Development of a green roof medium utilizing recycled materials

Background/Question/Methods

Green roofs are emerging worldwide as a low-impact development (LID) strategy. Artificial soils, more lightweight than natural soils, better accommodate the weight-bearing limit of buildings. A medium containing recycled crumb rubber supplemented with compost (RM) was compared to commercially available green roof media (CM). Leachate and solid samples were analyzed for culturable heterotrophs, coliforms and E. coli, bacterial DNA, nutrients, and community physiological diversity (Biolog) between the two media.

Microcosms filled with the appropriate medium were watered with synthetic rainwater at regular intervals. At the start of weeks 6 and 9, each system was spiked with E. coli to simulate environmental contamination.  Samples were assayed for heterotrophs and E. coli using MPN and MF techniques. DNA was extracted using the MoBio PowerWater® and PowerSoil® protocols, amplified through PCR using 16S ribosomal primers, and characterized through pyrosequencing. Biolog Ecoplates were used to assess the community physiological diversity present between the two media.  Nutrient tests included ammonia, total nitrogen, nitrate, total phosphorus, and metals.

Results/Conclusions

Culturable heterotrophic microbial numbers were found to be higher in the RM compared to CM in both leachate and media samples. There was a transient increase in the number of coliforms after the E. coli spike. Coliform levels returned to acceptable EPA levels more quickly in CM than RM. In both leachate and media samples, microbial DNA concentrations were initially higher in the CM and continued to increase over time. As the study progressed, DNA concentrations in the RM increased to levels similar to the CM.  E. coli was present in the final RM samples. Higher concentrations of nutrients were found in the CM leachate. Microbial community diversity appeared to be greater in RM samples.

Overall, the RM showed no increase in the release of pathogenic microbes or nutrient loads and does not exceed regulatory standards. Although the microbial communities vary between the two media types, the RM has potential to be a good alternative to current green roof materials. This offers significant advantages both environmentally and societally and should be investigated further.