Prediction of Flow Field And Oxygen Utilization Rate (Our) In Orbal Biological System (Obs) Using Cfd

• Main Author: Saad, Noor Aida Saad
• Format: Thesis
• Language: English
• Published: 2016
• Subjects: T Technology; TA1-2040 Engineering (General). Civil engineering (General)
• Online Access: http://eprints.usm.my/40719/1/Prediction_of_Flow_Field_And_Oxygen_Utilization_Rate_%28Our%29_In_Orbal_Biological_System_%28Obs%29_Using_Cfd.pdf; http://eprints.usm.my/40719/

Sistem Orbal Biologi (OBS), yang merupakan salah satu daripada parit pengoksidaan yang telah diubah suai, telah direka untuk memberikan proses olahan biologi yang optimum. Oleh kerana reka bentuk strukturnya yang berpotensi besar untuk menggalakkan proses olahan yang optimum, adalah sangat penting untuk kita mengkaji keseluruhan sistem operasinya. Hasrat ini selari dengan permintaan semasa yang mahukan sistem olahan air sisa yang lebih baik yang bukan sahaja fokus kepada pencapaian standard kualiti efluen yang ditentukan, tetapi juga mengambil kira aspek-aspek lain seperti ekonomi dan alam sekitar. Oleh sebab ini, kajian ini dijalankan untuk memahami sistem olahan OBS yang kompleks ini. Model proses awal dan model Computational Fluid Dynamics (CFD) telah digunakan untuk mendapatkan gambaran yang lebih baik tentang OBS. Model CFD 3 dimensi, 2 fasa yang bersaluran terbuka telah dibangunkan untuk mengkaji mekanisma operasi OBS. Dalam kajian ini, model CFD telah digunakan untuk mewakili OBS untuk mengatasi beberapa limitasi model proses awal. Keputusan simulasi telah digunakan untuk mengkaji corak aliran di seluruh parit, taburan halaju pada kedalaman yang berbeza, agihan halaju untuk keadaan operasi yang berbeza, pecahan isipadu udara dan air di seluruh parit, pengedaran tekanan merentasi saluran dan masa tahanan hidraulik. Keputusan model CFD juga digunakan untuk mengira kadar aliran isipadu yang lebih khusus, nilai Oxygen Utilization Rate(OUR) dalam parit dan penggunaan kuasa yang berkaitan dengan nilai-nilai OUR. Berdasarkan kepada kajian kesan kepelbagaian operasi shaf ke atas prestasi OBS, didapati bahawa bilangan shaf yang beroperasi mempengaruhi jumlah keseluruhan penggunaan kuasa dalam sistem ini. Penutupan kombinasi shaf 2(saluran luar) dan shaf 6 (saluran tengah dan dalam) menyumbang kepada penggunaan kuasa yang kurang tetapi masih mempunyai nila purata OUR yang sama (26,483 kg/d) seperti kes asal (26,594 kg/d). Keputusan menunjukkan, walaupun shaf ini ditutup, tetapi tindakbalas biologi di dalam parit masih berterusan seperti tindakbalas biologi bila kesemua shaf beroperasi. Ramalan penggunaan kuasa di masa hadapan yang dikaitkan dengan peningkatan nilai OUR, yang mungkin berlaku kerana standard kualiti efluen yang lebih ketat, juga telah dibentangkan dan dibincangkan dalam kajian ini. Berdasarkan keputusan, saluran paling luar merekodkan penggunaan oksigen tertinggi secara keseluruhannya, iaitu sebanyak 79% berbanding dengan saluran tengah dengan hanya 9% dan diikuti oleh saluran dalaman dengan 12%. Pada masa akan datang, jika standard kualiti efluen yang lebih ketat dikenakan, kadar penggunaan oksigen juga akan pasti meningkat. Jika kadar penggunaan oksigen pada masa kini iaitu 21109 kg/d meningkat kepada 30%, ia akan menyebabkan peningkatan penggunaan elektrik sebanyak 3,241 MW/tahun (hampir RM 1,092,284 setahun). Melalui kajian ini, peningkatan pemahaman tentang bagaimana simulasi seperti ini dapat digunakan dalam pemodelan proses olahan air sisa berjaya dilaksanakan. Ia amat berguna dalam proses permodelan proses olahan air sisa ddi masa akan datang. ________________________________________________________________________________________________________________________ Orbal Biological System (OBS), which is one of the modified oxidation ditches, has been designed to provide an optimized approach of the biological treatment process. Due to the structural design of the OBS that has great potential to promote optimal treatment processes, it is very crucial to study its operating system. This intention is in line with the current demand on wastewater treatment system, which not only focusing on the achievement of specified effluent quality standards, but also taking into consideration other aspects such as economic and environmental. Because of this reason, the study was conducted to understand the complicated process of OBS. The preliminary process model and Computational Fluid Dynamics (CFD) model were used to obtain a better picture of the OBS. Three-dimensional, two-phase and an open channel CFD-based model was developed to study the mechanism of OBS. In the study, the CFD model was used to represent the OBS to overcome some of the limitations of the preliminary process model. The simulation results were used to study the flow pattern across the ditch, the velocity distribution at different depths, velocity distribution for different operating conditions, the volume fraction of air and water in the ditch, the pressure distribution across the channels and hydraulic residence time. The results of CFD model were used to calculate more specific volumetric flow rate, the Oxygen Utilization Rate (OUR) of the ditch and the power consumption associated with the values of OUR. Based on the study on the effect of different operating shafts on the performance of OBS, it was found that turning off the combination of shaft 2 (outer channel) and shaft 6 (middle and inner channel) was contributing to less power consumption but still having almost the same average OUR value (26,483 kg/d) like the real case (26,594 kg/d). The results show that although these shafts are turned off, but the biological reactions inside the ditch still occur as the biological reactions when all the shafts are operated. Forecast future power consumption associated with the OUR values, which may occur due to effluent quality standards that are more stringent, was also presented and discussed in this study. Based on the results, the outermost channels recorded the highest overall oxygen consumption, which is 79% compared with the middle channel with only 9%, followed by inner channel with 12%. In the future, if more stringent effluent quality standards imposed, the rate of oxygen consumption will definitely increase. If the rate of oxygen consumption at present, namely 21,109 kg/d increased to 30%, it will lead to increased electricity use by 3,241 MW/year (nearly RM 1,092,284/year). The research provides a better understanding on how this simulation tool will be able to be applied within wastewater process modelling. It can considerably contribute to the further expansion of wastewater treatment process models.