Volatile Suspended Solids Removal Research Articles

Optimal design and modelling of sustainable bio-catalytic enzyme for wastewater treatment using response surface methodology and artificial neural network

ABSTRACT The accessibility of various wastewater treatment systems has become increasingly prominent in current times, coinciding with a rising concern regarding the substantial content of food waste that poses a significant challenge in waste management. In this context, ecoenzymes have emerged as a promising technique for water treatment due to their remarkable efficiency in pollutant removal and cost-effectiveness. Nevertheless, many experts encounter difficulties in determining the optimal combination of ecoenzyme variables to maximize pollutant removal. The objective of this study is to apply a comprehensive approach utilizing Response Surface Methodology (RSM), Artificial Neural Networks (ANN), and Multi-objective Genetic Algorithm (MOGA) methods for the modeling and optimization of ecoenzyme-based water treatment processes. The research outcomes demonstrate a noteworthy alignment between the actual removal rates of Total Suspended Solids (TSS), Volatile Suspended Solids (VSS), and Total Ammonia Nitrogen (TAN) with the values predicted by both RSM and ANN models, validating the robustness of the regression analysis (R2 >0.95). From the optimization results, it is elucidated that to attain the maximum removal of TSS, VSS, and TAN, the treatment process should be conducted over a period of 3.5 days, employing specific enzyme concentrations of 0.07 units/ml for protease, 0.027 units/ml for amylase, and 0.36 units/ml for lipase. This study employed machine learning approaches and MOGA optimization to predict the optimum ecoenzyme properties for maximum wastewater removal utilizing the concept of a low-cost, sustainable purification technology.

Wet oxidation and catalytic wet oxidation of pharmaceutical sludge

In this work, wet oxidation and catalytic wet oxidation of pharmaceutical sludge using homogeneous and heterogeneous catalysts were investigated. The results indicate that wet oxidation is a promising method for the highly efficient degradation of pharmaceutical sludge. Under optimal conditions, the highest removal efficiencies of volatile suspended solids (VSS) 86.8% and chemical oxygen demand (COD) 62.5% were achieved at 260 °C for 60 min with an initial oxygen pressure of 1.0 MPa. NaOH exhibited excellent acceleration performance on the VSS removal. The highest VSS removal efficiency of 95.2% was obtained at 260 °C for 60 min with an initial oxygen pressure of 1.0 MPa and 10 g·L−1 of NaOH. By using a Cu–Ce/γ-Al2O3 catalyst, the highest removal rates of VSS 87.3% and COD 72.6% were achieved at 260 °C for 60 min with an initial oxygen pressure of 1.0 MPa and 10 g·L−1 of catalyst. The wet oxidation reaction can be maintained itself owing to the exothermic heat. The produced low-molecular-weight carboxylic acids have potential commercial utilization as organic carbon sources in the biological wastewater treatment processes. The inorganic residues can be utilized for the building materials production. These results implied that the catalytic wet oxidation is a promising method for the volume reduction and resource utilization of pharmaceutical sludge.

Pretreatment of Slaughterhouse Effluent Treatment Plant Sludge Using Electro-Fenton Process for Anaerobic Digestion

Sludge management is an integral process of an effluent treatment plant (ETP). This study aimed at using the electro-Fenton (EF) process for pretreatment of a cattle-based slaughterhouse ETP sludge to enhance biogas production from anaerobic digestion. EF-oxidation experiments were conducted in 0.5 L beakers with mild-steel electrodes, to study the effect of factors, viz., H2O2 concentration, current density and reaction time on soluble chemical oxygen demand (sCOD) concentration, soluble extracellular polymeric substances (sEPS) concentration and volatile suspended solids (VSS) removal efficiency. This was followed by the quantification of biogas production from the raw and pretreated sludge in anaerobic digestion (AD). Experimental conditions for the EF process were optimized using response surface methodology (RSM). At optimized experimental conditions, EF pretreatment resulted in an increase in sCOD and sEPS concentrations, from 0.91 g/L to 6.1 g/L and 0.18 g/L to 1.4 g/L, respectively. VSS removal efficiency was 68.1%. Batch anaerobic digestion studies demonstrated an enhancement in the specific biogas yield, from 110 NmL/g-VS to 460 NmL/g-VS.

Biogas Production Under Different Inoculum to Palm Oil Mill Effluent Ratio

Palm oil mill effluent (POME) is a wastewater generated from palm oil industries that rich with organic and nutrients which can becomes an excellent substrate for biogas production. A comprehensive study was carried out to study the effect of different ratio of inoculum to POME substrate for biogas production. In addition, the removal efficiencies of biochemical oxygen demand (BOD), chemical oxygen demand (COD), ammoniacal nitrogen (AN), total nitrogen (TN) total suspended solid (TSS), volatile suspended solid (VSS) were also evaluated. Bio-methane potential (BMP) was used by manipulating temperature and HRT which were set to 28-32 °C and 30 days. The BMPs were operated under different ratio of inoculum to substrate at ratio of 20:80, 30:70 and 40:60. Highest cumulative biogas yield obtained was 1990 mL in the BMP containing 30:70 (inoculum:substrate) followed by the ratio of 40:60 with 1055 mL and 20:80 with 345 mL. Maximum TSS and VSS removal efficiency were 27% and 55%, recorded in 30:70 respectively, while in 40:60 and 20:80 were 23% and 12% and 8% and 51% respectively. The removal of TN was also high at 30:70 with 79% removal. Removal efficiency of COD was in BMP of 20:80 with 54% removal while BOD removal was seen the highest in 40:60 ratio BMP. Lastly, the AN were managed to be removed about 95% in 20:80 BMP. The results obtained in this study indicated that with different ratio od inoculum to POME substrate can enhance biogas production and quality of POME prior discharge to environment.

Avaliação da eficiência de reatores anaeróbios na remoção de poluentes em efluentes do processamento de pescado

ABSTRACT The objective of the present work was to evaluate the performance of a compartmentalized anaerobic reactor (CAR) followed by anaerobic filter in real scale, effluent treatment from fish fridge processing. The work was developed in a fish fridge located in the south of the state of Minas Gerais, which has an effluent treatment plant composed of static screen, grease removal device, CAR reactor and anaerobic filter. The monitoring happened trough physical-chemical parameters of fluids and effluents from all sampling points of the plant biweekly. The parameters evaluated were temperature, pH, chemical oxygen demand (COD), biochemical oxygen demand (BOD), total suspended solids (TSS), volatile suspended solids (VSS), volatile fatty acids (VFA) and alkalinity. Operational problems found in the primary treatment were not limiting for the system performance, which managed to meet the parameters of environmental legislation in Minas Gerais according to the efficiency of removal of COD, BOD and VSS from wastewater from fish processing.

Solid and Soluble Organic Compounds Changes During Thermophilic Aerobic Digestion of Sewage Sludge

Over the past decade, thermophilic aerobic digestion (TAD) has been considered a practical approach to treating highstrength wastewater because of its several advantages. In this study, batch-mode TAD of sewage sludge was conducted using a lab-scale bioreactor to evaluate the changes in solid and soluble organic compounds. The TAD process achieved rapid degradation of the solid compound at 55oC. The removal efficiency of volatile suspended solids (VSS) increased rapidly at the early operation stage and achieved 25% at 48 h. Then, the VSS removal efficiency increased gradually up to approximately 40% at the end of the digestion (168 h). As a result of VSS degradation, the soluble chemical oxygen demand (SCOD) increased significantly up to 12 g/L at 36 h and then showed a slow decline pattern. Furthermore, the pattern of protease activity was similar to that of the SCOD. This result indicates that the rapid degradation of VSS is highly linked to the increase of protease activity and thermophilic conditions. This study used acetic acid and propionic acid. The variation patterns were similar under both acetic acid and propionic acid. The concentrations of acetic acid and propionic acid rapidly increased up to 3.05 g COD/L at 48 h and 2.51 g COD/L at 36 h, respectively. Then, major volatile fatty acids concentrations declined sharply and became undetectable after 144 h. Similar to the SCOD variation pattern, the soluble total nitrogen and NH4+-N also decreased sharply after 72 h. This might be because ammonia stripping is enhanced by pH rising and continuous aeration during the TAD.

Upgrading combined anaerobic-aerobic UASB-FPU to UASB-DHS system: Cost comparison and performance perspective for developing countries

This study determined the performance of upgraded sewage treatment plant (STP) based on an anaerobic-aerobic system consisting Up-flow Anaerobic Sludge Blanket (UASB) reactor, Final Polishing Unit (FPU) and Down-flow Hanging Sponge (DHS) system, treating municipal wastewater located at Dhandhupura, Agra, Uttar Pradesh (India). The performance evaluation of STP were based on the removal efficiency of parameters like Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Suspended Solids (TSS) and Volatile Suspended Solids (VSS). The results exhibits improved BOD, COD, TSS and VSS removal efficiency for UASB-DHS system (92.01%, 82.26%, 91.02%, 92.88%) as compared to UASB-FPU (81.67%, 74.35%, 83.72%, 83.41%) respectively. In addition, this study compares the UASB-DHS system with other post-treatment methods based on their treatment cost and removal efficiency. The technology ranking analysis showed that the UASB-DHS system scored the highest desirability value of 6.77 followed by MBR and ASP technology having a score of 5.17 and 5.11 respectively. The results demonstrated that the UASB-DHS system outperformed the existing UASB-FPU system and met the Indian standards for effluent discharge i.e. BOD less than 30 mg/l. Therefore, the treated sewage can be considered for cultivation, irrigation purpose and safely discharged into water bodies. Moreover, the cost analysis indicates that the UASB-DHS system requires less energy in operation, low expenses in sludge handling and comparatively small area required for post-treatment operation. Hence, this work proposed an economical and efficient approach for the treatment of municipal wastewater particularly for developing countries.

Effect of the operating conditions on the anaerobic digestion of wheatgrass for chemicals and energy production

The aim of this study was to investigate anaerobic digestion of wheatgrass in the absence of hydrolytic pre-treatments. The effect of solids retention time (SRT) (1–64 days), inoculum acclimation (0–80 days acclimation), temperature (40–70 °C) and buffer capacity (20–200 mM phosphate buffer) on conversion of the feedstock, yield and composition of liquid-phase products (ethanol and short-chain organic acids, SCOAs) and COD removal was investigated in semi-continuous (intermittent feed) completely mixed reactors. SRT had the most important effect on process performance. Biodegradation of the feedstock was favoured at high SRT, with 61% removal of volatile suspended solids and 84% removal of total carbohydrates at SRT 64 days. However, low yield of liquid-phase products was observed at high SRT because of strong methanogenic activity (57% removal of the total COD). The highest yield of liquid-phase products was 20% (COD basis) at SRT 8 days. Although high biodegradation of the feedstock was observed after long-term batch acclimation (30 and 80 days), once the digestion conditions were switched to semi-continuous at short SRT (2 days), the biodegradation of the feedstock decreased considerably. The best process performance was observed at 40 °C.

Using straw as a bio-ethanol source to promote anaerobic digestion of waste activated sludge

Commercial ethanol production from straw is a series of complex processes that are energy-intensive and uneconomical. Corn straw was used as a bioethanol source to mix with waste activated sludge for improving anaerobic co-digestion (AcoD) in this study. Ethanol production from the straw after yeast fermentation was 1400–2200 mg COD/L, accounting for about 0.1% of the fermentative effluent, but methane production of the yeast-group increased by 36% compared to that of control-group with no ethanol production in advance. Volatile suspended solid removal achieved 60%, obviously higher than common anaerobic digestion (AD). Multiple lines of evidence including sludge conductance, effects of activated carbon on methanogenesis and microbial community demonstrated that ethanol from the straw fermentation stimulated direct interspecies electron transfer to be established in the digesters. The results suggested that using ethanol produced from straw was a cost-effective novel way for energy recovery from disposal of agricultural and municipal wastes.

A strategy for enhancing anaerobic digestion of waste activated sludge: Driving anodic oxidation by adding nitrate into microbial electrolysis cell

Cathodic reduction of CO2 and anodic oxidation of organic matters are crucial to methane-producing microbial electrolysis cell (MEC) applied in anaerobic digestion of waste activated sludge. However, cathodic CO2 reduction is usually restrained by slow metabolism rates of H2-utilizing methanogens and low electron-capturing capacity of CO2, which consequently slows down the anodic oxidation that participates to sludge disintegration. Herein, a strategy with adding nitrate as electron acceptor to foster electronic transfer between the anode and cathode was proposed to improve anodic oxidation. Results showed that the average efficiency of anodic oxidation in the nitrate-added MEC increased by 55.9%. Accordingly, volatile suspended solid removal efficiency in the nitrate-added MEC was 21.9% higher than that of control MEC. Although the initial cumulative methane production in the nitrate-added MEC was lower than that of control MEC, the cumulative methane production in 24 days was 8.9% higher. Fourier transform infrared spectroscopy analysis indicated that anodic oxidation of MEC with nitrate accelerated the disintegration of sludge flocs and cell walls. Calculation on current signal further revealed that anodic oxidation driven by cathodic nitrate reduction was the main mechanism responsible for the improved sludge digestion.

Rapid and efficient activated sludge treatment by electro-Fenton oxidation

Advanced oxidation process is one of the important process to improve the efficiency of activated sludge dewatering and digestion. In this study, electro-Fenton (EF) was investigated as a pretreatment method for improving activated sludge dewatering and disintegration in terms of specific resistance to filtration, volatile suspended solids removal and release of soluble organics. The morphology of sludge flocs and properties of extracellular polymeric substance (EPS) were investigated to understand the involved mechanisms. The results showed that EF could increase the dewaterability of activated sludge effectively in 40–60 min. The size of sludge flocs decreased after EF treatment, but zeta potential was elevated to near zero and floc structure became coarser with bigger holes. EF could enhance the sludge floc disintegration, released protein and polysaccharide to soluble EPS fraction, and promote the humification process. The kinetic analysis further indicated that EF increased the pseudo first-order EPS solubilization rate. EF had high operational stability by retaining over 90% initial activity even after five repeated use of dewatering filtrate. This study provides a rapid and efficient solution for improving sludge dewaterability by electro-Fenton.

Characterization and Utilization of Pulp and Paper Mill Sludge Digesting Thermophilic Bacteria in Composting Process

Pulp and paper mill sludge (PPMS) was found to be poorly colonised with thermophilic microorganisms. However, evidence to support the need for inoculation to facilitate PPMS composting has only been demonstrated in one instance. In this study, we aimed to: screen and identify PPMS digesting thermophilic bacterial strains; investigate effects of the mixture of selected thermophilic bacterial strains on PPMS digestion; and utilize this mixture as start inoculum in PPMS composting and assess the quality of compost product. The results showed that eleven thermophilic bacterial strains were isolated from Bai Bang PPMS by the enrichment culture method. Among these, three strains which reflected high growth rates on the plates of Minimal Media Agar supplemented with Bai Bang PPMS and showed hydrolytic and ligninolytic activities on the agar plates containing appropriate inductive substrates were selected. Based on the morphological, biochemical characteristics and 16S rRNA gene sequencing, they were identified as Bacillus subtilis. The inoculation with the mixture of selected strains enhanced remarkably Bai Bang PPMS digestion. The dry weight decrease, volatile suspended solids removal, dehydrogenase and protease activities in the inoculated sludge were 2.1-, 1.5-, 1.3- and 1.2- fold higher, respectively, compared to the non-inoculated sludge. The assessment of compost quality based on stability using the alkaline trap method and maturity using the germination and root elongation test showed that the inoculated compost was stable and mature while the non-inoculated compost was unstable and immature. These thermophilic bacterial strains therefore have great potential for Bai Bang PPMS composting.

VSS Degradation Kinetics in High Temperature Aerobic Digestion and Microbial Community Characteristics

Piggery wastewater is a kind of high concentration organic wastewater with high concentration of pollutants, large amount of emissions, and serious environmental pollution and is difficult to deal with. Piggery wastewater was treated with autothermal hyperthermia aerobic digestion process (ATAD) and its biodegradation kinetics was studied. The ATAD system was automatically heated up and the reaction temperature rose from ambient temperature of 20°C to a maximum temperature of 64°C. Based on Arrhenius formula, the empirical model is obtained through dimensional analysis. The removal of volatile suspended solids (VSS) was correlated with the initial VSS concentration, water inlet temperature, aeration rate, and agitation rate in the model. In the empirical model, the apparent activation energy was 2.827 kJ·mol−1. The exponentials for the initial VSS concentration, aeration rate, and stirring rate were 1.0587, −0.0976, and −0.1618, respectively. The correlation coefficient of the exponential factor was 0.9971. The VSS removal efficiency predicted by the model was validated with an actual test, showing a maximum relative deviation of 8.82%. Sludge systems show a lower diversity of microbial populations and Bacillus occupies a very important position in the reactor. The data obtained will be useful for optimizing piggery wastewater treatment process. The new model provided good theoretical guidance with good practicality.

Application of electrochemical peroxidation (ECP) process for waste-activated sludge stabilization and system optimization using response surface methodology (RSM).

In this study, the performance of the electrochemical peroxidation (ECP) process for removing the volatile suspended solids (VSS) content of waste-activated sludge was evaluated. The Fe2+ ions required by the process were obtained directly from iron electrodes in the system. The performance of the ECP process was investigated in various operational conditions employing a laboratory-scale pilot setup and optimized by response surface methodology (RSM). According to the results, the ECP process showed its best performance when the pH value, current density, H2O2 concentration and the retention time were 3, 3.2 mA/cm2, 1,535 mg/L and 240 min, respectively. In these conditions, the introduced Fe2+ concentration was approximately 500 (mg/L) and the VSS removal efficiency about 74%. Moreover, the results of the microbial characteristics of the raw and the stabilized sludge demonstrated that the ECP process is able to remove close to 99.9% of the coliforms in the raw sludge during the stabilization process. The energy consumption evaluation showed that the required energy of the ECP reactor (about 1.8-2.5 kWh (kg VSS removed)-1) is considerably lower than for aerobic digestion, the conventional waste-activated sludge stabilization method (about 2-3 kWh (kg VSS removed)-1). The RSM optimization process showed that the best operational conditions of the ECP process comply with the experimental results, and the actual and the predicted results are in good conformity with each other. This feature makes it possible to predict the introduced Fe2+ concentrations into the system and the VSS removal efficiency of the process precisely.

Pesticide removal through wastewater and advanced treatment: full-scale sampling and bench-scale testing.

Dieldrin and DDx removal through wastewater treatment, ozonation, and microfiltration was assessed for a water reuse project for groundwater replenishment in Monterey, California, USA. Full-scale sampling was conducted at the wastewater treatment plant, and physical wastewater treatment processes, ozonation, and microfiltration were tested at the bench scale. Removals observed through wastewater treatment, ozonation, and microfiltration were 84%, 44% to 63%, and 97% to 98%, respectively, for dieldrin, and 93%, 36% to 48%, and 92% to 94% for DDx. These were sufficient to meet California Ocean Plan water quality objectives after wastewater treatment alone. Levels in the secondary effluent, ahead of ozonation, microfiltration, reverse osmosis and advanced oxidation in the advanced water purification facility, met drinking water standards. Removal of dieldrin and DDx through wastewater treatment occurred by physical treatment processes; removal through the full-scale wastewater treatment plant, which included biological and physical treatment processes, matched removal through the physical bench-scale wastewater treatment processes. Dieldrin and DDx removal correlated with removal of volatile suspended solids, suggesting that volatile suspended solids could be used as an indicator for pesticide removal through wastewater treatment. Dieldrin and DDx concentrations were highest in the solids contact process, where biomass is accumulated for carbon removal.

Acidogenic fermentation of vegetable and salad waste for chemicals production: Effect of pH buffer and retention time

This study investigated the effect of pH buffer and solids retention time (SRT) on the anaerobic fermentation of vegetable and salad waste (VSW). Experiments were carried out in batch and semi-continuous reactors at 35°C. In the batch experiments, the effect of pH buffer on the production of volatile fatty acids (VFAs) and ethanol was investigated. Acetate and butyrate were the main fermentation products. The maximum total product concentration was 43.3 and 18.5gCODl−1 in the buffered and unbuffered batch reactors resulting in a yield of 62 and 27% (CODtotalproduct/CODfeed) respectively. Volatile suspended solids (VSS) removal was higher in the buffered semi-continuous reactor (57%) compared to the unbuffered-acidic reactor (39%), but similar yields (15%, CODtotalproduct/CODfeed) were observed because biogas production was stimulated in the buffered reactor. The effect of SRT on the VSS removal and product distribution in unbuffered systems was investigated at 10, 20 and 30days SRT. The VSS removal increased as the SRT increased, ranging between 18.2–49.1%, likewise the total product concentration, 9.1–19.4gCODl−1, and product yield, 7–24% (CODtotalproduct/CODfeed). Acetate and butyrate were the predominent fermentation products at all conditions followed by caproate although caproate to this was only detected at 20 and 30days SRT. Total COD removal ranged between 15.2 and 35.1% with the highest removal observed at 30days SRT.

Comparison of thermophilic bacteria and alkyl polyglucose pretreatment on two-stage anaerobic digestion with waste sludge: Biogas production potential and substrate metabolism process

To gain a better understanding of the influence on two-stage anaerobic digestion of waste sludge with thermophilic bacteria (TB) and alkyl polyglucose (APG) pretreatment, changing of soluble chemical oxygen demand (SCOD), carbohydrate and protein in extracellular polymeric substances (EPS) and dissolved organic matters (DOM) were analyzed. The excitation-emission matrix (EEM) with fluorescence regional integration (FRI) was also used to investigate compositional and structural characteristics of DOM. The highest hydrogen and methane yield of TB pretreated sludge was 12.2 ml/g VS (volatile suspended solid) and 124.7 ml/g VS, and that of APG pretreated sludge was 28.3 ml/g VS and 19.9 ml/g VS. The VS removal of TB pretreated sludge (36.7%) was higher than APG pretreated sludge (27.1%) in the two-stage anaerobic digestion. The APG pretreatment could inhibit the activity of methanogens and the substrate (such as volatile fatty acids (VFAs), protein and soluble microbial materials) was accumulated compared with TB pretreatment.

Harvesting direct electricity from municipal waste-activated sludge simultaneous with its aerobic stabilization process: Investigation and optimization

Despite the numerous studies that have been carried out on electricity generation from biomaterials, there is no systematic information about the influence of operational conditions on the performance of these systems. In addition, the carried out researches in the different studies have been done in various conditions and systems, leading to a large variation in the parameters reported for each system. In this regard, the main goal of this study was to determine the operational conditions which the electricity generating systems are able to demonstrate the desirable performance. Therefore, a reactor has been designed with the aim of generating electrical energy, directly from municipal waste-activated sludge simultaneous with its aerobic digestion process and its performance has been successfully optimized using response surface methodology (RSM). The results of the optimization showed that the best performance of the reactor for the volatile suspended solid (VSS) removal could be obtained when the temperature and the pH values are about to 35°C and 7.0, respectively. Under these conditions, the models estimated 42% VSS removal efficiency. Also, the generated electrical power density achieved its peak in 35°C and 7.0 for the temperature and the pH. The value obtained for the generated electrical power under these conditions was 2.81W/m3. The comparison of the results derived from the laboratory experiments and the optimization process proved the validity of the obtained models. Another noticeable point was that the determined optimized conditions for the stabilization and the electricity generation processes were conforming to each other.

Minimization of municipal sewage sludge by means of a thermophilic membrane bioreactor with intermittent aeration

The increase of sewage sludge production together with the high treatment and disposal costs in the last years has pushed to study different solutions aimed at sludge minimization. In this paper, the thermophilic membrane technology was evaluated as an alternative for municipal sewage sludge reduction. The experimentation (carried out by means of a pilot scale plant, 1 m3 volume) was divided into two steps: the first one was aimed at confirming the results obtained in a previous research focused on industrial sludge treatment; the second step was devoted to define the best process conditions (in particular the optimization of the aeration phases) and to the chemical permeate characterization. The results of the experiments highlight that the hydraulic retention time (HRT) and aeration conditions play a crucial role on the overall process performance. The volatile suspended solids removal efficiency was greater than 80% under the following conditions: HRT even lower than 15 d; 2 h of aeration - 6 h of non aeration cycles; and organic loading rate of 2.0 kgCOD m−3 d−1. The permeate showed a good biodegradability under mesophilic conditions thus being treatable by means of conventional biological processes. Moreover, ammonia (the permeate presenting high concentrations) could be recovered as a fertilizer (stripping and subsequent washing of the exhausted gas is an established technique). Finally, the ammonia-free permeate can be valorised as a carbon source in denitrification processes.

The influence of temperature and SRT on high-solid digestion of municipal sewage sludge

The influence of temperature and solids retention time (SRT) on high-solid digestion of municipal sewage sludge was investigated in laboratory-scale reactors. Digestion with high-solid concentration reduces the required digestion volume and is advantageous for urban areas. The experimental conditions comprised total suspended solids (TSS) in digested sludge between 4.0 and 4.6%, temperatures in a range of 33 to 41 °C and the SRT between 10 and 25 d. High-solid digestion operates with increased NH4-N concentrations released from organic compounds. The anaerobic process can be limited by high NH4-N concentration and toxic NH3. In this study a stable digestion was observed up to 2,000 mg L(-1) NH4-N and 75 mg L(-1) NH3. Volatile suspended solids (VSS) and chemical oxygen demand removal was 53% and 57% respectively. However, digestion with 10 d SRT led to a declined VSS removal of 49%. The removal at 41 and 37 °C showed minor differences, while reduced NH4-N release and reduced methane production were observed at 33 °C. For economic reasons, high-solid digestion at 41 °C is not recommended, but will not impair VSS removal. The outcomes of this study confirm that digestion with up to 7.8% TSS in the feed is feasible for the tested temperatures and SRT down to 15 d.