Mechanical Treatment of Storm Water Thomas Soerens University of Arkansas
Outline Fundamentals of Settling Catch basin sizing examples Alternative mechanical treatment technologies
Settling Example Regulation Storm water treatment should remove 80% of Total Suspended Solids (TSS). vague: what size solids? System 1: Removes 80% solids with d50 of 50 microns System 2: Removes 80% solids with d50 of 100 microns System 2 not remove 80% of solids with d50 of 50 microns In comparing systems, must see data side by side and compare apples to apples Example Basin (next slide)
A rectangular settling tank processes 48,000 m3/day, is 6 m wide, 36 m long, and 4 m deep. What is the average hydraulic retention time in the tank (hr)? t0 = Vol/Q = (6m x 4m x 36m) / 48000 m3/day = 0.018 day = 0.432 hr = 26 min Assuming horizontal flow, what is the flow (approach) velocity (m/d)? vx = Q/(w x h) = Q / (6m x 4m) = 2000 m/day = 1.4 m/min What is the overflow rate for the tank (m/d)? v0 = Q/(w x L) = Q / (6m x 36m) = 222 m/day = 0.15 m/min note: vo = 4 m / 0.018 day = depth / retention time
Does a particle settle out? If it enters 4 m above bottom, it has to drop 4 m in 26 min to hit bottom If particle has a settling velocity greater than the overflow rate (0.15 m/min), it will settle out. example: vs = 0.20 m/min in 26 minutes, it drops 0.20 x 26 = 5.2 m > depth to drop 4 m, it takes 4/0.20 = 20 min < t0 in 20 minutes, it travels 20 x 1.4 m/min = 28 m < L If the settling velocity is less than the overflow rate, it doesn’t hit bottom example: vs = 0.10 m/min in 26 minutes, it drops 0.10 x 26 = 2.6 m < depth to drop 4 m, it takes 4/0.10 = 40 min > t0 in 40 minutes, it travels 40 x 1.4 m/min = 56 m > L
If it doesn’t hit bottom? Approximately vs/vo fraction of particles will settle out example: vs = 0.10 m/min Removal =~ 0.10/0.15 = 0.65 = 65% removal note: this is for horizontal clarifiers note: turbulence happens
Settling Velocity – Stoke’s Law Stoke’s law for settling velocity of spheres: vs = [(rp – rw)d2g]/18m rp , rw = density of particle, water d = diameter of particle g = gravity m = viscosity A 100 micron particle will have a settling velocity 4 times that of a 50 micron particle side note for water or wastewater treatment: In Stoke’s Law, what can be changed? Do you see why we coagulate and flocculate
Basin Sizing Approaches Using d50 Set overflow rate of basin at design flow equal to d50 of a grain-size analysis of dirt you want to remove. Can have v0 up to 1/0.8 = 1.25 of settling velocity 100 micron particle
for Q = 0.17 m3/sec (6 cfs) choose aspect ratio: Length = 4 x width set vo = Q/Asurface = Q/(w x 4w) = 0.015 m/sec w = 1.7 m (5.5 ft) , L = 6.7 m (22 ft) will a 5 ft x 20 ft basin work? vo = Q/wL = 0.018 m/sec vs/vo = 0.015/0.018 = 0.82 82% removal okay for 80% removal disclaimer: the above process is a principle, not a regulation or a standard.
Wait, how deep is it? depth not involved in calculation choose depth based on practical considerations of separating clean water from dirt. 1 inch deep? 1.7 second retention time - solids only have to fall 1 in to reach bottom can’t separate 100 feet deep? 34 min retention time - solids fall 100 feet in 34 minutes impractical 4 feet deep? 1.4 min ret time, velocity = 16 ft/min, might be good
For an overflow rate of 7m/24 min (depth/to) at 24 min, 45% of particles have hit bottom (7m) 60% of particles have settled to 2 m; 75% to 0.6m avg settling velocity of 15% of particles between 45% and 60% contours is about 3.4 m in 24 min; for next interval it’s 1.3m/24min. removal rate = vs/vo overall removal = 45% + 15% x (3.4m/24min)/(7m/24min) + 15% x (1.3/7) + … = 45% + 7.3% + 2.8% =~ 55% note: could also take this approach with grain size analysis data
Questions? next: examples of mechanical storm water treatment systems
Advanced Drainage Systems (ADS) Water Quality and Underground Detention/Infiltration Units
ADS system 2 units in series Water Quality Unit (WQU) series of weirs from 60-in diameter HDPE pipe. two manholes for maintenance Detention/Infiltration Unit (DIU) three 40-ft sections of 48 in perforated HDPE pipe top and sides of excavation are wrapped in geotextile flow 1 cfs or less though WQU then DIU > 1 cfs bypass WQU and go into DIU prevents resuspension
ADS system WQU: WQU size: 5 ft x 20 ft catchment area: 1 acre peak flow 1 cfs treatment volume 3264 cf $50k per acre requires high maintenance
WAL-MART SITE SUSTAINABILITY INITIATIVEWATER GROUP Mechanical Treatment Thomas Soerens University of Arkansas 479-575-2494 Scott Franklin PACLAND 503-659-9500
OBJECTIVE Identify existing and emerging mechanical storm water treatment technologies and describe design and decision parameters.
EXISTING TECHNOLOGIESMechanical Treatment • Manholes • Stormceptor • Downstream Defender • Continuous Deflective Separation (CDS)
EXISTING TECHNOLOGIESMechanical Treatment • Manholes • Aquafilter and Aquaguard • BaySeparator and BayFilter
EXISTING TECHNOLOGIESMechanical Treatment • Manholes • V2B1 • StormGate
EXISTING TECHNOLOGIESMechanical Treatment • Vaults • Stormfilter • Stormvault • Storm Water Quality Unit
EXISTING TECHNOLOGIESMechanical Treatment • Vaults • StormTreat • Contech Vortech • Crystal Stream Vault
EXISTING TECHNOLOGIESMechanical Treatment • Inserts • Fabco StormX inserts • SmartSponge (AbTech) • Skimmers, inserts, or vault • EcoSense filters
EXISTING TECHNOLOGIESMechanical Treatment • Other • various inserts and screens
EXISTING TECHNOLOGIESMechanical Treatment • Other • ADS Retention Systems • Kleerwater Oil/Water Separators • More, see: http://www.epa.gov/ne/assistance/ceitts/stormwater/techs.html
DESIGN PARAMETERS Mechanical Treatment • Constituent parameters – design for % removal of • Trash • Solids • Oil and grease • Organics • Nutrients • Metals • Pathogens
DESIGN PARAMETERS Mechanical Treatment • Concrete manhole possible retrofit • Downstream Defender • SmartSponge Vault • Designed in or major reconstruction concrete manholes • BaySaver • Stormceptor • Larger vaults – Designed in or major reconstruction
EMERGING TECHNOLOGIES Mechanical Treatment • Emerging Technologies: • Membrane Processes - microfiltration • Dissolved Air Flotation – for oils and grease • Revolving Drum Screens • Other wastewater process • Example: Santa Monica Urban Runoff Recycling Facility
SMURRF Santa Monica Urban Runoff Recycling Facility Joint Santa Monica-Los Angeles Project • Reuse a local water resource. • Keep a pollution source out of Santa Monica Bay. • Reduce imported water & impacts on other watersheds. • Open, walk-through facility to educate the public. • Up to 500,000 gallons/day • 325,000 average • 3% of City’s daily water use. • $12 Million for 0.3 MGD • $175,000 O&M
Dissolved Air Floatation SMURRF Process Rotating Drum Screens Grit Chamber Membrane Microfiltration UV Disinfection
DESIGN LIMITATIONS Mechanical Treatment • Advance processes applications (e.g., SMURFF), are demonstration projects paid by grants • Not economically feasible at this time • Retrofit and construction issues • Inserts can be placed in, but are not as effective • Some manhole applications can be retrofit with relatively minor reconstruction
DESIGN LIMITATIONS Mechanical Treatment • Vault applications • Must be designed in. Retrofit is difficult. • Stormfilter and some other applications may allow changing or expanding treatment processes in the future. • Flexibility and upgradeability of systems should be considered.
PROS / CONS Mechanical Treatment • Pros: • more reliable, flexible than natural treatment or infiltration • Not sensitive to climate, soil, season • can remove hydrocarbons, metals, nutrients • designed for desired constituents and removal rates • Cons: • The most effective systems are expensive • O & M cost and effort can be considerable • difficult retrofits for the most effective systems
CLIMATE / REGIONAL RESTRICTIONS Mechanical Treatment • In general, no climate or regional restrictions • Ice, snow, deicing issues dealt with in site-specific design • StormTreat is a constructed wetland • Not as effective in Winter in some climates • A system in California had to be watered
RANKING OF ALTERNATIVES Mechanical Treatment • Natural treatment and infiltration are preferred when feasible and appropriate. Mechanical systems tend to be more expensive and require more operation and maintenance. • Mechanical treatment systems in addition to or instead of natural treatment can be designed to meet specific goals. • Vault systems (e.g., StormFilter), if affordable, may offer more flexibility and upgradeability than manhole systems. • Inserts can be retrofitted to remove trash, solids, and oils.
RECOMMENDED FOR DETAILED STUDYMechanical Treatment • Can a standard protocol be established to evaluate which natural treatment, infiltration, and mechanical treatment alternatives are most appropriate for each site? • Can a standard design of a mechanical treatment system be established that can be adapted to different site conditions including hydrology, water constituents, and discharge limits?