The effect of operational parameters of the process of sludge ozonation on the solubilisation of organic and nitrogenous compounds

Towards zero excess sludge discharge with built-in ozonation for wastewater biological treatment

In this study, a biological treatment process, which used a built-in ozonation bypass to achieve sludge reduction, was built to treat the industrial antifreeze production wastewater (mainly composed of ethylene glycol). The results indicated there is a positive correlation between ozone dosage and sludge reduction. At the laboratory level, the MLSS in the system can be stably controlled at around 3400 mg MLSS L-1 under the dosage of 0.18 g O3 g-1 MLSS. Ozonation can increase the compactness of sludge flocs (fractal dimension increased from 1.89 to 1.92). Ozone destroys microbial cell membranes and alters the structure of sludge flocs through direct oxidation through electrophilic reactions. It leads to the release of intracellular polysaccharides, proteins, and other biological macromolecules in microorganisms, thereby promoting the implicit growth of microbial populations. Some bacteria such as g_Pseudomonas, g_Gemmobacter, etc. have strong ethylene glycol degradation ability and tolerance to ozonation. The removal of ethylene glycol includes the glyoxylate cycle, glycine serine carbon cycle, and the glutamate-cysteine ligase pathway of assimilation. Gene KatG and gpx may be key factors in improving microbial tolerance to ozonation. The comprehensive evaluation from the perspectives of cost and carbon emission shows that choosing ozone cracking-implicit growth in wastewater treatment systems has significant cost advantages and application value.

In-situ sludge reduction based on Mn2+-catalytic ozonation conditioning: Feasibility study and microbial mechanisms

To improve the sludge conditioning efficiency without increasing the ozone dose, an in-situ sludge reduction process based on Mn2+-catalytic ozonation conditioning was proposed. Using ozone conditioning alone as a control, a lab-scale sequencing batch reactor coupled with ozonated sludge recycle was evaluated for its operating performance at an ozone dose of 75 mg O3/g VSS and 1.5 mmol/L Mn2+ addition. The results showed a 39.4% reduction in MLSS and an observed sludge yield of 0.236 kg MLSS/kg COD for the O3+Mn2+ group compared to the O3 group (15.3% and 0.292 kg MLSS/kg COD), accompanied by better COD, NH4+-N, TN and TP removal, improved effluent SS and limited impact on excess sludge properties. Subsequently, activity tests, BIOLOG ECO microplates and 16S rRNA sequencing were applied to elucidate the changing mechanisms of Mn2+-catalytic ozonation related to microbial action: (1) Dehydrogenase activity reached a higher peak. (2) Microbial utilization of total carbon sources had an elevated effect, up to approximately 18%, and metabolic levels of six carbon sources were also increased, especially for sugars and amino acids most pronounced. (3) The abundance of Defluviicoccus under the phylum Proteobacteria was enhanced to 12.0% and dominated in the sludge, they had strong hydrolytic activity and metabolic capacity. Denitrifying bacteria of the genus Ferruginibacter also showed an abundance of 7.6%, they contributed to the solubilization and reduction of sludge biomass. These results could guide researchers to further reduce ozonation conditioning costs, improve sludge management and provide theoretical support.

Techno-Economic Evaluation of Ozone Application to Reduce Sludge Production in Small Urban WWTPs

In Chile, small wastewater treatment plants (WWTPs) (treatment capacity of less than 4,800 m3/d) are normally not designed with consideration for the potential valorization of generated sludge. For this reason, they are generally operated at high solids residence times (SRT) (15 d) to promote the decay of biomass, promoting less sludge production and reducing the costs associated with biomass management. Operation at high SRT implies the need for a larger activated sludge system, increasing capital costs. The implementation of a sludge-disintegration unit by ozonation in future WWTPs could enable operation at an SRT of 3 d, with low sludge generation. In this work, we evaluate how the implementation of a sludge-ozonation system in small WWTPs (200–4000 m3/d) would affect treatment costs. Four scenarios were studied: (1) a current WWTP operated at an SRT of 15 d, without a sludge ozonation system; (2) a WWTP operated at an SRT of 15 d, with a sludge-ozonation system that would achieve zero sludge production; (3) a WWTP operated at an SRT of 3 d, with a sludge-ozonation system that would provide the same sludge production as scenario 1; (4) a WWTP operated at an SRT of 15 d, with a sludge-ozonation system that would achieve zero sludge production. Economic analysis shows that the treatment costs for scenarios 1 and 2 are similar, while a reduction in cost of up to 47% is obtained for scenarios 3 and 4.

Partial ozonation of returned sludge via high-concentration ozone to reduce excess sludge production: A pilot study

Partial ozonation of returned sludge via high and low concentration of ozone were compared to evaluate their efficiency in excess sludge production reduction. A pilot-scale system of anaerobic/anoxic/oxic (A/A/O) + ozonated sludge recycle (OSR) process was operated for 97 days, to investigate the effects of different ozone concentration (380 mg/L and 150 mg/L) on the nutrient removal capacity, sludge reduction rate, the excess sludge properties including settling, dewatering and anaerobic digestion (AD) performance. It was found that at the same total ozone dosage (13 mg/g MLSS, 25 mg/g MLVSS), the ozone of 380 mg/L achieved much higher organic matters and total excess sludge reduction (41.6% and 25.9%) than 150 mg/L applied (31.0% and 18.2%). It also laid less deterioration effect on the effluent quality and had better nutrient (COD, NH₄⁺-N, TN) removal capacity than 150 mg/L applied. Meanwhile, little difference was found in the settling, dewatering and AD properties of excess sludge from the two A/A/O + OSR processes. Meanwhile, sludge solubilization rate, BIOLOG ECO microplate, 16S rRNA sequencing were applied comprehensively to illustrate the reasons for above advantages of the elevated ozone dosage applied. It was clarified that compared to 150 mg/L, A/A/O + OSR with ozone of 380 mg/L had higher sludge solubilization rate, less impact on bacterial community distribution and utilization capacity of carbon sources in bioreactors.

Waste sludge disintegration, methanogenesis and final disposal via various pretreatments: Comparison of performance and effectiveness

This study compared the three wastewater pretreatments of ozonation, Fe²⁺-S₂O₈ ^2- and freeze-thawing (F/T) in the disintegration, anaerobic digestion (AD) and final disposal of the sludge. The F/T pretreatment increased the dewaterability and settleability of the sludge by 7.8% and 47.1%, respectively. The ozonation pretreatment formed more volatile fatty acids (VFAs), with a peak value of 320.82 mg SCOD/L and controlled the release of sulfides. The Fe²⁺-S₂O₈ ^2- pretreatment removed heavy metals through the absorption and flocculation of ferric particles formed in-situ. During the anaerobic digestion of the sludge, the ozonation pretreatment accelerated the hydrolysis rate (k) rather than the biochemical methane potential (B₀) of the sludge due to the high VFA content in the supernatant. Comparatively, the F/T pretreatment facilitated the B₀ with great economic efficiency by enhancing the solubilisation of the sludge. Although Fe²⁺-S₂O₈ ^2- pretreatment decreased the methane production, the ferric particle was a unique advantage in the disintegration and harmless disposal of the sludge. The digested sludge had more VFAs after ozonation pretreatment, which contributed to the recycling of carbon. In addition, the lower sludge volume could save the expense of transportation and disposal by ozonation pretreatment. Different pretreatments had different characteristics. The comparative study provided information allowing the selection of the type of pretreatment to achieve different objectives of the treatment and disposal of sludge.

Excess sludge disintegration by discharge plasma oxidation: Efficiency and underlying mechanisms

A huge amount of excess sludge is inevitably produced in wastewater treatment, and it is becoming more and more urgent to realize efficient sludge reduction. Discharge plasma oxidation was used to efficiently disintegrate excess sludge for sludge reduction in this study. Approximately 18.22% sludge disintegration and 27.8% reduction of total suspended solids (TSS) were achieved by discharge plasma treatment. The water content of the filter cake decreased from 81.9% to 76.0% and the bound water content decreased from 2.66 g/g dry solid to 0.73 g/g dry solid after treatment. The large quantities of reactive oxygen species (ROS) generated by discharge plasma played important roles in sludge disintegration by destroying flocs and promoting the transformation of organic substances. Concurrent cell lysis induced by ROS oxidation released intracellular organics and water into the liquid phase. The fraction of soluble extracellular polymer substances (S-EPS) was enhanced from 16.10% to 58.51%, whereas the tightly bound fraction was reduced from 70.62% to 28.91%. Migration and decomposition of EPS were the main processes for EPS changing at a low oxidation capacity, whereas cell lysis became important at a high oxidation capacity. In summary, the plasma treatment effectively improved sludge disintegration.

Degradation and Deactivation of Bacterial Antibiotic Resistance Genes during Exposure to Free Chlorine, Monochloramine, Chlorine Dioxide, Ozone, Ultraviolet Light, and Hydroxyl Radical

This work investigated degradation (measured by qPCR) and biological deactivation (measured by culture-based natural transformation) of extra- and intracellular antibiotic resistance genes (eARGs and iARGs) by free available chlorine (FAC), NH₂Cl, O₃, ClO₂, and UV light (254 nm), and of eARGs by ^•OH, using a chromosomal ARG ( blt) of multidrug-resistant Bacillus subtilis 1A189. Rate constants for degradation of four 266-1017 bp amplicons adjacent to or encompassing the acfA mutation enabling blt overexpression increased in proportion to #AT+GC bps/amplicon, or in proportion to #5'-GG-3' or 5'-TT-3' doublets/amplicon, with respective values ranging from 0.59 to 2.3 (×10¹¹ M^-1 s^-1) for ^•OH, 1.8-6.9 (×10⁴ M^-1 s^-1) for O₃, 3.9-9.2 (×10³ M^-1 s^-1) for FAC, 0.35-1.2(×10¹ M^-1 s^-1) for ClO₂, and 2.0-8.8 (×10^-2 cm²/mJ) for UV at pH 7, and from 1.7-4.4 M^-1 s^-1 for NH₂Cl at pH 8. For FAC, NH₂Cl, O₃, ClO₂, and UV, ARG deactivation paralleled degradation of amplicons approximating a ∼800-1000 bp acfA-flanking sequence required for natural transformation in B. subtilis, whereas deactivation outpaced degradation for ^•OH. At practical disinfectant exposures, eARGs and iARGs were ≥90% degraded/deactivated by FAC, O₃, and UV, but recalcitrant to NH₂Cl and ClO₂. iARG degradation/ deactivation always lagged cell inactivation. These findings provide a quantitative framework for evaluating ARG fate during disinfection/oxidation, and support using qPCR as a proxy for tracking ARG deactivation under carefully selected circumstances.

A new approach for excess sludge reduction by manganese dioxide oxidation: performance, kinetics, and mechanism studies

A considerable amount of excess sludge, a kind of hazardous waste, is produced from the conventional wastewater treatment systems such as activated sludge process, and efficient sludge reduction processes are needed. A new chemical method for sludge reduction was proposed by using manganese dioxide as oxidant in this study. A favorable condition for sludge reduction is determined as manganese dioxide dosage of 0.165 g g^-1 wet sludge, sulfuric acid concentration of 3 mol L^-1, and reduction temperature of 90 °C for 90 min, where the sludge reduction efficiency can reach 73.30%. Reaction kinetic study revealed that the sludge reduction rate was controlled by the surface chemical reaction and the reaction followed a shrinking core kinetic model with apparent activation energy of 37.76 kJ mol^-1. Furthermore, reaction process analysis indicated that the sludge hydrolysis included two steps, i.e., floc destruction and microbial cell disruption. Considering the high efficiency and short treatment time, manganese dioxide oxidation is suggested to be a feasible method for disintegration of excess activated sludge.

Release of phosphorus from sewage sludge during ozonation and removal by magnesium ammonium phosphate

The release rule of phosphorus from sewage sludge during ozonation and removal by the magnesium ammonium phosphate (MAP) method were investigated. The results showed that the concentrations of total phosphorus in aqueous phase (TP_(A)) and orthophosphate (PO₄^3--P) in ozonized sludge supernatant rose obviously with increasing ozone dose when ozone dose was below 61.2 mg O₃/gSS then almost kept constant. The TP_(A) and PO₄^3--P contents in the ozonized sludge supernatant were 70.9 and 63.3 mg/L when ozone dose was 61.2 mg O₃/gSS, respectively. Total phosphorus in the sludge solid (TP_(S)) was mostly distributed in inorganic phosphorus (IP) (more than 81.5% of TP_(S)), and non-apatite inorganic phosphorus (NAIP) was the major component of IP in the sludge (more than 78.7% of IP) during ozonation. The release contribution (RC) of IP to TP_(A) accounted for over 73.9%. The optimized conditions for the removal of phosphorus from ozonized sludge supernatant were set at an initial Mg²⁺/PO₄^3--P molar ratio of 1.8, pH 9.5, and reaction time of 5 min, under which the removal efficiencies of TP_(A) and PO₄^3--P were 43.1 and 52.2%, respectively.

Holistic sludge management through ozonation: A critical review

This paper critically reviews the multidimensional benefits of ozonation in wastewater treatment plants. These benefits include sludge reduction, removal of emerging trace organic contaminants (TrOC) from wastewater and sludge, and resource recovery from sludge. Literature shows that ozonation leads to sludge solubilisation, reducing overall biomass yield. Sludge solubilisation is primarily influenced by ozone dosage, which, in turn, depends on the fraction of ozonated sludge, ozone concentration, and sludge concentration. Additionally, sludge ozonation facilitates the removal of TrOCs from wastewater. On the other hand, by inducing cell lysis, ozonation increases the chemical oxygen demand (COD) and nutrient concentration of the sludge supernatant, which deteriorates effluent quality. This issue can be resolved by implementing resource recovery. Thus far, successful retrieval of phosphorous from ozonated sludge supernatant has been performed. The recovery of phosphorous and other resources from sludge could help offset the operation cost of ozonation, and give greater incentive for wastewater treatment plants to adapt this approach.

Interpreting the synergistic effect in combined ultrasonication-ozonation sewage sludge pre-treatment

The sequential combination of ultrasonication and ozonation as sewage sludge treatment prior to anaerobic digestion was investigated. Synergistic volatile suspended solids (VSS) solubilization was observed when low energy ultrasonication (⩽12kJg(-1) TS) was followed by ozonation. 0.048gO3g(-1) TS ozonation induced the maximum VSS solubilization of 41.3% when the sludge was pre-ultrasonicated at 9kJg(-1) TS; while, the same ozone dosage applied without prior ultrasonication only induced 21.1% VSS solubilization. High molecular weight (MW) components (MW>500kDa) were found to be the main solubilization products when sludge was only ozonated. However, solubilization products by ozone were mainly in the form of low MW components (MW<27kDa) when sludge was pre-ultrasonicated. The high MW products generated by ultrasound were effectively degraded in the subsequent ozonation. Anaerobic biodegradability increased by 34.7% when ultrasonication (9kJg(-1) TS) and ozonation (0.036gO3g(-1) TS) were combined sequentially. The maximum methane production rate increased from 3.53 to 4.32, 4.21 and 4.54mL CH4d(-1) after ultrasonication, ozonation and ultrasonication-ozonation pre-treatments, respectively.

The process of activated sludge ozonation: effect of ozone on cells, flocs, macromolecules and nutrient release

The purpose of this research is to investigate the activated sludge ozonation process. Results revealed that bacteria destruction and cell solubilization were not obvious when ozone dose was lower than 11 mgO₃/gMLSS (MLSS: mixed liquor suspended solids), while pores appeared on the sludge and bacterial disappeared from floc surface. In the range of 11-90 mgO₃/gMLSS, ozone had significant effect on cell permeabilization and disruption. Meanwhile, a large quantity of macromolecules and nutrients were released from bacteria cells. Additionally, efficiency of ozone utilization was low but specific solubilization related to cell lysis was high at this level. Greater than 90 mgO₃/gMLSS, the number of live cells and dead cells were both stable, and the quantity of material in bulk liquid increased slowly. The specific solubilization ratio decreased while the efficiency of ozone utilization began to increase. This indicated that ozone oxidized the macromolecules in the bulk liquid instead of bacteria cells at high ozone dose.

A full-scale plug-flow reactor for biological sludge ozonation

The reduction of biological excess sludge production using ozone is a well-known technology and is applied in several full-scale plants around the world. Nevertheless, optimisation of the process is not yet adequately documented in the literature. Operational parameters are usually chosen by assuming a direct proportionality between ozone dose and excess sludge reduction. This paper investigates the role of ozone concentration on process efficiency and demonstrates the (non-linear) inverse relationship between ozone dose and specific particulate chemical oxygen demand solubilisation in plug-flow contact reactors. The influence of total suspended solids concentration is also studied and described. No short-term lethal effects on heterotrophic biomass have been observed.

Fate of return activated sludge after ozonation: an optimization study for sludge disintegration

The effects of ozonation on sludge disintegration should be investigated before the application of ozone during biological treatment, in order to minimize excess sludge production. In this study, changes in sludge and supernatant after ozonation of return activated sludge were investigated for seven different ozone doses. The optimum ozone dose to avoid inhibition of ozonation and high ozone cost was determined in terms of disintegration degree as 0.05 g O3/gTS. Suspended solid and volatile suspended solid concentrations of sludge decreased by 77.8% and 71.6%, respectively, at the optimum ozone dose. Ozonation significantly decomposed sludge flocs. The release of cell contents was proved by the increase of supernatant total nitrogen (TN) and phosphorus (TP). While TN increased from 7 mg/L to 151 mg/L, TP increased from 8.8 to 33 mg/L at the optimum ozone dose. The dewaterability and filterability characteristics of the ozonated sludge were also examined. Capillary suction time increased with increasing ozone dosage, but specific resistance to filtration increased to a specific value and then decreased dramatically. The particle size distribution changed significantly as a result of floc disruption at an optimum dose of 0.05 gO3/gTS.

Improved solubilization of activated sludge by ozonation in pressure cycles

The generation of a large volume of activated sludge (AS) from wastewater treatment has increasingly become a great burden on the environment. Anaerobic digestion is routinely practiced for excess waste sludge; however, the process retention time is long because of kinetic limitation in the hydrolysis step. We tested the feasibility of applying ozone in pressure cycles to enhance the disintegration and solubilization of AS with the goal to prepare them for digestion using reduced ozone dose and contact time. The AS was subjected to repetitive pressure cycles in a closed vessel in which an ozone gas mixture was compressed into the slurry to reach 1040 kPa in the headspace to be followed by rapid venting. For a returned AS with total COD (tCOD) of 8200 mg L(-1), a dose of 0.01 gO(3)g(-1) total suspended solids (TSS) delivered via 20 pressure cycles within 16 min resulted in a 37-fold increase of the sCOD/tCOD ratio (due to increased soluble COD, i.e. sCOD) and a 25% reduction of TSS, in comparison to a dose of 0.08 gO(3)g(-1) TSS via bubbling contact over 15 min that resulted in a 15-fold increase of the sCOD/tCOD ratio and a 12% reduction of TSS. Sludge solubilization was evidenced by increased dissolved contents of total phosphorous (from 10 to 64 mg L(-1)), total nitrogen (from 14 to 120 mg L(-1)), and protein (from <15 to 39 mg L(-1)) in the sludge suspension after treatment, indicating significant solubilization of AS.

Behavior of inorganic elements during sludge ozonation and their effects on sludge solubilization

The behavior of inorganic elements (including phosphorus, nitrogen, and metals) during sludge ozonation was investigated using batch tests and the effects of metals on sludge solubilization were elucidated. A decrease of ∼ 50% in the ratio of sludge solubilization was found to relate to a high iron content 80-120 mgFe/gSS than that of 4.7-7.4 mgFe/gSS. During sludge ozonation, the pH decreased from 7 to 5, which resulted in the dissolution of chemically precipitated metals and phosphorus. Based on experimental results and thermodynamic calculation, phosphate precipitated by iron and aluminum was more difficult to release while that by calcium released with decrease in pH. The release of barium, manganese, and chrome did not exceed 10% and was much lower than COD solubilization; however, that of nickel, copper, and zinc was similar to COD solubilization. The ratio of nitrogen solubilization was 1.2 times higher than that of COD solubilization (R(2)=0.85). Of the total nitrogen solubilized, 80% was organic nitrogen. Because of their high accumulation potential and negative effect on sludge solubilization, high levels of iron and aluminum in both sewage and sludge should be considered carefully for the application of the advanced sewage treatment process with sludge ozonation and phosphorus crystallization.