Nitrogen management in nitrification-hydroponic systems by utilizing their pH characteristics

Abstract

In the past decade, a wave of articles was published on the liquid fertilizer by-product from the anaerobic-digestion-of-biomass process. This is not surprising given that the fertilizer (termed “digestate”) is highly nutrient dense and is produced at low cost from waste biomass. Digestate has been proposed as an alternative hydroponic nutrient solution, provided that a fraction of the ammonium in the digestate be converted to nitrate. Nitrification bacteria are abundant and they readily accomplish this task upon aeration of the digestate. Although much attention has been paid to these nitrification-hydroponic systems, the field is still within its infancy and standard control methodologies have not yet been established. In traditional hydroponics, nutrient concentrations are controlled through electrical conductivity measurements, but the imbalance of nutrients and the high salt content of the digestate makes this method ill-suited for nitrification-hydroponic systems. These systems have pronounced pH responses, however, which provide information about the nutrient uptake characteristics of the plants and the nitrifying bacteria. In this study, these pH characteristics were used to control the total nitrogen concentration in solution. This was accomplished by controlling the pH via hydroxide and ammonium dosing (synthetic digestate) at a ratio of 1.5 molar. High plant growth rates were maintained at nitrogen concentrations 10 times lower than in conventional operation, thus improving the system’s nitrogen use efficiency proportionally. Given the growth rate of this sector and its nutrient pollution potential, the results serve as steppingstones on the road to improved utilization of digestate as hydroponic feed solutions.