Evolution of Organic Matter and Organic Nitrogen in Leachates with Landfilling Age: A Size Distribution and Hydrophobicity Study

Landfill leachates are frequently discharged to publically owned treatment works (POTWs). The presence of UV quenching substances in leachate interferes with UV disinfection at the POTWs. Also, the biorefractory organic nitrogen contained in the leachate contributes to the plant's effluent nitrogen making it difficult to meet the tightening total nitrogen effluent regulations in some regions of the country. To study the changes in organic carbon and nitrogen with landfill age, leachate samples with landfilling ages ranging from two to 30 years from a large landfill in Kentucky were collected and fractionated on the basis of their particle size (molecular weight cut off) and hydrophobicity into humic acids (HA), fulvic acids (FA), and a hydrophilic (Hpi) fraction. The results identified the hydrophilic component, which was mostly < 1 kDa as the major source of the total organic carbon and organic nitrogen. This component tends to persist even in the older stabilized leachates whereas the HA fraction gets degraded or converted into FA fraction. The effectiveness of biological treatment for leachates at different ages was examined to develop an understanding that might help design on-site treatment methods. It was observed that on-site biological treatment was able to degrade organic matter measured as total organic carbon (TOC) but was not effective for removal of UV quenching substances, especially for the older leachates. This pointed to the possibility that the microorganisms with the capability to degrade the UV absorbing substances need a detention time of around 10-20 years, which cannot be provided by on-site biological treatment. It was also observed that one-third of the organic nitrogen was from proteins that were mainly concentrated in the Hpi fraction.