Application of anammox within an integrated approach to sustainable food waste management and valorization

Highlights

• Effluent from 2-stage anaerobic digestion of food waste (2sAD-FW) was treated by anammox after partial nitration (PN) step.

• Partial by-pass of PN reactor was used to regulate NO₂/NH₄ ratio in anammox influent.

• An alternative plant layout for 2sAD-FW wastewater treatment was proposed.

• Color evolution in granular anammox fed with real wastewater was investigated.

• FISH results correlated well with changes in operating conditions.

Abstract

In this study, the anammox process was applied for the first time to the treatment of ammonium-rich liquid residues produced by the two-stage anaerobic digestion of food waste (2sAD-FW); such residues may represent a significant environmental issue if not properly managed. A granular anammox reactor was fed with a progressively increasing share of partially nitritated 2sAD-FW wastewater. An alternative operating strategy based on partial by-pass of the partial nitritation unit was tested, in order to regulate the influent NO₂/NH₄ molar ratio without chemical addition. High nitrogen removal efficiency (89 ± 1%) and negligible nitrite discharge rates were achieved, together with high nitrogen removal rate/nitrogen loading rate (NRR/NLR, 97 ± 1%) and stable specific anammox activity (0.42 ± 0.03 gN₂-N/gVSS d). The observed NH_4-removed/NO_2-removed/NO_3-produced molar ratio was in agreement with anammox stoichiometry, as confirmed by the low contribution (<5%) of denitrification to nitrogen removal. Moreover, the possibility of using digital color characterization of granular biomass as a novel, simple tool for the monitoring of anammox biomass enrichment and process performance was investigated under dynamic conditions, using real wastewater: changes in granule color correlated well with the increasing share of 2sAD-FW wastewater in the influent (R² = 83%), as well as with the decrease of anammox biomass abundance in the reactor (R² = 68%). The results suggest that anammox may be successfully integrated into a 2sAD-FW system, thus enhancing its environmental sustainability.

Introduction

Every year, approximately 1200 Mt (Mt = 10⁶ metric tons) of the food produced worldwide are lost or wasted through the food supply chain, which causes significant social, environmental, and economic issues [1,2]. According to European waste statistics, approximately 245 Mt of municipal solid waste were generated in the EU-28 in 2016, out of which food waste (FW) accounted for 35% by weight [3]. Rather than being considered an environmental issue, FW may be seen as a potential source of material and energy which should be recovered within an eco-sustainable approach. The two-stage anaerobic digestion process (2sAD) aiming at the recovery of hydrogen (dark fermentation) and methane has proved to be a promising option, since it enhances overall energy recovery compared to conventional one-stage processes [4,5]. Moreover, the possibility of recovering energy as hydrogen and methane, rather than only methane, is of interest due to the positive environmental features of hydrogen as an energy carrier, especially if generated from renewable non-fossil sources [3].

However, maximization of energy recovery from FW by a 2sAD process must be accompanied by minimization of its potential environmental impacts, in order to promote an environmentally sound approach. Since FW consists mainly of carbohydrates, fats and proteins [6], the anaerobic digestion of such a substrate results in the transfer of nutrients from the solid to the liquid phase, for which the direct discharge into municipal sewers is not a viable option as it would unbalance the chemical oxygen demand/total Kjeldahl nitrogen/total phosphorus (COD/TKN/TP) ratio of municipal wastewater [7]. The use of the digested effluents in agriculture as organic fertilizer may also be limited by factors such as transport requirements, water content, or the presence of unwanted substances and pathogenic microorganisms [8].

The relatively low organic carbon to nitrogen ratio makes such effluents potentially suitable for treatment by completely autotrophic nitrogen removal processes, which combine partial nitritation (PN) and anaerobic ammonia oxidation (anammox). In the PN reactor, roughly 50–55% of influent ammonium should be converted into nitrite by ammonium oxidizing bacteria (AOB), so that the residual ammonium can be converted into dinitrogen gas in the anammox reactor by Planctomycetes (a distinct phylum in the Bacteria domain), using nitrite as electron acceptor [9]. PN/anammox represents a cost- and technically effective alternative to conventional biological treatment based on full nitrification and denitrification, as well as to chemical-physical processes, requiring less energy and fewer chemicals (no exogenous organic substrates such as methanol or glucose are required) [10,11]. Moreover, the PN/anammox footprint is low in terms of greenhouse gas emissions [12,13], which makes it an eco-sustainable option worthy of being investigated; compared with conventional wastewater treatment based on nitrification and denitrification, CO₂ emissions may be reduced by up to 80%, and N₂O is absent in anammox physiology (conversely, it is an intermediate in denitrification) [13].

Despite such promising features, limited work has been focused on the application of PN/anammox to the treatment of the liquid effluents originated by the one-stage anaerobic digestion of the municipal solid waste organic fraction (OFMSW) [7,14]: in one report, the possibility was successfully investigated of upgrading the Florence wastewater treatment plant by integrating the anaerobic co-digestion (waste activated sludge+OFMSW) with the completely autotrophic removal of nitrogen from the supernatant, using a nitritation membrane bioreactor and an anammox sequencing batch reactor [7]. The latter showed variable NH₄⁺-N and NO₂⁻-N removal rates, ranging from 1.3–47.1 mg N/L d and from 0.5–47.6 mg N/L d, respectively. Others [14] performed batch experiments to investigate the response of anammox biomass exposed to the liquid fraction of digested and co-digested OFMSW, using conductivity as an aggregate parameter to evaluate inhibition capacity: although the undiluted liquid fraction of digested OFMSW was found to have the strongest inhibitory capacity (anammox activity reduction of 73–89%), likely due to the high overall conductivity, anammox biomass was found to have a potential adaptation capability.

As for the anaerobic digestion of food waste in a two-stage system (2sAD-FW), so far only preliminary studies have been reported concerning the application of PN/anammox to the treatment of simulated (synthetic) 2sAD-FW wastewater [15,16]. In particular, anammox biomass was able to withstand the high nitrogen loading rates applied to the PN reactor (up to 1.5 g N/L d), showing high nitrogen removal efficiencies (NRE > 90%) and negligible nitrite discharge rates [16]. However, specific knowledge about anammox behavior with real 2sAD-FW wastewater is not available, so that an important gap has to be filled in view of possible process scale-up.

In this study, the liquid effluent produced by 2sAD-FW and treated in a PN unit was fed into a granular anammox sequencing batch reactor, in order to evaluate its applicability and achieve process optimization using real wastewater. An alternative process layout based on partial by-pass of the PN unit was tested in order to regulate the influent NO₂/NH₄ molar ratio without chemical (e.g. NH₄Cl) additions, which would represent a significant cost at pilot/full scale. A novel approach to anammox sludge characterization was also implemented, based on the determination of biomass color as a potentially quick, simple and cost-effective indirect measure of process performance and biomass enrichment.