Proposal for Nutrient Reduction Credits for Volume Pond Retrofits

Transcription

1 September 6, 2013 Proposal for Nutrient Reduction s for Volume Pond Retrofits Ryan Winston & Bill Hunt Biological and Agricultural Engineering NC State University

2 Proposed Wet Pond Retrofits Floating Treatment Wetlands Upflow Filters Vegetated (Aquatic or Littoral) shelves Iron Enhanced Sand Filtration

3 Floating Treatment Wetlands Two published studies with reliable data: Winston et al. (2013) Durham, NC Borne et al. (2013) Auckland, New Zealand

4 Upflow Filters Upflow filtration system to treat water as it discharges from drawdown orifice Ensures total saturation of filtration media Reduces filter clogging Easy to retrofit existing ponds (within outlet structure) Typically focused on removing phosphorus and metals residence time too short for N removal

5 Littoral (Aquatic) Shelves Required around all ponds in NC (new const.) Typically 10 ft wide, on 10:1 slope Benefits: Lessens potential for drowning Potential nutrient removal benefits (plant uptake)

6 Iron Enhanced Littoral Sand Filter Idea has been pioneered by Univ. of Minnesota Typically C33 sand with 5-8% iron filings by volume Top of sand typically placed at the normal pool elevation

7 Iron Enhanced Sand Filtration Fe Enhanced Sand Filter in Prior Lake, MN (Erickson et al., 2012)

8 Approach for ing Wet Pond Retrofits Mined data from NCSU studies (within NC) and other published studies from reliable sources (in US and Australasia). Conservative values for effluent concentrations chosen based on fieldcollected data. given based upon minimum standards provided for installation and maintenance of each retrofit.

9 FTW - Recommendations for Pollutan t Requirements for 20% surface coverage by FTWs. 80% of FTW surface area planted. Effluent concentration reduced TP Placed in line/s by 0.03 mg/l. perpendicular to flow. No benefit given to coverage beyond 20%. Between 20 and 50% of Sliding scale starting at 20% the pond surface must Coverage (which is assigned be covered by FTWs. a 0.0 mg/l reduction) and TN 80% of FTW surface increases linearly to 50%, area planted. Placed in which is assigned a 0.11 mg/l line/s perpendicular to reduction in effluent

10 FTW - Recommendations for Pollutant TSS Requirements for 20% surface Any pond designed for coverage by FTWs. 85% TSS removal that 80% of FTW surface has at least 20% FTW area planted. Placed coverage should then be in line/s considered equal to a perpendicular to flow. pond designed for 90% There is no benefit TSS removal (without an given if coverage is increase in pond surface more than 20% area)

11 Upflow Filters Recommendations for Pollutan t Requirements for TP An upflow filter with an approved media is installed and reliably maintained on fixed schedule. The design of the filter should not restrict flow such that more than 120 h are needed to dewater the water quality volume. Effluent concentration reduced by 0.02 mg/l. TN and TSS N/A None.

12 Littoral Shelves Recommendations for Review of published data proved fruitless Currituck Co. (NC) data on littoral shelf retrofit were inconclusive Recommendation: no additional credit at current time

13 Iron Enhanced Sand Filter Recommendations for Pollutan t Requirements for TP The designer must calculate the fraction of annual runoff that is treated by a littoral sand filter with iron enhancement Effluent concentration of all water treated by iron enhanced sand filter is 0.05 mg/l. Water not passing through the filter retains the average effluent concentration of an unamended pond. TN N/A None1 If at least 50% of annual inflow flows through the TSS An 85% TSS pond with littoral filters can be considered equivalent to a 90% TSS