15150 DPW siltprison memo_20231205
MEMORANDUM
TO: Douglas McDonald
Northampton Department of Public Works Stormwater Manager
FROM: Coleman Horsley, PE
DATE: December 5, 2023
RE: Smith College Lazarus-Wurtele SiltPrison Phosphorus Removal Effectiveness
Nitsch Project #15150
The purpose of this Memorandum is to provide an engineering opinion on the effectiveness of the SiltPrison
160 DP’s phosphorus removal in regard to questions from Douglas McDonald, the Stormwater Manager for
the Northampton Department of Public Works (DPW), as part of his review of the Smith College Lazarus-
Wurtele project’s application to the DPW for a stormwater permit.
SiltPrison’s total suspended solids (TSS) removal effectiveness was 3rd party tested by Alden Labs as part of
the MASTEP program and has MassDEP verification for TSS 1-1000 particle size distribution (PSD) of
62.6%. It is generally understood that 50-75% of phosphorus is particle bound. Based on the MassDEP
verification and this understanding SiltPrison claims the filtration alone before the granular media is
introduced will result in a 31-47% reduction in phosphorus. Additionally, the SiltPrison DP has a granular
media mix of Perlite, Vermiculite, and Zeolite which allows a portion of the dissolved phosphorous to also be
filtered out. This statement is backed up by research such as Application of natural zeolite for phosphorus
and ammonium removal from aqueous solutions by Nuray Karapinar published in the Journal of Hazardous
Materials (2009), Ammonia removal from sewage using natural Australian zeolite by N.A. Booker, E.L
Cooney, A.J. Priestley published in the journal Water Science and Technology (1996), and Organics and
phosphorus removal in circular flow corridor constructed wetland system by Anjishnu Biswas and S.K.
Patidar published in the Journal of Water Process Engineering (2022), among several other peer reviewed
articles.
SiltPrison estimates that the granular media filters out an additional 20-45% reduction of the dissolved
phosphorus. This results in an estimated total phosphorus removal between 44% (31% plus 13% [20% of the
69% remaining after 31% removal of filtration]) and 71% (47% plus 23.85% [45% of the remaining 53%
remaining after 47%]). Nitsch Engineering has conservatively estimated for this project that 50% of total
phosphorus in the system is filtered out by the SiltPrison 160 DP. This is based upon the above-mentioned
research articles, standard engineering practice, and technology claims by the manufacturer. As stated in
Appendix E, Long-Term Pollution Prevention Plan and Stormwater Operation and Maintenance Plan, of the
Stormwater report that was submitted as part of this application, Nitsch Engineering recommends that the Silt
Prison be maintained per manufacturer’s guidelines and recommendations. These recommendations from
the manufacturer include an annual replacement of the granular fill to maximize continued phosphorus
removal. This entails removing any excess water in the SiltPrison structure and then the material can be
easily removed with a minimum 5 horsepower shop vacuum with a minimum 1-1/2” hose. Then about 11-14
cubic feet of the specified granular material is replaced. Silt Prison offers maintenance of their structures and
their services run about $200 per structure for maintenance. If there are any further questions, please do not
hesitate to contact me at 857-206-8761 or chosley@nitscheng.com
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