Anaerobic Digestion Process: Nutrient Transformations & Management Tips

1. Pius Ndegwa Nutrient Management & Air Quality Specialist Biological Systems Engineering Washington State University AD101 – Nutrient Transformations, Nutrient Management, and Benefits

2. Anaerobic Digestion Process Types of AD Systems Nutrient Transformations Nutrient Availability & Management Benefits AD101 – Overview

3. AD101 – Anaerobic Digestion Process Organic Matter Protein Insoluble organic or inorganic compounds & Water Carbohydrate

4. Anaerobic bacteria break down or "digest" organic material in the absence of oxygenand produce "biogas" as a waste product Temperature. Anaerobic bacteria communities can endure temperatures ranging from below freezing to above 135°F (57°C), but they thrive best at temperatures of about 98°F (37°C) (mesophilic) and 130°F (54°C) (thermophilic) For AD operated in the thermophilic range, digestion and biogas production is faster than in the mesophilic range. However, the process is highly sensitive to disturbances, such as changes in feed materials or temperature. On the other hand, the AD operated in the mesophilic range must be larger (to accommodate a longer period of digestion within the reactor) but the process is less sensitive to upset or change in operating regimen. AD101 – Desirable AD Conditions

5. The AD pH. In most cases, the pH is self-regulating but bicarbonates are sometimes used to maintain consistent pH. Optimal pH range is between 6.8 to 8.5, i.e. slightly alkaline. Acid forming bacteria grow much faster than methane forming bacteria. This can reduce pH to unfavorable pH for methane forming bacteria thus inhibiting the activity of methanogens. This is referred to souring and may result in failure of the AD. AD101 – Desirable AD Conditions Reactor schematic: http://www.makingenergy.com/

6. Classification: Based on: Operational temperature or Design Operational Temperature: Low temperature digestion, commonly referred to mesophilic digestion (mediated by mesophile bacteria) occurs optimally between 37°-41°C or at ambient temperatures between 20°-45°C. High temperature digestion referred to thermophilic digestion (mediated by thermophile bacteria) takes place optimally at between 50°-52° and up elevated temperatures up to 70°C. AD101 – Types of AD Systems

7. Designs Classification AD101 – Types of AD Systems Reactor schematic: http://www.makingenergy.com/

8. AD101 – What Determines Choice of Design? http://www.rcminternationalllc.com http://www.rcminternationalllc.com http://www.cadyinc.com Reactor schematic: http://www.makingenergy.com/

9. The plug flow digester is best suited for handling manure with a solids content of 11% to 13% A complete mix digester operates best with a solids content of 3% to 10% Lagoon typically operates on very dilute waste streams of approximately 3% AD101 – Manure Collection and Handling System! Source: http://www.makingenergy.com Source: http://www.menschmfg.com

10. AD101 – Nutrient Transformations

11. AD101 – Nutrient Transformations

12. The process of AD bio-converts organic nutrients into inorganic nutrients Organic-Nitrogen (proteins) converts into ammonium-Nitrogen and biogas Organic-Phosphorus (lipids) is converted to Ortho-Phosphorus and biogas Both Ammonium-N and Ortho-P are both plant available, i.e. AD results in more bio-available plant nutrients. Implication: Application timing is an issue and should be carefully planned. Total Quantities: Total amounts of vital nutrients entering the AD process remains the same, i.e. nutrient are conserved during the AD process. Potassium (K): Does not undergo any transformations during AD of manure. AD101 – Nutrient Transformations

13. AD101 – Nutrient Availability & Management

14. The actual nutrient value of AD effluent depends two factors: The nutrient value of the raw manure, which in turn depends on diet, species of animal, method of collection, and storage of manure. The extent or completeness of digestion process. Chemical analysis or an equivalent analysis of each effluent should be obtained before applying manure to a field. AD101 – Nutrient Contents in AD Effluents

15. In general, availabilities (to plants) of N and P are increased during anaerobic digestion via mineralization of respective organic components. However, there can be considerable loss of N from the more volatile ammonium (NH4+) and ammonia (NH3)-N species that could reduce eventual N availability depending effluent storage. P and K on the other hand are not subject to volatilization loss! In lagoon storage of the effluent, significant P could settle with the sludge and thus reduce it availability in the lagoon liquid. Bottom Line: Chemical analyses is essential for proper nutrient management of the digester effluents. AD101 – Nutrient Availability and Management

16. Recap: AD Pros and Cons • Pros • Produces usable products: Biogas & nutrient-rich effluent • Reduce solids or biomass (compared to Aerobic process) • Reduces odors • Reduces emissions of greenhouse gases • Converts N and P to more plant available forms • Cons • Expensive to install • Requires additional resources to operate and maintain • By itself, does NOT address excess nutrient problem • Could result in excessive ammonia emissions

17. Thank You!Questions?