544
J. Lohmann
The magnitude and position of the cost minimum depends on local cost factors. Apart
from power costs, these mainly depend on the price of the trickling filter packing and of
the aeration basin.
High-rate activated sludge system
The use of commercial oxygen enables sludge with a much higher concentration to be
handled than with the classic activated sludge process. As a result of the higher oxygen
level, it may even be possible to achieve a higher degree of sludge activation. As the BOD
reduction capability is strongly affected not only by the space loading, but also by the
sludge loading (Fig. 1), the efficiency per unit of aeration basin volume may be improved
(Fig. 1, curve 3).
The savings through having an aeration tank of smaller volume will be partly or even
completely off-set by the expenditure for an oxygen generation unit.
We have compared the costs of a classic activated sludge unit with those of a high-rate
activated sludge system (Fig. 10).
We assume that the commercial oxygen will be used up except for a residual amount
equal to the oxygen concentration in the ambient air. We further assume that the oxygen
will be generated by the adsorption method. It can be seen that, owing to the BOD
reduction characteristics, the area representing the costs of a classic activated sludge
system (φχ 0 ) rises more steeply with an increasing degree of purification than in the case
of a high-rate activated sludge system. The cost areas intersect in the range of high degrees
of BOD reduction. At higher levels, the high-rate activated sludge system may well be
more economical. As the power consumption figures for this system are high, the
absolute value of the intersection curve depends on local power prices.
Special conditions, e.g. utilization of the nitrogen obtained in low-temperature oxygen
generation, can reduce the oxygen generation costs and this, in turn, would shift the
intersection curve in the direction of lower BOD reduction. It should be pointed out,
though, that low-temperature installations are only economical for large capacities.
The use of oxygen should be considered if very high degrees of BOD reduction are
desirable, as compared with the classic activated sludge process. Other processes have
deliberately not been considered here.
Incineration of concentrated wastewater
The specific incineration costs are determined by the calorific value of the wastewater
(this is proportional to the BOD 5 concentration) and by the plant capacity (Fig. 11).
Our cost calculations are based on a plant type, of which two have already been in
operation in Germany for some considerable time (5). The major factors are
pre-evaporation of the polluted wastewater and heat recovery after incineration.
Cost comparison between incineration and biological process is not easily possible. In
the incineration process, the wastewater is completely eliminated and there is no sludge
to be disposed of after incineration, whereas in the biological process, only BOD 5
impurities are removed from the wastewater. The disadvantage of the latter system is that
the elimination of the last traces of impurities is particularly costly.
In consideration of these criteria, incineration is to be preferred at concentrations
greater than 30,000 to 50,000 mg BOD s /l. These figures are only meant as a rough guide
and may fluctuate considerably depending on local conditions. We have, consequently,
not entered any boundaries in the diagram.
J. Lohmann
The magnitude and position of the cost minimum depends on local cost factors. Apart
from power costs, these mainly depend on the price of the trickling filter packing and of
the aeration basin.
High-rate activated sludge system
The use of commercial oxygen enables sludge with a much higher concentration to be
handled than with the classic activated sludge process. As a result of the higher oxygen
level, it may even be possible to achieve a higher degree of sludge activation. As the BOD
reduction capability is strongly affected not only by the space loading, but also by the
sludge loading (Fig. 1), the efficiency per unit of aeration basin volume may be improved
(Fig. 1, curve 3).
The savings through having an aeration tank of smaller volume will be partly or even
completely off-set by the expenditure for an oxygen generation unit.
We have compared the costs of a classic activated sludge unit with those of a high-rate
activated sludge system (Fig. 10).
We assume that the commercial oxygen will be used up except for a residual amount
equal to the oxygen concentration in the ambient air. We further assume that the oxygen
will be generated by the adsorption method. It can be seen that, owing to the BOD
reduction characteristics, the area representing the costs of a classic activated sludge
system (φχ 0 ) rises more steeply with an increasing degree of purification than in the case
of a high-rate activated sludge system. The cost areas intersect in the range of high degrees
of BOD reduction. At higher levels, the high-rate activated sludge system may well be
more economical. As the power consumption figures for this system are high, the
absolute value of the intersection curve depends on local power prices.
Special conditions, e.g. utilization of the nitrogen obtained in low-temperature oxygen
generation, can reduce the oxygen generation costs and this, in turn, would shift the
intersection curve in the direction of lower BOD reduction. It should be pointed out,
though, that low-temperature installations are only economical for large capacities.
The use of oxygen should be considered if very high degrees of BOD reduction are
desirable, as compared with the classic activated sludge process. Other processes have
deliberately not been considered here.
Incineration of concentrated wastewater
The specific incineration costs are determined by the calorific value of the wastewater
(this is proportional to the BOD 5 concentration) and by the plant capacity (Fig. 11).
Our cost calculations are based on a plant type, of which two have already been in
operation in Germany for some considerable time (5). The major factors are
pre-evaporation of the polluted wastewater and heat recovery after incineration.
Cost comparison between incineration and biological process is not easily possible. In
the incineration process, the wastewater is completely eliminated and there is no sludge
to be disposed of after incineration, whereas in the biological process, only BOD 5
impurities are removed from the wastewater. The disadvantage of the latter system is that
the elimination of the last traces of impurities is particularly costly.
In consideration of these criteria, incineration is to be preferred at concentrations
greater than 30,000 to 50,000 mg BOD s /l. These figures are only meant as a rough guide
and may fluctuate considerably depending on local conditions. We have, consequently,
not entered any boundaries in the diagram.