Biotechnological cleaning of polluted waters by means of a system producing large amounts of biodegradable organic matter

Several methods for treatment of acid mine waters exist, depending upon the volume of the effluents, the type and concentration of contaminants present. The most largely used method is connected with the chemical neutralization of the waters followed by the precipitation of metals. Such active treatment requires the installation of a plant with agitated reactors, precipitators, clarifiers and thickeners with high costs for reagents, operation, maintenance and disposal of the resulting metals laden sludge. The only alternative of such high-cost schemes are the passive treatment systems (Cambridge, 1995; Costello, 2003; Johnson and Hallberg, 2005; Groudev et al., 2007).

These systems have been developed on the basis of naturally occurring biological and geochemical processes in order to improve the quality of the influent waters with minimal operation and maintenance costs. The main advantage of these systems over chemical neutralization is that large volumes of sludge are not generated, the contaminants being precipitated mainly as sulphides.

A wide range of passive treatment systems is available currently. This paper contains some data about the treatment of acid drainage waters polluted by heavy metals, arsenic and sulphates by means of a laboratory-scale passive system initially consisting of an alkalizing limestone drain, a permeable reactive multibarrier (intended for microbial dissimilatory sulphate reduction, biosorption and additional chemical neutralization) and a constructed wetland arranged in a series. Howevevr, on the basis of the results obtained during this study, a second variant of the passive system was developed. In this variant the rich-in-dissolved biodegradable organic compounds effluents from the multibarrier were treated by a microbial fuel cell for electricity generation, and the effluents from this cell were subjected to purification by means of the constructed wetland.

The constructed wetland was a plastic vessel with rectangular form and a volume of 180 l (120 cm long, 60 cm wide and 25 cm deep). The bottom of the vessel was covered with a 20 cm layer consisting of a mixture of compost, soil with a high organic content, silt and sand. This layer was used to support the growth of the higher plants which were planted in the vessel, as well as a source of organic substances for the growth of the heterotrophic microorganisms which developed in this system. Water-torch (Typha latifolia) sod mast were placed in this layer, spaced evenly in the vessel, together with other emergent vegetation, mainly such related to the genus Zygnemophyta, as well as the water clover (Marsilea) were placed in the vessel which was filled with tap water. Fertilizers containing nitrogen, phosphorus and potassium in suitable forms were added to the vessel and artificial light was applied to enhance the growth of the vegetation. The vessel was maintained at 18-20⁰C in the course of two months. A stable biocenose developed during this period. Apart from the above-mentioned plants, the biocenose contained various microorganisms, protozoa, insects and other invertebrate organisms.

The microbial fuel cell was plexiglass rectangular container already described (Groudev et al., 2014). The feed stream ,i.e. the multibarrier effluents, was supplied to the bottom anodic section of the container at rates varying in range of 10-50 ml/h and the effluent passed through the cathodic section and exited at the top continuously. Air was injected during the treatment to the cathodic section.

Elemental analysis of the water samples was performed by atomic absorption spectrometry and inductively coupled plasma spectrometry. The isolation, identification and enumeration of microorganisms were carried out by the classical physiological and biochemical tests (Karavaiko et al., 1988) and by the molecular PCR methods (Sanz and Köchling, 2007).

It was found that an efficient removal of pollutants from the water being treated was achieved in the multibarrier even at dilution rates as high as 0.3-0.5 h^-1. The residual concentrations of pollutants in most cases were decreased below the relevant permissible levels for waters intended for use in agriculture and/or industry. This was due to different biological, chemical and physico-chemical processes but the main role was played by the microbial dissimilatory sulphate reduction. This conclusion was made on the basis of the data about the generation of hydrogen sulphide, the significant decrease of the concentration of sulphate ions and of the levels of redox potential (Eh), as well as about the increase of the number of sulphate-reducing bacteria, the level of the pH and the content of insoluble sulphides of the heavy metals and arsenic in the multibarrier. These heavy metals and the arsenic were precipitated mainly as very fine particles of the relevant sulphides. However, portions of these pollutants were precipitated mainly as hydroxides and carbonates or were removed as a result of their sorption by the organic matter in the multibarrier.

The multibarrier effluents during the first 3-4 months since the start of this experiment were subjected to the treatment for the removal of the dissolved organic compounds, ammonium and phosphate ions by the constructed wetland. This treatment was very successful and the concentrations of these components in the wetland effluents were decreased below the relevant permissible levels. At the same time, the preliminary tests revealed that the multibarrier effluents were very suitable for electricity generation by means of microbial fuel cells. It was also found that these effluents contained some electrochemically active microorganisms. Some of these microorganisms were able to transfer electrons from the organic substrates in the multibarrier directly to the anode of the microbial fuel cell used in this study. Some other microorganisms present in the multibarrier effluents, mainly sulphate-reducing bacteria (Table 3), were able to perform the electron transfer by means of secreted metabolites, such as the hydrogen sulphide. For that reason, some multibarrier effluents containing both soluble organic substrates and the relevant microorganisms were directly used for electricity generation in the microbial fuel cell constructed for this purpose. However, apart from this way of treatment, in some cases the multibarrier effluents were additionally inoculated with some electrochemically active microorganisms and then were directed to the microbial fuel cell.

The preliminary test revealed that such waters are suitable for electricity generation by means of the microbial fuel cell used in this study (Table 4).

The effluents from this cell still contained some dissolved organic compounds in total concentrations higher than the relevant permissible levels. However, these residual dissolved organic compounds were efficiently decreased to total concentrations below the permissible level by means of treatment in the constructed wetland.

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Costello, C., 2003. Acid Mine Drainage: Innovative Treatment technologies, U.S. Environmental Protection Agency, Washington.

Groudev, S.N., Georgiev, P.S., Spasova, I.I. and Nikolova, M.V., 2007. Bioremidiation of acid mine drainage in an uranium deposit, Advanced Materials Research, 20-21, 248-257.

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