Purple phototrophic bacteria for resource recovery: Challenges and opportunities.

Sustainable development is driving a rapid focus shift in the wastewater and organic waste treatment sectors, from a "removal and disposal" approach towards the recovery and reuse of water, energy and materials (e.g. carbon or nutrients). Purple phototrophic bacteria (PPB) are receiving increasing attention due to their capability of growing photoheterotrophically under anaerobic conditions. Using light as energy source, PPB can simultaneously assimilate carbon and nutrients at high efficiencies (with biomass yields close to unity (1 g CODbiomass·g CODremoved-1)), facilitating the maximum recovery of these resources as different value-added products. The effective use of infrared light enables selective PPB enrichment in non-sterile conditions, without competition with other phototrophs such as microalgae if ultraviolet-visible wavelengths are filtered. This review reunites results systematically gathered from over 177 scientific articles, aiming at producing generalized conclusions. The most critical aspects of PPB-based production and valorisation processes are addressed, including: (i) the identification of the main challenges and potentials of different growth strategies, (ii) a critical analysis of the production of value-added compounds, (iii) a comparison of the different value-added products, (iv) insights into the general challenges and opportunities and (v) recommendations for future research and development towards practical implementation. To date, most of the work has not been executed under real-life conditions, relevant for full-scale application. With the savings in wastewater discharge due to removal of organics, nitrogen and phosphorus as an important economic driver, priorities must go to using PPB-enriched cultures and real waste matrices. The costs associated with artificial illumination, followed by centrifugal harvesting/dewatering and drying, are estimated to be 1.9, 0.3-2.2 and 0.1-0.3 $·kgdry biomass-1. At present, these costs are likely to exceed revenues. Future research efforts must be carried out outdoors, using sunlight as energy source. The growth of bulk biomass on relatively clean wastewater streams (e.g. from food processing) and its utilization as a protein-rich feed (e.g. to replace fishmeal, 1.5-2.0 $·kg-1) appears as a promising valorisation route.

Model-based analysis and optimization of a full-scale industrial high-rate anaerobic bioreactor.

The objective of this paper is to present the model-based optimization results of an anaerobic granular sludge internal circulation reactor. The International Water Association Anaerobic Digestion Model No. 1 extended with phosphorus (P), sulfur (S), and ethanol is used to describe the main biological and physico-chemical processes. The high-rate conditions within the reactor are simulated using a flow + reactor model comprised of a series of continuous stirred tank reactors followed by an ideal total suspended solids separation unit. Following parameter estimation by least squares on the measured data, the model had a relative mean error of 13 and 15% for data set #1 and data set #2, respectively. Response surfaces show that the reactor performance index (a metric combining energy recovery in the form of heat and electricity, as well as chemicals needed for pH control) could be improved by 45% when reactor pH is reduced down to 6.8. Model-based results reveal that influent S does not impose sufficient negative impacts on energy recovery (+5.7%, in MWh/day,+0.20 M€/year when influent S is removed) to warrant the cost of its removal (3.58 M€/year). In fact, the process could handle even higher S loads (ensuring the same degree of conversion) as long as the pH is maintained above 6.8. Nevertheless, a higher S load substantially increases the amount of added NaOH to maintain the desired operational pH (>25%) due to the acidic behavior of HS - . CO 2 stripping decreases the buffer capacity of the system and hence use of chemicals for pH control. Finally, the paper discusses the possibilities and limitations of the proposed approach, and how the results of this study will be put into practice.

Predicting scale formation during electrodialytic nutrient recovery.

Electro-concentration of nutrients from waste streams is a promising technology to enable resource recovery, but has several operational concerns. One key concern is the formation of inorganic scale on the concentrate side of cation exchange membranes when recovering nutrients from wastewaters containing calcium, magnesium, phosphorous and carbonate, commonly present in anaerobic digester rejection water. Electrodialytic nutrient recovery was trialed on anaerobic digester rejection water in a laboratory scale electro-concentration unit without treatment (A), following struvite recovery (B), and following struvite recovery as well as concentrate controlled at pH 5 for scaling control (C). Treatment A resulted in large amount of scale, while treatment B significantly reduced the amount of scale formation with reduction in magnesium phosphates, and treatment C reduced the amount of scale further by limiting the formation of calcium carbonates. Treatment C resulted in an 87 ± 7% by weight reduction in scale compared to treatment A. A mechanistic model for the inorganic processes was validated using a previously published general precipitation model based on saturation index. The model attributed the reduction in struvite scale to the removal of phosphate during the struvite pre-treatment, and the reduction in calcium carbonate scale to pH control resulting in the stripping of carbonate as carbon dioxide gas. This indicates that multiple strategies may be required to control precipitation, and that mechanistic models can assist in developing a combined approach.

Modelling phosphorus (P), sulfur (S) and iron (Fe) interactions for dynamic simulations of anaerobic digestion processes.

This paper proposes a series of extensions to functionally upgrade the IWA Anaerobic Digestion Model No. 1 (ADM1) to allow for plant-wide phosphorus (P) simulation. The close interplay between the P, sulfur (S) and iron (Fe) cycles requires a substantial (and unavoidable) increase in model complexity due to the involved three-phase physico-chemical and biological transformations. The ADM1 version, implemented in the plant-wide context provided by the Benchmark Simulation Model No. 2 (BSM2), is used as the basic platform (A0). Three different model extensions (A1, A2, A3) are implemented, simulated and evaluated. The first extension (A1) considers P transformations by accounting for the kinetic decay of polyphosphates (XPP) and potential uptake of volatile fatty acids (VFA) to produce polyhydroxyalkanoates (XPHA) by phosphorus accumulating organisms (XPAO). Two variant extensions (A2,1/A2,2) describe biological production of sulfides (SIS) by means of sulfate reducing bacteria (XSRB) utilising hydrogen only (autolithotrophically) or hydrogen plus organic acids (heterorganotrophically) as electron sources, respectively. These two approaches also consider a potential hydrogen sulfide ( [Formula: see text] inhibition effect and stripping to the gas phase ( [Formula: see text] ). The third extension (A3) accounts for chemical iron (III) ( [Formula: see text] ) reduction to iron (II) ( [Formula: see text] ) using hydrogen ( [Formula: see text] ) and sulfides (SIS) as electron donors. A set of pre/post interfaces between the Activated Sludge Model No. 2d (ASM2d) and ADM1 are furthermore proposed in order to allow for plant-wide (model-based) analysis and study of the interactions between the water and sludge lines. Simulation (A1 - A3) results show that the ratio between soluble/particulate P compounds strongly depends on the pH and cationic load, which determines the capacity to form (or not) precipitation products. Implementations A1 and A2,1/A2,2 lead to a reduction in the predicted methane/biogas production (and potential energy recovery) compared to reference ADM1 predictions (A0). This reduction is attributed to two factors: (1) loss of electron equivalents due to sulfate [Formula: see text] reduction by XSRB and storage of XPHA by XPAO; and, (2) decrease of acetoclastic and hydrogenotrophic methanogenesis due to [Formula: see text] inhibition. Model A3 shows the potential for iron to remove free SIS (and consequently inhibition) and instead promote iron sulfide (XFeS) precipitation. It also reduces the quantities of struvite ( [Formula: see text] ) and calcium phosphate ( [Formula: see text] ) that are formed due to its higher affinity for phosphate anions. This study provides a detailed analysis of the different model assumptions, the effect that operational/design conditions have on the model predictions and the practical implications of the proposed model extensions in view of plant-wide modelling/development of resource recovery strategies.

A plant-wide aqueous phase chemistry module describing pH variations and ion speciation/pairing in wastewater treatment process models.

There is a growing interest within the Wastewater Treatment Plant (WWTP) modelling community to correctly describe physico-chemical processes after many years of mainly focusing on biokinetics. Indeed, future modelling needs, such as a plant-wide phosphorus (P) description, require a major, but unavoidable, additional degree of complexity when representing cationic/anionic behaviour in Activated Sludge (AS)/Anaerobic Digestion (AD) systems. In this paper, a plant-wide aqueous phase chemistry module describing pH variations plus ion speciation/pairing is presented and interfaced with industry standard models. The module accounts for extensive consideration of non-ideality, including ion activities instead of molar concentrations and complex ion pairing. The general equilibria are formulated as a set of Differential Algebraic Equations (DAEs) instead of Ordinary Differential Equations (ODEs) in order to reduce the overall stiffness of the system, thereby enhancing simulation speed. Additionally, a multi-dimensional version of the Newton-Raphson algorithm is applied to handle the existing multiple algebraic inter-dependencies. The latter is reinforced with the Simulated Annealing method to increase the robustness of the solver making the system not so dependent of the initial conditions. Simulation results show pH predictions when describing Biological Nutrient Removal (BNR) by the activated sludge models (ASM) 1, 2d and 3 comparing the performance of a nitrogen removal (WWTP1) and a combined nitrogen and phosphorus removal (WWTP2) treatment plant configuration under different anaerobic/anoxic/aerobic conditions. The same framework is implemented in the Benchmark Simulation Model No. 2 (BSM2) version of the Anaerobic Digestion Model No. 1 (ADM1) (WWTP3) as well, predicting pH values at different cationic/anionic loads. In this way, the general applicability/flexibility of the proposed approach is demonstrated, by implementing the aqueous phase chemistry module in some of the most frequently used WWTP process simulation models. Finally, it is shown how traditional wastewater modelling studies can be complemented with a rigorous description of aqueous phase and ion chemistry (pH, speciation, complexation).

Low pH anaerobic digestion of waste activated sludge for enhanced phosphorous release.

This paper assesses anaerobic digestion of waste activated sludge (WAS) at low pH to enhance phosphorous solubility. Batch biochemical methane potential tests were conducted at a pH range of 5 to 7.2 in two separate sets (two different WAS samples collected from municipal WWTP). Low pH (<5.7) caused a significant (p = 0.004) decrease in methane potential (B0) up to 33% and 3.6 times increase in phosphorus release compared to neutral pH (7-7.7), but with no major change in methane production rate coefficient (khyd). The loss in methane yield was mainly due to decrease in hydrolytic capability rather than inhibition of methanogenesis with volatile fatty acids being <300 mgCOD L(-1) and soluble COD <1300 mgCOD L(-1) even at low pH. While pH did not influence the acetoclastic community (Methanosaeta dominated), it was the primary driver for the remaining community (p = 0.004), and caused a loss of diversity and shift to Clostridia.

Biological phosphorus removal from abattoir wastewater at very short sludge ages mediated by novel PAO clade Comamonadaceae.

Recent increases in global phosphorus costs, together with the need to remove phosphorus from wastewater to comply with water discharge regulations, make phosphorus recovery from wastewater economically and environmentally attractive. Biological phosphorus (Bio-P) removal process can effectively capture the phosphorus from wastewater and concentrate it in a form that is easily amendable for recovery in contrast to traditional (chemical) phosphorus removal processes. However, Bio-P removal processes have historically been operated at medium to long solids retention times (SRTs, 10-20 days typically), which inherently increases the energy consumption while reducing the recoverable carbon fraction and hence makes it incompatible with the drive towards energy self-sufficient wastewater treatment plants. In this study, a novel high-rate Bio-P removal process has been developed as an energy efficient alternative for phosphorus removal from wastewater through operation at an SRT of less than 4 days. The process was most effective at an SRT of 2-2.5 days, achieving >90% phosphate removal. Further reducing the SRT to 1.7 days resulted in a loss of Bio-P activity. 16S pyrotag sequencing showed the community changed considerably with changes in the SRT, but that Comamonadaceae was consistently abundant when the Bio-P activity was evident. FISH analysis combined with DAPI staining confirmed that bacterial cells of Comamonadaceae arranged in tetrads contained polyphosphate, identifying them as the key polyphosphate accumulating organisms at these low SRT conditions. Overall, this paper demonstrates a novel, high-rate phosphorus removal process that can be effectively integrated with short SRT, energy-efficient carbon removal and recovery processes.

Phototrophic bacteria for nutrient recovery from domestic wastewater.

The organics and nutrients in industrial and domestic wastewater are increasingly being regarded as a valuable resource for energy and nutrient recovery. Emerging concepts to redesign wastewater treatment as resource recovery systems include the use of different bacteria and algae to partition carbon and nutrients to the particulate phase through assimilation or bio-accumulation. This study evaluates the use of purple phototrophic bacteria (PPB) (also known as purple non-sulphur bacteria or PNSB) for such a biological concentration process through a series of batch tests. The key objectives are to (a) demonstrate consistent selection and enrichment of PPB using infrared light in a non-sterile medium, and (b) achieve effective partitioning of soluble organics, ammonium and phosphate into the PPB culture. PPB were successfully enriched from pre-settled domestic wastewater within 2-3 days and identified as members of the order Rhodobacterales. Under anaerobic conditions with infrared irradiation the enrichment culture was able to simultaneously remove COD (63 ± 5%), NH4-N (99.6%-0.12 ± 0.03 mgN L(-1)) and PO4-P (88%-0.8 ± 0.6 mgP L(-1)) from primary settled domestic wastewater in 24 h. In this experiment, acetate was added as an additional carbon source to demonstrate the maximal nitrogen and phosphorous elimination potential. Almost all the COD removed was assimilated into biomass rather than oxidised to CO2, with the total COD actually increasing during the batch experiments due to phototrophic synthesis. NH4-N and PO4-P were also assimilated by the biomass rather than removed through destructive oxidation or accumulation. The process offers the opportunity to concentrate organics and macronutrients from wastewater in one solids stream that can be anaerobically digested to generate energy and recover nutrients from the concentrated digestate. Technical challenges include the design of a continuous reactor system, as well as efficient delivery of electrons, either through light or chemical sources.

Biochemical methane potential of beef feedlot manure: impact of manure age and storage.

Methane capture and use from intensive livestock industries is relatively new, and there is limited chemical and kinetic degradation information available for beef feedlot manure in Australia or internationally. This paper evaluates the biochemical methane (CH) potential, apparent first-order hydrolysis rate coefficient, and losses in organic content of manure as it ages on feedlot pads and in stockpiles. Chemical characterization of fresh, pad, and stockpiled manure is assessed. Biochemical CH potential on volatile solids (VS) almost always decreased significantly from fresh to pad and from pad to stockpile, ranging (in mL CH g VS) from 230 to 360 in fresh manure, from 70 to 280 in pads, and from 60 to 200 in stockpiles. Kinetics of degradation also varied with manure age (fresh: 0.12 ± 0.01 d; pad: 0.06 ± 0.02 d; and stockpiled: 0.05 ± 0.04 d). At least 50%, and up to 80%, of the original biochemical CH potential of the manure (i.e., degradable material) was lost on drying in pads, and the loss after stockpiling was much greater (>85%). The loss of N was 15 to 60% as manure dried on pads and was much greater after stockpiling (40-90%). Phosphorus loss, though lower than nitrogenous losses, was still significant (25-65% on pads and 35-85% in stockpiles). Although digestion of stockpiled manure is still feasible to generate energy, collection of fresh manure is important to maximize outcomes, with a possible order of magnitude increase in CH production achievable.

Operating aerobic wastewater treatment at very short sludge ages enables treatment and energy recovery through anaerobic sludge digestion.

Conventional abattoir wastewater treatment processes for carbon and nutrient removal are typically designed and operated with a long sludge retention time (SRT) of 10-20 days, with a relatively high energy demand and physical footprint. The process also generates a considerable amount of waste activated sludge that is not easily degradable due to the long SRT. In this study, an innovative high-rate sequencing batch reactor (SBR) based wastewater treatment process with short SRT and hydraulic retention time (HRT) is developed and characterised. The high-rate SBR process was shown to be most effective with SRT of 2-3 days and HRT of 0.5-1 day, achieving >80% reduction in chemical oxygen demand (COD) and phosphorus and approximately 55% nitrogen removal. A majority of carbon removal (70-80%) was achieved by biomass assimilation and/or accumulation, rather than oxidation. Anaerobic degradability of the sludge generated in the high-rate SBR process was strongly linked to SRT, with measured degradability extent being 85% (2 days SRT), 73% (3 days), and 63% (4 days), but it was not influenced by digestion temperature. However, the rate of degradation for 3 and 4 days SRT sludge was increased by 45% at thermophilic conditions compared to mesophilic conditions. Overall, the treatment process provides a very compact and energy efficient treatment option for highly degradable wastewaters such as meat and food processing, with a substantial space reduction by using smaller reactors and a considerable net energy output through the reduced aerobic oxidation and concurrent increased methane production potential through the efficient sludge digestion.

Nutrient solubilization and its availability following anaerobic digestion.

This study aims to investigate solubilization of elements (P, N, K, Ca and Mg) during anaerobic digestion (AD) of solid agriculture waste. It is important to maintain particularly phosphorous in the aqueous phase to be able to subsequently recover it in a concentrated form via crystallization. Batch AD was carried out at a mesophilic condition (37 °C) and pH 7.0 ± 0.2 on a variety of piggery and poultry solid waste streams. Less than 10% of the total P, Ca and Mg was in soluble form in the digestate. Most of the N and K remained soluble in the digestate. A bioavailability test (citric acid extraction) showed P, Ca and Mg in the digestate were totally available. Complete solubilization of P, Ca and Mg occurred below a threshold of pH 5.5. This indicates these nutrients were released during digestion, and then either bound to form inorganic compounds or adsorbed on solid surfaces in the digestate. These effects reduce the feasibility of post-digestion recovery of the nutrients via struvite crystallization. Strategies to improve nutrient solubilization and recovery during the AD include addition of complexing chemicals, operation at depressed pH, or otherwise modifying the operating conditions.

Phosphorus recovery from wastewater through microbial processes.

Waste streams offer a compelling opportunity to recover phosphorus (P). 15-20% of world demand for phosphate rock could theoretically be satisfied by recovering phosphorus from domestic waste streams alone. For very dilute streams (<10 mg PL(-1)), including domestic wastewater, it is necessary to concentrate phosphorus in order to make recovery and reuse feasible. This review discusses enhanced biological phosphorus removal (EBPR) as a key technology to achieve this. EBPR relies on polyphosphate accumulating organisms (PAOs) to take up phosphorus from waste streams, so concentrating phosphorus in biomass. The P-rich biosolids can be either directly applied to land, or solubilized and phosphorus recovered as a mineral product. Direct application is effective, but the product is bulky and carries contaminant risks that need to be managed. Phosphorus release can be achieved using either thermochemical or biochemical methods, while recovery is generally by precipitation as struvite. We conclude that while EBPR technology is mature, the subsequent phosphorus release and recovery technologies need additional development.