Cost Relationship of Biological and Thermal Processes
541
The conditions at which the total costs are equal are represented by the curve at the
intersection of the two areas. While the specific BOD reduction costs of the trickling filter
rise gradually with decreasing concentration and increasing plant capacity, those for the
activated sludge system drop sharply with increasing capacity.
The costs of an activated sludge unit φ 3 do not vary considerably with a greater degree
of BOD reduction (Fig. 5, η = 75%) whereas the trickling filter costs φ 4 are considerably
higher.
This shifts the curve of the intersection of the two areas in favour of the activated
sludge process; in other words, the activated sludge process becomes more and more
economical as the capacity increases. Conversely, the trickling filter process is still best
for small plants.
The cost areas no longer intersect when the degree of BOD reduction is 17 = 95% (Fig.
6), i.e. the costs of the trickling filter process sludge process φ 6 .
Thus, for the given BOD reduction curves and assumptions, the plastic trickling filter is
to be preferred for partial purification.
Combined plastic trickling filter/activated sludge process
In addition to the relationships already mentioned, the specific BOD reduction costs
of the combined process are mainly determined by the efficiency of the two plant
sections Th and η 2 .
C s = f ( F , C B 0 D ,τ?ο>ϊ?ι,τ? 2 )
In the following discussion, we have kept any two of the values constant and made
only the remaining three variable.
As can be expected, the specific BOD reduction costs drop with increasing plant
capacity (Fig. 7).
Depending on the partial efficiency, there will be a pronounced minimum of the cost
area φ η within the range of 771 = 50 - 70%. The cost reduction is mainly due to the low
power requirements of highly loaded trickling filters (4). This means that, compared with
the activated sludge process (171 = 0, η 2 = 1), the specific BOD reduction costs can be
lowered by about 40 - 50%. The cost reduction cannot be directly applied with respect
to the entire plant, since the cost of the activated sludge or trickling filter stage is only a
portion of the total costs of the wastewater purification plant.
The cost area minimum becomes more pronounced as the wastewater concentration
increases (Fig. 8). However, even at the relatively low BOD concentration of 200 mg/1,
the BOD reduction costs can be lowered in this case.
Cost reduction is limited by a certain maximum wastewater concentration. This, in
turn, depends on the type of plastic trickling filter used, since the limiting factor is the
lower limit of the hydraulic loading of the trickling filter to avoid clogging.
Apart from the wastewater concentration and plant size, the total efficiency will have
a major effect on the cost relationship (Fig. 9).
The BOD reduction costs in the case of a low degree of purification by means of the
combined process are higher than in the case of a single process. On the other hand, with
higher degrees of BOD reduction, the costs of a single process are higher than of a process
combination. Consequently, a combination of the two processes is particularly suitable
for plants with a high degree of purification, providing the BOD reduction characteristics
of the wastewater to be treated are similar to those described in Fig. 1.
541
The conditions at which the total costs are equal are represented by the curve at the
intersection of the two areas. While the specific BOD reduction costs of the trickling filter
rise gradually with decreasing concentration and increasing plant capacity, those for the
activated sludge system drop sharply with increasing capacity.
The costs of an activated sludge unit φ 3 do not vary considerably with a greater degree
of BOD reduction (Fig. 5, η = 75%) whereas the trickling filter costs φ 4 are considerably
higher.
This shifts the curve of the intersection of the two areas in favour of the activated
sludge process; in other words, the activated sludge process becomes more and more
economical as the capacity increases. Conversely, the trickling filter process is still best
for small plants.
The cost areas no longer intersect when the degree of BOD reduction is 17 = 95% (Fig.
6), i.e. the costs of the trickling filter process sludge process φ 6 .
Thus, for the given BOD reduction curves and assumptions, the plastic trickling filter is
to be preferred for partial purification.
Combined plastic trickling filter/activated sludge process
In addition to the relationships already mentioned, the specific BOD reduction costs
of the combined process are mainly determined by the efficiency of the two plant
sections Th and η 2 .
C s = f ( F , C B 0 D ,τ?ο>ϊ?ι,τ? 2 )
In the following discussion, we have kept any two of the values constant and made
only the remaining three variable.
As can be expected, the specific BOD reduction costs drop with increasing plant
capacity (Fig. 7).
Depending on the partial efficiency, there will be a pronounced minimum of the cost
area φ η within the range of 771 = 50 - 70%. The cost reduction is mainly due to the low
power requirements of highly loaded trickling filters (4). This means that, compared with
the activated sludge process (171 = 0, η 2 = 1), the specific BOD reduction costs can be
lowered by about 40 - 50%. The cost reduction cannot be directly applied with respect
to the entire plant, since the cost of the activated sludge or trickling filter stage is only a
portion of the total costs of the wastewater purification plant.
The cost area minimum becomes more pronounced as the wastewater concentration
increases (Fig. 8). However, even at the relatively low BOD concentration of 200 mg/1,
the BOD reduction costs can be lowered in this case.
Cost reduction is limited by a certain maximum wastewater concentration. This, in
turn, depends on the type of plastic trickling filter used, since the limiting factor is the
lower limit of the hydraulic loading of the trickling filter to avoid clogging.
Apart from the wastewater concentration and plant size, the total efficiency will have
a major effect on the cost relationship (Fig. 9).
The BOD reduction costs in the case of a low degree of purification by means of the
combined process are higher than in the case of a single process. On the other hand, with
higher degrees of BOD reduction, the costs of a single process are higher than of a process
combination. Consequently, a combination of the two processes is particularly suitable
for plants with a high degree of purification, providing the BOD reduction characteristics
of the wastewater to be treated are similar to those described in Fig. 1.