Evaluating the opportunities for mainstream P-recovery in anaerobic/anoxic/aerobic systems.

Mainstream P-recovery can help wastewater treatment plants (WWTPs) to effectively maintain good enhanced biological phosphorus removal (EBPR) while helping to recover P. In this study, a pilot-scale anaerobic-anoxic-aerobic (A2O) process was operated for simultaneous COD/N/P removal and P-recovery under different operational conditions. The operation with conventional extraction of waste activated sludge (WAS) from the aerobic reactor was compared to the mainstream P-recovery strategy of WAS extraction from the anaerobic reactor. Successful nutrient removal was obtained for both scenarios, but the anaerobic WAS extraction results improved polyphosphate accumulating organisms (PAOs) activity by increasing almost 27 % P concentration in the anaerobic reactor. WAS fermentation was also evaluated, showing that anaerobic WAS required only 3 days to reach a high P concentration, while the aerobic WAS fermentation required up to 7 days. The fermentation process increased the amount of soluble P available for precipitation from 24.4 % up to 51.6 % in the fermented anaerobic WAS scenario. Results obtained by precipitation modelling of these streams showed the limitations for struvite precipitation due to Ca2+ interference and Mg2+ and NH4+ as limiting species. The optimum precipitation scenario showed that P-recovery could reach up to 51 % of the input P, being 90 % struvite.

Exploring the stability of an A-stage-EBPR system for simultaneous biological removal of organic matter and phosphorus.

This work evaluates the performance and stability of a continuous anaerobic/aerobic A-stage system with integrated enhanced biological phosphorus removal (A-stage-EBPR) under different operational conditions. Dissolved oxygen (DO) in the aerobic reactor was tested in the 0.2-2 mgDO/L range using real wastewater amended with propionic acid, obtaining almost full simultaneous COD and P removal without nitrification in the range 0.5-1 mgDO/L, but failing at 0.2 mgDO/L. Anaerobic purge was tested to evaluate a possible mainstream P-recovery strategy, generating a P-enriched stream containing 22% of influent P. COD and N mass balances indicated that about 43% of the influent COD could be redirected to the anaerobic digestion for methane production and 66% of influent NH4+-N was discharged in the effluent for the following N-removal B-stage. Finally, when the system was switched to glutamate as sole carbon source, successful EBPR activity and COD removal were maintained for two months, but after this period settleability problems appeared with biomass loss. Microbial community analysis indicated that Propionivibrio, Thiothrix and Lewinella were the most abundant species when propionic acid was the carbon source and Propionivibrio was the most favoured with glutamate. Thiothrix, Hydrogenophaga, Dechloromonas and Desulfobacter appeared as the dominant polyphosphate-accumulating organisms (PAOs) under different operation stages.

A review on the integration of mainstream P-recovery strategies with enhanced biological phosphorus removal.

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.

Nitrite and nitrate inhibition thresholds for a glutamate-fed bio-P sludge.

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 HNO2-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.

Achieving simultaneous biological COD and phosphorus removal in a continuous anaerobic/aerobic A-stage system.

Recovering energy from wastewater in addition to its treatment is a hot trend in the new concept of water resource recovery facility (WRRF). High-rate systems operating at low solid retention time (SRT) have been proposed to meet this challenge. In this paper, the integration of Enhanced Biological Phosphorus Removal (EBPR) in an anaerobic/aerobic continuous high-rate system (A-stage EBPR) was evaluated. Successful P and COD removal were obtained operating at SRT 6, 5 and 4 days treating real wastewater, while a further decrease to 3 days led to biomass washout. The best steady state operational conditions were obtained at SRT = 4d, with high removal percentage of P (94.5%) and COD (96.3%), and without detecting nitrification. COD mineralization could be reduced to 30%, while 64 % of the entering carbon could be diverted as biomass to energy recovery. Regarding nitrogen, about 69±1% of the influent N was left as ammonium in the effluent, with 30% used for biomass growth. The aerobic reactor could be operated at low dissolved oxygen (DO) (0.5 mg/L), which is beneficial to decrease energy requirements. Biochemical methane potential (BMP) tests showed better productivity for the anaerobic sludge than the aerobic sludge, with an optimal BMP of 296±2 mL CH4/gVSS. FISH analysis at SRT = 4d revealed a high abundance of Accumulibacter (33±13%) and lower proportion of GAO: Competibacter (3.0±0.3%), Defluviicoccus I (0.6±0.1%) and Defluviicoccus II (4.3±1.1%).

Systematic comparison framework for selecting the best retrofitting alternative for an existing water resource recovery facility.

A systematic comparison framework for selecting the best retrofitting alternative for a water resource recovery facility (WRRF) is proposed in this work. The procedure is applied comparing different possible plant configurations to retrofit an existent anoxic/oxic (A/O) WRRF (Manresa, Spain) aiming to include enhanced biological phosphorus removal (EBPR). The framework for comparison was built on system analysis using a calibrated IWA ASM2d model. A multicriteria set of performance variables, as the operational and capital expenditures (OPEX and CAPEX, respectively) and robustness tests for measuring how fast the plant configuration refuses external disturbances (like ammonium and phosphate peak loads), were used for comparison. Starting from the existent WRRF, four plant configurations were tested: single A2 /O (only one anoxic reactor converted to anaerobic), double A2 /O (two anoxic reactors converted to anaerobic), BARDENPHO, and UCT. The double A2 /O plant configuration was the most economical and reliable alternative for improving the existent Manresa WRRF capacity and implementing EBPR, since the effluent quality increased 3.8% compared to the current plant configuration. In addition, the double A2 /O CAPEX was close to €165,000 which was at the same order of the single A2 /O and lower than the BARDENPHO and UCT alternatives. PRACTITIONER POINTS: Four configurations including EBPR were evaluated for retrofitting an A/O WRRF. A new multicriteria comparison framework was used to select the best configuration. Up to 13 criteria related to effluent quality, robustness and costs were included. A single function based on the combination of all the criteria was also evaluated.

Living on the edge: Prospects for enhanced biological phosphorus removal at low sludge retention time under different temperature scenarios.

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.

Evaluation of the integration of P recovery, polyhydroxyalkanoate production and short cut nitrogen removal in a mainstream wastewater treatment process.

Wastewater treatment systems are nowadays evolving into systems where energy and resources are recovered from wastewater. This work presents the long term operation of a demo-scale pilot plant (7.8 m3) with a novel configuration named as mainstream SCEPPHAR (ShortCut Enhanced Phosphorus and polyhydroxyalkanoate (PHA) Recovery) and based on two sequencing batch reactors (R1-HET and R2-AUT). This is the first report of an implementation at demo scale and under relevant operational conditions of the simultaneous integration of shortcut nitrification, P recovery and production of sludge with a higher PHA content than conventional activated sludge. An operating period under full nitrification mode achieved successful removal efficiencies for total N, P and CODT (86 ± 12%, 93 ± 9% and 79 ± 6%). In the following period, nitrite shortcut (with undetectable activity of nitrite oxidising bacteria) was achieved by implementing automatic control of the aerobic phase length in R2-AUT using ammonium measurement and operating at a lower sludge retention time. Similar N, P and CODT removal efficiencies to the full nitrification period were obtained. P-recovery from the anaerobic supernatant of R1-HET was achieved in a separate precipitator by increasing pH and dosing MgCl2, recovering an average value of 45% of the P in the influent as struvite precipitate, with a peak up to 63%. These values are much higher than the typical values of sidestream P-recovery (12%). Regarding PHA, a percentage in the biomass in the range 6.9-9.2% (gPHA·g-1TSS) was obtained.

Glutamate as sole carbon source for enhanced biological phosphorus removal.

Enhanced Biological Phosphorus Removal (EBPR) is based on the enrichment of sludge in polyphosphate accumulating organisms (PAO). Candidatus Accumulibacter is the bacterial community member most commonly identified as PAO in EBPR systems when volatile fatty acids (VFA) are the carbon source. However, it is necessary to understand the role of non-Accumulibacter PAO in the case of wastewater with low VFA content. This work shows the first successful long-term operation of an EBPR system with glutamate as sole carbon and nitrogen source, resulting in the enrichment of sludge in the genus Thiothrix (37%), the family Comamonadaceae (15.6%) and Accumulibacter (7.7%). The enrichment was performed in an anaerobic/anoxic/oxic (A2/O) continuous pilot plant, obtaining stable biological N and P removal. This microbial community performed anaerobic P-release with only 18-29% of the observed PHA storage in Accumulibacter-enriched sludge and with slight glycogen storage instead of consumption, indicating the involvement of other carbon storage routes not related to PHA and glycogen. Thiothrix could be clearly involved in P-removal because it is able of accumulating Poly-P, probably without PHA synthesis, but with glutamate involvement. On the other hand, Comamonadaceae could participate in degradation of glutamate and denitrification, but its involvement in P-uptake cannot be reliably concluded.

Enhanced Biological Phosphorus Removal at low Sludge Retention Time in view of its integration in A-stage systems.

The two-stage A/B WWTP configuration is being studied as a possible wastewater treatment with low energy consumption or even with a net energy generation. The first phase, A-stage, is designed to remove organic matter at very short Sludge Retention Time (SRT), while the B-stage is based on autotrophic nitrogen removal. However, P-removal in the A/B process usually only relies on precipitation. This work studies the potential inclusion of Enhanced Biological Phosphorus Removal (EBPR) in the A-stage phase. For this aim, the long-term operation of three different Sequencing Batch Reactors (SBR) enriched in Accumulibacter at low SRT was thoroughly monitored for more than three months each one. This work shows that EBPR can be sustained with a minimal SRT of 3.6 d at 25 °C. Lower values, SRT = 3 d, led to the PAO washout because of a reduction in P-release and P-uptake, an increase of the VSS/TSS ratio and a decrease of the P/C ratio. The Yobs could be related to the SRT with the parameters Y = 0.39 ± 0.05 gCODX·g-1CODS and kD = 0.06 ± 0.04 d-1 which leads to a 24% increase of biomass yield when SRT was reduced from 10 to 4 d.

Assessment of crude glycerol for Enhanced Biological Phosphorus Removal: Stability and role of long chain fatty acids.

Enhanced Biological Phosphorus Removal (EBPR) of urban wastewaters is usually limited by the available carbon source required by Polyphosphate Accumulating Organisms (PAO). External carbon sources as volatile fatty acids (VFA) or other pure organic compounds have been tested at lab scale demonstrating its ability to enhance PAO activity, but the application of this strategy at full-scale WWTPs is not cost-effective. The utilization of industrial by-products with some of these organic compounds provides lower cost, but it has the possible drawback of having inhibitory or toxic compounds to PAO. This study is focused on the utilization of crude glycerol, the industrial by-product generated in the biodiesel production, as a possible carbon source to enhance EBPR in carbon-limited urban wastewaters. Crude glycerol has non-negligible content of other organic compounds as methanol, salts, VFA and long chain fatty acids (LCFA). VFA and methanol have been demonstrated to enhance PAO activity, but there is no previous study about the effect of LCFA on PAO. This work presents the operation of an EBPR SBR system using crude glycerol as sole carbon source, studying also its long-term stability. The effect of LCFA is evaluated at short and long-term operation, demonstrating for the first time EBPR activity with LCFA as sole carbon source and its long-term failure due to the increased hydrophobicity of the sludge.

Cost and effluent quality controllers design based on the relative gain array for a nutrient removal WWTP.

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 (A(2)/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.

Experimental assessment and modelling of the proton production linked to phosphorus release and uptake in EBPR systems.

The modelling of the enhanced biological phosphorus removal (EBPR) process is a recent focus of interest. The pH profile is a promising output variable for EBPR modelling as it is very sensitive to the consumption or production of acid and base species (e.g. phosphate or VFA). pH-based EBPR modelling is based on the assumption that phosphorus is released and taken up as H(2)PO(4)(-), but this assumption has not been experimentally confirmed yet with enriched EBPR biomass. Therefore, the objective of this work was to assess the species in which P is released and taken up under different pH conditions. Several batch experiments were performed with an enriched culture of Accumulibacter (around 70+/-10% of total microorganisms). The total observed proton production, inorganic carbon, ammonium, phosphate and VFA were measured to evaluate the titrimetric contribution of anaerobic P-release and aerobic P-uptake over the total observed proton production. The results show that the only phosphorus form involved in P-release and P-uptake is equivalent in terms of proton production to H(2)PO(4)(-) in the pH range of 6.5-8.5. Finally, proton production and pH in several SBR cycles were modelled and resulted in good agreement with the experimental profiles.

Net P-removal deterioration in enriched PAO sludge subjected to permanent aerobic conditions.

Recently, some research in the field of enhanced biological phosphorus removal (EBPR) has been focused on studying systems where the electron donor (substrate) and the electron acceptor (nitrate or oxygen) are present simultaneously. This can occur, for example, in a full scale wastewater treatment plant during heavy rainfall periods when the anaerobic hydraulic retention time is temporarily shortened. To study this situation that could induce EBPR failure, the operation of a sequencing batch reactor (SBR) working under alternating anaerobic-aerobic conditions with an enriched EBPR population (50% Candidatus Accumulibacter phosphatis and less than 1% Candidatus Competibacter phosphatis) was shifted to strict aerobic operation. Seven cycle studies were performed during the 11 days of aerobic operation. Net P-removal was observed in this aerobic SBR during the first 4 days of operation but the system could not achieve net-P removal after this period, although the microbial composition, in terms of percentage of Accumulibacter and Competibacter, did not change significantly. The observed changes in the different compounds analysed (phosphorus, acetate, glycogen and PHB) as well as in the OUR profile indicate that metabolic changes are produced for the adaptation of PAO to aerobic conditions.