Single-sludge denitrification

Through single-sludge denitrification technology internally produced carbon are used to fuel the denitrification process. Recent work has shown potential in freshwater recirculating aquaculture systems, however knowledge on hydrolysis rates and compositions of bioavailable organic compounds under brackish and saltwater conditions are sparse.

Based on input-models, predictions of available carbon and nitrate can be made, providing information on the theoretically available C:N. The practically available C:N is less, since a large part of the organic matter in effluents from recirculating aquaculture systems is found in particulate form, not readily available for the denitrifying bacteria. 

Through a side stream incomplete anaerobic digestion process (hydrolysis) the organic matter can be transformed into readily available forms (VFAs and alcohols). Recent studies have demonstrated that only 20-25% of the particulate organic matter is transformed into readily available carbon sources and therefore BONUS CLEANAQ aims to optimize the hydrolysis process to achieve higher yields.

Different methods have previously been tested in freshwater sludge systems to increase the yields. e.g. positive temperature effects, effects of alkaline conditions and positive effects of hydrogen peroxide pre-treatment. These approaches will be investigated and evaluated in BONUS CLEANAQ.

Experimental set up for sludge test

Experimental set-up to test sludge from aquaculture water at different salinities. The effect of different environmental conditions (pH: 5,7, and 9; temperature: 10, 30, and 60⁰C), chemical treatment with hydrogen peroxide and fluctuating redox conditions (oxic/anoxic) on the yield of readily available carbon sources per amount of waste.

A C:N ratio of 4-6 (TCOD/NO3-N) is often stated as being required for optimal denitrification rates in real-life. However, lower optimum C:N ratios have been reported when using either a mixture of VFAs (2.37) or specific VFAs (1.72 - 2.05) as carbon-source. Using the readily available carbon sources produced from the fish waste may thus reduce the C:N ratio actually required providing increased denitrification rates compared to e.g. methanol, often used as an external carbon source in aquaculture.

These findings reported for wastewater (and freshwater) treatment will be investigated in the present project in order to quantify and evaluate how the different readily available carbon sources produced from fish waste influences denitrification in saline/brackish waters in terms of optimal C:N-ratio, denitrification rates and sludge production. The results will allow an optimized use of a residual resource (sludge) for removing nitrogen in effluents from recirculating aquaculture systems.
21 JUNE 2021