Page 728 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Biological Reactions and Kinetics 683

4000 papers had been published (with continuation by 22.1.5.3.3 Nitrogen
Professor James Alleman after the retirement of Professor Two sources of nitrogen in municipal wastewater are urea and
Bloodgood). The principles of the various unit processes proteins; ammonia is an end product of the breakdown of these
apply while management practices are most likely to be spe- compounds. Usually concentrations of ammonia are highest in
cific to the industry. the influent to a municipal wastewater treatment plant, e.g., in
the range of 10–30 mg NH 3 as N=L (Benefield and Randall,
22.1.5.3 Contaminants
1980, p. 88). During the course of treatment, ‘‘nitrification’’
Traditionally, before 1965, municipal wastewater treatment may occur (as stated by the conversion of ammonia to nitrite,
plants were designed to degrade the organic matter and NO 2 by the organism, Nitrosomonas). The second stage is the

reduce suspended solids and pathogens. The goal was to conversion of nitrite to nitrate, NO 3 ,by Nitrobacter, and is

maintain aerobic conditions in receiving waters, avoid rapid. A final step in nitrogen removal is ‘‘denitrification,’’ i.e.,
depositions of solids, and reduce the infectious disease
the conversion of NO 3 to nitrogen gas, N 2 ; usually, the
hazard. The increasing stringent legislation after 1965 first organism is Pseudomonas, and the rate of reaction is ‘‘fast.’’
placed limits on degradable organic matter and suspended
solids and then started to include ammonia, phosphorus, and
22.1.5.3.4 Ammonia
toxic substances.
A problem of ecology recognized during the 1970s was that
22.1.5.3.1 Organisms fish species are sensitive to low un-ionized ammonia, i.e.,
þ as pH
In raw sewage, organisms include such groups as hel- NH 3 , concentrations (NH 3 is favored over NH 4
increases where pH > pK a ¼ 9.27). Under most conditions
minthes, rotifers, protozoa, cysts, bacteria, and viruses. The
natural water bodies have pH ranges of 6.5–7.5, which
bacterium, E. coli is predominant, with concentrations on the
9
order of 10 =100 L, a calculation based on an estimate of per means that the NH 3 is concentration would be commensur-
9
capita coliform discharge of 100–400 10 =c=d (Tchobano- ately low. The pH usually increases with algae growths, but
usually pH < 8. Stream standards have been established such
glous and Burton, 1991, p. 93); dividing by 300 L=c=d
9
wastewater discharge gives the 10 =coliforms s=L. Also, to that the level of un-ionized ammonia does not harm aquatic
life (e.g., 0.06 mg=L as N). Therefore, ‘‘nitrification,’’ i.e.,
illustrate the diversity of organisms, Giardia lamblia cysts
conversion of ammonia to nitrate, has been required since the
and Cryptosporidium parvum oocysts have been measured in
early 1980s, which provides latitude for uncertainty in both
samples at 200–300#=L. Municipal wastewaters are always
the possible formation of NH 3 and for the sensitivity of fish
presumed to contain pathogenic organisms. Most are
species. In cell synthesis, the bacteria prefer ammonia as a
removed during traditional biological treatment because
source of nitrogen for building bacterial proteins.
they are not likely to survive long outside their niche, most
likely in the intestinal track of warm-blooded animals. Some
22.1.5.3.5 Synthetic Organics
organisms ‘‘encyst,’’ however, which equips them for longer
survival in a hostile environment. In biological treatment an New synthetic chemicals come to the forefront frequently and
ecosystem develops that includes rotifers, protozoa, and bac- have included antibiotics, personal-care products, endocrine
teria in a hierarchal structure that facilitates the degradation disruptors, etc. In a survey of pharmaceuticals and organic
of organic matter. contaminants in ambient waters, the USGS (http:==www.
toxics.usgs.gov=pubs=FS-027-02, 2009) found steroids, insect
22.1.5.3.2 Nutrients repellents, plasticizers, insecticides, nonprescription drugs,
Any organism requires a variety of nutrients for growth, prescription drugs, etc. Some compounds are persistent and
including sugars, fats, proteins for energy; phosphates for resistant to biological treatment. For reference, the number of
building ATP; nitrogen for protein synthesis; and sulfur, chemical substances cataloged in 2009 by the Chemical
certain metals at low concentrations, etc. These substances Abstract Service (CAS) registry exceeded 50 million. Many
are necessary for biological treatment and are present, as a synthetic organics are regulated (see, for example, Chapter 2)
rule in municipal wastewaters. In treating industrial waste- but the number is small compared to the total.
waters, it is often necessary to add nutrients in order to
compensate for a deficiency. At the same time, discharges of 22.2 CELL METABOLISM
these nutrients into ambient waters (even at the reduced con-
centrations after traditional biological treatment) may cause Biological reactions are better understood in the context of a
undesired organisms to thrive. This issue came into the fore- few fundamental notions from biochemistry. According to
front in the early 1960s when a focal point of the environ- Stryer (1981, p. 235), over a thousand chemical reactions
mental movement was Lake Erie, said to have been ‘‘dead,’’ occur in even a simple organism such as E. coli. The sum of
being choked with algae, which was attributed to phosphates these reactions constitute metabolism, which has two parts:
from various municipalities and industrial discharges, a con- (1) catabolism, which yields energy; and (2) anabolism, which
dition known as ‘‘eutrophication.’’ Because of its role as a is cell synthesis. These ideas are reviewed here, along with a
nutrient, phosphorous, along with nitrogen, has been consid- brief section on energy (thermodynamics). The term, ‘‘cell’’ as
ered a major pollutant. used in this text refers to a bacterium, which is a limited view.