Infrastructure Models for Ecological Wastewater Management - Restorative Redevelopment of Devastated Ecocultural Landscapes - page 402

E�uent destination Housing density

Irrigation
river

Irrigation
aquaculture
poultry

High
density

Medium
density

Low
density

Onsite irrigation,
use control,
regulation

Onsite: source
separation,
combined

Property
level

Spill over to
next level

Low-cost: wetlands,
waste stabilization,
duckweed ponds

Decentralized

Tertiary:
BNR

Centralized

Technologies Treatment level

FIGURE 20.1 Theoretical approach to decentralized wastewater management. Reprinted
with permission from Nhapi, I., H. J. Gijzen, and M. A. Siebel. 2003. A conceptual frame-
work for the sustainable management of wastewater in Harare, Zimbabwe. Water Science and
Technology 47(7-8):11-18.

cading upward from simple graywater irrigation systems (at the single-home level)
up to large-scale treatment works that primarily discharge to rivers.
Moving along the scale gradient produces a set of technical solutions that are dis-
tinctly different from the technology options offered at a previous scale. For instance,
sewage treatment for a large city could be provided at any of the scale levels. If the
decision is to centrally manage the wastewater, technologies most likely to be the
most cost-effective at this level include conventional sewers, pumping stations, and
one large mechanical treatment plant, operated by a full-time specialist staff sup-
ported by the government. This is a reasonable approach, provided that there is plan-
ning and organizational capacity capable of managing the wastewater infrastructure
from a regional perspective.
Another equally valid approach would be to divide the city into sections and
build a smaller treatment plant for each section of the city. This offers advantages
because less sewer pipe is needed, the treated effluent is physically closer to the
site(s) of water reuse, and the treatment facilities, which are smaller and less com-
plicated than large treatment plants, can be managed by local people. Technologies
likely to be cost-effective at this scale level include small-diameter sewers, ponds,
and water reuse. This is a reasonable approach if wastewater is managed at the
local level.
Finally, the city could opt to not construct any sewer system at all. Then by
default, responsibility to manage wastewater falls on individual families at the