- Hydrology & Groundwater Monitoring
- Engineering Design
BFI/Republic Services planned a landfill expansion for a 70-acre site that had been stripped of soil to the top of the limestone bedrock formation at the Middle Point Landfill near Murfreesboro, Tennessee.
CEC was retained by BFI to perform the hydrogeologic investigation and subsequent major permit modification of the landfill expansion. The investigation required characterization of the integrity of the limestone and location of the uppermost Karst aquifer beneath the site. The major permit modification included:
• Design drawings of an expansion over previously lined areas and sideslope riser sumps.
• Design of a subsurface leak detection system.
• Engineering Calculations including settlement, slope stability, leachate generation and collection, hydrology, hydraulic and pipe strength and sizing.
• Preparation of a groundwater monitoring plan in a karst aquifer.
• Preparation of a construction quality assurance plan.
• Preparation of a closure/post-closure plan.
The leachate generated by this site is typically much stronger than a “normal” landfill leachate, with ammonia concentrations upward of 2,200 mg/L, COD values above 5,000 mg/L, and very high total dissolved solids. The leachate is stronger because of the large volumes of aluminum dross (salt cake) disposed over the years. To replace the existing Breakpoint Chlorination (BPC) treatment system, CEC was retained to provide engineering to design a new leachate pretreatment plant.
Prior to commencing the design project, a pilot/treatability study was completed by the on-site plant operations group/consultant, and it was determined that biological treatment could consistently reduce the ammonia concentrations below 10 mg/L, however, the required long sludge ages can cause settling problems with the biomass. Therefore, CEC selected a membrane biological reactor (MBR) treatment system to prevent solids loss while allowing the system to carry a larger solids concentration, thereby reducing the reactor volume.
The leachate pretreatment facility consists of four 185,000-gallon storage tanks, two 450,000-gallon bioreactor tanks, two membrane tanks, and a volute sludge press. Designed to treat an average leachate volume of 100,000 gpd, with a maximum peak flow of 150,000 gpd, the new facility is incorporated with the existing leachate storage facility and BPC system. The previously existing BPC system remains available for operation should the need present itself. A complete SCADA system was designed to automate the MBR and allow everything to be controlled from the new office/laboratory. Upon commencing MBR operations, it met treatment expectations.