oxygenation and circulation as algae management · pdf fileoxygenation and circulation as...
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Oxygenation and Circulation as Algae
Management Strategies
Ken Wagner, Ph.D., CLM
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Why would we oxygenate or circulate water? Both processes increase oxygen, which is
important for ecology and water supply Oxygen helps prevent phosphorus release
from sediment which may fuel algae growth Mixing limits some algae and helps others
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Two related but separate processes Oxygenation – any process that adds
oxygen to target waters; mixing can do this if it increases contact with the atmosphere
Circulation – any process that blends water; can be mechanical or air driven, usually induces oxygenation
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Oxygenation is intended to: Increase oxygen levels to
avoid anoxia and accumulation of reduced compounds and/or provide habitat
Accelerate beneficial chemical changes to incoming loads of contaminants
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Circulation is intended to: Facilitate oxygenation through increased
transfer of oxygen from the atmosphere to surface water, and from bubbles
Homogenize water quality Disrupt algal growth processes
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Oxygen demand Bacterial respiration
between the thermocline and bottom of a reservoir can be greater than the input of oxygen from above, leading to oxygen depression
Oxygen demand is larger when more oxygen is available (consumption not static or linear)
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Adding oxygen to counter demand
Exchange is driven by concentration gradient, contact area, and contact time
Air is 21% oxygen, transfer from bubbles rarely exceeds 3% per meter of bubble rise
Pure oxygen will be more rapidly absorbed, small bubbles may never reach the surface
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Circulating water to counter demand Moving low oxygen water to the surface will facilitate
atmospheric transfer Moving high oxygen water to bottom supplies oxygen to
location of demand How well water mixes and oxygen transfers depends
largely on Relative Thermal Resistance to Mixing (RTRM)
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Circulation for homogenation
Depends on power supplied and gradients to be overcome, especially RTRM
Easier to mix water of similar density; whole lake mixing best started before stratification
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Circulation for algae control More time in the dark reduces growth (3X SDT
needed) Mixing limits advantages of buoyancy or motility Some algae depend on mixing to avoid settling Some algae prefer layered zonation Equilibration of pH alters C sources Possible exposure to enhanced grazing Possible exposure to viruses or other pathogens
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Keys to successful oxygenation Quantitatively counter oxygen demand Distribute the oxygen to where needed
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Keys to successful circulation If using air to mix, usually
need 1.3 scfm/acre, but can use as little as 0.8 scfm/ac in very deep water (50+ ft)
If using a pump system, move targeted water vol. every 10 days (10%/day)
To prevent stratification, start before it does
<3C difference top to bottom
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Experience with circulation Fountains – a form of
upflow mixing, aerate well, but usually a small volume per unit time; need enough capacity to move the target water volume
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Experience with circulation Diffuser systems highly varied, very common –
effective when properly sized, distributed and operated; compressors are usually the weak link
Oxygenation through interaction with atmosphere is usually more important than transfer from bubbles
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Experience with circulation Upflow mixers tend to be
small, low power systems, run by electricity, wind or solar
Intended to create compensatory circulation patterns at distance from mixer
Limited corroborating data
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Experience with circulation Downflow mixers with wide
size range, force oxygenated surface water deep
Can be solar or wind powered, but run efficiently on electricity
Data indicate substantial mixing and oxygenation if system large enough
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Circulation Constraints Must overcome density (usually thermal)
gradient to adequately mix water; can have both vertical and horizontal gradients
Sunlight constantly works to create gradients Warm water moved down will try to rise Cold water moved up will try to sink Enough power must be available to mix the target water volume if it is to be homogenized
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Wind
Standard diagrams of ideal circulation
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Upward pumping moves cold water that will try to plunge back down
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Downward pumping moves warm water that will try to rise back to the top
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Diffused air (left) and fountain aerators (below) have the same thermal density gradients to overcome; distribution of the power delivered is critical
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Experience with oxygenation Hypolimnetic chambers with air popular in 1980s-
1990s, effective but not overly efficient Non-destratifying, increased bottom DO, some
undersizing but more distribution/maintenance issues If internally supplied P is dominant source, may
control algae; need to understand seasonal P loading
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Experience with oxygenation One of the key findings of oxygenation projects
is that more than the measured demand has to be countered; addition of oxygen results in greater demand as a consequence of water movement that stimulates more demand
Pure oxygen raises the demand less than air (1.5 X vs. 2 to 5X, with variability)
Successful oxygenation must therefore deliver enough oxygen to the right places to counter demand
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Experience with oxygenation Layer Air (ECS creation) allows creation of a
thermally distinct, oxygenated layer Non-destratifying, usually just below
thermocline, creates barrier to vertical dissolved substance transport
Upper waters
Lower waters
“New” layer
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Experience with oxygenation Pure oxygen chambers more popular in the
west, can super-oxygenate in reservoir Distribution of oxygen is key issue; movement
across density gradient is limited
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Experience with oxygenation Can also super-oxygenate outside waterbody
and inject the water where desired Distribution remains the key issue, but density
differences might be used to advantage
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Experience with oxygenation Creates possibilities for placing oxygen rich
water only where it is needed
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Experience with oxygenation Pure oxygen diffusers - simplicity of diffuser
system with efficiency of oxygen transfer
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Experience with oxygenation Pure oxygen is typically released as small
bubbles through “soaker” hoses Can be done without pumps, can maintain
stratification, distribution is key issue
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Experience with oxygenation Before (left) vs. after (right)
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Experience with oxygenation and circulation Capacity and distribution are the keys
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Technical Conclusions Oxygenation and circulation are different approaches
with overlap – know the difference Adding oxygen can greatly improve water quality and
possibly limit algal growth if enough is added at the right places and internal P load is dominant
Circulation aids oxygenation, but can also provide homogenization and direct algae disruption that limits algal nuisances
Sizing and distribution are the primary determinants of success or failure
It is essential to understand the thermal structure of the lake to get best results
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Mixing observations from case histories Mixing must move the targeted water every 10
days to be effective; more frequent is better Mixing with air requires 1.3 scfm/ac, although
in deep water as little as 0.8 scfm/ac may work Oxygen addition through mixing is mainly by
atmospheric interaction Mixing to 3 X SDT likely to reduce algae Mixing to <2 X SDT will not eliminate algae, but
is likely to shift the types of algae present Intermittent mixing is worse than no mixing;
equipment failures must be avoided
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Oxygenation observations from case histories Non-destratifying oxygenation is most common where
coldwater habitat or deep intakes critical Oxygen demand rises by up to 5X measured demand
when oxygen is added, lower for pure oxygen (about 1.5X) than air (average about 3X)
Oxygen >2 mg/L will tend to prevent most water quality problems, including P release, but need 4-6 mg/L for habitat
Oxygenated water moves within density layers much more than across oxygen gradients; affects distribution approach
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Do you think the aerated or
mixed one tastes better?
QUESTIONS?
WRF report available late 2013