Effect of free nitrous acid pre-treatment on primary sludge at low exposure times

Potential of free nitrous acid (FNA) for sludge treatment and resource recovery from waste activated sludge: A review

The escalating production of waste activated sludge (WAS) presents significant challenges to wastewater treatment plants (WWTPs). Free nitrous acid (FNA), known for its biocidal effect, has gained a growing focus on sludge dewatering, sludge reduction, and resource recovery from WAS due to its eco-friendly and cost-effective properties. Nevertheless, there have been no attempts made to systematically summarize or critically analyze the application of FNA in enhancing treatment and resource utilization of sludge. In this paper, we provided an overview of the current understanding regarding the application potential and influencing factors of FNA in sludge treatment, with a specific focus on enhancing sludge dewatering efficiency and reducing volume. To foster resource development from sludge, various techniques based on FNA have recently been proposed, which were comprehensively reviewed with the corresponding mechanisms meticulously discussed. The results showed that the chemical oxidation and interaction with microorganisms of FNA played the core role in improving resource utilization. Furthermore, current challenges and future prospects of the FNA-based applications were outlined. It is expected that this review can refine the theoretical framework of FNA-based processes, providing a theoretical foundation and technical guidance for the large-scale demonstration of FNA.

A novel strategy for efficiently transforming waste activated sludge into medium-chain fatty acid using free nitrous acid

The global energy crisis is approaching due to rapid population growth and overexploitation of fossil fuels. Therefore, the development and use of new and renewable energy sources is already in the extreme urgency. This work developed a novel technology to efficiently produce renewable liquid bioenergy from discarded wastes, by effectively transforming sewage sludge into high-value medium chain fatty acids (MCFA). The maximum MCFA yield in the anaerobic sludge fermentation was revealed to be 10.6 times of control when utilizing sewage sludge with 1.78 mg-N/L free nitrous acid (FNA) pretreatment. The carbon flow from sewage sludge into MCFA in the fermentation system was significantly enhanced with appropriate levels (0.71-1.78 mg-N/L) of FNA pretreatment. Compared to FNA pretreatment, however, its direct addition severely inhibited total products (i.e., carboxylates and complex alcohols) generation because of the toxicity on live cells (decreasing to 8.3 %-13.9 %) in sludge. Kinetic models (one-substrate and two-substrate) were utilized to investigate the mechanism of MCFA promotion by FNA pretreatment on anaerobic sludge fermentation, in which linear relationship analysis between FNA-derived organic release and the fitted parameters were also performed. The results indicated that the conversion of refractory materials into rapidly bioavailable substrates for MCFA production contributed to increasing MCFA production rate and potential. Moreover, the relative abundances of functional microorganisms related to hydrolysis-acidification and chain elongation process increased under FNA pretreatment, further favoring the MCFA production. This study provides a novel and effective technology of sludge energy recovery that can achieve the next-generation sustainable sewage sludge management.

Combining high pressure homogenization with free nitrous acid pretreatment to improve anaerobic digestion of sewage sludge

Single pretreatment of sewage sludge, either physical, chemical or biological, has its own drawbacks in term of poor sanitization, energy intensity and high operational and capital cost. To tackle these drawbacks, combined high pressure homogenization (HPH) and free nitrous acid (FNA) pretreatment for sludge solubilization and further biodegradation in anaerobic digestion was investigated. Synergistic effect of combined HPH (40 MPa) and FNA (2.49 mg/L) pretreatment (HPH-FNA) for improving anaerobic digestion was evaluated, and its effect on archaeal and bacterial community structure was analyzed. Compared with single HPH and FNA pretreatments, HPH-FNA pretreatment efficiently solubilized wasted activated sludge (WAS), subsequently improved anaerobic digestion. Cumulative biogas production from sewage sludge pretreated with HPH-FNA was 154%, 108% and 284% more than that with single pretreatment of FNA, HPH and raw sludge, respectively. In addition, volumetric biogas production of combined pretreatment system (815 ml) was more than the sum from single pretreatment (710 ml). Methane content in biogas for raw sludge, FNA, HPH and HPH-FNA pretreated sludge was 45%, 51%, 55% and 65%, respectively. Illumina MiSeq sequencing analysis revealed that HPH-FNA pretreatment promoted bacterial growth of phyla Bacteroidetes, Firmicutes and Synergistetes and archaeal genera Methanospirillum and Methanosaeta. Overall, combined HPH-FNA pretreatment of sewage sludge, prior to anaerobic digestion, is an environmentally-friendly and potentially economic technology.

The origin of waste activated sludge affects the enhancement of anaerobic digestion by free nitrous acid pre-treatment

Anaerobic digestion is a common stabilization method for treating primary sludge (PS) and waste activated sludge (WAS). However, its application is often limited by the degradation of WAS. Recent studies have demonstrated FNA to be an effective pre-treatment for enhancing WAS degradability, while having limited effect on PS degradability. WAS characteristics are impacted by wastewater treatment plant (WWTP) configuration and this study is the first to compare the effectiveness of FNA pre-treatment on WAS from WWTP with and without primary treatment. In this study, WAS samples were collected from four full-scale WWTPs with or without primary treatment. Sludge characterization, biomethane potential tests and mathematical modeling were conducted to assess the impacts of FNA pre-treatment on anaerobic digestion. The results showed that FNA pre-treatment was consistently effective for WAS from different WWTPs, while the extent of enhancement varied between WWTPs. For WAS from WWTPs without primary treatment, FNA pretreatment increased the rate of hydrolysis by 54-66% compared to 22-33% increase for WAS without primary treatment. In contrast, WAS from WWTPs with primary treatment experienced greater increases in methane potential (22-24%) compared to WAS from WWTPs without primary treatment (14-16%). These variances could be associated with primary treatment impacting the wastewater COD/N ratio and thus portion of extracellular polymetric substances (EPS) and cells in WAS. FNA pre-treatment targets the destruction of polymetric substances and cells, therefore WAS with a higher proportion of cells (i.e., WAS with primary treatment) experienced greater improvements in methane yield. Similarly, greater improvements in hydrolysis rate were observed for WAS from WWTP without primary sedimentation which contain higher proportions of large EPS molecules. Despite its consistent effectiveness on WAS samples, FNA pre-treatment was ineffective for improving the digestibility of high-rate activated sludge (HRAS).

Enhancing anaerobic digestion using free nitrous acid: Identifying the optimal pre-treatment condition in continuous operation

Free Nitrous Acid (FNA) pre-treatment is a promising technology demonstrated effective in improving waste activated sludge degradability and anaerobic digestion (AD) performance. Pre-treatment conditions including FNA concentration and treatment duration determine operational and capital cost of full-scale implementation, which have not been studied in long-term experiments. The knowledge of FNA pre-treatment conditions improving the AD performance is urgently required to determine suitable conditions for the technology implementation. In this work, five different FNA concentrations (2.2, 4.4, 7.2, 12 mgN/L and nitrite only without pH adjustment) and three treatment durations (8, 24 and 48 h) were studied in four lab-scale semi-continuous AD reactors for over 300 days. FNA pre-treatment was shown under all tested conditions effective in enhancing AD performances, while its effectiveness and resulted benefits varied substantially amongst different pre-treatment conditions. The long-term experiment demonstrated that the methane production, sludge reduction and digested sludge viscosity of AD are positively correlated with the FNA concentration and durations, until an optimal condition is reached, which was identified in this work to be FNA concentration of 7.2 mgN/L and treatment duration of 24 h. Microbial community changes supported the apparent observation of enhanced sludge degradation at elevating FNA concentrations applied during pre-treatment. The short-term sludge solubilization results were inconsistent with the long-term AD performance, which was potentially caused by inhibitions from stringent FNA pre-treatment conditions applied (FNA = 12 mgN/L with 24-hour treatment & FNA = 7.2 mgN/L with 48-hour treatment). Overall, results suggested FNA pre-treatment at the optimized condition is highly beneficial to WWTPs and competitive with other pre-treatment technologies, e.g., thermal hydrolysis pre-treatment. This work comprehensively evaluated the key design parameters of FNA pre-treatment process, reached a major milestone in the development and applications of FNA technologies.

Removal of pathogen and antibiotic resistance genes from waste activated sludge by different pre-treatment approaches

In wastewater treatment plants, most of the pathogens and antibiotic resistant genes (ARGs) transferred into and concentrated in waste activated sludge (WAS), which would cause severe public health risks. In this study, the capabilities of several WAS pre-treatment approaches to inactivate coliforms/E. coli and ARGs, as well as the subsequent regrowth of coliforms/E. coli and ARGs/intI1 in treated sludge were investigated. The results showed that electro-Fenton (EF), with continuous hydroxyl radical generation, could efficiently inactivate coliforms/E. coli in 60 min (about 4 log units), followed by methanol (MT), anode oxidization (AO), and acidification (AT). Kinetic analysis showed that the inactivation mainly occurred in the first 10 min. However, the efficiencies of all studied pre-treatment approaches on inactivating ARGs/intI1 (<2 log units) were lower than coliforms/E. coli, whilst EF still had the highest efficiency of ARGs/intI1 reduction. Mechanical ultrasound treatment (ULS) could not inactivate coliforms/E. coli in WAS, but it could efficiently reduce ARGs/intI1. High regrowth rates of coliforms/E. coli were observed in the treated WAS in 10 days, but the abundances of ARGs/intI1 continuously reduced during the after-treatment incubation. Our study showed that EF could efficiently disinfect potential pathogens, however, the reduction of ARGs/intI1 in WAS need further investigation.

Improving wastewater management using free nitrous acid (FNA)

Free nitrous acid (FNA), the protonated form of nitrite, has historically been an unwanted substance in wastewater systems due to its inhibition on a wide range of microorganisms. However, in recent years, advanced understanding of FNA inhibitory and biocidal effects on microorganisms has led to the development of a series of FNA-based applications that improve wastewater management practices. FNA has been used in sewer systems to control sewer corrosion and odor; in wastewater treatment to achieve carbon and energy efficient nitrogen removal; in sludge management to improve the sludge reduction and energy recovery; in membrane systems to address membrane fouling; and in wastewater algae systems to facilitate algae harvesting. This paper aims to comprehensively and critically review the current status of FNA-based applications in improving wastewater management. The underlying mechanisms of FNA inhibitory and biocidal effects are also reviewed and discussed. Knowledge gaps and current limitations of the FNA-based applications are identified; and perspectives on the development of FNA-based applications are discussed. We conclude that the FNA-based technologies have great potential for enhancing the performance of wastewater systems; however, further development and demonstration at larger scales are still required for their wider applications.

Advances in enhanced volatile fatty acid production from anaerobic fermentation of waste activated sludge

Low acid production and acid-forming process instability are becoming the major issues to limit the popularization of anaerobic fermentation to produce volatile fatty acid. Considerable research efforts have been made to address these problems, from studying the microorganisms that are primarily responsible for or detrimental to this process, to determining their biochemical pathways and developing mathematical models that facilitate better prediction of process performance to identify the mechanism and optimization of process control. A limited understanding of the complex microbiology and biochemistry of anaerobic fermentation is the primary cause of acid production upset or failure. This review critically assesses the recent advances in enhanced volatile fatty acid production from anaerobic fermentation of waste activated sludge from micro to macro scale, particularly relating to the microbiology, biochemistry, impact factors, and enhancement methods. Previous results suggest that further studies are necessary to substantially promote the efficiency and stability of acid production. One of the promising directions appears to be integrating the existing and growing pretreatment technologies and fermentation processes to enhance metabolic pathways of acetogens but inhibit activities of methanogens, which this study hopes to partially achieve.

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.

Increasing capacity of an anaerobic sludge digester through FNA pre-treatment of thickened waste activated sludge

Free nitrous acid (FNA) pre-treatment has been previously demonstrated to be effective in enhancing methane production and volatile solids (VS) destruction in the anaerobic digestion of waste activated sludge for an equivalent hydraulic retention time (HRT). We hypothesise that, due to enhancement of hydrolysis kinetics, FNA pre-treatment will also allow reduction in the HRT while retaining performance. This would allow for improvement of capacity constrained digesters. Two anaerobic sludge digesters (control-experiment) were fed with the same thickened waste activated sludge (TWAS) from a full-scale plant for 6 months. With 24 h pre-treatment of TWAS at an FNA concentration of 6.1 mgN/L (NO₂-N = 250 mg/L, pH = 5.0, T = 25 °C), the HRT for the experimental anaerobic digester was progressively reduced from 15 days to 12 days and then to 7.5 days. In comparison, the control reactor was operated at a constant HRT of 15 days, representing typical loading conditions. With the shortened HRTs, the experimental AD reactor achieved VS destruction at 36.9 ± 0.8% (12 days) and 36.8 ± 1.0% (7.5 days), representing 30-40% relative increase in comparison to the control reactor (at 26.5 ± 0.8% and 28.3 ± 0.7%, respectively, in the same two periods). This was supported by a similar (31-35%) increase in the methane production per unit of VS fed. The volumetric methane production rate of the experimental digester was increased by 165% at HRT of 7.5 days compared with the control digester at HRT of 15 days. The results demonstrated that FNA pre-treatment of TWAS can substantially increase the capacity of an anaerobic sludge digester, with a highly favourable economic outcome.

Improvement of anaerobic digestion of sewage mixed sludge using free nitrous acid and Fenton pre-treatment

BACKGROUND

Recently, it has been indicated that free nitrous acid (FNA) and Fenton pre-treatment of waste activated sludge can enhance methane production in anaerobic digestion of waste activated sludge. In addition, it has been revealed that the substances used in these pre-treatments are both eco-friendly and economically attractive because not only are they produced in anaerobic digestion, but they are also low priced. Since primary sludge and waste activated sludge are mixed prior to anaerobic digestion in the majority of wastewater treatment plants, this study aims to assess the influence of combined FNA and Fenton on the anaerobic digestion of mixed sludge.

RESULTS

According to this study's results, methane generation from anaerobic digestion of mixed sludge was enhanced when using FNA and Fenton pre-treatment, affirming the effectiveness of the individual and combined pre-treatments in anaerobic digestion of mixed sludge. The enhanced methane production was significant in combined pre-treatments (up to 72%), compared with FNA and Fenton pre-treatment alone (25% and 27%, respectively). This corroborates the positive synergistic effect of the combined pre-treatments on methane production. The enhanced methane can be attributed to augmented soluble fractions of organic matter in addition to increased readily biodegradable organic matter, caused by the pre-treatments. Additionally, the amount of chemical oxygen demand (COD) was assessed during anaerobic digestion, and it was revealed that COD decreased considerably when the pre-treatment strategies were combined.

CONCLUSIONS

This study reveals that the pre-treatments are potentially applicable to full-scale wastewater treatment plants because a mixture of primary sludge and waste activated sludge was used for the pre-treatments. Additionally, combined FNA and Fenton pre-treatments prove more effective in enhancing methane production and organic removal than these pre-treatments alone. The enhanced methane production is important for two reasons: a higher amount of renewable energy could be generated from the enhanced methane production and the COD of digested sludge reduces in such a way that facilitates application of the sludge to agricultural lands and reduces sludge transport costs.

Energy efficiency: Importance of indigenous microorganisms contained in the municipal solid wastes

2016 was an extraordinary year for renewable energy, as it had the largest global capacity additions seen to date. However, challenges remain, particularly beyond the power sector. Overcoming these challenges means pursuing goals on development and optimization of strategies focused in causing an increase in bioenergy usage. Considering the seriousness of the challenge this paper has been developed. In the present study, indigenous microorganisms gathered from municipal solid waste will be analysed at to find out the role such organisms have on an anaerobic digester and its performance, with the aim of producing biogas in order for it to be used as electricity or treated to produce high quality fuel. The presence of such anaerobic microbiota can help avoid the two most tragic situations of an anaerobic digestion plant: overloading and washing out. The information of the present paper would have to be considered in future researchers about pre-treatments because most novelty studies are focused on hard pre-treatment to destroy microorganisms in the substrate (to increase the biogas production). In the present paper, it is underlined that the destruction of the microbiota in the substrate could produce adverse effects in the performance in the reactor.

Synthesis of Cr2O3/C3N4 composite for enhancement of visible light photocatalysis and anaerobic digestion of wastewater sludge

Visible light photocatalysts of Cr₂O₃/C₃N₄ composites (with different melamine concentrations) were prepared by high temperature calcination method. The composites samples were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy SEM, energy-dispersive X-ray spectroscopy (EDX), UV-visible spectroscopy and particle size analysis, which clearly indicated the coexistence of both Cr₂O₃ and C₃N₄ in the composites. The Cr₂O₃/C₃N₄ catalysts were tested for photocatalytic degradation of 2-chlorophenol in wastewater and solubilization of sludge in anaerobic digestion process to enhance biomethane production. The co-catalytic performance of Cr₂O₃, with 6% of melamine (precursor of C₃N₄), improved the photocatalytic degradation of 2-chlorophenol (k = 0.0156 min^-1) under visible light, where up to 94% removal was achieved at optimum pH 5.0, pollutant concentration of 60 mg/L, and time duration of 180 min. On another hand, application of Cr₂O₃/C₃N₄ for photocatalytic pretreatment of sludge released the soluble substances in solution in which sCOD was increased from 431 mg/L to 3666 mg/L after 6 h and VS content decrease by only 9.1%, which indicated that the short time pretreatment could avoid the further mineralization of organic to complete degradation. Thereafter, anaerobic digestion of solubilized sludge was achieved after 30 days with production of 634 ml kg^-1_VS of methane and 46% of organic matter removal efficiency (OMRE), compared with 472 ml kg^-1_VS and 402 ml kg^-1_VS of methane, 35 and 31% of OMRE respectively in photolytic and raw sludge (control) reactors. These results can provide a useful base and reference for the multi applications of visible light Cr₂O₃/C₃N₄ photocatalyst in enhancement of degradation of toxic pollutant in wastewater and sludge stabilization with bioenergy production in practice.

Optimization of free nitrous acid pre-treatment on waste activated sludge

The effectiveness of the Free Nitrous Acid (FNA) sludge treatment was tested in the range from 0 to 3.0 mg N-HNO₂/L with acidified and neutral pH. 4 h pre-treatment times were used and the specific methane production (SMP) investigated. Results show that between 50 and 100 mg/L of N-NO₂^-/L disappeared during the FNA pre-treatment, reducing its effectiveness. A minimum level of nitrite (174 mg N-NO₂^-/L tested in this study), independently of pH/FNA, was necessary to assure the presence of the chemical throughout the duration of the pre-treatment. Sludge viability was compromised while WAS solubilization and SMP were enhanced with nitrite concentrations of 174 mg N-NO₂^-/L or higher, even at low FNA levels (<0.15 mg N-HNO₂/L). Results show that acidified pH is not needed to enhance methane production, making the pretreatment more economically and environmentally attractive.

Exploring alternatives to reduce economical costs associated with FNA pre-treatment of waste activated sludge

Recent studies have shown the effectiveness of Free Nitrous Acid (FNA) pre-treatment in enhancing sludge biodegradability and improving its methane production potential. FNA is regarded as an environmental friendly pre-treatment which can be easily applied when a source of nitrite is present in wastewater treatment plants. However, when nitrite is not available and needs to be purchased, this treatment can become less attractive due to the costs associated to nitrite. In order to overcome this possible limitation, two different strategies to optimize the use of nitrite during FNA treatment were investigated: i) Recovering NO₂^- after the pre-treatment is completed; and ii) Concentrating the sludge before FNA pre-treatment. Results show that recovering NO₂^- from the pre-treated sludge is not suitable due to the loss of soluble organic matter present in the supernatant after the pre-treatment. However, concentrating the sludge before the pre-treatment seems a good strategy to optimize the use of nitrite.

Sludge treatment: Current research trends

Sludge is produced during wastewater treatment as a residue containing most insoluble and adsorbed soluble impurities in wastewaters. This paper summarized the currently available review papers on sludge treatments and proposed the research trends based on the points raised therein. On partition aspect, sludge production rate and the reduction of production rate and the fate and transformation of involved emergent contaminants including endocrine disrupting chemicals and pharmaceuticals and personal care products are widely studied. On release aspect, development of thermal processes on sludge with migration and transformation of heavy metals in sludge during treatment is a research focus. The use of detailed fluid and biological reaction models and advanced instrumentation and control systems is studied to optimize treatment performances. On recovery part, co-digestion of sludge with co-substrates at mesophilic and hyperthermophilic conditions and the recovery of phosphorus at low costs are research highlights.

Seeking to enhance the bioenergy of municipal sludge: Effect of alkali pre-treatment and soluble organic matter supplementation

The aim of this research is to enhance the mesophilic anaerobic digestion of municipal sludge from Cadiz-San Fernando (Spain) wastewater treatment plant at 20days hydraulic retention time (HRT). Two different strategies were tested to improve the process: co-digestion with the addition of soluble organic matter (1% v/v); and alkali sludge pre-treatment (NaOH) prior to co-digestion with glycerine (1% v/v). Methane production (MP) was substantially enhanced (from 0.36±0.09 L CH₄ l/d to 0.85±0.16 L CH₄ l/d), as was specific methane production (SMP) (from 0.20±0.05 L CH₄/g VS to 0.49±0.09 L CH₄/g VS) when glycerine was added. The addition of glycerine does not seem to affect sludge stability, the quality of the effluent in terms of pH and organic matter content, i.e. volatile fatty acids (VFA), soluble organic matter and total volatile solid, or process stability (VFA/Alkalinity ratio<0.4). Alkali pre-treatment prior to co-digestion resulted in a high increase in soluble organic loading rates (more than 20%) and acidification yield (more than 50%). At 20days HRT, however, it led to overload of the system and total destabilization of the mesophilic anaerobic co-digestion of sewage sludge and glycerine.