ABSTRACT
In Japan, a huge amount of municipal solid waste has been directly dumped in landfills before issuance of the waste management and public cleansing law in 1970. It is estimated that more than 1,500 such landfill sites are distributed nationwide. Leachate from waste-dumped landfills shows completely different water quality from ordinary groundwater runoff as well as from municipal sewage waters. Most of the organic matter and phosphorous are filtered out and oxygen is completely consumed in the course of retention, infiltration and through travelling underground. On the other hand, ammonia nitrogen is the dominant pollutant components with extremely significant loads. Several denitrification technologies have already being developed in the field of sewage water treatment, where carbon and phosphorous originally contained in the water play important roles in the treatment process. However such conventional techniques cannot be applied to the treatment of leachate, since both carbon and phosphorous are missing from the leachate.
The field site in this study is a municipal waste landfill located in a mountain area in Kobe City, where huge amounts of municipal solid waste was dumped in the 1960 to 70s. During the first twenty years after the landfill construction, the leachate yielded significant ammonia load. Ammonia concentration has now decreased to a certain level but it still exceeds the target level from time to time. So far, the leachate has been denitrified in a treatment plant where methanol and phosphorus are artificially fed to nourish the denitrification bacteria. Such a concentrated treatment facility is economically feasible in the case of a young landfill but it is no longer in a case with aged landfills like the present case. The plant is now aging and needs to be replaced as soon as possible by an alternative facility that is more economic, energy-saving and free from maintenance.
Nitrogen removal techniques consist of two stages; the nitrification and denitrification processes. This study is to focus on denitrification and a laboratory experiment was carried out in order to develop an alternative technology for leachate denitrification. It has already been found from the authors' preliminary experiment that hydrogen feeding agents show high performance in activating denitrification bacteria as a carbon resource as well as acting as a reducer. The experiment was carried out in a closed test tank. However, the prototype leachate reservoir is an open system in which leachate inflow is continuously loaded from the catchment to the reservoir. In order to simulate an experiment closer to the prototype, an improved experimental model was devised in this study, where leachate was continuously fed to the test tank. What was investigated was how the denitrification rate depended on relating parameters such as leachate inflow discharge, hydrogen feeding agent, microbe-bound carrier and other additives. A functional dependency of denitrification rate on inflow discharge was quantitatively confirmed, which provides useful information for determining the dimensions and capacity of the alternative treatment plant. In addition, the hydrogen feeding agent was found to be a promising material for constructing a cost-saving and maintenance-free facility for leachate treatment. Based on the present experiment, a design concept for the leachate treatment system was proposed as well.
