KARAKTERISTIK HIDRAULIK FILTRASI DAN CUCIBALIK FILTER BETON

BUDI KAMULYAN, Prof. Dr. Ir. Fatchan Nurrochmad, M.Agr.

2014 | Disertasi | S3 Teknik Sipil

Filter pasir merupakan alat penjernih air yang sampai saat ini masih digunakan secara luas di Indonesia. Filter pasir sangat efektif dalam menahan partikel suspensi, sehingga menghasilkan air jernih setelah proses filtrasi. Penggunaan filter pasir sering mengalami permasalahan penurunan kapasitas dan kemampuan filter menjernihkan air. Penurunan kapasitas disebabkan oleh sumbatan permukaan akibat stratifikasi partikel setelah proses cucibalik (backwashing). Penurunan kemampuan menjernihkan air diakibatkan oleh filter tidak bersih dan pengurangan tebal filter setelah cucibalik. Cucibalik filter pasir sangat kompleks dan mahal terkait dengan kebutuhan air pencuci, waktu dan energi. Filter dengan media beton porus (filter beton) diusulkan sebagai media alternatif / pengganti filter pasir yang diharapkan memberikan solusi dari permasalahan penggunaan filter pasir. Penelitian ditujukan untuk melakukan kajian: 1) perubahan diameter partikel dan porositas filter beton akibat penambahan semen sebagai bahan pengikat pasir; 2) perubahan sifat hidraulik filter beton dari sifat hidraulik filter pasir ditinjau dari kehilangan tinggi tekanan; 3) kapasitas produksi air dan kemampuan filter beton mereduksi kekeruhan air; dan 4) karakteristik cucibalik filter beton ditinjau berdasarkan tingkat kebersihan filter dan kecepatan pelepasan partikel suspensi. Pengembangan teori dilakukan untuk mendapat persamaan teoritis porositas (fb) dan diameter partikel (Dfb) filter beton berbasis porositas (p) dan diameter partikel (Dp) pasir penyusun filter beton. Filter beton tersebut dibuat dengan rasio pasir–semen (m) tertentu. Persamaan terapan Ergun diperoleh dengan memasukkan nilai fb dan Dfb pada persamaan kehilangan tinggi tekanan Ergun (1952). Faktor koreksi () ditambahan pada persamaan terapan Ergun, sehingga diperoleh persamaan teoritis kehilangan tinggi tekanan aliran melewati filter beton. Persamaan teoritis dikalibrasi dengan percobaan filtrasi air dengan 3 model filter beton dengan partikel pasir ukuran 0,425 – 0,85 mm (F-1); 0,85 – 1,00 mm (F-2); dan 1,00 – 2,00 mm (F-3). Uji filtrasi filter F-1, F-2 dan F-3 pada berbagai variasi kecepatan dilakukan untuk mengetahui kapasitas dan kemampuan filter beton mereduksi kekeruhan air. Persamaan teori pelepasan partikel suspensi selama cucibalik filter beton dimodifikasi dari persamaan Huang dan Basagoiti (1989) dengan memasukkan nilai fb dan Dfb. Persamaan teori konstanta pelepasan partikel suspensi (Ksb) dikembangkan dengan penambahan koefisien ()sebagai kuantifikasi faktor-faktor pengaruh pada pelepasan partikel suspensi. Persamaan teoritis Ksb dikalibrasi dengan percobaan cucibalik 3 model filter beton F-1, F-2 dan F-3 pada beberapa variasi kecepatan, sehingga diperoleh nilai . Hasil penelitian menunjukkan bahwa filter beton yang dibuat dengan rasio pasir–semen m<5 menyebabkan porositas filter beton (fb) menjadi 0,5 sampai 0,8 kali porositas filter pasir (p) dan diameter partikel filter beton (Dfb) menjadi 1,1 sampai 1,6 kali diameter partikel pasir (Dp). Rasio pasir–semen m>5 menyebabkan porositas dan diameter partikel filter beton mendekati nilai porositas dan diameter partikel pasir penyusun filter. Rasio pasir–semen m>5 merupakan zone optimal untuk perencanaan filter beton. Perubahan porositas dan diameter partikel meningkatkan kehilangan tinggi tekanan filter beton sebesar 2,4 kali kehilangan tinggi tekanan filter pasir. Perbedaan tinggi tekanan xxi memberikan faktor koreksi (kehilangan tinggi tekanan teoritis antara 0,46 dan 0,65. Gradasi partikel pasir menunjukkan pengaruh yang signifikan terhadap faktor koreksi. Gradasi yang semakin homogen menyebabkan faktor koreksi semakin besar perbedaan kehilangan tinggi tekanan filter. Faktor koreksi kehilangan tinggi tekanan menunjukkan nilai yang lebih besar untuk filter F-2 dengan gradasi yang lebih homogen dibandingkan filter F-1 dan F-3. Kapasitas filter beton mengalirkan air dipengaruhi oleh ukuran partikel pasir penyusun filter dan kecepatan filtrasi. Ukuran partikel dan kecepatan filtrasi yang semakin besar menyebabkan kemampuan filter semakin besar, namun menurunkan kemampuan filter beton dalam mereduksi kekeruhan air. Konstanta pelepasan partikel suspensi (Ksb) dipengaruhi oleh parameter yang menentukan waktu detensi cucibalik yaitu panjang filter, porositas filter dan kecepatan cucibalik. Kalibrasi persamaan teoritis dan hasil penelitian cucibalik filter F-1, F-2 dan F-3 pada kecepatan yang bervariasi menghasilkan nilai koefisien pelepasan partikel () sebesar 3,0. Kata kunci: rasio pasir–semen, faktor koreksi kehilangan tinggi tekanan, konstanta pelepasan partikel, koefisien pelepasan partikel.

Sand filter is a water purification apparatus that has been widely used in Indonesia. It effectively removes the suspension particle and produces clean water. The use of sand filter frequently faces the problems, such as decreasing capacity of clean water production due to surface clogging caused by sand particle stratification after the backwashing process, decreasing filter capability to purify water due to dirty filter resulted from the velocity and the time of backwashing, which are too low and too short, respectively. Sand filter backwashing is restricted by the velocity and pressure to avoid the sand particle from being transported out of the filter. Such restriction causes a longer time of backwashing process and the need for energy/electricity to generate the backwashing pump as well as to provide washing water. To cope with the above-mentioned problems, porous concrete media (concrete filter) is studied and proposed as an alternative to substitute the sand filter. The objective of this research is to study the characteristics of filtration hydraulic and concrete filter backwashing, including the following aspects : 1) the changing of particle diameter and porosity of concrete filter due to the additional cement as the sand binding substance; 2) the changing of hydraulic characteristics of the concrete filter to sand filter , 3) water production capacity and the capability of the concrete filter to reduce the turbidity; and 4) the characteristic of concrete filter backwashing overviewed from the filter cleanliness level and the velocity of suspension particle release. Theoretical equations of the concrete filter porosity (fb) and particle diameter (Dfb) was developed base on the original porosity (p) and particle diameter (Dp) of the filter composing material. The concrete filter is made from sand and cement at various sand to cement ratio. Ergun equation (1952) is modified by substituting p and Dp to fb and Dfb respectively. Dividing by w, the Ergun’s Equation is obtained as applied for concrete filter. A correction factor (is added to accommodate the changing of concrete filter characteristics. The applied Ergun’s Equation is re-formulated by adding the correction factor (to yield the theoretical headloss equation of the concrete filter. The value was calibrated using filtration experiments with three concrete filter models. These are concrete filter with particle size in between of 0.425 to 0.85 mm (F-1); between 0.85 to 1.00 mm (F-2); and in between 1.00 to 2.00 mm (F-3). Filtration experiment at various filtration velocities, suspension-particle concentration and turbidity were carried out to study the production capacity and the capability of the concrete filter in reducing turbidity. A model of suspension-particle released during backwashing of the concrete filter is modified from on the Huang and Basagoiti model (1989) by substituting p and Dp to fb and Dfb respectively. A theoretical particle released constant (Ksb) equation is developed and a particle released coefficient ( is added which is describes the undefined influencing factors. The value was calibrated using experiments at various backwashing velocities with those three concrete filters. The experiment results showed that at a sand to cement ratio (m) less than 5, fb decrease from 0.5 to 0.8 times to the original porosity (p) and Dfb increase from 1.1 to xxiii 1.6 times to the diameter of the original sand diameter (Dp). At the sand to cement ratio higher than 5, the changing of them were not significantly larger comparing the original porosity and particle diameter of the filter composing material. The value of m > 5 is the optimum value for the concrete filter designing. The changes of porosity and particle diameter increase the headloss through the concrete filter of about 2.4 times of that through the sand filter. The headloss differences yield a correction factor (of the theoretical headloss from 0.46 to 0.65. Particle grading affects the correction factor. More uniform grading cause larger correction factor which indicate the hydraulic characteristic of the concrete filter approaches that of the sand filter. Filter F-2 which is more uniform than filter F-1 and F-3 yields larger correction factor. The concrete filter capacity and filter capability in reducing turbidity depend on the filter particle size and filtration velocity. A larger filter particle size and higher filtration velocity yield larger capacity, however they reduce the concrete filter capability. The particle release constant (Ksb) is affected by the parameters defining the backwashing detention time, including the filter length, porosity and the backwashing velocity. The concrete filters (F-1, F-2 and F-3) which were backwashed at various velocities yield a coefficient Ksb ( ) of about 3.0. Keywords: sand to cement ratio, headloss correction factor, particle release constant, particle release coefficient.

Kata Kunci : rasio pasir–semen, faktor koreksi kehilangan tinggi tekanan, konstanta pelepasan partikel, koefisien pelepasan partikel.