Understanding the basic principles of water treatment media pilot trial design and evaluation

By Neal E. Megonnell

There are many methods to evaluate the use of activated carbon and other media for water treatment. These include a simple adsorption model, an adsorption isotherm, or a laboratory mini-column study, such as the rapid small scale column test (RSSCT). While these approaches can generate some valuable initial data, each comes with limitations.

Initial screening using these methods can provide a starting point, but to overcome their limitations, a pilot scale study conducted at the site is the best method. Properly designing, operating, and analyzing data from a pilot trial will yield valuable performance data, as well as other information that can be used to anticipate full-scale operation.

Pilot Column Design Considerations

A pilot column should be operated with the surface loading rate and empty bed contact time (EBCT) identical to the full-scale operation, regardless of media selection. These calculations are simple, knowing the full-scale flow rate and size of the adsorption vessel being used.

In the case of a 12-foot diameter vessel treating 500 gpm, the surface loading rate is determined by dividing the flowrate by the surface area of the vessel. In this case, a 12-foot diameter vessel has a 113 ft2 surface area. Therefore, dividing the 500 gpm flow rate by 113 gives a surface loading rate of 4.4 gpm/ft2.

Many bid specifications for large eight, 10, and 12-foot diameter vessels will include a reference to 20,000 lbs of carbon. The 20,000 lbs number comes from carbon products that have an apparent density of 0.5 g/cc.

Depending on the activated carbon being tested, the apparent density can range from 0.35 g/cc to over 0.6 g/cc. It is important to use the desired empty bed contact time in the full scale, to determine the volume of activated carbon required in the full scale and pilot system.

Again, assuming 500 gpm, and an empty bed contact time of 10 minutes, 5,000 gallons or approximately 670 ft3 of activated carbon would be required for the full scale. Knowing the density of the product then allows the weight of the activated carbon to be determined. The effect of the apparent density can result in a wide range of carbon weight.

Pilot units exist in various forms and configurations. For flexibility, it is best to have a pilot column unit capable of testing multiple products at the same time and be able to cover a wide range of surface loading rates and empty bed contact times.

Knowing the diameter of the pilot scale columns, as well as the full-scale surface loading rate and empty bed contact time, allows for the calculation of the pilot scale flow rate and amount of activated carbon or other media. Assuming a 4" inside diameter pilot column, 4.4 gpm/ft2 and 10-minute empty bed contact time, a flow rate of 0.4 gpm and 4 gallons or 0.5 ft3 of activated carbon are required.

Activated Carbon Preparation

Dry activated carbon as received, contains gas within its pore structure that must be removed by wetting the activated carbon prior to use. Water temperature and time have large effects on how quickly the activated carbon will wet, as do other properties such as carbon type and surface properties.

Once the activated carbon is dry loaded into the pilot column, wetted, and soaked for 24 hours, the column is ready for backwash to remove fine particles and stratify the activated carbon particles by size and density. The backwash rate is determined by product and temperature.

Pilot Unit Operation And Data Collection

Once the activated carbon, or other media, has been properly prepared, the columns are ready for downflow operation and data collection. Many pilot units are operated without taking full advantage of the amount of data that can be collected. Most pilot trials simply analyze the effluent concentration from the columns without using the opportunity to sample for other parameters that may be very important for initial startup and long-term operation. The pilot unit should be monitored for the following parameters:

• Activated carbon virgin carbon properties;

• Influent and effluent pH;

• Influent and effluent metals of interest such as arsenic and antimony;

• Breakthrough profiles for identifiable and measurable compounds;

• Pressure drop during operation;

• Non adsorbable total organic carbon (TOC) fraction;

• Potential biological activity specifically with biologically degradable compounds.

Data Analysis

Effluent water quality data can be used to determine the duration of certain events, such as the increase or spike in the pH in duration and magnitude, the duration and magnitude of target metals leaching, and breakthrough of specific compounds as well as aggregate measurements such as TOC.

The breakthrough curves can be analyzed for obvious specific events, such as the initial point of breakthrough and the point of breakthrough at which the treatment objective is exceeded, but a fully developed breakthrough curve can lead to far more valuable data.

A single compound breakthrough curve that is fully developed can be used to determine the length of the mass transfer zone. This calculation will show the portion of the activated carbon bed that is utilized, and can be used to design full-scale systems that fully utilize the adsorbent capacity.

It is also possible that individual compounds with concentrations below the treatment objective may exhibit what is known as “roll over”. This is a phenomenon that exists in an activated carbon adsorption system where a more strongly adsorbed compound will force a weakly adsorbed compound to desorb and show at higher concentrations in the effluent when compared to the influent.

Post Pilot Trial Analysis

A great deal of information can be obtained from a pilot trial that cannot be determined by modelling, isotherm, or mini-column testing. Aside from the obvious parameters outlined previously, more valuable information can be obtained after the pilot trial by analyzing the spent activated carbon utilized during the trial.

Spent carbon should be analyzed for parameters such as iodine number, ash content, and can also be utilized for labscale reactivation to determine the feasibility of reactivation and potential reuse.

Conclusions

Although adsorption modelling, isotherms, and mini-columns can generate valuable data, the limitations associated with each method leave many questions that can only be answered by conducting a properly designed pilot column.

With our limited knowledge of emerging contaminants, such as the growing list of PFAS compounds, endocrine disrupting compounds, and improving detection limits for well-known compounds, pilot trials can generate the highest quality data. These also generate side-by-side data not only with similar products, but in the case of PFAS compounds, various existing and experimental products.

It is important to properly design and operate the pilot unit to ensure the data will ultimately represent the fullscale operation. It is also imperative to analyze all media, both before and after testing, to ensure the media is representative of actual production and that the used media can be either reused or properly disposed.

Pilot trials can generate extremely valuable data when tests are properly designed and data is carefully analyzed. Bad inputs yield bad outputs, so spend the time to properly design the pilot, and outline and review what is being analyzed, as well as the overall goals.

Neal E. Megonnell is with AqueoUS Vets. For more information, visit: www.aqueousvets.com