Watershed Achievements Report

Minnesota Upper Mississippi River Basin River Basin Watershed Achievements Report 2008 2015
Twin Cities Watershed
Permeable Reactive Barrier for
Phosphorus Removal
The goal of this project was to determine the
effectiveness of using “spent lime”, created by local water
utilities during drinking water clarification, to reduce
phosphorus in storm water flow. Spent lime contains
calcium, iron, and/or aluminum, all three of which bind
P. Because of the potential for high phosphorus binding
capacity, we hypothesize that spent lime could be used
in a reactive barrier system to remove phosphorus from
storm water runoff in a cost effective manner. Laboratory
experiments were conducted to determine the optimum
conditions needed for effective phosphorus removal and
in what type of storm water a spent lime barrier would
be most effective. The results of these experiments were
the basis for the pilot study to determine the P removal
effectiveness under field conditions.
To effectively and permanently remove phosphorus,
phosphate must be incorporated into recalcitrant
organic material or be bound to cations such as
calcium, magnesium, or unreducible trace metals
such as aluminum. Spent lime is an abundant waste
byproduct of drinking water treatment and the primary
component of spent lime is calcium carbonate.
Fortunately, calcium chemically prefers to be bound
to phosphate over carbonate, and phosphate is readily
converted into calcium phosphate in the presence
of high concentrations of calcium carbonate (Stumm
and Morgan, 1996). Spent lime has an advantage over
limestone in that it consists of recently precipitated and
hence more available calcium carbonate. The use of
spent lime for stormwater treatment is a new concept.
A treatment cell with spent lime is not precipitating or
flocculating phosphate (e.g., like alum), and it is not
necessarily intended to filter as do sand filters. Rather, it
is a chemical “substitution” reaction whereby the newly
formed calcium phosphate simply resides in the cell
where the calcium carbonate once resided.
Goals
• Quantify the percentage improvement in phosphorus
removal that can be achieved with the addition of
spent lime to storm water compared to storm water
treated by traditional sedimentation practices.
Installation of the spent lime test cell on the NW corner of
Larpenteur Avenue and Prosperity Road in Maplewood,
Minnesota.
• 30 to 70% P removal improvement.
• Determine if this practice is warranted for a full scale
installation for stormwater treatment.
Results that count
• Monitoring was effectively completed to allow for an
accurate analysis of the treatment results and cost
effectiveness of the BMP.
• The demonstration treatment cell removed an average
of 63.6% total phosphorus and 63.9% of total dissolved
phosphorus.
• Our analysis and testing has confirmed that this is a
practical and cost effective BMP for the reduction of
total and dissolved phosphorus in urban settings.
Financial information
Funding type: Section 319
Grant amount: $89,200
In-kind: $0
Matching funds: $82,222.51
Contact information
Cliff Aichinger, Administrator
Ramsey-Washington Metro Watershed District
2665 Noel Drive
Little Canada, MN 55117
651-792-7957
cliff@rwmwd.org
MPCA Project Manager: Brooke Asleson
Minnesota Pollution Control Agency www.pca.state.mn.us 110