Towards new carbon-neutral food systems: Combining carbon capture and utilization with microbial protein production.

Alternative protein sources such as microbial protein (MP) are currently considered to alleviate the burden that food production exerts on the environment. Even though MP production is highly efficient in land and nutrient utilization, their carbon footprint should be improved. Here we propose the use of CCU as a driver for heterotrophic MP production. By comparing different MP production routes starting from liquid substrates derived from CO2 (i.e., formate, acetate, methanol, and ethanol) and their respective metabolic pathways, the potential of this concept as a carbon-neutral food or feed production process was estimated. Acetate and ethanol appear to be the most beneficial substrates for the integrated CCU-to-MP process in terms of electricity demand (acetate: 25 - 54 kWh/kgproduct, ethanol: 28 - 56 kWh/kgproduct). Moreover, recycling CO2 enables a carbon-negative protein production process by 2030 (considering the projected CO2 emissions from electricity in the EU: 0.096 kgCO2-eq/kWh) for formate, acetate, and ethanol (-1.1 up to 13 kgCO2-eq/kgproduct).

Combining partial nitritation and heterotrophic denitritation for the treatment of landfill leachate previous to an anammox reactor.

Landfill leachate can present extremely elevated concentrations of ammonium (up to 6,000 mg N-NH(4) (+) L(-1)) and a low biodegradable organic matter fraction. As an alternative to conventional systems, this wastewater can be treated on a more sustainable way by a fully autotrophic partial nitritation-anammox system. The operation of the first step of this system, the partial nitritation, is critical since the elevated concentrations of ammonium and nitrite in the reactor can severely inhibit ammonium oxidizing bacteria (AOB) activity. In this way, the inclusion of anoxic phases during the feeding events to promote the denitrification via nitrite can be a good option for upgrading the process performance and increasing the stability of the system. This paper deals with the evaluation of an anoxic-aerobic step-feed strategy for the operation of a partial nitritation SBR. Results of this study have revealed a decrease on the total nitrogen inside the reactor of more than 200 mg N L(-1) without prejudice on the partial nitritation process. Furthermore, this study has also allowed detecting an AOB activity reduction at the end of aerobic phases due to bicarbonate limitation and/or free nitrous acid inhibition.

Systematic model development for partial nitrification of landfill leachate in a SBR.

This study deals with partial nitrification in a sequencing batch reactor (PN-SBR) treating raw urban landfill leachate. In order to enhance process insight (e.g. quantify interactions between aeration, CO(2) stripping, alkalinity, pH, nitrification kinetics), a mathematical model has been set up. Following a systematic procedure, the model was successfully constructed, calibrated and validated using data from short-term (one cycle) operation of the PN-SBR. The evaluation of the model revealed a good fit to the main physical-chemical measurements (ammonium, nitrite, nitrate and inorganic carbon), confirmed by statistical tests. Good model fits were also obtained for pH, despite a slight bias in pH prediction, probably caused by the high salinity of the leachate. Future work will be addressed to the model-based evaluation of the interaction of different factors (aeration, stripping, pH, inhibitions, among others) and their impact on the process performance.

The effect of urban landfill leachate characteristics on the coexistence of anammox bacteria and heterotrophic denitrifiers.

Heterotrophic denitrification coexists with the anammox process contributing to N removal owing to the biodegradable organic matter supply from urban landfill leachate and the decay of microorganisms. Both biomasses consumed nitrite increasing the nitrite requirements of the system. The aim of this paper is the study of the causes which induce the system to decrease nitrogen removal efficiency. In this study, urban landfill leachate has been treated in an anammox Sequencing Batch Reactor (SBR) for 360 days. The anammox reactor treated on average 0.24 kgN m(-3) d(-1) obtaining nitrogen removal efficiencies up to 89%. The results demonstrated that i) a suitable influent nitrite to ammonium molar ratio is a crucial factor to avoid troubles in the anammox reactor performance; ii) an excess of nitrite implied nitrite accumulation in the reactor; iii) a lower nitrite supply than the necessary for the system could force a loss of specific anammox activity due to nitrite competition with denitrifiers. These results pointed out the importance of the previous partial-nitritation process control in order to obtain a correct influent nitrite to ammonium molar ratio for the anammox reactor. In addition, sudden variation of the leachate characteristics must be avoided.

Heterotrophic denitrification on granular anammox SBR treating urban landfill leachate.

The anammox process was applied to treat urban landfill leachate coming from a previous partial nitritation process. In presence of organic matter, the anammox process could coexist with heterotrophic denitrification. The goal of this study was to asses the stability of the anammox process with simultaneous heterotrophic denitrification treating urban landfill leachate. The results achieved demonstrated that the anammox process was not inactivated by heterotrophic denitrification. Moreover, part of the nitrate produced by anammox bacteria and part of the influent nitrite were removed by heterotrophic denitrifiers with associated biodegradable organic matter consumption. In this sense, the contribution on nitrogen removal of each process was calculated using a nitrogen mass balance methodology. An 85.1+/-5.6% of the nitrogen consumption was achieved via anammox process while the average heterotrophic denitrifiers contribution was 14.9+/-5.6%. Heterotrophic denitrification was limited by the available easily biodegradable organic matter.

Partial ammonium oxidation to nitrite of high ammonium content urban landfill leachates.

In an effort to treat N-rich streams in a more sustainable way, recent years have seen the development of new technologies, most of which are based on autotrophic denitrification via nitrite (anammox). In order to attain a suitable influent for that process, the wastewater must be treated by partially oxidising the ammonium to nitrite. In that aspect, this article presents the start-up and operation of a Partial Nitritation Sequencing Batch Reactor (PN-SBR) treating urban landfill leachates. Stable partial nitritation has been reached treating high ammonium loads (1-1.5 kg Nm(-3)d(-1)), demonstrating the feasibility of this technology as a previous step of anammox process. This study has also given away the importance of pH influence over ammonium oxidising bacteria (AOB) activity, thus it has been possible to determine the values of the half inhibition constants for free ammonia (k(I,FA)=605.48+/-87.18 mg N-NH L(-1)) and free nitrous acid (k(I,FNA)=0.49+/-0.09 mg N-HNO2 L(-1)), as well as the half-saturation constant for bicarbonate (k(HCO3-) = 0.01 +/- 0.16 mg CL(-1)).

Microbial electrosynthesis of butyrate from carbon dioxide.

This work proves for the first time the bioelectrochemical production of butyrate from CO2 as a sole carbon source. The highest concentration of butyrate achieved was 20.2 mMC, with a maximum butyrate production rate of 1.82 mMC d(-1). The electrochemical characterisation demonstrated that the CO2 reduction to butyrate was hydrogen driven. Production of ethanol and butanol was also observed opening up the potential for biofuel production.

Anoxic phases are the main N2O contributor in partial nitritation reactors treating high nitrogen loads with alternate aeration.

Partial nitritation (PN) reactors treating complex industrial wastewater can be operated by alternating anoxic-aerobic phases to promote heterotrophic denitrification via NO2(-). However, denitrification under stringent conditions can lead to high N2O production. In this study, the suitability of including anoxic phases in a PN-SBR treating real industrial wastewater was assessed in terms of process performance and N2O production. The PN-SBR was operated successfully and, when the HCO3(-):NH4(+) molar ratio was adjusted, produced a suitable effluent for a subsequent anammox reactor. 10-20% of the total influent nitrogen was removed. N2O production accounted for 3.6% of the NLR and took place mainly during the anoxic phases (60%). Specific denitrification batch tests demonstrated that, despite the availability of biodegradable COD, NO2(-) denitrification advanced at a faster rate than N2O denitrification, causing high N2O accumulation. Thus, the inclusion of anoxic phases should be avoided in PN reactors treating industrial wastewaters with high nitrogen loads.

Effect of temperature on AOB activity of a partial nitritation SBR treating landfill leachate with extremely high nitrogen concentration.

This study investigates the effects of temperature on ammonia oxidizing bacteria activity in a partial nitritation (PN) sequencing batch reactor. Stable PN was achieved in a 250 L SBR with a minimum operating volume of 111L treating mature landfill leachate containing an ammonium concentration of around 6000 mg N-NH(4)(+)L(-1) at both 25 and 35 °C. A suitable influent to feed an anammox reactor was achieved in both cases. A kinetic model was applied to study the influence of free ammonia (FA), the free nitrous acid (FNA) inhibition, and the inorganic carbon (IC) limitation. NH(4)(+) and NO(2)(-) concentrations were similar at 25 and 35 °C experiments (about 2500 mg N-NH(4)(+)L(-1) and 3500 mg N-NO(2)(-)L(-1)), FA and FNA concentrations differed due to the strong temperature dependence. FNA was the main source of inhibition at 25 °C, while at 35 °C combined FA and FNA inhibition occurred. DGGE results demonstrated that PN-SBR sludge was enriched on the same AOB phylotypes in both experiments.

Impact of influent characteristics on a partial nitritation SBR treating high nitrogen loaded wastewater.

The Anammox process allows a sustainable treatment of wastewater with high nitrogen content. Partial oxidation of ammonium to nitrite is a previous and crucial step. Given the variability on wastewater composition, the operation of sequencing batch reactors (SBR) for partial nitritation (PN) is very challenging. This work assessed the combined influence of influent characteristics and process loading rate. Simulation results showed that wastewater composition - Total nitrogen as ammonia (TNH) and total inorganic carbon (TIC) - as well as nitrogen loading rate (NLR) govern the outcomes of the reactor. A suitable effluent can be produced when treating wastewater with different ammonia levels, as long as the TIC:TNH influent molar ratio is around 1:1 and extreme NLR are avoided. The influent pH has a key impact on nitrite conversion by governing the CO(2)-bicarbonate-carbonate equilibrium. Finally, results showed that oxidation of biodegradable organic matter produces CO(2), which acidifies the media and limits process conversion.