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Significance
Constructed wetlands have been widely used as decentralized wastewater treatment systems. However, they are disadvantageous in one way or the other i.e. limited nutrients removal rate and large land footprint that raises concern on their effectiveness. Thus, development of alternative methods for improving oxygen transfer for boosting aerobic treatment in constructed wetlands is highly desirable. Denitrification being an anaerobic process, it is consequently important to improve also the anaerobic conditions of the system to enhance the nitrogen removal rate.
In the traditional constructed wetlands configurations, a combination of vertical flows to improve organic carbon removal and nitrification processes followed by horizontal flow to enhance denitrification processes is involved. Even though the alternated aerobic and anaerobic conditions allow for different metabolic pathways for carbon and nitrogen degradation, the multistage configuration is characterized by the potential depletion of organic matter which limits the denitrification process.
To this note, S. Cabred, Valeria Giunta Ramos, J.E. Busalmen, Dr. Juan Pablo Busalmen and Dr. Sebastian Bonanni from INTEMA in Argentina developed a new configuration for constructed wetlands for depuration of urban and domestic wastewater. Their main aim was to enhance the chemical oxygen demand and nitrogen removal efficiencies. The research work is currently published in Chemical Engineering Journal.
The reduced stack constructed wetland- as the new configuration was known-comprised of three stacked shallow stages. The 15 cm depths were operated under partial saturation, allowing the existence of both aerobic and anaerobic conditions on each stage. Sequential aeration steps were produced by a single-pulse of wastewater. This improved the overall convective oxygen transfer to enhance nitrification reactions, with denitrification also favored by the availability of carbon sources under anaerobic conditions.
The introduction of conductive fillings further led to an improvement in the treatment performance attributed to the direct interspecies electron transfer (DIET) among the bacteria in the bed.
As proof of the concept, it was necessary to analyze the removal of nitrogen species and chemical oxygen demand under high organic loads based on conductive and nonconductive fillings. Here, single-stage vertical flow constructed wetland filled with traditional and conductive materials were used as control units. Interestingly, the high removal rate did not require external energy inputs, additional carbon source or wastewater recirculation thus proving to be more effective than traditional strategies. The presented configuration recorded removal of 90% inlet chemical oxygen demand, 80% inlet ammonia, 55% of the total nitrogen which were much higher than those values reported in the control units. Besides, 85% of chemical oxygen demand degradation was observed under a high loading rate.
Based on the results, the poor performance in the traditional single-stage conductive units, as opposed to the excellent performance of the nonconductive reduced depth stacked constructed wetlands was a clear indication that high oxygen input and alternation of aerobic and anaerobic conditions has a significant role in the realization of high nitrogen and chemical oxygen demand degradation efficiencies. The strategy is a promising approach for decentralized household wastewater treatment. Dr. Sebastian Bonanni in a statement to Advance in Engineering mentioned their system is useful owing to its relative energy saving approach, low cost and high efficiency and the need of lower quantities of expensive materials in the case of conductive units. He further noted that its performance and cost can be improved by analyzing the effects of several design parameters that are currently under study. The group is currently working in the creation of a spin-off that will produce and commercialize household treatment systems based on the concepts presented in this work.
Reference
Cabred, S., Giunta Ramos, V., Busalmen, J., Busalmen, J., & Bonanni, S. (2019). Reduced depth stacked constructed wetlands for enhanced urban wastewater treatment. Chemical Engineering Journal, 372, 708-714.
