Search

Highlights: [•] Development of new control strategies for enhancing P-removal in WWTP. [•] Performance comparison of optimized open loop operation to the new control structures. [•] Control loops based on COD addition to the anaerobic reactor allow stable EBPR. [•] Optimized controllers allowed lower effluent pollutants and minimal operational costs. [Copyright &y& Elsevier]

• Mainstream P-recovery can be successfully implemented in WWTPs with EBPR. • High extraction ratio increases P recovery but hinders PAO activity. • A P-recovery efficiency of 60% is the top long-term limit goal for these systems. • Mainstream P-recovery can help the treatment of low COD/P ratio influents. Phosphorus (P), an essential nutrient for all organisms, urgently needs to be recovered due to the increasing demand and scarcity of this natural resource. Recovering P from wastewater is a feasible and promising way widely studied nowadays due to the need to remove P in wastewater treatment plants (WWTPs). When enhanced biological P removal (EBPR) is implemented, an innovative option is to recover P from the supernatant streams obtained in the mainstream water line, and then combine it with liquor-crystallisation recovery processes, being the final recovered product struvite, vivianite or hydroxyapatite. The basic idea of these mainstream P-recovery strategies is to take advantage of the ability of polyphosphate accumulating organisms (PAO) to increase P concentration under anaerobic conditions when some carbon source is available. This work shows the mainstream P-recovery technologies reported so far, both in continuous and sequenced batch reactors (SBR) based configurations. The amount of extraction, as a key parameter to balance the recovery efficiency and the maintenance of the EBPR of the system, should be the first design criterion. The maximum value of P-recovery efficiency for long-term operation with an adequate extraction ratio would be around 60%. Other relevant factors (e.g. COD/P ratio of the influent, need for an additional carbon source) and operational parameters (e.g. aeration, SRT, HRT) are also reported and discussed. [ABSTRACT FROM AUTHOR]

Abstract: Polyhydroxyalkanoate (PHA) is a biodegradable plastic synthesised by bacteria as energy and carbon storage material. PHA production is mostly based on pure cultures operated under sterile conditions, which increase the costs of this biopolymer. The use of inexpensive mixed culture biomass, such as activated sludge, to produce biodegradable plastics from renewable waste streams has been proposed as an alternative. The effect of carbon sources (acetate, propionate, butyrate and glucose) on the type and quantity of PHA synthesis obtained with different enhanced biological phosphorus removal (EBPR) microbial communities enriched with acetate and propionate are reported in this work. Two sequencing batch reactors (SBRs) were seeded with biomass withdrawn from a non-EBPR wastewater treatment plant (WWTP). The same operational conditions were kept, but using acetate or propionate as the sole carbon source of each reactor. These conditions produced two microbial communities with different P-removal capacity. The results presented in this study show the effect of the carbon source on the PHA composition (amount of polyhydroxybutyrate (PHB), polyhydroxyvalerate (PHV) and polyhydroxy-2-methylvalerate (PH2MV)), which differed not just between substrates but also between the two EBPR communities used. In addition, some monomers not always analysed contribute significantly to the total amount of PHA, especially when using butyrate, showing that if they are not considered this can lead to erroneous calculated yields. [Copyright &y& Elsevier]

The design of new wastewater treatment plants with the aim of capturing organic matter for energy recovery is a current focus of research. Operating with low sludge residence time (SRT) appears to be a key factor in maximizing organic matter recovery. In these new configurations, it is assumed that phosphorus is chemically removed in a tertiary step, but the integration of enhanced biological phosphorus removal (EBPR) into these short-SRT systems seems to be an alternative worth studying. A key point of this integration is to prevent the washout of polyphosphate accumulating organisms (PAO) despite the low SRT applied. However, the minimum SRT required to avoid PAO washout depends on temperature, due to its effects on reaction kinetics, gas transfer rates, biomass growth and decay rates. This work includes a wide range of short and long-term experiments to understand these interactions and shows which combinations of SRT and temperature are detrimental to PAO growth. For example, an EBPR system operating at 20 °C and SRT = 5 d showed good performance, but EBPR activity was lost at 10 °C. EBPR operated at SRT = 10 d had 86% P removal at 20 °C but decreased to 71% at 15 °C and progressively lost its activity at lower temperature. The temperature coefficient obtained for PAO show a low degree of temperature dependence (θ = 1.047 ± 0.014), and should be considered when designing short-SRT systems with EBPR. Image 1 • Study of long-term and short-term effects of SRT and temperature on EBPR. • Operation is evaluated at different SRT (3.5–15) and T (10-20 °C). • EBPR rates have a medium dependence on T (θ = 1.047 ± 0.014). • Using correct T coefficients is essential for designing A-stage systems with EBPR. • A correlation SRT-T is reported to calculate the minimum SRT required for PAO. [ABSTRACT FROM AUTHOR]

The new paradigm in wastewater treatment aims at diverting as much organic matter as possible to anaerobic digestion to boost energy recovery (i.e. biogas production) and to decrease operational costs (i.e. aeration requirements and sludge generation). To achieve this, short sludge retention time (SRT) systems could be implemented in the first stage of the wastewater treatment plants (WWTPs) that include biological phosphorus removal. This enables efficient removal of both carbon and phosphorus. This paper focuses on the biochemical methane potential (BMP) that could be reached when using bio-P sludge with different polyhydroxyalkanoate (PHA) content obtained under different SRTs and under anaerobic and aerobic environments. The PHA content of the biomass increased from 3.15 to 9.80 mmol C/g VSS when the SRT was decreased from 15 to 5 days and, on average, anaerobic sludge contained 45% more PHA than the aerobic sludge. BMP was correlated with the PHA content, following an approximate linear trend: BMP (mL CH 4 /g VSS) = 240 + 15.3 * PHA (mmol C/g VSS), independently of SRT and the anaerobic or aerobic extraction. The maximum BMP obtained was 401 mL CH 4 /g VSS with the biomass with the highest PHA content (9.80 mmol C/g VSS). When operating at low SRT and purging from the anaerobic phase, 28% of the influent COD content could be recovered as methane. This represented up to 2.5 times more energy than in the more conventional scenario tested: operating at high SRT with conventional aerobic purge. Image 1075 • BMP for Bio-P sludge is linearly correlated to its PHA content. • BMP (mLCH 4 /gVSS) can be calculated as 240 + 15.3 * PHA (mmolC/gVSS). • The correlation of BMP and the PHA content in bio-P sludge is independent of SRT. • PHA increased from 3.15 to 9.80 mmolC/gVSS when SRT decreased from 15 to 5 days. • 28% of influent COD was recovered as CH 4 purging from anaerobic phase at low SRT. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N 2 O), a significant contributor to the greenhouse effect, is generated during the biological nutrient removal in wastewater treatment plants (WWTPs). Developing mathematical models estimating the N 2 O dynamics under changing operational conditions (e.g. dissolved oxygen, DO) is essential to design mitigation strategies. Based on the activated sludge models (ASM) structure, this work presents an ASM2d-N 2 O model including all the biological N 2 O production pathways for a municipal anaerobic/anoxic/oxic (A 2 /O) WWTP with biological removal of organic matter, nitrogen and phosphorus, and its application in different dynamic scenarios. Three microbial N 2 O production pathways were considered: nitrifier denitrification, hydroxylamine oxidation, and heterotrophic denitrification, with the first two being activated by ammonia oxidizing bacteria (AOB). A stripping effectivity (SE) coefficient was added to reflect the non-ideality of the stripping model. Partial nitrification and high N 2 O production via nitrifier denitrification were observed when the range of DO in the aerobic compartment was 1.8 to 2.5 mg·L −1 . It could imply that low aeration strategies lead to low overall carbon footprint provided complete nitrification is not hindered. The model predicted high N 2 O emissions when low DO (∼1.1 mg L −1 ) and high ammonium concentration concurred. With the AOB prevailing over the nitrite oxidizing bacteria (NOB), nitrite was accumulated, triggering the activation of the nitrifier denitrification pathway. After suddenly increasing the influent ammonium load, the AOB had a greater growth compared to the NOB and the same pathway was considered as N 2 O hotspot. Especially under conditions promoting partial nitrification (i.e. low DO) and raising the stripping effect importance (i.e. high SEs), the highest N 2 O emission factors were predicted. [ABSTRACT FROM AUTHOR]

• Tracer experiments helped identify plant hydraulics for reliable modelling. • Sensitivity analysis helped to reveal an optimal parameter subset. • ASM2d-N 2 O model was successfully calibrated to describe a full-scale urban WWTP. • Trends in both nutrients removal and N 2 O emissions were described. Nitrous oxide (N 2 O) is a greenhouse gas (GHG) emitted during biological nitrogen removal from wastewater treatment plants (WWTPs). Some modelling tools have been proposed to predict N 2 O emissions during the design and operation of WWTPs. In this study, the novel ASM2d-N 2 O model, which accounts for the production of N 2 O in nutrient removal WWTPs, was used to study the associated emissions from a full-scale WWTP with two independent lines. Firstly, the hydraulics of the WWTP was characterized by a residence time distribution test, showing the flow was equally divided into the two treatment lines (49.3 vs. 50.7%), that each reactor worked as an ideal continuous stirred tank reactor and the secondary settler model flux was similar to a plug-flow reactor. The ASM2d-N 2 O model was then calibrated using experimental data obtained under dynamic conditions. A global sensitivity analysis was used to select, among 59 model parameters, five candidates that resulted to be related to nitrifying organisms. Different parameter subsets up to four parameters were evaluated, being the subset [µ NOB , q AOB_AMO , K O2_NOB , K NO2_NOB ] the best, achieving 53.3% reduction of the calibration cost function. The model fit obtained provided a reasonably description of nutrients and N 2 O emission trends, considering the inherent operational variability suffered in full-scale WWTPs. Finally, a simulation-based study showed that, for the given WWTP and operational conditions, an unbalanced distribution of flow-rate between the two treatment lines did not result in a significant increase on N 2 O emissions. The results obtained show that this model can be a suitable tool for predicting N 2 O emissions in full-scale WWTPs, and can therefore be used to find operational conditions that help to minimise these emissions. [ABSTRACT FROM AUTHOR]

Abstract: The main objective of this work was the design of different effluent quality controllers and a cost controller for WWTPs. This study was based on the relative gain array (RGA) analysis applied to an anaerobic/anoxic/aerobic (A2/O) configuration of a simulated WWTP, with combined removal of organic matter, nitrogen and phosphorus. The RGA analysis was able to point out the best pairing amongst the input and the output control variables of the plant to design low order and decentralized effluent quality controllers, such as proportional-integral controllers for each variable of interest (ammonium, nitrate and phosphate). In a second step, a cost controller to automatically search for the most economic setpoints of the effluent quality controllers was implemented based on the best decentralized control structure tested. The simulated plant was operated under different control modes that chronologically represent control configurations becoming gradually more complex: (i) in open loop; (ii) with dissolved oxygen (DO) control in the last aerobic reactor only; (iii) with the effluent quality controllers active; (iv) with the effluent quality controllers active and automatically receiving the setpoints from a cost controller. The effluent quality controllers alone and the cost control together with effluent quality controllers could save up to 42,000 Euros/year and 225,000 Euros/year, respectively, when compared to the operating costs of the plant operating with DO control (a reduction of 2.5% and 13% of the operating costs, respectively). The cost controller proved to be a good tool for automating the search of the most profitable setpoints of the effluent quality controllers for a given cost setpoint. [Copyright &y& Elsevier]

Enhanced biological phosphorus removal (EBPR) is an efficient and sustainable technology to remove phosphorus from wastewater. A widely known cause of EBPR deterioration in wastewater treatment plants (WWTPs) is the presence of nitrate/nitrite or oxygen in the anaerobic reactor. Moreover, most existing studies on the effect of either permanent aerobic conditions or inhibition of EBPR by nitrate or free nitrous acid (FNA) have been conducted with a " Candidatus Accumulibacter" or Tetrasphaera -enriched sludge, which are the two major reported groups of polyphosphate accumulating organisms (PAO) with key roles in full-scale EBPR WWTPs. This work reports the denitrification capabilities of a bio-P microbial community developed using glutamate as the sole source of carbon and nitrogen. This bio-P sludge exhibited a high denitrifying PAO (DPAO) activity, in fact, 56% of the phosphorus was uptaken under anoxic conditions. Furthermore, this mixed culture was able to use nitrite and nitrate as electron acceptor for P-uptake, being 1.8 μg HNO 2 –N·L−1 the maximum FNA concentration at which P-uptake can occur. Net P-removal was observed under permanent aerobic conditions. However, this microbial culture was more sensitive to FNA and permanent aerobic conditions compared to " Ca. Accumulibacter"-enriched sludge. [Display omitted] • A glutamate-fed bio-P sludge used nitrite/nitrate as electron acceptor for P-uptake. • Total P-uptake inhibition by FNA was observed at 2 μg HNO 2 –N L−1. • P-uptake rate was reduced by 72% at an initial concentration of 83 mg N–NO 3 - L−1. • Our bio-P sludge was more sensitive to oxygen than Accumulibacter -enriched cultures. [ABSTRACT FROM AUTHOR]