Exploring the recovery of potassium-rich struvite after a nitrification-denitrification process in pig slurry treatment

Application of a practical methodology for the selection of suitable value chains to produce circular fertilisers from secondary raw materials

Background

The growing demand for food products, driven by a growing world population, has increased Europe's dependence on conventional fertilisers, which have a high impact on the environment. In the last decade, new circular fertiliser value chains have appeared as promising alternatives to conventional fertilisers.

Methods

Because of the huge number of alternatives, this study aimed to develop a practical methodology that facilitates the analysis of data related to each value chain to identify and select the most promising circular fertiliser value chains to promote their wide-scale production and use in agriculture, replacing the dependence on conventional fertilisers in Europe. This methodology is based on two stages (funnelling process and scoring system) and considers the 16 criteria (e.g. technical viability, nutrient content, among others) defined in the study. The methodology was tested for 48 value chains identified during the mapping of secondary raw materials in Europe with the potential to be used as circular fertilisers, classifying them into seven different raw materials: urban wastewater (UWW), industrial wastewater (IWW), sewage sludge (SS), biowaste (BW), biological by-products (BBP), treated manure (TM), and digestate (DIG). The funnelling process is based on a GO/NO-GO approach that meets six criteria and allows the discarding of 18 value chains, from 30 to the second stage. The scoring system was a more complete analysis, including ten new scoring criteria.

Results

This system allowed the identification of the potential of the value chains analysed, concluding that struvite from UWW, struvite from IWW, stabilized sludge from SS, composted biowaste from BW, feather meal from BBP, solid fraction from DIG, and spent mushroom substrate from TM are the most promising options for agriculture.

Conclusions

The develop methodology was used to evaluate 48 different value chains with the potential to generate promising circular fertlizers. Seven value chains were finally selected.

Dynamic ammonium retention for nutrient separation from manure digestate

Extensive nitrogen emissions with negative impact on nature and the environment urge effective valorization of manure and fractionation of nutrients to enable precision fertilization. Typically, manure is fed to a digester to produce biogas. The remaining digestate is then mechanically separated into a solid phosphorous-rich fraction and a liquid fraction containing both NH4+ and K. These ions are difficult to separate due to their very identical size and charge. We show that with smart tuning of the pH to control the NH3/NH4+ equilibrium, membranes can produce dedicated N and K-rich streams. The increased pH switches the equilibrium towards the neutrally charged solute NH3 that permeates more easily through the membrane than charged NH4+ and K+ ions. Experiments with both artificial NH4Cl/KCl mixtures as well as real liquid digestate and four different membrane types, ranging from open nanofiltration (NF) to sea water reverse osmosis (RO) membranes were performed. At neutral pH, no N/K selectivity was observed, not for single components nor for mixtures. When the pH was increased towards alkaline environment, distinct selectivity for N/K was obtained both with model solutions and real liquid digestate. At a suitable pH of 10, with >80 % of the total ammonia present as NH3, the RO BW membrane showed a large N/K selectivity of 35 in the crossflow system. Additional RO steps at low pH allows subsequent concentration of the formed NH4+ and K+ fractions. The presented dynamic pH approach proofs that in a two-step RO system both N, and K-enriched fertilizers can be produced from real liquid digestate.

Pilot scale on-site demonstration and seasonality assessment of nitrogen recovery and water reclamation from pig's slurry liquid fraction

Livestock slurry has gathered significant interest as a secondary raw material for fertilisers industry due to its content on macronutrients -nitrogen, phosphorous, and potassium- and organic carbon. In this study, the performance of an on-site pilot plant composed by microfiltration, membrane-assisted stripping, and reverse osmosis for selective recovery of nitrogen as fertiliser and water reclamation was demonstrated for 2 years in a pig farm, referenced to 8 batches for seasonal assessment. Microfiltration mitigated the seasonal variation in the composition of pig slurry leading to stable process efficiency in the following steps. Membrane-assisted stripping resulted in the recovery of up to 56% of nitrogen as high-purity ammonium sulphate, and up to 42% of reclaimed water as reverse osmosis permeate. The proposed train of technologies reported proper performance and robustness during the whole demonstration period as it resulted in the production of reclaimed water and ammonium sulphate with no significant quality variations. The energy cost for both products obtained in this study was found in the average of the previous works reviewed with 12.49 kWh kg-1 NH3 produced, and 0.37 kWh m-3 of reclaimed water. The environmental assessment showed that nitrogen losses could be reduced by up to 90 kg N ha-1 d-1 by replacing manure spreading with precise fertilisation techniques, enabled by the selective recovery of nitrogen from SLF. Finally, the financial study showed that the scaling up of the proposed train of technologies would result in benefits for farms with more than 1600 pig heads.

Reagent-free phosphorus precipitation from a denitrified swine effluent in a batch electrochemical system

There is high interest in the recovery of phosphorus (P) from wastewater through crystallization processes. However, the addition of chemical reagents (e.g., sodium hydroxide) to raise the pH may result in high treatment costs and increased concentrations of undesired metal ions (e.g., sodium). As an alternative, in this research we considered electrochemical mediated precipitation at low current densities (0.4-1.2 A m-2) without using chemical reagents. For that purpose, a two-chamber electrochemical system was operated in batch for treating denitrified swine effluent (48 mg P L-1). By applying current at 1.2 A m-2, and targeting pH 11.5, a maximum P removal rate of 33.4 mmol P (L·d-1) was obtained while the P removal efficiency was above 90 %. New solids that formed mostly remained suspended in the catholyte. Before discharge, the catholyte effluent was recirculated to the anodic compartment to neutralize the pH, achieving a final pH of 6.4 ± 0.1. Chlorine (Cl2) production in the anodic compartment was favored by a small anode surface and a high initial pH of the catholyte. Although the production of chlorine achieved was limited (the highest concentration was 8.6 ± 0.1 mg Cl2 L-1) these findings represent a new opportunity for the recovery and onsite use of this side-product. Electrochemical impedance spectroscopy tests confirmed that the deposition of solids inside the cathodic compartment during the experimental period was limited. Membrane analysis revealed significant scaling of carbonate compounds. The electrochemical treatment described above was shown as a promising alternative to sodium hydroxide and sulfuric acid dosage for pH adjustment when crystallizing phosphate salts.

Nutrient recovery and valorisation from pig slurry liquid fraction with membrane technologies

Livestock slurry has been reported to be a potential secondary raw material as it contains macronutrients ‑nitrogen, phosphorus and potassium-, which could be valorised as high-quality fertilizers if proper separation and concentration of valuable compounds is performed. In this work, pig slurry liquid fraction was assessed for nutrient recovery and valorisation as fertilizer. Some indicators were used to evaluate the performance of proposed train of technologies within the framework of circular economy. As ammonium and potassium species are highly soluble at the whole pH range, a study based on phosphate speciation at pH from 4 to 8 was assessed to improve the macronutrients recovery from the slurry, resulting in two different treatment trains at acidic and alkaline conditions. The acidic treatment system based on centrifugation, microfiltration and forward osmosis was applied to obtain a nutrient-rich liquid organic fertilizer containing 1.3 % N, 1.3 % P₂O₅ and 1.5 % K₂O. The alkaline path of valorisation was composed by centrifugation and stripping by using membrane contactors to produce an organic solid fertilizer -7.7 % N, 8,0 % P₂O₅ and 2.3 % K₂O-, ammonium sulphate solution -1.4 % N- and irrigation water. In terms of circularity indicators, 45.8 % of the initial water content and <50 % of contained nutrients were recovered - 28.3 % N, 43.5 % P₂O₅ and 46.6 % K₂O - in the acidic treatment resulting in 68.68 g fertilizer per kg of treated slurry. 75.1 % of water was recovered as irrigation water and 80.6 % N, 99.9 % P₂O₅, 83.4 % K₂O was valorised in the alkaline treatment, as 219.60 g fertilizer per kg of treated slurry. Treatment paths at acidic and alkaline conditions yield promising results for nutrients recovery and valorisation as the obtained products (nutrient rich organic fertilizer, solid soil amendment and ammonium sulphate solution) fulfil the European Regulation for fertilizers to be potentially used in crop fields.

Towards advanced simultaneous nitrogen removal and phosphorus recovery from digestion effluent based on anammox-hydroxyapatite (HAP) process: Focusing on a solution perspective

In this paper, the state-of-the-art information on the anammox-HAP process is summarized. The mechanism of this process is systematically expounded, the enhancement of anammox retention by HAP precipitation and the upgrade of phosphorus recovery by anammox process are clarified. However, this process still faces several challenges, especially how to deal with the ∼ 11% nitrogen residues and to purify the recovered HAP. For the first time, an anaerobic fermentation (AF) combined with partial denitrification (PD) and anammox-HAP (AF-PD-Anammox-HAP) process is proposed to overcome the challenges. By AF of the organic impurities of the anammox-HAP granular sludge, organic acid is produced to be used as carbon source for PD to remove the nitrogen residues. Simultaneously, pH of the solution drops, which promotes the dissolution of some inorganic purities such as CaCO₃. In this way, not only the inorganic impurities are removed, but the inorganic carbon is supplied for anammox bacteria.

Second-Generation Magnesium Phosphates as Water Extractant Agents in Forward Osmosis and Subsequent Use in Hydroponics

The recovery of nutrients from wastewater streams for their later use in agricultural fertilization is an interesting approach. Wastewater recovered magnesium phosphate (MgP) salts were used in a forward osmosis (FO) system as draw solution in order to extract water and to produce a nutrient solution to be used in a hydroponic system with lettuces (Lactuca sativa, L.). Owing to the low solubility of the MgP salts (i.e., struvite, hazenite and cattiite) in water, acid dissolution was successfully tested using citric and nitric acids to reach pH 3.0. The dilution by FO of the dissolved salts reached levels close to those needed by a hydroponic culture. Ion migration through the membrane was medium to high, and although it did not limit the dilution potential of the system, it might decrease the overall feasibility of the FO process. Functional growth of the lettuces in the hydroponic system was achieved with the three MgP salts using the recovered water as nutrient solution, once properly supplemented with nutrients with the desired concentrations. This is an innovative approach for promoting water reuse in hydroponics that benefits from the use of precipitated MgP salts as a nutrient source.