Environmental Engineering Reference
In-Depth Information
consist of a modification of the conventional activated sludge systems and provision of
special conditions within the reactor for:
• Phosphorous release and consequent biological consumption by the biomass - the
release of phosphorous is enhanced in anoxic conditions, which precede the oxygen
reach stage, where the phosphorous is consumed by the biomass;
• Nitrification of ammonia to nitrates - it is an aerobic process, held parallel to the
removal process of the organic material in the aeration tank;
• Denitrification of nitrates to nitrogen gas and its release to the atmosphere - this is an
anaerobic process, which requires a carbon source.
In the activated sludge system, microbes are held in suspension and perform a wide range
of biological processes on the wastewater as it passes through the aeration tank. The
varying aerobic, anoxic and anaerobic conditions in the system encourage the growth and
activity of different microbes with specific action on the sewage. This results in a higher
rate of nutrients' removal during the biological process. The major advantages of BNR
plants are nutrient removal and compactness, while the major disadvantages are high
energy input for aeration, large number of mechanical equipment that requires skilled
labor and a high cost for operation and maintenance.
2.2
Sludge treatment and disposal methods
2.2.1 Sludge stabilization
Raw sludge contains a high concentration of organic matter and pathogenic
microorganisms. It is prone to degradation and putrefaction, which leads to malodors and
insect breeding. In addition, it provides a perfect environment for the growth of pathogen
microorganisms, thus posing a substantial public health problem in terms of diseases'
spreading. Sludge stabilization is the process of mineralization of the organic matter
contained in sludge. Usually, about 50-60% of the organic material is mineralized during
this process. Stabilized sludge does not degrade further when disposed on land. During
this process, partial removal of pathogen organisms is achieved as well.
The most widely applied method for sludge stabilization is the anaerobic digestion,
although in some cases aerobic stabilization methods, similar to the process of activated
sludge treatment, are applied too. The anaerobic digestion of sewage sludge is a process,
involving a complex mixture of different types of facultative and strictly anaerobic
bacteria. A schematic representation is shown in Figure 8.2. During the acid forming
stage, known also as acetogenesis, the end products are CO
2
and volatile organic acids,
which are extremely malodorous, producing the specific putrefaction smell. The organic
acids formed during the process of acidogenesis are further degraded to CH
4
by
methanogens. The limiting stage is the methanogenic one, as the strictly anaerobic
methanogens are very sensitive to the environmental conditions and require a narrow
range of pH variation (between 6-8) for their survival.
In cases of high organic loading or other unfavorable conditions, the acid-formers
produce large amounts of organic acids, leading to the accumulation of acids in the
reactor and lowering the pH, which additionally inhibits the process and if not controlled
on time could lead to its complete failure. Under such conditions, the sludge produced
