2.5. Energy calculations for different reactor combinations
Six operating combinations were assessed relative to energy use
for aeration, mixing, and pumping, and potential energy from biogas
production, which used operating data from experiments here
and previous PCP waste treatment work (Ahammad et al., 2013).
Relative energy use for each operating combination was based on
geometrically identical, scaled-up units treating 1000 L/d wastewater.
Energy use in aerated units was dominated by aeration and mixing needs (75%). Energy for aeration was calculated based
on liquid height and measured air flow rates needed to maintain
targeted DO concentrations in the reactors, whereas mixing energy
was based on equivalent paddle-mixing requirements for scaled
aerobic reactors. Energy also was estimated for feed pumps and
effluent recycle based on power ratings for characteristic industrial-
scale centrifugal pumps. Energy estimates for the anaerobic
units did not include aeration, but did include mixing energy as
well as energy required for heating the anaerobic units to 35 C.
However, potential energy recovery from anaerobic biogas production
was included in the calculations and was the useful energy
obtainable from industrial-scale gas turbines operating on biogas
(Ahammad et al., 2012). The net energy used by the anaerobic units
was the difference between energy for pumping and mixing minus
the potential energy obtained from biogas production.