Sulfide inhibition to nitrifying bacteria has prevented the integration of digestate nitrification and biogas desulfurization to simplify anaerobic digestion systems. In this study, liquid digestate with NaHS solution was treated using nitrifying sludge in a sequential-batch reactor with a long fill period, with an ammonium loading rate of 293 mg-N L-1 d-1 and a stepwise increase in the sulfide loading rate from 0 to 32, 64, 128, and 256 mg-S L-1 d-1. Batch bioassays and microbial community analysis were also conducted with reactor sludge under each sulfide loading rate to quantify the microbial acclimatization to sulfide. In the reactor, sulfide was completely removed. Complete nitrification was maintained up to a sulfide load of 128 mg-S L-1 d-1, which is higher than that in previous reports and sufficient for biogas treatment. In the batch bioassays, the sulfide tolerance of NH4+ oxidizing activity (the 50% inhibitory sulfide concentration) increased fourfold over time with the compositional shift of nitrifying bacteria to Nitrosomonas nitrosa and Nitrobacter spp. However, the sulfur removal rate of the sludge slightly decreased, although the abundance of the sulfur-oxidizing bacteria Hyphomicrobium increased by 30%. Therefore, nitrifying sludge was probably acclimatized to sulfide not by the increasing sulfide removal rate but rather by the increasing nitrifying bacteria, which have high sulfide tolerance. Successful simultaneous nitrification and desulfurization were achieved using a sequential-batch reactor with a long fill period, which was effective in facilitating the present acclimatization.

A novel coupling process using an aerobic bacterial reactor with nitrification and sulfur–oxidization functions followed by a microalgal reactor was proposed for simultaneous biogas desulfurization and anaerobic digestion effluent (ADE) treatment. ADE nitrified by bacteria has a potential to be directly used as a culture medium for microalgae because ammonium nitrogen, including inhibitory free ammonia (NH₃), has been converted to harmless NO₃⁻. To demonstrate this hypothesis, Chlorella sorokiniana NIES-2173, which has ordinary NH₃ tolerance, i.e., 1.6 mM of EC₅₀ compared with other species, was cultivated using untreated/treated ADE. Compared with the use of a synthetic medium, when using ADE with 1–10-fold dilutions, the specific growth rate and growth yield maximally decreased by 44% and 88%, respectively. In contrast, the algal growth using undiluted ADE treated by nitrification–desulfurization was almost the same with using synthetic medium. It was also revealed that 50% of PO₄³⁻ and most metal concentrations of ADE decreased following nitrification–desulfurization treatment. Moreover, upon NaOH addition for pH adjustment, the salinity increased to 0.66%. The decrease in metals mitigates the bioconcentration of toxic heavy metals from wastewater in microalgal biomass. Meanwhile, salt stress in microalgae and limiting nutrient supplementation, particularly for continuous cultivation, should be of concern.

Removal of dissolved oxygen from algal photobioreactors is essential for high productivity in mass cultivation. Gas-permeating photobioreactor that uses hydrophobic membranes to permeate dissolved oxygen (pervaporation) from its body itself is an energy-efficient option for oxygen removal. This study comparably evaluated the characteristics of various commercial membranes and determined the criteria for the selection of suitable ones for the gas-permeating photobioreactors. It was found that oxygen permeability is limited not by that in the membrane but in the liquid boundary layer. Membrane thickness had a negative effect on membrane oxygen permeability, but the effect was as minor as less than 3% compared with the liquid boundary layer. Due to this characteristic, the lamination of nonwoven fabric with the microporous film did not significantly decrease the overall oxygen transfer coefficient. The permeability in the liquid boundary layer had a significantly positive relationship with the hydrophobicity. The highest overall oxygen transfer coefficients in the water-to-air and water-to-water oxygen removal tests were 2.1±0.03 ×10-5 and 1.39±0.09 ×10-5 m s-1, respectively. These values were considered effective in the dissolved oxygen removal from high-density algal culture to prevent oxygen inhibition. Furthermore, hydrophobicity was found to have a significant relationship also with water entry pressure, which needs to be high to avoid culture liquid leakage. Therefore, these results suggested that a microporous membrane with strong hydrophobicity laminated with nonwoven fabric would be suitable characteristics for gas-permeating photobioreactor.