Environmental Engineering Reference
In-Depth Information
3.2 Factors affecting biodegradation
The microbial population follows a growth cycle that can be divided into
several distinct phases: the lag phase, exponential phase, stationary phase,
and death phase. In the lag phase, there is a delay in the microbial population
growth until the microbes have become adjusted to the food source, which
in many instances is the contaminant of interest, and surrounding conditions.
The microbes cannot consume the food source until they have developed
the required enzymes and metabolites necessary to break down the contam-
inant. After the necessary enzymes and metabolites have been produced, the
microbes enter the exponential phase of growth. The rate of exponential
growth is influenced by environmental conditions (e.g., temperature) as well
as by characteristics of the organism itself. However, exponential growth
cannot occur indefinitely. Generally, either an essential nutrient for growth
is used up or some waste product of the organism builds up to an inhibitory
level and exponential growth ceases. At this point, the population has
reached the stationary phase, in which there is no net increase or decrease
in cell number. If conditions worsen (i.e., toxins continue to build up or the
food source becomes depleted), the microbial population will enter the death
phase and the viable number of microbes will decrease (Atlas, 1984; Brock
et al., 1984).
Given this growth cycle, almost all organic compounds are degradable
provided the proper circumstances or time. However, a range of physical,
chemical, and biochemical conditions or materials can interfere with biore-
mediation. The biodegradability of wastes or specific waste constituents
must be defined in terms of a realistic period determined by the goals of the
treatment program. Biodegradability limits due to toxicity are a function of
concentration. If the concentration of the toxicant can be controlled or the
biomass is large enough, many highly toxic wastes or constituents can be
biodegraded. By incrementally increasing contaminant concentrations,
microbes can be challenged to degrade more wastes. The key is to slowly
increase the contaminant concentrations, allowing the microbe population
to adapt to the changing conditions and produce the required enzymes and
metabolites.
3.2.1 Metals
Metals can inhibit various cellular processes and their effects are often con-
centration dependent. Metal toxicity for microbes will usually involve spe-
cific chemical reactivity. Metals such as copper, silver, and mercury are typ-
ically very toxic, particularly as ions, whereas metals such as lead, barium,
and iron are usually benign to the microbes at levels typically encountered.
The nutrient metals are usually found naturally in the necessary amounts
for plants and microbes in fertile soils. The principal inorganic nutrients are
nitrogen and phosphorus; however, trace amounts of potassium, calcium,
sulfur, magnesium, iron, and manganese are also required for optimum
