Cities and operators of water treatment plants (WTPs) are constantly looking for ways to provide safe, reliable, high-quality drinking water in sufficient amounts for existing and new customers at a relatively low cost. While the overall cost of WTPs are dependent on the site, separating and disposing of residuals along with the consumption of chemicals to make potable water carries a hefty price tag, and these are two of the primary drivers of the cost of operation.
In fact, the transportation of residuals offsite can be one of the most expensive aspects of a WTP’s operation. WTPs have three primary ways to dispose of the residuals by using a conventional landfill, land application, or a monofill, each varying greatly in cost of disposal, methods of operation, and public perception.
Conventional residual dewatering technologies include thickening followed by dewatering methods using centrifuges or belt presses. The cake must then be disposed. Many utilities choose to dispose of cake at a landfill. While thickening and dewatering operations have high manpower requirements, the loading, transport, and offloading of residuals adds further complexity.
In some larger plants, dewatering requires 24/7 operations, yet most landfills operate within a 40-hour schedule, leaving WTPs at the mercy of a landfill’s hours of operation, having to haul and dispose 24 hours of cake production in an eight-hour shift. Because landfills accept all human waste, they require tighter regulatory controls — reflected in their tipping fees. Landfills are not popular in the public eye, whereas other options available are less visible to the public and reduce objections.
Land application is another sludge disposal option that requires the WTP to have a lot of available land to dump and spread out the sludge. The sludge is spread on the ground and a front-end loader or similar equipment is used to turn over the soil to help evaporate the water before consolidating and packing it. However, regulatory agencies limit the amount of sludge that can be dumped per square foot of a land application site, making this option difficult for cities with less available land. To mitigate regulatory agency concerns for groundwater contamination by free water released from the land application, normally a periodic groundwater testing and reporting requirement is added.
The Gulf Coast Water Authority (GCWA) sought to overcome these challenges by seeking an alternative solution to land application and landfill sludge disposal. The GCWA currently operates a 50 million-gallon-per-day (mgd) conventional WTP, where the sludge from a thickener is liquid applied to 150 acres at a land application site.
While this option required minimum capital costs initially, it was manpower intensive and presented safety concerns to the operators. The large acreage and wet sludge presented challenges for routine maintenance, which caused rapid overgrowth of vegetation and turned the site into a haven for mosquitoes, rodents, and snakes that personnel had to navigate around.
Tasked with alleviating the GCWA’s concerns, AECOM proposed to design lagoons and a monofill to meet its objective of utilizing a process that required low capital, life-cycle, transportation, and operational costs.
Performance of a sludge lagoon requires sufficient detention time to allow particles to settle, clarifying the effluent. Once the basin volume is partially filled with settled residuals, the flow to that lagoon is stopped, allowing the captured residuals to naturally dewater. The design is simple and conventional, but can be designed robust to accommodate future changes in water quality and increased residuals volume.
The separation process between solids and liquids eliminates the need for mechanical equipment such as belt filter presses and centrifuges, although it takes much more land space. There is no mechanical maintenance until the lagoon is filled, reducing maintenance activities and operator exposure to rotating equipment. Once the sludge has been dewatered, it is excavated and transported (onsite) from the lagoons to the monofill site. This lagoon-cleaning process only takes place once a year and eliminates the need to haul sludge to an offsite landfill weekly, lowering operational and transportation costs.
The GCWA monofill will be an exclusive landfill for residuals from this WTP only that will receive dewatered sludge from the lagoons on an annual basis. The monofill will be developed in four cells to improve access to the site and facilitate drainage and phase capital investment over the next 50 years as needed. A monofill is designed to offer a high degree of self-reliance, and it prevents leachate of metals below ground. With proper design, a monofill can have a smaller footprint and lower visibility than other methods of disposal. By transporting only onsite, roadway damage and transport risk are reduced for the utility.
A monofill’s success depends on how well it is designed and built. Basic considerations include:
- geotechnical investigations and recommendations for soil modifications for reducing permeability;
- design alternatives to soil modification such as the use of synthetic liner or natural soil;
- analysis of soil excavated to determine slope layback limitations and consideration of vegetative soil stabilizing techniques; and
- operational plan addressing residuals application methods, traffic patterns, maximum lift height, and adequate drying periods before further application.
Additionally, a monofill’s success is dependent on a good understanding of the unique monofill permitting process to prevent schedule delays for construction. The overall process can take as long as one year since it involves multiple public notices, waiting periods, and public hearings, in addition to technical review. Working closely with regulatory bodies through the entire permitting and design process is essential to achieving final acceptance without major change.
The GCWA realized the value of a monofill and, as a result, AECOM was able to minimize capital cost for its client while maintaining their level of service. The GCWA will be able to reliably dispose sludge for 50 years in approximately 25 acres of the site (a reduction in the original 150 acres used to dispose of sludge) while keeping life-cycle costs low. And, they’re not the only ones benefitting — this method of disposal helps GCWA continue to provide high- quality water to its customers at competitive rates, with the ability to plan for additional production capacity to support future growth of the region.
Keith O’Connor, P.E., is senior project manager with AECOM (www.aecom.com).