Anaerobic digestion is increasingly being considered a viable method for sustainable waste management in industry. Anaerobic digestion of industrial effluents is used to convert organic material that is at concentrations that are too low for economic recovery into methane gas. The process involves the breakdown of organic effluent materials using minerals and metals in a bio-available form. To date, research in the field of anaerobic digestion has been predominantly experimental and empirical. Currently, no studies provide guidelines for predicting the micro-nutrient requirements for a particular application beyond the general micro-nutrient-to-COD ratios.

Therefore, the WRC commissioned the WASH R&D Centre to develop a theoretical model to describe the partitioning of micro-nutrients between soluble, precipitate and potentially bound and adsorbed phases, which can be used to determine the amount of the micro-nutrient available for anaerobic digestion and therefore can be used to predict the microbial response to different micronutrient dosing strategies. The WASH R&D Centre has previously engaged in research to investigate micro-nutrient dosing requirements for anaerobic digestion, which resulted in a technique for assessing the fate of metals within the mixed liquor of an anaerobic digester. However, the research did not attempt to model the organically bound or adsorbed metals in the anaerobic liquor, nor was the relationship between metal concentration and anaerobic activity clearly elucidated.

This research has several aims, including 1) to develop a dynamic physico-chemical model of micro-nutrient partitioning in anaerobic mixed liquor; 2) to experimentally quantify the micro-nutrient concentration in different phases in an anaerobic sequencing batch reactor; 3) to undertake preliminary correlation analysis between concentrations of micro-nutrient in the different phases of anaerobic mixed liquor and anaerobic activity; 4) to quantify the departure of the solution inorganic ion concentrations from equilibrium concentrations during dynamic experiments; and 5) to develop a procedure for identifying non-bioavailable micro-nutrient sinks in different phases of anaerobic mixed liquor and for proposing dosing strategies to minimise micro-nutrient dose and sequestration of micro-nutrients into these sinks.

The expected outcome of this research is a modelling tool to assist in the design of anaerobic digestion applications for treatment of micro-nutrient limited industrial effluents. The resultant model may also be used to troubleshoot existing installations and to evaluate the dosing strategies of existing treatment systems. The model will consider the influence of all ionic species in anaerobic liquor and therefore will deal in an integrated way with macro-nutrients micro-nutrients and alkalinity since all of these factors influence partitioning and bio-availability of micro-nutrients. This would allow more realistic costing of anaerobic system design and operation, and may also reduce nutrient dosing costs in existing systems. Moreover, it is expected that the results of this research will enhance the appeal of anaerobic digestion as a sustainable effluent management option with beneficial reuse of waste through energy recovery.