Anaerobic Wastewater Treatment
tank, for example due to bad retention of the sludge or increased sloughing off of
the sludge from the filter media. The washout may also take place even if the
organic load of the process is unchanged. Disturbances caused by washout will
occur relatively slowly (within one sludge age, that is, perhaps 1-2 weeks).
Inhibition of the biomass may take place by means of internally produced inhibition
(fatty acids, ammonia, pH) or by means of substances supplied from the outside
environment (sulphate, ammonia, metals, specific organic materials). It will be
characteristic for the external inhibition that the whole biomass is inhibited however, still in such a way that the greatest effect can be observed on the methane
producing bacteria. External inhibition will normally occur quickly (in the course of
a few hours}, whereas produced inhibition occurs somewhat slower. Table 9.11 gives
a brief survey of typical disturbances, and how they influence the anaerobic process.
Notice that an observed abnormality in a control parameter may often be caused by
several different interferences (simultaneous or isolated). For example, the reason
for a quick increase in the content of volatile acids may be
- increased load of dissolved organic matter,
- temperature variations,
- (heavy) pH increase,
-pH decrease,
- inhibition caused by hydrogen sulphide (sulphate in the influent}, ammonia,
or other toxic substances.
It can further be seen from the table that the pH is an extremely bad control
parameter (apart from the fact that it is easy to measure).
Inhibition of anaerobic wastewater processes with substances from the outside
follows a pattern which deviates somewhat from what we know from anaerobic
sludge stabilization. Type of substance as well as exposure time and plant layout
play an essential role for the inhibition phenomenon in anaerobic wastewater
processes. Table 9.12 gives a survey of limits for inhibition for a number of toxic
substances.
For certain inhibitors, anaerobic wastewater processes can withstand much higher
concentrations than anaerobic sludge processes of wastewater. The reason is that the
hydraulic retention time for water is much shorter than the sludge age in the
anaerobic wastewater treatment methods. The micro organisms are therefore only
exposed to toxic substances for a shorter period of time. In this connection reactors
with plug flow have a special advantage, for example fixed filters.
In Table 9.12 we also note the difference between single doses and continually
applied inhibitors. For substances where the microorganisms can adjust (for example cyanide), they can withstand considerably higher concentrations by continual supply than by single doses.
For substances with a direct toxic effect and with little possibility of adaption (for
example nickel) we see the adverse effect, where a short exposure of a rather high
307
tank, for example due to bad retention of the sludge or increased sloughing off of
the sludge from the filter media. The washout may also take place even if the
organic load of the process is unchanged. Disturbances caused by washout will
occur relatively slowly (within one sludge age, that is, perhaps 1-2 weeks).
Inhibition of the biomass may take place by means of internally produced inhibition
(fatty acids, ammonia, pH) or by means of substances supplied from the outside
environment (sulphate, ammonia, metals, specific organic materials). It will be
characteristic for the external inhibition that the whole biomass is inhibited however, still in such a way that the greatest effect can be observed on the methane
producing bacteria. External inhibition will normally occur quickly (in the course of
a few hours}, whereas produced inhibition occurs somewhat slower. Table 9.11 gives
a brief survey of typical disturbances, and how they influence the anaerobic process.
Notice that an observed abnormality in a control parameter may often be caused by
several different interferences (simultaneous or isolated). For example, the reason
for a quick increase in the content of volatile acids may be
- increased load of dissolved organic matter,
- temperature variations,
- (heavy) pH increase,
-pH decrease,
- inhibition caused by hydrogen sulphide (sulphate in the influent}, ammonia,
or other toxic substances.
It can further be seen from the table that the pH is an extremely bad control
parameter (apart from the fact that it is easy to measure).
Inhibition of anaerobic wastewater processes with substances from the outside
follows a pattern which deviates somewhat from what we know from anaerobic
sludge stabilization. Type of substance as well as exposure time and plant layout
play an essential role for the inhibition phenomenon in anaerobic wastewater
processes. Table 9.12 gives a survey of limits for inhibition for a number of toxic
substances.
For certain inhibitors, anaerobic wastewater processes can withstand much higher
concentrations than anaerobic sludge processes of wastewater. The reason is that the
hydraulic retention time for water is much shorter than the sludge age in the
anaerobic wastewater treatment methods. The micro organisms are therefore only
exposed to toxic substances for a shorter period of time. In this connection reactors
with plug flow have a special advantage, for example fixed filters.
In Table 9.12 we also note the difference between single doses and continually
applied inhibitors. For substances where the microorganisms can adjust (for example cyanide), they can withstand considerably higher concentrations by continual supply than by single doses.
For substances with a direct toxic effect and with little possibility of adaption (for
example nickel) we see the adverse effect, where a short exposure of a rather high
307