying microbes. The performance of the grey water treatment system was returned to the original level in 1-2 weeks after the replacement of wood chips, which were used as carrier material. Preliminary results in the laboratory also indicate that nitrogen removal in grey water treatment can be enhanced by using inoculants. Biological waste water treatment based on RBBRs purified car wash waste waters efficiently, while the reduction of surfactants was at least 95% and the reduction of chemical oxygen demand (COD) between 87 and 95% during the sampling period. Efficient waste water treatment allows automatic car washes to recycle water used for washing. The main challenges for the quality of purified water seems to be optimal nutrient input and an on-line monitoring system for water quality. We conclude that waste water treatment technology based on microbial biofilms is an efficient alternative, at least in smaller units suitable for single family homes. Although not tested in VESITURVA, we believe that the units and the technology can be upscaled and adapted to at least cover the needs of several families or a small village. Furthermore, the results regarding removal of the micropollutants tested were promising, and it is likely that many other organic micropollutants would behave similarly in multi-phase biofilm treatment systems.