Can enhanced street sweeping be a cost-effective way to reduce urban stormwater P loads?
By Larry Baker
Throughout Minnesota, cities struggle with the goal of reducing nutrient inputs to urban lakes. The nearly completed Prior Lake Street Sweeping Study took a new look at an old practice, examining the potential of enhanced street sweeping as a source reduction BMP. The study is unique in several aspects: (1) it examined the effect of both percentage tree canopy and sweeping frequency (once, twice, and four times per month); (2) sweeping was done from snowmelt through snowfall, and therefore included autumn leaf fall (most previous studies did not); (3) tree leaves were analyzed separately from the finer sediments; and (4) costs were computed for each of nearly 400 sweeping runs.
The sweeping design was developed by Ross Bintner, formerly the Water Resources Engineer at the City of Prior Lake (now at the City of Edina). The University of Minnesota research team was led by Larry Baker (Bioproducts and Biosystems Engineering, BBE) and Sarah Hobbie (Ecology, Evolution, and Behavior, EEB), with funding provided by EPA’s Nonpoint Source Program, via the Minnesota Pollution Control Agency. Technician Chris Buyarksi led the analytical effort.
Preliminary findings, to be incorporated into a thesis by BBE graduate student Paula Kalinosky, show that the quantities of dry solids, phosphorus, and nitrogen removed increase with tree canopy and frequency of sweeping. For example, sweeping high canopy streets four times each month removed 3.9 times more phosphorus than sweeping low canopy streets once per month. Removal of nutrients followed a seasonal cycle, with larger quantities of nutrient removal during the spring and fall. For high canopy streets, 70-80% of the nitrogen and phosphorus removed during the spring and fall peaks was contained in the “coarse organics” – tree leaves, seeds, and other vegetative debris (see Figure 2). The economics also appear quite favorable: under certain circumstances (e.g., during fall and for streets with high canopy cover), the cost of P removal was less than $20 per pound but typically $40-50/lb P removed during the fall peak for higher canopy, compared with typical values of several hundreds of dollars per pound for stormwater ponds.
Results are being incorporated into an online spreadsheet planning tool that will allow public works departments to estimate potential nutrient and solids removals under various sweeping scenarios. The spreadsheet tool, along with final project findings, will be presented in several workshops to be held in July and August of 2013. Results will also be presented at the 2013 Low Impact Development Conference, being held August 8-13, 2013.
Proposed follow-up research will (1) determine the effectiveness of enhanced sweeping for other cities in Minnesota; (2) predict improvements in lake clarity under various sweeping regimes; and (3) optimize street sweeping, to achieve maximum benefits at minimum cost.