Innovative Hydrodynamic Disintegrator Adjusted to Agricultural Substrates Pre-treatment Aimed at Methane Production Intensification—CFD Modelling and Batch Tests

Life Cycle Assessment of sewage sludge mono-digestion and co-digestion with the organic fraction of municipal solid waste at a wastewater treatment plant

Wastewater treatment plants (WWTPs) aim to increase energy independence by intensifying biogas production. The study involved Life Cycle Assessment (LCA) comparing sewage sludge (SS) anaerobic co-digestion (AcD) with the organic fraction of municipal solid waste (OFMSW) at WWTP against SS anaerobic mono-digestion (AD). The LCA was based on the authors' research showing that methane production related to wet mass in AcD was higher than in AD by 86.4, 225.8, and 354.3 % for SS:OFMSW mixing ratios of 75:25, 50:50, and 25:75, respectively. The LCA was conducted for 1 kWh of biogas energy produced at a WWTP and included two subsystems: energy production (AD/AcD and CHP) and associated digestate management (drying, incineration with energy reuse, residual landfilling). CML-IA baseline 2013 and Recipe 2016 Midpoint (H) LCIA methodologies indicated the environmental impacts on abiotic depletion - fossil fuels (fossil resource scarcity), global warming, human toxicity (non-carcinogenic), freshwater, and marine aquatic ecotoxicity. AD of SS in all categories was characterized by the highest impact. The impact decreased with an increase in the OFMSW rate. Both methodologies showed a significant impact of AD and AcD on global warming, which indicated the need for the determination of the carbon footprint (CF) of 1kWh biogas energy production (gCO2 kWh-1) at WWTP, using IPCC 2021 GWP100 (incl. CO2 uptake). The CF was the highest for biogas energy production in SS AD (1509 gCO2 kWh-1) and it decreased with the increase of OFMSW content in AcD (872-481 gCO2 kWh-1). This was also observed for the management of associated digestate. CF changed from 1508 gCO2 kWh-1 for SS AD to 396 gCO2 kWh-1 for SS: OFMSW mixing ratio of 25: 75 in AcD. The environmental burdens associated with biogas energy production and digestate management were equally significant. Finally, the formula for economic and ecological comparison of SS AD and AcD with OFMSW was proposed.

The use of hydrodynamic cavitation for waste-to-energy approach to enhance methane production from waste activated sludge

Anaerobic digestion in wastewater treatment plants converts its unwanted end product - waste activated sludge into biogas. Even if the process is well established, pre-treatment of the sludge can further improve its efficiency. In this study, four treatment regimes for increasing methane production through prior sludge disintegration were investigated using lab-scale cavitation generator and real sludge samples. Three different cavitating (attached cavitation regime, developed cloud shedding cavitation regime and cavitation in a wake regime) and one non-cavitating regime at elevated static pressure were studied in detail for their effectiveness on physical and chemical properties of sludge samples. Volume-weighted mean diameter D[4,3] of sludge's particles decreased by up to 92%, specific surface area increased by up to 611%, while viscosity (at a shear rate of 3.0 s-1) increased by up to 39% in the non-cavitating and decreased by up to 24% in all three cavitating regimes. Chemical changes were more pronounced in cavitating regimes, where released soluble chemical oxygen demand (sCOD) and increase of dissolved organic matter (DOM) compounds by up to 175% and 122% were achieved, respectively. Methane production increased in all four cases, with the highest increase of 70% corresponding to 312 mL CH4 g-1 COD. However, this treatment was not particularly efficient in terms of energy consumption. The best energy balance was found for the regime with a biochemical methane potencial increase of 43%.

Effects of various rotational speeds of hydrodynamic disintegrator on carbon, nutrient, and energy recovery from sewage sludge

Until recently, sewage sludge produced in wastewater treatment processes was considered problematic waste. It currently constitutes a valuable substrate for raw materials and energy recovery. One of the methods of intensifying resource recovery from sludge is its pretreatment by means of disintegration methods. This study presents the CFD modelling and experimentally investigates the use of a hydrodynamic cavitation rotor operated with various rotational speeds (1500, 2500, and 300 rpm) for the recovery of organic compounds, nutrients, and energy. Rheological properties of raw sludge, a non-Newtonian fluid, were determined and used in the modelling calculations. Cavitation zones were observed for 2500 rpm and 3000 rpm, although a stronger cavitation effect occurred for a rotational speed of 3000 rpm. A rotational speed of 1500 rpm was too low to generate a pressure drop below 1705 Pa, and no cavitation was recorded. An increase in rotational speed from 1500 rpm to 3000 rpm for each analysed energy density caused an increase in SCOD and nitrogen concentration. Moreover, it was determined that at low energy densities (<105 kJ/L), mechanical tearing was the dominant factor responsible for carbon recovery, and at its higher values (≥105 kJ/L), the cavitation phenomenon became increasingly important. Rotation speed also had a significant effect on methane yield (YCH4). An increase in YCH4 by 6.2% was recorded only for disintegrated sludge at a rotational speed of 1500 rpm in reference to untreated sludge. Disintegration conducted at higher rotational speeds led to a decrease in YCH4 (-0.7% for 2500 rpm and -7.9% for 3000 rpm).

Re-flocculation reduces the effectiveness of sewage sludge pretreatment through hydrodynamic disintegration prior to anaerobic digestion

Circular economy model, based on the "make, use, reuse, remake, recycle" approach, is an alternative to progressive depletion of non-renewable fossil fuels. Sewage sludge can be a source of renewable energy obtained through the anaerobic conversion of their organic fraction into biogas. This process is mediated by highly complex microbial communities and its efficiency depends on the availability of substrates to microorganisms. Disintegration of the feedstock in the pre-treatment step may intensify the anaerobic digestion, but re-flocculation of disintegrated sludge (reassembly of the released fractions into larger agglomerates) may result in a reduced availability of the released organic compounds for microbes. Pilot-scale studies on re-flocculation of disintegrated sludge were conducted to select parameters for scaling-up the pre-treatment and intensifying the anaerobic digestion process in two large Polish wastewater treatment plants (WWTPs). Samples of thickened excess sludge from full-scale WWTPs were subjected to hydrodynamic disintegration at three energy density levels of 10 kJ/L, 35 kJ/L and 70 kJ/L. Microscopic analyses of disintegrated sludge samples were carried out twice: i) immediately after the disintegration process at a given energy density level, ii) and after 24-h incubation at 4 °C following the disintegration. Micro-photographs of 30 randomly selected fields of view were taken for each analysed sample. A method of the image analysis was developed as a tool to measure dispersion of sludge flocs to assess the re-flocculation degree. Re-flocculation of the thickened excess sludge occurred within 24 h after hydrodynamic disintegration. This was evidenced by a very high re-flocculation degree, reaching up to 86%, depending on the origin of the sludge and the energy density levels used for the hydrodynamic disintegration.

Hydrodynamic disintegration of thickened excess sludge and maize silage to intensify methane production: Energy effect and impact on microbial communities

The aim of this project was to study the combination of two methods to increase methane production: feedstock pretreatment by hydrodynamic disintegration and co-digestion of maize silage (MS) with thickened excess sludge (TES). Disintegration of TES alone resulted in a 15% increase in specific methane production from 0.192 Nml/gVS (TES + MS) to 0.220 Nml/gVS (pretreated TES + MS). The energy balance revealed additional energy (0.14 Wh) would cover only the energy expenditure for the mechanical pretreatment and would not allow for net energy profit. Identification of the methanogenic consortia by 16S rRNA gene amplicon sequencing revealed that Chloroflexi, Bacteroidota, Firmicutes, Proteobacteria and Actinobacteriota were five most abundant bacteria phyla, with Methanothrix and Methanolinea as the dominant methanogens. Principal component analysis did not show any effect of feedstock pretreatment on methanogenic consortia. Instead, the composition of inoculum was the decisive factor in shaping the microbial community structure.

Pretreatment of milled and unchopped sugarcane bagasse with vortex based hydrodynamic cavitation for enhanced biogas production

Anaerobic digestion can potentially valorise sugarcane bagasse to biogas and fertiliser. Pretreatment is however required to overcome recalcitrance and enhance the biogas yields. Literature reporting the investigation of various biomass pretreatments often use milled biomass as substrate rather than as-received fibrous biomass. This does not establish the true influence of the pretreatment type on biogas generation. Additionally, milling energy is also ignored when calculating net energy gains from enhanced biogas yields and are thus misleading. In this work, a vortex-based hydrodynamic cavitation device was used to enhance the biomethane yields from fibrous as-received biomass for the first time. Clear justification on why milled biomass must not be used as substrates for demonstrating the effect of pretreatment on biogas production is also discussed. The net energy gain from milled hydrodynamic cavitation pre-treated bagasse can be similar to as-received bagasse only when the specific milling energy is ≤700 kWh/ton.

Recent advances in hydrodynamic cavitation-based pretreatments of lignocellulosic biomass for valorization

Recently, the hydrodynamic cavitation (HC)-based pretreatment has shown high effectiveness in laboratories and even in industrial productions for conversion of lignocellulosic biomass (LCB) into value-added products. The pretreatment capability derives from the extraordinary conditions of pressures at ∼500 bar, local hotspots with ∼5000 K, and oxidation (hydroxyl radicals) created by HC at room conditions. To promote this emerging technology, the present review summarizes the recent advances in the HC-based pretreatment of LCB. The principle of HC including the sonochemical effect and hydrodynamic cavitation reactor is introduced. The effectiveness of HC on the delignification of LCB as well as subsequent fermentation, paper production, and other applications is evaluated. Several key operational factors (i.e., reaction environment, duration, and feedstock characteristics) in HC pretreatments are discussed. The enhancement mechanism of HC including physical and chemical effects is analyzed. Finally, the perspectives on future research on the HC-based pretreatment technology are highlighted.

Integral analysis of hydrodynamic cavitation effects on waste activated sludge characteristics, potentially toxic metals, microorganisms and identification of microplastics

Wastewater treatment plants, the last barrier between ever-increasing human activities and the environment, produce huge amounts, of unwanted semi-solid by-product - waste activated sludge. Anaerobic digestion can be used to reduce the amount of sludge. However, the process needs extensive modernisation and refinement to realize its full potential. This can be achieved by using efficient pre-treatment processes that result in high sludge disintegration and solubilization. To this end, we investigated the efficiency of a novel pinned disc rotational generator of hydrodynamic cavitation. The results of physical and chemical evaluation showed a reduction in mean particle size up to 88%, an increase in specific surface area up to 300% and an increase in soluble COD, NH₄-N, NO₃-N, PO₄-P up to 155.8, 126.3, 250 and 29.7%, respectively. Microscopic images confirmed flocs disruption and damage to yeast cells and Epistilys species due to mechanical effects of cavitation such as microjets and shear forces. The observed cell ruptures and cracks were sufficient for the release of small soluble biologically relevant dissolved organic molecules into the bulk liquid, but not for the release of microbial DNA. Cavitation treatment also decreased total Pb concentrations by 70%, which was attributed to the reactions triggered by the chemical effects of cavitation. Additionally, the study confirmed the presence of microplastic particles and fibers of polyethylene, polyethylene terephthalate, polypropylene, and nylon 6 in the waste activated sludge.

Investigation into cavitational intensity and COD reduction performance of the pinned disc reactor with various rotor-stator arrangements

In this study, the hydrodynamic cavitation and wastewater treatment performance of a rotary generator with pin disk for hydrodynamic cavitation are investigated. Various geometrical features and arrangements of rotor and stator pins were evaluated to improve the configuration of the cavitation device. The pilot device used to perform the experiments was upgraded with a transparent cover that allows visualization of the hydrodynamic cavitation in the rotor-stator region with high-speed camera and simultaneous measurement of pressure fluctuations. Based on the hydrodynamic characteristics, three arrangements were selected and evaluated with respect to the chemical effects of cavitation on a 200-liter wastewater influent sample. The experimental results show that the rotational speed and the spacing of the rotor pins have the most significant effect on the cavitation intensity and effectiveness, while the pin diameter and the surface roughness are less significant design parameters. Cavitation intensity increases with pin velocity, but can be inhibited if the pins are arranged too close together. At best configuration, COD was reduced by 31% in 15 liquid passes, consuming 8.2 kWh/kg COD. The number of liquid passes also proved to be an important process parameter for improving the energy efficiency.

The Enhancement of Energy Efficiency in a Wastewater Treatment Plant through Sustainable Biogas Use: Case Study from Poland

The improvement of energy efficiency ensuring high nutrients removal is a great concern for many wastewater treatment plants (WWTPs). The energy balance of a WWTP can be improved through the application of highly efficient digestion or its intensification, e.g., through the introduction of the co-substrates with relatively high energy potential to the sewage sludge (SS). In the present study, the overview of the energetic aspect of the Polish WWTPs was presented. The evaluation of energy consumption at individual stages of wastewater treatment along with the possibilities of its increasing was performed. Additionally, the influence of co-digestion process implementation on the energy efficiency of a selected WWTP in Poland was investigated. The evaluation was carried out for a WWTP located in Iława. Both energetic and treatment efficiency were analyzed. The energy balance evaluation of this WWTP was also performed. The obtained results indicated that the WWTP in Iława produced on average 2.54 GWh per year (7.63 GWh of electricity in total) as a result of the co-digestion of sewage sludge with poultry processing waste. A single cubic meter of co-substrates fed to the digesters yielded an average of 25.6 ± 4.3 Nm3 of biogas (between 18.3 and 32.2 Nm3/m3). This enabled covering the energy demand of the plant to a very high degree, ranging from 93.0% to 99.8% (98.2% on average). Importantly, in the presence of the co-substrate, the removal efficiency of organic compounds was enhanced from 64% (mono-digestion) to 69–70%.