Rusnandi Garsadi TU DEFT

GRADIENT VELOCITY PADA SISTEM MICRO HYDRAULIC FLOCCULATION DIKAJI MELALUI MODEL PILOT WATER TREATMENT PLANT DAN MODEL COMPUTER FLUID DINAMIC (CFD) MENGGUNAKAN AIR BAKU KANAL TU DELFT

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
PhD Theses from JBPTITBPP / 2009-05-14 15:39:47
Oleh : RUSNANDI GARSADI (NIM: 35003203), S3 - Engineering Sciences
Dibuat : 2008, dengan 13 file

Keyword : Gradient velocity(G), koagulasi, flokulasi, water treatment, koloid, koagulant, aggregasi, floc, break-up, laminar, model pilot plant, baffled, micro hydraulic, waktu detensi(Td), turbidity, acoustic doppler velocimetri (ADV), computer fluid dynamics (CFD)

Water Treatment Plant (WTP) konvensional merupakan instalasi pengolahan air yang menggunakan sistem koagulasi, flokulasi, sedimentasi, filtrasi dan disinfeksi melalui proses pembubuhan koagulant pada air baku tawar dengan polutan koloidal. WTP konvensional sampai saat ini merupakan sistem yang paling banyak dipergunakan, baik di Indonesia maupun di dunia. Di Indonesia dari kapasitas terpasang sistem air bersih 110.000 liter/detik, 80.000 liter/detik dari keseruhan kapasitas tersebut masih menggunakan WTP konvensional.

Permasalahan yang dijumpai pada WTP konvensional di Indonesia ialah bahwa umumnya WTP hanya dapat mengolah air bersih antara 65-70% dari sistem kapasitas disain. Hal ini terlihat dari hasil flokulasi dimana pembentukan floc tidak optimal dan tingkat kekeruhan dari hasil pengendapan sedimentasi masih tinggi. Waktu detensi sistem flokulasi WTP konvensional yang pada umumnya berdasarkan kriteria disain antara 20-30 menit. Sedangkan bila uji pengolahan air baku dilakukan dengan menggunakan jar test, floc optimal dapat dihasilkan dengan waktu detensi antara 5-10 menit. Perbedaan waktu detensi tersebut perlu menjadi perhatian agar dapat dilakukan penelitian lebih lanjut.

Proses koagulasi merupakan pencampuran cepat antara air baku koloidal dengan koagulan (Alum, Ferry chloride, PAC), yang merupakan proses destabilisasi dari gerak brownian partikel koloidal. Sedangkan proses flokulasi merupakan proses lanjutan dari proses koagulasi dimana partikel koloid(-) dan koagulan(+) saling mendekat dan menempel (forming a chemical bond), selanjutnya membentuk kumpulan floc yang semakin membesar, menggumpal atau agregasi antara partikel mikro floc dengan partikel koloid, dan dengan sesama partikel floc. Pada proses flokulasi dimana agregasi partikel koloidal terbentuk menjadi floc optimum, hal ini dipengaruhi oleh gerak brownian, kecepatan aliran (gradient velocity G) dan break-up.

Proses flokulasi merupakan inti, dari serangkaian tahapan proses pengolahan air pada suatu WTP konvensional, dimana bila pembentukan floc optimal, yaitu bila seluruh partikel koloid akan menggumpal sempurna membentuk floc yang padat dan cepat mengendap, sehingga efluent dari sistem clarifier atau sistem sedimentasi menghasilkan tingkat kekeruhan yang rendah. Dengan demikian clogging yang terjadi dan run time sistem kerja filter menjadi lebih panjang. disamping hal tersebut waktu pencucian media filter atau back wash akan menjadi lebih pendek.

Micro hydraulic flokulasi merupakan proses flokulasi dimana hidrodinamika kecepatan aliran ditinjau dalam tiga dimensi. Kecepatan aliran merupakan parameter utama yang menentukan besaran gradient velocity untuk menghasilkan pembentukan floc yang optimum. Teori dan formulasi gradient velocity (G) mengacu pada hasil penelitian von Smoluchowski (1917) dan (Camp-Stein, 1943), yaitu peneliti-peneliti yang pertama kali merumuskan proses koagulasi dan flokulasi, yang banyak dipakai sebagai acuan sampai saat ini. Kajian gradient velocity pada proses flokulasi, dilakukan melalui model fisik (reaktor pilot plant) dan model matematika computer fluid dynamicss (CFD-Fluent). Eksperimen proses flokulasi melalui reaktor pilot plant dilakukan melalui debit dan dosis optimum.

Sistem aliran reaktor flokulasi mengunakan aliran tangensial dengan aliran naik atau turun yang dilengkapi pengarah aliran baffle secara bertingkat. Pengukurun langsung kecepatan aliran tiga dimensi pada tabung reaktor vertikal, dengan menggunakan Acoustic Doppler Velocitymeter (ADV-Nortek) merupakan hal yang baru dengan tingkat kesulitan tinggi. Pengukuran ini menggunakan tabung reaktor berdiameter 200 mm dan tinggi 3.4 m, yang memerlukan teknik dan peralatan khusus untuk mengukur kecepatan aliran. Laser Doppler Velocitymeter (LDV) tidak dapat dipergunakan dalam pengukuran tersebut, karena permukaan tabung tidak rata. Demikian juga halnya dengan current meter tidak dapat dipergunakan, karena akan mengganggu aliran dan dimensi alat tersebut.

Penelitian dilakukan melalui percobaan menggunakan reaktor pilot plant flokulasi, yang dibuat khusus di laboratorium Water Technology, Delft University of Technology, the Netherlands. Kajian sistem flokulasi tersebut menggunakan sumber air baku Kanal Delft. Kondisi kekeruhan air baku antara 100-300 NTU, pH antara 7-8, temperatur minimum 6 oCelsius. Pengukuran kecepatan aliran dilakukan pada debit 500 liter/jam dan terjadinya pembentukan floc optimum. Titik pengukuran dilakukan pada tiga lokasi plan cut, yaitu pada elevasi +30 cm, +110 cm, dan +180 cm dari dasar tabung. Pada setiap plane cut dibuat grid titik pengukuran dengan selang 20 mm. Plane cut 1 pada proses terjadinya koagulasi dengan Td = 0.85 menit. Plane cut 2 pada proses awal pembentukan floc dengan Td = 0.34 menit. Plane cut 3 pada proses pembentukan floc dengan Td = 3.12 menit. Efluent reaktor flokulasi pada Td= 8.1 menit.

Verifikasi kecepatan aliran hasil model CFD dilakukan melalui perbandingan profil kecepatan dari hasil pengukuran ADV dan CFD pada tiga plane cut yang setara. Hubungan gradient velocity terhadap Td diperoleh dari kecepatan aliran hasil pengukuran ADV dan CFD. Hasil pola grafik hubungan antara gradient velocity dan Td menunjukkan bahwa pada kondisi sub-tropis grafik eksponensial menurun lebih curam dan Td lebih panjang, sedangkan pada kondisi tropis grafik lebih landai dan Td lebih pendek. Proses flokulasi optimum terjadi pada aliran laminer dengan bilangan Reynolds antara 200-2000. Karakteristika profil kecepatan aliran pada pembentukan floc optimum dapat diketahui, yang digambarkan melalui hubungan G dan Td. Sistem micro hydraulic flokulasi optimum dengan Td 8-10 menit, dapat dipertimbangkan untuk meningkatkan kapasitas sistem WTP yang ada, dan mini reaktor koagulasi dan flokulasi dapat dipertimbangkan sebagai pengganti fungsi alat jar test dan dapat dikaji dalam penelitian selanjutnya.


Deskripsi Alternatif :

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.

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.

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.

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.

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.

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.

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