Quiz 5 Flashcards
(15 cards)
Hypolimnetic aeration (2)
Uses compressed air or oxygen to circulate the hypolimnion and maintain thermal stratification
Oxygen is added mainly from the bubbles as there is minimal contact with the atmosphere
Seven basic designs of hypolimnetic aeration
Full lift
Partial lift
Deep oxygen bubble injection (DOBI)
Downflow air injection (DAI)
Side stream injection design (SSI)
Mechanical aeration design
Downflow bubble contact aeration (DBCA)
Full lift (6)
Water is lifted to the surface of the lake, air is added, then water is returned to the hypolimnion
Most common
Operates on air or oxygen
Used in St Mary Lake on Salt Spring Island and Langford Lake
Has separator box and inlet and outlet tubes
Airline runs from shore
Partial lift (4)
Useful in areas with heavy ice and surface activity
Difficult to service and monitor performance
Works best on oxygen and may cause N2 supersaturation on air
Partial lifts can be replaced by full lifts when they break (Medical Lake near Spokane)
Deep oxygen bubble injection (DOBI) (3)
Uses oxygen only
Thermocline decoupling of fine bubbles from plume will function as a hypolimnetic oxygenation system in deep lakes
Inject oxygen via a linear or point source defuser sitting on lake bottom
Downflow air injection (DAI) (3)
Simple design - has motor on surface, water is sucked in top of tube, oxygenated, then pushed out bottom on lake floor
Operates on air or oxygen
May cause N2 supersaturation on air
Side stream injection (SSI) (4)
Runs on oxygen
Used in fish farm industry
Very few installations
Pump and tank on bank, sucks up air at bottom of lake, oxygenates it, and shoots it through tubes to hypolimnion
Mechanical aeration (3)
Similar to side stream injection
Except it runs on air, not oxygen
Very few installations
Downflow bubble contact aeration (DBCA) (5)
Many installations and growing
Also called Speece cone design
Operates on oxygen only
Very high oxygen transfer (~>95%)
Requires large water pump and 3-phase power
Limitations of conventional full lift and partial lift systems (4)
Relatively low oxygen transfer efficiency (max 50%)
Floating structures may create boating hazards, attract birds and people (eg. Full lift)
Restricted to deeper (>10m) lakes and reservoirs
Non-standard designs have slowed acceptance by environmental engineers
The Speece Cone (5)
Superior oxygen transfer performance relative to full lift and partial lift systems
Designed in the late 1960s but the first installation was in Newman Lake, WA in 1992
Designed by Richard Speece
Very turbulent gas transfer process can produce extremely high dissolved oxygen concentrations (>100mg/L)
Even used to remove odour and corrosion in municipal sewer systems
Cryogenic oxygen supply (3)
Until the 1980s oxygen was generated at large cryogenic production facilities and delivered to locations by insulated tankers
Now it’s stored as a liquid in insulated tanks
As it evaporates, it self-pressurizes, so it can be used for lake oxygenation in sites where electricity isn’t available
PSA oxygen supply (4)
PSA oxygen generators separate O2 from air
PSA oxygen is generated by forcing dry, clean, compressed air through a Pressure Swing Adsorption (PSA) column
Pairing of the Speece Cone and PSA O2 generation is a significant technological combination
PSA oxygen generation accelerated the acceptance of Speece Cones as liquid oxygen was no longer required to be delivered or stored on site
Limitations of PSA oxygen (2)
Generation requires feed air at 100psi and delivers it at 40psi
Therefore max depth of use is limited to about 10m depth, without resulting to sophisticated oxygen compressors
Which aeration systems run on air, oxygen, or both? (7)
Air:
Mechanical aeration
Oxygen: Partial lift runs best on oxygen Deep oxygen bubble injection Downflow bubble contact aeration Side stream injection
Air or oxygen:
Full lift
Downflow air injection
