Modified Poly(acrylic acid)-Based Hydrogels for Enhanced Mainstream Removal of Ammonium from Domestic Wastewater.

Rapid and continuous ammonium adsorption from mainstream coupled with side-stream ammonium recovery and adsorbent regeneration could enable ammonium recovery from domestic wastewater. This study describes the use of tailored poly(acrylic acid)-based (NaPAA) hydrogels as effective sorbents for ammonium removal from domestic wastewater. Modified NaPAA hydrogels having 60% ionization and 4.8 mol % N',N'-methylenebisacrylamide as the cross-linker reduced the overall swelling by 92% from 407 to 31 g/g because of higher cross-linking density. At hydrogel loadings of 2.5-7.5 g/L, the NaPAA hydrogels achieved ammonium concentrations of 8.3 ± 0.6 to 10.1 ± 0.1 mg/L NH4-N, which corresponds to removal efficiencies of 53-77% after 10 min of contact time in real domestic wastewater. At the same hydrogel loadings, the ammonium removal efficiency of NaPAA hydrogels in synthetic wastewater was found to be comparable to that in real sewage (71% vs 69%, respectively), suggesting that the sorption performance is only marginally affected by organic constituents found in domestic wastewater. In addition, the NaPAA hydrogels removed 25-51% ammonium in 10 min from synthetic streams having 200-400% higher ionic strengths than those commonly observed in sewage. Furthermore, simulation studies showed that a discharge concentration of ∼1.9 mg/L NH4-N, well below the commonly applied discharge limits in most regions, can be achieved using mainstream ammonium removal by NaPAA hydrogels followed by biological assimilation from the growth of ordinary heterotrophic organisms.

Mainstream Ammonium Recovery to Advance Sustainable Urban Wastewater Management.

Throughout the 20th century, the prevailing approach toward nitrogen management in municipal wastewater treatment was to remove ammonium by transforming it into dinitrogen (N2) using biological processes such as conventional activated sludge. While this has been a very successful strategy for safeguarding human health and protecting aquatic ecosystems, the conversion of ammonium into its elemental form is incompatible with the developing circular economy of the 21st century. Equally important, the activated sludge process and other emerging ammonium removal pathways have several environmental and technological limitations. Here, we assess that the theoretical energy embedded in ammonium in domestic wastewater represents roughly 38-48% of the embedded chemical energy available in the whole of the discharged bodily waste. The current routes for ammonium removal not only neglect the energy embedded in ammonium, but they can also produce N2O, a very strong greenhouse gas, with such emissions comprising the equivalent of 14-26% of the overall carbon footprint of wastewater treatment plants. N2O emissions often exceed the carbon emissions related to the electricity consumption for the process requirements of WWTPs. Considering these limitations, there is a need to develop alternative ammonium management approaches that center around recovery of ammonium from domestic wastewater rather than deal with its "destruction" into elemental dinitrogen. Current ammonium recovery techniques are applicable only at orders of magnitude above domestic wastewater strength, and so new techniques based on physicochemical adsorption are of particular interest. A new pathway is proposed that allows for mainstream ammonium recovery from wastewater based on physicochemical adsorption through development of polymer-based adsorbents. Provided adequate adsorbents corresponding to characteristics outlined in this paper are designed and brought to industrial production, this adsorption-based approach opens perspectives for mainstream continuous adsorption coupled with side-stream recovery of ammonium with minimal chemical requirements. This proposed pathway can bring forward an effective resource-oriented approach to upgrade the fate of ammonium in urban water management without generating hidden externalized environmental costs.

Decoupling Livestock from Land Use through Industrial Feed Production Pathways.

One of the main challenges for the 21st century is to balance the increasing demand for high-quality proteins while mitigating environmental impacts. In particular, cropland-based production of protein-rich animal feed for livestock rearing results in large-scale agricultural land-expansion, nitrogen pollution, and greenhouse gas emissions. Here we propose and analyze the long-term potential of alternative animal feed supply routes based on industrial production of microbial proteins (MP). Our analysis reveals that by 2050, MP can replace, depending on socio-economic development and MP production pathways, between 10-19% of conventional crop-based animal feed protein demand. As a result, global cropland area, global nitrogen losses from croplands and agricultural greenhouse gas emissions can be decreased by 6% (0-13%), 8% (-3-8%), and 7% (-6-9%), respectively. Interestingly, the technology to industrially produce MP at competitive costs is directly accessible for implementation and has the potential to cause a major structural change in the agro-food system.

Electrochemical Production of Magnetite Nanoparticles for Sulfide Control in Sewers.

Recently, naturally occurring magnetite (Fe3O4) has emerged as a new material for sulfide control in sewers. However, unrefined magnetite could have high heavy metal contents (e.g., Cr, Zn, Ni, Sn, etc.) and the capacity to remove dissolved sulfide is reasonably limited due to relatively large particle sizes. To overcome the drawbacks of unrefined magnetite we used an electrochemical system with mild steel as sacrificial electrodes to in-situ generate high strength solutions of plate-like magnetite nanoparticles (MNP). MNP with a size range between 120 and 160 nm were electrochemically generated at 9.35 ± 0.28 g Fe3O4-Fe/L, resulting in a Coulombic efficiency (CE) for iron oxidation of 93.5 ± 2.8%. The produced MNP were found to effectively reduce sulfide levels in sewage from 12.7 ± 0.3 to 0.2 ± 0.0 mg S/L at a sulfide-to-MNP ratio of 0.26 g S/g Fe3O4-Fe. Subsequently, MNP were continuously generated with polarity switching at stable cell voltage for 31 days at 4.53 ± 0.35 g Fe3O4-Fe/L with a CE for iron oxidation of 92.4 ± 7.2%. The continuously produced MNP reduced sulfide at similar levels to around 0.2 mg S/L at a ratio of 0.28 g S/g Fe3O4-Fe.

Microbes and the Next Nitrogen Revolution.

The Haber Bosch process is among the greatest inventions of the 20th century. It provided agriculture with reactive nitrogen and ultimately mankind with nourishment for a population of 7 billion people. However, the present agricultural practice of growing crops for animal production and human food constitutes a major threat to the sustainability of the planet in terms of reactive nitrogen pollution. In view of the shortage of directly feasible and cost-effective measures to avoid these planetary nitrogen burdens and the necessity to remediate this problem, we foresee the absolute need for and expect a revolution in the use of microbes as a source of protein. Bypassing land-based agriculture through direct use of Haber Bosch produced nitrogen for reactor-based production of microbial protein can be an inspiring concept for the production of high quality animal feed and even straightforward supply of proteinaceous products for human food, without significant nitrogen losses to the environment and without the need for genetic engineering to safeguard feed and food supply for the generations to come.

Enhancing toxic metal removal from acidified sludge with nitrite addition.

The production of sludge (biosolids) during wastewater treatment is a major issue for water utilities. A main issue limiting its beneficial reuse on agricultural lands is the presence of toxic metals. The currently used metal reduction technologies achieve insufficient removal of metals that are bound to the organic fraction of the sludge. In this study, we propose and demonstrate a novel method that involves the addition of nitrite during sludge acidification to enhance metal removal. Using waste activated sludge collected from three full-scale wastewater treatment plants, we found that acidification to pH 2.0 achieved good Zn solubilization of around 70%, but only 3-7% of Cu was being dissolved. Nitrite addition to the acidified sludge at a concentration of 20 mg NO2(-)-N/L (equals to 19.2 mg HNO2-N/L), substantially enhanced Cu removal to 45-64%, while Zn removal was also increased to over 81%. Metal distribution analysis using sequential chemical extraction revealed that the improvement of Cu and Zn removal was mainly due to the release of the organically bound metal fraction. We hypothesize that free nitrous acid (HNO2, FNA) may assist in the (partial) disruption of extracellular polymeric substances (EPS) and the subsequent release and solubilization of fixed metals.

Scaling-Free Electrochemical Production of Caustic and Oxygen for Sulfide Control in Sewers.

Caustic shock-loading and oxygen injection are commonly used by the water industry for biofilm and sulfide control in sewers. Caustic can be produced onsite from wastewater using a two-compartment electrochemical cell. This avoids the need for import and storage of caustic soda, which typically represents a cost and a hazard. An issue limiting the practical implementation of this approach is the occurrence of membrane scaling due to the almost universal presence of Ca(2+) and Mg(2+) in wastewater. It results in a rapid increase in the cell voltage, thereby increasing the energy consumption of the system. Here, we propose and experimentally demonstrate an innovative solution for this problem involving the inclusion of a middle compartment between the anode and cathode compartments. Caustic was efficiently produced from wastewater over a period of 12 weeks and had an average Coulombic efficiency (CE) of 84.1 ± 1.1% at practically relevant caustic strengths (∼3 wt %). Neither membrane scaling nor an increase in the cell voltage was observed throughout the experiments. In addition, dissolved oxygen was produced in the anode, resulting in continuously oxygenated wastewater leaving the three-compartment cell. This membrane-scaling control strategy represents a major step forward toward practical implementation of on-site simultaneous electrochemical caustic and oxygen generation for sulfide control in sewers and also has the potential to be applied to other (bio)electrochemical systems receiving wastewater as source for product recovery.

Dynamically adaptive control system for bioanodes in serially stacked bioelectrochemical systems.

Microbial bioelectrochemical systems (BESs) use microorganisms as catalysts for electrode reactions. They have emerging applications in bioenergy, bioproduction, and bioremediation. BESs can be scaled up as a linked series of units or cells; however, this may lead to so-called cell reversal. Here, we demonstrate a cell balance system (CBS) that controls individual BES cells connected electrically in series by dynamically adapting the applied potential in the kilohertz frequency range relative to the performance of the bioanode. The CBS maintains the cell voltage of individual BES cells at or below a maximum set point by bypassing a portion of applied current with a high-frequency metal oxide semiconductor field-effect transistor switch control system. We demonstrate (i) multiple serially connected BES cells started simultaneously and rapidly from a single power source, as the CBS imparts no current limitation, (ii) continuous, stable, and independent performance of each stacked BES cell, and (iii) stable BES cell and stack performance under excessive applied currents. This control system has applications for not only serially stacked BESs in scaled-up stacks but also rapidly starting individual- and/or lab-scale BESs.

How can we possibly resolve the planet's nitrogen dilemma?

Nitrogen is the most crucial element in the production of nutritious feeds and foods. The production of reactive nitrogen by means of fossil fuel has thus far been able to guarantee the protein supply for the world population. Yet, the production and massive use of fertilizer nitrogen constitute a major threat in terms of environmental health and sustainability. It is crucial to promote consumer acceptance and awareness towards proteins produced by highly effective microorganisms, and their potential to replace proteins obtained with poor nitrogen efficiencies from plants and animals. The fact that reactive fertilizer nitrogen, produced by the Haber Bosch process, consumes a significant amount of fossil fuel worldwide is of concern. Moreover, recently, the prices of fossil fuels have increased the cost of reactive nitrogen by a factor of 3 to 5 times, while international policies are fostering the transition towards a more sustainable agro-ecology by reducing mineral fertilizers inputs and increasing organic farming. The combination of these pressures and challenges opens opportunities to use the reactive nitrogen nutrient more carefully. Time has come to effectively recover used nitrogen from secondary resources and to upgrade it to a legal status of fertilizer. Organic nitrogen is a slow-release fertilizer, it has a factor of 2.5 or higher economic value per unit nitrogen as fertilizer and thus adequate technologies to produce it, for instance by implementing photobiological processes, are promising. Finally, it appears wise to start the integration in our overall feed and food supply chains of the exceptional potential of biological nitrogen fixation. Nitrogen produced by the nitrogenase enzyme, either in the soil or in novel biotechnology reactor systems, deserves to have a 'renaissance' in the context of planetary governance in general and the increasing number of people who desire to be fed in a sustainable way in particular.

Influence of inoculum selection on the utilisation of volatile fatty acid and glucose in sulfate reducing reactors.

The capacity of three inocula (sewer biofilm, mangrove and estuary sediment) to utilise typical fermentation products of municipal solid waste for biological sulfate reduction was investigated. Each inoculum was used in two reactors, one fed a mixture of volatile fatty acids and another fed glucose to provide a suite of fermentation products via naturally occurring fermentation. Following 228 days of reactor operation, reactors inoculated with mangrove and estuary sediments exhibited higher sulfate reducing efficiencies (80-88%) compared to the biofilm-inoculated reactors (32-49%). Minimal use of acetate and its accumulation in the biofilm-inoculated reactors pointed to the high abundance of incomplete-oxidising sulfate reducing bacteria (SRB), Desulfovibrio and Desulfobulbus (90-96% of the sulfate reducing population). Although Desulfovibrio was also prominent in reactors inoculated with mangrove and estuary sediments, Desulfobacter, a known acetoclastic sulfate reducer, emerged from trace levels in these sediment (0.01% abundance in the estuary sediments and below detection in the mangrove sediments) to comprise 14%-70% of the sulfate reducing population at the end of reactor operation.

Electrochemical iron production to enhance anaerobic membrane treatment of wastewater.

Although iron salts such as iron(III) chloride (FeCl3) have widespread application in wastewater treatment, safety concerns limit their use, due to the corrosive nature of concentrated solutions. This study demonstrates that local, electrochemical generation of iron is a viable alternative to the use of iron salts. Three laboratory systems with anaerobic membrane processes were set up to treat real wastewater; two systems used the production of either in-situ or ex-situ electrochemical iron (as Fe2+ and Fe2+(Fe3+)2O4, respectively), while the other system served as a control. These systems were operated for over one year to assess the impact of electrochemically produced iron on system performance. The results showed that dosing of electrochemical iron significantly reduced sulfide concentration in effluent and hydrogen sulfide content in biogas, and mitigated organics-based membrane fouling, all of which are critical issues inherently related to sustainability of anaerobic wastewater treatment. The electrochemical iron strategy can generate multiple benefits for wastewater management including increased removal efficiencies for total and volatile suspended solids, chemical oxygen demand and phosphorus. The rate of methane production also increased with electrochemically produced iron. Economic analysis revealed the viability of electrochemical iron with total cost reduced by one quarter to a third compared with using FeCl3. These benefits indicate that electrochemical iron dosing can greatly enhance the overall operation and performance of anaerobic membrane processes, and this particularly facilitates wastewater management in a decentralized scenario.

Assessing the potential for up-cycling recovered resources from anaerobic digestion through microbial protein production.

Anaerobic digesters produce biogas, a mixture of predominantly CH4 and CO2 , which is typically incinerated to recover electrical and/or thermal energy. In a context of circular economy, the CH4 and CO2 could be used as chemical feedstock in combination with ammonium from the digestate. Their combination into protein-rich bacterial, used as animal feed additive, could contribute to the ever growing global demand for nutritive protein sources and improve the overall nitrogen efficiency of the current agro- feed/food chain. In this concept, renewable CH4 and H2 can serve as carbon-neutral energy sources for the production of protein-rich cellular biomass, while assimilating and upgrading recovered ammonia from the digestate. This study evaluated the potential of producing sustainable high-quality protein additives in a decentralized way through coupling anaerobic digestion and microbial protein production using methanotrophic and hydrogenotrophic bacteria in an on-farm bioreactor. We show that a practical case digester handling liquid piggery manure, of which the energy content is supplemented for 30% with co-substrates, provides sufficient biogas to allow the subsequent microbial protein as feed production for about 37% of the number of pigs from which the manure was derived. Overall, producing microbial protein on the farm from available methane and ammonia liberated by anaerobic digesters treating manure appears economically and technically feasible within the current range of market prices existing for high-quality protein. The case of producing biomethane for grid injection and upgrading the CO2 with electrolytic hydrogen to microbial protein by means of hydrogen-oxidizing bacteria was also examined but found less attractive at the current production prices of renewable hydrogen. Our calculations show that this route is only of commercial interest if the protein value equals the value of high-value protein additives like fishmeal and if the avoided costs for nutrient removal from the digestate are taken into consideration.

Magnetic poly(acrylic acid)-based hydrogels for rapid ammonium sorption and efficient sorbent separation from sewage.

Ammonium sorption and recovery processes typically take place in conventional packed columns, with a configuration that enables maximum sorption by the sorbents. However, batch or semi-continuous operations in packed columns have associated issues such as scaling and frequent backwashing requirements, which are economically prohibitive. As an alternative, ammonium sorption could occur in well-mixed continuously stirred tanks, which would allow for the ammonium sorption process to be retrofitted in existing wastewater treatment plants, provided that efficient sorbent separation can be achieved. This study demonstrates, for the first time, the preparation of magnetic poly(acrylic acid)-based (PAA) ammonium sorbents through the incorporation of magnetic (Fe3O4) nanoparticles (MNP) produced via scalable and cost-effective electrochemical synthesis. The MNP and PAA hydrogels were synthesized independently and the MNPs subsequently integrated into the PAA hydrogel network by particle diffusion and physical entrapment. No adverse effects on swelling and ammonium sorption following immersion in either synthetic or real sewage were observed after MNPs were incorporated into the hydrogels. Importantly, PAA-MNP hydrogels demonstrated high ammonium sorption efficiencies (80-93%) in real sewage and achieved rapid ammonium recovery of 73 ± 1.1% within 15 min of mild acid washing (pH 4) 15 min at a maximum recovery.

The impact of primary sedimentation on the use of iron-rich drinking water sludge on the urban wastewater system.

The impact of primary sedimentation on the multiple use of iron in an urban wastewater system was investigated. Our previous work showed that in-sewer iron-rich drinking water sludge (DWS) dosing exhibited multiple benefits in the downstream processes. However, the system studied did not include a primary settler. We hypothesised that primary sedimentation could significantly change the characteristics of the wastewater flowing to the bioreactor, particularly its particulate components. This could in turn influence the availability of iron for phosphate removal from wastewater and/or sulfide removal in the anaerobic sludge digester. Long-term (~4 months) experiments were carried out on two laboratory-scale wastewater systems, each comprising sewers reactors, a primary sedimentation tank, a wastewater treatment reactor, and an anaerobic sludge digester. It was found the majority (85%) of the Fe contained in the sewer effluent was present in the primary sludge with the remaining (15%) staying in the primary effluent. This significantly affected the flow-on effect of Fe on the phosphate removal during wastewater treatment, removing only 1.2 ± 0.1 mgP L-1, as compared to 3.5 ± 0.1 mgP L-1 achieved previously in the absence of a primary settler. However, the P to Fe removal ratio was 0.32 mgP/mgFe, similar to the ratio observed previously without primary sedimentation (0.36 mgP/mgFe). The dissolved sulfide removal in the anaerobic digester was 2.7 ± 0.5 mgS L-1, substantially lower than 7.2 ± 0.3 mgS L-1 previously attained without primary sedimentation. This suggests that Fe in the primary sludge was not completely available for dissolved sulfide removal in the digester. However, the dewaterability of the anaerobically digested sludge improved with a relative increase of 25.0 ± 0.9%, compared to the 21.7 ± 0.6%, previously observed without primary sedimentation. The results demonstrated that primary sedimentation reduced the effectiveness to deliver the benefits of the in-sewer DWS dosing strategy, but the results are still favourable.

Articulating the effect of food systems innovation on the Sustainable Development Goals.

Food system innovations will be instrumental to achieving multiple Sustainable Development Goals (SDGs). However, major innovation breakthroughs can trigger profound and disruptive changes, leading to simultaneous and interlinked reconfigurations of multiple parts of the global food system. The emergence of new technologies or social solutions, therefore, have very different impact profiles, with favourable consequences for some SDGs and unintended adverse side-effects for others. Stand-alone innovations seldom achieve positive outcomes over multiple sustainability dimensions. Instead, they should be embedded as part of systemic changes that facilitate the implementation of the SDGs. Emerging trade-offs need to be intentionally addressed to achieve true sustainability, particularly those involving social aspects like inequality in its many forms, social justice, and strong institutions, which remain challenging. Trade-offs with undesirable consequences are manageable through the development of well planned transition pathways, careful monitoring of key indicators, and through the implementation of transparent science targets at the local level.

Recovery of in-sewer dosed iron from digested sludge at downstream treatment plants and its reuse potential.

Iron-based coagulants are dosed in enormous amounts and play an essential role in various segments of our urban water infrastructure. In order for the water industry to become circular, a closed-loop management strategy for iron needs to be developed. In this study, we have demonstrated for the first time that in-sewer dosed iron, either in the form of FeCl3 or ferric-based drinking water sludge (Fe-DWS) as a means to combat sewer corrosion and odour, can be recovered in the form of vivianite in digested sludge in down-stream wastewater treatment plants. Importantly, about 92 ± 2% of the in-sewer dosed Fe was estimated to be bound in vivianite in digested sludge. A simple insertion of Neodymium magnet allowed to recover 11 ± 0.2% and 15.3 ± 0.08% of the vivianite formed in the digested sludge of the in-sewer dosed iron in the form of FeCl3 and Fe-DWS, respectively. The purity of recovered vivianite ranged between 70 ± 5% and 49 ± 3% for in-sewer dosed FeCl3 and Fe-DWS, respectively. Almost complete (i.e. 98 ± 0.3%) separation of Fe in the form of ferrihydrite was achieved from vivianite after alkaline washing. Subsequent batch experiments demonstrated that the recovered ferrihydrite can be directly reused for efficient sulfide control in sewers. At a ferrihydrite-Fe:S molar ratio of 1.2:1, sewage dissolved sulfide concentrations was reduced from 15 mgS/L to below 0.5 mgS/L within 1 h of reaction. Overall, the results obtained in our study flag a first step for utilities towards a closed-loop iron-based coagulant management approach.

The hydrogen gas bio-based economy and the production of renewable building block chemicals, food and energy.

The carrying capacity of the planet is being exceeded, and there is an urgent need to bring forward revolutionary approaches, particularly in terms of energy supply, carbon emissions and nitrogen inputs into the biosphere. Hydrogen gas, generated by means of renewable energy through water electrolysis, can be a platform molecule to drive the future bioeconomy and electrification in the 21st century. The potential to use hydrogen gas in microbial metabolic processes is highly versatile, and this opens a broad range of opportunities for novel biotechnological developments and applications. A first approach concerns the central role of hydrogen gas in the production of bio-based building block chemicals using the methane route, thus, bypassing the inherent low economic value of methane towards higher-value products. Second, hydrogen gas can serve as a key carbon-neutral source to produce third-generation proteins, i.e. microbial protein for food applications, whilst simultaneously enabling carbon capture and nutrient recovery, directly at their point of emission. Combining both approaches to deal with the intermittent nature of renewable energy sources maximises the ability for efficient use of renewable resources.

Effects of in-sewer dosing of iron-rich drinking water sludge on wastewater collection and treatment systems.

The use of coagulants and flocculants in the water and wastewater industry is predicted to increase further in the coming years. Alum is the most widely used coagulant, however, the use of ferric chloride (FeCl3) is gaining popularity. Drinking water production that uses FeCl3 as coagulant produces waste sludge rich in iron. We hypothesised that the iron-rich drinking water sludge (DWS) can potentially be used in the urban wastewater system to reduce dissolved sulfide in sewer systems, aid phosphate removal in wastewater treatment and reduce hydrogen sulfide in the anaerobic digester biogas. This hypothesis was investigated using two laboratory-scale urban wastewater systems, one as an experimental system and the other as a control, each comprising sewer reactors, a sequencing batch reactor (SBR) for wastewater treatment, sludge thickeners and anaerobic digestion reactors. Both were fed with domestic wastewater. The experimental system received in-sewer DWS-dosing at 10 mgFe L-1 while the control had none. The sulfide concentration in the experimental sewer effluent decreased by 3.5 ± 0.2 mgS L-1 as compared with the control, while the phosphate concentration decreased by 3.6 ± 0.3 mgP L-1 after biological wastewater treatment in the experimental SBR. The dissolved sulfide concentration in the experimental anaerobic digester also decreased by 15.9 ± 0.9 mgS L-1 following the DWS-dosing to the sewer reactors. The DWS-doing also enhanced the settleability of the mixed liquor suspended sludge (MLSS) (SVI decreased from 193.2 ± 22.2 to 108.0 ± 7.7 ml g-1), and the dewaterability of the anaerobically digested sludge (the cake solids concentration increased from 15.7 ± 0.3% to 19.1 ± 1.8%). The introduction of DWS into the experimental system significantly increased the COD and TSS concentrations in the wastewater, and consequently the MLSS concentration in the SBR, however, this did not affect normal operation. The results demonstrated that iron-rich waste sludge from drinking water production can be used in the urban wastewater system achieving multiple benefits. Therefore, an integrated approach to urban water and wastewater management should be considered to maximise the benefits of iron use in the system.

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.

Effects of aging of ferric-based drinking water sludge on its reactivity for sulfide and phosphate removal.

Recent studies demonstrated the practical potential of multiple beneficial reuse of ferric-rich drinking water sludge (ferric DWS) for sulfide and phosphate removal in wastewater applications. In practice, ferric DWS is often stored on-site for periods ranging from days to several weeks (or even months), which may affect its reuse potential through changes in iron speciation and morphology. In this study, we investigated for the first time the impact of ferric DWS 'aging' time on the iron speciation and morphology and its subsequent impact on its reactivity and overall sulfide and phosphate removal capacity. A series of coagulation tests were conducted to generate ferric DWS of a practically relevant composition by using raw influent water from a full-scale drinking water treatment plant (DWTP). A comparison with ferric DWS from 8 full-scale DWTPs confirmed the similitude. The presence of akaganeite (ß-FeOOH) was detected in ferric DWS (through XRD analyses), independent of the DWS storage time. However, the morphology of akaganeite changed over time from a predominant poorly-crystalline phase in 'fresh' DWS (8 ± 0.1% of total Fe) to a highly crystalline phase (76 ± 3% of total Fe) at a sludge aging time of 30 days which was confirmed by means of Rietveld refinement in XRD analyses (n = 3). Subsequent batch tests showed that its sulfide removal capacity decreased significantly from 1.30 ± 0.02 mmol S/mmol Fe (day 1) to 0.60 ± 0.01 (day 30), a decrease of 54 % (p < 0.05). The level of crystallinity however had no impact on sulfide removal kinetics, most sulfide being removed within 10 minutes. Upon aeration of sulfide-loaded ferric DWS in activate sludge, amorphous iron oxides species were formed independent of the initial DWS crystallinity which resulted in efficient P removal at capacities similar to that of conventional FeCl3 dosing.