Page 69 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

24 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

have been contemplated, for a variety of other situations such a low level of turbidity does not ensure that the water is safe,
as for water reuse for agriculture, water reuse for drinking but a high level is grounds for rejection based on palatability
water, etc. and on the presumption that the water is not safe.
One of the earliest surrogates in wastewater treatment was the
2.1.4.2 Normative Standards 5 day BOD, i.e., BOD5 (or simply BOD, with the 5 day incu-
There is nothing absolute about water quality standards. Such bation period understood). The BOD is a measure of biodegrad-
standards are determined both by the degree of perceived risk able organic matter. The difference between BOD
that a society is willing to accept and by the knowledge of
particular contaminants and their effects, i.e., criteria. It is the
norms of a society that determine which contaminants are of TABLE 2.2
concern and at what levels. Thus, water quality standards are Examples of Surrogates in Water Treatment
normative in nature.
Surrogate Measurement
2.1.4.3 Standards as Targets for Treatment Wastewater
Water quality standards define the performance requirements BOD a Organic matter that is subject to biodegradation by biological
for treatment plants. A problem is that such standards have b treatment
evolved and changed over the decades since their inception, SS Solids that are subject to settling and that will remain in
suspension
i.e., since the 1962 USPHS drinking water standards, consti-
MLSS c Total suspended matter in an activated sludge reactor
tuting a ‘‘moving target,’’ so to speak. Planning a treatment d
VSS Index of suspended matter that is organic carbon
train should, then, build in the flexibility to meet more strin-
Coliforms e Indicator of the presence of pathogens
gent standards in the future.
The overall ‘‘goal’’ in drinking water treatment has been, Potable water
f
since its inception, ‘‘to produce a safe, palatable water,’’ as Turbidity Suspended matter 1 mm
expressed by the American Water Works Association. Trad- Index of palatability
itional objectives of treatment that support the health goal Color g Index of palatability
have been to remove disease-causing organisms. Removal h Index of TOC, organic matter, and fulvic acids
of color, odor, and turbidity has supported the goal of a TOC Collective or group measure of organic matter
TTHMFP i THM precursors
palatable water. The concern about chemicals that could
j Index of TOC and TTHMFP
be carcinogenic has led to additional objectives, first to UV 254
Coliforms Indicator of the presence of pathogens
remove trihalomethane precursors (expressed in the 1974
MPA k Indicator of whether protozoan cysts could be present in
Safe Drinking Water Act, i.e., PL93-523). This objective
finished water
was expanded to include removal of disinfection by-products
Particle Indicator of quality of finished water
(DBPs). Subsequent to this, in later amendments, the SDWA l
counts
provided the mandate to remove an array of organic chemicals
and metals. Thus, while the goals have remained the same, the a BOD: Biochemical oxygen demand.
objectives defined to achieve those goals have expanded to b SS: Suspended solids as filtered by filter paper.
c
accommodate increasing knowledge about contaminants, MLSS: Mixed liquor suspended solids is a gross parameter of active
lower detection limits and increased accuracy in sampling biomass and requires oven-drying a sample; MLVSS: mixed liquor
and analysis, and more encompassing normative standards. volatile suspended solids, is considered more accurate measure of active
biomass, but requires a placing a solids sample, after oven-drying, in a
laboratory furnace at 6008C to combust the organic solids.
d
2.1.5 SURROGATES VSS: Volatile suspended solids.
e
Coliforms: Bacteria of the coliform group that ferments lactose. Two
Most measures of water quality involve surrogates, i.e., a subgroups of the coliform group are (1) Aerobacter aerogenes and (2)
quantity that is relatively easy to measure and may be used fecal coliforms.
as an index of the quantity of specific interest. To illustrate, f Turbidity: A measure of the light-scattering property of a liquid.
two surrogates—turbidity and coliform bacteria—have been g Color: A measure of the color of a water as measured by the cobalt–
institutionalized in their use. Regarding the latter, there are platinum standard.
h TOC: Total organic carbon as measured by converting non-purgable
many enteric pathogens, e.g., hepatitis A virus, ECHO 12
virus, Coxsakie virus, polio virus, Vibrio cholera bacterium, carbon to carbon dioxide, which is measured by infrared absorbance.
i TTHMFP: Total trihalomethane formation potential, which is the chlorine
Salmonella typhosi bacterium, Shigella dysenteriae bacterium,
consumption of a given water in mg=L over a specified period of time, e.g.,
Endamoeba histolytica, Giardia lamblia cysts, Cryptosporid-
24 or 96 h.
ium parvum oocysts, etc. By definition, ‘‘enteric’’ means that
j
UV 254 : Absorbance of a water sample by an instrument emitting
such organisms may be found in municipal sewage. Therefore,
wavelength 254 nm (in the UV range).
the coliform group of bacteria (more specifically, the fecal k
MPA: Microscopic particulate analysis.
coliform subgroup) serves as a surrogate for enteric pathogens l Particle counts: Counts of particles in #=mL in the water source being
in general, and such presence would indicate the presence of measured.
pathogens. Turbidity is a general indicator of water quality;