Renzun Zhao

Advanced Oxidation Processes (AOPs) in Wastewater Treatment

Advanced oxidation processes (AOPs) were first proposed in the 1980s for drinking water treatment and later were widely studied for treatment of different wastewaters. During the AOP treatment of wastewater, hydroxyl radicals (OH·) or sulfate radicals (SO4·−) are generated in sufficient quantity to remove refractory organic matters, traceable organic contaminants, or certain inorganic pollutants, or to increase wastewater biodegradability as a pre-treatment prior to an ensuing biological treatment. In this paper, we review the fundamental mechanisms of radical generation in different AOPs and select landfill leachate and biologically treated municipal wastewater as model wastewaters to discuss wastewater treatment with different AOPs. Generally, the treatment efficiencies rely heavily upon the selected AOP type, physical and chemical properties of target pollutants, and operating conditions. It would be noted that other mechanisms, besides hydroxyl radical or sulfate radical-based oxidation, may occur during the AOP treatment and contribute to the reduction of target pollutants. Particularly, we summarize recent advances in the AOP treatment of landfill leachate, as well as advanced oxidation of effluent organic matters (EfOM) in biologically treated secondary effluent (BTSE) for water reuse.

Characterization and treatment of organic constituents in landfill leachates that influence the UV disinfection in the publicly owned treatment works (POTWs).

Landfill leachates strongly quench UV light. When discharged to POTWs, leachates can interfere with UV disinfection. To investigate the UV quenching problem of landfill leachates, a variety of landfill leachates with a range of conditions were collected and characterized. The UV blocking component was found to be resistant to biological degradation so they pass through wastewater treatment plants and impact the subsequent UV disinfection system. Leachate samples were fractionated into humic acids (HAs), fulvic Acids (FAs) and hydrophilic (Hpi) fractions to investigate the source of UV absorbing materials. Results show that for all leachates examined, the specific UV254 absorbance (SUVA254) of the three fractions follows: HA>FA>Hpi. However, the overall UV254 absorbance of the Hpi fraction was important because there was more hydrophilic organic matter than humic or fulvic acids. The size distribution was also investigated to provide information about the potential for membrane treatment. It was found that the size distribution of the three fractions follows: HA>FA>Hpi. This indicates that membrane separation following biological treatment is a promising technology for removal of humic substances from landfill leachates. Leachate samples treated in this manner could meet the UV transmittance requirement of the POTWs.

Application of Fenton's reagent as a polishing step for removal of UV quenching organic constituents in biologically treated landfill leachates.

When landfill leachate, with or without biological pretreatment, is discharged to publically owned treatment works (POTWs), it can interfere with the installed treatment facilities. Biological treatment is ineffective for the removal of some of the bio-refractory organic matter, including UV₂₅₄ quenching substances. Fentons reagent treatment for biologically treated landfill leachates is examined in this study as a polishing step to make landfill leachates acceptable to POTWs. The optimum conditions for the Fentons reagent treatment are explored. The molecular weight and hydrophobic-hydrophilic nature based fractions of the Fentons treated leachate samples are analyzed to provide insight into the leachate fractions targeted by the Fentons reagent. The results indicate that Fentons reagent can act as a good compliment to biological treatment as it can remove leachate fractions which are widely considered to be bio-refractory. It exhibited good UV₂₅₄ absorbance removal by removing larger molecular weight humic substances and thus, can help solve the UV₂₅₄ quenching problem due to leachates discharged to POTWs.

Variation in organic matter characteristics of landfill leachates in different stabilisation stages

This study investigates the effect of landfill age on landfill leachate characteristics; two aspects are focused here. One is ultraviolet absorbance at 254 nm (UV254) property, as the discharge of landfill leachates to publically owned treatment works can cause interference with UV254 disinfection. The other is biorefractory organic nitrogen in leachates, as it can contribute to effluent nitrogen making it difficult to meet stringent effluent nitrogen regulations. To study variation in UV254-absorbing organic carbon and organic nitrogen, leachate samples ranging from cells with ages 2 to 30 y from a large landfill in Kentucky, were collected and fractionated on a basis of their molecular weight and chemical nature into humic acids, fulvic acids and a hydrophilic fraction. The effectiveness of long term landfilling and membrane treatment for organic matter and organic nitrogen removal was examined. Humic materials, which were the major UV254-absorbing substances, were mainly >1 kDa and they degraded significantly with landfill age. The hydrophilic organic fraction, which was the major contributor to organic nitrogen, was mainly <1 kDa and it became increasingly recalcitrant with landfill age. This study provides insight into the characteristics of the different leachate fractions with landfilling age that might aid the design of on-site leachate treatment techniques.

Evolution of nitrogen species in landfill leachates under various stabilization states

In this study, nitrogen species in landfill leachates under various stabilization states were investigated with emphasis on organic nitrogen. Ammonium nitrogen was found to be approximately 1300mg/L in leachates from younger landfill units (less than 10years old), and approximately 500mg/L in leachates from older landfill units (up to 30years old). The concentration and aerobic biodegradability of organic nitrogen decreased with landfill age. A size distribution study showed that most organic nitrogen in landfill leachates is <1kDa. The Lowry protein concentration (mg/L-N) was analyzed and showed a strong correlation with the total organic nitrogen (TON, mg/L-N, R2=0.88 and 0.98 for untreated and treated samples, respectively). The slopes of the regression curves of untreated (protein=0.45TON) and treated (protein=0.31TON) leachates indicated that the protein is more biodegradable than the other organic nitrogen species in landfill leachates. XAD-8 resin was employed to isolate the hydrophilic fraction of leachate samples, and it was found that the hydrophilic fraction proportion in terms of organic nitrogen decreased with landfill age. Solid-state 15N nuclear magnetic resonance (NMR) was utilized to identify the nitrogen species. Proteinaceous materials were found to be readily biodegradable, while heterocyclic nitrogen species were found to be resistant to biodegradation.

Treatment of organic matter and methylated arsenic in landfill biogas condensate.

To explore the feasible treatment alternatives for organic contaminant, especially organic arsenic species in the landfill gas (LFG) condensate, a variety of treatment approaches were examined and evaluated in this study. Biological degradation, conventional and advanced oxidation, and physical absorption showed limited effectiveness to convert the methylated arsenic to inorganic arsenic. Reverse osmosis (RO) was found to be able to remove the organic arsenic and meet the discharge limits. Maximum removal efficiency and cost level were summarized for all treatment approaches tested, which can be a reference for the organic arsenic treatment method selection under different circumstances.

A review of landfill leachate induced ultraviolet quenching substances: Sources, characteristics, and treatment

Landfill leachate contains extremely diverse mixtures of pollutants and thus requires appropriate treatment before discharge. Co-treatment of landfill leachate with sewage in wastewater treatment plants is a common approach because of low cost and convenience. However, some recalcitrant organic compounds in leachate can escape biological treatment processes, lower the UV transmittance of waste streams due to their UV-quenching properties, and interfere with the associated disinfection efficacy. Thus, the leachate UV quenching substances (UVQS) must be removed or reduced to a level that UV disinfection is not strongly affected. UVQS consist of three major fractions, humic acids, fulvic acids and hydrophilics, each of which has distinct characteristics and behaviors during treatment. The purpose of this review is to provide a synthesis of the state of the science regarding UVQS and possible treatment approaches. In general, chemical, electrochemical, and physical treatments are more effective than biological treatments, but also costlier. Integration of multiple treatment methods to target the removal of different fractions of UVQS can aid in optimizing treatment. The importance of UVQS effects on wastewater treatment should be better recognized and understood with implemented regulations and improved research and treatment practice.

Characterization of ultraviolet-quenching dissolved organic matter (DOM) in mature and young leachates before and after biological pre-treatment

The discharge of municipal landfill leachate into publicly owned treatment works (POTWs) is a common leachate management practice in the United States. However, the benefits of this option have diminished in many POTWs because leachate can significantly interfere with UV disinfection due to the introduction of UV-quenching substances (UVQS). This study aims to characterize UV-quenching dissolved organic matter (DOM) in different raw and sequencing batch reactor (SBR) pre-treated landfill leachates. Nine leachate samples, including four raw mature leachates, four SBR pre-treated mature leachates, and one young raw leachate, were investigated. The mature leachates, regardless of raw or SBR pre-treated ones, had lower biodegradability (low BOD5/COD), higher specific UV absorbance (SUVA), greater hydrophobic fraction, and more high molecular weight (MW) organic compounds than the young leachate. SUVA served as a useful indicator for the characterization of UV-quenching DOM, which exponentially declined with increasing BOD5/COD. The high SUVA of the mature leachates suggests a highly aromatic and hydrophobic character with a high fraction of aquatic humic matter and high MW compounds. In contrast, the low SUVA of the young leachate implies the presence of a high fraction of non-humic matter, a highly aliphatic character, and more low MW molecules. After the mature leachates were biologically pre-treated, the SUVA increased due to the partial degradation of high MW compounds. Results from fluorescence excitation–emission matrix (EEM) analysis could effectively reflect the variation of hydrophobic DOM, but poorly indicated the presence of hydrophilic DOM.

Pilot scale study of sequencing batch reactor (SBR) retrofit with integrated fixed film activated sludge (IFAS): nitrogen removal and design consideration

Excessive nitrogen is a typical limiting factor for wastewater discharge. The IFAS process has been widely applied to meet more stringent organics and nitrogen discharge limits. However, very few studies have been conducted for the application of the IFAS process in sequencing batch reactors (SBRs). This study coupled the IFAS process with SBR operation in a pilot-scale reactor, aiming to verify the feasibility of IFAS-SBR combination and observe the process performance. Phased tests were conducted for different operations, such as nitrification only, modified Ludzack–Ettinger (MLE), etc. The results showed that IFAS-SBR was effective for organics removal with over 90% CBOD removal and an effluent CBOD of less than 25 mg L−1. Also, excellent NH4+-N removal, characterized by 96% removal efficiency and an average effluent concentration of 0.8 mg L−1, was achieved under low N loading (23.6 g-N per m3 per day) and sufficient solid retention time (SRT, 4.34 days). It was found that nitrification occurred with a SRT less than the minimum nitrifying SRT, which was attributed to the biofilm and its seeding effect. Kinetics tests showed that the IFAS surface NH4+-N removal rate was 0.69 g-N per m2 per day at 15 °C, and 31% of the NH4+-N removal capacity was contributed by the suspended-growth biomass, while 69% by the biofilm. The NH4+-N removal improvement was attributed to both attached-growth biomass on media carriers and the “seeding effect”. In addition, having an anoxic zone in SBR systems can reduce the number of trains for nitrogen removal applications compared to the pre-denitrification conventional SBR systems.