By considering various approaches in sump design, landfill operators can tailor condensate management to their site’s unique requirements, improving environmental control, and ensuring compliance with regulations.
By Scott McGourty, PE
Proper management of landfill gas condensate is vital to effective operation of landfill gas collection and control systems (GCCS). The function of landfill gas condensate sumps is similar to the role of kidneys in the human body. Kidneys remove excess fluids and waste from the bloodstream, supporting the circulatory system and other systems in the human body. Landfill gas condensate sumps facilitate removal of liquids and suspended solids from GCCS piping, supporting air emissions control systems and compliance in other regulatory obligations. An ineffective condensate removal system will result in landfill gas pipes becoming clogged with liquids, which impairs system vacuums and results in a myriad of negative operational and compliance impacts.
For landfills with an existing GCCS, or sites preparing for the installation of a new GCCS proper planning and design of condensate management infrastructure is an essential step. Effective design of condensate sumps involves consideration of several factors including:
• Estimated Condensate Generation Rates
• Condensate Accumulation (Emergency Conditions)
• Serviceability Requirements
• Site-Specific Condition
• Geotechnical Considerations
• Maintenance and Accessibility
• Contingency Planning
• Future Expansion
Estimated Condensate Generation Rates
Condensate forms as landfill gas cools while traveling in gas system laterals and headers. The rate at which condensate forms in the gas system depends on several factors, including: the temperature of the gas at the point of generation, ambient weather temperatures, the gas flow rate, and the vacuum pressure in the system piping. Condensate sump sizing should account for peak generation rates during worst-case weather conditions (typically during the coldest weather conditions at the site).
Condensate Accumulation (Emergency Conditions)
During the GCCS design process, site operators and engineers should identify potential emergency conditions. These conditions may include instances when the sump pump is inoperable. In such cases, (either due to pump malfunction, or loss of pneumatic pressure or electric power), the presence of redundant pumps in the sump does not decrease the required sump size; the sump should have ample capacity to contain all condensate generated from landfill gas until the pump is repaired or replaced. For example, the simulated length of time that the pump may be down could consist of a 12-hour period to simulate a pump that malfunctions shortly after the site is closed for the day, but is not discovered until the site reopens. The maximum length for which the pump can be down should account for site-specific factors such as site hours of operation, the availability of spare pumps and parts, and maintenance technician inspection schedules.
The sump size and pump selection should meet the serviceability requirements related to the frequency and duration of the sump pump’s operation. The minimum daily runtime for a sump pump depends on the ratio of the condensate generation rate and the pump rate. Serviceability requirements may include a maximum number of operating hours pumps may run before being pulled for inspection, maintenance, or replacement. The presence of redundant pumps in a sump can decrease the length of time, which each pump must run. Design engineers should select a pump able to accommodate the typical condensate generation rates while accounting for anticipated pump maintenance service cycles and lifespan. Manufacturers may offer guidelines regarding pump maintenance frequency, however, site-specific physical and chemical characteristics of condensate such as pH or solids content may dictate maintenance requirements on a sump-specific basis.
The sump size should consider landfill gas composition, condensate generation rates, waste type, landfill age, climate, and operating conditions specific to the landfill site to optimize condensate management. Sites with high temperatures in the waste mass, large fluctuations in weather conditions, or that accept waste with high moisture content are likely to require larger sumps. In such cases, sumps may need to be spaced more frequently along the header.
Geotechnical aspects are also important to consider during sump design. A deep, relatively small diameter sump will create high bearing pressure on the materials underneath the sump compared to a larger diameter, shallow sump. This consideration becomes especially important when the sump is situated within the waste footprint or on soft soils. In such cases, the bearing capacity of the surrounding materials should be assessed to ensure that they can adequately support the increased pressure exerted by the sump. Sump dimensions or base plate design may also need to be adjusted.
Maintenance and Accessibility
Ensuring easy access and maintenance of the sump pump(s) and associated equipment is an important consideration during design. Easy access simplifies the tasks of operations and maintenance personnel and reduces downtime during servicing. A well-designed sump should provide ample space for personnel and equipment to work comfortably and safely. For example, where feasible, the sump should be placed in areas with access roads in close proximity, minimal obstructions or overgrowth that could impede access, and adequate distance from steep slopes to accommodate maintenance activities. Incorporating safety features, such as lighting or warning signage, can enhance the overall accessibility and safety during maintenance operations.
Contingency planning is an important consideration in sump design, ensuring a reliable condensate removal system even during unforeseen circumstances. To prepare for pump maintenance, repairs, or emergencies when a pump is offline, the design team should explore options such as incorporating backup or redundant pumping systems in the sump or purchase of spare pumps staged onsite. Having these backups ready can prevent disruptions in condensate removal, maintaining the smooth operation of gas collection and control. Moreover, implementing sump risers with camlock ports allows for easy servicing by vacuum trucks, providing a swift response during critical situations.
Properly considering future expansion and changes in gas collection system capacity is a fundamental element in sump design. The design team should take a forward-looking approach, strategically placing sumps in areas not intended for future waste filling operations in the near term whenever possible. Additionally, sizing sumps for larger flow rates than currently anticipated allows them to handle increased condensate volumes, resulting from expanded gas collection operations, which minimizes the need for frequent modifications and system upgrades, optimizing efficiency and cost-effectiveness. Moreover, considering the direction of flow during ultimate header build-out ensures that the sumps are strategically placed to efficiently collect condensate during all stages of landfill development. By integrating these proactive measures into sump design, landfill operators can establish a flexible and adaptable condensate management system, aligning with future growth and changes in gas collection needs.
Incorporating a vacuum break allows for servicing the sump without interrupting the operation of the GCCS. This feature can become important for sites with challenging minimum run times for flares as a permit condition, where uninterrupted gas collection is essential to compliance. By implementing a vacuum break, maintenance personnel can safely access and service the sump without impacting gas collection activities, ensuring continuous condensate removal and environmental compliance.
Another consideration for sites requiring enhanced environmental control is the use of dual containment with a liquid level indicator. Dual contained sumps are commonly used when located outside the liner footprint. This setup provides an additional layer of protection against potential leaks, managing the risk of releases. The liquid level indicator facilitates easy inspection via a water level sounder, enabling prompt action in case of any anomalies or excessive condensate accumulation in the secondary containment space.
Exploring alternatives to sumps, such as drip legs, presents another avenue for efficient condensate management. Drip legs can be a viable option in specific scenarios, offering a simple and effective means of removing condensate from the gas stream and reinjecting it into the waste mass. Drip legs may be appropriate for some closed sites where gas generation and condensate generation are low or anticipated to decrease, and site inspections are relatively infrequent. By considering these various approaches in sump design, landfill operators can tailor condensate management to their site’s unique requirements, improving environmental control, and ensuring compliance with regulations. | WA
Scott McGourty, PE is a Senior Project Manager at Civil & Environmental Consultants with more than 16 years of engineering experience. Scott can be reached at (208) 908-1573 or e-mail [email protected].