Abstract:
A biological early warning system (EWS) was developed to screen wastewater
containing nitrification inhibitors and identify nitrifying bacteria activity reduction
without relying on absolute values of sensor signals. To do so, numerous sensors were
evaluated using a tiered approach to aid the analysis and made it easier to convey the
current state of the technology. The research then produced a framework for the
development of an EWS and the applicability of sensors to the wastewater matrix. The
research identified a need for the development of a strategy and guidance that can help
in the prevention and detection of nitrification inhibitors. Initial tests focussed on sewer
biofilm N2O emissions, however, despite average nitrification rates of 19.5 g-NH4
+
-
N.m-
2
.d-
1
the response was unreliable due to inadequate control. To address this, a
circulating floating bed biofilm reactor (CFBBR) was designed as a sidestream. The
CFBBR biofilm’s toxicity response was compared to the sewer biofilm, a 2850 mg.L-
1
MLSS culture and a 10.5 mg.L-
1
MLSS culture (with equivalent biomass concentration
to the CFBBR biofilm). The cultures responded differently with an inhibitory effect
scale of Cu2+ > ATU > Ni2+ > Cr6+ for CFBBR biofilm, ATU > Cu2+ > Ni2+ > Cr6+ for
2850 mg L-
1
MLSS, ATU > Ni2+ > Cr6+ > Cu2+ for 10.5 mg.L-
1
MLSS and ATU > Cu2+
> Cr6+ > Ni2+ for sewer biofilm. This was firstly attributed to suspended growth
nitrification stimulation by Cu2+ doses up to ~45 mg.L-
1
resulting in a lower inhibitory
effect. Secondly, very high Cr6+ and Ni2+ doses were required for biofilm nitrification
inhibition, due to diffusion limitations and slow transport through cell membranes. The
CFBBR biofilm response to heavy metals was characterised through N2O and CO2
spikes and a post shock emissions recovery period was observed with the trend Ni2+ >
Cr6+ > Cu2+ . A 10 minute hydraulic retention time allowed quick detection and steady
state nitrification rates of 0.4 g-NH4
+
-N.m-
2
.d-
1
despite high organic loading rates.
Additionally, a suspended growth based monitor (Nitritox) was assessed as an inlet
works toxicity detector. Incorporation of a Nitritox with a CFBBR based sewer monitor
offered increased robustness over a CFBBR only system and was shown to be viable
system in catchments >200,000 population equivalent. This information is useful to
water utilities so that they can plan for and experiment with upset early warning
protocols. It is also useful to manufacturers as they can determine product performance
needs.
