Study on ammonia generation from digested sludge by subcritical water treatment

Sewage sludge and livestock manure have attracted attention as new renewable energy sources for the formation of a circular economy (Kirchher et al. 2017; Oladejo et al. 2019), and technology to recover energy from these sources is currently being developed (Almås & Singh 2017; Kacprzak et al. 2019). This technology mainly utilizes the organic matter contained in sludge to generate biogas, biomethane, crude oil, and biochar, which are then used as energy sources. On the other hand, the use of nitrogen components in sludge is limited to fertilizers for agriculture. Because the agricultural use of sludge is not widespread in Japan, nitrogen is mostly disposed of. Ammonia, a nitrogenous component found in large amounts in sewage sludge, is not only available as a fuel (Kurata et al. 2017), but it has attracted attention as a storage and carrier of hydrogen (Lamb et al. 2019). According to Japanese sewage statistics, sewage treatment plants overall receive about 484,000 t-N/year of nitrogen components, and they release about 174,000 t-N/year as effluent and about 134,000 t-N/year as sludge (Kojima et al. 2021). The difference, about 176,000 t-N/year, is released into the atmosphere by denitrification, which consumes a large amount of energy. If all of the approximately 134,000 t-N released as sludge from sewage treatment plants in a year were converted into ammonia, the energy potential would be about 1.02 billion kWh, which is about 15.8% of the annual electricity consumption of all sewage treatment plants in Japan, based on an estimate of 382.6 kJ/mol for the calorie of ammonia combustion. Thus, the recovery of nitrogen components from sewage sludge via replacement with ammonia is considered a new and effective utilization technology for sewage sludge.

The ammonia stripping method is one way to release ammonia from sewage sludge (Kinidi et al. 2018). However, this method can only recover soluble ammonia. The nitrogen component in sewage sludge can be divided into solids and dissolved nitrogen. Even in digested sludge, where the amount of nitrogen dissolved in the water in the sludge is considered high, the amount of nitrogen derived from solids is crucial because it accounts for about 51.3% of the total (Kojima et al. 2021). Therefore, to recover nitrogen components from sewage sludge and use them as energy, nitrogen derived from solids must also be recovered, and pretreatment (e.g., through solubilization) is necessary. Solubilization methods include hydrothermal treatment and microbial degradation (Park et al. 2019), but most are aimed at increasing methane recovery in anaerobic fermentation, and few at recovering nitrogen components.

In this paper, we examined a technology for converting solid nitrogen components in sewage sludge into ammonia under subcritical conditions. Water at higher temperatures and pressures than the critical point (374.2 °C, 22.1 MPa) is known as supercritical water, which has excellent hydrolytic resolution. Subcritical conditions are not clearly defined. However, they are generally understood to refer to lower temperatures and pressures near the critical point. For example, Qian et al. showed that adding an excess of oxidants to supercritical water can remove and completely degrade more than 99% of chemical organic demand (COD), total organic carbon (TOC), and NH₃ in municipal sewage sludge (Qian et al. 2016). Ammonia produced by heat treatment was considered a waste product that had to be removed. Therefore, ammonia was either decomposed and treated as nitrogen using a catalyst that decomposes ammonia, treated at even higher temperatures, or treated for longer periods of time (Sumikura et al. 2006; Li et al. 2020). In Sumikura's work, the ammonia generation rate from the methane fermentation residue was the highest under 450 °C, 15 MPa, 20 min. Under higher temperature and pressure conditions, ammonia was decomposed into nitrogen. Therefore, equivalent or lower temperature and pressure conditions are considered suitable for ammonia recovery. The objective of this research is to identify the superior conditions for generating ammonia from digested sludge by subcritical water treatment. Through this research, we will confirm that subcritical water treatment is a superior method compared to the existing method of ammonia stripping. If it can be confirmed that a greater amount of ammonia is produced than that contained in the original digested sludge, it suggests that more ammonia can be generated from the sludge than with ammonia stripping. The amount of ammonia generated was investigated for the combustion condition, in which hydrogen peroxide was added to oxidize nitrogen-containing organic matter, and the pyrolysis condition, in which less energy input is required and the organic matter after treatment is expected to be utilized.

The digested sludge used in the experiment was provided by the Northern Sludge Materialization Center of Yokohama City in 2017. Table 1 shows the properties of the sludge. In the digested sludge used in this study, the nitrogen component in the solids was 1,728 mg/kg and that in the water was 1,362 mg/kg. Water content was determined from the decrease in weight when heated at 105 °C. Among the elemental contents, C, H, N, and S were determined by an elemental analyzer (Analytik Jena, 2400 II). Cl was measured according to the Wastewater Examination Method (Yamaguchi 2012) published by the Japan Sewage Works Association, and ash by intense thermal loss at 600 °C. In addition, the content of O elements was calculated by subtracting the C, H, N, S, Cl, and ash values from 100%. The total N in filtered solution was determined by a combustion catalyst-based elemental analyzer (Shimadzu, TOC-L), and -N by ion chromatography (ThermoFisher, ICS-1600). Small batch-type experimentals were conducted. Digested sludge and H₂O₂ as combustion agents were placed in the reaction vessel (10 mL, SUS316) and sealed. The amount of each input was determined from the pressure, volume, and temperature (P–V–T) relationship to achieve the target pressure under the target temperature. All the gases inside each reaction vessel were replaced with argon gas. Reaction vessels with samples were put in a sand bath (TECHNE) preheated at reaction temperature. The reaction time refers to the time in which the vessel was put into the sand bath, and the reaction temperature was assumed to be the same as the set temperature of the sand bath. The reaction pressure was calculated from the total water content in the digested sludge and H₂O₂ in the reaction vessel, the gas volume in the reaction vessel, the internal volume of the reaction vessel, and the reaction temperature using the P–V–T relationship for water. After a predetermined reaction time, the reaction was stopped by removing the vessel from the sand bath and cooling it in running water. After cooling, the decomposition products in the reaction vessel were filtered by a 0.45 μm cellulose acetate filter (ADVANTEC, 25CS045AN), and was determined by ion chromatography. The ammonia generation rate was calculated by comparing the total nitrogen content in the digested sludge and -N after treatment. Each experiment was repeated in triplicate.

The oxygen ratio in this paper refers to the ratio between the minimum stoichiometric amount of oxygen required for the complete combustion of C and H in organic sludge to produce 1 mol of ammonia and the quantity of oxygen contained in the material enclosed in the reaction vessel. The former was calculated from Equation (1), which shows that 7.6 mol of oxygen is required to obtain 1 mol of ammonia, or 46.1 mmol of oxygen per gram of sludge. The latter was calculated by the amount of oxygen contained in the digested sludge itself, water, and hydrogen peroxide. Note that this study did not analyze the composition of organic matter in detail, and the oxygen demand may differ from actual values. An oxygen ratio of 0 indicates pyrolysis under subcritical conditions, and the other ratio (0.3–2.0) indicates combustion conditions. Similar tests were conducted using ammonia water to confirm the transformation of ammonium into other nitrogen forms under subcritical treatment conditions.

Under pyrolysis treatment in subcritical conditions, when the temperature was increased, ammonium nitrogen generation experiences a decreasing trend from 69.9 to 62.7%. When the pressure was varied, no significant differences were found; however, conditions under 1 MPa had the lowest mean value of 54.9%. When the reaction time was increased until 5 min, the rate of ammonium nitrogen generation increased from 40.2 to 64.6%. For reaction times longer than 5 min, the rate was 64.2–64.7%.

The rate of ammonium nitrogen generation will vary depending on the material compositions. Previous studies have shown that in the treatment of food residues with subcritical water, the products after treatment vary depending on the constituents, and amino acids are the main source of ammonia (Posmanik et al. 2017). Sewage sludge with a high nitrogen content, such as amino acids, would be more desirable to increase the amount of ammonia created by this method.

The rate of ammonia nitrogen generation was higher in the combustion condition than in the pyrolysis condition for all the conditions in this study. As to the supercritical treatment of sewage sludge (450 °C, 25 MPa, 2 min), the solid content decreased, and the NH₃ removal rate increased as the amount of hydrogen peroxide added was increased (Xu et al. 2013). Our conditions (400 °C, 10 MPa, 5 min) were milder than these because no ammonia decomposition occurred, we believe that the higher the amount of H₂O₂ added, the higher the rate of ammonium nitrogen generation. Another approach, focusing on energy recovery, involves the effective use of the organic matter contained in sludge by utilizing the residue after methane fermentation or by suppressing the decomposition of organic carbon under pyrolysis. Furthermore, we find that a more detailed study of the composition of the sludge would lead to further efficiency improvements.

This study investigated the superior conditions for the creation of ammonia from nitrogen components in digested sludge by subcritical treatment. The conditions under which this paper was conducted were as follows: the ammonia generation rate was 84.0% at 400 °C, 10 MPa, 5 min, and an oxygen ratio of 1.2. This result suggested that 74.3% of the nitrogen component and soluble organic nitrogen contained in the solid content of digested sludge could be converted into ammonia, presenting the possibility of recovering more nitrogen components as ammonia compared to ammonia stripping, which can only recover soluble ammonia.

The digested sludge used in this study was provided by the Northern Sludge Materialization Center of Yokohama City. We would like to take this opportunity to express our gratitude to the Center.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.