Polyhydroxyalkanoate production from fermentation of domestic sewage sludge monitoring greenhouse gas emissions: A pilot plant case study at the WRRF of Palermo University (Italy)

VFA extraction through silicone membrane fosters PHA production from nutrient-rich biowaste

This study presents a novel four-stage process for polyhydroxyalkanoates (PHA) production from nutrient-rich sheep cheese whey (CW). The key advancement was the integration of a volatile fatty acid (VFA) extraction stage into the conventional three-stage PHA production process. Application of membrane separation to fermented cheese whey resulted in the generation of a "retentate" stream containing both organic acids and nutrients, suitable for microbial culture selection, and a VFA-rich but nutrient deprived "permeate" stream, ideal for PHA accumulation. Thus, the carbon-to-nitrogen (C/N) ratio was optimized for both the selection and accumulation stages, which is crucial for efficient PHA production and for eliminating the need for exogenous nitrogen addition. The integrated process resulted in significantly higher yields (0.55 vs 0.26 gC-PHA gC-OA-1) and PHA content (37% vs 28%) than the control, where fermented cheese whey was directly used as feedstock for the accumulation stage. The results highlight the potential of this approach for optimizing PHA production from sub-optimal, nutrient-rich substrates.

The role of carbon and nitrogen ratio on sewage sludge microbiota for producing polyhydroxyalkanoates

The products of an advanced sewage sludge fermentation process can be used to generate polyhydroxyalkanoates (PHAs), precursors of bioplastics considered excellent candidates for replacing petroleum-derived plastics. The aerobic feast-anoxic famine cycling strategy has proven to be an efficient method for enriching sewage sludge microbiota with PHA-producing microorganisms. This work evaluated the effect of different carbon to nitrogen ratios (C/N) of 3.5, 2, and 1 g COD/g N for modulating the structure of sewage sludge microbiota to improve PHA production. The study was executed on a pilot plant scale using wasted activated sludge as an organic carbon source derived from an oxic-settling anaerobic plant that collects wastewater from various facilities at the University of Palermo campus. PHA production performance was monitored over three experimental periods characterized by a different C/N ratio. The results showed that lower C/N ratios reduced PHA production with 20, 24, and 26 % w/w of PHAs for COD/N values of 1, 2, and 3.5 g COD/g N, respectively. In parallel, the metataxonomic analysis revealed a higher abundance of PHA-producing microorganisms at the ratio of 3.5 g COD/g N, such as Proteobacteria, specifically Betaproteobacteria. In addition, the analysis showed an increase in fungal abundance and diversity as decrease the ratio C/N decreased. Thus, these findings demonstrate the utility of metataxonomics in elucidating the relationships between operating conditions, bacterial and fungal microbiota structure and the achievement of specific outputs. The insights gained from this study demonstrated a positive correlation between C/N ratios, PHA-producing microorganisms, and PHA yields.

Potential for nutrients reuse, carbon sequestration, and CO2 emissions reduction in the practice of domestic and industrial wastewater recycling into agricultural soils: A review

This review assesses the feasibility of reusing treated wastewater for irrigation in agricultural soils as a strategy for nutrients recycling and mitigation of CO2 emissions. Through a literature review, it was examined wastewater sources enriched with carbon and nutrients, including municipal wastewater and associated sludge, vinasse, swine wastewater, as well as wastewater from the food industry and paper and pulp production. The review also explores the dynamics of organic matter within the soil, discussing the aspects related to its potential conversion to CO2 or long-term storage. It was found that industrial wastewaters, owing to their higher organic matter and recalcitrance, exhibit greater potential for carbon storage. However, the presence of pollutants in wastewater necessitates careful consideration, particularly concerning their impact on soil quality. Toxic metals, microplastics, and organic compounds emerged as significant contaminants that could accumulate in the soil, posing risks to ecosystem health. To mitigate the environmental impacts, it was evaluated various wastewater treatment technologies and their associated carbon emissions. While advanced treatments may effectively reduce the contaminant load and mitigate soil impacts, their adoption is often associated with an increase in CO2 emissions. Membrane bioreactors, microfiltration, ultrafiltration, and up-flow anaerobic sludge blanket reactors were identified as promising technologies with lower carbon footprints. Looking ahead, future research should aim to enhance the understanding of carbon dynamics in soil and validate the environmental impacts of treated wastewater disposal. Despite remaining uncertainties, the literature indicates a positive outlook for wastewater recycling in soil, offering a viable strategy for carbon storage and mitigation of greenhouse gas emissions.

Assessing the aerobic/anoxic enrichment efficiency at different C/N ratios: polyhydroxyalkanoate production from waste activated sludge

Polyhydroxyalkanoates (PHA) can be produced using fermentation products of an excess sewage sludge fermentation process. An efficient method to enrich a PHA-producing community is an aerobic-feast/anoxic-famine enrichment strategy. The effect of different carbon to nitrogen (C/N) feed ratios of 1, 2 and 3.5 g COD/g N on the process performance was studied. The study was executed on a pilot plant scale using fermented waste activated sludge as the organic carbon source. The system's performance was monitored in terms of removing contaminants, producing PHA, and reducing N2O emissions. The results indicated that a lower C/N ratio results in lower PHA production, with PHA content in the sludge of 20, 24 and 36 % w/w for C/N ratios of 1, 2 and 3.5 g COD/g N, respectively. At the lowest C/N ratio, the highest nitrite accumulation rate (77 %), nitrification efficiency (89 %) and denitrification efficiency (89 %) were observed, but the N2O production was also the highest (0.77 mg N2O-N/L). The long-term comprehensive monitoring carried out in this study revealed high carbon and ammonia removal efficiencies (never below 80 %) despite the C/N shifts and high COD and ammonia concentrations. At the same time, the system showed relatively low PHA production and high environmental impact in terms of high gaseous N2O emission. These findings question the sustainability of the aerobic-feast/anoxic-famine enrichment strategy for PHA production in full-scale plants.

A review on microbes mediated resource recovery and bioplastic (polyhydroxyalkanoates) production from wastewater

BACKGROUND

Plastic is widely utilized in packaging, frameworks, and as coverings material. Its overconsumption and slow degradation, pose threats to ecosystems due to its toxic effects. While polyhydroxyalkanoates (PHA) offer a sustainable alternative to petroleum-based plastics, their production costs present significant obstacles to global adoption. On the other side, a multitude of household and industrial activities generate substantial volumes of wastewater containing both organic and inorganic contaminants. This not only poses a threat to ecosystems but also presents opportunities to get benefits from the circular economy. Production of bioplastics may be improved by using the nutrients and minerals in wastewater as a feedstock for microbial fermentation. Strategies like feast-famine culture, mixed-consortia culture, and integrated processes have been developed for PHA production from highly polluted wastewater with high organic loads. Various process parameters like organic loading rate, organic content (volatile fatty acids), dissolved oxygen, operating pH, and temperature also have critical roles in PHA accumulation in microbial biomass. Research advances are also going on in downstream and recovery of PHA utilizing a combination of physical and chemical (halogenated solvents, surfactants, green solvents) methods. This review highlights recent developments in upcycling wastewater resources into PHA, encompassing various production strategies, downstream processing methodologies, and techno-economic analyses.

SHORT CONCLUSION

Organic carbon and nitrogen present in wastewater offer a promising, cost-effective source for producing bioplastic. Previous attempts have focused on enhancing productivity through optimizing culture systems and growth conditions. However, despite technological progress, significant challenges persist, such as low productivity, intricate downstream processing, scalability issues, and the properties of resulting PHA.

Sludge reduction, nitrous oxide emissions, and phosphorus removal by oxic-settling-anaerobic (OSA) process: the effect of hydraulic retention time

This paper presents a study on reducing sewage sludge by an oxic-settling-anaerobic (OSA) pilot plant compared to the conventional activated sludge (CAS) process in view of resource recovery and moving towards plant carbon neutrality. The OSA plant was supplied with real wastewater and the anaerobic reactor was operated under two hydraulic retention times (HRT) (4 and 6 h). Greenhouse gas (GHG) emissions were monitored for the first time to determine the OSA process's production mechanism. The results highlighted that under the lowest HRT (4 h), the removal efficiencies of COD and PO4-P, increased from 75 to 89% and from 39 to 50% for CAS and OSA configurations, respectively. The observed yield coefficient was reduced from 0.58 gTSS gCOD-1 (CAS period) to 0.31 gTSS gCOD-1 (OSA period). A remarkable deterioration of nitrification efficiency under OSA configuration was obtained from 79% (CAS) to 27% (OSA with HRT of 6 h). The huge deterioration of nitrification significantly affected the GHG emissions, with the N2O-N fraction increasing from 1% (CAS) to 1.55% (OSA 4 h HRT) and 3.54% (OSA 6 h HRT) of the overall effluent nitrogen, thus suggesting a relevant environmental implication due to the high global warming potential (GWP) of N2O.

Effect of bacteria-algae ratio on treatment of anaerobic digested wastewater by symbiotic coupling of bacteria and algae under the background of carbon neutralization

Environmental pollution is a growing concern, particularly the impact of sewage treatment gas on the atmosphere's greenhouse effect. Efficient sewage resource recycling is crucial to achieving carbon neutrality. The bacteria-algae symbiotic sewage treatment system combines wastewater treatment, carbon dioxide fixation, and biomass energy recovery to achieve the goal of carbon neutrality, environmental protection, and the transformation of high-value added products. This paper presents the construction of a sequencing batch photobiological reaction system that utilizes a microbial-algae symbiotic relationship. The system was used to analyze the degradation effects of sCOD, TN, AN, and TP in anaerobic digestion wastewater by varying the microbial-algae ratios. Additionally, changes in the microbial community were analyzed to explore the system's potential for reducing carbon emissions. The study's findings indicate that: 1)When the ratio of bacteria to algae was 2:3, the removal rates of TN, AN, sCOD, and TP were 81.38%, 94.28%, 75.33%, and 96.56%. 2)Changing the ratio of bacteria to algae would affect the bacterial concentration in the mixed system, but not the bacterial community structure. The results indicate that a ratio of 2:3 enhances the removal of pollutants by bacteria and algae symbionts.3) Under the context of carbon neutralization, this paper investigates the reduction of carbon emissions in ADE treated by bacteria-algae symbiosis at the optimal bacteria to algae ratio. The experimental process can reduce 177.03 mg CO2 compared to complete nutrient consumption treatment, which is equivalent to a reduction of 355.08 g CO2 per 1 m3 of ADE. For full anaerobic treatment, this experimental process can reduce 228.35 mg of CO2 equivalent CH4, which translates to a reduction of 456.71 g of CO2 equivalent CH4 per 1 m3 of ADE.