When developing a liquid treatment system design at TerraTherm, we keep in mind that the system will be temporary and have an overall short duration; typically 12 months or less. This means that we need to design and select a process configuration that minimizes the cost of meeting applicable discharge requirements.
Cost elements taken into consideration include:
To design the most cost-effective treatment system for each project, we rely on site owners and their consultants to provide detailed soil and groundwater characterization and identify the specific discharge requirements that will apply to the thermal remediation program.
Liquid phase activated carbon systems are often used as a key component of the overall compliance strategy for a thermal remediation project. The full cost impact of a carbon system must consider any necessary pre-treatment steps that are needed to ensure carbon performance/utilization and to avoid excess maintenance requirements.
Activated carbon is available in many forms – granular, powder, pellet, fiber etc. The granular form is typically used in thermal remediation because of its suitability & applicability to the wastewater (liquid stream) generated during operations.
There may be numerous sources of wastewater generated during operations, including
• condensate from soil vapor extraction,
• separated entrained liquid in sump tanks or moisture separators, and
• recovered groundwater without or with free phase liquid (also referred to as NAPL).
In steam enhanced extraction remediation sites, steam also ends up being a significant component of the combined liquid stream that needs to be treated.
Steps in the design process:
1. First, a carbon system design needs to consider the short term nature of thermal remediation (operations typically last for 6-12 months) and the degree of operator presence planned for this period of the project.
Vessel sizing and piping configuration needs to consider
• contaminant loading rates,
• compliance and performance sampling schedules,
• laboratory turnaround on data,
• media change frequency, and
• media management approaches planned for the project.
2. The next step is to carefully examine site characterization data (specific contaminants present and their mass estimate in the soil and the groundwater) and translate this data into a design basis for the carbon system. Predicting how these contaminants will end up in the liquid streams that require treatment, and the range of contaminant concentrations expected throughout operations, is often a challenging task given the nature and operational history of many remediation sites.
Some contaminants tend to break down (biologically and/or chemically) in the subsurface with time, creating breakdown byproducts. Also, as thermal operations progress, elevated temperatures in the soil and groundwater may alter the composition and concentrations of contaminants in the liquid streams and this all needs to be taken into consideration.
Contaminants characterization should include
• detailed analysis of volatile (VOC),
• semi-volatile (SVOC) and
• non-volatile organic compounds.
Naturally occurring Organic Matters (NOM) should also be measured using
• Biological Oxygen Demand (BOD),
• Chemical Oxygen Demand (COD) and
• Total Organic Carbon (TOC) analysis.
NOM typically are not considered contaminants however these will compete with organic compounds for carbon surface area and increase carbon usage. Characterization should also include analysis for metals, particulate matters (suspended solids) and water chemistry (pH, Total Dissolved Solids, alkalinity, hardness etc.) to determine what pre-treatment steps may be necessary to ensure efficient carbon operations.
3. The next step is to carefully consider the effluent discharge criteria. Any limitations imposed by downstream treatment processes or existing treatment facilities, where treated liquid may be discharged to, need to be considered. Remember that not all compounds in the liquid stream may be contaminants of concern at the site, but they may have a significant impact on treatment system operational performance.
4. Once the treatment goals have been determined, the next step is to select the type of carbon based on the contaminants and to predict the carbon usage for the project. Coal and coconut based activated carbons are commonly used and have varying properties. They are available as Virgin or Re-activated carbon. The type of carbon selected should be based either on experience or in consultation with carbon manufacturer.
There are various methods of predicting carbon usage – using isotherms, performing column test, pilot test etc.
Using Isotherms (published curves or equations) is a quick way to determine theoretical adsorption capacity (and carbon usage) for a particular contaminant based on expected concentrations.
Each contaminant has its own isotherm and carbon usage of each contaminant added together is a good method to predict the total carbon usage for the liquid stream. It should be noted that Isotherms represent equilibrium adsorption capacities at standard conditions and do not account for interfering or competitive adsorption. So, a safety factor should be used when predicting carbon usage. Isotherms also do not provide any information on required contact time or depth of carbon. However, they are a quick, relatively easy and often practical method for designing carbon systems for remediation sites.
Column or pilot tests will provide additional information not obtained from Isotherm method but they often are time consuming, can be expensive, and have their own limitations.
5. Once the carbon usage is predicted, the next step should be to evaluate if activated carbon (and the LGAC system with multiple carbon vessels/bed) is a feasible and economical solution. For very poorly adsorbed contaminants (e.g. vinyl chloride), carbon may not be a solution and other media or approaches will need to be investigated.
For sites with mainly moderate to high VOC loadings, use of an Air Stripper followed by activated carbon for final polishing may be a better solution than a carbon system alone. The stripped VOCs can be treated by vapor phase activated carbon or other methods (e.g. oxidizer).
Stripping VOCs from water and treating these contaminants via vapor phase carbon may make sense because vapor phase carbon often has a much higher adsorptive capacity for a given contaminant than liquid phase carbon does. Air Stripper can however add complexity to the operations and increase operations & maintenance costs. So the feasibility of a solution should be evaluated based on the overall cost, the ease of operations, and any site imposed restrictions.
Carbon systems are often considered simple compliance tools, but there’s a lot that goes into the design of an optimal system. The complex topic of pre-treatment is a whole different topic for perhaps a later blog.