WWTP. During the monitoring period 1.35 kg CH4/d were emitted from
the bioreactor which is much lower than the CH4 emissions reported by
Daelman et al. (2012) from a study on a WWTP in The Netherlands. In
their WWTP, the CH4 emissions from the plug–flow and carrousel reactors
were 48 kg CH4/d and 264 kg CH4/d respectively. This was due to
the higher dissolved CH4 flowing at the headworks level (influent and reject
wastewater streams) in Daelman's study (192 kg CH4/d).
In this study, calculations taking into account the hydraulic
residence time (HRT) of the different sites of the bioreactor and measurements
of the dissolved CH4 in the influent wastewater streamindicated
that the daily CH4 peak emissions detected at the bioreactor were
related with the normal influent wastewater flow dynamics, although
some deviations could be observed. In some of the cases recorded
such as the one shown in Fig. 2, CH4 emissions continued increasing
during night-time, even though the flow of influent wastewater decreased.
This is probably due to the fact that reject water was released
into the influent stream during night-time also, increasing the concentration
of CH4 when the volume of wastewater was smaller and modifying
the CH4 emissions pattern from the reactor. In terms of concentration,
the CH4 content of the reject wastewater was over 90% larger than the
concentrationmeasured in influent wastewater. However, the flow of reject
wastewater was around 99% smaller than the average influent wastewater
flow of the plant diminishing its contribution on the overall CH4
emissions detected.
Mitigation strategies could be implemented from the design to the
operational level of a wastewater treatment system in order to favour
the conversion of CH4 before it is stripped due to aeration. As reported,
a large portion of the CH4 that is emitted fromWWTPs enters the system
dissolved in the influent wastewater streams and is previously generated
in the sewer pipes bymethanogenic organisms. Thus, any measures taken
in order to minimize or control the activity of methanogens may contribute
to the overall minimization of CH4 emissions. Research on this
topic has successfully been carried out in recent years, mostly focusing
on the addition of chemicals that inhibit methanogenic activity in the
sewer networks