Performance Evaluation of Hydrocyclone in Ballasted Flocculation Process for Particle Separation using Computational Fluid Dynamics (CFD)

Abstract

The ballast flocculation (BF) process is widely used in water treatment process in order to cope with deterioration of water quality due to climate change. Microsand and magnetite are commonly used, which act as a seed in ballasted floc. The recovery of the ballast in the BF process can affect the operating cost and system sustainability. In this study, we evaluate various sizes of magnetite particle with the specific gravity of 5.57 and sludge particle with the specific gravity of 1.06 in order to apply the hydrocyclone as a ballast recovery system in the BF process. The conventional purpose of the hydrocyclone is to separate solids and liquids, while the ideal recovery system in the BF process should separate the ballast from the sludge and liquid. In order to estimate the optimal conditions for the ballast recovery and the sludge removal ratio, the CFD modeling was prepared. The average ballast recovery ratio showed up to 99.9 % varying with the inlet flow velocity. As the inlet flow velocity increased, the average sludge removal ratio decreased until 2 m/s, but then again increased after 2 m/s. The highest ratio was approximately 91 % at 3.5 m/s of the inlet flow velocity. These differences are due to the Reynolds number. Although Reynolds numbers in hydrocyclones has been estimated only by the inlet flow velocity, in order to clarify the fish-hook, in the present study, we considered the flow velocity change inside of the hydrocyclone and the weight of the particles. The separation efficiency of each particle shows a fish-hook shape related with the Reynolds number. In the BF process, the ballast recovery ratio should be maintained high, while the sludge recovery ratio should be maintained low. The criteria for the ballast and sludge recovery can be reached by maintaining the range of particle size, simultaneously satisfying this requirement at a certain inlet flow velocity. In other respects, for an optimal ballast recovery and sludge removal, above a certain size of ballast should be used, and the sludge needs to be pulverized to a certain size as the pretreatment of the hydrocyclone. Furthermore, the results on the distribution of three-dimensional velocities confirmed that hydrocyclones in the BF process have to operate at an inlet flow velocity that is higher than the conventional one. The possibility of increasing the sludge removal ratio was newly suggested according to the geometric design combination of hydrocyclone cones. |가중응집침전 공정(Ballasted flocculation process)은 기후 변화로 인한 수질 악화에 대처하기 위해 수처리 공정에서 널리 사용되고 있는 기술 중 하나이다. 가중응집제란 가중응집 플럭에서 핵 역할을 하는 물질로 마이크로샌드(Micro-sand)와 마그네타이트(Magnetite)가 일반적으로 사용된다. 가중응집침전 공정에서 가중응집제의 회수는 시스템 운영 비용과 지속성에 영향을 줄 수 있다. 본 연구에서는 가중응집침전 공정에서 가중응집제 회수 장치로 하이드로사이클론을 적용하기 위해 비중이 각각 5.57, 1.06 인 마그네타이트와 슬러지 입자를 다양한 크기로 평가한다. 하이드로사이클론의 통상적 인 목적은 고체와 액체를 분리하는 것이며, 가중응집침전 공정의 이상적인 회수 시스템은 가중응집제를 슬러지와 액체로부터 분리해야한다. 가중응집제 회수 및 슬러지 제거 비율에 대한 최적 조건을 산정하기 위하여 CFD 해석을 수행하였다. 평균 가중응집제 회수율은 하이드로사이클론의 유입 속도에 따라 최대 99.9%를 나타내며, 유입 속도가 증가함에 따라 평균 슬러지 제거율은 2 m/s 지점까지 감소하지만 2 m/s 지점 이후부터 증가하는 경향을 나타냈다. 최대 평균 슬러지 제거율은 3.5 m/s의 유입 속도에서 약 91% 이다. 이러한 차이는 레이놀즈 수의 변화에 의해 나타난다. 하이드로사이클론의 레이놀즈 수는 입구 유속에 의해서만 추정되어 왔지만, Fish-hook 패턴을 명확하게 분석하기 위해서는 하이드로사이클론 내부의 유속 변화와 입자의 중량을 고려해야 한다. 한편, 각 입자의 분리 효율은 레이놀즈 수와 관련된 Fish-hook 패턴을 나타낸다. 가중응집침전 공정에서 가중응집제 회수율은 높게 유지해야하며, 슬러지 회수율은 낮게 유지해야 한다. 가중응집제 및 슬러지 회수 기준은 특정 유입 속도에서 이 요구 사항을 동시에 충족시키는 입자 크기 범위를 유지함으로써 달성 될 수 있다. 최적의 가중응집제 회수 및 슬러지 제거를 위해, 특정 크기 이상의 가중응집제를 사용해야하며, 슬러지는 회수 시스템의 전처리로써 특정 크기로 분쇄 되어야한다. 다른 측면에서, 가중응집침전 공정에서 하이드로사이클론은 기존의 것에 비해 높은 유입 속도로 운전해야 한다는 것을 3차원 속도 분포 분석을 통해 다시 한번 확인되었다. 또한 하이드로사이클론 콘의 설계 조합에 따라 보다 작은 유입 속도에서도 슬러지 제거율을 향상시킬 수 있는 가능성이 제시되었다.; The ballast flocculation (BF) process is widely used in water treatment process in order to cope with deterioration of water quality due to climate change. Microsand and magnetite are commonly used, which act as a seed in ballasted floc. The recovery of the ballast in the BF process can affect the operating cost and system sustainability. In this study, we evaluate various sizes of magnetite particle with the specific gravity of 5.57 and sludge particle with the specific gravity of 1.06 in order to apply the hydrocyclone as a ballast recovery system in the BF process. The conventional purpose of the hydrocyclone is to separate solids and liquids, while the ideal recovery system in the BF process should separate the ballast from the sludge and liquid. In order to estimate the optimal conditions for the ballast recovery and the sludge removal ratio, the CFD modeling was prepared. The average ballast recovery ratio showed up to 99.9 % varying with the inlet flow velocity. As the inlet flow velocity increased, the average sludge removal ratio decreased until 2 m/s, but then again increased after 2 m/s. The highest ratio was approximately 91 % at 3.5 m/s of the inlet flow velocity. These differences are due to the Reynolds number. Although Reynolds numbers in hydrocyclones has been estimated only by the inlet flow velocity, in order to clarify the fish-hook, in the present study, we considered the flow velocity change inside of the hydrocyclone and the weight of the particles. The separation efficiency of each particle shows a fish-hook shape related with the Reynolds number. In the BF process, the ballast recovery ratio should be maintained high, while the sludge recovery ratio should be maintained low. The criteria for the ballast and sludge recovery can be reached by maintaining the range of particle size, simultaneously satisfying this requirement at a certain inlet flow velocity. In other respects, for an optimal ballast recovery and sludge removal, above a certain size of ballast should be used, and the sludge needs to be pulverized to a certain size as the pretreatment of the hydrocyclone. Furthermore, the results on the distribution of three-dimensional velocities confirmed that hydrocyclones in the BF process have to operate at an inlet flow velocity that is higher than the conventional one. The possibility of increasing the sludge removal ratio was newly suggested according to the geometric design combination of hydrocyclone cones.