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150 Chapter 4 Quantities of Water and Wastewater Flows

3. Minimum daily: 0.32 105 33.6 gpcd 0.302 MGD
with Eq. 4.24: Q min day 105 0.2(9) 1>6 30 gpcd 0.27 MGD
Solution 2 (SI System):
1. Average daily spent water: 0.7 568 Lpcd 397 Lpcd 3.573 MLD
2. Maximum daily: 3.5 397 Lpcd 1,389.5 Lpcd 12.505 MLD
with Eq. 4.23: Q max day 397 5 (9) 1>6 1,377 Lpcd 12.393 MLD
3. Minimum daily: 0.32 397 Lpcd 127.04 Lpcd 1.14 MLD
with Eq. 4.24: Q min day 397 0.2(9) 1>6 115 Lpcd 1.035 MLD

Expected low flows in spent-water and combined sewers are as meaningful as
expected high flows because suspended solids are deposited or stranded as flows and ve-
locities decline. Flow may be obstructed and malodorous, and dangerous gases may be
released from the accumulating detritus.

4.8 DESIGN FLOWS FOR WASTEWATER INFRASTRUCTURE DEVELOPMENT
In the state of New York, USA, Table 4.18 can be used as a basis for the design of sewage
treatment and disposal facilities for new developments, and for existing establishments
when the hydraulic loading cannot be measured. Alternatively, water usage data can be
used to estimate wastewater flow, if it is available for an establishment. Adjustments
should be made for infiltration and for water that will not reach the sewer (such as boiler
water). For commercial establishments variations in flow may be extreme; in these cases it
is necessary to examine the significant delivery period of the wastewater and base the peak
design flow on this information to prevent an excessive rate of flow through the treatment
system. It may be desirable to include an equalization basin prior to the treatment system.
New York State’s environmental conservation law mandates the use of water-saving
plumbing facilities in new and renovated buildings. Hydraulic loading, as determined from
Table 4.18, may be decreased by 20% in those installations serving premises equipped
with certified water-saving plumbing fixtures. A combination of new and old fixtures can
be considered on a pro rata basis.
New toilets, which use as little as 0.5 gal (1.89 L) of water per flush, are becoming
available on the market and the reduction of wastewater flow attributable to these and other
new technologies is considered on a case-by-case basis. The reduction allowance depends
in part on the ability of the builder or owner to ensure adequate maintenance and/or
replacement in kind when necessary.
Once the design wastewater flow is chosen, an environmental engineer may design the
sanitary sewers to carry peak residential, commercial, institutional, and industrial flows, as
well as infiltration and inflow. Gravity sewers are designed to flow full at the design peak
flow, in accordance with U.S. Environmental Protection Agency rules. As discussed previ-
ously, design flows are based on various types of developments. Table 4.18 provides a list
of design flows for various development types. The design for a long-lived wastewater
infrastructure should consider serviceability factors, such as ease of installation, design
period, useful life of the conduit, resistance to infiltration and corrosion, and maintenance