Civil Engineering Reference
In-Depth Information
the surface would simply be a tradeoff of waters within the study area, and would
have no impact on the water budget. If treated wastewater is discharged to a river and
carried across a study area boundary or to the ocean, however, there would be a water
budget benefit through reuse.
Treated wastewater is one of the most reliable sources of water, because of its
consistent availability and yield increase with population growth. The obvious down-
side is quality and public perception. Even though water may be highly treated, the
public or marketplace may demand that it not be used for some purposes. Also, unless
the wastewater is de-salted in some manner, salt is recycled back through the system,
leading to a long-term buildup in salt in the soil or aquifer(s).
Stormwater
Rainfall and snowmelt within a study area can sometimes be captured for beneficial
use, reducing outflows from the study area and increasing the water budget. Stormwater
can be used in the same manner as treated wastewater. Some uses may require physical
treatment. Stormwater can be utilized for groundwater recharge by capturing storm-
water in detention or retention basins and allowing the water to percolate into the
ground, or releasing the water slowly to a leaky outlet channel.
Reliability is relatively poor for this water supply. Stormwater, by definition, comes
at a time when irrigation demands are minimal. Effective utilization requires storage.
Because stormwater is generally impractical as a potable water source, quality is not
as serious an issue. The National Pollutant Discharge Elimination System (NPDES)
permitting program requires best management practices (BMPs) to minimize urban
runoff pollutants. Detention and retention ponds are examples of BMPs. Typically,
urban runoff doesn't have the dissolved solids load that treated wastewater has, so salt
buildup is not as serious an issue. Metals and other pollutants tend to build up in the
surface sediments of the ponds, which can be managed if removed in a regular main-
tenance program.
Conservation and Demand Management
Reductions in water demand are often considered as new water supplies. This is done
primarily for comparison against other water supply alternatives. Typical water con-
servation measures that reduce inside demands include low-flush toilets, low-flow
showerheads, automatic cutoff faucets, toilet dams, and horizontal axis (front-loading)
washing machines. Measures for outside water conservation include recycle systems
on car washes, irrigation systems that minimize runoff and deep percolation, low-
water-use landscaping, and watering restrictions. Finally, a number of water pricing
mechanisms, public education programs, and system-wide pressure reduction can be
used to reduce water use.
Nearly all water used inside the home exits the home via the sewer. Very little is
actually consumed. Therefore, inside conservation reduces both water use and waste-
water production. Whether inside conservation actually reduces the strain on the water
budget depends on whether treated wastewater leaves the study area. Inside conser-
vation produces economic benefits, regardless of whether there are water budget ben-
efits.
