Challenges in Treatment of Digestate Liquid Fraction from Biogas Plant. Performance of Nitrogen Removal and Microbial Activity in Activated Sludge Process

Aleksandra Chuda,Krzysztof Ziemiński

doi:10.3390/en14217321

Abstract

Even thoughdigestate, which is continually generated in anaerobic digestion process, can only be used as fertilizer during the growing season, digestate treatment is still a critical, environmental problem. That is why the present work aims to develop a method to manage digestate in agricultural biogas plant in periods when its use as fertilizer is not possible. A lab-scale system for the biological treatment of the digestate liquid fraction using the activated sludge method with a separate denitrification chamber was constructed and tested. The nitrogen load that was added tothe digestate liquid fraction accounted for 78.53% of the total nitrogen load fed into the reactor. External carbon sources, such as acetic acid, as well as flume water and molasses, i.e., wastewater and by-products from a sugar factory, were used to support the denitrification process. The best results were obtained using an acetic acid and COD (Chemical Oxygen Demand)/NO3–N (Nitrate Nitrogen) ratio of 7.5. The removal efficiency of TN (Total Nitrogen), NH4–N (Ammonia Nitrogen) and COD was 83.73%, 99.94%, 86.26%, respectively. It was interesting to see results obtained that were similar to those obtained when using flume water and COD/NO3–N at a ratio of 8.7. This indicates that flume water can be used as an alternative carbon source to intensify biological nitrogen removal from digestate.

Highlights

The increase in organic waste generated in food industry plants observed in recent years and the emphasis on sustainability has significantly contributed to the growing interest in anaerobic digestion as a method for waste management
The most commonly used biological method in industrial wastewater treatment plants is still a conventional activated sludge process that involves autotrophic nitrification and heterotrophic denitrification [13,14,15]. The stability of this process and the treatment outcome depend on the microbial community structure, the amount of functional groups belong to the activated sludge, their activity (Nitrate Utilization Rate—NUR, Ammonia Utilization Rate—AUR, Oxygen Utilization Rate—OUR), and the ratio of AOB (Ammonia–Oxidizing Bacteria) to NOB (Nitrite–Oxidizing Bacteria) [14,16,17], while the microbial activity is influenced by operating parameters, such as pH, temperature, and oxygen concentration as well as the nitrogen and COD loading rate and COD/TN ratio in the influent [1,13,15,18]
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Summary

Introduction

The increase in organic waste generated in food industry plants observed in recent years and the emphasis on sustainability has significantly contributed to the growing interest in anaerobic digestion as a method for waste management. Due to the moderate effectiveness and high costs of these methods, which can range between 5.4–7.0 EUR/m3 of digestate for membrane drying and stripping, their full-scale applications are limited [7,9] This is the reason why if the agricultural management of digestate is not possible, its biological treatment, which is regarded as the most economical process for nitrogen removal, may be a rational solution [7,10]. The problems associated with the fragile resistance of anammox bacteria to environmental changes, the low growth rate of these bacteria, and limits placed on maximum nitrogen removal efficiency constitute major obstacles to the wide range application for these processes [12] For this reason, the most commonly used biological method in industrial wastewater treatment plants is still a conventional activated sludge process that involves autotrophic nitrification and heterotrophic denitrification [13,14,15]. Several factors have to be considered when selecting a carbon source, including the costs, denitrification rate, degree of utilization, sludge production, and adaptation time of the activated sludge [19,20]

Methods

Results

Conclusion