Oxygenation and Circulation as Algae

Management Strategies

Ken Wagner, Ph.D., CLM

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

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

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

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

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)

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

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)

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

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

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

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

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

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

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

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

Wind

Standard diagrams of ideal circulation

Upward pumping moves cold water that will try to plunge back down

Downward pumping moves warm water that will try to rise back to the top

Diffused air (left) and fountain aerators (below) have the same thermal density gradients to overcome; distribution of the power delivered is critical

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

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

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

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

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

Experience with oxygenation Creates possibilities for placing oxygen rich

water only where it is needed

Experience with oxygenation Pure oxygen diffusers - simplicity of diffuser

system with efficiency of oxygen transfer

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)

Experience with oxygenation and circulation Capacity and distribution are the keys

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

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

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

Do you think the aerated or

mixed one tastes better?

QUESTIONS?

WRF report available late 2013