Syngas Purification Research Articles

The effect of the number of syn-gas purification nozzles of the water scrubber method on the characteristics of gasification combustion results

The product of the gasification process contains several impurities, including tar, ash, CO2, and other contaminants. To achieve efficient syngas results, the syngas must undergo a filtration or purification process, which can increase its energy density. The purpose of this study was to determine the effect of the number of purification nozzles on the characteristics of the resulting flame, the duration of the flame produced during the gasification process, and the rate of heat absorption in the water generated from the syngas purification process. The water scrubber method employed in the syngas purification process utilizes an updraft gasification reactor. The biomass used in this study is rubber wood, with a venturi nozzle size of 0.15 mm and a pressure of 0.12 Pa. The tests varied the number of purification nozzles to 2, 3, and 4. The use of 4 nozzles had a significant effect on the combustion characteristics, resulting in a blue flame with a duration of 33 minutes. The configuration with 3 nozzles produced a flame that exhibited a mixture of blue and orange colors, with the blue flame being more dominant, lasting 38.2 minutes. In contrast, the configuration with 2 nozzles resulted in a bluish-red flame, predominantly red in color, with a duration of 45 minutes. The heat absorption rates in the water produced for the configurations with 2, 3, and 4 nozzles were measured at 539 J/s, 449.1 J/s, and 414.62 J/s, respectively.

Evaluation of an alternative process for the production of hydrocarbons from CO2: Techno-economic and environmental analysis

This study aims to evaluate and compare two integrated processes (Plant A and B) for hydrocarbon production from CO2. They involve proton exchange membrane water electrolysis for H2 production, reverse water-gas shift reaction for syngas production, pressure swing adsorption for syngas purification, Fischer-Tropsch synthesis for diesel-range hydrocarbons from CO2, and atmospheric distillation for product separation. Plant B additionally integrates a hydrocracking-based upgrading section. The simulations were performed using Aspen Plus® v10, Aspen Adsorption® v10, and Aspen Custom Modeler® v10. The evaluation covers technical, economic, environmental, and multi-criteria assessment (GREENSCOPE). The proposed processes demonstrate the potential to convert CO2 to hydrocarbons, with carbon conversion rates of 75% and 71% for Plants A and B, respectively. Process energy intensity is 155 and 170 MJ/kgLP for Plants A and B, respectively. Environmental assessment reveals CO2 reduction, resulting in negative global warming potentials of −2.20 and −0.84 kgCO2-eq/kgLP for Plants A and B, respectively. The sustainability degree indices for Plants A and B are 1.00 and 0.06, respectively, indicating Plant A as the more sustainable process alternative based on both quantitative and qualitative metrics. Economic analysis shows project unviability in the Brazilian context due to reliance on H2 production and prevailing pricing conditions. Economic viability requires a price increase of liquid products by 266% and 302% or CO2-eq abatement costs ranging from 2.05 to 3.43 US$/kgCO2-eq for Plants A and B, respectively. GREENSCOPE methodology indicates better performance for Plant A across all scores, highlighting a clear connection between the hydrocracking unit and performance indicators. The findings show that these processes align with sustainable development goals (SDGs) targets for climate action (SDG 13), clean energy (SDG 7), industry, innovation, and infrastructure (SDG 9), decent work and economic growth (SDG 8), and responsible consumption and production (SDG 12). Overall, the research moves forward SDGs by offering solutions to cut carbon emissions and promote sustainable industrial practices.

Demonstrating Pilot-Scale Gas Fermentation for Acetate Production from Biomass-Derived Syngas Streams

Gas fermentation is gaining attention as a crucial technology for converting gaseous feedstocks into value-added chemicals. Despite numerous efforts over the past decade to investigate these innovative processes at a lab scale, to date, the evaluation of the technologies in relevant industrial environments is scarce. This study examines the fermentative production of acetate from biomass-derived syngas using Moorella thermoacetica. A mobile gas fermentation pilot plant was coupled to a bubbling fluidized-bed gasifier with syngas purification to convert crushed bark-derived syngas. The syngas purification steps included hot filtration, catalytic reforming, and final syngas cleaning. Different latter configurations were evaluated to enable a simplified syngas cleaning configuration for microbial syngas conversion compared to conventional catalytic synthesis. Fermentation tests using ultra-cleaned syngas showed comparable microbial growth (1.3 g/L) and acetate production (22.3 g/L) to the benchmark fermentation of synthetic gases (1.2 g/L of biomass and 25.2 g/L of acetate). Additional fermentation trials on partially purified syngas streams identified H2S and HCN as the primary inhibitory compounds. They also indicated that caustic scrubbing is an adequate and simplified final gas cleaning step to facilitate extended microbial fermentation. Overall, this study shows the potential of gas fermentation to valorize crude gaseous feedstocks, such as industrial off-gases, into platform chemicals.

Development of Heat Exchanger for Syngas Purification in a Smart Mini-Grid Gasification Power Plant

The shell and tube heat exchanger was designed using a stainless pipe with a diameter of 203.2 mm (8 inches) and 3 mm thick. The design calculation results indicates that heat transfer rate was about and a tube length of 0.7 m was used and the number of tubes Nt is 17, this is required to cool the syngas from 350℃ to 80°C theoretically. Results obtained from CFD ANSYS software 2022 version showed that the verification of the model was achieved by comparing the theoretical temperatures of the syngas with the temperatures predicted from the simulation.

Purification of syngas from Refuse-Derived Fuel (RDF) gasification: Techno-economic analysis

Purification of syngas from Refuse-Derived Fuel (RDF) gasification: Techno-economic analysis

Optimal integration modeling of Co – Electrolysis in a power-to-liquid industrial process

High temperature co-electrolysis using solid-oxide electrolysis cells is a highly efficient pathway for green syngas production owing to the possibility of heat integration with other processes. Therefore, this study described and evaluates a flexible and efficient configuration for producing sustainable synthetic fuels using electricity from renewables and captured CO2 by integrating co-electrolysis in a power-to-liquid industrial plant. Thereafter, novel and efficient technologies were implemented for green syngas production and its subsequent purification, increasing the overall process efficiency and achieving a significant reduction in the carbon footprint compared to mature synthetic crude production processes. Catalytic partial oxidation and dual pressure swing adsorption were integrated with co-electrolysis and Fischer–Tropsch synthesis in a scaled industrial plant, using residual streams from the complex or those of renewable origin as feed, which allowed the continuous operation of the process independent of renewable power generation. The mass and energy balance, performance, and efficiency estimations were also included in this study. A solid-oxide electrolytic cell (SOEC) plant using renewable electricity and heat input from thermal integration with the outlet syngas stream of the catalytic partial oxidation reactor was selected as a case study. Both the performance and efficiency analyses of the co-electrolysis unit demonstrated the benefits of such thermal integration in comparison with current solutions. In this study, both the thermal integration of the process streams, as well as the energy and heat consumed by the syngas purification process were considered.

4E analysis of two novel H2S concentrators in dual-stage Selexol process of integrated gasification combined cycle

Syngas purification unit of integrated gasification combined cycle (IGCC) has a significant impact on environmental and economic benefits. Dual-stage Selexol process of IGCC can simultaneously capture both CO2 and H2S. Two novel H2S concentrators in dual-stage Selexol process had been studied. Pressure drop method as the focus of this paper, 4E analysis method of two cases (depressurization combined with stripping and single depressurization) was applied. Two models of Selexol process were developed by ASPEN PLUS and validated by existing reports. The simulated results indicated that depressurization combined with stripping method had lower utilities consumption, 6.89 MW less in exergy destruction, 6.51 million dollars less in total annual cost, and better environmental benefit, comparing with single depressurization method, and this method also consumes less energy and costs up to 9.6 % less on capital cost than the traditional single stripping method. However, single depressurization method had better performance in H2 recovery and less system complexity. The sensitivity analysis is also employed to study the impact of some key parameters on 4E analysis. This study can be used as a comparative reference for advanced syngas purification technologies.

Performance analysis of the acid gas removal from syngas based on self-heat recuperation technology

Considering the high energy consumption of the acid gas removal (AGR) unit in integrated gasification combined cycle (IGCC) systems, this paper innovatively combines the two self-heat recuperation technology (SHRT) forms with the syngas purification process. The AGR unit and the power generation unit in this paper are simulated by Hysys V11. By establishing the energy balance and exergy balance equation for each component, the change of energy consumption and exergy loss of the unit before and after the process modification is analyzed. Compared with the two improved methods, Case1 uses low-pressure water as the working medium of SHRT process, which has the characteristics of high COP of heat pump and low throttling loss, and achieves good results in terms of operating cost and exergy loss. Case1 has 3.75% lower operating cost compared with the basic process, the annual operating cost of H2S removal is reduced by 144,200$, and the exergy loss of the AGR unit is reduced by 23.48 kW. Case2 does not have advantages under the two evaluation indexes. Subsequently, sensitivity analysis of the inlet pressure of Case1 heat pump exchanger further improves the energy-saving potential of this retrofit solution. The article explains the effect of the SHRT process in the AGR unit on the exergy loss of the rich solvent regeneration process and the distillate cooling process, and verifies the energy-saving potential of the SHRT in the acid gas removal unit of IGCC.

Acid Gas and Tar Removal from Syngas of Refuse Gasification by Catalytic Reforming

The existence of acid gas and tar in syngas of municipal solid waste gasification limits its downstream utilization as a clean energy source. Here, we investigated the catalytic removal of HCl and tar. The key parameters affecting the catalytic reaction, including space velocity, temperature, the amounts of active metals in the catalyst and the carrier material, were studied, targeting optimized operating conditions for enhanced syngas purification. The morphology, mineral phases, surface area and pore size before and after the reaction were investigated to understand the mechanism to dominate the reaction. The results showed that the removal rate of CaO adsorbent and HCl reached 96% at 400 °C. When the space velocity ratio was 1.0 and the temperature was 400 °C, HCl removal (97%) by NaAlO2 was even better. Nevertheless, clogging was observed for NaAlO2 via the BET test after reaction to jeopardize its durability. A level of 25% Ni doping on Zr1-x(Cex)O2 support provides high stability for tar removal. This is because the Zr1-x(Cex)O2 carrier has higher carbon deposition resistivity than the Al2O3 carrier. The EDX results confirmed that a large amount of C (79.3%) was accumulated on the commercial catalyst surface supported by Al2O3 (25% Ni-based). As for the temperature, a temperature higher than 800 °C could not enhance the efficiency of tar removal, likely due to catalyst deactivation. Carbon deposition and agglomeration are the two main causes of catalyst deactivation. At 800 °C, 25% Ni-based synthetic catalyst can convert 48.5 ± 19.4% tar to low molecular weight organic compounds. By contrast, such a conversion rate under the same temperature only accounted for 5.0 ± 6.8% based on a commercial catalyst. These insights point to the important role of catalyst support materials.

Handling model uncertainty in control of a pressure swing adsorption unit for syngas purification: A multi-model zone control scheme-based robust model predictive control strategy

The application of a robustly stabilizing model predictive control (MPC) strategy to a pressure swing adsorption (PSA) unit, hitherto unexplored in the literature, is addressed in this work. Here, a set of linear models to handle model uncertainties describes the PSA nonlinear dynamic behaviour. Each model represents a different operating condition of the PSA, generating a multi-model uncertainty description. In addition to incorporating the multi-model uncertainty, the proposed MPC control law deals with a controlled output zone tracking scheme to systematically accommodate typical cyclic steady states of a PSA process. From the industrial standpoint, economic targets on process variables (controlled outputs, manipulated inputs, or both) are also incorporated into the robust control method. A Real-Time Optimizer usually defines these targets, making up a two-layer control scheme. The effectiveness of the robust MPC is evaluated in a syngas purification process via a single bed, six-step PSA unit composed of a porous amino-functionalized titanium terephthalate MIL-125-type MOF adsorbent.

Proposal and assessment of a solar-coal thermochemical hybrid power generation system

The thermochemical complementarity is an efficient path to improve the utilization efficiency of coal and address the intermittence of solar energy. In this paper, a solar-coal thermochemical hybrid power generation system based on supercritical water gasification is proposed, and the feature is that the low gasification temperature of 650 °C makes it feasible to use concentrated solar energy to provide reaction heat for the gasification process. The energy and exergy analysis methods are used to evaluate the thermodynamic performance of the system with a net power output of 500 MW. The results show that the net power generation efficiency and exergy efficiency of the proposed system can reach 53.06 % and 52.24 %, respectively, which are approximately 6.94 and 6.83 percentage points higher than those of the reference system. The significant role of thermochemical method is that the chemical energy of syngas produced by the proposed system is 851.57 MW, which is increased by approximately 29.70 %. Through the energy utilization diagram analysis, it is found that the key to performance improvement is that the exergy destruction of the fuel conversion process and heat exchange process are decreased by 10.36 % and 54.68 %, respectively. Finally, the proposed system has better performance and lower exergy destruction. The solar thermal energy of low energy level is converted into syngas chemical energy of high energy level, and the upgrading of solar energy is achieved. In addition, the proposed system is simpler, and eliminates the air separation unit and syngas purification unit. This work provides a promising method for the complementary utilization of solar energy and coal.

Dynamic Simulation and Process Design Analysis of Syngas Purification and The Utilization of Gas Permeation

Abstract: Syngas generation from alternative energy sources, notable methanation, is currently gaining popularity. The technique saves energy by converting it into a chemical product. It is not suited for use directly in distribution grids, where a greater purity of methane is necessary for maximum power density. Various syngas purification levels should be supplied for various methane grades (for heat, power, and automotive fuels uses). Water damages the mechanical components of an engine, thus it must be removed before it can be utilized as a fuel. CO2 removal should also be performed to increase methane heat quality and minimize pollution. This research evaluated the performance of a syngas purification system using a flash separator and hollow fiber membranes. A flash separator model condensed the water from the wet feed. The CO2 removal from methane was then investigated using hollow fiber membranes. As a consequence, Aspen Plus® V8.6 includes a FORTRAN user model for unit operations. The greatest result was filtering methane up to 98 percent vol. using a two stage pervaporation system with recycling streams. The model scheme may help in the implementation and quality evaluation of a complex methanation plant system

Chemical looping partial oxidation (CLPO) of toluene on LaFeO3 perovskites for tunable syngas production

Tar is the by-product of coal and biomass gasification which decreases the purity of the primary syngas and causes blockages in the pipes. Chemical looping partial oxidation (CLPO) is a promising technology to convert tar to high-purity syngas in a sustainable way. Here, we synthesized LaFeO3 perovskites as oxygen carriers with five different wet chemistry synthesis methods and tested their performance for toluene CLPO in a fixed-bed reactor. The template synthesis method presents the highest purity, the most oxygen species adjacent to oxygen vacancy, the best resistance to the carbon deposition, and exhibits the best performance in CLPO, with up to ∼100 % toluene conversion efficiency, up to ∼100 % CO selectivity, and the best recyclability. The H2/CO ratio in the syngas products varies between 0.86 and 2.2 in different stages of the toluene CLPO process. By adjusting the reaction time of toluene catalytic cracking, the H2/CO ratio is tunable within this range. Based on these results, we designed a new in-situ CLPO process for tar-containing syngas purification that can achieve tunable H2/CO ratio for syngas. In the process, toluene cracking occurs in front of toluene partial oxidation by using reverse flow design to simultaneously achieve ∼100 % toluene conversion ratio and high conversion rate. This work shows that LaFeO3 is highly efficient in converting toluene to high-purity syngas, and it has the potential to convert tar-containing syngas in-situ to achieve energy- and cost-effective manufacture for high-value-added syngas products.

Unravelling the significance of catalyst reduction stage for high tar reforming activity in the presence of syngas impurities

Ni catalysts are used widely in tar reforming for syngas purification. This study investigates the aim of the reduction stage toward Ni catalysts during tar reforming in the presence of syngas impurities. A NiAl2O4 catalyst was used for the reforming of a mixture of tar model compounds (naphthalene, toluene, and styrene) in simulated syngas containing 50 ppmv H2S and 500 ppmv HCl. The catalyst exhibited activity in tar reforming even without the pre-reduction treatment, but it was deactivated rapidly due to the favored reaction between H2S and Ni2+ species, causing increased Ni agglomeration and carbon deposition. Pre-reduction of catalysts enhanced the catalyst stability even in the presence of syngas impurities. Higher reforming activity was achieved by increasing the reduction temperature from 800 °C to 900 °C. However, transformation in the support structure at higher reducing temperature could lead to decreased exposure of Ni due to the collapse of pore structures.

A 4E analysis of a novel coupling process of syngas purification and CO2 capture, transcritical CO2 power and absorption refrigeration

To achieve the goal of CO2 emission peak and carbon neutrality, a novel process coupling syngas purification and CO2 capture, transcritical CO2 power and absorption refrigeration is proposed. The purpose is to realize energy conservation and CO2 emission reduction by coupling medium and low-grade waste heat, electricity and cold water in the process. Based on verifying the accuracy of the model and simulation, the proposed system is studied from four aspects: energy, exergy, economy and environment (4E). The results show that the process can meet the industrial requirements of syngas purification and CO2 products. The thermal efficiency and exergy efficiency are 35.58% and 35.96% respectively. The annual total cost is 2.45 million dollars, and the annual CO2 emission reduction is 113.6 kt. By exploring the exergy destruction and cost distribution of components, the parts that need attention and improvement in the process are determined. In addition, the effects of heat transfer temperature difference and refrigeration temperature on the performance of the process are also studied. This provides a reference for the comprehensive performance analysis and application feasibility of the coupling process of syngas purification, CO2 capture and waste heat utilization.

INFLUENCE OF THE OXIDIZER ON THE FORMATION AND PURIFICATION EFFICIENCY OF ACID GASES PRODUCED DURING ASPHALTENE GASIFICATION

Link for citation: Ermolaev D.V., Daminov A.Z. Influence of the oxidizer on the formation and purification efficiency of acid gases produced during asphaltene gasification. Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2022, vol. 333, no. 4, рр. 215-223. In Rus.
 The relevance of the study is determined by the need to understand the influence of the oxidizer on the formation of acid gases (CO2, H2S, COS and CS2) during thermal decomposition of high-viscosity hydrocarbons. This is important for predicting the purification efficiency of the produced gasification products and estimating the economic costs. The aim: using the simulation to study the effect of an oxidizer in the form of steam on the composition and properties of asphaltene gasification products obtained from natural bitumen, as well as to determine the cleaning efficiency depending on the amount of steam and the absorbent based on NaOH water-alkaline solution. Object: asphaltene of natural bitumen of Ashalchinskoe field of the Tatarstan Republic (Russia), oxidizer in the form of steam, the value of which varied from 0,1 to 1 depending on the amount of asphaltene. Methods: simulation of asphaltene gasification and acid gas absorption taking into account influence of an oxidizer in a form of steam with regard for basic chemical kinetics, ultimate analysis and TGA. Simulation results of gasification and absorption showed that steam used as an oxidizer during asphaltene gasification has a significant influence on the composition and properties of gasification products, as well as on the purification of syngas. With the increase of steam, a parabolic dependence of the concentrations of syngas components is observed, which values decrease with time, except for CO2. The calorific value of syngas decreases from 11,3 to 7,2 MJ/m3 and the cold gas efficiency increases from 53,4 to 62,5 % due to growth of syngas yield. As the amount of steam increases, the amount of absorbent decreases and the purification efficiency of acid gases rises. Thus, the amount of absorbed CO2 increases by 20,7 % while the absorbent decreases by 6,7%. At the same time the amount of absorbed H2S increased by 0,39 % with decrease of NaOH by 40,9 %.

Comparison of Syngas-Fermenting Clostridia in Stirred-Tank Bioreactors and the Effects of Varying Syngas Impurities.

In recent years, syngas fermentation has emerged as a promising means for the production of fuels and platform chemicals, with a variety of acetogens efficiently converting CO-rich gases to ethanol. However, the feasibility of syngas fermentation processes is related to the occurrence of syngas impurities such as NH3, H2S, and NOX. Therefore, the effects of defined additions of NH4+, H2S, and NO3− were studied in autotrophic batch processes with C. autoethanogenum, C. ljungdahlii, and C. ragsdalei while applying continuously gassed stirred-tank bioreactors. Any initial addition of ammonium and nitrate curbed the cell growth of the Clostridia being studied and reduced the final alcohol concentrations. C. ljungdahlii showed the highest tolerance to ammonium and nitrate, whereas C. ragsdalei was even positively influenced by the presence of 0.1 g L−1 H2S. Quantitative goals for the purification of syngas were identified for each of the acetogens studied in the used experimental setup. Syngas purification should in particular focus on the NOX impurities that caused the highest inhibiting effect and maintain the concentrations of NH3 and H2S within an acceptable range (e.g., NH3 < 4560 ppm and H2S < 108 ppm) in order to avoid inhibition through the accumulation of these impurities in the bioreactor.

A novel standpoint of Pressure Swing Adsorption processes multi-objective optimization: An approach based on feasible operation region mapping

Optimization of cyclic adsorption processes is a challenging issue due to the dynamic-operating complexity of these processes. In this context, this work proposes a new approach for multi-objective optimization of Pressure Swing Adsorption (PSA) units, extending the concept of the Pareto front to Pareto region. The proposed methodology, hitherto unexplored in the literature, consists of integrating a likelihood test, an arrangement from the Fisher–Snedecor test to the solution of a multi-objective problem provided by a Swarm Particle Optimization technique. The Pareto region is divided into operating sub-regions that meet the optimization constraints and prioritize a determined objective by a clustering process. These sub-regions make the operation more flexible. Furthermore, the analysis of the operating variables feasible operation interval demonstrated to be an important tool to provide information regarding the system behavior. As a study case, it is presented the optimization of a PSA process for syngas purification. The results demonstrate that the methodology proposed here uses the feasible operation region map and the clustering strategy to exploit the multi-objective optimization. Therefore, providing a more reliable and precise optimization of PSA units, while providing an important tool for making decisions in the PSA system.

Warm Plasma Application in Tar Conversion and Syngas Valorization: The Fate of Hydrogen Sulfide

Warm plasma techniques are considered a promising method of tar removal in biomass-derived syngas. The fate of another problematic syngas impurity—hydrogen sulfide—is studied in this work. It is revealed that processing simulated syngas with a microwave plasma results in hydrogen sulfide conversion. For different gas flow rates (20–40 NLPM) and hydrogen sulfide concentrations ranging from 250 ppm to 750 ppm, the conversion rate varies from ca. 26% to 45%. The main sulfur-containing products are carbon disulfide (ca. 30% of total sulfur) and carbonyl sulfide (ca. 8% of total sulfur). Besides them, significantly smaller quantities of sulfates and benzothiophene are also detected. The main components of syngas have a tremendous impact on the fate of hydrogen sulfide. While the presence of carbon monoxide, methane, carbon dioxide, and tar surrogate (toluene) leads to the formation of carbonyl sulfide, carbon disulfide, sulfur dioxide, and benzothiophene, respectively, the abundance of hydrogen results in the recreation of hydrogen sulfide. Consequently, the presence of hydrogen in the simulated syngas is the main factor that determines the low conversion rate of hydrogen sulfide. Conversion of hydrogen sulfide into various sulfur compounds might be problematic in the context of syngas purification and the application of the right technique for sulfur removal.

Integrating biodegradation and ozone-catalysed oxidation for treatment and reuse of biomass gasification wastewater

Syngas purification via wet-scrubbing processes generates a relevant amount of wastewater, which requires proper treatments before its disposal or reuse. Integrating chemical with cost-saving biological approaches represent a valuable alternative to traditional chemical-physical treatments. This study investigated the effectiveness of the BIO&CHEM treatment scheme for the treatment and reuse of wastewater collected from a wet-scrubber unit (chemical oxygen demand, COD = 2600 mg/L) containing several hazardous pollutants such as phenol (110 mg/L) and xylene (64 mg/L). The BIO&CHEM system included a sequencing batch biofilter granular reactor (SBBGR) and an ozonation unit that were operated at different hydraulic retention times (HRTs, from 5 to 1 day) and transferred ozone doses (TODs, 400, 280 and 250 mg/Linf), respectively. When a 4 d-HRT was selected, biologically treated wastewater was characterized by a COD of 140 mg/L and its toxicity on Daphnia magna was decreased from 100% to 5%. The integration between the biological SBBGR-based treatment (HRT = 1.5 d) and the ozonation (TOD = 280 mg/Linf) lowered effluent COD to 33 mg/L and erased toxicity on Daphnia magna. This operating condition resulted the most sustainable enabling multiple wastewater reuse and a significant reduction of plant size comparing with the biological treatment. Furthermore, the investigated aromatic and polyaromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs; including benzene, toluene, xylene and phenol) were completely removed. The analysis of the reactor biomass, performed at the end of the experimental trial, excluded also their mere accumulation within the reactor due to absorption or adsorption on the sludge.