Description
According to the United States Environmental Protection Agency (USEPA), there are over 3000 active landfills in the United States, and over 10,000 inactive landfills. As of 2010, landfills were being used to dispose of over 135 million tons of municipal solid waste annually. Disposal of municipal solid waste in landfills results in the formation of landfill leachate, which contains a complex mixture of organic as well as inorganic chemicals, many of them toxic. Landfill leachate is produced in significant quantities and must be treated prior to discharge. Currently, the most common manner of treatment and disposal of landfill leachate is to route it through the municipal sewage system to the nearest publicly owned treatment works (POTW) or wastewater treatment plants.
The difficulty of effectively treating comingled landfill leachate and municipal wastewater is presenting a serious challenge, both for landfill and POTW operations. The potentially toxic and biologically recalcitrant nature of landfill leachate makes it highly resistant to treatment by conventional biological treatment processes, such as activated sludge or anaerobic digestion that are typically employed at wastewater treatment utilities. In addition, as UV disinfection has been gaining popularity for treatment of the final effluent, it has become clear that inclusion of landfill leachate can interfere with efficacy of disinfection, resulting in violations of fecal coliform maximum contaminant levels (MCLs). This problem stems from the tendency of landfill leachate to absorb ultraviolet light, a process known as “UV quenching,” which effectively diminishes the dose available for disinfecting microbes.
Recent research has begun to make advances in understanding which characteristics of leachate are most problematic for UV quenching. In particular, hydrophobic humic substances, especially humic and fulvic acids, are well-known for their UV-absorptive properties. Certain hydrophilic substances in leachate also absorb UV light. Several treatment options have been explored recently targeted specifically at reducing such UV-quenching compounds in leachate. These include, chemical oxidation, reverse osmosis (RO), ultrafiltration, ion exchange, coagulation, and activated adsorption. Unfortunately, none of these options are optimal, each having been deemed either ineffective, economically infeasible, or logistically infeasible from the standpoint of residuals handling.
The purpose of this research was to systematically investigate a promising and cost-effective biological treatment approach for reducing UV-quenching substances in landfill leachate. The approach employs a submerged Anaerobic Membrane Bioreactor (AnMBR), which retains biomass in the reactor at a near infinite solids retention time using an ultrafiltration (UF) membrane, and imposes sequential thermophilic and mesophilic digestion conditions to simulate acceleration of the natural landfill leachate aging process. The AnMBR was operated for over a year, demonstrated feasibility for on–site pretreatment of landfill leachate prior to discharge to wastewater treatment plants.
The AnMBR system consisted of two-stage biological treatment. The first stage was anaerobic thermophilic digestion, operating at 55 ± 2°C, and the second stage was equipped with a biomass-retaining membrane, operating at 37 ± 2°C and serving as the primary AnMBR stage. Raw Landfill leachate from a young, active landfill cell was fed to the anaerobic, thermophilic reactor, whose effluent was then fed into the mesophilic AnMBR, subsequent to filtration to remove solids larger than 2.5μm. The hydraulic retention time (HRT) for the thermophilic step was 25 ± 5 days while for the AnMBR, the SRT was 40 ± 5 days. The volume of the thermophilic reactor was 1.8 L while the AnMBR was 2 L. Not only did using the membrane as the final step reduce the solids in the end product, but it also prevented the need for wasting biomass. The reactors were sealed airtight and were mixed by recirculating headspace gas back in through the bottom. The rectors were fed and the contents were wasted as required without opening the seal. This was accomplished by means of automated peristaltic pumps.
The AnMBR system was monitored over a seven-month period of operation. Parameters examined included chemical oxygen demand (COD), pH, solids and metals content. The humic and fulvic acids, as well as hydrophilic substances (a group including species such as water – soluble carbohydrates and proteins), were also quantified and documented. Detailed analyses of molecular weight distributions of these substances has advanced understanding of the fundamental mechanisms contributing to UV-quenching in landfill leachate. The biological process was found to be effective at removing recalcitrant UV-quenching substances. Specifically, there was approximately 20% removal of humic acids and fulvic acids and a reduction of over 50% in the hydrophilic content. The removal of these substances reduced the overall UV absorption by 30 – 40%. Reduction in UV absorbance was in the range of 10 – 20% in the first 2 – 3 months of operation, increasing to 30 – 40% over time and continuing to trend towards improved removal. An interesting observation was an increase in fulvic acids in the thermophilic step. This suggested the possibility of interconversion between the UV-quenching substances present in leachate due to thermophilic conditions. However, in the mesophilic step the concentrations of humic, fulvic, and hydrophilic substances reduced. However, a comparable mesophilic digester (without a membrane) did not achieve the same levels of UV absorbance reduction. A one-stage MBR without thermophilic pretreatment was also not as effective, validating the functionality of the thermophilic step and the importance of including this stage in the treatment process. This also showed the effectiveness of the membrane in contributing to a reduction in UV absorbance by means of retaining biomass in the reactor.
In the final stage of this research, next generation DNA sequencing is being applied to gain insight into the functionality of the microbial communities inhabiting the AnMBR system. Specifically, a shot-gun metagenomic sequencing approach involving DNA extraction and purification followed by illumina sequencing is being applied as a powerful tool to directly examine the suite of functional genes that are dominant in the system. Initial metagenomics results have provided iv
insight into the carbon metabolism in the thermophilic and mesophilic stages and have provided a second line of evidence of the importance of the membrane for building up high levels of active biomass in the system to enhance degradation of UV-quenching organics. Given the vast nature of next-generation DNA sequencing data sets and numerous questions that can be posed, the metagenomics data will continue to be explored in order to better understand the functionality of the AnMBR system for removal of UV-quenching substances from landfill leachate.
Our team’s prior research has indicated that even when the leachate comprises 1% or less of the total flow into the POTWs, disinfection can be inhibited. Thus, it will be essential that wastewater treatment facilities carefully regulate the flow of landfill leachates into the plant until effective treatment options can be identified. Likewise, landfill corporations should work towards developing effective, economical, on–site treatment of landfill leachates prior to discharge. Given the promising performance of the AnMBR for removal of UV-quenching substances, it is recommended that this technology be examined further at pilot-scale.
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