Saturday, 15 July 2006

Issues with Re-Using Organic Based Wastewaters in Soilless Applications.

R. Michitsch1, C. Chong2, B.E. Holbein3, and R.P. Voroney2. (1) Dalhousie Univ, 14-200 Court Street, Box 143, Truro, NS B2N 3H7, Canada, (2) Univ of Guelph, Guelph, ON N1G 2W1, Canada, (3) SUBBOR, Etobicoke, ON M8V 3Y3, Canada

Nutrients in liquid wastes (ie wastewaters) are ‘lost’ resources if not re-used. Due to increasing food demands of the expanding human population, re-use applications provide a cost-effective disposal measure since the recycled nutrients reduce fertilization costs. Soil-less environments, typically as hydroponic applications, have the advantage of eliminating poor soil structure or non-uniform texture, drainage, weed and soil-borne pathogen growth, while reducing nutrient and labour requirements. These systems can be located in unused space (eg hotel lobby, abandoned building), providing wastewater treatment, plant and biofuel production, and aesthetic value. As well, hydroponic culture is useful to quickly indicate problems or sensitivities to nutrient deficiencies (or toxicities) due to direct contact of nutrients (in solution) with plant roots.

However, re-use of liquid wastes also represent unique problems when re-used in soil-less applications. Physical issues include continual monitoring of the growing environment , provision of fertilizer sources, appropriate media for plant positioning and aeration, refilling of water due to evapotranspirational losses, threat of apparatus failure, high infrastructure costs, poor climatic conditions or inapproptriate timing during the year. Keeping in this context, non-physical issues include minimal buffer action , pH fluctuation, nutrient imbalance and fluctuating levels, form of nitrogen, soluble salts and excessive electrical conductivity level, presence of phytotoxic or degradatory compounds (eg root exudation compounds) and heavy metals, residual dissolved organic carbon, low disease immunity, lack of soil micro-organisms and potential heavy metal partitioning to toxic levels. The overall interactive effect of these issues often leads to differences in biomass production between tops and roots of plants, or further symptoms of stress.

Research in this area is ongoing, but generally lacking or preliminary. For example, the use of zeolites, struvite or effective micro-organisms are unique and recent initiatives. Research comparing growth response and mineral nutrient status of plant growth in hydroponic culture will be detailed to primarily discuss the aforementioned ‘problematic’ issues. Recent research findings will be discussed to better evaluate the role of these issues in soil-less environments.


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