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Secondary treatment
Published in Rumana Riffat, Taqsim Husnain, Fundamentals of Wastewater Treatment and Engineering, 2022
The basic activated sludge process consists of three components, as illustrated in Figure 8.3. They are, (i) a biological reactor where the microorganisms are kept in suspension and aerated, (ii) a sedimentation tank or clarifier, and (iii) a recycle system for returning settled solids from the clarifier to the reactor. Wastewater flows continuously into the aeration tank or biological reactor. Air is introduced to mix the wastewater with the microorganisms and to provide the oxygen necessary to maintain aerobic conditions. The microorganisms degrade the organic matter in wastewater and convert them to cell mass and waste products. The mixture then goes to the secondary clarifier, where clarification of effluent and thickening of settled solids takes place. The clarified effluent is discharged for further treatment or disposal. The thickened solids are removed as underflow. A portion of the underflow is wasted (called waste activated sludge WAS), while the remainder (20–50%) is returned to the aeration tank as return activated sludge (RAS). The return sludge helps to maintain a high concentration of active biomass in the aeration tank.
Design and Operation of Biological Phosphorus Removal Facilities
Published in Richard Sedlak, Phosphorus and Nitrogen Removal from Municipal Wastewater, 2018
In most activated sludge treatment systems, the RAS is removed from the secondary clarifiers and pumped back to the upstream end of the aeration basin. This is generally a continuously pumped flow stream. The RAS feed to the Phostrip process may either be diverted from the pumped RAS line through a flow control valve, or a separate pumping system may be utilized drawing from a common RAS wet well. Since the total RAS withdrawal rate from the secondary clarifiers influences the operation and performance of the clarifiers, it is preferable to divert flow from the pumped RAS line to the Phostrip process to allow better control over clarifier operation. The use of a separate pumping system increases the complexity of controlling the RAS removal rate from the clarifiers. Flow metering should be provided on the RAS diversion line to allow control and monitoring of the flow. The RAS should enter the stripper basin below the water surface. A free, plunging discharge into the basin should be avoided as it will entrain air and inhibit the anaerobic processes occurring in the stripper.
Hybrid phase error analysis for dual-polarized reflectarray antenna using asymmetric split patch element
Published in Electromagnetics, 2022
Min Wang, Yuxin Mo, Xuan Li, Nan Hu, Wenqing Xie, Zhengchuan Chen
Reflectarray antenna (RA) as one of the strong competitors of high-gain antenna, which combines the advantages of parabolic reflector antennas and microstrip array antennas (Huang and Encinar 2008; Nayeri, Yang, and Elsherbeni 2018). Compared with the traditional phased array antenna, the RA does not require complicated feeding network, which makes the cost and complexity of antennas lower. Even the RA has higher space utilization, lower loss and simpler structure than transmitarray antenna (TA). Thus, the RAs have the advantages of easy-to-fabricate structure, low-profile, low-cost and high efficiency. It becomes a good candidate for various application scenarios, such as satellite communication (Abdollahvand et al. 2020; Deng et al. 2018; Henderson and Ghalichechian 2020; Peng, Qu, and Xia 2020; Phua, Lim, and Chung 2020; Zhang et al. 2021b), airborne communication, radar beam steering (Wu et al. 2021; Yang et al. 2017; Yin et al. 2021) and 5 G millimeter-wave communication (Mei, Zhang, and Pedersen 2020), etc.
Sludge Pre-Treatment through Ozone Application: Alternative Sludge Reuse Possibilities for Recirculating Aquaculture System Optimization
Published in Ozone: Science & Engineering, 2019
Desislava Bögner, Frederike Schmachtl, Björn Mayr, Christopher P. Franz, Sabine Strieben, Gregor Jaehne, Kai Lorkowski, Matthew J. Slater
Large amounts of sludge are produced in RAS systems and its disposal is costly both in economic terms and in its impact on overall water use. In the current study, sludge composition was determined with regard to applicable nitrogen and carbon availabilities in response to ozonation with a long-term view to sludge recycling as a carbon source, e.g. for denitrification. Ozonation resulted in an increase on the SAFA:MUFA-PUFA fatty acid ratio in solid or liquid phases profiles, in significant increases in available dissolved carbon and nitrogen organic compounds, as well as ammonium, and otherwise resulted in a total depletion of nitrite and a marked reduction in the turbidity of the samples due to a reduction in solid contents.
Comparison and strategy of nitrogen removal at different low temperatures in a pilot-scale A2/O system
Published in Environmental Technology, 2019
Xiaoying Liu, Ao Zeng, Yatao Wang, Peiju Liu, Yuan Chen, Yinghe Jiang
To improve nitrogen removal efficiency at low temperatures (<15°C), several methods have been proposed: (1) strengthening the activities of nitrification microorganism by increasing sludge return ratio, (2) increasing DO concentration during the aerobic process, and (3) extending the residence time of the aerobic stage [13,14]. In addition, previous studies have indicated that by providing enough microbial biomass and easily degradable organic compounds, ideal denitrification can be achieved [15–18]. In the A2/O process, return activated sludge (RAS) includes external RAS and internal RAS. Increasing external RAS means to supply more microorganisms to degrade the nutrients. The optimal value is 75–100% [19]. Increasing internal RAS would stimulate denitrification of the system and effectively reduce the effluent concentration, leading to the low effluent TN. The optimal internal RAS is 100–300% [11,12,20–22]. However, high RAS would sharply elevate investment and running costs [23]. DO concentration has acted as a key factor for the nitrification mechanism. For example, low DO (<1.0 mg L−1) might induce partial nitrification and denitrification [24]. In large-scale applications, the control of low DO is difficult. In order to ensure complete nitrification, DO concentration should be over 2.0 mg L−1 [25,26]. Furthermore, oxygen transfer efficiency, which has a direct effect on the DO concentration of bulk solution, is associated with the aeration device and mixed liquor suspended solid (MLSS). For example, in the biofilm reactor, when DO concentration is up to 3.5–7 mg L−1, excellent nitrification performance is achieved, which indirectly shows that oxygen transfer is very vital [27]. Increasing the residence time of the aerobic stage would supply sufficient time for complete nitrification and need a large reactor volume, which would lead to high capital costs [28,29]. Obviously, high microbial biomass would favour the growth of nitrification bacteria, and thus improve nitrification and denitrification efficiencies [26,28–30]. Previous researches about the influence of low temperature on nitrogen removal mainly focus on the DO and RAS. These studies provide a theoretical and practical basis for nitrogen removal at low temperatures [26,28]. However, these results are obtained primarily through the lab-scale experiments and so lack instructive suggestions for practical operations. Furthermore, the investigations of nitrogen removal in the A2/O process with low temperature and RAS, which would effectively decrease the running costs of WWTPs, are very few.