- Nikole Stone & Alyson Fortune
Mass removal is a major focal point for all parties involved with an in situ thermal remediation (ISTR) project, but it is often not well defined or understood. Although the basis for calculating mass removal seems simple, it can be far more complicated than most clients and regulators might expect. If it is so complicated and time consuming, is it really necessary to calculate mass removal? In this blog post, we will explain why it is important for regulators and property owners (and therefore environmental consultants), as well as why your contractor needs this information, too. We will also cover some of the key challenges and considerations associated with calculating mass removal.
If you would like to learn more about mass removal, join us for next week’s webinar, Mass Removal: Why It Is Important and How to Calculate It. We will dive deeper into the importance of understanding contaminant mass, as well as the various field and laboratory analytical methods that provide a foundation for those calculations.
Mass removal is often regarded as one of the defining factors for project success. Thermal technologies are widely known for their robust design that allows for removal of large quantities of contaminant mass in a relatively short period of time. Regulators, property owners, and their consultants are often (if not always) interested in analyzing the return on remediation investment (RORI)—that alone puts mass removal at the top of the list in terms of valuable data.
Additionally, on many thermal projects there is a high level of community involvement and oversight. Mass removal from the thermal treatment zone (TTZ) is a straightforward and understandable metric for clients and regulators to demonstrate the benefits of thermal treatment to the public.
Understanding the mass present within the subsurface before thermal treatment and afterward is definitely important for the experts with whom you partner.
Your thermal team relies on the initial mass estimate within the TTZ for designing and pricing the right thermal treatment for the job. Mass estimates are critical for sizing equipment and quantifying project consumables. If the mass estimate is inaccurate or incomplete, it could result in extended operating time during the thermal remedy.
The mass removed during treatment may also be part of meeting contractual payment milestones. It is also one of the primary parameters used to determine appropriate timing to initiate confirmatory soil sampling and, ultimately, system shutdown at the end of the project.
Mass removal in the vapor phase is the most common phase in which mass removal is calculated, and is completed for all thermal remedies. It is most commonly tracked at the inlet to the vapor treatment system after vapor conditioning (i.e., condensate removal). This location is most amenable to a reliable flow measurement, as well as field screening with instruments such as a photoionization detector (PID) or flame ionization detector (FID). These instruments provide frequent concentration data that can be used to demonstrate daily fluctuations in the mass removal, which is critical for the dynamic and fast-moving nature of thermal projects.
To confirm the field instruments’ readings, a laboratory analytical sampling program is always implemented. As a general rule, volatile organic compound (VOC) vapor analytical samples are collected on a regularly scheduled interval at the same location as the field screening, either on a weekly or bi-weekly basis. It is not uncommon to detect significant vapor phase concentrations of less volatile compounds with higher boiling points, such as semi-volatile organic compounds (SVOCs), polychlorinated biphenyls (PCBs), pesticides, or dioxin/furans, when present at the site.
Liquid (or dissolved) phase mass removal is most commonly calculated using a selected laboratory analytical method (or methods) and flow rates through the above ground liquid treatment system. These samples are typically collected after free-phase product separation to minimize overlap in mass calculations.
Tracking of free product is generally less complex, and focuses mainly on the physical accumulated volume of product (quantified via direct tank measurements) and an assumed or calculated density. Depending on the specific gravity of the free product and other physical and chemical properties, it can be difficult to separate non-aqueous phase liquid (NAPL) from water being treated through the system.
Other challenges include tracking the rate of NAPL accumulation over time, due to the nature of thermal remedies, and batch-removal of recovered product. It is also possible for further separation of phases to occur over time in holding tanks after measurements have been collected, which can cause an overall drop in product volume from one measurement to the next.
There is a lot to know about how contaminant mass impacts thermal treatment design and operation, but if you feel like you need more than this brief overview, register for next week’s webinar, Mass Removal: Why It Is Important and How to Calculate It. We will discuss methods for calculating mass removal, and share case studies about how mass removal has been calculated during operations at various thermal sites. After our presentation, there will time for a Q&A, so bring your questions.