An innovative microelectrode fabricated using photolithography for measuring dissolved oxygen distributions in aerobic granules

Performance of nitrogen removal in an alternating activated sludge reactor for full-scale applications

A novel wastewater treatment process, known as an alternating activated sludge reactor (AASR), is proposed to treat wastewater in full-scale operations. The AASR is a technical development based on the sequencing batch reactor (SBR) and cyclic activated sludge technology (CAST). The performance of AASR was evaluated in this study and found to be effective for the removal of pollutants. The average effluent NH₄⁺-N, TN, TP, and COD concentrations were 0.5, 17, 0.8, and 40 mgL^-1, respectively. The corresponding average removal efficiencies were 97%, 59%, 83%, and 83%, respectively, indicating that the AASR was also a successful operating system for the removal of organic matter. The AASR has many advantages, such as successive filling, high removal efficiency, high stability and reliability, low area requirement, no sludge circulation reflux, and low construction costs. The operation mode of the alternating anoxic, anaerobic, and aerobic conditions displayed a higher efficiency for nitrification than that of conventional SBR. The effective mode for denitrification was a step-feed. The control program of the AASR is highly flexible and can easily be modified by a plant manager to meet various loading requirements.

A Minimally Invasive Microsensor Specially Designed for Simultaneous Dissolved Oxygen and pH Biofilm Profiling

A novel sensing device for simultaneous dissolved oxygen (DO) and pH monitoring specially designed for biofilm profiling is presented in this work. This device enabled the recording of instantaneous DO and pH dynamic profiles within biofilms, improving the tools available for the study and the characterization of biological systems. The microsensor consisted of two parallel arrays of microelectrodes. Microelectrodes used for DO sensing were bare gold electrodes, while microelectrodes used for pH sensing were platinum-based electrodes modified using electrodeposited iridium oxide. The device was fabricated with a polyimide (Kapton^®) film of 127 µm as a substrate for minimizing the damage caused on the biofilm structure during its insertion. The electrodes were covered with a Nafion^® layer to increase sensor stability and repeatability and to avoid electrode surface fouling. DO microelectrodes showed a linear response in the range 0–8 mg L⁻¹, a detection limit of 0.05 mg L⁻¹, and a sensitivity of 2.06 nA L mg⁻¹. pH electrodes showed a linear super-Nernstian response (74.2 ± 0.7 mV/pH unit) in a wide pH range (pH 4−9). The multi-analyte sensor array was validated in a flat plate bioreactor where simultaneous and instantaneous pH and DO profiles within a sulfide oxidizing biofilm were recorded. The electrodes spatial resolution, the monitoring sensitivity, and the minimally invasive features exhibited by the proposed microsensor improved biofilm monitoring performance, enabling the quantification of mass transfer resistances and the assessment of biological activity.

Robust electrochemical analysis of As(III) integrating with interference tests: a case study in groundwater

In Togtoh region of Inner Mongolia, northern China, groundwater encountered high concentrations As contamination (greater than 50 μg L(-1)) causes an increasing concern. This work demonstrates an electrochemical protocol for robust (efficient and accurate) determination of As(III) in Togtoh water samples using Au microwire electrode without the need of pretreatment or clean-up steps. Considering the complicated conditions of Togtoh water, the efficiency of Au microwire electrode was systematically evaluated by a series of interference tests, stability and reproducibility measurements. No obvious interference on the determination of As(III) was observed. Especially, the influence of humic acid (HA) was intensively investigated. Electrode stability was also observed with long-term measurements (70 days) in Togtoh water solution and under different temperatures (0-35 °C). Excellent reproducibility (RSD:1.28%) was observed from different batches of Au microwire electrodes. The results obtained at Au microwire electrode were comparable to that obtained by inductively coupled plasma atomic emission spectroscopy (ICP-AES), indicating a good accuracy. These evaluations (efficiency, robustness, and accuracy) demonstrated that the Au microwire electrode was able to determine As(III) in application to real environmental samples.

Quantification of the shear stresses in a microbial granular sludge reactor

Since a certain level of hydrodynamic shear force is needed in the formation of microbial granules for wastewater treatment, a method for quantifying the shear stresses in a microbial granular sludge reactor is highly desirable. In this work a novel energy-dissipation-based model was established and validated to quantitatively describe the shear stresses in a granular sludge sequencing batch reactor (SBR). With this model, the shear stress at the solid-liquid interface in an SBR was estimated and the relative magnitudes of shear stresses induced by fluid, gas bubble and collision on granules were evaluated. The results demonstrate that the effect of reactor geometry on the global shear stress was significant. Both the shear stress at the microbial granule surface and the biomass-loss rate increased with an increase in biomass concentration in the SBR. The gas bubble and the collision were found to be the main source for the shear stress at the granule surface.

Measurement of dissolved oxygen and its diffusivity in aerobic granules using a lithographically-fabricated microelectrode array

A novel gold microelectrode array (MEA) was manufactured with microfabrication techniques and applied on the measurement of dissolved oxygen profile in an aerobic granule. The MEA contained five gold microelectrodes, which had a good linear response to dissolved oxygen and typically had a lifetime of more than 10 days. Dissolved oxygen microprofiles near the surface of an aerobic granule were monitored with this MEA. Based on the measurements, an oxygen effective diffusivity in the upper 100 microm layer of the aerobic granule was estimated to be 1.19 x 10(-9) m2/s. The experimental results demonstrate that the MEA was able to measure the DO levels in aerobic granules accurately and precisely and that the MEA could be used to determine constituents, profiles, and functions in situ in small spaces. Moreover, since the device shape and microelectrode arrangement were all defined by photolithography, the proposed fabrication procedure was flexible and appropriate for fabrication of various types of MEAs.

Characterization, modeling and application of aerobic granular sludge for wastewater treatment

Recently extensive studies have been carried out to cultivate aerobic granular sludge worldwide, including in China. Aerobic granules, compared with conventional activated sludge flocs, are well known for their regular, dense, and strong microbial structure, good settling ability, high biomass retention, and great ability to withstand shock loadings. Studies have shown that the aerobic granules could be applied for the treatment of low- or high-strength wastewaters, simultaneous removal of organic carbon, nitrogen and phosphorus, and decomposition of toxic wastewaters. Thus, this new form of activate sludge, like anaerobic granular sludge, could be employed for the treatment of municipal and industrial wastewaters in near future. This chapter attempts to provide an up-to-date review on the definition, cultivation, characterization, modeling and application of aerobic granular sludge for biological wastewater treatment. This review outlines some important discoveries with regard to the factors affecting the formation of aerobic granular sludge, their physicochemical characteristics, as well as their microbial structure and diversity. It also summarizes the modeling of aerobic granule formation. Finally, this chapter highlights the applications of aerobic granulation technology in the biological wastewater treatment. It is concluded that the knowledge regarding aerobic granular sludge is far from complete. Although previous studies in this field have undoubtedly improved our understanding on aerobic granular sludge, it is clear that much remains to be learned about the process and that many unanswered questions still remain. One of the challenges appears to be the integration of the existing and growing scientific knowledge base with the observations and applications in practice, which this paper hopes to partially achieve.

Simultaneous determination of nitrate and dissolved oxygen under neutral conditions using a novel silver-deposited gold microelectrode

In this work a novel gold-based microelectrode was successfully fabricated using photolithographic techniques and electrochemical deposition for a simultaneous determination of nitrate and dissolved oxygen (DO) under neutral conditions. Three-dimensional tree-shaped silver nanorods were formed on the gold surface through electrochemical deposition and they had an electrochemical catalytic reductive activity for both nitrate and oxygen under neutral conditions. Thus, the silver nanorods served as the active center of the microelectrode. The microelectrode could be renewed over five times. Linear sweep voltammetry was employed to quantitatively analyze the nitrate and DO in solution. The microelectrode was used to measure the nitrate and DO microprofiles in a nitrifying aerobic granule from a sequencing batch reactor, which shows that denitrification did not occur in the tested granule. The measurement results demonstrate that the microelectrode was able to simultaneously determine the nitrate and DO levels in the granules under neutral conditions accurately and precisely.

Innovative solid-state microelectrode for nitrite determination in a nitrifying granule

Nitrite is an intermediate of both nitrification and denitrification in biological removal of nitrogen from wastewater, and in situ measurement of nitrite concentration in a biofilm or microbial granules is highly desirable. However, a solid-state microelectrode for nitrite determination is not available yet In this work, a solid-state microelectrode was manufactured through electrochemical codeposition of Pt--Fe nanoparticles on a gold microelectrode fabricated using photolithography for in situ nitrite determination. This gold-based microelectrode could be used as a more cost-effective, efficient, and reliable alternative to the liquid membrane microelectrode. Nanoparticles with an average diameter of 50 nm were observed on the surface of the chemically modified electrode. A sigmoid peak at ca. 0.7 V (vs Ag/AgCl) was found on the linear sweep voltammogram in nitrite solutions by using the fabricated microelectrode. The peak height of the first-order derivative of the sigmoid peak was proportional to the nitrite concentration of 0.001--0.05 M and could be used for quantitative determination of nitrite. The detection limits (S/N = 3) were approximately 3 x 10(-5) M. The nitrite microprofiles of aerobic granules from a nitrifying reactor were measured with the microelectrode to demonstrate its potential applications with high spatial resolution.