Exergy saving and exergy production in municipal wastewater ...

Exergy saving and exergy production in municipal wastewater treatment
- Isothermal microcalorimetry
- Accelerated rate
- Constant pressure
- Constant volume calorimeter (bomb calorimeter).
The same as exergy, the concept of calorimetry can be used to calculate
the mass and energy fluxes of the biological processes inside the
treatment plant. One calorie (Cal) is the energy needed to increase the
temperature of one kilogram of water at 15°C up to one degree of
Celsius. This definition is used in food science but the method to
measure this amount is by burning completely the food in a device called
bomb calorimeter. The human body cannot do this; only part of the
food is absorbable and in fact it is the exergy content of the food that
body uses to work with (Zvolinschi, 2006). Calorific value of a specific
mass of a fuel is the amount of the heat it releases when it is burned with
oxygen in a container with a constant volume under high pressure of
oxygen (30 atm). This process happens in bomb calorimeter.
1.16. Bomb calorimeter
The sample in this device is combusted under almost 30 atm of oxygen
pressure. The differences in the temperature of the bomb will be
measured and multiplied to the specific heat value of the bomb. This
way the amount of calorie content of the sample is specified. In the
wastewater treatment, use of this device can be for measurement of
calorific value in the influent and the effluent of each process. The
difference in the calorific value will give the removal efficiency. It has
been tried to find a correlation between these values and COD content
in some food samples. Additionally, the amount of protein, carbohydrate
and fat in the flow can be specified if one of them is known to the
operator. These amounts can play roles in deciding what type of
processes can be useful for treatment.
In food science the following energy values is generally being used:
- Protein= 17kJ/g.
- Fat= 37 kJ/g
- Carbohydrate= 17 kJ/g.
The mean amount of nitrogen in the protein (mainly as amino acid) has
been specified as 16%.
1/0.16=6.25 (19)
The amount of nitrogen content can be calculated by dividing the
protein content of the influent by 6.25 (FAO food and nutrition
report, 2002).
Studies have been done on measuring the energy content of urine
(mainly contain nitrogen) which has showed bomb calorimetry is a useful
method for such measurements. The difference between the energy
content of the food and the urine and stools gives the metabolizable
energy of the food.
The metabolizable energy content of the influent by the treatment plant
can also be measured this way (Rodrigues et al, 2005). However, this
should be considered that there is a difference between nitrogen in the
food and in urine. The nitrogen in the food mainly is in the protein.
However, in urine from a healthy human body there is only a trace
amount of protein. The un-oxidized matter in the urine produces 7.9
calories which is equal to almost 1.25 calories per gram of absorbed
protein (Merrill and Watt, 1973):
(7.9/6.25)≈1.25 (20)
15