A/O process for wastewater treatment

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I. A/O Process 1. Basic Principle 　A/O is an abbreviation for Anoxic/Oxic. Its advantage lies in the fact that, in addition to decomposing organic pollutants, it also has the capability to remove nitrogen and phosphorus. It utilizes anaerobic hydrolysis technology as a pretreatment step for activated sludge; therefore, the A/O process represents an improved version of the activated sludge method. 　　The A/O process connects a preceding anoxic stage with a subsequent aerobic stage; in the A stage, the DO level is no more than 0.2 mg/L, while in the O stage, the DO level is 2–4 mg/L. In the anoxic zone, heterotrophic bacteria hydrolyze suspended pollutants such as starch, fibers, and carbohydrates, as well as soluble organic substances in wastewater, into organic acids. This process breaks down large molecular organic compounds into smaller ones, and converts insoluble organic substances into soluble ones. When these products resulting from anoxic hydrolysis enter the aerobic tank for aerobic treatment, it enhances the biodegradability of the wastewater and improves the efficiency of oxygen utilization ； In the anoxic zone, heterotrophic bacteria convert pollutants such as proteins and fats into ammonia by releasing ammonia (NH3, NH4+) from the nitrogen in organic chains or the amino groups in amino acids. Under conditions of sufficient oxygen supply, autotrophic bacteria carry out nitrification, oxidizing NH3-N (NH4+) to NO3-. This nitrogen is then returned to tank A through recirculation. In the anoxic zone, denitrifying bacteria reduce NO3- to gaseous nitrogen (N2), thus completing the cycle of C, N, and O in the ecosystem and enabling the harmless treatment of wastewater. 2. Characteristics of the A/O internal circulation biological nitrogen removal process Based on the description of the basic process for biological nitrogen removal above, and taking into account years of experience in removing nitrogen from coking wastewater, we have concluded that the (A/O) biological nitrogen removal process has the following advantages: (1) High efficiency. This process achieves high removal efficiency for organic matter and ammonia nitrogen in wastewater. When the total retention time is greater than 54 hours, the effluent after biological nitrogen removal is further treated through coagulation and sedimentation, which allows the COD level to be reduced to below 100 mg/L; other parameters also meet the discharge standards, with a nitrogen removal rate of over 70%. 　　(2) The process is simple, requires less investment, and has low operating costs. This process uses the organic matter in wastewater as a carbon source for denitrification, thus eliminating the need to use expensive carbon sources such as methanol. In particular, after a device for removing fixed ammonia was installed in the ammonia vaporization tower, the carbon-to-nitrogen ratio increased, and the alkalinity generated during denitrification correspondingly reduced the amount of alkali required for the nitrification process. 　　(3) The anoxic denitrification process exhibits a high degradation efficiency for pollutants. For parameters such as COD, BOD5, and SCN-, the removal rates in the anoxic zone were 67%, 38%, and 59%, respectively; while the removal rates for phenols and organic matter were 62% and 36%, respectively. Therefore, denitrification is the most economical and energy-saving degradation process. 　　(4) High volumetric load. By employing enhanced biochemical processes in the nitrification stage and membrane technology with high sludge concentrations in the denitrification stage, the sludge concentration during both nitrification and denitrification is effectively increased, resulting in a higher volume load compared to similar processes abroad. 　　(5) The anoxic/aerobic process has a strong capacity to withstand load shocks. When the quality of the incoming water varies greatly or the pollutant concentration is high, this process can still operate normally, making its operation and management quite simple. Through the comparison of the above processes, it is easy to see that the biological denitrification process itself not only removes nitrogen but also degrades organic substances such as phenols, cyanides, and COD. Taking into account the characteristics of water volume and quality, we recommend the anaerobic/aerobic (A/O) biological nitrogen removal (internal circulation) process, so that the wastewater treatment system can not only meet the nitrogen removal requirements but also ensure that other parameters comply with the emission standards. 3. Disadvantages of the A/O process 　　1. Due to the lack of an independent sludge return system, it is not possible to develop sludge with unique functions, resulting in a low degradation rate for hard-to-degrade substances ； 　　2. To improve denitrification efficiency, it is necessary to increase the internal circulation ratio, which in turn raises operating costs. Furthermore, the recirculating liquid comes from the aeration tank and contains a certain level of DO, which makes it difficult to maintain an ideal anoxic condition in section A, affecting the denitrification process and making it hard to achieve a nitrogen removal rate of 90%. 　　3. Influencing factors: Hydraulic retention time (nitrification > 6 hours, denitrification < 2 hours); sludge concentration MLSS (>3000 mg/L); sludge age (>30 days); N/MLSS load rate (<0.03); total nitrogen concentration in the influent water (<30 mg/L). II. A2/O process: 1. Basic principle: The A2/O process is an abbreviation for Anaerobic-Anoxic-Aerobic; it refers to the biological nitrogen and phosphorus removal process that involves anaerobic, anoxic, and aerobic conditions. The treatment efficiency of this process generally reaches: 90%~95% for BOD5 and SS, over 70% for total nitrogen, and around 90% for phosphorus. It is suitable for medium and large-scale municipal wastewater treatment plants that require nitrogen and phosphorus removal. However, the capital and operating costs of the A2/O process are higher than those of the conventional activated sludge method, and it requires high levels of operational management. Therefore, given China’s current circumstances, this process is only employed when the treated wastewater is discharged into enclosed or slowly flowing water bodies, causing eutrophication that in turn affects water supply sources. 2. Characteristics of the A2/O process: 　　(1) High efficiency in removing pollutants, stable operation, and good resistance to shock loads. 　 　　(2) Good sludge settling performance. 　　(3) The organic combination of three different environmental conditions—anaerobic, anoxic, and aerobic—and various types of microbial communities enables the simultaneous removal of organic matter as well as nitrogen and phosphorus. 　　(4) The denitrification efficiency is affected by the reflux ratio of the mixed liquid, while the phosphorus removal efficiency is influenced by the DO and nitrate oxygen carried in the recycled sludge; therefore, the efficiency of both denitrification and phosphorus removal cannot be very high. 　　(5) In the process of simultaneous deoxygenation, phosphorus removal, and organic matter removal, this process has the simplest flow scheme, and its total hydraulic retention time is also less than that of other similar processes. 　　(6) Under alternating anaerobic-anoxic-aerobic conditions, filamental bacteria do not proliferate in large numbers; the SVI is generally less than 100, and sludge bulking does not occur. 　　(7) The phosphorus content in sludge is high, generally above 2.5%. 3. Disadvantages of the A2/O process · The volume of the reaction tank is even larger than that in the A/O nitrogen removal process ； 　　·The backflow rate of sludge is high, resulting in higher energy consumption ； 　　·The cost is relatively high for small and medium-sized sewage treatment plants ； 　　·The economic benefits of biogas recovery and utilization are poor ； 　　·Sludge leachate requires chemical phosphorus removal. III. Oxidation Ditch 1. Basic Principle The oxidation ditch, also known as an oxidation channel, is so named because its structure takes the form of a closed circular ditch. It is a variant of the activated sludge process. Because the wastewater and activated sludge continuously circulate in the aeration channel, it is sometimes referred to as a “circular aeration tank” or a “non-terminal aeration tank”. The oxidation ditch features a long hydraulic retention time and a low organic load; it essentially belongs to a delayed aeration system. An oxidation ditch generally consists of a ditch body, aeration equipment, inlet and outlet devices, as well as flow guidance and mixing equipment. The planar shape of the ditch body is usually circular, but it can also be rectangular, L-shaped, circular, or of other shapes; the end faces of the ditch are often rectangular or trapezoidal in shape. 2. Characteristics of the oxidation ditch process 　　(1) Diversity in structural forms 　　In its basic form, the aeration tank of an oxidation ditch is in the shape of a closed channel, and the shape and structure of such channels vary; they can be circular, oval, or other shapes. It can be a single-groove system or a multi-groove system ； A multi-groove system can be a set of concentric, interconnected grooves, or it can be a set of grooves that are parallel to each other and have the same size. There are oxidation ditches that are constructed separately from secondary sedimentation tanks, as well as those that are built together; the latter can be further divided into internal and external types, and so on. The variety of design configurations endows the oxidation ditch with flexible operating capabilities, allowing it to function in any mode typical of activated sludge systems, and it can be combined with other process units to meet various requirements regarding the quality of the effluent. 　　(2) Diversity of aeration equipment Commonly used aeration equipment includes rotating brushes, rotary disks, surface aerators, and jet aerators. Different aeration devices result in various types of oxidation ditches, such as the Carrousel oxidation ditch that uses surface aerators, and the Pasville oxidation ditch that employs rotating brushes. Unlike other activated sludge processes, aeration devices are installed only at one or several locations within the ditch; their number depends on the scale of the treatment facility, the quality of the raw wastewater, and the design of the oxidation ditch. In addition to supplying sufficient oxygen, these aeration devices are also responsible for maintaining a water flow velocity of at least 0.3 m/s within the ditch, thereby ensuring circulation and keeping the activated sludge in suspension. 　　(3) Adjustable aeration intensity The aeration intensity in an oxidation ditch can be adjusted in two ways. The first method is adjustment through the outlet overflow weir: by adjusting the height of the overflow weir, the water depth in the channel is changed, which in turn alters the submersion depth of the aeration device, allowing its oxygenation level to meet the requirements of operation. Changes in the submersion depth also affect the driving force of the aeration equipment, thereby allowing for some regulation of the inflow velocity ； The second method is to adjust the speed of the aerators directly: thanks to the development of mechanical and electrical equipment as well as automatic control technologies, it is now possible to adjust the speed of the aerators in oxidation ditches, thereby controlling the intensity of aeration. 　　(4) Simplification of pretreatment and sludge treatment. The hydraulic retention time and sludge age in an oxidation ditch are longer than those in conventional biological treatment methods, allowing both suspended and soluble organic matter to be thoroughly stabilized; as a result, a primary sedimentation tank is not necessary in an oxidation ditch. Due to the long sludge age and low load in the oxidation ditch process, the excess sludge discharged is highly stable, and its amount is also low. Therefore, anaerobic digestion is no longer necessary; only concentration and dewatering are required. 3. Disadvantages of the oxidation ditch process: (1) Sludge bulking problem – When there is an abundance of carbohydrates in the wastewater, an imbalance in N and P levels, a low pH value, an excessive sludge load in the oxidation ditch, insufficient dissolved oxygen concentration, and poor sludge removal, it is easy to trigger filamentous sludge bulking ； Non-filamentous sludge bulking mainly occurs when the wastewater temperature is low and the sludge load is high. The microbial load is high, and bacteria absorb large amounts of nutrients. Due to the low temperature, the metabolic rate is slow, resulting in the accumulation of large quantities of highly viscous polysaccharides. This increases the amount of water attached to the surface of the activated sludge, leading to a high SVI value and sludge bulking. 　　(2) Foam problem: Due to the large amount of grease present in the influent water, the treatment system is unable to remove it completely and effectively; some of this grease accumulates in the sludge. As a result of oxygenation and stirring by the rotating brushes, a large amount of foam is generated ； A long sludge age and aged sludge also tend to cause foaming. 　　(3) Sludge floating issue: When the oil content in the wastewater is too high, the sludge in the entire system becomes lighter, making it difficult to control its retention time in the secondary sedimentation tank. This can lead to oxygen deficiency and the floating of decomposed sludge ； When aeration lasts too long, high levels of nitrification occur in the tank, resulting in high nitrate concentrations; denitrification then takes place easily in the secondary sedimentation tank, producing nitrogen gas that causes the sludge to float to the surface ； Furthermore, if the oil content in the wastewater is too high, the sludge may carry oil and float to the surface. 　　(4) Issues of uneven flow velocity and sludge deposition: In an oxidation ditch, in order to achieve its unique mixing and treatment effects, the mixed liquid must circulate within the ditch at a certain flow velocity. It is generally believed that the minimum flow velocity should be 0.15 m/s, and the average flow velocity required to prevent sedimentation should be 0.3–0.5 m/s. The aeration equipment in oxidation ditches is typically made up of aeration rotary brushes and aeration rotary disks; the immersion depth of the rotary brushes is 250–300 mm, while that of the rotary disks is 480–530 mm. Compared to the water depth of the oxidation ditch (3.0–3.6 m), the rotating brush accounts for only 1/10 to 1/12 of this depth, while the rotating disk accounts for only 1/6 to 1/7. As a result, the flow velocity at the upper part of the oxidation ditch is relatively high (around 0.8–1.2 m, or even higher), whereas the flow velocity at the bottom is very low (especially below 2/3 or 3/4 of the water depth, where there is almost no flow in the mixed liquid). This leads to significant sediment accumulation at the bottom of the ditch (sometimes with a sediment thickness of up to 1.0 m), **which reduces the effective volume of the oxidation ditch, diminishes its treatment efficiency, and affects the quality of the treated water.** IV. SBR Process 1. Process Principle A certain amount of activated sludge is pre-cultured in the reactor. When wastewater enters the reactor and comes into contact with the activated sludge in the presence of oxygen, microorganisms utilize the organic substances in the wastewater for metabolism, breaking down these organic substances while simultaneously causing the microbial cells to proliferate. The microbial cell material is separated from water by precipitation, thereby treating the wastewater. Its treatment process is mainly accomplished through several purification steps, including initial removal and adsorption, microbial metabolism, and the formation of flocs along with their flocculation and sedimentation properties. 2. Characteristics of the SBR process 　　(1) The ideal plug-flow process increases the driving force for biochemical reactions, improving efficiency; anaerobic and aerobic conditions alternate within the tank, resulting in good purification effects. 　　(2) It has stable operating performance: the wastewater settles in an ideal stationary state, requiring little time, high efficiency, and producing water of good quality. 　　(3) It can withstand shock loads; the retained treated water in the tank acts to dilute and buffer the wastewater, effectively resisting the impact of water volume and organic pollutants. 　　(4) The various steps in the processing process can be adjusted according to water quality and volume, allowing for flexible operation. 　　(5) It requires few processing devices, has a simple structure, and is easy to operate and maintain. 　　(6) A concentration gradient of DO and BOD5 exists in the reaction tank, effectively controlling activated sludge bulking. 　　(7) The SBR process system is also suitable for modular construction methods, facilitating the expansion and renovation of wastewater treatment plants. 　　(8) Denitrification and phosphorus removal are achieved by appropriately controlling the operating mode to alternate between aerobic, anoxic, and anaerobic conditions, resulting in excellent denitrification and phosphorus removal effects. 　　(9) The process flow is simple and the cost is low. The main equipment consists of only one sequencing batch reactor; there is no secondary sedimentation tank or sludge return system, and the equalization tank and primary sedimentation tank can also be omitted, resulting in a compact layout and reduced land use. 3. Disadvantages of the SBR process 　　(1) Intermittent operation cycle, requiring high levels of automatic control ； 　　(2) Operation at variable water levels results in increased power consumption ； 　　(3) The denitrification and phosphorus removal efficiency is not very high ； 　　(4) The stability of sludge is not as good as that in anaerobic nitrification. V. CAST Process 1. Principle of the CAST Process The CASS biological treatment method is short for Cyclical Activated Sludge System; a CASS tank consists of a pre-reaction zone and a main reaction zone. In the pre-reaction zone, microorganisms can rapidly adsorb most of the soluble organic matter in wastewater through an enzyme-mediated transfer mechanism, undergoing a process of rapid accumulation of high loads of substrate. This provides good buffering effects against variations in water quality, volume, pH, and the presence of toxic substances. It also inhibits the growth of filamentous bacteria, thereby effectively preventing sludge bulking ； Subsequently, a matrix degradation process at a lower load occurs in the main reaction zone. The CASS process integrates reaction, precipitation, drainage, and functional operations. The degradation of pollutants occurs over time as a result of fluid flow, while microorganisms are subjected to periodic changes between aerobic, anoxic, and anaerobic conditions, thereby enabling the removal of pollutants. It also possesses effective functions for nitrogen and phosphorus removal. 2. Characteristics of the CAST process 　　(1) Flexible and reliable operation 　　　　● The biological selector can operate in three modes – aerobic, anoxic, and anaerobic – depending on the quality of the wastewater. The selector can operate at a constant volume or a variable volume. ● Its operating state can be adjusted as needed to take advantage of the physiological characteristics of different microorganisms. ● The variable volume of the selector helps prevent sludge bulking, thereby enhancing the reliability of the system. ● It has strong resistance to shock loads, making it suitable for treating industrial wastewater and municipal sewage. (2) Fewer treatment structures are required, resulting in a simpler process. ● The total volume of the tanks is reduced, which lowers the costs associated with civil engineering work. ● There is no need for secondary sedimentation tanks or sludge scraping equipment, nor for sludge return pump stations. (3) Phosphorus and nitrogen removal can be achieved. ● By adjusting the sequence of aeration and non-aeration in the biological selector with variable volume, the efficiency of biological phosphorus and nitrogen removal is improved. (4) Investment is saved. ● Fewer structures are needed, reducing the land area required. ● The equipment and control systems are simple. ● The aeration intensity is low, so no large-scale gas supply equipment is necessary. ● Operating costs are low. 3. Process disadvantages (1) Intermittent operation is required, which demands high levels of automatic control ； 　　(2) Operation at variable water levels results in increased power consumption ； 　　(3) Lower volume utilization rate ； 　　(4) The stability of sludge is not as good as that in anaerobic nitrification.