Nitrogen dynamics in soils fertilized with digestate and mineral fertilizers: A full field approach.

Highly stabilized digestate from sewage sludge and digestate-derived ammonium sulphate (RFs), were used in a comparison with synthetic mineral fertilizers (SF) to crop maize in a three-year plot trial in open fields. RFs and SF were dosed to ensure the same amount of mineral N (ammonia-N). In doing so, plots fertilized with digestate received much more N (+185 kg ha-1 of organic N) because digestate also contained organic N. The fate of nitrogen was studied by measuring mineral and organic N in soil at different depths, ammonia and N2O emissions, and N uptake in crops. Soil analyses indicated that at one-meter depth there was no significant difference in nitrate content between RF, SF and Unfertilized plots during crop season indicating that more N dosed with digestate did not lead to extra nitrate leaching. Ammonia emissions and N content in plants and grains measured were also similar for both RF and SF. Measuring denitrification activity by using gene makers resulted in a higher denitrification activity for RF than SF. Nevertheless, N2O measurements showed that SF emitted more N2O than RF (although it was not statistically different) (7.59 ± 3.2 kgN ha-1 for RF and 10.3 ± 6.8 kgN ha-1 for SF), suggesting that probably the addition of organic matter with digestate to RF, increased the denitrification efficiency so that N2 production was favoured. Soil analyses, although were not able detecting N differences between SF and Rf after three years of cropping, revealed a statistical increasing of total carbon, suggesting that dosing digestate lead to carbon (and maybe N) accumulation in soil. Data seem to suggest that N2O/N2 emission and organic N accumulation in soil can explain the fate of the extra N dosed (organic-N) in RF plots.

Microbial recycling cells: First steps into a new type of microbial electrochemical technologies, aimed at recovering nutrients from wastewater.

The aim of this work were to study terracotta-based porous air-water separators (4 mm thickness) in microbial recycling cells (MRCs) fed with cow manure (CM), swine manure (SM) and dairy wastewater (DW). Over 125 days, besides the removal of 60-90% of soluble-COD, considerable fractions of the main macronutrients (C, N, P, K, Fe, Mn, Ca, Mg) were removed from the wastewater and deposited on the terracotta separators as both inorganic salts and biomass deposits. Water evaporation at air-water interface as well as the high cathodic pH (10-12), induced by oxygen reduction to OH-, were the predominant factors leading to precipitation. The separators were saturated of up to 10 g per kg of terracotta of the main macronutrients, with negligible concentrations of the main inorganic contaminants. These materials could be directly reused as nutrients-enriched solid conditioners for agricultural soils.

Microbial recycling cells (MRCs): A new platform of microbial electrochemical technologies based on biocompatible materials, aimed at cycling carbon and nutrients in agro-food systems.

This article reviews the mechanisms that drive nutrients and carbon sequestration from wastewaters by microbial electrochemical technologies (METs). In this framework, a new generation of METs is also presented (to be called microbial recycling cells, MRCs), based on 100%-recyclable materials (biomass-derived char coal, clay, terracotta, paper, ligno-cellulosic plant materials, etc.), which can act as bio-electrodes, separators and structural frames. In traditional METs architectures (based on technological materials such as carbon cloths, plastic panels, membranes, binders), inorganic salts precipitation and adsorption, as well as biofouling due to organic-matter deposition, are considered as main drawbacks that clog and hinder the systems over relatively short periods. In MRCs, these mechanisms should be maximized, instead of being avoided. In this perspective, both inorganic and organic forms of the main nutrients are sequestered from wastewater and deposited on METs modules. Once the systems become saturated, they can entirely be recycled as agricultural soil conditioners or as base for organic-mineral fertilizers.

Bioelectrochemical Nitrogen fixation (e-BNF): Electro-stimulation of enriched biofilm communities drives autotrophic nitrogen and carbon fixation.

A new approach to microbial electrosynthesis is proposed, aimed at producing whole biomass from N2 and inorganic carbon, by electrostimulation of complex microbial communities. On a carbon-based conductor under constant polarization (-0.7 V vs SHE), an electroactive biofilm was enriched with autotrophic nitrogen fixing microorganims and led to biomass synthesis at higher amounts (up to 18 fold), as compared to controls kept at open circuit (OC). After 110 days, the electron transfer had increased by 30-fold, as compared to abiotic conditions. Metagenomics evidenced Nif genes associated with autotrophs (both Archaea and Bacteria) only in polarized biofilms, but not in OC. With this first proof of concept experiment, we propose to call this promising field 'bioelectrochemical nitrogen fixation' (e-BNF): a possible way to 'power' biological nitrogen fixation, organic carbon storage and soil fertility against desertification, and possibly a new tool to study the development of early prokaryotic life in extreme environments.

Single-chamber microbial fuel cells as on-line shock-sensors for volatile fatty acids in anaerobic digesters.

In anaerobic digesters (AD), volatile fatty acids (VFAs) concentration is a critical operative parameter, which is usually manually monitored to prevent inhibition of microbial consortia. An on-line VFAs monitoring system as early-warning for increasing concentrations would be of great help for operators. Here, air-cathode membraneless microbial fuel cells (MFCs) were investigated as potential biosensors, whose electrical signal instantaneously moves from its steady value with the accumulation of VFAs in the anodic solution. MFCs were operated equipping four lab-scale ADs with carbon-based electrodes. Reactors were filled with the digestate from a full-scale AD and fed in batch with four kinds of feedstock (cheese whey, kitchen waste, citrus pulp and fishery waste). The MFC signal initially increased in parallel to VFAs production, then tended to a steady value for VFAs concentrations above 1000mgAcL-1. Peak concentrations of tVFAs (2500-4500mgAcL-1) and MFCs potentials were negatively correlated (r=0.916, p<0.05), regardless of the type of substrate. Inhibition of the MFC system occurred when VFAs increased fast above 4000mgAcL-1. Polarization curves of electrodes stressed that electroactive bacteria on bioanodes were strongly subjected to inhibition. The inhibition of electroactivity on bioanode trended like typical shock-sensors, opening to direct application as early-warning monitoring system in full-scale ADs.

A study of microbial communities on terracotta separator and on biocathode of air breathing microbial fuel cells.

Recently, terracotta has attracted interest as low-cost and biocompatible material to build separators in microbial fuel cells (MFCs). However, the influence of a non-conductive material like terracotta on electroactive microbiological communities remains substantially unexplored. This study aims at describing the microbial pools developed from two different seed inocula (bovine and swine sewage) in terracotta-based air-breathing MFC. A statistical approach on microbiological data confirmed different community enrichment in the MFCs, depending mainly on the inoculum. Terracotta separators impeded the growth of electroactive communities in contact with cathodes (biocathodes), while a thick biofilm was observed on the surface (anolyte-side) of the terracotta separator. Terracotta-free MFCs, set as control experiments, showed a well-developed biocathode, Biocathode-MFCs resulted in 4 to 6-fold higher power densities. All biofilms were analyzed by high-throughput Illumina sequencing applied to 16S rRNA gene. The results showed more abundant (3- to 5-fold) electroactive genera (mainly Geobacter, Pseudomonas, Desulfuromonas and Clostridia MBA03) in terracotta-free biocathodes. Nevertheless, terracotta separators induced only slight changes in anodic microbial communities.

Improving methane production and anaerobic digestion stability of food waste by extracting lipids and mixing it with sewage sludge.

Anaerobic digestion (AD) of FW shows instability due to both the presence of high lipids and accumulation of volatile fatty acids. In this study, AD of food waste (FW) was optimized by removing lipids (LRFW) and by co-digestion with sewage sludge (1:1w/w on dry matter). The results obtained showed that lipids extraction increased FW methane yield from 400 to 418mL-gVSadded-1 under mesophilic conditions (35°C) and from 426 to 531mL-gVSadded-1 in thermophilic conditions (55°C). Two degradation phases (k1 and k2) described FW and LRFW degradation. In the thermophilic, LRFW-k1 (0.1591d-1) was slightly higher than that of FW (k1 of 0.1543d-1) and in the second stage FW-k2 of 0.0552d-1 was higher than that of LRFW (k2 of 0.0117d-1). The majority of LRFW was degraded in the first stage. FW and sewage sludge co-digestion reduced VFA accumulation, preventing media acidification and improving process stability.