Advanced Ecotechnology for Decentralized Wastewater Treatment and Reuse - Restorative Redevelopment of Devastated Ecocultural Landscapes - page 389

with small areas that have to process large flows of wastewater require more energy-
intensive, conventional treatment systems.
Ecotechnology for wastewater treatment is typically composed of constructed
pond or wetland systems. The discussion will start with ponds and then move on to
wetlands (see also chapters 17 and 19).
As described earlier, pathogens are a primary concern in any type of reuse sys-
tem. Each pathogen in wastewater is a biological particle with a known size and
predictable physical characteristics. Large particles, such as Ascaris eggs (diameter
[d] = 70 microns [μm]), will settle to the bottom of large, quiescent ponds. Any
pathogen can stick to larger particles and settle. This latter mechanism is especially
important for viruses (d < 0.1 μm), which have no tendency to settle in water as indi-
vidual particles.
Classic engineering theory accounts for the removal of pathogens in ponds by
means of settling, but it misses the key ecological observation that pathogens are
biological particles that are food for other microorganisms. Particles in suspension of
d < 10 μm, a size class covering most pathogens, have little or no tendency to settle
and are removed principally by means of predation by zooplankton in pond sys-
tems (Figure 19.6). Plankton ecology provides mechanistic insight into the pathogen
removal properties of wastewater treatment lagoons.
Although seldom recognized as such, the lagoon systems favored by WHO
(Figure 19.7) for agricultural reuse are a de facto ecotechnology because of pathogen
removal by zooplankton. Their suitability for decentralized reuse is limited by large

Larger pathogens settle

HELMINTH OVA

C
O
N
S
U
M
E
R
S

(a)

PROTOZOA

(b)

BACTERIA

(c)

0.1 1 10 100

Pathogen diameter, µm

FIGURE 19.6 Zooplankton predation of biological particles by size class. Each pathogen
class has a distinct size range. Representative consumers graze a particular size class of bio-
logical particles: Adult microcrustaceans, such as Daphnia or copepods (a), consume par-
ticles in the approximate range of 1 to 25 µm (Dodson and Frey 1991), rotifers (b) in the 4 to
17 µm size range (Wallace and Snell 1991), and protozoa (c) in the 0.1 to 2.0 µm size range
(Taylor and Saunders 1991). Viruses are left out of this figure but are also subject to preda-
tion by smaller classes of protozoa (Kim and Unno 1996). Images (a) and (b) courtesy of the
National Oceanic and Atmospheric Administration.