| dc.description.abstract | Microalgal treatment of anaerobic digestate has attracted research attention due to algae’s capability for nutrient removal and recovery. However, microalgae rarely tolerate full-strength anaerobic digestate, and this problem is frequently handled with dilution. According to the literature, it has been hypothesized that high ammonia, turbidity, and heavy metals are the main inhibitors. However, our previous research suggested that additional inhibitors may be even more important. In past work, we developed a pretreatment method using aerobic bacteria that alleviated algal inhibition, and we hypothesized that inhibitory organic compounds are present in digestate. This approach enables algae to reach very high culture densities before nutrients are exhausted from the digestate. When algae reach a high density, light penetration diminishes, slowing down or halting growth before all the nutrients are fully removed from the digestate. This results in incomplete nutrient recovery and limits the efficiency of the process. To address this issue, implementing multi-stage algae cultivation can help maintain optimal growth conditions, allowing for nutrient removal to occur over multiple batches of algae growth and enhancing the overall effectiveness of the treatment system.
In the first chapter of this study, we aim to 1) develop two different pretreatment strategies (activated sludge and biochar pretreatment) and test their alleviation of inhibition, 2) determine metabolites that are removed during pretreatment using LCMS-MS and 3) Analyze the inhibitory effect of identified metabolites on the growth of algae using a dose-response approach. Both pretreatments significantly enhanced biomass productivity (412 mg L-1 d-1 after activated sludge pretreatment, 292 mg L-1 d-1 in biochar pretreatment, and only 40 mg L-1 d-1 without pretreatment, P < 0.05). Additionally, total nitrogen removal efficiency was 63%, 21%, and 5% with activated sludge pretreatment, biochar pretreatment, and no treatment, respectively. LCMS-orbitrap analysis revealed the removal of many phenolic compounds, antibiotics, and animal hormone metabolites after pretreatment, and we showed that six of the twelve compounds tested are inhibitory to algae at < 1 mg L-1 levels in dose-response trials (Butylparaben, Salicylic acid, Bisphenol A, Bisphenol F, Tiamulin, Androsterone).
In the second chapter, we developed and tested a two-stage semi-continuous reactor system (two sequencing reactors with clarifiers) for long-term algal cultivation in pretreated anaerobic digestate, aiming to enhance nutrient removal efficiency and biomass yield over single-stage systems. Results indicate that the multi-stage reactor setup improves nutrient recovery and mitigates light limitations due to high cell density, a common limitation in single-stage systems. This study achieved some of the highest algae productivities ever achieved on full-strength anaerobic digestate (0.59-0.78 g L-1 d-1 and 0.15-0.92 g L-1 d-1 for stages 1 and 2, respectively). Plus, adding a second stage of algae cultivation enabled significant (P<0.05) additional N and P removal, resulting in an unprecedented 95% reduction in influent NH4-N, reducing the value from ~2,400 mg L-1 to <200 mg L-1. P was also reduced from ~550 mg L-1 to ~100 mg L-1 across the multi-stage process.
Overall, this study suggests that organic compounds in digestate are important inhibitors of algal growth and shows the effectiveness of the two pretreatments in removing them. This study also contributes to developing sustainable waste-to-resource strategies, highlighting the potential of algae cultivation to treat nutrient-rich waste streams while producing high-value biomass | en_US |
