Lide Chen

Hydrothermal carbonization (HTC) of digested dairy manure produces hydrochar and a nitrogen-rich post-liquid. This study evaluated a submerged tubular expanded polytetrafluoroethylene (ePTFE) gas-permeable membrane (GPM) system for ammonia recovery from post-HTC liquid derived from digested dairy manure, focusing on the effects of feed temperature, acid circulation rate, and feed volume-to-membrane surface area ratio (FV/MS). Compared with filtered digested manure, the post-HTC liquid had a similar total ammoniacal nitrogen (TAN) concentration (1151 vs. 1164 mg N L⁻¹) but a slightly higher pH (8.38 vs. 7.94), which favored ammonia transfer. Increasing feed temperature from 20 to 60 °C raised 24 h TAN recovery from 63.5% to 99.6% and average TAN transfer flux from 11.1 to 17.7 g m⁻² d⁻¹, although it also increased water vapor crossover and diluted the acid trapping solution. Increasing acid circulation from 10 to 30 mL min⁻¹ improved 48 h TAN recovery from 85.2% to 94.0%, while a further increase to 50 mL min⁻¹ produced only a small additional gain. In contrast, elevating FV/MS from 0.015 to 0.045 m³ m⁻² reduced 48 h TAN recovery from 94.0% to 59.6% while increasing average TAN transfer flux from 8.2 to 16.2 g m⁻² d⁻¹ because larger feed volumes sustained the concentration driving force for a longer period. Nitrogen mass balance showed that more than 94–99% of the initial TAN was accounted under most conditions, with only a small fraction lost to volatilization.

The dairy industry produces large amounts of dairy wastewater containing ammonia nitrogen (NH₃-N). Sustainable treatment technologies are needed which can reduce the environmental pollution caused by NH₃-N emissions from dairy wastewater. Chemical coagulation combined with the photo-electro-Fenton (PEF) treatment process has been considered a promising technology that can effectively remove NH₃-N from dairy wastewater. In this study, Taguchi design was used first to narrow down the operating factors from five to three. The three most influential factors were then further optimized for an optimum NH₃-N removal efficiency using response surface methodology (RSM) coupled with Box–Behnken design. Both RSM and artificial neural network (ANN) models were developed to predict the NH₃-N removal efficiency. Under the optimal conditions of 0.51 mM Fe²⁺, 49.44 mA/cm² current density, and 118.60 min treatment time, removal of 92.13% NH₃-N from dairy wastewater with 90% N₂ selectivity was achieved during validation experiments. The ANN model showed a superior predictive performance to the RSM model. The NH₃-N degradation rate was calculated at 0.0229 min⁻¹ based on a pseudo-first-order kinetic model. These findings demonstrate the applicability of the integrated chemical coagulation and PEF process for significantly reducing ammonia nitrogen in dairy wastewater.

This study integrated post-hydrothermal carbonization (HTC) liquids from raw and digested dairy manure with a tubular expanded polytetrafluoroethylene (ePTFE) gas permeable membrane (GPM) system. The effects of HTC temperature (180-240 °C) and residence time (1-4 h) on total ammoniacal nitrogen (TAN) concentration and pH in post-HTC liquids were first investigated. The post-HTC liquids, together with filtered manures, were subsequently applied in a tubular ePTFE GPM reactor to evaluate ammonia recovery performance. TAN concentrations in post-HTC liquids generally increased with higher HTC temperatures and longer residence times. HTC of digested manure yielded liquids with higher TAN concentrations (1150-1210 mg N L-1) and mildly alkaline pH (8.2-8.4), whereas post-HTC liquids derived from raw manure were acidic (pH 5.4-7.1) and exhibited lower TAN concentrations (725-764 mg N L-1). All post‑HTC liquids exhibited strong buffering capacity, indicating that bulk chemical pH adjustment would require substantial alkali inputs. The GPM system recovered ammonia from all feed streams, but recovery was consistently higher for post‑HTC liquids from digested manure. Residual heat from the HTC process can benefit the subsequent GPM process, as increasing post-HTC liquid temperature enhances ammonia recovery. Over 24 h, post‑HTC liquids from digested manure achieved a daily average TAN transfer flux of 11.05-14.46 g m-2 d-1, compared with 3.92-5.13 g m-2 d-1for liquids derived from raw manure. Overall, the results indicate that an integrated anaerobic digestion-HTC-GPM configuration, using digestate as the HTC feedstock and relying on aeration-based pH adjustment is a technically promising pathway for nitrogen and phosphorus recovery from dairy manure.

This study focused on the optimization of hydrothermal carbonization (HTC) of nutrient-rich dairy manure digestate using response surface methodology based on a face-centered central composite design (FCCD-RSM), aiming to simultaneously maximize hydrochar (HC) yield and nutrient recovery. FCCD-RSM was employed to systematically evaluate the impact of HTC reaction temperature (180–240 °C) and reaction time (20–90 min) on HC yield, phosphorus (P) content in HC, and total ammonia-nitrogen (TAN) concentration in the HTC liquid. Results showed that the P content in HC and the TAN concentration in the HTC liquid increased with increasing HTC temperature and reaction time, whereas HC yield decreased. Based on experimental data, quadratic polynomial models were developed. Using RSM with equal weighting assigned to the three responses, the optimized conditions were determined to be 216.5 °C and 84.1 min, resulting in an HC yield of 79.03 ± 2.61%, P content in HC of 1.76 ± 0.04%, and TAN concentration in the HTC liquid of 891.67 ± 16.67 mg/L. These values were close to the predicted values and fell within the 95% confidence intervals. Therefore, the optimized HTC parameters can simultaneously and effectively produce P-rich HC and TAN-rich liquid, providing insights into a more sustainable approach to circular waste management and nutrient recycling on dairy farms.

Dairy manure, a significant source of phosphorus (P), can potentially cause environmental risk due to P runoff when dairy manure is directly applied to cropland. Thus, there is an increasing interest in mitigating P loss from manure prior to land applications. This study aimed to investigate the potential of hydrochar produced by hydrothermal carbonization (HTC) for P recycling from dairy manure with and without the addition of CaO, focusing on the plant bioavailability, stabilization, and transformation of P in the resultant hydrochar. Hydrochar was prepared under different temperatures (180-240°C). The effect of CaO addition (0-10% of raw manure on dry wt. basis) was also evaluated at 220°C. Results showed that water-soluble P (WSP), a key indicator of P runoff loss, was significantly reduced in hydrochar, particularly with CaO addition. In addition, the plant available P in hydrochar increased with HTC temperature increase till 220°C, which accounted for ∼90% of total P content, then decreased with temperatures higher than 220°C. The addition of CaO slightly reduced plant bioavailability when compared to hydrochar produced at 220°C without additive. The P fractionation and speciation analyses indicated the transformation of P into Ca-associated apatite P. Hydrochar produced at 220°C with 10% CaO addition resulted in a high P recovery (∼85%) and a reduced runoff risk by 97%. The results demonstrate the efficacy of P recycling through hydrochar produced from dairy manure through HTC, which offers a sustainable approach to managing dairy waste while mitigating the potential environmental risks of P runoff.

Dairy wastewater (DW) contains a high concentration of organic and inorganic pollutants. In recent years, extensive research has been conducted to develop more efficient techniques for the treatment of DW. Electrochemical advanced oxidation processes (EAOPs) have gained significant attention among the various treatment approaches. EAOPs rely on electrochemical generation of hydroxyl radicals (•OH) which are considered highly potent oxidizing compounds for the degradation of pollutants in DW. In this paper, we provide an overview of the treatment of DW using various EAOPs, including anodic oxidation (AO), electro-Fenton (EF), photo electro-Fenton (PEF), and solar photo electro-Fenton (SPEF) processes, both individually and in combination with other techniques. Additionally, we discuss the reactor design and operating parameters employed in EAOPs. The variation in degradation efficiency is due to different oxidizing agents produced in specific approaches and their pollutant degradation abilities. In AO process, •OH radicals generated on electrode surfaces are influenced by electrode material and current density, while EF procedures use Fe²⁺ to create oxidizing agents both on electrodes and in the DW solution, with degradation mechanisms being affected by Fe²⁺, pH, and current density; additionally, PEF and SPEF approaches enhance oxidizing component production and pollutant degradation using ultraviolet (UV) light. Integration of EAOPs with other biological processes can enhance the pollutant removal efficiency of the treatment system. There is a scope of further research to exhibit the effectiveness of EAOPs for DW treatment in large scale implementation.

Dairy manure adds a substantial amount of nitrogen to wastewater due to its high levels of associated nutrients. Removal and recovery of ammonia nitrogen (NH₃-N) from raw liquid dairy manure (RLDM) is greatly valued. This study was focused on the vacuum thermal stripping–acid absorption (VTS-AA) process for NH₃-N from RLDM, followed by modeling and optimization. Using the response surface methodology (RSM)-based central composite design (CCD) approach, the critical operational parameters of the vacuum thermal stripping process, including temperature (50–70 °C), pH (9–11), vacuum pressure (35–55 kPa), and treatment time (60–90 min), were optimized. With the specified parameters set at temperature 69.9 °C, pH 10.5, vacuum pressure 53.5 kPa, and treatment time 64.2 min, the NH₃-N removal efficiency attained was 98.58 ± 1.05%, aligning closely with the model prediction. Furthermore, the recovered ammonium sulfate ((NH₄)₂SO₄) closely matched their commercial counterparts, confirming the effectiveness of the VTS-AA process in recovering NH₃-N from RLDM. The distinct advantage of the employed technology lies in the concurrent energy demand reduction achieved by introducing a vacuum system. These findings contribute valuable insights into the practical implementation of the VTS-AA process for treating raw dairy manure, particularly in large-scale operational contexts.

The release of anaerobically digested dairy wastewater (ANDDW) without a treatment can lead to severe environmental pollution, prompting the exploration of effective and sustainable treatment methods. Amidst various wastewater treatment approaches, the electro-oxidation (EO) process is considered as a promising, clean, and adaptable solution. In this study, the major operational parameters viz. current density, electrolyte concentration, treatment time, and mixing speed of an EO comprising Ti/PbO₂ anode and stainless-steel cathode, were optimized using response surface methodology (RSM) for efficient removal of chemical oxygen demand (COD), ammonia nitrogen (NH₃-N), total phosphorus (TP), orthophosphate (OP), total nitrogen (TN), and total Kjeldahl nitrogen (TKN) from ANDDW. Optimal conditions were identified as a current density of 90 mA cm⁻², 0.08% electrolyte concentration, 180 min treatment time, and 400 rpm mixing speed. Under the optimum conditions, the COD, NH₃-N, TP, OP, TN, and TKN removal efficiencies were 78.36, 63.93, 87.41, 92.39, 67.01, and 81.42%, respectively. Furthermore, the reaction rate followed the first-order kinetic model for the pollutants removal with correlation coefficients (R²) close to 1. The findings highlight the potential of using the EO process to treat high pollutant-laden ANDDW and encourage further studies to confirm the corresponding outcomes on a pilot scale. Graphical abstract