Kanikah Mehndiratta, MSc, University of Glasgow
The ever-growing world population poses serious concerns for food shortage, until and unless the situation is targeted through newer approaches. These approaches to food supply should meet the demand for essential nutrients such as potassium, nitrogen, and phosphorus. Domestic wastewater, also called black water (BW) can be considered one source for recovery and reuse of nutrients. Thermophilic anaerobic digestion can be considered a promising strategy to treat concentrated black water. It can simultaneously help in the recovery of essential nutrients and the removal of pathogens for safe usage. A team of researchers from the Netherlands has devised a novel approach for the treatment of BW via thermophilic and hyper-thermophilic bacteria.
Thermophilic anaerobic digestion:
Mesophilic bacteria have long been used to treat wastewater. Thermophilic bacteria at 55°C and hyper-thermophilic bacteria at 70°C, can be used for anaerobic digestion or mechanization and COD (organic compounds) removal. BW has a high concentration of organic nutrients which could be converted to methane via anaerobic digestion. A concentrated BW stream contains enough value of COD for matching with the energy required for high-temperature associated activity of hyper-thermophilic bacteria.
There exists a lot of legislative, technological, and social barriers against such an approach. The major restriction at the legislative level is heavy metal and pathogen load in fecal matter. At the technological level, the dilution of waste streams could challenge the recovery of nutrients. A source-separated selection of BW would make it easier for treatment by anaerobic bacteria. The ultra-low flush volume of BW via vacuum collection can make the BW stream more concentrated.
Previous studies have successfully conducted anaerobic digestion of BW via an up-flow sludge blanket reactor at a temperature of 25°C. These had reported a hydraulic retention time (HRT) of about 8 days and a loading rate of organic matter (OLR) of 1 kg COD/m3/day. This study came up with the following values, 78% COD removal conversion and 54% COD to methane. For ensuring safe usage of the essential nutrients recovered and effective pathogen removal, the usage of bacteria that can survive at high temperatures has been the study approach. A high value of efficiency and lower retention period can prove useful in applying the technique at the industrial level. Smaller reactors can also offer lower energy costs. The usage of ultra-low volume toilets with vacuum technology can help in obtaining a concentrated BW.
Research Strategy adopted:
BW for the experimentation was obtained from Wageningen University and stored in a stirred tank with an HRT of 7 days. Flush volume was fixed at 0.2 and 0.5 L/ flush. The temperature was set at 4-7°C initially. In the thermophilic and hyper-thermophilic reactors, BW has flowed at a concentration of 4-6 L/hr. Peristaltic pumps were used for supplying pulse to the reactors. The quantification of Biogas was done at 25°C via a drum-type gas flowmeter. The whole study had a total run time of about 719 days. COD concentration was gradually increased in higher phases of trials.
Conclusions from the study:
In comparison to mesophilic anaerobic digestion, a much higher OLR can be applied during thermophilic anaerobic digestion. Thermophilic treatment of BW resulted in a COD removal of about 70% with a start-up time of 12 days. Hyper-thermophilic digestion led to 38% mechanization of organic matter in BW. The study also concluded a shift from anaerobic digestion via acetoclastic methanogens to syntropic oxidation of acetate. The major bacteria involved in the sludge as deduced via 16S rRNA gene analysis were of the order Clostridiales, Methanobacteriales, and Bacteriodales.
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References:
1. Moerland, M. J., Castañares Pérez, L., Ruiz Velasco Sobrino, M. E., Chatzopoulos, P., Meulman, B., de Wilde, V., Zeeman, G., Buisman, C. J. N., & van Eekert, M. H. A. (2021). Thermophilic (55 °c) and hyper-thermophilic (70 °c) anaerobic digestion as novel treatment technologies for concentrated black water. Bioresource Technology, 340, 125705. https://doi.org/10.1016/j.biortech.2021.1257
2. Zhang, Q., Zhang, L., Guo, B., & Liu, Y. (2020). Mesophiles outperform thermophiles in the anaerobic digestion of blackwater with kitchen residuals: Insights into process limitations. Waste Management, 105, 279–288. https://doi.org/10.1016/j.wasman.2020.02.018
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Author info:
Kanikah Mehndiratta is an avid researcher in the field of Genetics with a background in Biotechnology. She is a postgraduate from the University of Glasgow in their Medical Genetics and Genomics program. Currently, based in Chandigarh as a scientific writer, she involves herself mainly in projects related to neurological disorders. Outside of academics, she likes to read novels, travel and is involved in volunteer work mostly.
LinkedIn profile- https://www.linkedin.com/in/kanikah-mehndiratta-301830171
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