Advanced Ecotechnology for Decentralized Wastewater Treatment and Reuse - Restorative Redevelopment of Devastated Ecocultural Landscapes - page 390

Oxygen Oxygen Oxygen

Q in Q out

Anaerobic Facultative Aerobic

FIGURE 19.7 Schematic of lagoons in series. The actual number of lagoons may vary.
Passive lagoons in series meet World Health Organization (1989) treatment requirements
for irrigation reuse. Wastewater (Q) entering the system consumes available oxygen in the
first lagoon, making the first lagoon anaerobic. The second lagoon shifts back and forth
between aerobic and anaerobic conditions. The final, or polishing, lagoon should be aerobic.
Zooplankton (consumers of pathogens) need aerobic conditions. All lagoons have large algae
populations because of sunlight and ample nutrients. The algae produce oxygen during the
day, but consume it at night. Water entering the first lagoon should take several weeks to pass
through all lagoons (Crites, Reed, and Middlebrooks 2006), providing ample time for zoo-
plankton consumption of pathogens.

TABLE 19.1
Energy Use Comparison

Area Footprint,
m 2 /m 3 of Wastewater System

Lagoon 150

Surface-low wetland 94

Horizontal subsurface-flow wetland 30

Aerated or tidal-flow wetland 16

Activated sludge < 1

Note: The comparison was done for 200 m 3 /d (1,000 people at 200
liters per capita use per day) systems, all designed to achieve
the same reuse and nitrogen removal standards.

land requirements (Table 19.1) and odor. The first stage of the lagoon system is typi-
cally anaerobic and may produce foul odors.
Constructed wetlands are a class of ecotechnology well suited to decentralized
applications (see chapter 17). They are much smaller than lagoons (Table 19.1), and
they have features that control odor.
Surface-flow wetlands (Figure 19.8) are similar to wetlands in nature. Shallow
water facilitates thick growth of wetland plants. Wastewater is treated as it flows
through plant thatch. As with all treatment wetlands, plants themselves do not sub-
stantially contribute directly to treatment, but create an environment where a micro-
bial ecosystem can utilize wastewater as a growth substrate and thereby treat it to
design standards.