1. Land Requirements
The size and availability of land are the primary
criteria determining the choice of wastewater treatment
process. The area needed for a municipal system
usually depends on the level of treatment, types of
treatment systems and processes, and the size of a
treatment plant [8].
Figure 2 presents average values of land requirements
per design flow (m3
d-1) for different processes
from 28 different treatment plants. These data
represent the entire area of the facilities including
plant footprints, pathways, offices, etc. The overall
data show, as expected, that more advanced processes
such as AS and OD require less area than the natural
treatment systems. WSP requires the most space per
m
3
of daily design flow, or about 10 times more area
than the space required by AS and OD. Figure 2 also
shows the average area required per m3
of daily design
flow for plants ranging in size from less than 5,000 to
100,000 m3. There is a presumption that larger plants
generally require less land per unit served due to an
economy of scale. Graphs in Fig. 2 clearly show a
strong economy-of-scale trend in WSP process, for
which the amount of land required per m3
of design
flow decreases as the size of treatment plant increases.
Within the context of wastewater treatment projects
developed in Thailand, data show that the selection
of treatment processes for municipalities were attributed
to their levels of urbanization and availabilities
of lands. In highly urbanized municipalities and
tourism areas, such as Pataya, Hua Hin and Phuket,
where most of the economic activities are concentrated
around these cities, land availability for a
wastewater treatment project is very limited and expensive.
The choices of treatment process and plant
size tend to be a function of land cost. Data show that
an energy-intensive process particularly the AS process
is one of the most preferable treatment alternatives
in those areas. In lesser developed municipalities
where large plots of land are inexpensive and available,
pond or lagoon systems are commonly used. Beside
the urbanization factor, the choices of treatment
process and plant size also depend on the availability
of government land. In order to avoid buying private
owned land, the government usually provides land to
local authorities for wastewater treatment projects at
no financial cost.
In Fig. 3, the values of land requirements in m2
for the four processes are graphically presented as a
function of design capacity (m3
d-1). Models for landrequirements give a good fit to the data of all processes
except for OD. With very high R2
values, area
requirements for AS and WSP can be well explained
by their design capacities. Nevertheless, the moderately
high R2 of 0.64 for AL and 0.60 for OD models
could indicate an acceptable goodness of fit between
the design capacity and land area requirement variables,
suggesting a fairly strong predictive potential
for the models. Based on graphical data in Fig. 3, the
equations produced to express the land requirements
are summarized in the form of L = aQb
in Fig. 3 (Table
1).