"Carbon Symbiosis" - a Concept Integrating Reforestation, Indoor Carbon Dioxide Control and Urban Farming.

Is it possible to reforest, control indoor carbon dioxide level and promote urban farming using a single approach? Reforestation programs play an important role in carbon capture and sequestration. However, the need to develop tree saplings before they can be deployed on reforestation sites can delay the start of this sequestration process. This study proposes a novel method to reduce this time wastage by erecting a special "active" hydroponic greenhouse on the reforestation site, while waiting for the saplings to be ready. During service, exhaust from widely used onsite generators is stored and channeled into these greenhouses, where special plant sequesters are given time to remove the carbon dioxide in the exhaust (this is termed "active" plant-based sequestration). In the first of this three-part study, experiments and theoretical modeling of a scaled-up version of such a greenhouse - using Eichhornia Crassipes (i.e. water hyacinth) as the plant sequester - showed that it can potentially remove more carbon dioxide than some common species of tropical trees that are fully grown and occupy the same land area; furthermore, the greenhouse is able to achieve this removal rate in a much shorter time. That is, this greenhouse can kick-start and augment the sequestration function of the reforestation program. In the second part of this study, the used carbon dioxide-dosed water hyacinth in the greenhouse was converted into biochar by pyrolysis (thus "locking in" the carbon in the biomass). Preliminary test results indicated that biochar made from this feedstock (i.e. carbon-enhanced biochar) can adsorb twice as much carbon dioxide from the environment (over 5 days) as biochar made from undosed water hyacinth. This is caused by additional oxonium functional groups on the surface of the biochar, which also increase its anion exchange capacity. The potential of using such carbon-enhanced biochar in wall cavities to control indoor carbon dioxide levels in buildings was demonstrated. It was found that this setup can remove as much as 1 - 4 mmol of carbon dioxide per gram of biochar, which implies that it is possible to reduce indoor carbon dioxide concentration from 1,000ppm to 500 ppm within 1-2 hours of application. In the third part, the effect of carbon-enhanced biochar on the growth of lettuce in tropical urban farming was investigated. Our preliminary studies show that a mixture of carbon-enhanced biochar and soil does not support growth as well as mixture of soil and normal biochar (made from undosed water hyacinth) due to inhibition of release of certain nutrients in the soil. In summary, this work explores the use of "active" hydroponic greenhouse to realize "carbon symbiosis" as a holistic sustainable strategy to utilize carbon, by integrating reforestation, indoor carbon dioxide control and urban farming.