Thesis - faculty.ait.ac.th - Asian Institute of Technology

activity to degrade the pollutants. Lead is one of the important toxicant due to its abilities
to causes devastating and irreversible neurological damage to children, leading to learning
disabilities and damage to the brain and nervous system. Exposures at high doses of lead
can lead to coma, convulsions and death (LaGrega, et al., 1994). Inhibitory effects on
biological treatment can be observed by reduced organic removal efficiency, and poor
settling characteristics of the microorganism in the biological process. The toxicity of the
pollutant depends on the concentration and type of organism present. In this context, the
importance of toxicity or inhibition can not be neglected.
Various methods have been described in the literature to determine the toxicity of
chemicals to microorganisms. Normally, the toxicity of the compound is evaluated with
organism such as algae, water flea, and fish, which is costly and time consuming. The
focus of these methods is to mainly investigate the inhibition of microbial respiration in
relation to rate of oxygen consumption.
In biological treatment, toxicity is generally monitored by measuring certain
activities of the microorganisms. This may be observed by changes in respiratory activity
or biochemical tests which measure the concentration of certain biochemical agents
(Talinli and Tokta, 1994; Chen, et al., 1997; Madoni, et al., 1999). It is possible to
determine the inhibitory effects of compounds with the help of feed from activated sludge
batch reactors in which a biological seed and various concentrations of inhibitors are
mixed. Respiratory response is a sensitive determinant which provides a faster and more
accurate estimate of which is acceptable toxicity studies (Morgan and de Villiers, 1978).
Toxicity detection system uses a variety of biological responses and process
variables, for a wide range of biological species. The measurement of oxygen uptake,
organic removal efficiency, or enzymatic activity indicates biological responses to the
various conditions. A variety of toxic agents can cause different patterns of inhibition (i.e.,
activity per unit biomass vs toxicant concentration), on the other hand, various activity
indicators may show different inhibition patterns for a single toxic agent (Patterson, et al.,
1970).
DO concentration is an important variable in the operation of the biological treatment.
The toxicity could be tested by comparing the respiration rate before and after addition of
toxicant (Temmink, et al., 1993; Madoni, et al., 1999). The results obtained from toxicity
test could tell the extent to which the efficiency and operation of biological treatment could
be affected.
The respiration rate of activated sludge depends on the activity of biomass which are
also depends on some operating conditions in the activated sludge process. The operating
conditions include mean cell residence time, organic loading rate, substrate limitation,
environmental conditions such as pH, temperature, and toxic substances. The maximum
respiration rate should be constant under normal operating conditions. In the presence of
toxicant in the system, the maximum respiration rate and the performance of the system
will decrease (Kim, et al., 1994).
Temmink, et al. (1993) had conducted a study of copper (Cu) toxicity. During the
experiment, the copper concentration in the wastewater was increased from 25 to 200 mg/L.
It was found that the respiration rate decreased about 30% at the copper concentration of
50 mg/L. The sludge had been completely inactivated when the copper concentration in the
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