Environmental Engineering Reference
In-Depth Information
but it does seem clear that the influence of Islamic authorities on civil affairs, such
as reuse, will be important. Evidence from scripture, actual practice, and attitudes
in neighboring societies leads one to conclude that the influence of Islam will be to
promote wastewater reuse.
WASTEWATER REUSE: WHY DO IT?
The overall rationale for wastewater reuse is to control water pollution, to augment
existing water supplies, and/or to create an alternative water resource to preserve
higher quality water sources (Asano and Levine 1998). Water reuse can be an impor-
tant part of overall water management because only a small percentage of daily
water use within a city actually requires potable quality water. Wastewater treated to
reuse standards is also a consistent source of water supply because of the relatively
constant use of water by people. As such, reused wastewater is well suited to applica-
tions with consistent demands or a structured succession of uses that come and go
with the seasons.
Each potential reuse application is a function of local needs, opportunities, and
constraints. For instance, landscape irrigation is most meaningful where cultural val-
ues prize green parks or common public green space, agricultural irrigation may not
be well suited to urban areas, and so on. Except for direct potable use, highly treated
wastewater is best considered like any other water resource in terms of appropriate-
ness of applications, but is an “extra” source of water in terms of satisfying overall
water demand. Like any valuable resource, reuse demand can exceed supply.
Control of pollution provides a rationale for wastewater reuse. The high treatment
standards required for reuse also protect the environment. Removal of pathogens,
either entirely (California-Florida model) or below set concentrations (WHO model),
entails a high-level treatment for other wastewater constituents that are harmful to
the environment. Thus, a well-designed and managed reuse system is environmen-
tally beneficial.
WHAT IS DECENTRALIZED REUSE?
Large sewer networks that bring wastewater to a central treatment facility typically
serve cities. If these central facilities treat wastewater to reuse standards, a large
network of pipes is needed to redistribute the treated effluent to users. In contrast,
decentralized wastewater reuse systems operate over smaller areas and with much
smaller flows. The savings in pipe capital costs can be large.
In terms of flows, the border between large, central treatment systems and small,
decentralized treatment systems is somewhat arbitrary. Crites and Tchobanoglous
(1998) set it at 3,750 cubic meters per day (1.0 million gallons per day). Above this
flow, large-scale technologies, such as activated sludge, are as cost-effective and reli-
able. Below this flow, other technologies tend to be more cost-effective and reliable
than scaled-down versions of large-scale technologies. In particular, small commu-
nities in the United States often struggle with the high operational costs and incon-
sistent performance of activated sludge systems under this size (EPA 2000).
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