Keeping Your Head Above Water
Traditional Product Development Models
Nearly every product that is engineered and manufactured -- from toothbrushes to nuclear reactors -- goes through some variation of the same product development progression. First a market need is identified, and a product is conceived that will fill that need. Prototypes are built and tested in order to refine the concept and production processes. Once usability and performance data has been collected, and the product has reached a marketable form, it is launched on a limited basis, usually in combination with mechanisms to measure the success or failure of the product. Based on this limited production, the product is either released on a broader scale or refined and re-tested.
One would expect the same product development process to apply to the stormwater treatment market. However, regulatory requirements have created an environment where developers and contractors frequently use products that have bypassed some of the important performance verification steps. Non-proprietary land-based treatment systems, such as wet ponds and bioswales (slight depressions, sometimes swampy, in the midst of generally level land), are almost universally accepted by permit review agencies that generally regard their performance to be unquestionable. Non-proprietary structural best management practice (BMP), such as sand filters are frequently accepted as well.
Unfortunately, some BMPs manufactured to treat stormwater have skipped some of the important product development steps. In some cases, hundreds of installations occur based on limited lab data and without any verification of field performance. The result is that proprietary BMPs are generally lumped together and shunned by engineers and regulators who consider them all unproven. At first glance they may seem relatively similar in operation and a fair evaluation may be difficult because test data may be hard to compare or lacking altogether. A closer look would expose significant differences in design philosophies and treatment capabilities that would help design engineers select the appropriate technology for their application. This closer look would also separate those technologies that have been adequately tested from those that apparently skipped important steps in the product development process.
Laboratory testing is the most viable means of product evaluation and development.
The Market for Stormwater Treatment Systems
Stormwater treatment is a growth industry, with some estimates projecting 40 to 50 percent annual increases. Stormwater runoff has emerged as a leading cause of surface water quality impairment, degradation, and the recent release of Phase II stormwater regulations will require more land uses and smaller lots to obtain National Pollutant Discharge Elimination System (NPDES) permits. As land values increase and open space disappears, the market for structural BMPs that can be installed below grade -- leaving land available for other uses -- is predicted to grow proportionally faster than the traditional BMP market. The advent of watershed specific Total Maximum Daily Loads (TMDLs) will also drive the market for retrofit solutions.
Several basic ideas are important in conceptualizing stormwater treatment systems. For example, in addition to not being land intensive, a manufactured BMP must be
- Easy to install;
- Easy to maintain;
- Capable of handling a wide range of flows;
- Effective in removing targeted pollutants; and
- Reasonably priced.
Virtually all proprietary stormwater BMPs satisfy most, if not all of these criteria, to some extent.
Proof of Concept
Early in the development of a proprietary stormwater treatment product, laboratory testing is the most viable means of product evaluation and development. Lab tests enable product innovators to repeat tests and control isolate variables like flow rate, and pollutant concentrations in ways that would take years to replicate in the field. Laboratory testing also allows for a testing schedule that is independent of natural precipitation patterns. This provides the opportunity to gather data at a rapid rate. It can take months and thousands of dollars in the field just for the equipment calibration necessary before usable data is collected. Initially, lab testing is a far better investment.
Field testing protocols and equipment that yield informative data have been slow to develop.
Typically, bench scale tests are a good starting point because they demonstrate the basic operating principles of a system without significant capital expenditure. It is also usually easier and cheaper to make modifications to a small model than a full-scale unit. Performance data can be gathered at this stage and used to evaluate the relative effects of various designs changes; however, this data should not be used exclusively to predict full-scale performance. Some parameters, such as hydraulics and particle size, do not scale down well, which can compromise the initial bench scale tests.
Once the product concept is validated through bench scale testing, full scale lab testing should commence. This should be performed on a model that is functionally equivalent to the product that will go to market.
At this stage, several performance benchmarks should be established, including
- Peak hydraulic capacity;
- Headloss through the unit throughout the full range of operating rates;
- Removal efficiency over the full range of operating rates for all targeted pollutants; and
- Identification of factors affecting removal efficiency, such as
- Operating rate; and
- Pollutant concentration.
Every opportunity to simulate typical field pollutant concentrations, forms and sizes should be taken in the lab. For tests like total suspended solids (TSS), and oil and grease, this is relatively easy. However, it may be difficult to create test water for other parameters like nutrients and metals that may be found in different forms depending on sources and environmental conditions. Complex biological interactions of natural BMPs can be nearly impossible to simulate in a lab. However, proprietary BMPs manufactured to treat stormwater generally lack biological components and are better suited to laboratory investigation.
Using the benchmark performance data generated in the laboratory in combination with historical precipitation and runoff rates, it should be possible to generate a model to predict field performance. This model should provide a link between the lab and the "real world."
Prototype Field Testing
The most important step in the product development process is to find out how the product performs in an actual application. It provides an opportunity to evaluate the manufacturing process and the applicability of lab data and field performance model to field conditions. Unfortunately, the time and expense involved with field testing proprietary stormwater BMPs means that field testing has rarely been done prior to introduction of the products.
Virtually all proprietary BMPs sales are in response to some treatment requirement. Since BMPs are typically designed to satisfy these requirements, field testing protocols should be designed to measure compliance with these rules. For instance, typical rules that require specific load reductions on a net annual basis dictate a testing protocol that monitors influent and effluent concentrations for all runoff over an entire year. Rules concerning effluent limits dictate a simpler testing protocol where only effluent quality is measured.
Specifiers should look for full scale lab testing and field performance model testing that incorporate lab testing, as well as anticipated site conditions.
Since regulations are generally moving toward including net load reduction language, field testing protocols are also converging in the same manner. For example, several notable protocols developed in the last few yearsdeveloped by organizations like the National Science Foundation and the American Public Works Association are based on gathering a large amount of data over a long period of time. The reality is that field testing in this manner is of stormwater treatment systems continues to be an expensive, long-term commitment with the potential to push time limits, patience and budgets to the breaking point.
Field Testing Challenges
Typical annual monitoring costs for stormwater treatment systems are in the range of $50,000, including the purchase of automatic samplers, flow meters and sample analysis. In many cases, a good portion of the first year is spent in the calibration phase where storm frequency dictates progress. This is due in part to the fact that field testing protocols and equipment that yield informative data have been been slow to develop. Aside from equipment issues, there are still several areas of debate about testing protocols for stormwater systems, underscoring the "frontier" aspect of the industry.
First, there is some debate about the ability of standard peristaltic pump-driven automatic samplers to draw representative samples from stormwater. Depending on the suction head, the intake velocity of these samplers may be up to four feet per second (ft/s). This intake velocity is probably not sufficient to draw in heavy solids that may be traveling at speeds up to twice that velocity. Additionally, the diameter of the strainer and suction lines block the passage of particles larger than 1.4-inches in diameter. The importance of accurately sampling larger particles in stormwater is emphasized by recent research suggesting that the majority of solids transported by stormwater are larger than 550 microns. (Sansalone, et. al., 1998)
Peristaltic pumps may also deliver inconsistent sample volumes throughout a storm which may lead to significant error when compositing many samples. Vacuum driven samplers tend to reduce these problems, but are not as widely used. Other options exist, like the Coshocton wheel, but require a cascading head drop at each sampling point.
Another current debate in the industry surrounds accepted measures for solids in treated water. TSS has traditionally been the analysis of choice to determine the concentration of solids transported by stormwater. Virtually all regulations that address solids removal are written in terms of TSS removal. Recently, some serious doubt about the suitability of this method has emerged based on comparisons with the suspended solids concentration method (SSC) advocated by the United States Geological Survey (USGS). The major difference between the methods are that the U.S. Environmental Protection Agency (EPA) and Standard Methods protocols for TSS analysis involve the withdrawal and analysis of an aliquot of the original sample, and the SSC method requires that the entire sample be analyzed.
There also appears to be a lack of consistency in sample preparation methods between laboratories that conduct TSS tests. One reason may be the interpretation of the disclaimer "Exclude large floating particles or submerged agglomerates of non-homogeneous materials from the sample if it is deemed that their inclusion is not representative" (Standard Method 2540 D). The SSC test approach includes all sampled solids. TSS tests show a negative bias when compared to SSC results from the same site.
The compounded effect of sampler inadequacies and TSS analysis bias is to systematically underestimate the mass of sand-sized particles, which can dramatically lower the calculated removal efficiency of a BMP.
How Testing Programs Guide Product Evaluation
Assuming that all the previous development work has been completed, once lab and field testing has been completed, performance verification is simply a matter of calibrating the field performance model against actual field results. If a discrepancy between calculated and actual performance exists, then adjusting the model is one approach. The other is to refine the operation of the BMP itself.
Evaluating engineered systems to be specified for applications can be most effectively accomplished by reviewing the manufacturers' testing protocols. While not all manufacturers are equipped yet with a full range of testing data, specifiers should look for full scale lab testing and field performance model testing that incorporates lab testing, as well as anticipated site conditions. Some products are now involved in field tests, which will offer specifiers independent verification of the field performance model.
Among the things that specifiers should consider in evaluating stormwater treatment systems are claims by manufacturers that systems remove particles down to a specific particle size rather than of the net annual load as required. Other claims that merit scrutiny are the use of mathematical models in place of lab testing, lab testing through only a portion of the systems operating capacity and bench scale tests used to represent full-scale performance.
As the stormwater treatment industry grows, and more engineered BMPs become available as options for treating stormwater, specifiers will need to be savvy in evaluating performance claims and sorting through testing data to determine validity and applicability for individual sites.
American Public Health Association. (1998), Standard Methods for the Examination of Water and Wastewater, 20th Edition, 2-57.
Sansalone, J.J., J. M. Koran, J. A. Smithson, and S. G. Buchberger, 1998, "Physical Characteristics of Urban Roadway Solids Transported During Rain Events," Journal of Environmental Engineering, ASCE, 124, 5, 427.
Stormwater News -- www.stormwater-resources.com
Stormwater Manager's Resource Center -- www.stormwatercenter.net
The Clean Water Network -- www.cwn.org
This article appeared in the November 2001 issue of Environmental Protection, Vol. 12, No. 11, on page 25.
This article originally appeared in the 11/01/2001 issue of Environmental Protection.