Encyclopedia of Environmental Science and Engineering, Volume I and II

EUTROPHICATION 397

In addition, since algae are organic in nature, they exert

a biochemical oxygen demand (BOD) on the stream oxygen

resources as does other materials which are organic.

Extensive tests were run on the Fox River in Wisconsin

by Wisniewski in 1955 and 1956 to examine the infl uence

of algae on the purifi cation capacity on rivers. In the most

general terms, the studies indicate that algae increase the

B.O.D. by adding organic matter capable of aerobic bacte-

rial decomposition and by the respiration of the live cells

which utilize oxygen during the absence of light. In the

presence of light, algae produce oxygen and as a result

may cause a “negative” B.O.D. for a production of oxygen

in excess of that required for the normal B.O.D. require-

ments or aerobic bacterial stabilization. In addition to the

above, the following specifi c conclusions were drawn from

the tests:

1) The oxidation rate resulting from the respiration

of live algae was much lower than that obtained

by the biological oxidation of the dead algae.

2) The ultimate B.O.D. of live algae was practically

the same as for dead algae.

3) A linear relationship was found to exist between the

five-day B.O.D. of suspended matter and volatile

suspended solids concentration.

4) The B.O.D. increases with increases in suspended

solids, the latter consisting largely of algae.

Additional work was done in this area and reported

in 1965 by O’Connell and Thomas. They note that the

oxygen produced by photosynthetic plants is affected

greatly by changes in the availability of light due to cloud

cover, turbidity in the water, etc., and therefore it may be

too variable to be used as a reliable factor in evaluating

the oxygen resources of a river. Another variable may be

the loss of oxygen to the atmosphere during the daylight

hours, caused by excess oxygen production and localized

supersaturation.

An important consideration is the type of photosynthetic

plants which are prevalent in a river. According to the above

authors, if benthic algae and/or rooted aquatic plants are

predominant (in lieu of phytoplankton), there will be little

benefi cial effect on the oxygen balance. In addition night-

time absorption of oxygen through respiration can seriously

reduce daily minimum concentrations of dissolved oxygen.

Determination of the effects of the benthic algae oscillato-

ria along a fi vemile stretch of the Truckee River in Nevada

indicated that on the average of the organism produced 72.5

pounds/acre/day of oxygen through photosynthesis. Oxygen

uptake for these same organisms amounted to an average of

65.4 pounds/acre/day.

An examination of the oxygen profi les indicated that the

oxygen variation throughout the day ranged from 2 (at night)

to 13 (during daylight) parts per million.

It is dissolved oxygen variations such as the above which

has been responsible for the disappearance of high quality

game fi sh in many of our natural waterways.

CONTROL METHODS TO PREVENT

EUTROPHICATION

There are a number of methods which attempt to limit the

amounts of nutrients in bodies of water once the point of

eutrophy has been reached. Some of these include dredging

and removing bottom sediments with an inert liner, harvesting

the algae, fi sh, aquatic weeds, etc., and diluting the standing

water with a water of lower nutrient concentration. Although

these methods may have their proper application, if eutrophi-

cation is to be decelerated, nutrient removal must start before

wastes are permitted to enter the receiving waters.

Regarding the specifi c nutrients necessary to be removed,

most researchers have placed the blame of eutrophication in

waters to the inorganic forms of phosphorus and nitrogen.

A smaller number of researchers are claiming that the algae–

bacteria symbiosis relationship might be responsible for the

rapid growth of blooms and that the amount of algae pres-

ent in natural waters is in direct balance with the amount of

carbon dioxide and/or bicarbonate ions in the waters. They

further argue that an external supply of the above elements

is necessary for the growth of algae populations. Since nei-

ther theory has been proved conclusively to date, the control

methods given will be for the removal of nitrogen and phos-

phorus since it is these nutrients which most researchers lay

to the blame of eutrophication and which have been there-

fore subsequently studied in detail.

NITROGEN REMOVAL

Land Application

It has been found that nitrogen-bearing waters, when perco-

lated through soil are subjected to physical adsorption and

biological action which removes the nitrogen in the ammo-

nium form. It appears, however, that the nitrate form of

nitrogen remains unaffected. At present this process is only

at the theoretical stage, and to the author’s knowledge no

full-scale application has been attempted. Considerable land

area would be involved which may prove a deterrent.

Anaerobic Denitrification

In this process, the nitrate present in sewage is reduced by

denitrifying bacteria to nitrogen and nitrous oxide gases which

are allowed to escape into the atmosphere. In order to satisfy

the growth and energy requirements of the bacteria, methanol

in excess of 25 to 35% must be added as a source of carbon.

The removal effi ciency ranges from 60 to 95%. The

major advantage to anaerobic denitrifi cation is that there are

no waste products requiring disposal. This process is still

primarily in the experimental stage at this date.

Ammonia Stripping

Ammonia stripping is an aeration process modifi ed by fi rst

raising the pH of the wastewater above 10.0. At this pH the

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