Co-digestion and co-treatment of sewage and organic waste in mainstream anaerobic reactors: operational insights and future perspectives.

The global shift towards sustainable waste management has led to an intensified exploration of co-digestion and co-treatment of sewage and organic waste using anaerobic reactors. This review advocates for an integrated approach where organic waste is treated along with the sewage stream, as a promising solution to collect, treat, and dispose of organic waste, thereby reducing the environmental and economic burden on municipalities. Various efforts, ranging from laboratory to full-scale studies, have been undertaken to assess the feasibility and impacts of co-digestion or co-management of sewage and organic waste, using technologies such as up-flow anaerobic sludge blankets or anaerobic membrane bioreactors. However, there has been no consensus on a standardized definition of co-digestion, nor a comprehensive understanding of its impacts. In this paper, we present a comprehensive review of the state-of-the-art in liquid anaerobic co-digestion systems, which typically operate at 1.1% total solids. The research aims to investigate how the integration of organic waste into mainstream anaerobic-based sewage treatment plants has the potential to enhance the sustainability of both sewage and organic waste management. In addition, utilizing the surplus capacity of existing anaerobic reactors leads to significant increases in methane production ranging from 190 to 388% (v/v). However, it should be noted that certain challenges may arise, such as the necessity for the development of tailored strategies and regulatory frameworks to enhance co-digestion practices and address the inherent challenges.

Sustainability of the anaerobic digestion of oil refinery secondary sludge.

Among the waste generated at oil refineries, secondary sludge from biological wastewater treatment processes (activated sludge systems) stands out. This paper aimed to assess the use of anaerobic digestion (AD) to treat sludge by SWOT (Strength, Weakness, Opportunity, and Threat) analysis, ranking the different factors based on sustainability criteria. Additionally, the SWOT factors were matched (TOWS matrix) to help interpret the results. AD was found to be compatible with sustainability. The results demonstrated that the strength of AD (reduced organic load) compensates for its weaknesses (need for operational control and initial implementation costs), thereby avoiding the threat (sludge composition) and making the most of the opportunity (lower disposal cost). AD and co-digestion (added with food waste) used to treat oil refinery sludge showed that around 60% of the factors analyzed were confirmed experimentally. It was concluded that AD should be considered in the sustainable treatment of oil refinery waste activated sludge, especially when mixed with other readily biodegradable wastes.

Anaerobic digestion (AD) is a potential treatment for secondary sludge from refineries.AD is compatible with sustainability.Anaerobic co-digestion of refinery sludge contributed positively to sustainable treatment.The strength (reduced organic load) of AD counteract its weaknesses (operational control and implementation costs).The threat of AD (sludge composition) should be avoided and the opportunity (low disposal cost) leveraged.

Extracellular hydrolytic potential drives microbiome shifts during anaerobic co-digestion of sewage sludge and food waste.

Bacterial community structure and dynamics in anaerobic digesters are primarily influenced by feedstock composition. It is therefore important to unveil microbial traits that explain microbiome variations in response to substrate changes. Here, gene and genome-centric metagenomics were used to examine microbiome dynamics in four laboratory-scale reactors, in which sewage sludge was co-digested with increasing amounts of food waste. A co-occurrence network revealed microbiome shifts in response to changes in substrate composition and concentration. Food waste concentration correlated with extracellular enzymes and metagenome-assembled genomes (MAGs) involved in the degradation of complex carbohydrates commonly found in fruits and plant cell walls as well as with the abundance of hydrolytic MAGs. A key role was attributed to Proteiniphillum for being the only bacteria that encoded the complete pectin degradation pathway. These results suggest that changes of feedstock composition establish new microbial niches for bacteria with the capacity to degrade newly added substrates.

Anaerobic co-digestion of oil refinery waste activated sludge and food waste.

The technology of anaerobic co-digestion to treat the excess biological sludge discharged from activated sludge systems in oil refineries was evaluated in bench scale experiments. Mixing food waste rich in fruits and vegetables with this sludge increased the reduction of volatile solids and biogas yield. An experimental design indicated that the best co-digestion condition was the use of waste activated sludge without previous dewatering (3.5% total solids) and food waste in an 80:20 ratio (% v/v), without the addition of inoculum. After 45 days at 35 °C, this condition resulted in volatile solid (VS) removal of 52% and biogas yield of 80.7 mL biogas/g VSadded, against only 19% and 38.5 mL biogas/g VSadded in mono-digestion of sludge alone. Anaerobic co-digestion demonstrates promising results and the potential for a simple and effective treatment method for excess biological sludge generated at refineries.

Shifts of acidogenic bacterial group and biogas production by adding two industrial residues in anaerobic co-digestion with cattle manure.

Anaerobic biodigestion figures as a sustainable alternative to avoid discharge of cattle manure in the environment, which results in biogas and biofertilizer. The anaerobic bioconversion of biomass to methane via anaerobic biodigestion requires a multi-step biological process, including microorganisms with distinct roles. Here, the dynamics of acidogenic bacterial populations by classical microbiology, as well as biogas productivity by gasometer and chromatography, in the anaerobic co-digestion process were studied. This paper presents a performance evaluation of co-digestion systems for biogas production using cattle manure and wastes from the Sewage Treatment Station of a brewery and ricotta cheese whey. The search revealed that the type of substrate added in co-digestion with cattle manure, Carbon/Nitrogen ratio, and Ammonia Nitrogen were the most influential factors that explained many of the variations of the microbiota in the biodigesters fed. This study demonstrated a good potential for the use of ricotta cheese whey in the production of biogas and its further conversion into energy. These findings could provide some fundamental and technical information for the co-treatment of industrial derived wastes in centralized anaerobic biodigestion facilities in a sustainable manner with high process capacity and methane recovery.

Operational and biochemical aspects of co-digestion (co-AD) from sugarcane vinasse, filter cake, and deacetylation liquor.

This work performed co-AD from the vinasse and filter cake (from 1G ethanol production) and deacetylation liquor (from the pretreatment of sugarcane straw for 2G ethanol production) in a semi-Continuous Stirred Tank Reactor (s-CSTR) aiming to provide optimum operational parameters for continuous CH4 production. Using filter cake as co-substrate may allow the reactor to operate throughout the year, as it is available in the sugarcane off-season, unlike vinasse. A comparison was made from the microbial community of the seed sludge and the reactor sludge when CH4 production stabilized. Lactate, butyrate, and propionate fermentation routes were denoted at the start-up of the s-CSTR, characterizing the acidogenic phase: the oxidation-reduction potential (ORP) values ranged from -800 to -100 mV. Once the methanogenesis was initiated, alkalizing addition was no longer needed as its demand by the microorganisms was supplied by the alkali characteristics of the deacetylation liquor. The gradual increase of the applied organic load rates (OLR) allowed stabilization of the methanogenesis from 3.20 gVS L-1 day-1: the highest CH4 yield (230 mLNCH4 g-1VS) and average organic matter removal efficiency (83% ± 13) was achieved at ORL of 4.16 gVS L-1 day-1. The microbial community changed along with the reactor operation, presenting different metabolic routes mainly due to the used lignocellulosic substrates. Bacteria from the syntrophic acetate oxidation (SAO) process coupled to hydrogenotrophic methanogenesis were predominant (~ 90% Methanoculleus) during the CH4 production stability. The overall results are useful as preliminary drivers in terms of visualizing the co-AD process in a sugarcane biorefinery integrated to scale. KEY POINTS: • Integration of 1G2G sugarcane ethanol biorefinery from co-digestion of its residues. • Biogas production from vinasse, filter cake, and deacetylation liquor in a semi-CSTR. • Lignocellulosic substrates affected the biochemical routes and microbial community. • Biomol confirmed the establishment of the thermophilic community from mesophilic sludge.

Effect of Agitation on Anaerobic Co-Digestion of Swine Manure and Food Waste

Abstract The synergism of food waste associated with swine manure can provide an increase in biogas production, besides promoting greater stability in the anaerobic co-digestion process. To verify this effect, co-digestion tests were performed in two reactors, one with agitation, and the other without agitation. In both systems, gasometers were used to measure biogas production in an experiment lasting two hydraulic retention times (HRT). On each feeding day, the temperatures of the ambient and of the effluent taken from the reactors were measured, and samples of the food waste and effluent were collected to perform analysis of pH, total solids (TS), volatile solids (VS), fixed solids (FS), volatile acidity (AV), and total alkalinity (TA). In addition, the chemical oxygen demand (COD) was determined every five days, and gas composition was determined at the beginning of the second HRT. As important results, in both reactors a decrease in pH was verified due to the weakening of the buffer effect of the medium. This was due to the low alkalinity found in the food waste, causing an increase in acidity in the contents of the reactors. The volume of biogas produced was higher in the reactor with agitation, which meant an increased efficiency of the process. Finally, a low methane content was verified through chromatographic analyses, indicating a reduction in the activity of the microorganisms present in the medium. Thus, it is concluded that agitation linked to anaerobic co-digestion of swine manure with food waste exerted a positive effect on biogas production.

Co-digestión anaerobia de la fracción orgánica de residuos sólidos urbanos y su lixiviado / Anaerobic co-digestion of the organic fraction of urban solid waste and its leachate

RESUMEN Los residuos sólidos urbanos (RSU), al ser vertidos sin tratamiento apropiado, ocasionan daños severos al medio ambiente influyendo en la calidad de vida de la población; por esta razón actualmente se le presta atención al adecuado manejo de estos residuos. En este trabajo se estudia la reducción de la fracción orgánica (FORSU), de residuos sólidos generados por un grupo poblacional, tomando como referente una de las residencias estudiantiles de la Universidad de Oriente. La generación de FORSU en la residencia es de 0,06 kg/habdía, con un contenido de sólidos totales de 30,9 ± 5,3 %, de los cuáles el 81,7 ± 0,6 % son sólidos volátiles. Se evalúa la digestión anaerobia la FORSU, mediante un sistema de tratamiento en dos etapas. La primera etapa se realiza en un reactor en lote de lecho escurrido, que permitió un tratamiento discontinuo e in situ de la FORSU, en co-digestión anaerobia con un 25 % de estiércol vacuno. En una segunda etapa, se evalúa el tratamiento anaerobio para los lixiviados que se generan en el reactor en lote, empleando un reactor UASB. En el reactor en lote se alcanzó una remoción de sólidos volátiles del 44,1 %, mientras que en el reactor UASB se removió un 81,2 % de DQO. Como resultado la evaluación del sistema de tratamiento, se alcanzó una productividad total de 5,37 LCH4-kgSV -1-d-1.

ABSTRACT Urban solid waste (MSW), when are discharged without proper treatment, causes severe damage to the environment, influencing the population's quality of life. For this reason, attention is currently paid to the proper handling of this waste. This paper studies the reduction of the organic fraction of solid waste (OFMSW), generated by a population group, taking as reference one of the Student Residences of the Universidad de Oriente. The generation of OFMSW in the residence is 0.06 kg/inhabday, with a total solid content of 30.9 ± 5.3%, of which 81.7 ± 0.6% are volatile solids. The anaerobic digestion of OFMSW is evaluated through a two-stage treatment system. The first stage is performed in a drained-bed batch reactor, which allowed discontinuous and in-situ treatment of OFMSW, in anaerobic co-digestion with 25% of cow manure. In a second stage, the anaerobic treatment for the leachates generated in the batch reactor is evaluated, using a UASB reactor. In the batch reactor a removal of volatile solids of 44.1% was achieved, while in the UASB reactor 81.2% of COD was removed. As a result of the evaluation of the treatment system, total productivity of 5.37 L CH4 -kgSV -1-d-1 was achieved.

Enzymatic pretreatment and anaerobic co-digestion as a new technology to high-methane production.

The population growth is causing an increase in the generation of effluents (mainly organic fraction of municipal solid waste (OFMSW) and agro-industrial waste), which is an old problem in agro-industrial countries such as Brazil. Contrastingly, it is possible to add value to these residual biomasses (residues) through the application of new technologies for the production of bioenergy. Anaerobic digestion (AD) of sewage sludge is being applied in many effluent treatment plants for the sustainable and economically viable production of biogas. However, the biogas produced from AD (sludge) or co-digestion (sludge with other residues) presents a concentration of methane between 60 and 70% on average, which is relatively low. This review is aimed at analyzing studies involving (i) production of lipases by solid-state fermentation (SSF) by different microorganisms for the application in enzymatic pretreatments prior to the anaerobic treatment of effluents; (ii) pretreatment followed by AD of various residues, with an emphasis on OFMSW and sewage sludge; and (iii) more recent studies on anaerobic co-digestion (AcoD) and hybrid technologies (pretreatment + AD or AcoD). There are many studies in the literature that demonstrate the enzymatic pretreatment or AcoD applied to the optimization of methane production. Nevertheless, few studies report the combination of these two technologies, which can improve the process and reduce or eliminate the costs of biogas purification, which are major challenges for the viability of this route of bioenergy production. KEY POINTS: • Municipal and agro-industrial wastes have potential as medium for lipase production. • Enzymatic pretreatment and anaerobic co-digestion are low cost for high-methane production. Graphical abstract Interactions among various factors optimization methane production from enzymatic pretreatment and AcoD.

Evaluation of biomethanization during co-digestion of thermally pretreated microalgae and waste activated sludge, and estimation of its kinetic parameters.

The maximum methane yield that can be obtained from anaerobic co-digestion of microalgae and waste activated sludge (WAS) mixtures, after thermal pretreatment at 65 °C during 4 h, was investigated. Furthermore, the fitting of the experimental data by five kinetic models (first-order, second-order, modified Gompertz, Logistic, and two-substrate) was evaluated. Thermal pretreatment increased the methane yield of single microalgae and WAS digestion by ≈ 44 and by ≈ 52%, respectively. The results also showed that up to 60% of WAS can be co-digested with microalgae without impairing the methane yield, producing up to 338 mLCH4 gVS-1. Data from digestion of non-pretreated microalgae and WAS were well described by all kinetic models, but digestion of thermally pretreated microalgae, WAS, and their co-digestion mixtures, was best fitted by means of a two-substrate model, indicating that after pretreatment it is necessary to take into account the contribution of both rapidly and slowly biodegradable fractions.

Towards centralized biogas plants: Co-digestion of sewage sludge and pig manure maintains process performance and active microbiome diversity.

The aim of this study is to assess the performance of anaerobic digestion against co-digestion systems during the start-up stages based on key process parameters and biological indicators. Two parallel experiments treating sewage sludge alone or co-digested with low concentration of pig manure (8% vol., 2-3% in COD basis) were carried out in two lab-scale CSTR at mesophilic conditions. Same inoculant and organic loading rate sequences were applied for two consecutive runs of 79 and 90 days. According to the removal efficiencies achieved, no significant differences were encountered amongst mono-digestion and co-digestion. This observation was reinforced with the analysis of the total/active microbiome, sequencing 16S rRNA genes and transcripts. The addition of a co-substrate at low concentration had a negligible effect on the total/active microbial communities; they evolved following the same pattern. This might be an advantage in order to upgrade existing wastewater treatment plants to become centralized biogas facilities.

Methane production by co-digestion of poultry manure and lignocellulosic biomass: Kinetic and energy assessment.

Six typical Brazilian lignocellulosic biomasses (rice straw, corn cob, peanut shell, sawdust, coffee husk and sugarcane bagasse) were evaluated for methane production by solid-state anaerobic co-digestion with poultry manure. The results showed the highest methane production was obtained with corn cob and poultry manure (126.02 Nm3 CH4. ton residue-1) using a food to inoculum ratio of 0.5, which lowered volatile fatty acids accumulation. In this condition, the thermal energy production (1.73 MJ.kg live chicken-1) would be able to replace 53.2% of the energy with firewood in poultry farming. The high hemicellulose and low lignin content in corn cob seem to explain the biomethanation of such biomass, and this agrees with the microbial analysis which revealed the predominance of bacteria related to plant polysaccharides hydrolysis and carbohydrate conversion in the inoculum. The methane production was best modelled by Groot's multi-stage model, and the microbial adaptation to lignin might explain this.

Biomethane production by thermophilic co-digestion of sugarcane vinasse and whey in an AnSBBR: Effects of composition, organic load, feed strategy and temperature.

This work investigated the application of a thermophilic (55 °C) anaerobic reactor with immobilized biomass, mechanically stirred and operated in sequential batch and fed batch (AnSBBR) for environmental compliance and methane production by co-digesting cheese whey (W) and sugarcane vinasse (V). The assays were performed in four steps. In the first step the composition of 75%W:25%V (on a COD basis) was determined to be the most adequate for the anaerobic process. In the second step the applied volumetric organic load (AVOL) was increased and in the third step the feed strategy was modified achieving best results at AVOL of 25 gCOD.m-3.d-1, in which the removed organic matter efficiency was 72%, the molar productivity was 278 molCH4.m-3.d-1 and methane yield was 15.3 mmolCH4.gCOD-1. In the fourth step the temperature was modified to 50 °C and 45 °C, achieving worse results. From the kinetic model adjusted to experimental data it was identified that the acetoclastic route was predominant in methane generation. The estimated energy recovered by co-digesting cheese whey and sugarcane vinasse using industrial information was 2.2 × 104 MW h per month, equivalent (in Brazil) to the electricity consumption of about 135 × 103 inhabitants or monthly savings of US$ 1,653,000 replacing the diesel oil consumed in the industry.

Improvement of Sugarcane Stillage (Vinasse) Anaerobic Digestion with Cheese Whey as its Co-substrate: Achieving High Methane Productivity and Yield.

This study investigated methane production in an anaerobic sequencing batch biofilm reactor (AnSBBR) by co-digesting sugarcane vinasse and cheese whey. The assessment was based on the influence of feed strategy, interaction between cycle time and influent concentration, applied volumetric organic load (OLRA), and temperature over system stability and performance. The system showed flexibility with regard to the feed strategy, but the reduction of cycle time and influent concentration, at the same OLRA, resulted in lower methane productivity. Increasing organic load, up to the value of 15.27 gCOD L-1 day-1, favored the process, increasing methane yield and productivity. Temperature reduction from 30 to 25 °C resulted in worse performance, although increasing it to 35 °C provided similar results to 30 °C. The best results were achieved at an OLRA of 15.27 gCOD L-1 day-1, cycle time of 8 h, fed-batch operation, and temperature of 30 °C. The system achieved soluble COD removal efficiency of 89%, methane productivity of 208.5 molCH4 m-3 day-1 and yield of 15.76 mmolCH4 gCOD-1. The kinetic model fit indicated methanogenesis preference for the hydrogenotrophic route. At the industrial scale estimative, considering a scenario with a sugarcane ethanol plant with ethanol production of 150,896 m3 year-1, it was estimated energy production of 25,544 MWh month-1.

Semi-continuous anaerobic co-digestion of flotation sludge from broiler chicken slaughter and sweet potato: Nutrients and energy recovery.

Energy production based on the proper allocation of environmental liabilities is in line with the concept of sustainability. Flotation sludge (S) is a type of waste derived from the physical treatment of the wastewater generated in significant quantities during chicken slaughter in Brazil. If not treated, this wastewater may contribute to pollution, but further treatment provides clean energy and nutrient recycling. The present study aimed at evaluating the reduction of (S) organic load by means of mono and co-digestion with sweet potatoes (P) while promoting its conversion into energy (methane) and nutrients (digestate). Semi-continuous reactors (60 L capacity) were used with a hydraulic retention time of 25 days. The reactors were fed daily with 2.4 L consisting of 60% digestate recirculation, 40% non-chlorinated water and 4.5% total solids (TS). Using nine reactors and six progressive periods, eleven conditions were evaluated with three replicates each. The percentages of (P) and (S) varied from 0 to 100. The best observed condition in terms of energy recovery and TS removal was 60% of P + 40% of S (p ≤ 0.05), as it presented values of at least an increase of 92% in total biogas volume, an increase of 123% in specific methane production, an increase of 98% in specific methane yield and an increase of 44% in TS removal efficiency compared to mono-digestions. The fertilizer potential of the digestate generated in the different conditions was calculated and evaluated according to the area of (P) production. The results varied from 3.6 to 10.8 ha of (P) using 100 m3 of digestate. A multivariate analysis showed that higher amounts of (P) in substrate composition favor energy recycling while higher concentrations of (S) enhance the production of a digestate with valuable agronomic characteristics.

Use of anaerobic co-digestion as an alternative to add value to sugarcane biorefinery wastes.

In this study the anaerobic co-digestion (AcD) of sugarcane biorefinery by-products, i.e. hemicelluloses hydrolysate (HH) (obtained by hydrothermal pretreatment of sugarcane bagasse), vinasse, yeast extract (YE) and sugarcane bagasse fly ashes (SBFA), was optimized by means of biochemical methane potential experiments. The best experimental conditions of AcD (25-75% HH-to-vinasse mixture ratio; 1.0 g L-1 YE; 15 g L-1 SBFA and 100-0% HH-to-Vinasse; 1.5 g L-1 YE; 45 g L-1 SBFA) led to the production of 0.279 and 0.267 Nm3 of CH4 per kg of chemical oxygen demand (COD) with an energy surplus of 0.43 and 0.34 MJ kg SB-1, respectively. Adsorption experiments using SBFA were carried out and showed this residue could adsorb up to 61.71 and 17.32 mg g-1 of 5-hydroxymethyl-2-furfuraldehyde and 2-furfuraldehyde, thereby reducing toxicity and improving biogas production.

Analysis of the behavior for operation parameters in the anaerobic digestion process with thermal pretreatment, using fuzzy logic.

This article presents a study that identifies the variables with greatest impact on the biogas and methane production over a process with thermal pretreatment, to accelerate anaerobic digestion process in sewage sludge in a water treatment plant, for a poultry processing factory, by using fuzzy logic. The designed fuzzy logic model includes 688 inference rules, with a correlation of 99.3% between prediction data against experimental data, for the biogas variable; and 97% for the methane variable. The predictions of the fuzzy logic model were analyzed with response surface models, and it is concluded that the temperature and operating time variables are mutually determining in the biogas and methane production. Likewise, this research provides a methodology for the design of an expert decision support system that allows to evaluate and optimize a mesophilic anaerobic digestion process through a previous thermal treatment in order to improve the yields of biogas and methane in the treatment of effluent sludge from agroindustry. These results propose to diffuse logic as a reliable tool to make comparisons, and predictions for operation variables management on the treatment of residual sludge with thermal pretreatment on anaerobic digestion.

Methane Production by Co-Digesting Vinasse and Whey in an AnSBBR: Effect of Mixture Ratio and Feed Strategy.

The most common approach to deal with vinasse (sugarcane stillage) is fertigation, but this technique compromises soil structure and surrounding water bodies. A possible solution is to transport vinasse to local cheese whey producers and perform the co-digestion of these wastewaters together, reducing their organic load and generating bioenergy. Therefore, this study investigated the application of an AnSBBR (anaerobic sequencing batch biofilm reactor) operated in batch and fed-batch mode, co-digesting vinasse and whey at 30 °C. The effect of influent composition and feeding strategy was assessed. In all conditions, the system achieved high organic matter removal (approximately 83%). Increasing the percentage of vinasse from 0 to 100% in the influent resulted in a decrease in methane productivity (76.3 to 51.1 molCH4 m-3 day-1) and yield (12.7 to 9.1 molCH4 kgCOD-1), but fed-batch mode operation improved reactor performance (73.0 molCH4 m-3 day-1 and 11.5 molCH4 kgCOD-1). From the kinetic metabolic model, it was possible to infer that, at the best condition, methane is produced in a similar way from the acetoclastic and hydrogenotrophic routes. A scheme of four parallel reactors with a volume of 16,950 m3 each was proposed in the scale-up estimation, with an energy recovery estimated in 28,745 MWh per month.

Effect of pretreatments on corn stalk chemical properties for biogas production purposes.

Different pretreatments were evaluated on corn stalk (Zea mays) applied as a lignocellulosic source in anaerobic co-digestion with swine manure, using sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) for biogas production purposes. Using H2SO4 we achieved a 75.1% removal of the hemicellulose fraction, in low acid concentrations (0.75% v.v-1). However, this technique inhibited the co-digestion process. Pretreatment with 12% of H2O2 (pH 11.5) increased the cellulose fraction by 73.4% and reduced the lignin content by 71.6%. This pretreatment is recommended for biogas production, as it increased the final volume of biogas by 22% and reduced the digestion time by one third. So, a promising alternative was obtained in order to facilitate the anaerobic digestion of the carbohydrates present in this biomass.

Co-digestion of sewage sludge with crude or pretreated glycerol to increase biogas production.

Anaerobic co-digestion of sewage sludge and glycerol from the biodiesel industry was evaluated in three experimental stages. In the first step, the addition of higher proportions of crude glycerol (5-20% v/v) to the sludge was evaluated, and the results showed a marked decrease in pH and inhibition of methane production. In the second step, co-digestion of sludge with either a lower proportion (1% v/v) of crude glycerol or glycerol pretreated to remove salinity resulted in volatile acid accumulation and low methane production. The accumulation of volatile acids due to the rapid degradation of glycerol in the mixture was more detrimental to methanogenesis than the salinity of the crude glycerol. In the third step, much lower amounts of crude glycerol were added to the sludge (0.3, 0.5, 0.7% v/v), resulting in buffering of the reaction medium and higher methane production than in the control (pure sludge). The best condition for co-digestion was with the addition of 0.5% (v/v) crude glycerol to the sewage sludge, which equals 0.6 g glycerol/g volatile solids applied. Under this condition, the specific methane production (mL CH4/g volatile solids applied) was 1.7 times higher than in the control.