Behaviour of Bulk and Trace Organics in a Full Scale Membrane Bioreactor for Landfill Leachate Treatment
Landfill leachates are generally highly pollutant wastewaters which have percolated through a deposit of solid waste. In many states it is now required to collect and treat landfill leachates to avoid contamination of surface and groundwater bodies. A state of the art technology for landfill leachate treatment uses membrane bioreactor technology prior to potential polishing with dense membrane processes, activated carbon adsorption or chemical oxidation to make the effluent suitable either for discharge. Landfill leachate contains a wide range of potentially hazardous chemical contaminants.
This paper investigates contaminant removal mechanisms in the membrane bioreactor, particularly the impact of ultrafiltration on the retention of bulk organics, which are characterized by size exclusion chromatography, and endocrine disrupting trace organics. The xenoestrogenic chemical bisphenol A deals as an indicator substance and is analysed in raw leachate, activated sludge and final effluent. Bisphenol A removal rates of up to 99% were found. Some retention of the low-molecular bisphenol A compound in ultrafiltration of activated sludge biomass has been observed and attributed to an association with macromolecules and colloids, which are retained by the porous membrane.
Landfill leachates are defined as liquids percolating through agglomerations of solid waste and are considered as high strength wastewater with specific treatment needs. Christensen et al., 2001 characterised landfill leachate as water based solution of four groups of pollutants:
- Dissolved organic matter – including more refractory compounds (e.g. fulvic-like and humic-like compounds)
- Inorganic macro-compounds
- Heavy metals
- Xenobiotic organic compounds
Large molecular refractory compounds (i.e. humic substances) were the major chemical oxygen demand (COD) components of aged raw landfill leachate (Wang et al., 2006). Several organic trace contaminants have been detected in landfill leachates (Paxeus, 2000; Wenzel, 1998; Baun et al., 2004; Behnisch et al., 2001; Benfenati et al., 2003; Castillo et al., 2001; Schwarzbauer et al., 2002; Yamamoto et al., 2001).
In Germany there are very strict regulations concerning the collection, treatment and discharge of landfill leachates. Koss and Trapp (2003) have given a detailed description of the situation for the federal state of Northrhine-Westfalia. Up to now 55 landfills equipped with leachate treatment exists in Northrhine-Westfalia. The applied treatment concepts are very different (mechanical, biological, chemical and/or chemical/physical). In the late 1980s ultrafiltration (UF) and reverse osmosis (RO) systems were being used to clean leachate by separating and concentrating (Robinson, 2005). Koss and Trapp, 2003 forecasted an increasing application of membrane processes for the future treatment of landfill leachates. A similar outlook is given by the review of MBR applications within the sector of landfill leachate treatment (Robinson, 2005). Different researchers compared MBR systems with sequence batch reactors (SBRs) (Robinson 2005, Laitinen et al. 2006). Because of the advantages of sidestream MBRs he reasoned that this system will gradually become a more dominant leachate treatment process. As a further type of application Robinson (2005) described the installation of RO after a MBR process if there is a need to remove low molecular weight compounds from the treated leachate.
Among the wide range of contaminants in landfill leachate are endocrine disrupting compounds (EDC), which have achieve wide spread attention as priority coumpounds in the aquatic environment. There are many different substances, which can disruptively interact with the hormone system of humans and animals. Natural and synthetic estrogens belong to the most effective endocrine disrupting chemicals (EDCs). Potentially endocrine disrupting industrial chemicals are, for example, Nonylphenol (NP), Bisphenol A (BPA), and tributyl-tin. Xenoestrogens like NP, which is a degradation product of alkylphenolethoxylates (APEO) and BPA are not as estrogenically active as natural hormones, such as Estradiol (E2), Ethinylestradiol (EE2), but can be found in much higher concentrations in environmental compartments due to their widespread production (Körner et al., 2000). The European Commission has suggested that NP should be included in a list of priority substances for pollution prevention policies in the water sector (Commission EC, 1999).