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Quebec facility evaluates anaerobic digester performance sensor

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By Patrick Kiely

ASENTRY sensor system developed by Island Water Technologies (IWT) was installed in July 2017 at the CNETE (Centre National en Electrochimie et en Technologies Environnementales) testing facility in Quebec on two duplicate anaerobic digesters processing waste activated sludge.

These sensors measure microbial electron transfer (MET), which is an instantaneous measurement of microbial activity in the wastewater. As the exo-electrogenic microbes digest wastewater they respire electrons onto the SENTRY electrode. In higher strength wastewater streams, this MET measurement correlates well with volatile fatty acid, and information can be used to predict fluctuating concentrations over time.

A pretreatment step was added to one of the reactors, and the differences between the systems were analyzed, using a SENTRY system installed in each. The sensor is a key tool in understanding the conditions in test and control anaerobic digestion systems.

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For continuing anaerobic digestion activities, the sensor could be applied to characterizing the impact of pretreatment on influent wastewater streams, providing correlations to real-time biogas production, or optimizing feed cycle times to ensure removal of bio-available carbon.

Pretreatment accounted for a 22.9% increase in biogas production between the test and control system. Further analysis of fractionation of the remaining organic material could be beneficial for understanding the full effect of the pretreatment.

Pretreated wastewater showed more consistent activity throughout the week, suggesting the pretreatment was having the beneficial effect of maximizing the biological activity in the reactor. This also suggested that a potentially longer hydraulic retention time (HRT) could produce even higher biogas production and removal rates.

Biogas production (and therefore reactor activity) trended well with SENTRY output. This shows great promise as a way to fill in the gaps in data that exist in daily sampling. This biogas trend was especially helpful for understanding Reactor A (no pretreatment) as the batch mode beginning and end was clearly noticed (typically between noon and 7 pm). The pretreatment step increased the potential for biogas production of the microbes based on the current HRTs of the reactor.

SENTRY sensors were able to pick up distinct batch run cycles for each system. Three times higher average biological activity (MET) and longer cycles were noted in the reactor receiving the pre-treated influent (Reactor B), suggesting a more biologically active feed material.

The sensor displayed the weekly patterns of the feed cycle and response to the feed cycle in real time, with sensors placed inside the reactors. Feed dates/times could be observed, as well as when reactions inside the system had slowed down. SENTRY data provides operators the option to optimize feeding cycles more closely to MET output. Waiting for data to return to baseline would be a suitable strategy for optimized cycle times.

Output from the sensors was demonstrated to be strongly correlated to biogas production from the test and control reactors. The sensors helped in understanding both the impact of pretreatment on the suitability of the waste organics for anaerobic digestion processing and understanding/optimizing feeding cycles to maximize utilization of bio-available organics.

Patrick Kiely is with Island Water Technologies. This article appears in ES&E Magazine’s August 2019 issue.

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