GRADIENT VELOCITY IN MICRO HYDRAULIC FLOCCULATION SYSTEM, ANALYZED WITH PILOT WATER TREATMENT PLANT AND COMPUTER FLUID DYNAMIC (CFD) MODELS, USING TU DELFT'S RAW CANAL WATER

• Main Author: GARSADI , RUSNANDI
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/11356
• Institution: Institut Teknologi Bandung

Conventional Water Treatment Plant (WTP) is a water treatment installation which uses coagulation, flocculation, sedimentation, filtration and disinfection systems by the process of adding coagulant to plain raw water containing colloidal pollutant. Up till now conventional WTP is still the most widely used system in Indonesia and abroad. In Indonesia, out of the capacity of installed clean water system of 110.000 liter/second, as much as 80.000 liter/second still use conventional WTP. The problem found in conventional WTP in Indonesia is, generally the WTP is only capable to do clean water processing up to 65-70% out of the designed system capacity. This can be seen from the flocculation result in which the floc formation is not optimum and the degree of turbidity of the resulting sedimentation precipetate is still high. The detention time of the flocculation system in a conventional WTP is generally 20-30 minutes, based on its design criteria. But if raw water treatment analysis is done by using jar test, the optimum floc formation can be produced with the detention time of 5-10 minutes. The difference of these detention times should be considered for implementing a further study.<p>Coagulation process is a quick mixing of colloidal raw water with coagulants such as Alum, Ferric Chloride or PAC, which is a process of destabilization of the colloidal particles' Brownian motion. Flocculation process is continuation of the coagulation process, in which the colloidal particle (-) and coagulant (+) come together and stick to each other (forming a chemical bond). Further on, they form floc masses which grow bigger, coagulate or form an aggregation between the micro floc particles and the colloidal particles, and with mutual floc particles. In the flocculation process where the aggregated colloidal particles grow into flocs, it is influenced by Brownian motion, gradient velocity (G) and breakup.<p>The flocculation process is the core of a series of steps in water treatment process using a conventional WTP, this is the process when the floc formation reaches its optimum, i.e. when all of the colloidal particles completely coagulate to form solid flocs which precipitated rapidly, so the effluent of the clarifier or sedimentation systems will produce a low turbidity degree. In this way the resulted clogging and run time of the filter working system become longer. Besides, the filter medium washing time or back wash will be shorter. Micro hydraulic flocculation is a flocculation process in which the hydrodynamics of the flow velocity is observed in three dimensions. Flow velocity is the main parameter which determines the measure of gradient velocity to produce optimum floc formation. The theory and formulation of gradient velocity (G) are referred from the research results of Smoluchowski, (1917) and (Camp-Stein, 1943), they were the first researchers who formulated the coagulation and flocculation processes, which have been widely used as a reference up till now. The analysis of gradient velocity in flocculation process has been done with a physical model (a pilot plant reactor) and a mathematical model of computer fluid dynamics (CFD-Fluent). The flocculation process experiment was done through the pilot plant reactor with optimum debit and dosages.<p>The flocculation reactor system used was a tangential flow with upward or downward flows which were equipped with a graded flow driver. Direct measurement of the three-dimensional flow velocity in the vertical reactor tube, using Acoustic Doppler Velocitymeter (ADV-Nortek) was a new method with a high level of difficulty. This measurement was done by using a reactor tube 200 mm in diameter and 3.4 m high, which required special techniques and means for measuring the flow velocity. Laser Doppler Velocitymeter (LDV) could not be used in the measurement, because the tube's surface was not smooth. A current meter could not be used either, because it would hinder the flow and dimension of the apparatus.<p>The research was done by experimenting with a flocculation pilot plant reactor, which was specially designed at the Water Technology Laboratory, Delft University of Technology, the Netherlands. Analysis of the flocculation system was done by using raw water from the TU Delft's canal. The raw water's turbidity condition was between 100-300 NTU, pH between 7-8, the minimum temperature was 6 oC. Measurement of the flow velocity was done at the optimum debit of 500 L/hour at the location of floc formation. Measurement points were taken at three plane cut locations, they were +30 cm, +110 cm, and +180 cm from the base of the tube. Measurement point grids were made on every plane cut at 20 mm intervals: plane cut 1 at the process of coagulation with Td = 0.85 minutes; plane cut 2 at the early process of floc formation with Td = 0.34 minutes; and plane cut 3 at the floc formation process with Td = 3.12 minutes. The flocculation reactor effluent was reached at Td= 8.1 minutes.<p>The verification of flow velocity as the result of the CFD model was done by comparing the velocity profile of the ADV and CFD measurement results at the three equal plane cuts. The relation between the gradient velocity v.s Td was obtained from the flow velocity measurement results by using ADV and CFD. The resulting graph pattern of the relation between the gradient velocity v.s Td showed that at sub-tropical condition the exponential graph had a sharper decrease and the Td was longer, while at tropical condition the graph was more sloped and the. Td was shorter. The optimum flocculation process was observed at the laminary flow with Reynolds number between 200-2000. By this method, the flow velocity profile characteristic of optimum floc formation could be obtained, it was plotted as a relationship between G v.s Td. The micro hydraulic flocculation system worked optimally at Td 8-10 minutes. This result can be put into consideration for increasing the capacity of existing WTPs, and the mini coagulation and flocculation reactors can be put into consideration to replace the function of jar test apparatus and will be able to be analyzed in further research. <br />