Finding Sustainable, Cost-effective and Practical Solutions for Wastewater Solids Disposal at the City of Rio Rancho's Facilities -- A Case Study

The City of Rio Rancho currently disposes of its dewatered solids to a private landfill that has several challenges both for its current operations and viability of its long-term operations. These include the risk of the hauled away solids being turned away from the disposal site under wet weather conditions jeopardizing the daily operations for the City. With the combination of aging infrastructure, increasing tipping fees, and lack of availability of other reliable solids disposal options, the City commissioned a study to investigate sustainable and cost-effective disposal options. The City owns three wastewater treatment facilities that produce dewatered sludge using belt filter press with an average concentration of 14 percent solids. The project team evaluated a variety of disposal options that will provide the client with long-term practical solutions to provide reliability and offer a sustainable solids management plan. The team's goal was to evaluate a range of alternatives that consider both economic (capital investment and operational costs) and non-economic factors (ease of access, ease of overall operation and maintenance (O&M) and independence from other entities). The three main disposal options considered were surface disposal, composting, and landfilling. These options directly align with the National Pollutant Discharge Elimination System (NPDES) regulations that prescribe the various methods Rio Rancho can employ to dispose of their dewatered solids. These regulations categorize the requirements into three different elements. Element 1 (Land Application) and Element 2 (Surface Disposal) are regulated under EPA 40 CFR 503 Parts B and C. These two elements require stabilization of the sludge to Class A or Class B standards. Since the City does not currently stabilize their solids, the team did not prioritize land application or surface disposal as feasible alternatives due to stabilization requirements under Elements 1 and 2. Element 3 (Municipal Solid Waste Landfill Disposal) does not require stabilization, making landfill disposal of the dewatered solids the most desired choice for the City under the current operational scheme of things. The team analyzed seven different disposal alternatives and determined a final recommendation by performing a 20-year net present worth (NPW) using the following criteria: tipping fees, distance of the disposal location from the plants generating the solids, number of truck loads required per day, and the labor and maintenance costs associated with landfill disposal. The selected option was another landfill located a few further miles away compared to the current private landfill facility. However, being a county (government) operated landfill, the facility not only provided reliability and peace of mind but also offered a lower tipping fee than the current option under a government-to-government rate structure. So even with the longer hauling distance involved, the county landfill facility proved to be a better option in terms of overall costs of disposal under a NPW analysis. The second part of the project focused on evaluating alternatives to optimize operations. The team evaluated four alternatives for enhancing dewatering that could produce a higher concentration of solids and potentially stabilizing the solids along with the dewatering process. The options include: 1. Optimize the existing process and focus on operational efficiencies by changing the type of polymer and the polymer dose used to achieve a solids concentration of 16%. This 'low hanging fruit' approach could decrease overall disposal costs even by marginally increasing the dewatering performance for little to no major capital investment. 2. Replace the City's 25-year-old belt filter press equipment in a 'like for like' replacement to increase solids concentration and improve reliability. This would entail a slight capital cost but provide an opportunity to work with new equipment that has higher efficiency and reliability. 3. Add an electro-dewatering system after the belt filter press that could produce up to 35% solids. This would have a reasonable capital cost and higher O&M but produce a dewatered sludge (not stabilized) that has much higher solids, thereby significantly reducing disposal costs. 4. Add a thermal dryer following the belt filter press that could increase solids concentration up to 90%. This option has the highest capital and O&M expenses but also produces a Class A product with minimal disposal costs and provides the opportunity to harness a value-added product and create a revenue stream. This option also provides the best solution to address future regulatory issues related to emerging contaminants and Per- and polyfluoroalkyl substances (PFAS), if needed. Lastly, a combination of a belt filter press, electro-dewatering, and thermal dryer could further optimize the selection of the system. This combination option allows for maximum long-range planning and operational flexibility depending on the desired end goals - disposal of dewatered solids at the minimize costs, regulatory and/or sustainability drivers or production of a value-added product and can be implemented in a phased manner to manage capital spend and outlay. Considerations while evaluating and implementing new and innovative technologies include public perception, sustainability drivers, challenges such as finding the skills to operate and maintain these systems and the desire to prepare for a changing and evolving regulatory landscape. Additionally, site conditions and space availability can impact equipment sizing and access for O&M. The team completed a 20-year NPW analysis as part of the evaluation. Figure 1 provides a summary of this analysis and compares the quantity of solids offset per year and the associated number of truckloads for 14%, 16%, 35%, and 90% solids for 2024 (current year) and 2043 (20-year outlook). The number of truckloads of dewatered solids and overall quantity of solids can significantly decrease with increasing solids concentration and potentially provide the City with significant cost savings. Figure 1. Note: One truckload is based on ten cubic yards and 2043 projections are based on a 2% population increase. Overall, the team recommends that the City consider implementing a project that will not only provide a cost effective and reliable disposal option in the near and long term but also reduce the total quantity of solids produced thereby providing a more comprehensive, holistic, and sustainable approach to solids management. This will not only help improve operations in the long run but also offer independence and control over the solids handling and management process with less reliance on external stakeholders.