Percent Removal: Is it an Accurate Performance Measure?

Recent developments of the National Pollutant Discharge Elimination System (NPDES) have forced an ever-increasing number of regulatory agencies and municipalities to require the treatment of stormwater runoff from paved surfaces, rooftops and landscaped areas. As these agencies impose water quality treatment measures to meet NPDES regulations, they face the dilemma of how to regulate performance with relation to a set of specific pollutants that have been identified as significant problems withi

Recent developments of the National Pollutant Discharge Elimination System (NPDES) have forced an ever-increasing number of regulatory agencies and municipalities to require the treatment of stormwater runoff from paved surfaces, rooftops and landscaped areas. As these agencies impose water quality treatment measures to meet NPDES regulations, they face the dilemma of how to regulate performance with relation to a set of specific pollutants that have been identified as significant problems within their watershed.

There are different approaches commonly used by agencies to determine treatment requirements. Many agencies, such as states, cities, counties, departments of transportation and special districts, will provide a list of approved Best Management Practices (BMPs) with the notion that use of these practices constitutes regulatory compliance.

As a basis for meeting water quality criteria, acceptance of the BMP is frequently predicated on percent removal of target pollutants. For example, the Unified Sewerage Agency of Hillsboro, Ore., has a 65 percent removal standard for total phosphorus (TP), while the City of Portland is adopting a 70 percent removal standard for total suspended solids (TSS). Using these standards, it is up to the engineer or manufacturer to demonstrate that a particular technology or combination of technologies will meet the percent removal criteria.

Part of the challenge in developing a product for municipal market is the need to gain agency approval and demonstrate that a technology can reach a designated percent removal of a particular pollutant. There are some inherent problems with using percent removal as a sole measure of performance. The efficiency of the treatment facility can appear to increase when denser solids are washed into the system.

When evaluating a water quality facility, the following factors which influence percent removal need to be considered:

  • Percent removal will vary with influent concentration – as influent concentrations increase, removal percentages also increase.
  • At low influent concentrations, sampling and analytical errors can have significant impacts on percent removals.
  • Chemical and physical characteristics of the parameter being analyzed can strongly influence percent removal. Factors such as particle size distribution and chemical complexing can lead to misinterpretation of performance.
  • Extreme variability of climate conditions, antecedent conditions, storm intensity and duration, and facility maintenance practices will impact performance.

Stormwater Management manufactures a stormwater runoff filtration system which is used throughout the United States as a stormwater treatment BMP. The filtration system, called a StormFilter™, uses various types of media depending on the types of pollutants being targeted, such as total suspended solids (TSS), soluble metals, soluble phosphorus, and oil and grease.

Figure 1 shows a scatter plot of influent concentration vs. percent removal of TSS samples taken from various locations over the past few years. Based on the mass weighed average, the removal efficiency is approximately 83 percent.

The trend presented on Figure 1 demonstrates that pollutant removal percentages rise with an increase in influent concentration. In stormwater this phenomenon is probably associated with storm intensity, i.e. more intense (or energetic) storms detach solids and increase flows to transport heavier solids to the water quality facility. Heavier solids will settle quickly and also represent a large fraction of the total mass of solids being transported. Hence, the efficiency of the treatment facility appears to increase.

On the other end of the spectrum, low TSS values tend to be associated with lower flows and low storm energy. Low-energy storms will tend to transport only finer particles, which take longer to settle or can be more difficult to filter out. To illustrate how performance can be overrated, assume a high-energy storm with higher flows resulting in a high TSS concentration, where a large fraction of the TSS mass is sand. As the suspended solids flow through a treatment facility, the bulk of the sand falls out, resulting in what appears to be a high TSS removal. However, since the bulk of the pollutants are attached to the finer clay and silt particles or in soluble form, the actual pollutant removal characteristic can be poor. Therefore the measure of straight percent removal may not take into consideration important factors such as particle size, flows and concentration.

A good example of how low concentrations affect data can be seen with the analysis of heavy metals. Figure 2 shows how zinc removal varies with concentration. The CSF® leaf media developed by Stormwater Management has a cation exchange capacity which removes soluble metals, and the data on Figure 2 are from various studies with this media. It is evident that there is a strong relationship between percent removal and concentration. As with many media types, physical sorption increases with pollutant concentration due to equilibrium effects.

Analytical and sampling errors are apt to occur particularly at low concentrations that approach detectable limits of the instruments. As an example of a sampling error, assume a rain event with discrete parcels of influent containing anywhere from 10 to 20 ug/l of zinc. The water runs through a water quality facility that’s being monitored. If the influent sampler takes a sample from a 10 ug/l parcel and the effluent sampler takes a sample from a 20 ug/l parcel, analysis may show an influent concentration of 10 ug/l and an effluent concentration of 15 ug/l. This results in an erroneous negative (-)50 percent removal efficiency. These types of errors could be eliminated by multiple samples and statistical analysis. However, since there is so much variance in stormwater, along with limited time and budgets necessary to collect adequate samples, sometimes these types of results are used to interpret total system performance.

Another factor to consider is whether the pollutant concentrations being measured are posing a threat to the environment. Though the chronic exposure to low-level concentrations of pollutants impacts the environment, most researchers would agree that spikes of high-strength pollutants pose the highest risk. High concentrations of BOD (biological oxygen demand), petroleum hydrocarbons, nutrients and heavy metals can quickly impact the aquatic environment before there is a chance to assimilate it.

Accordingly, it is most desirable to have water quality facilities that function at their best when the pollutant concentrations are at their highest. Therefore, evaluating BMP performance in terms of percent removals should be at concentration levels that are most detrimental to the environment as well as at concentrations where the possibility of error can be diminished.

Conclusion

As our understanding of water quality facility operations increases, we need to continuously improve the methodology being used to measure performance. Perhaps an envelope curve showing percent removal vs. influent concentration can be developed along with correction factors to account for error, particle size distribution or chemical complexing. From there, this curve could be overlaid with land use to allow for the proper placement of BMPs with respect to an anticipated pollutant loading and expected maintenance requirements.

Conclusion

Accordingly, it is most desirable to have water quality facilities that function at their best when the pollutant concentrations are at their highest.

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