Semi-continuous Anaerobic Digestion Research Articles

Effects of long-term exposure to zinc on performances of anaerobic digesters for swine wastewater treatment under various organic loading rates

The long-term performance of anaerobic digestion (AD) often decreases substantially when treating swine wastewater contaminated with heavy metals. However, the toxicological characteristics and mechanisms of continuous exposure to heavy metals under different organic loading rates (OLR) are still poorly understood. In these semi-continuous AD experiments, it was found that zinc concentrations of 40 mg/L only deteriorated the reductive environments of AD. In comparison, a concentration of 2.0 mg/L probably facilitated the reproduction of microorganisms in the operating digesters with a constant OLR of 0.51 g COD/(L·d). Nevertheless, when the OLR was increased to 2.30 g COD/(L·d), 2.0 mg/L zinc inhibited various life activities of microorganisms at the molecular level within only 10 days. Hence, even though 2.0 mg/L zinc could promote AD performances from a macroscopic perspective, it had potential inhibitory effects on AD. Therefore, this study deepens the understanding of the inhibitions caused by heavy metals on AD and the metabolic laws of anaerobic microorganisms in swine wastewater treatment. These results could be referred to for enhancing AD in the presence of zinc in practical swine wastewater treatment.

Semi-continuous mesophilic anaerobic digestion of date palm (Phoenix dactelifyra L.) leaflets of the H'mira cultivar from the Adrar region of Southern Algeria using an alkaline pre-treatment

This research paper aim to investigate the enhancement potential of mesophilic Anaerobic Digestion (AD) of Algerian date palm (Phoenix dactylifera L.) leaflets from H'mira cultivars using an alkaline pretreatment approach. The experiment was conducted for the first time in semi-continuous digesters. NaOH was applied at various percentages (6 %, 12 % and 18 %) to chemically pretreat the substrate for 5 days at 37 °C. The chemical pretreatment results were interpreted for the first time with in-depth FTIR spectroscopic analyses. The Hydraulic Retention Time (HRT) was fixed at 15 days, and the Organic Loading Rate (OLR) at 2 g MO/L/day. The results showed that the methane (CH4) production values were stabilized with a clear distinction at averages of 211.85, 177.26, 143.36 and 123.07 ml of CH4/day, i.e. a CH4 yield of 105.92, 88.63, 71.68 and 61.53 ml of CH4/gVS/day for the reactors containing substrates pre-treated with 12 %, 6 %, 18 % NaOH and the substrate without pre-treatment respectively. Compared to the substrate without pre-treatment, the CH4 production rates were improved 1.72, 1.44 and 1.16 times for the reactors containing 12 %, 6 % and 18 % NaOH respectively. These findings were perfectly in accordance with FTIR spectroscopy results. These findings demonstrate that the semi-continuous AD process can be an attractive energy recovery solution from such waste compared to batch digesters, (on one day, it produce 77–105 % of the CH4 produced during 21–55 days in batch digesters), as well as the importance of adequate pre-treatment prior to the AD. FTIR Spectroscopic analysis technology can also be widely used to analyze pre-treatment data for lignocellulosic biomass, as it gives very encouraging results.

Lipid-extraction of Tribonema minus avoids foaming facilitating anaerobic co-digestion with molasses vinasse

Tribonema minus, a filamentous microalgae, has great potential as an anaerobic digestion (AD) substrate, owing to its relatively low cultivation and harvesting costs. This is the first study to explore the effect of T. minus as an anaerobic co-digestion (AcoD) substrate with molasses vinasse (MV). To analyze the potential effects of the structure and composition of T. minus on the AD process, we conducted batch and semi-continuous AD using simple dehydrated T. minus (Fresh T. minus, FTR) and lipid-extracted and pulverized T. minus (Pretreated-T. minus, PTR). The results showed that the maximum methane production potential of FTR was 158.9 ± 2.9 mL CH4 g−1 VS, which was reduced by 25% after pretreatment. However, the methane production potential increased by 18% in the batch AcoD with MV. Meanwhile, FTR led to the accumulation of fatty acids and lipids, including palmitic acid, in semi-continuous AcoD with MV. Lipid accumulation further increased the polysaccharide and protein content in the extracellular polymeric substances, resulting in a decrease in liquid surface tension and an increase in foam formation, which ultimately led to a decrease in methane production potential and soluble chemical oxygen demand removal rate. Microbial community analysis demonstrated that PTR effectively mitigated the inhibitory effects caused by filamentous microalga (FM). Our results further elucidated the potential mechanism of foam formation during the AD of FTR and revealed that pretreatment can lead to effective recovery of products from T. minus while promoting methane production potential and optimizing the microbial community composition during AcoD. Therefore, our study provides novel insights into the utilization of FM.

Unleashing the potential of leather waste: Biogas generation and cost savings through semi-continuous anaerobic co-digestion

Leather processing is notorious for generating substantial amounts of polluting and putrescible organic waste, usually ending up in landfills or incineration. This study aimed to assess the potential of biogas production, waste treatment efficiency, and energy and cost savings benefits when scaling up the anaerobic co-digestion of leather processing wastes. The study focused on three waste types: tannery primary sludge (TPS), leather fleshings waste (LFW), and tannery wastewater (TWW). Semi-continuous anaerobic digestion experiments were conducted using three bench-scale reactors (R1-R3) over five phases; phases I and II were the startup and stabilization. In phase III, with an organic loading rate (OLR) of 0.6 kgVS/m3/d, the R2 reactor achieved an average of 0.323 m3 methane/kgVS added/day. This performance surpassed the other reactors by 39.14% (R1) and 41.29% (R3). In phases IV and V, using TWW instead of tap water for substrate dilution at different ratios (25–100%) reduced methane yield by 3.32–11.73% compared to the reactor that used tap water. The study further revealed that a medium-sized anaerobic reactor treating tannery wastes could reduce electric energy consumption by 3.64% and thermal energy consumption by 5.20%. This showcases the energy-saving benefits of co-digesting tannery wastes rather than disposing them in landfills. Using tannery wastes instead of traditional disposal methods resulted in approximately 94.68% savings in electric consumption and 45.03% in thermal energy consumption. This study offers a promising approach for sustainable leather waste treatment, biogas production, and considerable energy and cost savings compared to conventional disposal methods like landfilling.

Metagenomic absolute quantification of antibiotic resistance genes and virulence factor genes-carrying bacterial genomes in anaerobic digesters

Sewage treatment works have been considered as hotspots for the dissemination of antibiotic resistance genes (ARGs). Anaerobic digestion (AD) has emerged as a promising approach for controlling the spread of ARGs while destroying biomass in sludge. Evaluating the impact of AD on ARG removal relies on the absolute quantification of ARGs. In this study, we quantified the ARG concentrations in both full-scale and lab-scale AD systems using a cellular spike-ins based absolute quantification approach. Results demonstrated that AD effectively removed 68 ± 18 %, 55 ± 12 %, and 57 ± 19 % of total ARGs in semi-continuous AD digesters, with solid retention times of 15, 20, and 25 days, respectively. The removal efficiency of total ARGs increased as the AD process progressed in the batch digesters over 40 days. A significant negative correlation was observed between digestion time and the concentrations of certain ARG types, such as beta-lactam, sulfonamide, and tetracycline. However, certain potential pathogenic antibiotic resistant bacteria (PARB) and multi-resistant high-risk ARGs-carrying populations robustly persisted throughout the AD process, regardless of the operating conditions. This study highlighted the influence of the AD process and its operating parameters on ARG removal, and revealed the broad spectrum and persistence of PARB in AD systems. These findings provided critical insights for the management of microbial hazards.

Modelling of technical, environmental, and economic evaluations of the effect of the organic loading rate in semi-continuous anaerobic digestion of pre-treated organic fraction municipal solid waste

The study concerned technical feasibility, economic profitability, and carbon footprint (CF) analysis of semi-continuous anaerobic digestion (sAD) of organic fraction of municipal solid waste (OFMSW). The research assessed the pre-treatment effect on sAD by varying organic loading rates (OLR) from 3.38 to 6.75 kgvs/m3d. Three sAD configurations were investigated: hydrodynamic-cavitated (HC-OFMSW), enzymatically pre-treated (EN-OFMSW), and non-pre-treated (AD-OFMSW). Principal Component Analysis and Supervised Kohonen's Self-Organizing Maps combined the experimental, economic, and environmental evaluations.The sAD configurations were grouped predominantly according to the OLR however, within each OLR group the configurations were clustered according to the pre-treatments.The finding highlighted that pre-treatments offset inhibition in sAD of OFMSW due to the OLR increase, being economically profitable and CF negative up to 4.50 kgvs/m3d for EN-OFMSW and to 5.40 kgvs/m3d for HC-OFMSW. Whereas sAD-OFMSW remained economically and environmentally viable only up to 3.87 kgvs/m3d.HC-OFMSW reached the highest performance. In detail, for HC-OFMSW the NPV and CF ranged from 17679.30 to 43827.12 euros and from −51.08 to −407.210 kg CO2eq/1 MWh daily produced, by decreasing the OLR from 5.40 to 3.87 kgvs/m3d.These results are fundamental since pre-treatment is usually expensive due to additional energy or chemical requirements.

Effects of Short Retention Times and Ultrasound Pretreatment on Ammonium Concentration and Organic Matter Transformation in Anaerobic Digesters Treating Sewage Sludge

Anaerobic digestion of sewage sludge is limited at the hydrolysis stage of the process. The goal of this study was to assess the effects of sludge retention times and ultrasound pretreatment on the ammonium concentration and organic matter transformation in anaerobic digesters treating sewage sludge. To achieve this, two laboratory-scale semicontinuous anaerobic digesters were operated for a period of over 70 d, including a control reactor and another fed by pretreated sludge. Both anaerobic systems were fed with mixed sludge (50%/50% primary/secondary treatment) in mesophilic conditions (37 °C), with solid retention times (SRT) of 7.5 d (Phase I) and 3 d (Phase II). The performance of the anaerobic digestion process was assessed in terms of the methane yield and the total and soluble chemical organic demand, total solids, and volatile solids removal. The results showed that the ultrasound pretreatment caused an increase of around 22.2% in CODt removal for an SRT of 7.5 d. Meanwhile, an SRT of 3 d resulted in a decrease of up to 92.4% in CODt removal. The performance in terms of biogas production and organic matter removal was significantly affected by the SRT reduction to 3 d, showing that the process is not viable in these conditions.

Application of biochar to anaerobic digestion versus digestate: Effects on N emissions and C stability

Research attention is growing for biochar as amendment for anaerobic digestion (AD), as it may improve both the AD process and digestate properties. In this study, two biochars (from insect frass or the woody fraction of green waste, both pyrolyzed at 450 °C) were added (5 % w/w) during semi-continuous AD of organic kitchen waste and chicken manure. Biochar was mixed either during the AD process or to the digestate post-treatment. The impact of biochar addition on the AD process, NH3 and greenhouse gas emissions, as well as C and N mineralization following soil application, was examined. Biochar did not affect the biogas yield, but did lead to a reduction in NH3 levels in the biogas and NH4+-N sorption onto the biochars, reducing the risk of AD inhibition. N sorbed as NH4+-N onto the biochar was more available for mineralization than N sorbed as NH3. N sorption on biochar-amended digestates did not result in trade-offs when these digestates were applied to soil: the sorbed N was not released as NH3 or N2O after soil application and was not released as mineral N in either the short or medium term. The C-rich biochars increased the OC content of the digestate, which enhanced its value as soil improver. Additionally, mixing biochar with digestate decreased the soil CO2 emissions up to 33 % when it was added after AD; no priming effects were observed. These findings highlight the potential benefits of incorporating biochar into AD systems and the subsequent application of biochar-amended digestate in agricultural practices.

Enhancing kitchen waste minimization and energy generation at 20 °C: a psychrophilic anaerobic co‐digestion study

AbstractBackgroundIn developing countries, anaerobic digestion still represents an emerging technology for solid waste management, despite the great energy potential of methane in biogas for heating purposes, especially in low‐temperature regions. This research evaluated the effect of hydraulic retention time (40–120 days) on the anaerobic co‐digestion of 35% w/w waste activated sludge and 65% w/w organic kitchen waste at a low temperature (20 °C). The anaerobic co‐digestion was conducted in five 11‐L semicontinuous anaerobic digesters. Additionally, the expected energy production from organic kitchen waste was calculated.ResultsThe specific methane yield (SMY) ranged from 0.20 to 0.27 LCH4 g‐VSadd−1, comparable to the SMY reported for meso‐ and thermophilic anaerobic digestion. Moreover, the expected energy production was 1.06 MJ day−1 of equivalent liquefied petroleum gas, which could satisfy ≤6.6% of the daily water heating requirements of an average Mexican household. The hydraulic retention time in the range of 60–120 days did not seem to significantly affect the performance of the digesters.ConclusionsThese digesters could easily be installed in low temperature regions for domestic use. The waste activated sludge substrate can be replaced with other nitrogen‐rich substances for indoor use. The data shown in this research set the basis for further investigations and domestic applications. © 2023 Society of Chemical Industry (SCI).

Advanced steam explosion as thermal hydrolysis process for high solids anaerobic digestion and enhanced methane yield: Proof of concept

Sewage sludge management is one of the major issues faced by wastewater treatment plant (WWTP) operators. Though anaerobic digestion (AD) is one of the most preferred sludge management techniques, the complex microbial floc structure of the sludge creates physical and chemical barriers that limit the rate of microbial degradation resulting in a longer hydraulic retention time (HRT) of 25–30 days. The thermal hydrolysis process (THP) can be used as a pretreatment technique to enhance the rate of biodegradation which results in reduced retention time and increased methane yields. This work investigated the effects of the THP on the semi-continuous anaerobic digestion of high solids sludge (15% total solids, TS) obtained from a municipal wastewater treatment plant (WWTP) operating at higher solids retention time (SRT) of 40 days. The thermal hydrolysis process achieved the effective release of organics into the liquid phase resulting in 37% chemical oxygen demand (COD) solubilization. Two semi-continuous digesters, one fed with untreated sludge (control digester, 5% TS) to mimic the conventional anaerobic digester and the other fed with THP sludge (160 °C, 30 min, 6 bar, 10% TS) were operated under a mesophilic regime and a hydraulic retention time (HRT) of 15 days. The methane yield from the THP digester (0.366 ± 0.010 m3/kg VSadded, 70 ± 1.13% CH4) was 47% higher than the control digester (0.248 ± 0.013 m3/kg VSadded). The volatile solids removal (VSR) was 51 ± 0.6% in the THP digester, i.e., 43% higher over control digester (35.7 ± 1.3%). The fecal coliforms concentration in the digestate of control and THP digesters were 9200 and 200 MPN/g, respectively. Thus, the THP sludge meets the Class A biosolids criteria (<1000MPN/g). THP integrated anaerobic digestion achieved higher solids reduction with enhanced energy recovery, high solids loading with shorter retention time, and generated Class A biosolids.

Microbial Population Dynamics during Unstable Operation of a Semicontinuous Anaerobic Digester Fed with a Mild-Treated Olive Mill Solid Waste

This research evaluates process instability together with microbial population dynamics of the startup of an anaerobic digestion of a mild pretreated solid olive oil waste. The pretreatment consisted of a mild thermal treatment called thermo-malaxation and a subsequent dephenolized process of the olive mill solid waste. The anaerobic digestion process of the mild pretreated and partially dephenolized biomass was studied for three Hydraulic Retention Times (HRTs), with 21 days each HRT, with an organic load rate of 1 g VS/L d, carried out at mesophilic temperature (35 ± 1 °C). The average value of methane yield decreased from 204 ± 9 mL CH4/g VS d on day 21, the last day of the first HRT, to 87 ± 24 mL CH4/g VS d on day 60, the last day of the third HRT. The alkalinity decreased drastically, indicating instability of the anaerobic digestion process. Although phenolic compounds were partially extracted in the pretreatment, the observed increase in phenolic compounds during reactor operation might be contributed to the methane production decay. Interestingly, volatile fatty acids decreased with time, indicating that not only the methanogenic stage but also the hydrolysis stage was affected. Indeed, the microbial analysis showed that the abundance of hydrolytic bacteria decreased over time. It is also worth noticing that hydrogenotrophic methanogens, while present during the first two HRTs, were not observed at the end of the last HRT. This observation, together with the increase in the relative abundance of acetoclastic methanogens, showed a shift in the methane production pathway from hydrogenotrophic methanogenesis to acetotrophic methanogenesis.

Sustainable Bioprocess Combining Fed-Batch Pretreatment Followed by Semi-Continuous Anaerobic Digestion of Brewer’s Spent Grains for Biomethane Production

Sustainable Bioprocess Combining Fed-Batch Pretreatment Followed by Semi-Continuous Anaerobic Digestion of Brewer’s Spent Grains for Biomethane Production

Valorization of strawberry extrudate waste: Recovery of phenolic compounds by direct-hydrothermal treatment and subsequent methane production by mesophilic semi-continuous anaerobic digestion

Strawberry extrudate (SE) is an underused by-product from strawberry industry. Recovery of the phenolic compounds present in SE would represent a very interesting valorisation option. Two main challenges need to be solved, firstly, the solubilisation and recovery of the phenolic compounds contained in SE, and, after that, the stabilisation of the resulted de-phenolized SE. The present research evaluates the potential of a biorefinery process combining a hydrothermal pre-treatment, followed by a phenolic extraction process and, finally, the anaerobic digestion of the remaining SE for producing energy that will contribute to compensate the energy requirements of the whole system. Following the hydrothermal pre-treatment at 170 °C for 60 min, an extraction of 0.6 ± 0.1 g of gallic acid per kilogram of SE was achieved using an adsorbent resin, representing a recovery rate of 64 %. Long-term semi-continuous anaerobic digestion of de-phenolized SE was evaluated at different organic loading rates to evaluate the stability of the process. The anaerobic digestion of pre-treated SE achieved a stable methane production value of 243 ± 34 mL CH4·g volatile solids−1·d−1 at an organic loading rate (ORL) of 1.25 g volatile solids·L−1·d−1. During the operation at this ORL, the control parameters including pH, alkalinity, soluble chemical organic demand (sCOD), and volatile fatty acid (VFA) remained stable and consistently constant. Specifically, the VFA in the reactor during this stable period achieved a value of 102 ± 128 mg O2/L. Also, an economic balance showed that the minimal price of the generated phenolic extract for having benefited from the proposed biorefinery system was 0.812 €·(g of gallic acid equivalents)−1, a price within the range of phenolic compounds used in the food industry.

Investigates of substrate mingling ratio and organic loading rate of KOH pretreated corn stover and pig manure in batch and semi-continuous system: Anaerobic digestion performance and microbial characteristics

The effects of substrate mingling ratio (SMR) (1:1, 1:2, 1:3, 3:1, and 2:1) and organic loading rate (OLR) (50–90 g total solids per liter per day) on anaerobic co-digestion performance and microbial characteristics were investigated for pig manure (PM) and pretreated/untreated corn stover in batch and semi-continuous anaerobic digestion (AD) system. The results showed that SMR and pretreatment affected co-digestion performance. The maximum cumulative methane yield of 428.5 ml⋅g−1 (based on volatile solids (VS)) was obtained for PCP13, which was 35.7% and 40.0% higher than that of CSU and PM. In the first 5 days, the maximum methane yield improvement rate was 378.1% for PCP13. The daily methane yield per gram VS of PCP13 was 11.4%–18.5% higher than that of PCU13. Clostridium_sensu_stricto_1, DMER64, and Bacteroides and Methanosaeta, Methanobacterium, and Methanospirillum had higher relative abundance at the genus level. Therefore, SMR and OLR are important factor affecting the AD process, and OLR can affect methane production through volatile fatty acids.

Combined effects of oxytetracycline concentration and organic loading rate on semi-continuous anaerobic digestion of swine wastewater

High concentrations of antibiotics in swine wastewater raises concerns about the potential adverse effects of anaerobic digestion (AD). Current studies mainly focused on the effects of various antibiotic concentrations. However, these studies didn’t take into account the fluctuation of swine wastewater quality and the change of reactor operating conditions in practical engineering applications. In this study, it was found that in the operating systems with COD of 3300 mg/L and hydraulic retention time (HRT) of 4.4 days, the continuous addition of oxytetracycline for 30 days had no effect on the AD performance. Nevertheless, when COD and HRT were changed to 4950 mg/L and 1.5 days respectively, oxytetracycline at 2 and 8 mg/L increased the cumulative methane yield by 27% and 38% at the cost of destroying cell membrane, respectively, while oxytetracycline at 0.3 mg/L improved the performance and stability of AD. These results could be referred for practical engineering applications.

Ultrasonication as anaerobic digestion pretreatment to improve sewage sludge methane production: Performance and microbial characterization

A large amount of sludge is inevitably produced during sewage treatment. Ultrasonication (US) as anaerobic digestion (AD) pretreatment was implemented on different sludges and its effects on batch and semi-continuous AD performance were investigated. US was effective in sludge SCOD increase, size decrease, and CH4 production in the subsequent AD, and these effects were enhanced with an elevated specific energy input. As indicated by semi-continuous AD experiments, the mean daily CH4 production of US-pretreated A2O-, A2O-MBR-, and AO-AO-sludge were 176.9, 119.8, and 141.7 NmL/g-VSadded, which were 35.1%, 32.1% and 78.2% higher than methane production of their respective raw sludge. The US of A2O-sludge achieved preferable US effects and CH4 production due to its high organic content and weak sludge structure stability. In response to US-pretreated sludge, a more diverse microbial community was observed in AD. The US-AD system showed negative net energy; however, it exhibited other positive effects, e.g., lower required sludge retention time and less residual total solids for disposal. US is a feasible option prior to AD to improve anaerobic bioconversion and CH4 yield although further studies are necessary to advance it in practice.

Enhanced hydrogen and methane production from date fruit wastes using semi continuous two-stage anaerobic digestion process with increasing organic loading rates

Biological conversion of date wastes under continuous conditions is a challenge due to their high content of biodegradable organic matter. In this study, hydrogen (H2) and methane (CH4) production in a two-stage process to increase the energy recovery from date pulp wastes (DPW) was investigated. In the first stage, the effect of increasing organic leading rate (OLR) on dark fermentation (DF) performances was evaluated in a semi continuously stirred tank reactors (SCSTR) without nutrients addition. The highest volumetric H2 production rate (VHPR) and specific H2 yield (SHY) of 0.76 L/L.d and 81 LH2/kg volatile solid (VS), respectively were achieved at high OLR of 11.74 gVS/L.d. In the second stage, the DF effluent was used as substrate to feed the SCSTR of anaerobic digestion (AD). The highest volumetric methane production rate (VMPR) of 2.92 L/L.d was achieved with a high OLR (4.25 gVS/L.d) and short hydraulic retention time (HRT, 8 days), producing specific methane yield (SMY) of 530 LCH4/kgVS. Thus, the two-stage process can be an effective strategy for high rate renewable energy recovery (13 kJ/gVSinlet) and organic waste processing.

Long-term effect of free ammonia pretreatment on the semi-continuous anaerobic primary sludge digester for enhancing performance: Towards sustainable sludge treatment

Primary sludge (PS) is one of the major sludge sources for anaerobic digesters in wastewater treatment plants. Although the impact of free ammonia (FA) pretreatment on methane production from anaerobic PS digestion was previously investigated using batch biochemical methane potential tests, these tests could not fully represent the continuous/semi-continuous anaerobic digestion that is currently used in practice. This study comprehensively evaluated the impact of FA pretreatment on the performance of anaerobic PS digestion for the first time using semi-continuous systems that run for over 120 days. FA pretreatment (560 mg NH3-N/L, 24 h) improved the volatile solids (VS) removal of PS by 12.2 % from 60.5 % to 67.9 %, with a similar improvement in total chemical oxygen demand removal of 14.9 % and methane production of 16.1 %. FA pretreatment increased the biomass-specific hydrolysis rate of digesters by 23.5 %. Model-based analysis revealed that the enhanced anaerobic digestion performance may be due to both the increased apparent hydrolysis rate (increased by 26.7 %) and the enhanced degradability extent (increased by 9.5 %) of PS, caused by FA pretreatment. The dewaterability of digested sludge was enhanced by 14.0 % due to FA pretreatment, which is also supported by the reduced capillary suction time from 15.1 s to 10.9 s. Removals of Fecal Coliform and E. Coli were enhanced by 0.6 and 1.4 log Most Probable Number/g vS by FA pretreatment. This study firstly manifested that FA pretreatment is a favourable approach to improve the performance of anaerobic PS digestion with extra benefits in pathogen removal and dewaterability.

Methane Production from Food Garbage Under the Batch and Semi-Continuous Anaerobic Digestion

Methane Production from Food Garbage Under the Batch and Semi-Continuous Anaerobic Digestion

A modelling approach for the assessment of energy recovery and impact on the water line of sludge pre-treatments

A simple, easy-to-use, first-order model was elaborated to predict the methane production and the release of ammoniacal nitrogen (N–NH3) to the digestate in full-scale anaerobic digestion (AD) processes. The study used long-term, semi-continuous AD tests, carried out with samples of primary sludge (PS), raw waste activated sludge (WAS), WAS after a thermo-alkali pre-treatment (90 °C, 90 min, 4 g NaOH/100 g TS) and mixed sludge (PS/treated WAS), to calibrate and validate the model. The results of both the experimental activities and the phase of model tuning demonstrated that the proposed model was capable to provide reliable information to completely characterize the AD process, thus overcoming the limitations due to discontinuity of experimental tests. Furthermore, it was demonstrated that low-temperature thermo-alkali pre-treatments could increase the values of the model parameters, namely methane production after an infinite time (B0, +70%) and hydrolysis constant (k, +450%), and made them comparable to those obtained by the application of commercial, high-energy demanding treatments (e.g. Cambi). Finally, the issue concerning the release of N–NH3 to digestate was deemed to be very worthy to being investigated because, after pre-treatments, the cost for nitrogen removal in the water line, through the traditional processes of nitrification – denitrification, could increase even by 140%.