Prior to any extensive experimental efforts, I first needed to obtain a fundamental understanding of landfilling operations overall: I quickly found out that I had to visit a landfill and speak with experts to grasp information that was too variable to be generalized for all landfills. So in late November 2015, myself, an undergraduate student, and one of our co-advisors on the project, Dr. Hinsby Cadillo-Quiroz, visited the Salt River Landfill and the formerly operated Tri-Cities Landfill located in Northern Scottsdale, AZ off of the Beeline Highway. The goal of our visit was to obtain aggregated landfill gas (LFG) and topsoil that is used as a daily cover. During the visit, we also were able to discuss gas generation dynamics with an employee (Rick Ribar) of an onsite gas-to-energy company, DTE energy. Mr. Ribar is responsible for maintaining the vacuum on the well heads that are used to collect the LFG. By adjusting the flow rate of the vacuum to ensure it is in equilibrium with methanogenesis, his efforts are critical to maintain when considering uncontrolled methane fluxes to the atmosphere along with persistent methanogenic activity during landfill operations.
After a successful first visit, it was deduced that refuse age and composition are the most deterministic variables regarding how units such as the Environmental Protection Agency (EPA) model predicted methane emissions given historic LFG production and landfill design. Once this was elucidated via various technical reports, it quickly became clear that refuse age and composition would become important in our planned analyses, too. Considering that landfills progress through distinct biological phases representative of natural decomposition of organic matter, the goal to increase methane production rates specifically in municipal solid waste could gain some bioinspiration solely from obtaining gas samples that (in theory) cover the range of phases in landfill progression: namely, 1) aerobic, 2) fermentative, 3) acidophilic, 4) methanogenic, and 5) air intrusion phases.
Thereafter, I visited the Salt River Landfill two more times during the Spring 2016 term with two new goals. Firstly, I wanted to meet with the CEO of the landfill and former chief engineer, Richard Allen P.E., to speak further about landfilling operations that did not pertain exclusively to gas generation. For instance, the temporal progression of these phases and associated age of the waste, differential groundwater liners used in Salt River landfill versus the Tri Cities landfill, the economic feasibility/scalability of waste-to-energy operations, and a list of individual contacts that can be useful to know perhaps later on in this work. What was perhaps most intriguing from the discussion with Mr. Allen was the efforts the managerial team plans to execute regarding converting one of the relatively new cells into what is known as a “bioreactor” cell. The idea of a bioreactor landfill centralizes around liquid (in the form of water, LFG condensate, or leachate) or air addition back into the landfill to promote accelerated degradation. Moreover, I additionally learned that the exhaustion of biogas from a landfill is indeed desired from an operator perspective, due primarily what is known as the aftercare period (i.e. when a landfill closes and no longer accept waste). It is during this aftercare period where regulatory pressure from agencies such as the EPA increases to ensure gas generation levels or potential to contaminate underlying groundwater (i.e. via accumulation of excess leachate) can be minimized effectively. The bioreactor-style landfill is certainly one way to minimize the aftercare period by promoting rapid stabilization, as the figure below depicts contrasting net LFG generation in a bioreactor compared to a traditional landfill.
Preliminary conclusions from the analyzed samples suggest the sampled wells were all in the methanogenic phase (even the 0-6 months and 6-12 months samples), given their isotopic signatures. This is an indication that the methanogenic phase develops rather quickly in this bioreactor cell, perhaps due to some microbial community dynamics that warrants further investigation. Another revelation that was confirmed from this early analysis was that the isotopic signature of the total carbon dioxide produced is perhaps a more reliable proxy for accelerated methane production rates than the isotopic signature of carbon dioxide produced from methanogenesis alone. The figure below shows the relationship between the isotopic signature of the total carbon dioxide pool along the progression of a landfill cell.
Mark received his B.S. in Biological Sciences from the University of California Merced. His prior research experience includes a project pertaining to methane production and removal rates in high elevation lakes in Yosemite National Park.