Exergy saving and exergy production in municipal wastewater ...

Exergy saving and exergy production in municipal wastewater treatment
100% COD influ
28% COD eff
72% COD biodegradable
20% COD anoxic processes
dewatered digested
(nitrificationdenitrification
&
sludge
anammox)
8% COD inert
anearobic processes
(biogas)
aerobic processes
Fig. 3. Typical COD flowchart in advanced wastewater
treatment.
- Nitrogen removed in digested sludge (assumed to be 0.45 g N/g TS
removed as digested sludge)
1.23. Nitrogen removed as gas (no COD consumption)
Either as nitrogen gas during the denitrification and anammox processes
or as free ammonia. Instead of measuring COD, treatment plants in
Stockholm are trying to implement TOD measurement. COD
measurements need materials which are toxic and therefore this index is
being replaced by TOD. Hultman has also tried to relate the exergy
concept to TSS and has introduced a new way of measurement to the
treatment processes.
1.24. Exergy calculation for a treatment plant using environmental data
based on BOD measurements
Treatment processes efficiencies can be assessed by measuring the
amount of the BOD being removed; therefore, the following calculations
can be considered in these kinds of units:
1 g organic matter=1.39 g BOD 5
1 g organic matter=18.7 kJ exergy
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1 g BOD 5=13.45 kJ exergy
According to the typical values of removal mentioned above for COD,
and considering the relationship between COD and BOD the
calculations can be applied for data based on BOD measurements (BOD
= 0.5 COD).
Table 2 biogas energy values.
COD value if one L biogas (2/3)×(4/1.4) 1.9 g
Gibb’s free energy of 1 L
biogas
0.0095×(2/3)MWh/L
0.0063 MWh/L
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