Microbiology community changes during the start-up and operation of a photosynthetic bacteria-membrane bioreactor for wastewater treatment

Material biosynthesis, mechanism regulation and resource recycling of biomass and high-value substances from wastewater treatment by photosynthetic bacteria: A review

The environmental-friendly and economic benefits generated from photosynthetic bacteria (PSB) wastewater treatment have attracted significant attention. This process of resource recovery can produce PSB biomass and high-value substances including single cell protein, Coenzyme Q₁₀, polyhydroxyalkanoates (PHA), 5-aminolevulinic acid, carotenoids, bacteriocin, and polyhydroxy chain alkyl esters, etc. for application in various fields, such as agriculture, medical treatment, chemical, animal husbandry and food industry while treating wastewaters. The main contents of this review are summarized as follows: physiological characteristics, mechanism and application of PSB and potential of single cell protein (SCP) production are described; PSB wastewater treatment technology, including procedures and characteristics, typical cases, influencing factors and bioresource recovery by membrane bioreactor are detailed systematically. The future development of PSB-based resource recovery and wastewater treatment are also provided, particularly concerning PSB-membrane reactor (MBR) process, regulation of biosynthesis mechanism of high-value substances and downstream separation and purification technology. This will provide a promising and new alternative for wastewater treatment recycling.

Purple phototrophic bacteria are outcompeted by aerobic heterotrophs in the presence of oxygen

There is an ongoing debate around the effect of microaerobic/aerobic conditions on the wastewater treatment performance and stability of enriched purple phototrophic bacteria (PPB) cultures. It is well known that oxygen-induced oxidative conditions inhibit the synthesis of light harvesting complexes, required for photoheterotrophy. However, in applied research, several publications have reported efficient wastewater treatment at high dissolved oxygen (DO) levels. This study evaluated the impact of different DO concentrations (0-0.25 mg·L^-1, 0-0.5 mg·L^-1 and 0-4.5 mg·L^-1) on the COD, nitrogen and phosphorus removal performances, the biomass yields, and the final microbial communities of PPB-enriched cultures, treating real wastewaters (domestic and poultry processing wastewater). The results show that the presence of oxygen suppressed photoheterotrophic growth, which led to a complete pigment and colour loss in a matter of 20-30 h after starting the batch. Under aerobic conditions, chemoheterotrophy was the dominant catabolic pathway, with wastewater treatment performances similar to those achieved in common aerobic reactors, rather than those corresponding to phototrophic systems (i.e. considerable total COD decrease (45-57% aerobically vs. ± 10% anaerobically). This includes faster consumption of COD and nutrients, lower nutrient removal efficiencies (50-58% vs. 72-99% for NH₄⁺-N), lower COD:N:P substrate ratios (100:4.5-5.0:0.4-0.8 vs. 100:6.7-12:0.9-1.2), and lower apparent biomass yields (0.15-0.31 vs. 0.8-1.2 g COD_biomass·g COD_removed^-1)). The suppression of photoheterotrophy inevitably resulted in a reduction of the relative PPB abundances in all the aerated tests (below 20% at the end of the tests), as PPB lost their main competitive advantage against competing aerobic heterotrophic microbes. This was explained by the lower aerobic PPB growth rates (2.4 d^-1 at 35 °C) when compared to common growth rates for aerobic heterotrophs (6.0 d^-1 at 20 °C). Therefore, PPB effectively outcompete other microbes under illuminated-anaerobic conditions, but not under aerobic or even micro-aerobic conditions, as shown by continuously aerated tests controlled at undetectable DO levels. While their aerobic heterotrophic capabilities provide some resilience, at non-sterile conditions PPB cannot dominate when growing chemoheterotrophically, and will be outcompeted.

Responses of microbial structures, functions, metabolic pathways and community interactions to different C/N ratios in aerobic nitrification

The responses of microbial structures, functional profiles and metabolic pathways during nitrification to four C/N ratios (0, 5, 10 and 15) were investigated in four parallel SBRs denoted as S₀, S₅, S₁₀, S₁₅. Results indicated that microbial diversities were affected by C/N ratios, while the same dominant taxa were observed, mainly including Proteobacteria, Betaproteobacteria, Rhodocyclales, Rhodocyclaceae, Zoogloea. The unique biomarkers were identified in each sludge sample through LEfSe analysis. Functional genera/enzymes responsible for removing organics and nitrogen coexisted in four SBRs at different abundances, except for that ammonia oxidizing bacteria (AOB) Nitrosomonas (0.33%-0.66%) and ammonia monooxygenase (amo) (9.4 × 10^-7-2.8 × 10^-6) were only detected in S₀. Moreover, PICRUSt analysis indicated similar overall patterns of metabolic pathways in four sludge samples. The network analysis revealed that total nitrogen removal positively correlated with hcp (Spearman's ρ of 0.853), and ammonia oxidizing rate was associated with amo (Spearman's ρ of 0.096).

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.

Elucidating salinity adaptation and shock loading on denitrification performance: Focusing on microbial community shift and carbon source evaluation

To understand the denitrification efficiency and microbial community shift with increasing salinity in salinity adaptation and shock loading process, nitrate (NO3--N), nitrite (NO2--N) and chemical oxygen demand (COD) removal efficiencies were monitored feeding acetate and primary sludge fermentation liquid. During adaptation process, salinity had little effect on NO3--N removal efficiency (>99.0%) with acetate-fed, while for fermentation liquid-fed, it decreased to around 97% at high salinity (>2.5%). Effluent NO2--N was lower than 0.1 mg/L, though obvious fluctuation of NO2--N was observed with fermentation liquid-fed when salinity change. During shock loading process, denitrification process all had slight decrease when the salinity abruptly increased to 5.0%. Traditional denitrifier of Thauera was the dominant genus, and a specialized microbial community of Azoarcus in salinity adaptation and Paracoccus in shock loading for denitrification showed high salinity tolerant. Meanwhile, microbial diversity was enriched with fermentation liquid-fed at high salinity condition.

Biokinetic and biotransformation of nitrogen during photosynthetic bacteria wastewater treatment

Photosynthetic bacteria (PSB) show a strong potential to degrade pollutants and meanwhile produce valuable biomass, thus realising both wastewater treatment and nutrient recovery. This paper detailed the study of nitrogen distribution forms, biokinetics and transformation during PSB wastewater treatment. The results showed that the total nitrogen (TN) and ammonium (NH₄⁺) removal reached 90.5% and 95.3%, respectively. The nitrogen removal and biomass production fit a quadratic and a cubic regression equation with an R² of 0.9793 and 0.9730, respectively. Mass balance showed that the influent nitrogen was transformed into valuable PSB cells (63.2%), gaseous N₂ (26.9%) and residual NH₄⁺, nitrate (NO₃^-) and organic nitrogen in the effluent (8.9%). In addition, 53.0% of the influent nitrogen was bio-transformed to protein nitrogen. Excellent nitrogen resource recovery was thus achieved.

Photosynthetic bacteria-based technology is a potential alternative to meet sustainable wastewater treatment requirement?

A paradigm shift is underway in wastewater treatment from pollution removal to resource or energy recovery. However, conventional activated sludge (CAS) as the core technology of wastewater treatment is confronted with severe challenges on high energy consumption, sludge disposal and inevitable greenhouse gas emission, which are posing a serious impact on the current wastewater industry. It is urgent to find new alternative methods to remedy these defects. Photosynthetic bacteria (PSB) have flexible metabolic modes and high tolerance, which enhance the removal of nutrients, heavy metals and organic contaminants efficiency in different wastewater. The unique phototrophic growth of PSB breaks the restriction of nutrient metabolism in the CAS system. Recent studies have shown that PSB-based technologies can not only achieve the recovery of nutrient and energy, but also improve the degradation efficiency of refractory substances. If the application parameters can be determined, there will be great prospects and economic effects. This review summarizes the research breakthroughs and application promotion of PSB-based wastewater treatment technology in recent years. Comparing discussed the superiority and inferiority from the perspective of application range, performance differences and recovery possibility. Pathways involved in the nutrient substance and the corresponding influencing parameters are also described in detail. The mode of PSB biodegradation processes presented a promising alternative for new wastewater treatment scheme. In the future, more mechanical and model studies, deterministic operating parameters, revolutionary process design is need for large-scale industrial promotion of PSB-based wastewater treatment.

Recycling of sugar industry wastewater for single-cell protein production with supplemental carotenoids

The bioconversion of sugar-industry wastewater to value-added products is a prominent topic in biotechnology. This work cultured a carotenoid-producing photosynthetic bacterium, Rhodopseudomonas faecalis, in a photo-bioreactor containing different wastewater from wastewater treatment ponds of a Thai sugar company. The cultivated R. faecalis produced single cell protein (SCP) with supplemental carotenoids. The cultivation boosted the growth and dehydrogenase activity of R. faecalis from all wastewater sources, while significantly reducing the total sugar concentration. The cellular protein concentration and carotenoid production of R. faecalis was maximised in wastewater collected from anaerobic pond and secondary mechanically aerated pond, respectively. At the end of the cultivation, the chemical oxygen demand was reduced by 80% and the protein content in the dry biomass exceeded 50%, within the acceptable ranges of SCP production. The biomass contained all essential amino acids and the leucine and lysine proportions were above the SCP guideline values. This study reveals that sugar-industry wastewater can be recycled in SCP production with supplemental carotenoids. The SCP is a potential commercial product for the sugar industry.

Bio-oxidation of Elemental Mercury into Mercury Sulfide and Humic Acid-Bound Mercury by Sulfate Reduction for Hg0 Removal in Flue Gas

Bioconversion of elemental mercury (Hg⁰) into immobile, nontoxic, and less bioavailable species is of vital environmental significance. Here, we investigated bioconversion of Hg⁰ in a sulfate-reducing membrane biofilm reactor (MBfR). The MBfR achieved effective Hg⁰ removal by sulfate bioreduction. 16 S rDNA sequencing and metagenomic sequencing revealed that diverse groups of mercury-oxidizing/sulfate-reducing bacteria (Desulfobulbus, Desulfuromonas, Desulfomicrobium, etc.) utilized Hg⁰ as the initial electron donor and sulfate as the terminal electron acceptor to form the overall redox. These microorganisms coupled Hg⁰ bio-oxidation to sulfate bioreduction. Analysis on mercury speciation in biofilm by sequential extraction processes (SEPs) and inductively coupled mass spectrometry (ICP-MS) and by mercury temperature programmed desorption (Hg-TPD) showed that mercury sulfide (HgS) and humic acid-bound mercury (HA-Hg) were two major products of Hg⁰ bio-oxidation. With HgS and HA-Hg comprehensively characterized by X-ray diffraction (XRD), excitation-emission matrix spectra (EEM), scanning electron microscopy-energy disperse spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR), it was proposed that biologically oxidized mercury (Hg²⁺) further reacted with biogenic sulfides to form cubically crystallized metacinnabar (β-HgS) extracellular particles. Hg²⁺ was also complexed with functional groups -SH, -OH, -NH^-, and -COO^- in humic acids from extracellular polymeric substances (EPS) to form HA-Hg. HA-Hg may further react with biogenic sulfides to form HgS. Bioconversion of Hg⁰ into HgS was therefore achieved and can be a feasible biotechnique for flue gas demercuration.

Bio-conversion of photosynthetic bacteria from non-toxic wastewater to realize wastewater treatment and bioresource recovery: A review

Generating or recycling water and resources from wastewater other than just treating wastewater is one of the most popular trends worldwide. Photosynthetic bacteria (PSB) wastewater treatment and resource recovery technology is one of the most potential methods. PSBs are non-toxic and contain lots of value-added products that can be utilized in the agricultural and food industries. They are effective to degrade pollutants and synthesize useful biomass, thus realizing wastewater treatment, bioresource production, and eliminating waste sludge. If all the nutrients in wastewaters could be bio-converted by PSB, then pollutant reductions and economic benefits would be achieved. This review paper firstly describes and summarizes this technology, including PSBs classification, metabolism, and the market application. The feasibility, technical procedures, bioreactors, pollutant removal, and bioresource production are also summarized, compared and evaluated. Issues that concern the advantages and industrialization of this technologies at the plant scale are also discussed.

A special light-aerobic condition for photosynthetic bacteria-membrane bioreactor technology

The combined photosynthetic bacteria (PSB) and membrane bioreactor (MBR) technology has the great advantage of simultaneously realizing wastewater purification and bio-resource recovery and has attracted increasing attention in recent years. Light-oxygen conditions are the most vital factor in wastewater treatment. The special light-aerobic condition was first applied to PSB-MBR wastewater treatment, and it was compared with three typical light-oxygen conditions. The results showed that the highest chemical oxygen demand (COD) removal efficiency (96.28%) and the highest biomass production (1.12 g/L/d) were simultaneously obtained under light-aerobic condition. This phenomenon overcame the limitations whereby optimal pollutant removal and bio-resource recovery could not be achieved at the same time. An analysis of the microbial community showed that different light-oxygen conditions caused large variations in the microbial community composition of PSB-MBR. The microbial diversity was lower when light and oxygen co-existed.

N2O emission and bacterial community dynamics during realization of the partial nitrification process

In this study, greenhouse gas emissions and microbial community succession during the realization of the partial nitrification (PN) process were studied. The results show that N₂O emission mainly occurred in the aerobic stage and the PN reactor released about 20 mg of N₂O gas each cycle. There is a positive correlation between the dissolved N₂O concentration and the temperature of a typical cycle. High-throughput sequencing was used to illustrate succession in the microbial community structure. The most significant microfloral change during the PN startup process was that some aerobic bacteria were relatively enriched and some anaerobic bacteria were weeded out. The ammonia oxidizing bacteria (AOB) like Nitrosomonadaceae were enriched on account of the suitable external environment. Pseudomonas whose main function is denitrification declined and Planctomyces (anammox) showed the same tendency. This study comprehensively demonstrates the fluctuations of dissolved and emitted N₂O while researching the succession of the microbial community in the culture of the PN process.

In this study, greenhouse gas emissions and microbial community succession during the realization of the partial nitrification (PN) process were studied.