Application and improvement methods of sludge alkaline fermentation liquid as a carbon source for biological nutrient removal: A review

Alkaline fermentation can reduce the amount of waste activated sludge and prepare sludge alkaline fermentation liquid (SAFL) rich in short-chain fatty acids (SCFAs), which can be used as a high-quality carbon source for the biological nutrient removal (BNR) process. This review compiles the production method of SAFL and the progress of its application as a BNR carbon source. Compared with traditional carbon sources, SAFL has the advantages of higher efficiency and economy, and different operating conditions can influence the yield and structure of SCFAs in SAFL. SAFL can significantly improve the nutrient removal efficiency of the BNR process. Taking SAFL as the internal carbon source of BNR can simultaneously solve the problem of carbon source shortage and sludge treatment difficulties in wastewater treatment plants, and further reduce the operating cost. However, the alkaline fermentation process results in many refractory organics, ammonia and phosphate in SAFL, which reduces the availability of SAFL as a carbon source. Purifying SCFAs by removing nitrogen and phosphorus, directly extracting SCFAs, or increasing the amount of SCFAs in SAFL by co-fermentation or combining with other pretreatment methods, etc., are effective measures to improve the availability of SAFL.

Comparison of primary and secondary sludge carbon sources derived from hydrolysis or acidogenesis for nitrate reduction and denitrification kinetics: Organics utilization and microbial community shift

Seeking available and economical carbon sources for denitrification process is an intractable issue for wastewater treatment. However, no study compared different types of waste sludge as carbon source from denitrification mechanism, organics utilization and microbial community aspects. In this study, primary and secondary sludge were pretreated by thermophilic bacteria (TB), and its hydrolysis or acidogenic liquid were prepared as carbon sources for denitrification. At C/N of 8-3, the variations of NO₃^--N and NO₂^--N were profiled in typical cycles and denitrification kinetics was analyzed. Primary sludge achieved a competitive NO_X-N removal efficiency with less dosage than secondary sludge. Fourier transform infrared (FTIR) spectroscopy was introduced to analyze organic composition from functional-group perspective and the utilization of organic matters in different sludge carbon sources was investigated. To further analyze the microbial community shift in different denitrification systems, high-throughput sequencing technology was applied. Results showed that denitrifier Thauera, belonging to Proteobacteria, was predominant, and primary sludge acidogenic liquid enriched Thauera most intensively with relative abundance of 47.3%.

Woodchips bioretention column for stormwater treatment: Nitrogen removal performance, carbon source and microbial community analysis

This study chose Oak woodchips and gravel as media filter to enhance the denitrification in the bioretention system (saturated zone 7.7 L) treating synthetic stormwater runoff. It revealed that the denitrification process mainly occurred during the drying phase and enlarging volume of saturated zones to retain more stormwater during storm event was the direct method to promote nitrogen removal of the bioretention system. Nevertheless, it was noted that the nitrogen and dissolved organic carbon would be released into the effluent during the wetting period. The denitrification rate with different nitrate nitrogen (NO₃-N) concentrations did not show the obvious change with zero order kinetics constant of 2.91 mg/L∙d on average. Furthermore, it confirmed that woodchips were degraded and converted to volatile fatty acids (VFAs), especially acetic acid as carbon source, further utilized by the denitrifying bacteria, such as Dechloromonas, Acidoborax, Pseudomonas, Denitratisoma and Acinetobacter. In addition, genera of Lachnospiraceae and Lactobacillus, which had the ability to degrade the macromolecular organic components into low molecular VFAs, were observed in the woodchips bioretention system.

Effects of chemical oxygen demand concentration, pH and operation cycle on polyhydroxyalkanoates synthesis with waste sludge

To reduce the cost of polyhydroxyalkanoate (PHA) production and dispose the amount of waste sludge simultaneously, chemical oxygen demand (COD) concentration, pH and operation cycle were investigated to find the optimal PHA synthesis conditions with waste sludge in this study. The maximum PHA content (31.3% of the cell dry weight (CDW)), as well as the highest PHA conversion rate (0.30 mg COD/mg COD) and PHA-specific synthesis rate (6.12 mg COD/mg CDW·h), was achieved with initial COD concentration, pH value and operation cycle: 6000 mg/L, 8.5 and 24 h. In order to further investigate the process of PHA synthesis, COD removal rate and CDW were also introduced. This study could provide valuable information for increasing the production of PHA with waste sludge.

Research on the enhanced biological nitrogen removal of wastewater by the ultrasound-hydrolysis acidification of excess sludge

To simultaneously improve the removal of nitrogen and phosphorus from wastewater with a low C/N ratio and reduce excess sludge production, in this paper, excess sludge ultrasound-hydrolysis acidification (UHA) pretreatment was coupled with the anaerobic-anoxic-oxic (AAO) process to provide carbon source and enhance biological nitrogen removal performance, and the experimental results can be summarized as follows. First, the total nitrogen (TN) concentrations in the effluent of the system decreased from 16.94 mg/L to 5.74 mg/L, and the removal rate of TN increased by 25.5%. In addition, the concentrations for ammonia nitrogen (NH₃ -N) in the system decreased 12.59 mg/L, and the removal rate of this index increased by 29.0%. Furthermore, the specific oxygen uptake rate (SOUR) in the anoxic zone increased significantly because the application of UHA products enhanced the microbial activity, and the addition of UHA products had an effect on the microbial community structure in the system. The amounts of denitrifying bacteria such as Betaproteobacteria and Alphaproteobacteria also increased, which enhanced the nitrogen removal efficiency of wastewater biological treatment. PRACTITIONER POINTS: Treatment of excess sludge in UHA device as an additional carbon source. Nitrogen removal efficiency was greatly improved after adding UHA products. Input of UHA products enhanced microbial activity in AAO system. Denitrifying bacteria increased with the addition of UHA products.

Characteristics of dissolved organic nitrogen in effluent from a biological nitrogen removal process using sludge alkaline fermentation liquid as an external carbon source

The addition of sludge alkaline fermentation liquid (SAFL) to biological nutrient removal (BNR) processes has been widely shown to reduce the dissolved inorganic nitrogen (DIN) in the effluent. However, knowledge regarding the effect of using SAFL in a BNR as an additional carbon source on the characteristics of dissolved organic nitrogen (DON) in the effluent is limited. This study investigated the effect of SAFL addition on effluent DON features and microbial community dynamics in a BNR process treating municipal wastewater. The performance of SAFL was compared with other two reactors (i.e., without an external carbon source and with the addition of sodium acetate). The results showed that the addition of SAFL can significantly reduce effluent DON (p < 0.05). The effluent DON was slightly higher with SAFL than with sodium acetate, but the bioavailable DON of the two reactors was similar (1.06 ± 0.11 vs 1.04 ± 0.12 mg/L, respectively, p > 0.05). The SAFL addition led to a decreased percentage of low molecular weight DON (p < 0.05) as well as an increased ratio of fulvic-like and humic-like substances to proteins-like substances. Moreover, the SAFL addition resulted in a lower percentage of substances resembling proteins/amino sugars and a higher percentage of lignin-like molecules than sodium acetate. These features accounted for the low DON bioavailability. The SAFL promoted the increased abundance of Bacteroidetes, Chloroflexi, Comamonadaceae and Rhodocyclaceae, which could be associated with the decreased effluent DON and its bioavailability. This study indicates that using SAFL as a BNR carbon source not only improves the removal of DIN but also reduces effluent DON and specifically DON bioavailability. These results facilitate the acquisition of comprehensive knowledge regarding the use of SAFL as an alternative external carbon source in the BNR process.

Effect of magnetic powder on denitrification using the sludge alkaline fermentation liquid as a carbon source

This work evaluates the impact of the different concentrations of Fe₃O₄ on nitrate removal and organic matters utilization in the sequencing batch reactors (SBRs) using the sludge alkaline digestion supernatant as external sludge carbon source. Results indicated that the optimal concentration of Fe₃O₄ was 1 g/L for enhancing denitrification with NO₃^--N removal efficiency of 93.13% (up to a 11.93% increase) and without NO₂^--N accumulation after 18 days. The changes of soluble chemical oxygen demand (SCOD), protein, and carbohydrate during denitrification process were analyzed to gauge the utilization of sludge fermentation products by denitrifiers. The SCOD was consumed for organisms involved in NO₃^--N removal and the Fe₃O₄ could promote the utilization of carbohydrate better than protein by denitrifiers during denitrification process. Denitrification rate (V_DN) and the nitrate-to-nitrite transformation ratio (NTR), as the kinetics parameters, were also investigated in different concentrations of Fe₃O₄.

Characterization of the refractory dissolved organic matters (rDOM) in sludge alkaline fermentation liquid driven denitrification: Effect of HRT on their fate and transformation

Enhanced biological denitrification for nitrogen removal using sludge alkaline fermentation liquid (SAFL) as an alternative carbon source has been widely reported in previous studies, while limited studies focused on the degradation of the organics presented in SAFL. In this study, an SAFL driven anoxic denitrification sequencing batch reactor (SBR) was established, the mechanism of organics utilization was characterized and the refractory dissolved organic matters (rDOM) was identified. Denitrification could rapidly proceed with the presence of volatile fatty acids (VFAs) initially, while the denitrification rate largely decreased after the VFAs depleted. A great deal of rDOM, which was hard to be utilized by denitrifying microorganism, was found in the effluent. A prolonged hydraulic retention time (HRT) led to the further transformation of particles and colloids to smaller colloids and soluble organics. Extended HRT promoted the degradation of soluble microbial by-product (SMP), but had minor effect on the removal of humic-like, and fulvic acid-like substances. The characterization of the effluent demonstrated the building blocks, were dominated in the rDOM (43.79%-48.78%), followed by high molecular weight protein (HMW-PN) (13.37%-17.39%), HMW polysaccharide (HMW-PS) (12.84%-15.9%), low molecular weight (LMW) neutrals (11.28%-13.65%), and hydrophobic dissolved organic carbon (HO-DOC) (8.0%-12.62%). Moreover, it was found that the building blocks were relatively easy to be degraded with the extension of HRTs, followed by LMW-PS, LMW-PN, LMW neutrals, HMW-PN, and HMW-PS. However, further extended HRT >24 h could not improve the removal of building blocks, LMW-PS and LMW neutrals. This study, for the first time, provided insights into the transformation of organic matters produced by SAFL in a denitrification system and acted as a guide for the subsequent advanced treatment.

The cotreatment of landfill leachate and high-nitrate wastewater using SBRs: evaluation of denitrification performance and microbial analysis

Resourceful disposal of landfill leachate has always been an intractable worldwide problem. This study was conducted to investigate the feasibility of biologically treating a combined waste stream of landfill leachate and high-concentration nitrate nitrogen (high-nitrate) wastewater. Raw landfill leachate was pretreated using anaerobic fermentation and ammonia stripping to improve biodegradability. The control sequencing batch reactor (SBR, named R0) was fed only with synthetic high-nitrate wastewater with sodium acetate as the carbon source, whereas the other experimental SBR (named R1) was loaded with mixtures containing leachates. Excessive increase in leachate adversely affected the cotreatment, and it was concluded that the landfill leachate volume ratio should never exceed 7.5% of the total wastewater (14% of the initial COD) based on further batch experiments. The maximum specific denitrification rate of 58.05 mg NO₃⁻-N (gVSS h)⁻¹ was attained in R1, while that of 32.32 mg NO₃⁻-N (gVSS h)⁻¹ was obtained in R0. Illumina MiSeq sequencing revealed that adding landfill leachate did not change the fact that Pseudomonas, Thauera, and Pannonibacter dominant in the sodium acetate supported the denitrification systems, but led to the adjustment of their relative abundance. Moreover, the narG, nirK, nirS, and norB denitrifying genes exhibited increased abundance by 138–980% in the cotreated system, which was confirmed by q-PCR analyses. These findings reveal that the denitrification efficiency of activated sludge in SBR cotreated with landfill leachate and high-nitrate wastewater significantly improved, and this may contribute toward the understanding of the molecular mechanisms of biological denitrification under the blending treatment of leachate and high-nitrate wastewater.

Resourceful disposal of landfill leachate has always been an intractable worldwide problem.

Optimization of polyhydroxyalkanoates (PHA) synthesis with heat pretreated waste sludge

To reduce the cost of polyhydroxyalkanoates (PHA) production and disposal amount of waste sludge simultaneously, the feasibility of using different heat pretreated sludge (60 °C, 80 °C, 100 °C, 120 °C) as external carbon source to synthesize PHA was examined in this study. The maximal PHA accumulation (24.1% of the dry cell weight) was achieved with 60 °C pretreated waste sludge, with the utilization efficiency of COD, proteins, carbohydrate and VFAs were 74.3%, 82.3%,47.2%,81.4%, respectively. Both of VFAs and non VFAs organics could be used as carbon source for PHA synthesis. The results of kinetic parameter analysis showed that the highest PHA production rate (0.23 mg COD/mg X·h) and the PHA conversion rate (0.46 mg COD/mg COD) all occurred when using 60 °C pretreated waste sludge. In order to further investigate the utilization of sludge carbon source for PHA synthesis, the three-dimensional fluorescence excitation-emission matrix (EEM) spectroscopy with fluorescence regional integration (FRI) analysis were introduced.

Enhancing denitrification with waste sludge carbon source: the substrate metabolism process and mechanisms

Using waste sludge internal carbon source for nitrogen removal in wastewater has drawn much attention, due to its economic advantages and sludge reduction. In this study, the performance of enhanced denitrification with waste sludge thermal hydrolysate and fermentation liquid as carbon sources at different SCOD/N (soluble chemical oxygen demand/NO₃^--N) was investigated. The optimum SCOD/N was 8 for sludge thermal hydrolysate and 7 for fermentation liquid, with NO₃^--N removal efficiency of 92.3 and 98.9%, respectively, and no NO₂^--N accumulation. To further understand the fate of sludge carbon source during denitrification, the changes of SCOD, proteins, carbohydrates, and volatile fatty acids (VFAs) were analyzed, and three-dimensional fluorescence excitation-emission matrix (EEM) spectroscopy with fluorescence regional integration (FRI) analysis was introduced. The utilization of SCOD was consistent with NO₃^--N reduction, and the utilization efficiency of different organic matter was as follows: VFAs > proteins > carbohydrates. The soluble organic-like materials (region IV) were the most readily utilized organic matter according to three-dimensional fluorescence EEM spectroscopy. Regarding denitrification mechanisms, the denitrification rate (V_DN), denitrification potential (P_DN), heterotroph anoxic yield (Y_H), and the most readily biodegradable COD (S_S) were also investigated.

Effects of hydraulic retention time (HRT) on denitrification using waste activated sludge thermal hydrolysis liquid and acidogenic liquid as carbon sources

Waste activated sludge (WAS) internal carbon source can efficiently and economically enhance denitrification, and hydraulic retention time (HRT) is one of the most important operational parameters for denitrification. The effects of HRT on denitrification were investigated with WAS thermal hydrolysis liquid and acidogenic liquid as carbon sources in this study. The optimal HRT was 12h for thermal hydrolysis liquid and 8h for acidogenic liquid, with NO₃^--N removal efficiency of 91.0% and 97.6%, respectively. In order to investigate the utilization of sludge carbon source by denitrifier, the changes of SCOD (Soluble chemical oxygen demand), proteins, carbohydrates, and VFAs (Volatile fatty acids) during denitrification process were analyzed and three-dimensional fluorescence excitation-emission matrix (EEM) spectroscopy with fluorescence regional integration (FRI) analysis was introduced. The kinetics parameters of denitrification rate (V_DN), denitrification potential (P_DN) and heterotroph anoxic yield (Y_H) were also investigated using sludge carbon source at different HRT.