Organic degrading bacteria and nitrifying bacteria stimulate the nutrient removal and biomass accumulation in microalgae-based system from piggery digestate

Highlights

• Raw piggery digestate was used for Desmodesmus sp. CHX1 cultivation.

• Inoculation with nitrifying bacteria showed better performance.

• Microalgae aggregating bacteria dominated in the optimized systems.

Abstract

The microalgae-based system has been applied in anaerobic digestate treatment for nutrient removal and biomass production. To optimize its performance in treating piggery digestate, here, commercial bacterial agents, including organic degrading bacteria (Cb) and nitrifying bacteria (Nb), were inoculated into the microalgae-based system dominated by Desmodesmus sp. CHX1 (D). Reactor DN (inoculated with D and Nb) and DCN (inoculated with D, and Cb to Nb at a ratio of 1:2) have better performance on NH₄⁺-N removal, with a final efficiency at 40.26% and 39.87%, respectively, and no NO₃⁻-N or NO₂⁻-N accumulations. The final total chlorophyll concentration, an indicator of microalgal growth, reached 4.74 and 5.47 mg/L in DN and DCN, respectively, three times more than that in D. These results suggested that high NH₄⁺-N removal was achieved by the assimilation into high microalgal biomass after the inoculation with functional bacteria. High-throughput sequencing showed that the richness of microbial community decreased but the evenness increased by inoculating functional microorganisms. Microalgae aggregating bacteria were Cellvibrio, Sphingobacterium, Flavobacterium, Comamonas, Microbacterium, Dyadobacter, and Paenibacillus. This study revealed that the inoculation with functional bacteria reconstructed the microbial community which benefited for the microalgal growth and nutrient removal, providing a promising strategy for treating highly-concentrated digestate.

Introduction

Microalgae-based system has been applied in waste stabilization ponds for several decades, which can not only remove nutrients from wastewater but also can produce algal biomass as an important raw material for biodiesel and animal feed (Katiyar et al., 2017). Microalgae have a 10–50 folds higher photosynthesis potential than terrestrial plants and an extensive adaptability of diverse wastewater, making it practical to combine the microalgae cultivation and wastewater treatment (Hom-Diaz et al., 2017). Indeed, various types of wastewater have been successfully utilized for microalgae cultivation to recover nutrients and produce biomass such as municipal wastewater (Su et al., 2011), seafood processing wastewater (Gao et al., 2018), and anaerobic digestate (Singh et al., 2011), where the biomass production was even higher than that in microalgal culture media (Luo et al., 2016).

Anaerobic digestate is a byproduct of biogas fermentation with high concentrations of nutrients like nitrogen and phosphorous (Xia and Murphy, 2016). It can be applied as agricultural fertilizer, but limited by the seasonality of crops and long distance of farmlands (Solé-Bundó et al., 2017). Additionally, removal of nutrients in anaerobic digestate through conventional wastewater treatment process requires a high cost and complex process like Anaerobic-Anoxic-Oxic. Recently, microalgae-based system has been applied to recover the nutrients in anaerobic digestate. Uggetti et al reported that digestate was an efficient substrate for microalgae cultivation, where the high concentration of digestate increased the biomass accumulation (Uggetti et al., 2014). Marcilhac et al. found that phosphorous concentration would not affect the nitrogen assimilation into microalgae when cultivated with digestate (Marcilhac et al., 2015). However, the high ammonia in anaerobic digestate is detrimental to microalgal growth because of its toxicity, usually at NH₄⁺-N > 100 mg/L (Parakh et al., 2019, Prandini et al., 2016). The general strategy to alleviate the toxicity of digestate is dilution, whereas a large amount of water are needed to afford the dilution with 10–20 folds (Wang et al., 2015). Microalgal-bacterial consortia was reported enhancing the overall uptake of nutrients and resistant with high ammonia concentration (Gonçalves et al., 2017), which is also a good way to protect microalgae away from high NH₄⁺-N toxicity. Naturally, the self-cleaning is always accomplished by the consortia of microalgae and prokaryotes (Ji et al., 2018). Su et al. used a wastewater-born and settleable algal-bacterial culture to treat municipal wastewater and produce microalgae biomass simultaneously, and found that the removal efficiencies of chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN) and total phosphorous (TP) were 98.2%, 88.3% and 64.8% within 8 days, respectively, along with the total increased suspended solid (TSS) (Su et al., 2011). Although natural consortia are easily obtained, the establishment of consortia is slow and may bring about some unexpected bacteria with negative influence. Conversely, artificial inoculation with high-performance species promotes microalgal growth via bioaugmentation (Yu and Mohn, 2002). A novel algal-bacterial symbiosis system by inoculating aerobic sludge was developed for enhancing nutrients (N, P) removal from domestic wastewater, of which the results showed that the TN and TP removal increased by 7.64% and 50.13%, and the chlorophyll a (Chl-a) accumulation in biofilm increased to 4.80 ± 0.08 mg/g (Tang et al., 2018). However, challenges, including poor identification of functional species and weak control of the community composition, still await resolution.

Microbial community in a microalgae-based wastewater treatment system is complex. A greater understanding of the symbiotic interactions between microalgae and prokaryotes is required for optimizing system performance (Wang et al., 2018). Bacteria belonging to Flavobacterium, Micropruina and Comamonadaceae, and microalgae related to Chlorella and Scenedesmus, were found responsible for the high efficient nutrient removal in a domestic wastewater treatment (Tang et al., 2018). Recently, activated sludge and commercial microbial agent were used for bio-augmentation of microalgae-based wastewater treatment, which make the improvement of both biomass production and effluent quality, as well as good control of microbial community and short time of start-up (Luo et al., 2019, Praveen et al., 2018).

In this work, two commercial bacterial agents organic degrading bacteria (Cb) and nitrifying bacteria (Nb) were inoculated into the microalgae-based system treating raw piggery digestate. Different ratios of bacterial agents were first set to optimize nutrients removal. Then the optimized systems were selected for further investigations including biomass production and nutrients removal especially inorganic nitrogen. Finally, 16S rRNA gene sequencing was used to identify the dominant bacteria and reveal their role in the optimized system with better performance. In doing so, we successfully construct an artificial microalgal-bacterial consortium possessing high nutrient removal and biomass production, and explain the underlying mechanism of the optimized performance.