Algal biomass production and nutrient removal from high strength anaerobic digestate

Abstract

The integration of microalgal biomass production with nutrient removal from the liquid portion of anaerobic digestate holds the potential to close the loop on waste. However, algal growth inhibition in anaerobic digestate has greatly suppressed the development of growing microalgae in anaerobic digestate at scale. Typically, 10-50 fold dilution were used to overcome the inhibition in lab scale studies which tried to grow microalgae in anaerobic digestate, but it is not cost-effective using dilution as the primary approach for inhibition alleviation in large scale algae-digestate treatment systems when considering the expansion of reactor volume, the large amount of freshwater usage, and the increased land occupancy. This dissertation focuses on alleviating algal growth in anaerobic digestate by a non-dilution biological pretreatment process. The algal inhibitory effects from anaerobic effluent were observed shortly after the attempts of growing microalgae in the digestate. The inhibition was severe and ubiquitous for a variety of microalgae in different types of anaerobic digestate based on our own work. The most common hypothesis for inhibition on digestate is the high total ammonia nitrogen (TAN) in the digestates. However, TAN inhibition did not fully explain the observations from algal growth in anaerobic digestate. High ammonium tolerating algae strain such as Chlorella sorokiniana can be robust in a chemical medium with 3500 mg L-1 ammonium. Moreover, the meta-analysis also revealed relationships between cultivation factors (e.g., light intensity, solid-separation, initial TAN, dilution factor, axenic condition etc.) and algal productivity in anaerobic digestate. Interestingly, neither TAN nor dilution were significant factors. In contrast, the use of chemical or biological pretreatment of digestate, solids removal, increased light intensity, and lower pH also resulted in significantly higher algal productivity. This analysis suggests that the development of non-dilution pretreatment approaches is essential for scale-up of algae-digestate treatment systems. Biological wastewater treatment such as activated sludge is a relatively mature technique in most wastewater treatment plants. The use of aerobic bacteria can be effective in removing organic and inorganic pollutants. First objective in this dissertation was to use aerobic bacteria as a pretreatment process for anerobic digestate before the inoculation of microalgae. A consortium of bacteria obtained from an activated sludge wastewater treatment plant was used to pretreat digestate prior to algae growth. No dilution of digestate was used. C. sorokiniana achieved very high biomass productivity of 250-500 mg L-1 day-1 in bacteria-pretreated municipal sludge digestate and food waste digestate whereas little to negative productivity was observed in the digestates without pretreatment. Pretreatment also led to significant increase in nutrient removal rate compared to the non-pretreated ones. The second objective of this research was to understand what cultivation factors contribute to successful pretreatment of digestate prior to algae growth… The performance of aerobic bacteria pretreatment for alleviating algal growth inhibition in undiluted anaerobic digestate was tested with two different strains of algae (C. sorokiniana and A. protothecoides) in two different strengths of anaerobic digestate. Both digestate types were obtained from a sludge digester at a municipal wastewater treatment plant but were collected at two different times: one digestate contained 1372 mg/L NH4-N (high strength) and the other contained 433 mg/L NH4-N (low strength). In high strength municipal sludge digestate, both strains of algae benefited from pretreatment, but in low strength digestate, the growth of C. sorokiniana was suppressed due to nutrient limitations. The performance test also revealed that longer pretreatment period generally had positive effect on alleviating algal growth inhibition from the digestate. Interestingly, the xenic (vs. axenic) condition was not a significant factor in this experimental result which is consistent with the result of multiple regression model from the meta-analysis study. Up until this point, model strains of algae were used in all experiments. However, such strains are unlikely to be practical in real-world systems given concerns about introducing non-native organisms to the environment. Consequently, the third objective of this dissertation was to adapt locally obtained consortia of algae to pretreated digestate and test the adapted community’s growth and nutrient removal performance. Local consortia were collected from local fishponds and the biofloc solids from Auburn University’s aquaponics system. The consortia were initially inoculated in 10% aerobic bacteria pretreated dairy manure anaerobic digestate, and gradually increased to 100% pretreated digestate. A complete restructuring of the algal community was observed in which the initial eukaryotic community was 95% Euglena the final was 70% Coelastrum with complete die-out of Euglena. Although the adapted consortium had 75% of the growth productivity of C. sorokiniana in the pretreated digestate, it did not grow in the non-pretreated digestate. This result reinforced the importance of digestate pretreatment for this native consortium. In summary, aerobic bacteria pretreatment is confirmed to be effective and critical for algal biomass production and nutrient removal in undiluted anaerobic digestate.