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Filamentous foam disintegration with free nitrous acid: Effect on anaerobic digestion
Abstract
Sludge foaming is a common problem in wastewater treatment plants negatively affecting operation of anaerobic digestion reactors. Therefore, in common practice, foam is removed from reactors without being fermented, leading to increase in sludge mass for disposal. However, foam is rich in lipids and can be a good source of methane if operational problems can be overcome. In this paper, in a two-stage experiment, we show that foam disintegration with free nitrous acid (FNA) can boost methane production and decrease foaming potential. In the first stage, the biochemical methane potential (BMP) of foam was evaluated to be higher by 19–63% (191-263NmL/gVS) than the BMP of waste activated sludge (WAS) (161 ± 1NmL/gVS) confirming previous assumptions. The main findings of the second stage (continuous experiments) are: (1) foam and WAS co-digestion leads to sludge stratification and thickened biomass accumulation in the upper part of the reactor, (2) FNA disintegration destroyed foam structure, resulting in lower biomass stratification and 14% higher methane production (134 mL/gVS) than observed in the reference reactor, (3) FNA disintegration of both substrates (foam and WAS) does not provide noticeable benefits in terms of biomass stratification. However, it does enhance methane production to 140 mL/gVS and sludge mineralization efficiency. A significantly higher impact of FNA on methane yield from foam than WAS was attributed to the high content of M.parvicella and the ability of these bacteria to adsorb and accumulate lipids. Anaerobic digestion of FNA disintegrated foam leads to substantial benefits in terms of methane production, reactor volume, and reagents consumption.
... However, since foam has considerable lipids composition, it can serve valuable purpose as substrate for methane generation if its operational issues are properly managed [138]. FNA has been shown to cause foam disintegration and decrease foaming potential to boost methane production [129]. This further demonstrates its potential for sludge management and high energy recovery. ...
Considering recent research trends, increasing resource recovery from the anaerobic fermentation of organic materials demands more efficient biodegradability of substrates, as well as suppressing the activities of undesirable microbes while promoting the activities of the desired ones. The utilization of substrate pretreatment technologies has been the predominant control strategies to accelerate the start-up process and enhance the efficiency of anaerobic digestion systems. Notwithstanding, many traditional pretreatment technologies have major drawbacks such as augmenting substrates degradability with little modulatory effects on microbial function in the process. Additionally, their high energy costs and concomitant environmental consequences necessitate the development of non-polluting, less energy intensive and sustainable pretreatment candidates. Along these lines, applications have emerged where free nitrous acid is used to augment the breakdown of organic wastes and inactivate microbes of competing pathways, resulting in enhanced bioresource yields from anaerobic fermentation. Free nitrous acid is produced from nitritation of digester liquors, ensuring its renewable supply in biological systems and making it particularly attractive for advancing circular economy. Hence, free nitrous acid utilization is a promising alternative method to beneficially exploit substrate and microbial activities in the different stages of anaerobic digestion such as hydrolysis, acidogenesis and methanogenesis for the dual benefits of sustainable waste management and enhanced resources yield. Overall, the findings of this review summarize for the first time the potential roles of free nitrous acid as a green pretreatment technology to profoundly enhance the efficiency of anaerobic digestion systems.
Candidatus Microthrix is one of the most common bulking filamentous microorganisms found in activated sludge wastewater treatment plants (WWTPs) across the globe. One species, Ca. M. parvicella, is frequently observed, but global genus diversity, as well as important aspects of its ecology and physiology, are still unknown. Here, we use the MiDAS ecosystem-specific 16S rRNA gene database in combination with amplicon sequencing of Danish and global WWTPs to investigate Ca. Microthrix spp. diversity, distribution, and factors affecting their global presence. Only two species were abundant across the world confirming low diversity of the genus: the dominant Ca. M. parvicella and an unknown species typically present along with Ca. M. parvicella, although usually in lower abundances. Both species were mostly found in Europe at low-to-moderate temperatures and their growth was favored in municipal WWTPs with advanced process designs. As no isolate is available for the novel species, we propose the name “Candidatus Microthrix subdominans.” Ten high-quality metagenome-assembled genomes recovered from Danish WWTPs, including 6 representing the novel Ca. M. subdominans, demonstrated high genetic similarity between the two species with a likely preference for lipids, a putative capability to reduce nitrate and nitrite, and the potential to store lipids and poly-P. Ca. M. subdominans had a potentially more versatile metabolism including additional sugar transporters, higher oxygen tolerance, and the potential to use carbon monoxide as energy source. Newly designed fluorescence in situ hybridization probes revealed similar filamentous morphology for both species. Raman microspectroscopy was used to quantify the in situ levels of intracellular poly-P. Despite the observed similarities in their physiology (both by genomes and in situ), the two species showed different seasonal dynamics in Danish WWTPs through a 13-years survey, possibly indicating occupation of slightly different niches. The genomic information provides the basis for future research into in situ gene expression and regulation, while the new FISH probes provide a useful tool for further characterization in situ. This study is an important step toward understanding the ecology of Ca. Microthrix in WWTPs, which may eventually lead to optimization of control strategies for its growth in this ecosystem.
Due to rapid urbanization, the number of wastewater treatment plants (WWTP) has increased, and so has the associated waste generated by them. Sustainable management of this waste can lead to the creation of energy-rich biogas via fermentation processes. This review presents recent advances in the anaerobic digestion processes that have led to greater biogas production. Disintegration techniques for enhancing the fermentation of waste activated sludge can be apportioned into biological, physical and chemical means, which are included in this review; they were mainly compared and contrasted in terms of the ensuing biogas yield. It was found that ultrasonic- and microwave-assisted disintegration provides the highest biogas yield (>500%) although they tend to be the most energy demanding processes (>10,000 kJ kg−1 total solids).
One of the problems in wastewater treatment technologies is the formation of foam/scum on the surface of bioreactors. The foam elimination/destruction can be carried out by various methods among which disintegration is included. Hybrid disintegration (chemical decomposition and hydrodynamic cavitation) of the foam microorganisms results in phosphates, ammonium nitrogen, magnesium and potassium transferred from the foam solids into the liquid phase. Application of both methods as a hybrid pre-treatment process caused in an increased concentration of phosphates of about 677 mg PO43- L-1, ammonium nitrogen about 41 mg N-NH4+ L-1. The concentration of Mg2+ and K+ in the solution increased from 6.2 to 31.1 mg Mg2+ L-1 and from 22.4 to 102.0 mg K+ L-1, respectively. The confirmation of physicochemical changes and release of cellular matter as a result of cellular lysis (hybrid disintegration) was IR analysis. It was demonstrated that the disintegration of foam permits removal of a part of nutrients in the form of struvite.
Large quantities of sludge are produced in biological wastewater treatment. Sludge has a high putrescibility and therefore has to be stabilizsed in order to enable an environmentally safe utilization or disposal. The anaerobic digestion is the standard technique to stabilize the sludge. Digestion produces biogas and reduces the amount of organic matter. The amount of excess sludge production from wastewater treatment plants is continuously increasing. Therefore, minimization of excess sludge is becoming more and more important. The aim of bacterial cell disintegration is the release of the cell contents as an aqueous extract. Mechanical disintegration can activate the biological hydrolysis process and therefore, significantly increase the stabilization rate of the secondary sludge. It has been shown that when subjecting the sludge to 30 min of mechanical disintegration, the COD concentration in the supernatant increased from 61 mg/l to more than 144 mg/l in activated sludge and from 68 to more than 688 mg/l in foam. This study presents an application of mechanical disintegration on sewage sludge (mainly surplus activated sludge) and foam to enhance biodegradability and biogas yield in anaerobic digestion.
The addition of readily available high strength organic wastes such as fats, oils, and grease (FOG) from restaurant grease abatement devices may substantially increase biogas production from anaerobic digesters at wastewater treatment facilities. This FOG addition may provide greater economic incentives for the use of excess biogas to generate electricity, thermal, or mechanical energy. Co-digestion of FOG with municipal biosolids at a rate of 10–30% FOG by volume of total digester feed caused a 30–80% increase in digester gas production in two full scale wastewater biosolids anaerobic digesters (Bailey, 2007; Muller et al., 2010). Laboratory and pilot scale anaerobic digesters have shown even larger increases in gas production. However, anaerobic digestion of high lipid wastes has been reported to cause inhibition of acetoclastic and methanogenic bacteria, substrate, and product transport limitation, sludge flotation, digester foaming, blockages of pipes and pumps, and clogging of gas collection and handling systems. This paper reviews the scientific literature on biogas production, inhibition, and optimal reactor configurations, and will highlight future research needed to improve the gas production and overall efficiency of anaerobic co-digestion of FOG with biosolids from municipal wastewater treatment.
A pilot-scale membrane-enhanced biological phosphorus removal process accumulated substantial quantities of stable foam on the surface of the anoxic zone. The foam contained 4 to 6% dry matter, with specific nitrogen and phosphorus contents that were similar to those of the underlying anoxic zone mixed liquor. Kinetic studies demonstrated that the specific rate of phosphorus release from the foam was only 25 to 30% of that observed with mixed liquor from the aerobic zone. Molecular techniques demonstrated that the calculated similarity of the microbial communities in the foam and the underlying mixed liquor was approximately 80%, with two phylotypes (Gordonia amarae and Microthrix parvicella) being uniquely enriched in the foam and one phylotype (Epistylis sp.) more abundant in the underlying mixed liquor. The production of foam was demonstrated to be a consistent phenomenon that depended on the concentration of the suspended solids in the bioreactor.
Current data about disintegration of waste activated sludge (WAS) with free nitrous acid (FNA) do not provide information about the efficiency of concentrations higher than 6.1 mgHNO2-N/L and exposure times over 72h. Moreover, current references do not represent disposable soluble chemical oxygen demand (SCOD) release as data on COD consumption for denitrification of residual nitrites are not presented. In this study, WAS from a large wastewater treatment plant (WWTP) was exposed to an FNA concentration ranging from 0.0 to 352.7 mgHNO2-N/L for as long as 6 days to provide missing data. FNA concentrations higher than previously reported have been shown to release greater amounts of organic matter, even when the requirement for denitrification is taken into account. Therefore higher concentrations may be beneficial for WWTPs, especially when a drop in costs of sludge disposal is considered. Demand of organic compounds for nitrites denitrification can constitute up to 86% of total released SCOD and therefore this process has to be included. Besides, tests for specific biomethane potential (SBMP) showed that increasing FNA concentrations above 6.4 mgHNO2-N/L did not benefit the growth of SBMP anymore, while FNA at least up to 3.7 mgHNO2-N/L did not increase the methane potential compared to control.
Foaming is a common operational problem in anaerobic digestion (AD) systems, where hydrophobic filamentous microorganisms are usually considered to be the major cause. However, little is known about the identity of foam-stabilising microorganisms in AD systems, and control measures are lacking. This study identified putative foam forming microorganisms in 32 full-scale mesophilic digesters located at 12 wastewater treatment plants in Denmark, using 16S rRNA gene amplicon sequencing with species-level resolution and fluorescence in situ hybridization (FISH) for visualization. A foaming potential aeration test was applied to classify the digester sludges according to their foaming propensity. A high foaming potential for sludges was linked to the abundance of species from the genus Candidatus Microthrix, immigrating with the feed stream (surplus activated sludge), but also to several novel phylotypes potentially growing in the digester. These species were classified to the genera Ca. Brevefilum (Ca. B. fermentans) and Tetrasphaera (midas_s_5), the families ST-12K33 (midas_s_22), and Rikenellaceae (midas_s_141), and the archaeal genus Methanospirillum (midas_s_2576). Application of FISH showed that these potential foam-forming organisms all had a filamentous morphology. Additionally, it was shown that concentrations of ammonium and total nitrogen correlated strongly to the presence of foam-formers. This study provided new insight into the identity of putative foam-forming microorganisms in mesophilic AD systems, allowing for the subsequent surveillance of their abundances and studies of their ecology. Such information will importantly inform the development of control measures for these problematic microorganisms.
In this study, waste activated sludge obtained from two full‐scale treatment plants with foaming issues was pretreated with acid/alkali treatment, acid‐phase fermentation, thermal treatment, ultrasonic treatment and metal salt treatment to investigate their effect on foam mitigation. Waste activated sludge was characterized for foaming index prior and after each pretreatment method. Among all pretreatment methods investigated, acid/alkali treatment and acid phase fermentation showed highest reduction of 53% in foaming and in inactivation of Gordonia amarae filaments. Pretreatment methods that resulted in sharp decrease in the foaming index concurred with higher amounts of dead foam formers as confirmed using live/dead staining and the PMA‐qPCR technique. Pretreating with iron(III)chloride gave good foaming reductions with 54% decrease in foaming potential at a concentration of 260 mg/L but did not result in a decline of foam formers as confirmed by live/dead staining. Ultrasonic treatment did not prove effective in lowering the foaming index or killing the G. amarae filaments.
Sludge digestion process can be optimized applying the methods of sludge disintegration. This article analyses the efficiency of the digestion process of disintegrated sludge by methods of ultrasound or high voltage in comparison to the digestion process of non-disintegrated sludge. The sewage sludge from two Lithuanian wastewater treatment plants as used for the research. Both wastewater treatment plants use analogous sludge treatment technology, in both of them the municipal wastewater is treated and the influence of industry on the quality of sewage is analogous. Due to this it has been assumed that the quality of excess sludge is also analogous. The results showed that both analysed methods of disintegration had the same effect on the increase of specific destruction of volatile solids (VSS) and increase of specific biogas production with the maximum values of 22% and 12–13% respectively. In case of digestion of disintegrated excess activated sludge applying both methods, the maximum methane content in biogas was 71.9%, whereas in case of digestion of non-disintegrated sludge it was 58.3% only. The total operation expenses and capital expenditures of ultrasonic disintegration method are more than 8 times higher comparing to the total expenses and expenditures of high voltage disintegration method.
In this work, the spent liquor of pine sawdust (L1), canola oil waste (L2), olive oil waste (L3) and vineyards waste (L4) from hydrothermal carbonization (220 °C, 1 h) process were studied as substrates for biogas production. Kinetic degradation using three kinetic models (Gompertz model, Hill model and Chapman model) was also analyzed. The batch experiments showed that the highest methane yield (253 NmLCH4/gCODadded) and higher soluble Chemical Oxygen Demand (sCOD) removal efficiency (71%) was achieved for the spent liquor of L1, reaching up to 81% of the theoretical methane yield. The liquid fractions of L2, L3 and L4 resulted in lower yields (24 ̶ 36% of the theoretical yield) and lower sCOD removal (40 ̶ 45%), which can be related to the presence of recalcitrant nitrogen materials formed during the HTC. Gompertz model better represents the performance of the liquor fractions (L1 and L2), having long lag phase (5–7 days), while Chapman model adjusted the accuracy of the behavior of L3 and L4 (lag phase < 1 days). As a conclusion, the spent liquor coming from HTC process of lignocellulosic biomass waste can be used as resource to recover energy through anaerobic digestion.
In this study, the effects of free nitrous acid (FNA) pre-treatment on the rheological properties of digested sludge were investigated at a pilot-scale, along with the improvement in volatile solids (VS) destruction and biogas production. Two pilot-scale anaerobic sludge digesters were operated for one year, one receiving thickened waste activated sludge (TWAS) without pre-treatment (control) and one receiving TWAS pre-treated for 24 h at an FNA concentration of 4.9-6.1 mgN/L (nitrite = 250 mgN/L, pH = 5.0, T = 22-30 °C). The results confirmed the enhancing effect of FNA pre-treatment on methane production (37 ± 1%), consistent with previous laboratory studies. Equally importantly, FNA pre-treatment substantially reduced the shear viscosity of TWAS by 51 ± 8% at 100 s-1 and 49 ± 7% at 250 s-1, likely due to the solubilization of the TWAS (11.1 ± 0.8%). Similarly, FNA pre-treatment also reduced these viscosity parameters of the digested sludge by 80 ± 4% and 78 ± 4%, respectively, caused by both enhanced VS destruction and disintegration of the digested sludge. The dewaterability of digested sludge, assessed by dewatered solids content, capillary suction time and specific resistance to filtration, was not improved by FNA pre-treatment. The polymer requirement for dewatering was reduced by 24 ± 0.6% due to the lower solids concentration in the digested sludge achieved with FNA pre-treatment. The changes to sludge rheological properties revealed in this study further enhances the business case for the FNA pre-treatment technology.
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.
Candidatus Microthrix parvicella has been frequently detected as the dominant filamentous bacteria in bulking sludge and thus seriously affects the stable operation of activated sludge processes. The extremely low growth rate of Ca. M. parvicella and its sensitivity to environmental variations greatly limit the development of effective techniques to control filamentous bulking. Based on previous investigations, a variety of restrictive substrates, operating and culture conditions, environmental factors and other potential inhibitors have varying degrees of impact on the growth of this microorganism. This review systematically summarizes the key factors affecting Ca. M. parvicella growth with a focus on the influencing mechanism. Recent filamentous bulking control strategies are also critically reviewed and discussed. Additionally, research needs for the next few years are proposed with the aim of establishing effective and specific control strategies for filamentous sludge bulking.
The methane productivity and long chain fatty acids (LCFAs) degradation capability of unacclimatized seed sludge (USS) and acclimatized seed sludge (ASS) at different substrate ratios of fats oil and grease (FOG) and mixed sewage sludge were investigated in this study. Biogas produced in ASS in initial phase of anaerobic digestion had higher methane content (65-76%) than that in USS (26-73%). The degradation of major LCFAs in the ASS was 22-80%, 33-191%, and 7-64% higher for the substrate ratios of 100:10, 100:20, and 100:30, respectively, as compared to the LCFAs' degradation in USS. Microbial acclimatization increased the population of Firmicutes (40%), Bacteroidetes (32%), Synergistetes (10%), and Euryarchaeota (8%) in ASS, which supported the faster rate of LCFAs degradation for its later conversion to methane. The significant abundance of Syntrophomonas and Methanosarcina genera in ASS supported faster generation rate of methane in an obligatory syntrophic relationship.
Secondary sludge pre-treatment with free nitrous acid (FNA) has been proven to enhance methane production during anaerobic digestion. However, it is still unclear if the same enhancement can be achieved only using nitrite, without sludge acidification. In this paper, secondary sludge was pre-treated during 5 h with nitrite within the range of 50–250 mg NO2⁻-N/L at neutral pH (6.7). Results obtained from biochemical methane potential tests (BMPs) indicated that sludge pre-treatment at 150 mg NO2⁻-N/L presented the best enhancement of methane production (24% as compared to the control). These conditions were used to pre-treat sludge added in a continuous lab-scale anaerobic digester that operated in parallel to another digester receiving sludge pre-treated with FNA (250 mg NO2⁻-N/L at pH 5.5). Results showed a very similar performance in terms of methane enhancement in both reactors, indicating that sludge acidification is not needed to improve methane yield. A preliminary economic assessment also highlights the need for assessing real chemical costs and national power prices before the implementation of these pre-treatment steps as the associated benefits can significantly change depending on the country where the wastewater treatment plant is located.
The in situ physiology of the filamentous bacterium Microthrix parvicella was investigated in anaerobic-aerobic dynamic phases in activated sludge with focus on the uptake of long chain fatty acids (LCFA) and growth. When 14C-labeled oleic acid was added to activated sludge with an excessive growth of M. parvicella, only little 14C-CO2 was produced under anaerobic conditions while a lot was produced under aerobic conditions. Microautoradiographic studies revealed that M. parvicella took up oleic acid under both anaerobic and aerobic conditions, while only a few floc formers were able to take it up under anaerobic conditions. Extraction and separation of the radioactive biomass into different lipid fractions showed that the oleic acid was stored mainly as neutral lipids under anaerobic conditions, whereas conversion to membrane phospholipids occurred almost exclusively under aerobic conditions, indicating growth. The surface properties of M. parvicella and other bacteria were characterized by hydrophobic fluorescent microspheres, which showed that M. parvicella was relatively hydrophobic. Furthermore, a surface-associated extracellular lipase activity was observed, indicating the ability of M. parvicella to degrade lipids near the filament surface. The results support the hypothesis that uptake and storage of LCFA as lipids under anaerobic conditions provide an effective competition strategy against bacteria that can only take up LCFA under aerobic conditions. Thus, M. parvicella seems to be a specialized lipid consumer with a physiological potential analogous to PAOs and GAOs being able to take up LCFA (but not short chain fatty acids or glucose) under anaerobic conditions and subsequently use the storage material for growth when nitrate or oxygen are available as electron acceptors.
Ozone was applied to return activated sludge in full-scale to study how ozone impacts filamentous bacteria viability (Live/Dead®). Additionally, the ozonated sludges were subjected to anaerobic digestion trials and analysis of micropollutants. Ozone treatment (3-4.8 g O3/kg TSS) improved the settling properties of the sludge by lowering the diluted sludge volume with 7-35%. Ozone inactivated filamentous bacteria outside the floc structures and the fraction of inactivated filaments increased with an increasing ozone dose. It was observed that ozone treatment may act selectively towards different types of filaments. With respect to the two dominating morphotypes present, Type 0041 filamentous bacteria were found to be more resistant to ozone attack than Microthrix parvicella. Thus, higher ozone doses may be required to mitigate sludge bulking caused by Type 0041 filaments. No effects could be discerned by ozone addition on neither the methane production of the sludge nor on the concentrations of micropollutants analysed for this study. The lack of effect on both methane production and micropollutant removal was deemed to be caused by insufficient ozone doses.
Previous work has demonstrated that pre-treatment of waste activated sludge (WAS) with free nitrous acid (FNA i.e. HNO2) enhances the biodegradability of WAS, identified by a 20-50% increase in specific methane production in biochemical methane potential (BMP) tests. This suggests that FNA pre-treatment would enhance the destruction of volatile solids (VS) in an anaerobic sludge digester, and reduce overall sludge disposal costs, provided that the dewaterability of the digested sludge is not negatively affected. This study experimentally evaluates the impact of FNA pre-treatment on the VS destruction in anaerobic sludge digestion and on the dewaterability of digested sludge, using continuously operated bench-scale anaerobic digesters. Pre-treatment of full-scale WAS for 24 h at an FNA concentration of 1.8 mg NN/L enhanced VS destruction by 17 ± 1% (from 29.2 ± 0.9% to 34.2 ± 1.1%) and increased dewaterability (centrifuge test) from 12.4 ± 0.4% to 14.1 ± 0.4%. Supporting the VS destruction data, methane production increased by 16 ± 1%. Biochemical methane potential tests indicated that the final digestate stability was also improved with a lower potential from FNA treated digestate. Further, a 2.1 ± 0.2 log improvement in pathogen reduction was also achieved. With inorganic solids representing 15-22% of the full-scale WAS used, FNA pre-treatment resulted in a 16-17% reduction in the volume of dewatered sludge for final disposal. This results in significantly reduced costs as assessed by economic analysis.
In this study, Illumina sequencing was used for the identification of bulking and foaming bacteria in industrial wastewater treatment plants. The reliable identification of bulking and foaming bacteria represents the first step in developing effective and specific control strategies to avoid disturbances in activated sludge systems. Illumina sequencing revealed 432 16S rRNA operational taxonomic units, representing phylotypes and including 21 bulking and foaming bacteria in the two investigated industrial wastewater treatment plants. Foaming represents the most severe problem in the cascade biology system. Up to 22.5% of all sequencing reads are bulking and foaming bacteria, including Chryseobacterium, Candidatus Microthrix parvicella and Gordonia sp. as the dominant bulking and foaming bacteria which are known for foam formation. Moreover, Illumina sequencing revealed an increase in Candidatus Microthrix parvicella and Gordonia sp. reads from activated sludge to foam and scum samples, indicating a preferred flotation and/or growth advantages in the foam and scum layers. Analyses of the taxonomic assignment and distribution showed that the phylum Actinobacteria is the most dominant phylum, underlining the key role of Actinobacteria in bulking and foaming. Multivariate data analysis was applied, revealing that the dominant bulking and foaming bacteria are positively correlated with the sludge age and influent flow and negatively correlated with the dissolved oxygen level and the temperature. In terms of developing a specific control strategy, the positive linear relationships to the fatty acid and surfactant sludge loadings are highlighted and the removal of lipid compounds from the wastewater influent could avoid an overgrowth of bulking and foaming bacteria.
This study investigates the influence of chemical composition on the biochemical methane potential (BMP) of twelve different batches of fruit and vegetable waste (FVW) with different compositions collected over one year. BMP ranged from 288 to 516 LN CH4 kg VS⁻¹, with significant statistical differences between means, which was explained by variations in the chemical composition over time. BMP was most strongly correlated to lipid content and high calorific values. Multiple linear regression was performed to develop statistical models to more rapidly predict methane potential. Models were analysed that considered chemical compounds and that considered only high calorific value as a single parameter. The best BMP prediction was obtained using the statistical model that included lipid, protein, cellulose, lignin, and high calorific value (HCV), with R² of 92.5%; lignin was negatively correlated to methane production. Because HCV and lipids are strongly correlated, and because HCV can be determined more rapidly than overall chemical composition, HCV may be useful for predicting BMP.
The effectiveness of low free nitrous acid (FNA) pre-treatment times (PTs) (<8 h) on waste activated sludge (WAS) is not known. This study explores the effectiveness of four different FNA concentrations (0, 2.49, 3.55 and 4.62 mg N-HNO2/L) and three low PTs (2, 5 and 8 h) on WAS characteristics and methane generation. Increasing FNA concentrations and PTs resulted in an increase in the solubility of the organic matter (chemical oxygen demand, proteins and polysaccharides). Cell viability was below 15% in all cases at PTs higher than 2 h. Biochemical methane production tests (BMP) showed a significant increase (20–27%) on specific and total methane production (SMP and MP) when the sludge was pretreated with 2.49- and 3.55 mg N-HNO2/L during 5 h and 8 h. Increasing PT (>5 h) resulted in a decrease in MP due to a volatile solid reduction on WAS during the pretreatment. The highest FNA concentration tested (4.62 mg N-HNO2/L) did not further improve MP and SMP. This study clearly shows the effectiveness of the FNA sludge pretreatment at low exposure times.
Phosphorus recovery and solids reduction potential from foam generated in the anoxic reactor of membrane-enhanced biological phosphorus removal system were studied using the microwave-enhanced advanced oxidation process (MW/H2O2-AOP). The MW/H2O2-AOP-treated foam at different experimental conditions showed significant improvement in solubilisation of orthophosphate, ammonia, volatile fatty acids and soluble chemical oxygen demand as well as settling and dewatering ability. The most significant factors maximising orthophosphate release in the order of most to least affecting were (i) microwave heating temperature, (ii) combined effect of microwave heating temperature and hydrogen peroxide addition and (iii) addition of sulfuric acid. At temperature 180°C, pH 3 and hydrogen peroxide dosage 0.5% v/v, nutrient solubilisation, settling, and dewatering properties were at their maximum with 67% of the total phosphate solubilised as orthophosphate.
This study underlines the significance of long chain fatty acid (LCFA) content in wastewater influents as an influencing factor promoting the growth of Candidatus 'Microthrix parvicella' (M. parvicella), the most common filamentous bacteria causing foam in activated sludge systems worldwide. Quantification of M. parvicella by real-time polymerase chain reaction (real-time PCR) and analysis of LCFAs by means of two-dimensional gas chromatography coupled with mass spectrometry (GCxGC/qMS), involving solid phase micro-extraction (SPME) to enhance sensitivity, were combined for the first time as a monitoring tool. The results indicate a highly significant correlation between the abundance of M. parvicella and the total LCFA loading (r = 0.96) and linolenic acid C18:3 (r = 0.98) in particular. Additionally, comparison of slope values for the direct correlations of all significant LCFAs found in the analyses showed that the influence of LCFAs on M. parvicella growth increases with an increasing degree of unsaturation of carbon chains. These findings suggest that by removing lipid compounds from the incoming waters, substrate availability would be limited for M. parvicella.
To improve anaerobic sludge digestion efficiency, the effects of high-pressure homogenization (HPH) conditions on the anaerobic sludge digestion were investigated. The VS and TCOD were significantly removed with the anaerobic digestion, and the VS removal and TCOD removal increased with increasing the homogenization pressure and homogenization cycle number; correspondingly, the accumulative biogas production also increased with increasing the homogenization pressure and homogenization cycle number. The optimal homogenization pressure was 50 MPa for one homogenization cycle and 40 MPa for two homogenization cycles. The SCOD of the sludge supernatant significantly increased with increasing the homogenization pressure and homogenization cycle number due to the sludge disintegration. The relationship between the biogas production and the sludge disintegration showed that the accumulative biogas and methane production were mainly enhanced by the sludge disintegration, which accelerated the anaerobic digestion process and improved the methane content in the biogas.
Anaerobic co-digestion of grass silage, sugar beet tops and oat straw with cow manure was evaluated in semi-continuously fed laboratory continuously stirred tank reactors (CSTRs). Co-digestion of manure and crops was shown to be feasible with feedstock volatile solids (VS) containing up to 40% of crops. The highest specific methane yields of 268, 229 and 213 l CH4 kg−1 VSadded in co-digestion of cow manure with grass, sugar beet tops and straw, respectively, were obtained with 30% of crop in the feedstock, corresponding to 85–105% of the methane potential in the substrates as determined by batch assays. Including 30% of crop in the feedstock increased methane production per digester volume by 16–65% above that obtained from digestion of manure alone. Increasing the proportion of crops further to 40% decreased the specific methane yields by 4–12%, while doubling the loading rate from 2 to 4 kg VS m−3 day−1 decreased the specific methane yields by 16–26%. The post-methanation potential of the digestates corresponded to 0.9–2.5 m3 CH4 t−1 wet weight of digestate and up to 12–31% of total methane production in northern climatic conditions, being highest after co-digestion of manure with straw.
Inhibition of ammonium oxidation and nitrite oxidation by free ammonia (FA) and free nitrous acid (FNA) was studied using three different sludges. An uncompetitive inhibition model fit the experimental data well when the reactions were under FA inhibition, whereas a noncompetitive model fit well under FNA inhibition. The estimates of the inhibition constant (KI) of nitrite oxidation were 46 μM for FA and 1.7–6.8 μM for FNA, each of which was significantly smaller than that of ammonium oxidation, which were 290–1600 μM for FA and 12 μM for FNA. The much smaller values of KI for nitrite oxidation reflected the susceptibility of that reaction to inhibition by FA and FNA, which could lead to accumulation of nitrite during nitrification. A kinetic model for simultaneous inhibition by FA and FNA was derived. The model predicted that nitrite oxidation should be affected more seriously than ammonium oxidation by the simultaneous inhibition, which would accelerate the accumulation of nitrite in a strong nitrogenous wastewater treatment. It also indicated that a complete removal of ammonia could be achieved with high accumulation of nitrite in a sequencing batch reactor, which is impossible in a continuous-flow reactor.
The performance of mesophilic anaerobic digesters of four large Italian wastewater treatment plants without primary sedimentation were studied. Only the waste activated sludge is stabilised by means of the mesophilic (35–37 °C) anaerobic digestion process. The anaerobic digesters generally worked with a hydraulic retention time in a range of 20–40 days and an organic loading rate of some 1 kg VS/ day. The solids content of the sludge fed to the digesters was in the range 2.6–3.9% and the gas produced per kilogram of volatile solids added was in the range 0.07–0.18 m3/kg VSfed. The specific gas production per kilogram of volatile solids destroyed was in the range 0.5–0.9 m3/kg VSdestroyed and the reduction of the volatile solids concentration was in the range 13–27% (average 18%). These figures are particularly significant when designing anaerobic digesters for the treatment of waste activated sludge as single substrate. Moreover, it was observed that the higher the applied solid retention time in the activated sludge process for wastewater treatment, the lower the gas production. In particular, the specific gas production decreased from 0.18 to 0.07 m3/kg VSfed when increasing the solid retention time in the wastewater treatment line from 8 to 35 days. Finally, a mathematical model for the prediction of biogas production on the solid retention time applied in the wastewater treatment process was developed.
The impact of increasing organic load on anaerobic digestion foaming was studied at both full and bench scale. Organic loadings of 1.25, 2.5 and 5 kg VS m(-3) were applied to bench-scale digesters. Foaming was monitored at a full scale digester operated in a comparable organic loading range over 15 months. The bench scale batch studies identified 2.5 kg VS m(-3) as a critical threshold for foam initiation while 5 kg VS m(-3) resulted in persistent foaming. Investigation of a full scale foaming event corroborated the laboratory observation that foaming may be initiated at a loading rate of ≥ 2.5 kg VS m(-3). Experimental findings on foam composition and differences in the quality characteristics between foaming and non-foaming sludges indicated that foam initiation derived from the combined effect of the liquid and gas phases inside a digester and that the solids/biomass ultimately stabilized foaming.
Waste activated sludge (WAS) requires a long digestion time because of a rate-limiting hydrolysis step - the first phase of anaerobic digestion. Pretreatment of WAS facilitates the hydrolysis step and improves the digestibility. This study examined the effects of ultrasonic, chemical, and combined chemical-ultrasonic pretreatments on WAS disintegration and its subsequent digestion at different solids retention times (SRTs). The efficient conditions for each pretreatment were evaluated based on per cent soluble chemical oxygen demand (%SCOD). The results showed that the combined chemical-ultrasonic pretreatment resulted in better WAS disintegration, based on %SCOD release, compared with individual chemical and ultrasonic pretreatments. At the optimum operating conditions of the combined chemical-ultrasonic pretreatment (NaOH dose of 10 mg g(-1) TS (total solids) and specific energy input of 3.8 kJ g(-1)TS), the %SCOD release was 18.1% +/- 0.5%, whereas 13.5% +/- 0.9%, 13.0% +/- 0.5% and 1.1% +/- 0.1% corresponded to individual chemical (50 mg g(-1) TS) and ultrasonic (3.8 kJ g(-1) TS) pretreatments and control (without pretreatment), respectively. The anaerobic digestion studies in continuous stirred tank reactors showed an increase in methane production of 23.4% +/- 1.3% and 31.1 +/- 1.2% for digesters fed with WAS pretreated with ultrasonic and combined chemical-ultrasonic, respectively, with respect to controls at the effective SRT of 15 days. The highest total solids removal was achieved in the digester fed with ultrasonic pretreated WAS (16.6% +/- 0.3%), whereas the highest volatile solids removal was achieved from the digester fed with combined chemical-ultrasonic pretreated WAS (24.8 +/- 0.4%). The findings from this study are a useful contribution to new pretreatment techniques in the field of sludge treatment technology through anaerobic digestion.
This study was to develop a simple and reliable method for quantifying Microthrix parvicella 16S rRNA gene copies and its application to activated sludge samples collected from wastewater treatment plants (WWTP) with and without foaming problems.
The relative frequency of M. parvicella was determined by combining real-time PCR assays for quantification of total bacterial 16S rRNA gene copies and M. parvicella 16S rRNA gene copies. The developed method was applied to analyse 32 activated sludge samples obtained from German WWTP. The level of M. parvicella 16S rRNA gene copies in the 18 nonfoaming samples was below 3% of the total number of 16S rRNA gene copies and in the range of 0-18% for the 14 foaming samples.
The described method allows reliable monitoring of the amount of M. parvicella in activated sludge samples.
The described method may become an important component of a warning system for forthcoming bulking and foaming episodes.
Minireview Waste lipids to energy : how to optimize methane production from long-chain fatty acids (LCFA)
- Alves
Alves, M.M., Pereira, M.A., Sousa, D.Z., Cavaleiro, A.J., Picavet, M., Smidt, H., Stams, A.
J.M., 2009. Minireview Waste lipids to energy : how to optimize methane production
from long-chain fatty acids (LCFA). Microbal. Biotechnol. 2, 538-550. https://doi.
org/10.1111/j.1751-7915.2009.00100.x.
Standard Methods for the Examination of Water and Wastewater 23rd edition
- R B Baird
- A D Eaton
- E W Rice
Baird, R.B., Eaton, A.D., Rice, E.W., 2013. Standard Methods for the Examination of
Water and Wastewater 23rd edition, American Public Health Association, American
Water Works Association, Water Environment Federatioon. 10.1016/B978-0-12-382165-2.00237-3.
Identification and control of filamentous micro-organisms in industrial wastewater treatment plants
- D H Eikelboom
- V Tandoi
- J Krooneman
- A Borger
- K Thelen
- C Kragelund
- P H Nielsen
Eikelboom, D.H., Tandoi, V., Krooneman, J., Borger, A., Thelen, K., Kragelund, C.,
Nielsen, P.H., 2006. Identification and control of filamentous micro-organisms in
industrial wastewater treatment plants, IWA Publishing.
Identification of microorganisms responsible for foam formation in mesophilic
- C Jiang
- S J Mcilroy
- R Qi
- F Petriglieri
- E Yashiro
- Z Kondrotaite
- P H Nielsen
Jiang, C., McIlroy, S.J., Qi, R., Petriglieri, F., Yashiro, E., Kondrotaite, Z., Nielsen, P.H.,
2021. Identification of microorganisms responsible for foam formation in mesophilic
anaerobic digesters treating surplus activated sludge
anaerobic digesters treating surplus activated sludge. Water Res. 191, 116779.
https://doi.org/10.1016/j.watres.2020.116779.
The intensification of sewage sludge anaerobic digestion by partial disintegration of surplus activated sludge and foam, in: Integration and Optimization of Sanitation Systems
- A Machnicka
- J Suschka
- K Grűbel
Machnicka, A., Suschka, J., Grűbel, K., 2005. The intensification of sewage sludge
anaerobic digestion by partial disintegration of surplus activated sludge and foam,
in: Integration and Optimization of Sanitation Systems In Urban Areas: Proceedings of
Polish-Swedish Seminars, Cracow, March 17-18. pp. 87-94.