The Stormwater Blogs

The NJDEP is developing a re-suspension test procedure for those manufactured treatment devices (MTDs) used as an online water-quality device. Until the test procedures are developed, all approved MTDs shall be used in an offline configuration for all storms that exceed the water quality storm.

This article is a related update to previous articles I wrote on MTDs that appeared in the January/February 2006 and the July/August 2004 issues of Stormwater. Recent changes in the approval and use of manufactured treatment devices (MTDs), particularly in New Jersey, warranted a quick overview and highlight of a recent decision by the New Jersey Department of Environmental Protection (NJDEP) on the use of MTDs.

The MTDs are generally defined as prefabricated stormwater treatment structures that utilize settling, filtration, absorptive/adsorptive materials, vortex separation, vegetative components, or other appropriate technology to remove pollutants from stormwater runoff. In New Jersey, the evaluation and certification is performed by the NJDEP Division of Science, Research & Technology (DSRT) in coordination with the New Jersey Corporation for Advanced Technology (NJCAT). The NJDEP DSRT is ultimately responsible for certifying final pollutant removal rates for all manufactured treatment devices. All conditional interim certifications are effective only for a limited time period, as determined on a case-by-case basis by the NJDEP. In addition, NJDEP has typically attached certain conditions for use of MTDs that have interim certifications, such that they only are used as part of a “treatment train” or where it is proven that no other conventional water-quality best management practice (BMP) can be incorporated into the design. It is important to note that devices may be “NJCAT Verified” but not receive full certification from NJDEP.

An overview of the process currently in place in New Jersey for evaluating and certifying MTDs was also presented in the previous articles.   The reader is encouraged to visit the NJCAT Web site and the NJDEP Website for detailed information and the specific conditions on the use of these structures.

At the time of the writing of this article, 12 MTDs have received conditional interim certification from NJDEP. Six were approved in 2006 and only two approved in 2005.  The filtration-type MTDs are currently approved for an 80% total suspended solids (TSS) removal rate and the gravity-separation-type MTDs are approved for a 50% TSS removal rate. It is important to note that the NJDEP has provided all the gravity-separation-type MTDs with the same TSS removal rate.

The NJDEP is currently developing a resuspension test protocol for those MTDs that seek approval for use as an online water-quality device. Until such protocol is developed and testing has been submitted and deemed acceptable under that protocol, all MTDs approved by NJDEP shall be used offline for all storms that exceed the NJDEP water-quality design storm effective April 11, 2008. Once the new protocol is in place, vendors may complete testing, and if the results justify doing so the MTDs may be approved for online use.

Figures 1 and 2 show a typical offline drainage system layout and a typical configuration of a diversion structure.

Figure 1.

Placement of offline MTDs will require additional space and hydraulic evaluations to model the diversion structure and friction and structural losses as a result of the additional pipe and structures. A few vendors have prefabricated diversion structures they market with their MTDs and provide technical guidance on the hydraulic design of the diversion structure.

There has been a lot of work performed to date related to the ongoing development of standards and approval methods of MTDs.  There is an American Society of Civil Engineers (ASCE)/Environmental and Water Resources Institute (EWRI) task force committee currently studying the certifications of manufactured stormwater BMPs. The task force committee is reviewing existing certification programs for various manufactured stormwater BMPs nationwide. This review and input will be used to help develop new guidelines. Additional information can be found at the Rutgers University-based Water Technology Website.

The development and use of MTDs continues to increase in order to meet escalating water-quality requirements and as the available land to develop becomes limited.  This will focus the need to continue to evaluate and measure the performance of the various MTDs in the market.

References

New Jersey Corporation for Advanced Technology, Verification Process Website. Accessed May 2008.

New Jersey Department of Environmental Protection, Division of Science, Research & Technology Website. Accessed May 2008.

Rutgers University, School of Engineering, Water Technology Website . Accessed May 2008.