Landfill Challenges & Biosolids Disposal Trends Analysis

Background and Project Goals Reduced acceptance of wastewater solids at landfills, paired with increased disposal tip fees, are a problem noted by wastewater utilities across the nation and one that has worsened in the past two years throughout the Mid-Atlantic and Northeast. A biosolids landfill study was conducted for a wastewater utility that treats 220 million gallons of wastewater per day, generates 105,000 wet tons of solids per year, and covers a service area with a population of nearly 900,000 residents. Approximately 18% of production is managed through landfill disposal. Regional landfills have enacted limitations on the utility over a two-year period that range from reduced tonnage and disposal times to complete cessation of acceptance. In response, a biosolids trends analysis project was initiated with a focus on landfill challenges and subsequent disposal policies. The primary purpose of the project was to establish the sustainability of landfill as a routine outlet for the utility's solids, and if regional capacity exists for potential emergency situations. A secondary goal was to provide understanding of the causes of extreme landfill heating (subsurface internal temperatures 250 °F up to 1500 °F) and slope failures at landfills — challenges that landfills reported as the main cause of biosolids volume restrictions. The project included interviews with industry experts to investigate causes of extreme landfill heating and slope failure, and surveys of regional landfills to understand limitation policies and identify potential disposal opportunities. Project Findings The most noteworthy finding of the project is that 'existing' approved landfill capacity can change from week to week. Verbal reports regarding capacity and acceptance by landfill representatives is unreliable. Figure 1 shows the monthly solids production volume that would go to landfills if other management programs failed in comparison to the ever-changing 'available' and 'approved' capacity reported by regional landfill representatives. The graphic shows approved capacity at the start of the project in January 2020 (6,380 wet tons per month) compared to approved capacity in August 2020 (2,910 wet tons per month). Also displayed is the available capacity reported during project interviews conducted from March to June 2020 (11,815 wet tons per month) versus capacity landfills were willing to accept post-completion of the project in October 2020 (4,190 wet tons per month). Total approved monthly landfill capacity dropped by 54% over the course of eight months and actual capacity secured at regional landfills post-project was 65% lower than what was indicated by representatives during project surveys. When interviewed for the project, representatives from six landfills reported available capacity but only one provided an approval to the utility beyond one load per day. Additionally, four landfills reduced existing approvals to 'emergency use only' though this was not indicated during landfill interviews. Even more surprising was three of those same landfills had also reported an abundance of available capacity to the County the year prior in a pledged long term 10-year plan submission. The reasons cited to the utility consistently involved wet waste restrictions and alleged connection between biosolids and extreme landfill heating and/or slope instability. In fact, all sixteen landfills participating in surveys noted operational changes and disposal restrictions based on those issues — with only two experiencing either challenge at their site. Another key finding from the project, based on interviews with industry experts, is that biosolids are not a direct standalone cause of either of these landfill challenges. Industry experts reported that extreme landfill heating and slope instability challenges are both caused by a combination of operational, biological, and chemical factors. Saturated waste masses do increase the chances of both landfill challenges — a primary reason the landfill limits are targeting reduction of 'wet wastes' as a preventative measure. While biosolids do add to the wet waste ratio, experts reported that a ratio of 20% wet waste to 80% municipal solid waste (MSW) is acceptable as long as sound operational practices are in place. Despite this, Waste Management is enforcing a strict 5% wet waste limit as a company policy and others (i.e., Republic, Advanced Disposal) are set at 10 - 15% in a conservative effort to avoid either challenge. Although the exact mechanisms are still under investigation, primary contributors to extreme landfill heating include poor liquid management, inefficient landfill gas collection systems, and exothermic reactions between wastestreams. Elevated temperatures can lead to drastic operational challenges including melting of leachate and landfill gas collection system piping, rapid decomposition and settling within the waste mass, and geysers of leachate due to pressure built up (Figure 2). One of the most interesting findings about extreme landfill heating is that one cause is the reaction between ash (i.e., fly ash, MSW incinerator ash) and organic wastes. The oxides in the ash mixed with organic material (including biosolids) can cause exothermic reaction over time — and lead to pockets of subsurface trapped heat. In other cases, subsurface extreme heating is occurring at landfills that used mixtures of ash and organics as an alternative daily or intermediate landfill cover, leading to a disproportionate ratio of reactive wastestreams in the deep waste mass. Extreme landfill temperatures can lead to slope instability and/or failure but are only one of the reported causes. Figure 3 shows a portion of a 12-acre slope failure at a landfill due primarily to inadequate design and stability analysis. As in the case of elevated temperatures, limiting biosolids acceptance is not the key answer to assuring slope stability. Sound operational handling practices including mixing wet wastes with solid wastes, spreading wet wastes out, and avoiding wet waste disposal at the toe of slope or at the interface of two landfill cells — can help to ensure safe disposal of wet wastes including biosolids. Conclusion As more is discovered about the mechanisms of the reactions leading to extreme heating and root causes of massive slope failures are identified — the industry is shifting recommendations away from over-saturating landfills, encouraging more focus on segregation of wastestreams that could lead to exothermic reactions, and homing in on how important it is to properly mix and spread out wet wastestreams. As a result, landfills are taking precautions by setting even stricter than recommended wet waste ratio limits and in some cases ceasing acceptance altogether. The utility is bearing the results of this with the most recent restrictions effectively reducing the approved disposal sites from eight to three sites over a period of three months. The main takeaway is that even a large utility, one with 900,000 resident's dependent on their facility, can essentially be refused disposal of solids, and landfill approvals can change at any moment — making landfill disposal an unreliable long-term biosolids management plan.