• Energy Research

Abstract Triacylglyceride (TAG) recovery from algal biomass is primarily limited by the rigid algal cell envelope (cell wall and cell membrane). In this work, the effect of free nitrous acid (FNA) pre-treatment on TAG recovery from algal biomass with six different FNA concentrations is reported. Results show that at a range of low FNA concentrations (0.24–2.25 mg HNO2–N L−1) TAG recovery was strongly enhanced with increasing FNA concentration. An FNA concentration of around 2 mg HNO2–N L−1 resulted in a 3.3-fold increase in fatty acid recovery over untreated algae, but higher FNA concentrations (13.49 and 26.98 mg HNO2–N L−1) were detrimental to TAG recovery. Analysis of the fatty acid profile revealed that the higher FNA concentrations caused a reduction in polyunsaturated fatty acids. Also, the ratio of extracted fatty acids to total lipids was significantly reduced when high FNA concentration were applied, and only non-fatty acid lipids potentially benefited from more intense FNA pre-treatments.

This study investigated biofilm establishment, biofilm structure, and microbial community composition of biofilms in three laboratory-scale moving bed biofilm reactors. These reactors were filled with three types of plastic carriers with varied depths of living space for microbial growth. The reactors were operated under the same influent and operational conditions. Along with the operation, the results showed that carriers with grids of 50 μm in height delayed the biofilm development and formed the thinnest biofilm and a carpet-like structure with the lowest α-diversity. In comparison, another two carriers with grids of 200 and 400 μm in height formed thick biofilms and large colonies with more voids and channels. Quantified properties of biofilm thickness, biomass, heterogeneity, portion of the biofilm exposed to the nutrient, and maximum diffusion distance were examined, and the results demonstrated that they almost (except for heterogeneity) strongly correlated to the α-diversity of microbial community. These illustrate that depth of living space, as an important parameter for carrier, could drive the formation of biofilm structure and community composition. It improves understanding of influencing factors on biofilm establishment, structure and its microbial community, and would be helpful for the design of biofilm processes.

The effectiveness of an aerobic, anoxic/anaerobic strategy for maintaining the activity of activated sludge performing biological nitrogen and phosphorus removal during long-term starvation is investigated. A lab-scale sequencing batch reactor (SBR) treating abattoir wastewater and achieving high-levels (>95%) of nitrogen, phosphorus and COD removal was used. The reactor was put twice into a so-called "sleeping mode" for a period of 5-6 weeks when the abattoir, where the wastewater was sourced, was closed down for annual maintenance. The "sleeping mode" operation consisted of 15 min aeration in a 6 h SBR cycle. The sludge was allowed to settle in the remaining time of the cycle. The decay rates for ammonia oxidising bacteria (AOB) and nitrite oxidising bacteria (NOB) were determined to be 0.017 and 0.004 d(-1), respectively. These decay rates correlated well with AOB and NOB population quantified using molecular techniques (FISH). There was negligible phosphate accumulation in the reactor during the first 1-2 weeks of starvation, which was followed by a linear net release of phosphate in the remaining 4-5 weeks at a very slow rate of 1-2 mgP gVSS(-1)d(-1). A sudden decrease in the aerobic activities of polyphosphate accumulating organisms (PAOs), observed via anaerobic/aerobic batch tests, occurred after 2 weeks of starvation. This correlated with a dramatic increase of several metal ions in the liquid phase. The underlying reasons are not clear. A resuscitation period with a gradual increase of the wastewater load was applied during the re-startup of the reactor after both "sleeping mode" periods. Each time, the performance of the reactor in terms of nitrogen and phosphorus removal fully recovered in 4 days.

The treatment of wastewater containing sulfides in bioelec-trochemical systems (BES) causes deposition of sulfur on the anode as a result of a solely electrochemical process. In this study, we investigate whether microorganisms can use this sulfur, ratherthan the anode or soluble sulfate, as an electron acceptor for the oxidation of acetate. Our results indicate that microorganisms use electrodeposited sulfur as preferable electron acceptor over the anode and sulfate and produce sulfide irrespective of electrochemical conditions. Bioelectrochemical and biological sulfide generation pathways were studied under different electrochemical conditions. The obtained results show that the sulfide generation rate at open circuit condition (anode potential -235 +/- 5 mV versus standard hydrogen electrode, SHE)was higher in comparison to the electrochemical sulfide generation even at a lower potential of -275 mV (vs SHE), confirming that sulfide is produced through biological processes without any current generation. However, during closed circuit operation, the overall Coulombic efficiency (97% +/- 2%) is not affected as the produced sulfide (originating from the reduction of deposited sulfur) is spontaneously reoxidized to sulfur when a favorable potential is maintained. This confirms the mediator role of sulfur during acetate oxidation in BES. A diagrammatic representation of the mechanism is proposed to characterize the interactions between acetate oxidation and sulfur conversions on the anode.

We are witnessing an enormous growth in biological nitrogen removal from wastewater. It presents specific challenges beyond traditional COD (carbon) removal. A possibility for optimised process design is the use of biomass-supporting media. In this paper, attached growth processes (AGP) are evaluated using dynamic simulations. The advantages of these systems that were qualitatively described elsewhere, are validated quantitatively based on a simulation benchmark for activated sludge treatment systems. This simulation benchmark is extended with a biofilm model that allows for fast and accurate simulation of the conversion of different substrates in a biofilm. The economic feasibility of this system is evaluated using the data generated with the benchmark simulations. Capital savings due to volume reduction and reduced sludge production are weighed out against increased aeration costs. In this evaluation, effluent quality is integrated as well.

For decades, glycolysis has been generally accepted to supply the reducing power for the anaerobic conversion of volatile fatty acids (VFAs) to polyhydroxyalkanoates (PHAs) by polyphosphate accumulating organisms (PAOs). However, the importance of the tricarboxylic acid (TCA) cycle has also been raised since 1980s. The aim of this study is to demonstrate the involvement of the TCA cycle in the anaerobic metabolism of PAOs. To achieve this goal, the glycogen pool of an activated sludge highly enriched in Candidatus Accumulibacter Phosphatis (hereafter referred to as Accumulibacter), a putative PAO was reduced substantially through starving the sludge under intermittent anaerobic and aerobic conditions. After the starvation, acetate added was still taken up anaerobically and stored as PHA, with negligible glycogen degradation. The metabolic models proposed by Pereira, Hesselmann and Yagci, which predict the formation of reducing power through glycolysis and the full or partial TCA cycle, were used to estimate the carbon fluxes. The results demonstrate that Accumulibacter can use both glycogen and acetate to generate reducing power anaerobically. The anaerobic production of reducing power from acetate is likely through the full TCA cycle. The proportion of TCA cycle involvement depends on the availability of degradable glycogen.