Greg Wilson, PE
Senior Water Resources Engineer | Barr Engineering Co.
Problem Statements
Aging stormwater ponds are not functioning as intended.
- Phosphorus is being released from bottom sediments, similar to eutrophic lakes.
- There are approximately 17,000 stormwater ponds in Minnesota
Lime water softening results in spent lime byproduct
- Currently being trucked to agricultural fields at high expense
Photo of dirty stormwater pond
Overall Goals and Objectives
- Evaluate whether beneficial re-use of spent lie can reduce sediment phosphorus release from ponds
- Conduct lab dosing experiments
- Apply spent lime to three ponds
- Conduct post-treatment monitoring to assess whether phosphorus release has been controlled
- Proof of concept - assess feasible application methods, costs and permit considerations
Background
- Phosphorus is stored in lake/pond bottom sediments and released under low oxygen.
- Decay of organic phosphorus also occurs.
- Alum treatments are normally done to bind phosphorus.
- Calcium from spent lime could substitute for aluminum.
Graphic of phosphorus released from sediment
Graphic of phosphorus binding to alum
Graphic of phosphorus binding to calcium
Spent Lime
Filter cake currently trucked to agricultural farms - used as soil amendment.
- $350 per truckload
- $1,000,000 total cost per year to SPRWS
Several other water utilities around the State use lime softening and generate spent lime.
Image of spent lime
Image of spent lime slurry (City of White Bear Lake)
Image of person spraying spent lime into pond
Application of Spent Lime Slurry to Stormwater Ponds
Typically dropped total phosphorus in half, with no sign of internal phosphorus release. Transparency readings typically doubled.
Image of pond during and immediately after application
Image of pond 6 hours after application, appearing clearer
Image of pond ~30 hours after application, even clearer than previous
Results, Considerations, and Comparisons
Graphs of Monitoring results - Wakefield Pond
Proof of Concept - Permit Considerations
- Trace chemicals in spent lime do not exceed MPCA's Tier 2 Soil Reference Values (SRVs) or Soil Leaching Values (SLVs). Spent lime does not represent a concern for release of contaminants.
- Barr (2014) compared aquatic toxicity of spent lime treated stormwater to untreated stormwater using sensitive test species (Ceriodaphnia dubia, a zooplankton). Spent lime did not result in unwanted toxic conditions.
- MPCA granted our permit on an experimental basis - continuous pH monitoring to confirm compliance with standard.
Photo of splash in pond
Proof of Concept - Costs/Feasibility
| Pond (area) | Spent lime Volume Applied (gallons) | Limitations | Contractor Cost (applied) |
| Wakefield Pond (0.4 acres) | 3,250 | Upper pH permit limit | $0.38 per gallon |
| Ash Street Pond (0.55 acres) | 6,150 | Schedule | $0.38 per gallon |
| Oak Knoll Pond (4.5 acres) | 88,000 | Center area applied w/ boat | $0.69 per gallon |
Photo of sediment core samples taken on three ponds
Comparison of Treatment Options for Sediment Phosphorus Release Control
| Treatment Option | Benefits | Drawbacks |
| Dredging | Restores treatment capacity | Upper pH permit limit |
| Alum | Not sensitive to anoxia; proven | pH considerations Sulfate |
| Iron Filings | Cheaper than alum | Redox sensitive; Raw material costs; Longevity unknown |
| Spent lime | Cost effective; Beneficial reuse; Insensitive to redox | pH considerations |
Summary
Lab tests demonstrated that Ca-P may be formed in sediments
- Insensitive to redox conditions; pH will increase, but effects can be mitigated
- Ca-P formation is likely slower than other metal-phosphate reactions (Al-P)
Spent lime applied to three ponds, to date
- Provided uniform sediment coverage, improved transparency
Post-treatment monitoring - results confirmed expectations
- Typically dropped pond TP concentrations in half
- Sediment core sampling also documented sediment-water interface changes (lower sediment oxygen demand)
Proof of concept
- Compared application methods - cost-benefit and permit considerations
- Alum applications typically cost five to twenty times higher
Logo for Barr Engineering
Acknowledgments
- Ramsey-Washington Metro Watershed District
- Vadnais Lake Area Water Management Organization
- City of White Bear Lake
- City of Maplewood
- City of Lino Lakes
- St. Paul Regional Water Services
- Minnesota Pollution Control Agency
- Minnesota Stormwater Research Council (Clean Water Fund)