Investigating the true limits of anaerobic treatment of wastewater at low temperature using a cold-adapted inoculum

Abstract

Anaerobic batch reactors were inoculated with cold-adapted biomass (seed) to treat the organic material (COD) of domestic wastewater at 4, 8 & 15oC. The substrate was pre-UV sterilized to preclude competition between the cells thriving in the seed and the autochthonous, originated from wastewater cells. The performance in terms of organic removal showed that the specific cold-adapted inoculum efficiently treats anaerobically raw domestic wastewater at all temperatures based on the UWWTD (Urban Waste Water Treatment Directive) (91/271/EEC). The observed methanogenic taxa were Methanomicrobiales, Methanosaetaceae, and Methanosarcina during the whole experimentation. Methanomicrobiales were predominant at lower temperatures (4, 8oC) followed by Methanosaetaceae; at 15oC there was no distinct difference amongst them. Longer enrichment showed that further investigation may be required to clearly point the predominance between methanogens. Specific cellular activity was calculated (via qPCR, FISH) to enable scale-up & design simulation. The specific methanogenesis values showed that the activities at low temperatures are at least similar to those of typical mesophiles using a conservative cellular weighing reference to convert the cells to VSS. Higher specific activities were observed after acclimation of the cells at 4oC compared to 15oC regardless of the operational temperature (4 or 15oC). Acclimation at 4oC also resulted in a formation of a community that can be hardly disturbed from the competition of the wastewater cells when the seed:substrate ratio is low. This was not evident after acclimation at 15oC and it manifests that anaerobic treatment start-up at 4oC results in a sturdy and highly active methanogenic community. The CODRAW:CH4 conversion at 4oC was approximately 50% and reached up to 80% of the theoretically expected for sterile and non-sterile wastewater feed respectively. It is likely that the conversion was boosted from the synergy of the indigenous bacterial communities from wastewater and the cells originated from the seed. Enzymes (lipases) assays showed that the wastewater-originated group of cells (bacteria) contributed to the hydrolysis of insoluble organic material (lipids) and led to richer formation of intermediates that were subsequently utilized by the methanogenic populations of the seed. Limited lipid hydrolysis accounted for the organic material that remained insoluble. The lipases assays demonstrated that on equal temperatures (37oC) the specific activity of the enzymes secreted from the cells at low temperature (4oC) is higher than those secreted from cells at 15oC. This proves that the formation of a sturdier and of higher wastewater treatment performance community is likely when this is developed at low temperatures. The assay also demonstrated that a 4-degree temperature increase (from 4-8oC) is adequate to trigger the lipid:CH4 bio-conversion. Thus, for a complete anaerobic wastewater treatment using the specific inoculum, the temperature limit lies in-between 4oC and 8oC. A scale up designation based on the differentiation of the specific methanogenic activity according to temperature shouted that this limit lies at 5oC. For operation at lower temperature (<5oC), the required vessel volume and the hydraulic retention time (HRT) become extremely high and consequently financially unattainable. The results suggest that inoculating digesters for low temperature operation with cold-adapted communities is a promising way to treat wastewater and an appropriate solution for the investigation of the process limits. Hence, my recommendation for successful low temperature carbon neutral wastewater treatment is the inoculation of anaerobic reactors with cold adapted or psychrophilic biomass strategy, acclimation at low temperature and operation at a temperature >5oC.