Leachate recirculation in bioreactor landfills: Field-scale testing and modeling

Leachate recirculation (LR) in municipal solid waste (MSW) landfills operated as bioreactors offers significant economical and environmental benefits. Conventional leachate recirculation systems (LRSs) consisting of horizontal trenches and vertical wells are commonly used for LR in MSW landfills. However, need for excavation, high installation costs and non-uniform distribution of recirculated leachate are the key disadvantages of these conventional methods. In this dissertation, a new design concept called permeable blankets (PBs) was evaluated numerically and tested in the field using instrumented test sections. This design concept consists of placing a relatively thin and high hydraulic conductivity material on a relatively flat or inclined waste surface in a landfill. A perforated pipe is embedded in the PB in the direction parallel to the shorter or longer plan view dimension of the PB where leachate is injected under a positive pressure. PBs require no excavation, can substitute multiple horizontal trenches or vertical wells and achieve a relatively uniform wetting of waste. PBs made up of granular materials can also provide an ideal platform to embed sensors for monitoring the pressure, temperature, and migration of injected leachate. Field-scale testing of PBs was conducted at an active MSW landfill located in Jackson Michigan where three 60-m-wide by 10-m-long PBs made up of crushed recycled glass, shredded tires and a geocomposite drainage layer (GDL) were constructed. About total 50 sensors were embedded in these blankets to measure moisture content, pore-water pressure, temperature, and vertical stress. Leachate was injected at flow rates ranging from 0.9 m 3/hr to 3.6 m3/hr per unit meter length of the injection pipe. The data collected from the sensors during the period from September 2003 to May 2005 indicated that the injected leachate traveled across the entire width of the blankets in time periods ranging from a few minutes to a few hours depending upon the injection rate. The key conclusion of this study is that permeable blankets can be used as an new LRS in MSW landfills operated as bioreactors. The key parameters that influence pore water pressure in the blanket are: leachate injection rate; and (2) the hydraulic properties and water contents of the PB and waste. The pore water pressure in PBs does not exceed the leachate injection pressure head as long as perched water tables do not exist in the landfill.