Microwave-enhanced Advanced Oxidation Process Research Articles

Anaerobic digestion of thickened secondary sludge after microwave advanced oxidation pretreatment

A microwave-enhanced advanced oxidation process (MW-AOP) was used to treat thickened waste secondary sludge prior to anaerobic digestion (AD). A large amount of soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFAs) was formed in the solution after pretreatment. Both single- and two-phase systems loaded up to 18.2 g volatile solids/(l day) while achieving high methane production, as well as high overall treatment efficiency. Both the upflow anaerobic blanket reactor and the continuous stirred tank reactor could operate at very short hydraulic retention times as short as 2 days without any evidence of souring. The results indicated that an acid reactor in the two-phase system was not required, due to very high levels of SCOD and VFAs produced in the MW-AOP-pretreated solution. As such, a single AD should be able to handle this easily biodegradable substrate following microwave pretreatment.

Anaerobic digestion of liquid dairy manure pretreated by the microwave-enhanced advanced oxidation process

ABSTRACT The microwave-enhanced advanced oxidation process (MW-AOP) was utilized for the treatment of liquid dairy manure, prior to anaerobic digestion (AD). A significant amount of soluble chemical oxygen demand (SCOD) was formed in the treated solution, but there was no or little increase in volatile fatty acid (VFA). In this study, up-flow anaerobic sludge blanket (UASB) reactors were used in both the conventional AD system and as the methane phase reactor in a two-phase anaerobic digestion (TPAD) system. Both AD systems were capable of operating at very short hydraulic retention times (HRTs) to as low as about 3 days, and very high methane production rates were achieved. However, much higher methane production was obtained in the TPAD system. The phase separation of the acidogenesis and the methanogenesis in the TPAD system not only increased methane production but also maintained reactor stability throughout the experimental period. Thus, the combination of MW-AOP treatment and TPAD appears to be an effective means of energy recovery from dairy manure.

Evaluation of impact of sludge types and solids content on sludge treatment using microwave enhanced advanced oxidation process

ABSTRACT The microwave enhanced advanced oxidation process (MW-AOP) has been applied to pre-treat different sludge types and high solids content. Secondary sludge not only had the highest solids and nutrient content but also yielded higher treatment efficiency than primary or mixed sludge. In the case of secondary sludge with 4% total solids (TS), the total suspended solids (TSS) concentration was reduced by 32% while soluble chemical oxygen demand concentration increased from 1% to 40% after treatment at 110°C. A high level of nutrient release was also achieved; about 65% total phosphate (TP) solubilized at 110°C. The degree of secondary sludge disintegration was dictated by temperature and hydrogen peroxide dosage. The optimal operating temperature for the system was 110°C, and sludge containing TS up to 8% was treated effectively. Secondary sludge with 8% TS had a TSS reduction of 41% after treatment at 110°C while COD solubilization was about 45%; about 55% TP was solubilized at 10 min holding time. Treatment of sludge with higher solids content would allow for handling larger amounts of sludge at a given period and reduce heating cost per unit of treated sludge. The inter-relationship between the degree of sludge disintegration and changes in chemical and physical properties was also clearly demonstrated here. The treated sludge would be an ideal substrate for anaerobic digestion or phosphorous recovery processes. High levels of nutrients (phosphorus and nitrogen) and metal release, and solids disintegration from sludge containing high solids content would make subsequent resource recovery processes more effective and economical.

Progress in applications of advanced oxidation processes for promotion of biohydrogen production by fermentation processes

Advanced oxidation processes (AOPs) are powerful methods for treating substrates using radicals that are generated in situ. This study reviewed applications of AOPs in enhancement of biohydrogen production. The AOPs are applied in substrate pretreatment because of their ability to break the complex structure of lignocellulosic biomass for ease of subsequent hydrolysis. The mechanism of solubilization of complex organics resulting in increased biodegradability of substrate during pretreatment has been suggested. Documented studies indicate that up to 98% color removal from organic wastewater is possible by the use of AOPs. Furthermore, a combination of AOPs with biological processes can achieve more than 90% COD removal from biohydrogen production effluent. Sonication, microwave-enhanced AOPs, and electrochemical treatment are the most applied AOPs in enrichment of biohydrogen with up to fivefold increase in biohydrogen yield achieved after electrochemical pre-treatment. The mechanism of enhancement of hydrogen yield in dark fermentation after pretreatment of the substrate and inoculum with AOPs has been proposed. The excess sludge produced during hydrogen fermentation can be pretreated with ozone and ultrasound before biomethanation process. More studies on co-production of biohydrogen and electricity through electrochemical oxidation in fuel cells are necessary. This study proposes the integration of AOPs with conventional processes in biorefinery production approach with aim of improving biohydrogen yields, co-producing it with other biofuels, and reducing the process costs. Future studies should focus on the scale-up of AOPs for commercial applications. Comparative studies on energy requirements for various AOPs applications are lacking and should be carried out. Graphical abstract

Microwave enhanced advanced oxidation treatment of fat, oil and grease (FOG) with organic co-substrates

ABSTRACT Mixtures of fats, oils and grease (FOG) either with dairy manure or with thickened waste secondary sludge (TWSS) were treated using microwave enhanced advanced oxidation process (MW-AOP). For both dairy manure and TWSS mixtures, the maximum increase in soluble COD (SCOD) resulted from the 1:1 mixture by total solids (TS) weight. In the TWSS mixtures, production of volatile fatty acid (VFA) increased with greater FOG content, while there was a decreasing production trend in VFA in dairy manure mixtures. Nutrients and metals were also released for all sets. The degradation followed peroxidation mechanism to produce lower molecular weight substrates such as short-chain fatty acids which would be less inhibitory to microbes. Nutrients and metals in the treated solution would sustain microbial growth in a biological system. FOG content for the mixtures in the MW-AOP treatment should be less than 75% by TS weight to prevent oxidation to CO2.

Microwave-enhanced advanced oxidation process of biogas slurry from cow manure anaerobic digester

ABSTRACT In order to improve the sedimentation performance of biogas slurry and reduce the COD, microwave-enhanced advanced oxidation process (MW/H2O2-AOP) was used to pretreat the cow manure biogas slurry. A laboratory-scale microwave reactor was used to strengthen the H2O2 advanced oxidation process to achieve the solid–liquid separation of biogas slurry which was difficult to be settled. The results showed that the maximum COD removal efficiency reached 68.8% when pH, H2O2/total solid, microwave time and microwave power were 3, 0.4, 5 min and 300 W, respectively. COD was reduced from 24,157 mg/L to 7536 mg/L, and the settlings volume was reduced to 38%, while other pollutants such as the total solid and total dissolved solid were significantly reduced. Total solid and total dissolved solid removal efficiency reached 50.0% and 62.0%, respectively. Since no other agents were added, MW/H2O2-AOP can effectively reduce secondary pollution and reduce the cost of treatment.

Microwave enhanced advanced oxidation treatment of dairy manure

The microwave enhanced advanced oxidation process (MW-AOP) was used to treat dairy manure in a continuous-flow 915 MHz microwave wastewater treatment system. The treatment efficiency increased with an increase in temperature, as well as hydrogen peroxide dosage. The settling property was also improved in all treated sets, regardless of temperature applied. The system operated at temperatures >100 °C had a much higher soluble chemical oxygen demand than at temperatures <100 °C. The highest soluble carbonaceous compounds, orthophosphate and ammonia were obtained at 110 °C and 0.6%H2O2 per % of total solids content. The process should be operated at higher temperatures and higher hydrogen peroxide dosages for maximizing solids disintegration, nutrient release and energy efficiency. An energy fingerprint correlating the cumulative energy consumption and temperature rise was developed. The results demonstrated that the custom designed MW-AOP system is suitable for the effective treatment of dairy manure. The system can readily be scaled up and integrated into a dairy farm manure treatment and resource recovery system.

Microwave-Enhanced Advanced Oxidation Treatment of Lipids and Food Wastes

Fats, oils, and grease (FOG) and source separated organics (SSO) were treated with the microwave-enhanced advanced oxidation process (MW-AOP) at 90 and 110 °C, with varying amounts of hydrogen peroxide dosages. The treatment efficiency, in terms of soluble substrates and volatile fatty acids (VFA), increased with an increase in both temperature hydrogen peroxide dosages. Fatty acids and compounds with carbonyl group and/or hydroxyl group in both initial and treated FOG samples were identified by gas chromatography-mass spectrometry. MW-AOP treatment temperatures and hydrogen peroxide dosages dictated the formation of degradation products. The degradation followed peroxidation mechanism to produce lower molecular weight substrates such as short chain fatty acids which would be less inhibitory to microbes. After the MW-AOP treatment, both SSO and FOG comprised of more soluble and low molecular weight compounds. These compounds included VFA and nutrients that would be readily available for bacterial or plant uptake.

Microwave-enhanced advanced oxidation processes for the degradation of dyes in water

Contamination of water bodies with dyes has become a serious global concern affecting the whole biosphere. Microwave irradiation and advanced oxidation processes (AOP) have recently gained importance as efficient remediation methods. Indeed, microwave offers special features such as fast, uniform, selective and controlled heating that result in a rapid reaction rate, whereas advanced oxidation processes are able to mineralize any organic pollutant. Here we review microwave-enhanced advanced oxidation processes for the removal or degradation of synthetic dyes, and we compare with conventional treatment methods. We found that microwave-enhanced AOP improve the efficiency of dye degradation. However, cost analyses reveal an economic limitation of microwave-enhanced processes.

Degradation of 4-Chlorophenol by Microwave-Enhanced Advanced Oxidation Processes: Kinetics and Influential Process Parameters

In this study, the degradation of 4-chlorophenol (4-CP) is investigated by using a combination of microwave (MW) irradiation and hydrogen peroxide as oxidant (the MW–H2O2-process). The influence of the critical parameters on the efficiency of the process is examined by applying Partial Least Square Regression (PLS) on a Design of Experiments (DOE). Also, a kinetic evaluation of the process is carried out. Results showed that the MW–H2O2-process is effective in the degradation of 4-CP. The most influential parameter of the MW–H2O2-process is the reaction temperature. Besides this parameter, the reaction time, initial 4-CP concentration, and initial hydrogen peroxide concentration have a (minor) influence on the 4-CP degradation. A maximum degradation efficiency within the levels of the DOE (remaining percentage of 4-CP of 34%) was achieved with an initial 4-CP concentration of 1000 mg/L, an initial H2O2 concentration of 11 g/L, reaction temperature of 180 °C, and reaction time of 20 min. The process follows pseudo first order reaction kinetics.

Microwave enhanced advanced oxidation treatment of sewage sludge from the membrane-enhanced biological phosphorus removal process

This study investigated the effectiveness of bench and pilot-scale microwave enhanced advanced oxidation process (MW-AOP) for treatment of sewage sludge in neutral, acidic (pH 4) and alkaline (pH 9) conditions. The MW-AOP treatments were very effective: total suspended solids reduction and chemical oxygen demand solubilization were better in the sets at pH 9 than at pH 4. Alkaline conditions would be more favorable for the subsequent anaerobic digestion for biogas production. Release of orthophosphate, particle size distribution, dewatering properties and settling of the sets at pH 4 were better than at pH 9. The supernatants obtained from the sets at pH 4 had a high orthophosphate concentration and a low total suspended solids content, which were more suitable for struvite (a fertilizer) recovery process. Energy consumption was lower for the pH adjusted treatment sets compared to those of MW only and MW-AOP at neutral conditions; energy for the pH adjusted sets varied between 0.32 and 0.41 kWh/L of sludge treated. The results indicated that the MW-AOP could reduce the volume of sludge for disposal, and produce a high quality treated effluent suitable for further resource and energy recovery. Depending on the needs of each wastewater treatment plant’s treatment priority, the experimental conditions applied in the process can be adjusted accordingly.

Nutrient recovery from foam with microwave treatment

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.

Recovery of phosphorus from dairy manure: a pilot-scale study

Phosphorus was recovered from dairy manure via a microwave-enhanced advanced oxidation process (MW/H2O2-AOP) followed by struvite crystallization in a pilot-scale continuous flow operation. Soluble phosphorus in dairy manure increased by over 50% after the MW/H2O2-AOP, and the settleability of suspended solids was greatly improved. More than 50% of clear supernatant was obtained after microwave treatment, and the maximum volume of supernatant was obtained at a hydrogen peroxide dosage of 0.3% and pH 3.5. By adding oxalic acid into the supernatant, about 90% of calcium was removed, while more than 90% of magnesium was retained. As a result, the resulting solution was well suited for struvite crystallization. Nearly 95% of phosphorus in the treated supernatant was removed and recovered as struvite.

Briefing: A continuous-flow 915-MHz microwave treatment of dairy manure

A pilot-scale (5 kW) 915-MHz microwave-enhanced advanced oxidation process (MW/H2O2-AOP) was custom designed and tested to treat dairy manure. The objective of the study was to verify the performance efficiency of the new system in terms of nutrient release, solids disintegration and energy consumption and compare it with an existing pilot-scale (5 kW) 2450-MHz microwave system. The 915-MHz MW/H2O2-AOP resulted in a release of about 60% of total phosphorus as orthophosphate, and about 25% of total chemical oxygen demand was disintegrated in the treated solution. The treatment performance was similar to that of 2450-MHz MW/H2O2-AOP albeit using lower H2O2 dosage. Research and development work currently underway with the 915-MHz MW/H2O2-AOP will optimise the treatment and energy efficiency.

Microwave enhanced advanced oxidation process for treating dairy manure at low pH

This study investigated the treatment of dairy manure using the microwave enhanced advanced oxidation process (MW-AOP) at pH 2. An experimental design was developed based on a statistical program using response surface methodology to explore the effects of temperature, hydrogen peroxide dosage and heating time on sugar production, nutrient release and solids destruction. Temperature, hydrogen peroxide dosage and acid concentration were key factors affecting reducing sugar production. The highest reducing sugar yield of 7.4% was obtained at 160°C, 0 mL, 15 min heating time, and no H2O2 addition. Temperature was a dominant factor for an increase of soluble chemical oxygen demand (SCOD) in the treated dairy manure. The important factors for volatile fatty acids (VFA) production were microwave temperature and hydrogen peroxide dosage. Temperature was the most important parameter, and heating time, to a lesser extent affecting orthophosphate release. Heating time, hydrogen peroxide dosage and temperature were significant factors for ammonia release. There was a maximum of 96% and 196% increase in orthophosphate and ammonia concentration, respectively at 160°C, 0.5 mL H2O2 and 15 min heating time. The MW-AOP is an effective method in dairy manure treatment for sugar production, nutrient solubilisation, and solids disintegration.

Effects of Irradiation Intensity and pH on Nutrients Release and Solids Destruction of Waste Activated Sludge Using the Microwave‐Enhanced Advanced Oxidation Process

Using the microwave-enhanced advanced oxidation process (MW/H2O2-AOP), the pH and irradiation intensity on waste activated sludge samples were investigated to provide insight to the athermal effects on nutrients release, solids destruction, particle size distribution and dewaterability, and to demonstrate their interrelationships. Carbonaceous matters and nutrients released into solution depended on the irradiation intensity and time. Higher irradiation levels tended to be more effective in the solubilization of nutrients and had more pronounced effects in the dewaterability of sludge. In terms of particle size distribution, detectable particles increased in size for treatments in acidic conditions, while the dewaterability of treated sludge was improved. In treatments under neutral and alkaline conditions, the particle size range increased, with more small particles formed, thereby significantly deteriorating the dewaterability of sludge treated in alkaline conditions. The best results for the solubilization of nutrients were in alkaline conditions with high irradiation power, but dewaterability of the sludge was compromised. Sludge treatment with the MW/H2O2-AOP in acidic conditions with high irradiation power yielded the best dewaterable sludge and significant nutrient solubilization; therefore, it is the recommended treatment condition for activated sludge.

The effects of irradiation intensity on the microwave-enhanced advanced oxidation process

The microwave enhanced advanced oxidation process (MW/H2O2-AOP) was adopted in successfully treating a multitude of organic slurries. The factor of ramp rate was incorporated in this study to investigate its correlations with different nutrients release into solution from waste activated sludge. Besides the ramp rate, the amount of time the samples were held at the treatment temperature to a total of 10 minutes aided in the isolation of held temperature from ramp rates, since different ramp rates will yield different reaction times. It was found that with the exception of orthophosphate and chemical oxygen demand release, ammonia and volatile fatty acids depended highly on the ramp rates. All constituents, chemical oxygen demand, orthophosphate, ammonia and volatile fatty acids, solubilized better with longer reaction times coupled with the highest ramp rate. Besides treatment time and ramp rates, treatment temperature and hydrogen peroxide dosage also play important roles in the release of various nutrients into solution by the MW/H2O2-AOP. The results from this study can aid in design parameters for the scaling up of a continuous microwave system for the MW/H2O2-AOP.

Factors affecting microwave-enhanced advanced oxidation process for sewage sludge treatment

The microwave-enhanced advanced oxidation process (MW/H2O2-AOP) was applied to sewage sludge for nutrient solubilization and solids' disintegration. Four factors, temperature, hydrogen peroxide dosage, mixing, and solids concentration were chosen for a screening experiment, and were ranked according to their significance of influence on the process. In general, temperature and hydrogen peroxide dosage are the two significant factors affecting the process, while mixing is the least significant factor. Temperature was the most significant factor for the release of orthophosphate, and hydrogen peroxide dosage was most important in ammonia release. Solids disintegration, in terms of soluble chemical oxygen demand (SCOD), was largely dependent on temperature and hydrogen peroxide dosage. For volatile fatty acids (VFA) release, mixing was the most significant factor. At higher temperatures with mixing, more VFA was released into the headspace, resulting in less VFA retained in the solution. The best results of solids' disintegration and nutrient release were obtained at 120°C, and 0.80 g H2O2/g dry sludge.

Treating solid dairy manure using microwave-enhanced advanced oxidation process

The microwave enhanced advanced oxidation process (MW/H2O2-AOP) was used to treat separated solid dairy manure for nutrient release and solids reduction. The MW/H2O2-AOP was conducted at a microwave temperature of 120°C for 10 minutes, and at three pH conditions of 3.5, 7.3 and 12. The hydrogen peroxide dosage at approximately 2 mL per 1% TS for a 30 mL sample was used in this study, reflecting a range of 0.53–0.75 g H2O2/g dry sludge. The results indicated that substantial quantities of nutrients could be released into the solution at pH of 3.5. However, at neutral and basic conditions only volatile fatty acids and soluble chemical oxygen demand could be released. The analyses on orthophosphate, soluble chemical oxygen demands and volatile fatty acids were re-examined for dairy manure. It was found that the orthophosphate concentration for untreated samples at a higher % total solids (TS) was suppressed and lesser than actual. To overcome this difficulty, the initial orthophosphate concentration had to be measured at 0.5% TS.

Solubilization of municipal anaerobic sludge using microwave-enhanced advanced oxidation process

The effect of microwave enhanced advanced oxidation process on the release of nutrients and solids disintegration from municipal anaerobic sludge was investigated. Three levels of temperature (160, 120, and 80°C) and two sets of pH, (7.8 and 4) were selected and tested at a microwave heating time of 5 minutes and hydrogen peroxide dosage of approximately 1 mL per 1% total solids. Total chemical oxygen demand concentrations in samples decreased after the treatments at pH 4, while they remained the same as the initial at pH 7.8. The soluble chemical oxygen demand concentrations increased in all sets, regardless of treatment conditions. There was an increase in orthophosphate concentration with an increase of temperature at pH 4, while a decrease of orthophosphate at pH 7.8 was observed. Ammonia concentration also increased with an increase of temperature at pH 4. A small amount of ammonia was converted to nitrates/nitrites in the absence of acid. Acid addition helped in the destruction of sludge solids. Low pH and high temperature gave the highest nutrient release and solids destruction.