Encyclopedia of Environmental Science and Engineering, Volume I and II

398 EUTROPHICATION

ammonia nitrogen present is readily liberated as a gas and

is absorbed into the atmosphere. Aeration is usually accom-

plished in a packed tray tower through which air is blown.

This process is suited to raw sewage where most of the

nitrogen is either in the ammonia form or may be readily

converted to that form. In secondary treatment processes

the conversion of ammonia nitrogen to nitrate nitrogen can

be retarded by maintaining a high organic loading rate on the

secondary process.

Effi ciency of nitrogen removal by ammonia stripping is

excellent with 80 to 98% reported. There is also the advantage

that there are no waste materials which must be disposed of.

PHOSPHORUS REMOVAL

Chemical Precipitation

Precipitation of phosphorus in wastewater may be accom-

plished by the addition of such coagulants as lime, alum,

ferric salts and polyelectrolytes either in the primary or sec-

ondary state of treatment, or as a separate operation in ter-

tiary treatment. In general, large doses in the order of 200

to 400 ppm of coagulant are required. However, subsequent

coagulation and sedimentation may reduce total phosphates

to as low as 0.5 ppm, as in the case of lime. Doses of alum

of about 100 to 200 ppm have reportedly reduced orthophos-

phates to less than 1.0 ppm.

Phosphorus removal by chemical coagulation generally

is effi cient with removals in the order of 90 to 95% reported.

Additional benefi ts are gained in the process by a reduction

in B.O.D. to a value of less than 1.0 ppm. Both installation

and chemical costs are high, however, and the sludges pro-

duced are both voluminous and diffi cult to dewater.

Sorption

Sorption is the process of passing wastewater down-

ward through a column of activated alumina whereby the

common form of phosphate are removed by ionic attraction.

Regeneration of the media is accomplished by backwash-

ing with sodium hydroxide followed by acidifi cation with

nitric acid.

Contrary to alum treatment, this process has the advantage

in that sulfate ions are removed and thus the sulfate concentra-

tion is not increased. Since no salts are added, the pH and the

calcium ion concentration remain unchanged. The process is

effi cient with more than 99% removal reported. The process

should be limited to wastewater with a moderate amount of

solids so as not to clog the media.

REMOVAL OF NITROGEN AND PHOSPHORUS

Biological (secondary) Treatment

In the secondary method of sewage treatment, bacteria uti-

lize soluble organic materials and transform them into more

stable and products. In the process nitrogen and phosphorus

are removed from the wastes, utilized to build new cellular

materials, and the excess is stored within the cell for future

use. For each pound of new cellular material produced,

assuming the material to be in the form of C 5 H 7 NO 2 , about

0.13 pounds of nitrogen and about 0.026 pounds of phospho-

rus would be removed from the sewage. In the actual opera-

tion of this process not all of this nitrogen is removed unless

additional energy material in the form of carbohydrates

is added. Although it may be possible to eliminate all the

nitrogen, a considerable amount of soluble phosphorus may

remain, possibly because of the high ratio of phosphorus

to nitrogen in sewage, attributable to synthetic detergents.

Much of this phosphorus can be removed by absorption on

activated sludge fl oc when it is later separated and removed.

This process offers a 30 to 50% removal of nitrogen and

about a 20 to 40% removal of phosphorus without the spe-

cial addition of carbohydrates.

Reverse Osmosis

The process of reverse osmosis consists of passing wastewa-

ter, under pressures as high as 750 psi, through a cellulose

acetate membrane. The result is the separation of water and all

ions dissolved therein. In actual practice the process has been

plagued with diffi culties primarily due to membrane fouling

or premature failure of the membrane. In addition some nitrate

and phosphate ions escape through the membrane.

Removal effi ciency ranges from between 65 to 95% (for

nitrogen).

Electrodialysis

Like reverse osmosis, electrodialysis is a non-selective

demineralization process which removes all ions which

would include the nitrate and phosphate ions. Essentially

an electric current is used in conjunction with a membrane

inserted in the line of current fl ow to separate the cations

and anions.

The problems that have developed in the operation of

this process include membrane clogging and precipitation

of low-solubility salts of the membrane. Acidifi cation of the

water and removal of some of the solids prior to treatment

has been effective in minimizing these problems, although it

adds to the cost.

Removal effi ciency ranges from between 30 to 50% (for

nitrogen).

Ion Exchange

In the ion exchange process wastewater is passed through

a media bed which removes both anionic phosphorus and

anionic nitrogen ions and replaces them with another ion

from the media. Regeneration of ion exchangers is com-

monly accomplished with inexpensive sodium chloride, and

frequently the salt is salvaged by recycling the backwash

water.

Diffi culties in the process may be caused by fouling of

the exchange resin due to organic material and reduction in

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