Exergy saving and exergy production in municipal wastewater ...

Exergy saving and exergy production in municipal wastewater treatment
human or the system will disappear. This heat drain has been called as
the energy tax which if it gets eliminated the system will collapse
(Bendoricchio, 1997).
The system tends to increase the negative entropy or negentropy
(produce more ordered or higher quality material) inside itself in order to
survive. This kind of system is called dissipative system as Prigogine
defines and it dissipates energy and entropy. The methods that are used
in the food science for analyzing the calorie-contents can also be used
for the wastewater treatment plant in order to evaluate the processes,
flows and storages (Bendoricchio, 1997). The rate of flow of useful
energy is power and in SI system it can be measured by joules per unit of
time or watt
1.6. Thermodynamic view of the treatment plant
In biological treatment units in a treatment plant, the bacteria digest the
biodegradable material flowing in the treatment plant and use them to
run the catabolic and anabolic reactions of their cells.
If the microorganism is imagined as a human body its metabolism can be
described with the above described view using classical thermodynamic
concepts like entropy, Gibb’s free energy and enthalpy. This way
thermodynamic science can tell if a specific biological reaction is possible
to happen (Von Stockar et al, 2006). The biological life in the treatment
plant can also be a basis for description of treatment plant as a human
body which metabolizes wastewater.
Another view to the treatment plant can be achieved from the
description of the ecosystem by Odum where in his book he says “an
entity can be considered an ecosystem as long as the major components
are present and operate together to achieve some sort of functional
stability, even if for only a short period of time.” He mentions that the
main feature of all the ecosystems is that in all of the ecosystems
autotrophic and heterotrophic components have interaction with each
other (Odum et al, 2004). This is similar to what is happening in the
treatment plant. The pollution entering the treatment plant introduces
entropy into the facility which turns into information and ATP (exergy)
in the microorganisms through their respiration process.
The Second law of thermodynamic which describes the entropy change
of an open system is as follows:
d S(t) = d e S(t) + d i S(t) (2)
d e S(t) = Entropy provided by the surrounding to the system (Entropy
flow) associated with the flow of energy and matter
d i S(t) = The entropy produced inside the system by
irreversible processes
d i S(t) = dQ(t)/T(t) (3)
dQ (t)= heat production caused by irreversible processes inside
the system
T (t) = current temperature (◦K) at a given point in the system
Β = dS/dt = (d e S)/dt+ (d i S)/dt (4)
β = rate of entropy production or energy dissipation
(dissipative function)
This law says d iS must be zero for reversible transformations and should
be positive for irreversible transformation of the system. However,
can be zero, positive or negative for an open system. For adiabatic
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