320
F. Hell and J. Lahnsteiner
(Rohmann and Sontheimer 1985). As organic substances, mainly ethanol or acetic
acid are applied. Basically, the biological processes consist of a substrate dosing, a
fixed bed or fluidized bed bioreactor and an aerobic post-treatment for the removal
of residual microorganisms. The investment costs are relatively low and, when
properly operated, the biological denitrification processes are highly reliable and
flexible (Lahnsteiner and Hell 1993). Nevertheless, biological processes require
continuous operation. Therefore, seasonal operation is hardly applicable. Energy
consumption and chemical consumption are quite low and practically no other
consumables are required. Hence also operating costs are low.
Another possibility for nitrate degradation is catalytic nitrate reduction, which
was developed for drinking water application (Tacke and Vorlop 1993). This
process is similar to autotrophic biological denitrification. Hydrogen gas is
utilized for the chemical reduction of nitrate. In contrast to the biological system,
not a bacteriological enzyme system but a chemical catalyst supports the reaction.
After saturation with hydrogen, the raw water passes a fixed-bed reactor. The
fixed bed consists of a metal-coated carrier material. Also this process needs an
aerobic post-treatment. The process requires a highly sophisticated catalyst, in
order to stop the reduction process at the stage of molecular nitrogen. An
overreaction leads to production of ammonia. Also the stability of the catalysts
must be high so that a metal contamination of the drinking water is avoided. A
main cost factor for this process is the cost for the catalysts.
Processes for nitrate separation from drinking water are ion exchange, reverse
osmosis and electrodialysis.
With the ion-exchange process the raw water passes an ion-exchange resin,
where the nitrate ion is exchanged with another anion, mainly chloride or
bicarbonate. The disadvantage of anion exchangers in the chloride form is the
increase in chloride concentration in the drinking water. Moreover, due to the
regeneration with sodium chloride, the total salt balance is increased. To
overcome this problem, the CARlX process was developed (Hagen 1991). This
process utilizes a mixed bed ion exchanger with a weak acidic cation exchanger in
the H+ form and a strong basic anion exchanger in the HC03- form. Thus, a neutral
ion exchange takes place and hence not only nitrate ions but also hardness are
removed from the drinking water. Regeneration of the resins is accomplished by
means of carbon dioxide, which is dissolved in the rinsing water.
The principle of reverse osmosis is that water passes a tight membrane when a
pressure higher than the corresponding osmotic pressure is applied. The salts are
retained in a concentrate. Due to the extremely high desalination, the treated water
must be blended with raw water so that the final drinking water still contains
essential salts. Finally, posttreatment for the adjustment of the calcium carbonate
equilibrium is required. Discharge of the concentrate is normally no problem,
when the local recipient is large enough. Moreover, the total salt balance regarding
raw water, consumed water and concentrate is practically the same. The main cost
parameters of reverse osmosis are the membrane costs and electric power. Even
though low-pressure membranes were developed in order to reduce the
consumption of power markedly, reverse osmosis is characterized by the highest
F. Hell and J. Lahnsteiner
(Rohmann and Sontheimer 1985). As organic substances, mainly ethanol or acetic
acid are applied. Basically, the biological processes consist of a substrate dosing, a
fixed bed or fluidized bed bioreactor and an aerobic post-treatment for the removal
of residual microorganisms. The investment costs are relatively low and, when
properly operated, the biological denitrification processes are highly reliable and
flexible (Lahnsteiner and Hell 1993). Nevertheless, biological processes require
continuous operation. Therefore, seasonal operation is hardly applicable. Energy
consumption and chemical consumption are quite low and practically no other
consumables are required. Hence also operating costs are low.
Another possibility for nitrate degradation is catalytic nitrate reduction, which
was developed for drinking water application (Tacke and Vorlop 1993). This
process is similar to autotrophic biological denitrification. Hydrogen gas is
utilized for the chemical reduction of nitrate. In contrast to the biological system,
not a bacteriological enzyme system but a chemical catalyst supports the reaction.
After saturation with hydrogen, the raw water passes a fixed-bed reactor. The
fixed bed consists of a metal-coated carrier material. Also this process needs an
aerobic post-treatment. The process requires a highly sophisticated catalyst, in
order to stop the reduction process at the stage of molecular nitrogen. An
overreaction leads to production of ammonia. Also the stability of the catalysts
must be high so that a metal contamination of the drinking water is avoided. A
main cost factor for this process is the cost for the catalysts.
Processes for nitrate separation from drinking water are ion exchange, reverse
osmosis and electrodialysis.
With the ion-exchange process the raw water passes an ion-exchange resin,
where the nitrate ion is exchanged with another anion, mainly chloride or
bicarbonate. The disadvantage of anion exchangers in the chloride form is the
increase in chloride concentration in the drinking water. Moreover, due to the
regeneration with sodium chloride, the total salt balance is increased. To
overcome this problem, the CARlX process was developed (Hagen 1991). This
process utilizes a mixed bed ion exchanger with a weak acidic cation exchanger in
the H+ form and a strong basic anion exchanger in the HC03- form. Thus, a neutral
ion exchange takes place and hence not only nitrate ions but also hardness are
removed from the drinking water. Regeneration of the resins is accomplished by
means of carbon dioxide, which is dissolved in the rinsing water.
The principle of reverse osmosis is that water passes a tight membrane when a
pressure higher than the corresponding osmotic pressure is applied. The salts are
retained in a concentrate. Due to the extremely high desalination, the treated water
must be blended with raw water so that the final drinking water still contains
essential salts. Finally, posttreatment for the adjustment of the calcium carbonate
equilibrium is required. Discharge of the concentrate is normally no problem,
when the local recipient is large enough. Moreover, the total salt balance regarding
raw water, consumed water and concentrate is practically the same. The main cost
parameters of reverse osmosis are the membrane costs and electric power. Even
though low-pressure membranes were developed in order to reduce the
consumption of power markedly, reverse osmosis is characterized by the highest