Disposal of residuals, such as sludge, is one of the primary cost drivers of operating water treatment plants. Water project manager Keith O’Connor makes the case for low-cost, low-tech solutions for managing sludge disposal.
Municipalities and operators of water treatment plants (WTPs) are constantly looking for ways to provide safe, reliable, high-quality drinking water in sufficient amounts, and at a relatively low cost, for existing and future customers. The overall cost of operating WTPs depends on the site, but the process of making potable water by separating and disposing of residuals, along with the consumption of chemicals, carries a hefty price tag and drives operation costs.
In fact, transporting residuals to an offsite location can be one of the most expensive aspects of a WTP’s operation. WTPs have three primary ways to dispose of residuals, and each varies greatly in terms of disposal costs, operating methods and public perception.
1. Conventional landfill. Conventional residual dewatering technologies include thickening followed by dewatering methods using centrifuges and belt presses. The resulting “cake” gets disposed of in a landfill. This process requires a high degree of manpower and complex loading, transporting and offloading operations, and garners negative perceptions as landfills are typically not popular in the public eye.
2. Land application. The biggest issue here is having enough available land to dump and spread out the sludge. Regulatory agencies limit the amount of sludge that can be dumped per square foot of a land application site, and periodic groundwater testing and reporting is required.
3. Lagoons and a monofill. An alternative approach, the performance of a sludge lagoon requires sufficient detention time to allow particles to settle. Once the basin volume is partially filled with settled residuals, the flow to that lagoon is stopped, allowing the capture residuals to naturally dewater. The dewatered sludge is then excavated and transported from the lagoon to an adjacent monofill, essentially an on-site, dedicated landfill.
Case study – Innovative lagoon and monofill solution for sludge disposal
The Gulf Coast Water Authority (GCWA) hired AECOM to devise a cost-efficient solution to overcome the challenges it faced operating a 50-million gallon per day (MGD) conventional WTP. The scope of work included devising a long-term plan for comprehensive sludge management that was easy to operate, delivered a safe work environment, alleviated lifecycle costs, allowed for zero discharge of sludge and water, and provided regulatory compliance.
In their previous WTP design, the GCWA disposed of sludge by using the land application and conventional landfill methods. The sludge from a thickener was liquid applied to a 150-acre land-application site. While this option initially required minimum capital costs, it was manpower intensive. Additionally, safety concerns arose from the large acreage being covered with wet sludge, which caused rapid overgrowth of vegetation and turned the site into a haven for mosquitoes, rodents and snakes.
Tasked with alleviating the GCWA’s concerns, we proposed to design lagoons and a monofill to meet the operator’s objective of enacting a process that required low capital and provided lifecycle cost savings, while minimizing transportation and operational costs.
The sludge lagoon design is simple and conventional, but provides a robust design to accommodate future changes in water quality and increased volume of residuals. 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. Until the lagoon is filled there is no mechanical maintenance required, which reduces 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 takes place once a year, eliminating the need to haul sludge to an offsite landfill each week, which lowers operational and transportation costs.
A monofill is designed to offer a high degree of self-reliance and prevent leachate of metals below ground. With skilfull design, a monofill can have a smaller footprint and lower visibility than other methods of sludge disposal. And because sludge is transported onsite from the lagoons to the monofill, roadway damage and transport risk are reduced for the water utility.
The GCWA monofill is an exclusive landfill for residuals from this WTP only, which receives dewatered sludge from the lagoons on an annual basis. The monofill was developed in four cells to improve access to the site, facilitate drainage and phase capital investment as needed over the next 50 years. Due to AECOM’s experience and understanding of the unique monofill permitting process, our team was able to work closely with regulatory bodies through the entire permitting and design process, which proved essential to achieving final acceptance without major changes or construction scheduling delays.
Through this lagoon and monofill design, GCWA’s capital costs were minimized, while maintaining the water authority’s level of service. The GCWA will be able to reliably dispose of sludge for 50 years in approximately 25 acres of the site — a reduction from the original 150 acres used to dispose of sludge — while keeping lifecycle costs low. Ultimately, this disposal method helps GCWA continue providing high water quality to its customers at competitive rates, with the ability to plan ahead for additional production capacity to support the region’s anticipated growth.
This article is part of our ongoing series examining the top water issues that need to be solved in the state of Texas, and beyond.
For further reading:
Water in Texas: Top five issues we need to solve now