Use of a passive bioreactor to reduce water-borne plant pathogens, nitrate, and sulfate in greenhouse effluent

Abstract

The goal of this study was to evaluate the use of passive bioreactors to reduce water-borne plant pathogens (Pythium ultimum and Fusarium oxysporum) and nutrient load (NO⁻ ₃ and SO²⁻ ₄) in greenhouse effluent. Sterilized and unsterilized passive bioreactors filled with a reactive mixture of organic carbon material were used in three replicates. After a startup period of 2 (sterilized) or 5 (unsterilized) weeks, the bioreactor units received for 14 weeks a reconstituted commercial greenhouse effluent composed of 500 mg L⁻¹ SO²⁻ ₄ and 300 mg L⁻¹ NO⁻ ₃ and were inoculated three times with P. ultimum and F. oxysporum (10⁶ CFU mL⁻¹). Efficacy in removing water-borne plant pathogens and nitrate reached 99.9% for both the sterilized and unsterilized bioreactors. However, efficacy in reducing the SO²⁻ ₄ load sharply decreased from 89% to 29% after 2 weeks of NO⁻ ₃-supply treatment for the unsterilized bioreactors. Although SO²⁻ ₄ removal efficacy for the sterilized bioreactors did not recover after 4 weeks of NO⁻ ₃-supply treatment, the unsterilized bioreactor nearly reached a similar level of SO²⁻ ₄ removal after 4 weeks of NO⁻ ₃-supply treatment compared with affluent loaded only with SO²⁻ ₄, where no competition for the carbohydrate source occurred between the denitrification process and sulfate-reducing bacteria activity. Performance differences between the sterilized and unsterilized bioreactors clearly show the predominant importance of sulfate-reducing bacteria. Consequently, when sulfate-reducing bacteria reach their optimal activity, passive bioreactors may constitute a cheap, low-maintenance method of treating greenhouse effluent to recycle wastewater and eliminate nutrient runoff, which has important environmental impacts.

Keywords:

• Denitrification
• Fusarium oxysporum
• organic carbon
• Pythium ultimum
• sulfate reduction
• sulfate-reducing bacteria

Acknowledgments

This work was funded by a grant from the BMP Innovation and Plant-Prod-Industry programs as well as Canada's Organic Science Cluster, which in turn is funded by the Canadian Agri-Science Clusters Initiative of Agriculture and Agri-Food Canada's Growing Forward Policy Framework and its industry partners. This study was performed within the framework of the Microbial Horticulture postgraduate school, FORMAS, Stockholm, Sweden. The authors would like to thank Réjean Bacon, Edith Tousignant, and Claudine Ménard for their technical assistance.