Simultaneous nitrification and denitrification in microbial community-based polyhydroxyalkanoate production

Demonstrating performance in scaled-up production and quality control of polyhydroxyalkanoates using municipal waste activated sludge

Significant progress has been made over the past decade with pilot scale polyhydroxyalkanoate (PHA) production by direct accumulation using municipal waste activated sludge (WAS). However, industrial upscaling experiences are still lacking in the research literature. In this study, a demonstration scale (4 m3) PHA production process was operated using industrially relevant equipment and compared favourably to those from parallel pilot scale (200 L) production runs. WAS grab samples from a Dutch full scale municipal wastewater treatment plant (WWTP) was used as the biomass source. Final biomass PHA contents and production yields, that are critical for technology viability, were statistically the same between the experiments conducted at pilot scale (0.41 ± 0.02 gPHA/gVSS and 0.42 ± 0.02 gCOD/gCOD) and demonstration scale (0.45 ± 0.05 gPHA/gVSS and 0.39 ± 0.07 gCOD/gCOD). The results furthermore aligned with previous 1 m3 piloting experiences and five year old historical data that similarly used WAS sourced from the same WWTP. Scalability for the technology and a robustness of the applied PHA production methods using WAS were demonstrated. Temperature and foaming control were identified to be critical to upscaled process engineering and design towards successful industrial implementations. The results of the present study, combined with previously produced PHAs and those historical data, support that feedstock quality predictably determines both the average PHA co-monomer content, as well as the blend distribution. PHA solvent extraction from WAS is inherently a blending process. Extraction homogeneously mixes polymer contributions from collectively stored granules from all species of microorganisms in the biomass. Dried PHA-rich biomass batches can be stockpiled and batches can be blended in extraction processes for both recovery and formulation to reach consistent polymer qualities across production batches. More centralized extraction facilities are therefore anticipated to offer economic benefits due to scale and greater opportunities for product quality specification and control. Research findings are presented herein of the production scale comparative study along with practical perspectives of technological readiness for realizing WAS based industrial scale PHA production, quality control, and the supply chains that will be necessary for successful commercial implementation.

Role of intracellular storage polymers in simultaneous biological nutrient removal and resources recovery

Simultaneous biological nutrient removal (SBNR) using an anaerobic-anoxic-oxic phase is the key feature of advanced wastewater treatment plants (WWTPs). Removing ammonia, total nitrogen, and phosphorus concurrently with organic matter and suspended solids from wastewater is essential to meeting stringent effluent discharge standards via SBNR in WWTPs. More insight into the mechanisms of SBNR, i.e., simultaneous nitrification-denitrification (SND) and enhanced biological phosphorus removal (EBPR) processes, the intracellular carbon reserves, i.e., polyhydroxyalkanoates (PHA) and specifically poly-β-hydroxybutyrates (PHB), will play a critical role in nutrients removal and resource recovery in WWTPs. Volatile fatty acids (VFA) in wastewater are the preferable source of PHA formation. However, municipal wastewater could not supply sufficient VFA fractions owing to short sewer lines; therefore, developing pre-fermentation chambers and other technological integration in the WWTPs can play an effective role in VFA production from raw sewage, resulting in the effective formation of PHA. On the other hand, PHA is a value-added biochemical, i.e., a potential substitute for fossil fuel plastics. WWTPs complying with SBNR are the bio-refineries for PHA (bioplastic precursors) production using diverse microbial populations. This review enlightens three dimensions of progressive systems and engineering-based viewpoints: (i) Increasing the SBNR by optimizing operational conditions subject to the substrate storage mechanisms of treatment systems; (ii) Technical solutions to enhance the VFA availability in sewage in WWTPs to achieve effective SBNR; and (iii) production of PHB (PHA) in WWTPs.

Calcium enhances polyhydroxyalkanoate production and promotes selective growth of the polyhydroxyalkanoate-storing biomass in municipal activated sludge

Activated sludge from municipal wastewater treatment processes can be used directly for the production of biodegradable polyesters from the family of polyhydroxyalkanoates (PHAs). However, municipal activated sludge typically cannot accumulate PHAs to very high levels and often low yields of polymer produced on substrate are observed. In the present work, it was found that the presence of calcium promotes selective growth and enrichment of the PHA-storing biomass fraction and significantly improved both PHA contents and yields. Calcium addition resulted in PHA contents of 0.60 ± 0.03 gPHA/gVSS and average PHA yields on substrate of 0.49 ± 0.03 gCOD_PHA/gCOD_HAc compared to 0.35 ± 0.01 gPHA/gVSS and 0.19 ± 0.01 gCOD_PHA/gCOD_HAc without calcium addition. After 48 h, three times more PHA was produced compared to control experiments without calcium addition. Higher PHA content and selective biomass production is proposed to be a consequence of calcium dependent increased levels of passive acetate uptake. Such more efficient substrate uptake could be related to a formation of calcium acetate complexes. Findings lead to bioprocess methods to stimulate a short-term selective growth of PHA-storing microorganisms and this enables improvements to the techno-economic feasibility for municipal waste activated sludge to become a generic resource for industrial scale PHA production.

Process conditions affect properties and outcomes of polyhydroxyalkanoate accumulation in municipal activated sludge

The developments of mixed culture polyhydroxyalkanoate production has been directed to maximize the biomass PHA content with limited attention to polymer quality. Direct comparison of PHA accumulation literature is challenging, and even regularly contradicting in reported results, due to underlying differences that are not well expressed. A study was undertaken to systematically compare the commonly reported process conditions for PHA accumulation by full-scale municipal activated sludge. A biomass acclimation step combined with a pulse-wise feeding strategy resulted in maximum average PHA contents and product yields. pH control and active nitrification did not result in observable effects on the PHA productivity. Under these conditions a high molecular weight polymer (1536 ± 221 kDa) can be produced. Polymer extraction recoveries were influenced by the PHA molecular weight. A standard protocol for an activated sludge PHA accumulation test including downstream processing and standardized extraction has been developed and is available as supplementary material.

A novel process for anammox pretreatment of municipal wastewater: semi-partial nitrification, biological phosphorus removal and recovery

Achieving simultaneous semi-partial nitrification and deep phosphorus removal is a preferred process technology for Anammox pretreatment. In this study, semi-partial nitrification combined with in-situ phosphorus recovery (PNPR) was used to treat municipal wastewater. The SRT conflict between the nitrification and phosphorus removal was resolved by in-situ phosphorus recovery every 20 cycles of Anaerobic/Oxid, and a supernatant with more than 10 times the influent phosphorus concentration was obtained, thus achieving bio-enhanced phosphorus removal and recovery with satisfactory semi-partial-nitrification effluent. Interestingly, the results showed that phosphorus removal and recovery process could improve the activity of AOB. The PNPR system's nitrite accumulation rate (NAR) and phosphorus removal rate (PRR) were more than 90% each, whereas the relative abundance of AOB and PAOs increased from 0.04% to 0.74% and from 0.25% to 0.70%, respectively (P < 0.01). Furthermore, on average, the NO₂^--N_eff/NH₄⁺-N_eff value was 1.96, which laid the foundation for the subsequent anammox treatment.

Exploring the Limits of Polyhydroxyalkanoate Production by Municipal Activated Sludge

Municipal activated sludge can be used for polyhydroxyalkanoate (PHA) production, when supplied with volatile fatty acids. In this work, standardized PHA accumulation assays were performed with different activated sludge to determine (1) the maximum biomass PHA content, (2) the degree of enrichment (or volume-to-volume ratio of PHA-accumulating bacteria with respect to the total biomass), and (3) the average PHA content in the PHA-storing biomass fraction. The maximum attained biomass PHA content with different activated sludge ranged from 0.18 to 0.42 gPHA/gVSS, and the degree of enrichment ranged from 0.16 to 0.51 volume/volume. The average PHA content within the PHA-accumulating biomass fraction was relatively constant and independent of activated sludge source, with an average value of 0.58 ± 0.07 gPHA/gVSS. The degree of enrichment for PHA-accumulating bacteria was identified as the key factor to maximize PHA content when municipal activated sludge is directly used for PHA accumulation. Future optimization should focus on obtaining a higher degree of enrichment of PHA-accumulating biomass, either through selection during wastewater treatment or by selective growth during PHA accumulation. A PHA content in the order of 0.6 g PHA/g VSS is a realistic target to be achieved when using municipal activated sludge for PHA production.

Modelling Mixed Microbial Culture Polyhydroxyalkanoate Accumulation Bioprocess towards Novel Methods for Polymer Production Using Dilute Volatile Fatty Acid Rich Feedstocks

Volatile fatty acid (VFA) rich streams from fermentation of organic residuals and wastewater are suitable feedstocks for mixed microbial culture (MMC) Polyhydroxyalkanoate (PHA) production. However, many such streams have low total VFA concentration (1–10 gCOD/L). PHA accumulation requires a flow-through bioprocess if the VFAs are not concentrated. A flow through bioprocess must balance goals of productivity (highest possible influent flow rates) with goals of substrate utilization efficiency (lowest possible effluent VFA concentration). Towards these goals, dynamics of upshift and downshift respiration kinetics for laboratory and pilot scale MMCs were evaluated. Monod kinetics described a hysteresis between the upshift and downshift responses. Substrate concentrations necessary to stimulate a given substrate uptake rate were significantly higher than the concentrations necessary to sustain the attained substrate uptake rate. A benefit of this hysteresis was explored in Monte Carlo based PHA accumulation bioprocess numerical simulations. Simulations illustrated for a potential to establish continuous flow-through PHA production bioprocesses even at a low (1 gCOD/L) influent total VFA concentration. Process biomass recirculation into an engineered higher substrate concentration mixing zone, due to the constant influent substrate flow, enabled to drive the process to maximal possible PHA production rates without sacrificing substrate utilization efficiency.

Nitrous oxide formation during simultaneous phosphorus and nitrogen removal in aerobic granular sludge treating different carbon substrates

The impact of different substrates on N₂O dynamics and gene expression of marker enzymes (nirS, nirK and nosZ) involved in denitrifying enhanced biological phosphorus removal (d-EBPR) was investigated. Aerobic granular sludge fed with VFAs led to an anoxic P-uptake (27.7 ± 1.2 mg PO₄^3--P.gVSS^-1) and N₂O emissions up to 80.7 ± 3.4% N₂O-N. A decisive role of Accumulibacter in N₂O formation was observed. Dosage of amino acids (12.0 ± 1.2 mg PO₄^3--P.gVSS^-1) and glucose (1.5 ± 0.9 mg PO₄^3--P.gVSS^-1) as sole substrate did not support d-EBPR activity. Presence of NO₂^- resulted in higher N₂O formation in comparison to nitrate and a nosZ/(nirS + nirK) ratio lower than 0.3. A linear correlation (R² > 0.95) between the nosZ/(nirS + nirK) ratio and the N₂O reductase rate was found only when dosing the same type of substrate. This suggests an interplay between the microbial community composition and different polyhydroxyalkanoates derivatives, when dosing different substrates.