Photoautotrophic removal of hydrogen sulfide from biogas using purple and green sulfur bacteria.

Biogas desulfurization based on anoxygenic photosynthetic processes represents an alternative to physicochemical technologies, decreasing the risk of O2 and N2 contamination. This work aimed at assessing the potential of Allochromatium vinosum and Chlorobium limicola for biogas desulfurization under different light intensities (10 and 25 klx) and H2S concentrations (1 %, 1.5 % and 2 %) in batch photobioreactors. In addition, the influence of rising biogas flow rates (2.9, 5.8 and 11.5 L d-1 in stage I, II and III, respectively) on the desulfurization performance in a 2.3 L photobioreactor utilizing C. limicola under continuous mode was assessed. The light intensity of 25 klx negatively influenced the growth of A. vinosum and C. limicola, resulting in decreased H2S removal capacity. An increase in H2S concentrations resulted in higher volumetric H2S removal rates in C. limicola (2.9-5.3 mg L-1 d-1) tests compared to A. vinosum (2.4-4.6 mg L-1 d-1) tests. The continuous photobioreactor completely removed H2S from biogas in stage I and II. The highest flow rate in stage III induced a deterioration in the desulfurization activity of C. limicola. Overall, the high H2S tolerance of A. vinosum and C. limicola supports their use in H2S desulfurization from biogas.

Assessment of the performance of an anoxic-aerobic microalgal-bacterial system treating digestate.

The performance of an anoxic-aerobic microalgal-bacterial system treating synthetic food waste digestate at 10 days of hydraulic retention time via nitrification-denitrification under increasing digestate concentrations of 25%, 50%, and 100% (v/v) was assessed during Stages I, II and III, respectively. The system supported adequate treatment without external CO2 supplementation since sufficient inorganic carbon in the digestate was available for autotrophic growth. High steady-state Total Organic Carbon (TOC) and Total Nitrogen (TN) removal efficiencies of 85-96% and 73-84% were achieved in Stages I and II. Similarly, PO43--P removals of 81 ± 15% and 58 ± 4% were recorded during these stages. During Stage III, the average influent concentrations of 815 ± 35 mg TOC·L-1, 610 ± 23 mg TN·L-1, and 46 ± 11 mg PO43--P·L-1 induced O2 limiting conditions, resulting in TOC, TN and PO43--P removals of 85 ± 3%, 73 ± 3%, and 28 ± 16%, respectively. Digestate concentrations of 25% and 50% favored nitrification-denitrification mechanisms, whereas the treatment of undiluted digestate resulted in higher ammonia volatilization and hampered nitrification-denitrification. In Stages I and II, the microalgal community was dominated by Chlorella vulgaris and Cryptomonas sp., whereas Pseudoanabaena sp. was more abundant during Stage III. Illumina sequencing revealed the presence of carbon and nitrogen transforming bacteria, with dominances of the genera Gemmata, Azospirillum, and Psychrobacter during Stage I, II, and III, respectively. Finally, the high settleability of the biomass (98% of suspended solids removal in the settler) and average C (42%), N (7%), P (0.2%), and S (0.4%) contents recovered in the biomass confirmed its potential for agricultural applications, contributing to a closed-cycle management of food waste.

A systematic optimization of piggery wastewater treatment with purple phototrophic bacteria.

The increase in natural water bodies pollution caused by intensive animal farming requires the development of innovative sustainable treatment processes. This study assessed the influence of piggery wastewater (PWW) load, air dosing, CO2/NaHCO3- supplementation and pH control on PWW treatment by mixed cultures of purple phototrophic bacteria (PPB) under infrared radiation in batch photobioreactors. PPB was not able to grow in raw PWW but PWW dilution prevented inhibition and supported an effective light penetration. Despite the fact that PPB were tolerant to O2, carbon recovery decreased in the presence of air (induced by stripping). CO2 supplementation was identified as an effective strategy to maximize the removal of carbon during PPB-based PWW treatment with removal efficiencies of 72% and 74% for TOC and VFAs. However, the benefits derived from CO2 addition were induced by the indirect pH control exerted in the cultivation medium. Thus, PPB supported an optimal pollutant removal performance at pH 7, with removal efficiencies of 75%, 39% and 98% for TOC, TN and VFAs.