Page 41 - Water and Wastewater Engineering Design Principles and Practice

1-12 WATER AND WASTEWATER ENGINEERING

• Groundwater infiltration.
• Raw water mineral composition.

Water quality standards to be met. Early consideration of the water quality standards provides
the basis for elimination of treatment technologies that are not appropriate. The standards are pre-
scribed by the regulating agency. The standards require that the treatment facility provide potable
water or discharge wastewater that meets numerical requirements ( performance standards). They
are based on statutory requirements. For example, regulations specify the concentration of coli-
form organisms that may be delivered to consumers or discharged into a river. For wastewater,
modeling studies of the stream or river may also be required. For the river, the regulating agency
will focus on the critical seasonal parameters for the stream or river. Normally, this will be in the
summer dry-season because the flow in the river or stream will be low (reducing the capacity for
assimilation of the treated wastewater), the oxygen carrying capacity of the stream will be low
(stressing the aquatic population), and the potential exposure from recreational activities will be
high. The potable water or wastewater effluent standards do not prescribe the technology that is
to be used in meeting the standards, but they do establish the goals that the engineer uses to select
the appropriate treatment processes.
Other requirements. In addition to the numerical standards, other requirements may be
established by the regulatory agency as well as the owner. For example, drinking water limits on
taste and odor, and specific minerals such as calcium, magnesium, iron, and manganese may be
specified. For wastewater, in addition to the numerical standards, the absence of foam, floating
material, and oil films may be required.

System reliability. System reliability refers to the ability of a component or system to perform
its designated function without failure. Regulatory reliability requirements are, in fact, redun-
dancy requirements. True reliability requirements would specify the mean time between failure
for given components or processes. This is difficult, if not impossible, criteria to specify or, for
that matter, to design, for the wide range of equipment and environmental conditions encountered
in municipal water and wastewater projects.
For water supply systems, some redundancy recommendations of the Great Lakes–Upper
Mississippi River Board of State and Provincial Pubic Health and Environmental Managers are
shown in Table 1-3 (GLUMRB, 2003).
There are three “reliability” classes for wastewater treatment facilities established by the
U.S. Environmental Protection Agency (EPA). Class I reliability is required for those plants that
discharge into navigable waters that could be permanently or unacceptably damaged by effluent
that was degraded in quality for only a few hours. Class II reliability is required for those plants
that discharge into navigable waters that would not be permanently or unacceptably damaged
by short-term effluent quality, but could be damaged by continued (several days) effluent qual-
ity degradation. Class III reliability is required for all other plants (U.S. EPA, 1974). Table 1-4
provides EPA guidance on minimum equipment to meet reliability requirements. In reviewing
the design that is submitted by the engineer, the regulatory agency uses this guidance to estab-
lish prescriptive requirements prior to the issuance of the permit to construct the facility. Some
states may require more stringent requirements than the federal guidance. For example, Michigan
requires Class I reliability for all plants.