Organic Waste Recycling

340 Organic waste recycling: technology and management

Aquatic treatment systems are analogous to many more common treatment
systems. The principal removal mechanisms are physical sedimentation and
bacterial metabolic activity, the same as is the case in conventional activated
sludge and trickling filter systems. Because the plants provide a support medium
for bacterial attachment and growth, aquatic treatment systems have thus been
equated with a slow-rate, horizontal trickling filter.
The fundamental difference between aquatic systems and more conventional
technology is the rate of treatment. In conventional systems wastewater is
treated rapidly in highly managed environments whereas in aquatic systems
treatment occurs at a relatively slow rate in essentially unmanaged natural
environments. The consequences of this difference are:

• Conventional systems require more construction and equipment but
  less land than aquatic systems.


• Conventional processes are subject to greater operational control
  and less environmental influence than aquatic process.

Assuming land is available, the comparative economics of conventional and
aquatic systems are contrasted in Table 7.6.

Table 7.6 Costs and energy requirements of conventional and aquatic treatment systems
(Adapted from Stowell et al. 1981)

Treatment plant size

378.5m^3 /day

(0.1m.g.d)

1892.5 m^3 /day

(0.5 m.g.d)

3785 m^3 /day
(1.0 m.g.d)
Item Conva Aquaticb Conv Aquatic Conv Aquatic
Capital cost, US$ ×10-6 0.71 0.37 1.23 0.55 1.60 0.90
O & M cost, US$/ year
×10-3

35 21 78 48 117 74

Energy, kJ/year×10-9 0.93 0.53 3.32 1.27 5.06 2.19
aActivated sludge + chlorination
b Primary clarification + artificial wetlands + chlorination

m.g.d = million gallons per day
O & M = operation and maintenance
kJ = kilo-joules

Aquatic treatment systems should not be confused with other types of
systems that may involve the application of wastewater to wetlands for reasons
other than wastewater treatment:

• Aquaculture (growth of organisms having economic value).


• Environmental enhancement (creation of habitat for wildlife).


• Wetlands effluent disposal (non point source disposal of treated
  wastewater).