Advances in pretreatment strategies to enhance the biodegradability of waste activated sludge for the conversion of refractory substances

Investigating enhancement of protease and lysozyme combination pretreatment on hydrolysis of sludge organics under humic acid inhibition

This study investigated the impact of humic acid (HA) on enzymatic pretreatment efficiency, focusing on sludge properties and HA molecular structure. The results showed that enzymatic pretreatment alleviates HA inhibition, improving hydrolysis efficiency. In the presence of HA, soluble proteins and polysaccharides in the enzyme-cocktail group reached 27.7 mg/L and 23.9 mg/L, 1.4 and 1.3 times higher than the blank group, respectively. The enzyme-cocktail group also had the highest soluble DNA concentration (19.4 mg/L) and the lowest viable cell proportion (69.3 %), indicating effective cell lysis. Enzyme-cocktail pretreatment reduced electrostatic repulsion, enhancing the mobility of extracellular organics. Enzyme interactions with HA released internal hydrolases and decreased amide groups on the HA surface, increasing the availability of biodegradable substrates. Overall, enzymatic pretreatment proves effective in mitigating HA-induced inhibition, thereby improving sludge biodegradation and enhancing carbon recovery in anaerobic fermentation.

Lysozyme coupling protease pretreatment to relieve the humic acid inhibition on excess sludge anaerobic fermentation

The asynchronous dosed protease and lysozyme combination pretreatment was proved to be effective in enhancing the anaerobic fermentation of waste activated sludge (WAS). However, humic acid (HA) in the sludge could interact with hydrolase and restrain the hydrolysis efficiency, thus inhibiting short-chain fatty acids (SCFAs) production. This study investigated the effectiveness and mechanism of enzymatic pretreatment against HA. Results showed that the enzyme cocktail method increased the extracellular bioavailable contents by 34 %, which raised SCFAs production by 89.69 % (1269.65 mg COD /L). The balanced ratio of hydrolysis and fermentation communities suggested that the small molecular organics generated by the hydrolysis community could be sufficiently utilized by fermentation communities. The metabolism of amino acids and glucose was facilitated, and the activities of key enzymes were enhanced. These results clarified the effect of asynchronous enzyme cocktail pretreatment against HA inhibition and contributed to SCFAs production, which offered fresh perspectives on carbon recovery.

Influence of residual anaerobic granular sludge (AnGS) from anaerobically digested molasses wastewater in aerobic granular sludge reactor

This study investigated the impact of residual anaerobic granular sludge (AnGS) from anaerobic digesters treating molasses wastewater on ammonium reduction in a downstream aerobic granular sludge (AGS) reactor. Two conditions were tested: raw (high AnGS concentration) and settled (low AnGS concentration) anaerobically digested molasses wastewaters were fed into the AGS reactor. With the introduction of raw wastewater, enhanced nitrite accumulation at 30 % and improved total inorganic nitrogen (TIN) removal at 11 % were observed compared to 1 % nitrite accumulation and 8 % TIN removal with the introduction of settled wastewater. However, AnGS adversely affected other aspects of reactor performance, increasing effluent solid content and decreasing soluble chemical oxygen demand removal efficiency from 20 % in the low AnGS condition to 11 % in the high AnGS condition. Despite the observed retention of AnGS in the reactor, no significant bioaugmentation effects on the microbial community of the AGS were observed. Aerobic granular sludge was consistently observed in both conditions. The study suggests that AnGS may act as a nucleus for granule formation, helping to maintain granule stability in a disturbed environment. This study offers a systematic understanding of the impact of AnGS on subsequent nitrogen removal process using AGS, aiding in the decision making in the treatment of high solid anaerobic digestate.

Feasibility of biogas upgrading at a WWTP after pre-treatment application: Techno-economic assessment validation with pilot test data

Improving the efficiency of anaerobic digestion (AD) of sewage sludge (SS) is a critical step toward the achievement of energy neutrality in wastewater treatment plants (WWTPs), as required by the European Green Deal. This study used a comparative techno-economic assessment (TEA) to evaluate the feasibility of producing biomethane, at a WWTP, through upgrading biogas with a double-stage permeation membrane plant. The biogas was originally generated from the AD of a mixture of primary sludge (PS) and either raw or pre-treated waste activated sludge (WAS), where biological or thermo-alkali pre-treatments were applied to increase the WAS intrinsic low degradability. The TEA was supported by the results of pilot-scale tests, carried out on WAS, which mimicked (i) a traditional mesophilic AD process; (ii) a two-stage AD process, where a temperature-phased anaerobic digestion (TPAD, 3 days, 55 °C + 20 days, 38 °C) was performed to biologically pre-treat WAS; (iii) a traditional mesophilic AD process preceded by a thermo-alkali (4 g NaOH/100 g TS, 90 °C, 90 min) pre-treatment. The TEA was carried out in two phases. In the first, the minimum size of the WWTP capable of making the costs necessary for the implementation of the biogas upgrading plant equal to the revenues coming from selling biomethane (at 62 €/MWh) in 10 years was calculated in the absence of pre-treatments. It resulted of 500,000 equivalent inhabitants (e.i.). In the second phase, for the WWTP size found previously, the effect of either biological or thermo-alkali pre-treatments on the economic balance was evaluated, that is the gain (or the loss) associated to the selling of biomethane, compared to the reference price of 62 €/MWh. It was found that the TPAD increased the biogas productivity by only 23.6%, too little to compensate the amount of heat necessary for the pre-treatment and the purchase cost of the additional reactor. Conversely, the thermo-alkali pre-treatment, which enhanced the WAS biogas productivity by 110%, increased the biomethane revenues by approx. 10 €/MWh, compared to the scenario without pre-treatments. This study offers useful data to WWTP managers who want to introduce WAS pre-treatments, combined with interventions for biogas upgrading, in a new or existing sludge line of a WWTP.

Improved anaerobic sludge fermentation mediated by a tryptophan-degrading consortium: Effectiveness assessment and mechanism deciphering

The hydrolysis of extracellular polymeric substances (EPS) represents a critical bottleneck in the anaerobic fermentation of waste activated sludge (WAS), while tryptophan is identified as an underestimated constituent of EPS. Herein, we harnessed a tryptophan-degrading microbial consortium (TDC) to enhance the hydrolysis efficiency of WAS. At TDC dosages of 5%, 10%, and 20%, a notable increase in SCOD was observed by factors of 1.13, 1.39, and 1.88, respectively. The introduction of TDC improved both the yield and quality of short chain fatty acids (SCFAs), the maximum SCFA yield increased from 590.6 to 1820.2, 1957.9 and 2194.9 mg COD/L, whilst the acetate ratio within SCFAs was raised from 34.1% to 61.2-70.9%. Furthermore, as TDC dosage increased, the relative activity of protease exhibited significant increments, reaching 116.3%, 168.0%, and 266.1%, respectively. This enhancement facilitated WAS solubilization and the release of organic substances from bound EPS into soluble EPS. Microbial analysis identified Tetrasphaera and Soehngenia as key participants in WAS solubilization and the breakdown of protein fraction. Metabolic analysis revealed that TDC triggered the secretion of enzymes associated with amino acid metabolism and fatty acid biosynthesis, thereby fostering the decomposition of proteins and production of SCFAs.

Enhanced biological wastewater treatment supplemented with anaerobic fermentation liquid of primary sludge

The wastewater treatment performance in an inverted A2/O reactor supplemented with fermentation liquid of primary sludge was explored comparing to commercial carbon sources sodium acetate and glucose. Similar COD removal rate was observed with the effluent COD stably reaching the discharge standard for those 3 carbon sources. However, the fermentation liquid distributed more carbon source in the anaerobic zone. Fermentation liquid and sodium acetate tests achieved better nitrogen removal rate than glucose test. The fermentation liquid test showed the best biological phosphorus removal performance with the effluent phosphorus barely reaching the discharge standard. The microbial community characterization revealed that the fermentation liquid test was dominated by phylum Proteobacter in all the anoxic, anaerobic and aerobic zones. Denitrifying phosphorus accumulating organisms (PAOs) (i.e., genera Dechloromonas and unclassified_f__Rhodocyclaceae) were selectively enriched with high abundances (over 20%), which resulted in improved phosphorus removal efficiency. Moreover, the predicted abundances of enzymes involved in nitrogen and phosphorus removal were also enhanced by the fermentation liquid.

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

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

Degradation of tetracyclines via calcium peroxide activation by ultrasonic: Roles of reactive species, oxidation mechanism and toxicity evaluation

Tetracyclines (TC) frequently detected in the aqueous environment pose threats to humans and ecosystems. The synergistic technology coupling ultrasound (US) and calcium peroxide (CaO₂) has a great potential to abate TC in wastewater. However, the degradation efficiency and detailed mechanism of TC removal in the US/CaO₂ system is unclear. This work was carried out to assess the performance and mechanism of TC removal in the US/CaO₂ system. The results demonstrated that 99.2% of TC was degraded by the combination of 15 mM CaO₂ with ultrasonic power of 400 W (20 kHz), but only about 30% and 4.5% of TC was removed by CaO₂ (15 mM) or US (400 W) alone process, respectively. Experiments using specific quenchers and electron paramagnetic resonance (EPR) analysis indicated that the generation of hydroxyl radicals (•OH), superoxide radicals (O₂^-•), and single oxygen (¹O₂) in the process, whereas •OH and ¹O₂ were mainly responsible for the degradation of TC. The removal of TC in the US/CaO₂ system has a close relationship with the ultrasonic power, the dosage of CaO₂ and TC, and the initial pH. The degradation pathway of TC in the US/CaO₂ process was proposed based on the detected oxidation products, and it mainly included N,N-dedimethylation, hydroxylation, and ring-opening reactions. The presence of 10 mM common inorganic anions including chloridion (Cl^-), nitrate ion (NO₃^-), sulfate ion (SO₄^2-), and bicarbonate ion (HCO₃^-) showed negligible influences on the removal of TC in the US/CaO₂ system. The US/CaO₂ process could efficiently remove TC in real wastewater. Overall, this work firstly demonstrated that •OH and ¹O₂ mainly contributed to the removal of pollutants in the US/CaO₂ system, which was remarkable for understanding the mechanisms of CaO₂-based oxidation process and its future application.

Boosting short-chain fatty acids production from co-fermentation of orange peel waste and waste activated sludge: Critical role of pH on fermentation steps and microbial function traits

The anaerobic co-fermentation of orange peel waste (OPW) and waste activated sludge (WAS) for useful short-chain fatty acids (SCFAs) generation presents an environmentally friendly and efficient method for their disposal. This study amied to investigate the effects of pH regulation on OPW/WAS co-fermentation, and found that the alkaline pH regulation (pH 9) significantly enhanced the promotion of SCFAs (11843 ± 424 mg COD/L), with a high proportion of acetate (51%). Further analysis revealed that alkaline pH regulation facilitated solubilization, hydrolysis, and acidification while simultaneously inhibiting methanogenesis. Furthermore, the functional anaerobes, as well as the expressions of corresponding gene involved in SCFAs biosynthesis, were generally improved under alkaline pH regulation. Alkaline treatment might played a critical role in alleviating the toxicity of OPW, resulting in improving microbial metabolic activity. This work provided an effective strategy to recover biomass waste as high-value products, and insightful understanding of microbial traits during OPW/WAS co-fermentation.

The role of pretreatments in handling antibiotic resistance genes in anaerobic sludge digestion - A review

Sludge is among the most important reservoirs of antibiotic resistance genes (ARGs), which would cause potential environmental risks with the sludge utilization. Currently, anaerobic digestion (AD) is effective to simultaneously realize the resource recovery and pollutants removal, including antibiotic resistance genes (ARGs), and various pretreatments are used to enhance the performance. Recently, plentiful publications have focused on the effects of pretreatment on ARGs removal, but the contradictory results are often obtained, and a comprehensive understanding of the research progress and mechanisms is essential. This study summarizes various pretreatment techniques for improving AD efficiency and ARGs reduction, investigates promising performance in ARGs removal when pretreatments combined with AD, and analyzes the potential mechanisms accounting for ARGs fates. The results showed that although thermal hydrolysis pretreatment showed the best performance in ARGs reduction during the pretreatment process, the significant rebound of ARGs would occur in the subsequent AD process. Conversely, ozone pretreatment and alkali pretreatment had no significant effect on ARGs abundance in the pretreatment stage, but could enhance ARGs removal by 15.6-24.3 % in the subsequent AD. Considering the efficiency and economic effectiveness, free nitrous acid pretreatment would be a promising and feasible option, which could enhance methane yield and ARGs removal by up to 27 % and 74.5 %, respectively. Currently, the factors determining ARGs fates during pretreatment and AD processes included the shift of microbial community, mobile genetic elements (MGEs), and environmental factors. A comprehensive understanding of the relationship between the fate of ARGs and pretreatment technologies could be helpful for systematically evaluating various pretreatments and facilitating the development of emerging and effective pretreatment techniques. Moreover, given the effectiveness, economic efficiency and environmental safety, we called for the applications of modern analysis approaches such as metagenomic and machine learning on the optimization of pretreatment conditions and revealing underlying mechanisms.

Distinct effects of typical sludge pretreatment approaches on the antibiotic resistance genes variations, associated bacterial community dynamics and metabolic activities during anaerobic fermentation process

Anaerobic fermentation is effective for waste activated sludge (WAS) disposal to realize resource generation and pollutants reduction, and various pretreatments were commonly applied to improve the performance. This work mainly investigated the effects of typical WAS pretreatment approaches on the antibiotic resistance genes (ARGs, as emerging contaminants) removal during anaerobic fermentation processes and unveiled the underlying mechanisms. The results indicated that all the pretreatment strategies exhibited evident effects on the overall ARGs removal with the order of Fe²⁺ activated persulfate (PS/Fe²⁺) > pH 10 > Ultrasonication > Heat, and showed selective removal tendency for the specific ARGs (namely easily removed (aadA1 and sul1) and persistent ARGs). Mechanistic analysis demonstrated that the pretreatments disrupted the extracellular polymeric substances (EPS) and rose the cell membrane permeability (particularly for PS/Fe²⁺ and Heat). Then the increased ARGs release benefitted the subsequent reduction of mobile genetic elements (MGEs) and extracellular ARGs (especially for PS/Fe²⁺ and pH10), resulting the ARGs attenuation. Pretreatments significantly shifted the microbial community structure and the abundances of potential ARGs hosts (i.e., Sulfuritalea, and Denitratisoma). Also, the different pretreatments exhibited distinct effects on the microbial metabolic traits related with ARGs proliferation (i.e., ABC transporters, two-component system and bacterial secretion systems), which also contributed to the ARGs attenuations during WAS fermentation. The partial least-squares path modeling (PLS-PM) analysis indicated that the bacterial community (total effects = 0.968) was key factor determining ARGs fates.