Effects of hydrogen peroxide on an upward flow biological filter bed (BFB) containing manganese dioxide fillers

Abstract

Generally, there is some residual hydrogen peroxide (H₂O₂) present in treated wastewater from the Fenton and Fenton-like oxidation processes. We investigated the influence of residual H₂O₂ on a lab-scale upward flow biological filter bed (BFB) containing manganese dioxide (MnO₂) particles. The H₂O₂ in the feed wastewater was rapidly decomposed into oxygen due to the catalytic role of the MnO₂ particles in the bottom layer of the BFB, resulting in a significant increase in the efficiency of chemical oxygen demand (COD) removal. A concentration of 120 mg L⁻¹ H₂O₂ in the feed wastewater increased the COD removal efficiency by 39%. This increase can be attributed to the generation of dissolved oxygen (DO) from H₂O₂ decomposition due to aerobic microorganism growth.

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1. Introduction

In recent years, Fenton and Fenton-like advanced oxidation technologies have garnered attention due to their treatment efficiency and extensive adaptability.^1–4 However, their high operating costs always perplex wastewater engineers. A frequently used strategy to decrease the cost is to combine Fenton or Fenton-like advanced oxidation with biological technologies.^5–10 In these combination processes, the treated wastewater from the Fenton or Fenton-like reactor usually contains some residual hydrogen peroxide (H₂O₂). However, this residual H₂O₂ has a negative effect on the biological process due to its strong oxidative power.¹¹ Therefore, a regulation pool is typically built between the Fenton reactors and the biological reactors to remove the residual H₂O₂.^12–14 H₂O₂ can be catalytically decomposed into H₂O and O₂ by certain metal oxides, as shown in reaction (1).^15–18 If metal oxide particles are used as the bottom fillers of an upward flow biological filter bed (BFB), H₂O₂ in the wastewater can be decomposed into oxygen as the H₂O₂-containing wastewater flows through the filler layer. Thus, it can be expected that metal oxide filler particles can not only reduce the regulation pool but also provide dissolved oxygen to enhance the growth of microorganisms inside the BFB.

2H₂O₂ → O₂ + 2H₂O (1)

In the paper, a lab-scale upward-flow BFB was constructed, as shown in Fig. 1. In the BFB, MnO₂ particles (0.075–0.15 mm in diameter) were used as catalysts for the reaction (1) because MnO₂ is reported to be an efficient catalyst for H₂O₂ decomposition.^19–21 In this paper, we mainly focus on the effects of H₂O₂ in the BFB. These effects include changes in DO, COD removal, microbial populations and void volume. Our aim was to develop a high efficient Fenton-BFB joint process.

Fig. 1 The BFB experimental system.

2. Materials and methods

2.1 BFB operation

The experiment was performed in a lab-scale upward-flow BFB. The experiment took place in a greenhouse on SYSU campus in Guangzhou, China. The temperature was in the range of 18–30 °C. The BFB was constructed using a polypropylene cylindrical container with a diameter of 19 cm and a height of 100 cm, as shown in Fig. 1. The fillers were divided into three layers from the bottom to the top: (i) a 20 cm high pavestone layer with a diameter range of 15–25 mm, (ii) a 50 cm high red brick layer with a diameter of about10 mm and (iii) a 20 cm high river sand layer. All of the containers equipped with valves for sampling in stratified at different substrates. A certain amount of MnO₂ was evenly added to the first layer of the reactor to facilitate H₂O₂ decomposition.

The feed wastewater of the BFB was prepared with glucose (C₆H₁₂O₆), ammonium sulfate {(NH₄)₂SO₄}, monopotassium phosphate (KH₂PO₄) and water. The COD concentration was 250 ± 20 mg L⁻¹. The ammonia concentration was 25 ± 3 mg L⁻¹. The total phosphorus concentration was 5.1 ± 0.4 mg L⁻¹. The experiment was divided into six stages with different dosages of H₂O₂ (20 mg L⁻¹, 40 mg L⁻¹, 60 mg L⁻¹, 80 mg L⁻¹, 120 mg L⁻¹ and 160 mg L⁻¹) under a fixed hydraulic retention time (HRT) of 8 h, flow of 23.6 mL min⁻¹, organic loading of 0.3 kg per m³ per day. The experiment was conducted from Mar. 2012 to Oct. 2013.

2.2 Catalytic decomposition of H₂O₂ with MnO₂

The catalytic decomposition of H₂O₂ with various amounts of MnO₂ was conducted in beakers with magnetic stirring at a speed of 100 rpm. In each experiment, a 100 mL solution containing 160 mg L⁻¹ H₂O₂ was employed.

2.3 Sample analysis

Samples were taken at the A1 (inlet), A2, A3 and A4 (outlet) location every two days. The samples were analyzed after filtration. COD was analyzed by the oxidation method using potassium dichromate.²² The DO in the different layers was measured using a DO (YSI 550A, USA) meter. H₂O₂ concentration was determined by colorimetric methods using titanium oxalate.²³ Microbial populations were observed using a microscope (NMM-820TRF, China).

3. Results and discussion

3.1 Catalytic decomposition of H₂O₂ by MnO₂

Fig. 2 shows the decomposition of hydrogen peroxide with different dosages of MnO₂ at different times. The decomposition efficiency of H₂O₂ was only 3.1% after 60 min when no MnO₂ was added. The decomposition efficiency reached 98% after 60 min when the dosage of MnO₂ was 0.2 g L⁻¹. This result confirms that MnO₂ can efficiently decompose H₂O₂. The inset of Fig. 1 presents the change in the decomposition efficiency of 160 mg L⁻¹ H₂O₂ over 10 min with the addition of MnO₂. The decomposition efficiency of H₂O₂ increased with increasing MnO₂ dosage, reaching a plateau at a dosage of 0.5 g L⁻¹. The decomposition efficiency was close to 100%, i.e., the H₂O₂ was completely decomposed. Consequently, the HRT of the wastewater in the MnO₂ catalytic layer was designed to last approximately 10 minutes because 0.5 g L⁻¹ MnO₂ was sufficient to decompose a concentration of less than 160 mg L⁻¹ H₂O₂.

Fig. 2 Decomposition efficiency of H₂O₂ (160 mg L⁻¹) in the presence of MnO₂.

3.2 Changes in H₂O₂ and DO concentration in the BFB

The top section of Fig. 3 shows the concentrations of H₂O₂ at various heights of the upward-flow BFB. The H₂O₂ concentrations in the wastewater rapidly decreased with increasing bed height. At a bed height of 20 cm, when the inlet concentration of H₂O₂ was less than 40 mg L⁻¹, the residual H₂O₂ was close to zero, i.e., almost all of the H₂O₂ was decomposed in the MnO₂-containing substrate layer at the bottom. Although the inlet concentration of H₂O₂ reached as high as 120 mg L⁻¹, the residual H₂O₂ was only 16 mg L⁻¹ at a height of 20 cm, i.e., the decomposition efficiency was 86.7%. This result indicates that H₂O₂ decomposition is focused at the bottom layer of the upward-flow BFB. As predicted, the H₂O₂ in the wastewater had no negative effect on the growth of the microorganism in the upper parts of the BFB. The vast majority of the H₂O₂ added in the experiment decomposed into H₂O and O₂ as reaction (1), and there was extremely small amounts of H₂O₂ generated HO₂˙, O₂˙, and –OH˙.

Fig. 3 Changes in DO and H₂O₂ concentrations at different heights. A1, A2, A3 and A4 are the sampling positions.

The DO concentration in the BFB also increased, as shown in the bottom section of Fig. 3. Without additional H₂O₂, the DO concentration at the inlet was 8.3 mg L⁻¹; however, the DO rapidly decreased with an increase in the height inside the BFB, reaching only 0.4 mg L⁻¹ at a bed height of 20 cm. This result indicates an anoxic state inside of the BFB. When H₂O₂ was added to the wastewater, the DO concentration inside of the BFB was obviously higher than that without additional H₂O₂, especially when the additional H₂O₂ was above 120 mg L⁻¹. At these higher H₂O₂ concentrations, the DO concentration at a bed height of 20 cm reached 15.5 mg L⁻¹, indicating a favorable state for aerobic microorganism growth.

According to reaction (1), the oxygen yield should be 0.47 times the H₂O₂ concentration if all of the H₂O₂ is decomposed. Consequently, the oxygenation efficiency (OE) of H₂O₂ decomposition can be calculated with the actual DO data using eqn (2).

OE (%) = increased DO/0.47(additional H₂O₂ − residual H₂O₂) × 100 (2)

As shown in the inset of Fig. 3, the oxygenation efficiencies were 20.9–43.8% for feed H₂O₂ concentrations of 20 mg L⁻¹ to 160 mg L⁻¹. The oxygenation efficiency of H₂O₂ was significantly higher than the reaeration rate of atmosphere in the traditional BFB, which generally contains less than 10% oxygen.^24,25 The high oxygenation efficiency of H₂O₂ is dependent on the characteristics of the liquid oxygen resource. H₂O₂ can be completely mixed with wastewater, homogeneously spread between fillers and is capable of producing pure oxygen. The low oxygenation efficiency of air may be due to the association of oxygen with other gases, such as nitrogen. Thus, H₂O₂ possesses a few advantages as an oxygen resource the Fenton-biological coupling reactor, especially for environments such as wetlands, where aeration is inconvenient.

3.3 Effect of H₂O₂ on COD removal

Fig. 4 shows the changes in COD removal efficiency with H₂O₂ concentration over the course of a stable 60 day run. Without additional H₂O₂, the COD removal efficiency of the BFB was only 40 ± 11% with small fluctuations. The addition of H₂O₂ increased the COD removal efficiency of the BFB. For example, when the wastewater containing 120 mg L⁻¹ H₂O₂ was fed into the BFB, the mean COD removal efficiency reached 79 ± 8%. This increase was directly proportional to the DO concentration when the concentration of the feed H₂O₂ was below 120 mg L⁻¹, as shown in Fig. 5. Thus, it can be inferred that the increase in COD removal efficiency is due to an enhancement in microbial metabolism due to an increase in the DO rather than from direct oxidation by H₂O₂ and oxygen radicals. It can also be observed from Fig. 5 that the DO in the BFB rapidly increases but that the COD removal does not correspondingly increase when the concentration of the feed H₂O₂ is over 120 mg L⁻¹. This result indicates that the DO concentration from the decomposition of 120 mg L⁻¹ H₂O₂ is sufficient for COD loading. Consequently, the concentration of feed H₂O₂ was controlled below 120 mg L⁻¹ in the subsequent experiments.

Fig. 4 COD removal efficiency at different H₂O₂ concentrations.

Fig. 5 Dependence of COD removal efficiency on H₂O₂ and DO concentrations.

3.4 Changes in void volume

Clogging is a common problem for fixed bed-type reactions. Fig. 6 gives the changes in BFB void volume over 16 months of operation. The void volume decreased from 40.1% to 36.1% after 16 months. This slight decrease suggests that there is no serous clogging. In combination with the results for COD removal, we can infer that the decomposition of H₂O₂ does not cause excessively fast growth of aerobic microorganisms. The microorganisms were observed to exist in aerobic biofilms even with increases in DO in the BFB.

Fig. 6 Changes in the BFB void volume.

3.5 Changes in the microbial populations

Fig. 7 shows images of the microorganism populations for various dosages of H₂O₂ at a bed height of 20 cm during stable operation of the BFB. It can be observed from Fig. 7a that no metazoans were present when the feed wastewater did not contain H₂O₂, which showed that anaerobic and facultative anaerobic bacteria were the dominant populations. However, when the feed wastewater contained 40 mg L⁻¹ H₂O₂, a larger number of nematodes were observed (Fig. 7b), indicating that the water was in a hypoxic state and that the dominant microbial species were facultative aerobes. When the dosage increased to 80 mg L⁻¹, rotifers begin to appear (Fig. 7c), suggesting that the dominant microbial species were aerobic microorganisms. Microscopic analysis showed that H₂O₂ did not have adverse impacts on the growth of microorganisms. This result is because H₂O₂ primarily decomposed at the paving stone layer.

Fig. 7 Images of microorganism populations for various dosages of H₂O₂ at a height of 20 cm during the stable running of BFB.

Additionally, as H₂O₂ decomposed and reaeration, the microorganisms at the pavestone layer gradually changed from anaerobic and facultative anaerobic populations to aerobic populations. These observations were consistent with the change in the DO concentration shown in Fig. 2.

3.6 Analysis of economic and application

In China, the cost of conventional BFB for sewage treatment is around 0.5 yuan/m³. And the cost of industrial H₂O₂ (35% mass fraction) is around 0.8 yuan/kg. The cost of H₂O₂ was 0.27 yuan/m³, when the feed concentration of H₂O₂ was 120 mg L⁻¹. The proportion of expense increased by 54%. However, the COD removal efficiency improved from 40% to 79%, the proportion of COD removal efficiency increased to 97.5%. Compared to the increased removal efficiency, the additional cost is acceptable, within a reasonable range. This process has a high economic value.

The improved BFB progress could be widely used in municipal sewage treatment, rural domestic sewage treatment and industrial wastewater deep treatment, due to its remarkable treatment effect, easy operation, on and simple installation. Especially, this progress could combine with Fenton or Fenton-like advanced oxidation technologies, which can not only use the residual H₂O₂ to improve the removal efficiency but also can reduce the cost of H₂O₂.

4. Conclusion

A lab-scale upward flow BFB containing MnO₂ particles was constructed to efficiently decompose H₂O₂ in feed wastewater. The decomposition process not only eliminated the detrimental strong oxidant effect of H₂O₂ but also converted it into DO to boost the aerobic microbial populations, leading to an increase in COD removal efficiency in the BFB. These findings will aid in the development of an efficient Fenton-biological combination process.

Acknowledgements

This research was supported by the Nature Science Foundations of China (21107146), Nature Foundations of Guangdong Province (92510027501000005), Science and Technology Research Programs of Guangzhou City (2012J4300118), the Project of Education Bureau of Guangdong Province (cgzhzd1001), and the Fundamental Research Funds for the Central Universities (121pgy20).

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