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Nutrient solubilization and its availability following anaerobic digestion
Chirag M Mehta1, Damien J Batstone
1Advanced Water Management Centre, The University of Queensland, St Lucia, Australia. c.mehta@uq.edu.au
Summary
Nutrient solubilization in agricultural waste during anaerobic digestion (AD) is key for recovery. Most phosphorus, calcium, and magnesium were insoluble, hindering recovery, but nitrogen and potassium remained soluble.
Area of Science:
- Agricultural Science
- Environmental Engineering
- Biogeochemistry
Background:
- Nutrient recovery from agricultural waste is crucial for sustainable resource management.
- Anaerobic digestion (AD) is a common treatment for agricultural waste, but nutrient recovery from digestate can be challenging.
- Phosphorus (P) recovery, particularly via struvite crystallization, requires P to be in a soluble form.
Purpose of the Study:
- To investigate the solubilization of key elements (P, N, K, Ca, Mg) during the anaerobic digestion of solid agricultural waste.
- To assess the impact of AD conditions on nutrient solubility and bioavailability.
- To identify factors limiting post-digestion nutrient recovery.
Main Methods:
- Batch anaerobic digestion (AD) conducted at mesophilic conditions (37 °C) and controlled pH (7.0 ± 0.2).
- Analysis of solid and liquid phases of digestate from piggery and poultry waste.
- Bioavailability testing using citric acid extraction to determine nutrient availability.
- Investigation of nutrient solubility across a range of pH conditions.
Main Results:
- Less than 10% of total P, Ca, and Mg were soluble in the digestate; N and K remained largely soluble.
- P, Ca, and Mg in the digestate were found to be totally bioavailable via citric acid extraction.
- Complete solubilization of P, Ca, and Mg was observed only below a pH threshold of 5.5.
- Nutrients likely became bound or adsorbed to solid phases post-release during digestion, reducing aqueous concentration.
Conclusions:
- Standard mesophilic AD conditions (pH 7.0) limit the solubility of P, Ca, and Mg, hindering their recovery through methods like struvite crystallization.
- Strategies such as adding complexing agents, operating at lower pH, or modifying AD conditions are necessary to enhance nutrient solubilization and recovery.
- Optimizing AD processes for nutrient recovery requires careful control of pH and potentially the use of chemical additives to maintain nutrient solubility.
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