Detail Cantuman Kembali
Evaluating Bacteria Removal Potential by Slow Sand Filtration: Effects of Rhine and Lava Sand and Operation Mode
- Bagikan:
Evaluating Bacteria Removal Potential by Slow Sand Filtration: Effects of
Rhine and Lava Sand and Operation Mode
Thesis Summary
In partial fulfillment of the requirements for the Master degree
Master Program of System Engineering
Field Study of Engineering Sciences
by:
Ekha Yogafanny
09/305580/PTK/06802
To
MASTER DEGREE PROGRAM
GADJAH MADA UNIVERSITY
YOGYAKARTA
2011
Table of Contents
Chapter I ............................................................................................................................. 2
Introduction .......................................................................................................................... 2
Chapter II............................................................................................................................ 3
Literature Review................................................................................................................. 3
Chapter III .......................................................................................................................... 5
Research Methods................................................................................................................. 5
Chapter IV.......................................................................................................................... 7
Result and Discussion ........................................................................................................... 7
4.1. Effect of filter media on bacteria removal ............................................................ 7
4.1.1. Effect of grain size distribution ........................................................................ 7
4.1.2. Effect of sand type ........................................................................................... 8
4.2. Effect of influent water quality (bacteria concentration)....................................... 9
4.3. Effect of intermittent operation mode on bacteria removal ................................. 10
4.3.1. Effect of higher HLR (0.1 – 0.2 m/h) ............................................................. 10
4.3.2. Effect of lower HLR (0.03 m/h) ..................................................................... 11
Chapter V .......................................................................................................................... 13
Conclusions and Suggestions .............................................................................................. 13
5.1. Conclusions....................................................................................................... 13
5.2. Suggestions ....................................................................................................... 13
References......................................................................................................................... 15
1
Chapter I
Introduction
Water is the most essential resource for everyone in the world, especially for drinking
water and sanitation. Nowadays, many people still do not have a safe and sustainable access
neither to drinking water nor to sanitation (WHO & UNICEF, 2010). Indonesia is one of the
Southeast Asian Countries and has some regions where the water cannot be easily accessed
by people. One of these regions is Gunung Kidul District, Yogyakarta Special Province. This
region is located in the southern part of Yogyakarta Special Province. Most of Gunung Kidul
District areas are situated in a karst landform zone. Gunung Kidul karst area occupies 65% of
the Western Gunung Sewu (Thousand Hills) karst (Haryono & Day, 2004). The special
feature of karst formation leads people living in this area suffer from water scarcity, mainly
during the dry season. Karst formations consist of carbonate and gypsum rock that have a
high solubility rate and also high infiltration rate. As a result, this region undergoes an
extreme water shortage especially during the dry seasons. This situation could be enhanced
by pumping up the water from underground to the surface (Nestmann et al., 2011). Since the
water quality is not good, the water in the surface should be treated first before its
distribution. Slow Sand Filtration (SSF) was selected as the most appropriate technology
considering several factors including regulatory requirements, cost, and operation.
The aim of this study is to evaluate the performance of SSF regarding bacteria
removal considering some variables such as filter media (sand type and grain size
distribution) and operation condition (intermittent mode and hydraulic loading rate (HLR)).
Furthermore, to achieve the main objective of this research, specific objectives need to be
accomplished and are defined as follows:
1. To ascertain whether SSF might be also used to remove (reduce) the bacteria content
from the raw water.
2. To analyze the effect of filter media (sand type and grain size distribution) and
operation condition (intermittent mode and filtration rate) on filter performance in
regard to bacteria removal.
3. To identify the mechanisms that might influence the bacteria removal.
2
Chapter II
Literature Review
The slow sand filtration, a method to gain clean water, has been known since 1804. It
was firstly designed and built by John Gibb in Scotland and he successfully sold the treated
water to the public at very cheap price per gallon. During the years of 1860s and 1870s until
1885, the discoveries were developed to take account of bacteriological examination
(Huisman & Wood, p. 15, 1974). During 1970s until 1990s, the researchers showed the high
efficiency of slow sand filtration to control the microbial contamination that was unknown
before (Logsdon et al., 2002).
Slow sand filtration is a water treatment process that permits raw water to percolate
from the surface through the fine sand medium and then drained out from the bottom. The
effective grain size applied in this system is finer than that in rapid sand filtration, usually the
effective size (d10) is in the range of 0.15 – 0.30 mm with the uniformity coefficient (Cu) is
less than 5 and preferably less than 3 (Visscher, 1990). The suspended solids, colloidal
matter, and also the bacteria from the raw water are accumulated at the very top layer which
is not regularly cleaned out, thus let the purifying bacteria become well developed in the top
layer and play an important role in slow sand filtration process to produce a good quality of
water. It is the reason why then a slow sand filtration is also called “biological” filtration
(Huisman & Wood, p. 18, 1974).
In a continuous slow sand filtration (SSFc), the process starts in the supernatant where
the water is retained for several hours and some particles are agglomerated and precipitated.
The supernatant water should be maintained around 0.3 – 1.5 m above the sand surface in
order to avoid disturbing the Schmutzdecke (Eng. dirty layer) in the upper layer of sand bed.
The feeding is conducted continuously as the filter is operated continuously a whole day,
week, and month until it is being clogged (Langenbach, p 11, 2010). During the operation,
suspended solids and microorganisms such as bacteria, parasites, and viruses are retained and
accumulated on the surface layer. This accumulation is very active with the microorganisms
living inside it is called Schmutzdecke or bio layer. This accumulation needs around weeks or
months to grow. This layer is believed to play an important role at reducing impurities,
turbidity, and pathogenic bacteria in the raw water, thus provides an excellent treated water
quality (Huisman & Wood, p. 21, 1974). SSFC is permanently water-saturated with the
3
supernatant water above the filter bed. Consequently the surface area is much smaller and the
oxygen supply depends on the dissolved oxygen in the influent.
Intermittent slow sand filtration (SSFi) is a modification to the normal operation of
SSFc and is commonly used for secondary and tertiary wastewater treatment. It differs from
SSFc mainly because of a feeding interval. The SSFc does not have the feeding interval
because of its continuously feeding, while SSFi does have the feeding interval. The SSFi does
not need a continuously feeding of the filter bed. SSFi is operated in usually 1 – 12 cycles of
feeding and draining. SSFi has a temporary operation in filtering the water. One filter run is
started with flooding the filter bed with the water and then letting the water percolate down
through the sand bed. After the feeding cycle the surface of filter bed dries. During this
interval or pause period, the filter surface allows diffusion of oxygen into the thin layer of
film covering the sand grains.
In the sand filter, the physical, chemical, and biological treatment process occur in
some ways. Complex forces contribute to these processes which are then defined as transport,
attachment, and purification mechanisms. The mechanisms of particles that are transported
and carried to get in touch with the sand grains consist of straining or screening,
sedimentation, inertial and centrifugal forces, diffusion, mass attraction, and electrostatic and
electrokinetic attraction (Huisman & Wood, p. 27, 1974). Huisman & Wood (p.30, 1974)
mentioned that the attachment mechanisms occur when the particles have gotten in contact
with the sand grains. These mechanisms are electrostatic attraction, Van der Waals force, and
adhesion. The two important processes in purification mechanism are chemical and
microbiological oxidation, but other biological processes of animal and vegetable life may
also play a noteworthy part. The Schmutzdecke layer or biofilm consist of bacteria derived
from the raw water.
4
Chapter III
Research Methods
Laboratory tests were conducted in filter columns. The experimental setup consisted
of 5 columns with a diameter of 5.2 cm and a height of 120 cm containing a layer of sand
supported by a layer of gravel. Each column has a valve at the bottom outlet connected with a
hose and a clamp. The clamp was applied in each column in order to easily be able to control
the filtration velocity at the outlet. The bacteria removal was analyzed based on three sand
filter configurations within two different sand types and under intermittent operation mode.
These three configurations of sand filter were C1 (d10 = 0.13 mm, Cu = 3.7), C2 (d10 = 0.07
mm, Cu = 4.2), and configuration of control filter (d10 = 0.2 mm, Cu = 2.1). in the beginning
of this experiment, all filter columns were operated under HLR 0.1 – 0.2 m/h and at the end,
those columns were operated with HLR 0.03 m/h.
These sands with their configurations were poured into five filter columns. A gravel
layer (5 cm) was placed at the bottom of each column, supporting the sand bed (50 cm), in
order to avoid the sand flowing out of the filter. An additional gravel layer (5 cm) was placed
upon the surface of the sand bed to reduce or avoid the disturbance of surface layer of sand
when the water was introduced into the column. The columns were constructed as follows:
filter columns F1 and F2 followed the configuration C1, filter columns F3 and F4 followed
the configuration C2, and filter column F5 was filled with sand directly from the nature.
Filter columns F1, F3, and F5 were filled with Rhine sand while filter columns F2 and F4
were filled with Lava sand.
During column construction, the sand characteristics i.e. porosity, permeability, and
sand surface area were calculated by some equations.
where n is porosity, Vvoid or Vv is volume or the pore space, and Vtotal or V is the total volume
of sample, γd is the dry unit weight, Gs is the specific gravity, γw is the water unit weight, Ws
is the dry sample weight, D is the diameter of the filter column, and H is the sand bed height.
For permeability, a is the cross-sectional area of the standpipe, A is the cross-sectional area of
5
the specimen, L is the length of the specimen, ho is the elevation above the datum of water in
the standpipe at the beginning of the experiment (t = 0), and hf is the elevation above the
datum of water in the standpipe at time t. For Sand surface area, As is specific surface area
(m2/m3), ds is the specific grain diameter, and p is porosity, A is the total sand surface area
(m2), d is the inner diameter of the filter column, and l is the bed depth.
Intermittent operation mode was applied in this experiment by feeding the columns
with the raw water once in a day and five days in a week. By doing so, pause periods of a
minimum of 24 hours could be achieved in this time interval. The feeding of 1 l raw water
into each column was conducted in the morning and then the water flowed out from the
columns through the sand filter until the supernatant water was down to the surface of the top
gravel layer. Water samples were taken from influent and effluent so that the microbiological
test could be conducted to measure the bacteria concentration in the water. An additional
measurement was conducted to determine the average filtration rate by taking the amount of
filtrate water (ml) during 20 minutes.
Evaluating the bacteria removal is one of the main objectives in this thesis. To
evaluate the bacteria removal, the concentration of bacteria from influent and effluent in
every time feeding must be known. The colilert-18 was used to measure the bacteria
concentration from influent and effluent water. The colilert-18 measurement was done every
afternoon so that the result could be measured by the next day in the morning. The influent
water was made by mixing the wastewater and the tap water with the dilution of 1:10. Colony
forming units (CFU) per 100 ml were calculated from the number of positive wells in the
Quanti-Tray/2000 then multiplied by the dilution factor (if any). The bacteria concentrations
were then transformed to the log10-units using the equation below.
log reduction = log(cfu/100ml)inf – log(cfu/100ml)eff
6
Chapter IV
Result and Discussion
4.1. Effect of filter media on bacteria removal
4.1.1. Effect of grain size distribution
The overall performance of these columns regarding bacteria removal was good
achieving 1.6 – 4.7 log-units or 97.7 – 99.998% removal of total coliforms and 1.6 – 5 log-
units or 97.6 – 99.999% removal of E. coli. The best performance with the consistent result
was attained by filter column F4 which consisted of Lava sand and had the configuration C2
(d10 = 0.07 mm and Cu = 4.2).
Ausland et al. (2002), Langenbach et al. (2009), and Bellamy et al. (1985) found out
that a decrease in grain size leads to an increase in treatment efficiency. From the results
obtained in this study, these two indicator bacteria seem to follow the same trend, i.e. the
highest bacteria removal corresponded to the finest grain size. The difference of bacteria
removal becomes evident when configuration C1 and C2 are compared as can be seen in
Figure 4.1.
5.0
4.0
3.0
2.0
1.0
0.0
F1(d10 :
F2 (d10 :
F3 (d10 :
F4 (d10 :
F5 (d10 :
0.13, Cu: 3.7) 0.13, Cu: 3.7) 0.07, Cu: 4.2) 0.07, Cu: 4.2) 0.2, Cu: 2.1)
Figure 4.1. The removal of total coliforms bacteria in five filter columns
In the filter with Lava sand (F2 and F4), a higher removal of both total coliforms and E. coli
was attained by filter column F4 that had sand with d10 of 0.07 mm and Cu of 4.2. This trend
was also found in the filters with the Rhine sand as sand type (F1 and F3). Filter column F3
(d10 of 0.07 mm and Cu of 4.2) appeared to have a better performance compared to filter
column F1, achieving a higher log-removal. On the other hand, when comparing filter
columns F1, F3, and F5 that had the same sand type (Rhine sand), filter column F5 (d10 0.2
mm and Cu 2.1) achieved better results (2.7 log-units of both total coliforms and E. coli)
compared to filter columns F1 and F3. It was expected that filter column F5 would show
7
good removal efficiency as its media characteristics are within the recommended range (d10 =
0.15 – 0.30 mm and Cu = 24 hours), and one week (>24 hours) resulted in a lower bacteria removal in the
first day operation resumed after the weekend. It meant that filter ability on bacteria removal
10
after filter operation resumed decreased. The same trend is clarified by Bouwer et al. (1984)
that the bacteria concentrations were always high in the effluent after the operation resumed.
Cited from Auset et al. (2002), “Bacteria removal decreased after the drying interval of the
filter and the longer the drying period the greater the deterioration. The time was needed to
recover the removal ability, a few hours for one day drying and more than 2 days for longer
drying periods. Due to the highly sensitive to alternate operation and drying period, to
enhance the removal ability, the drying period must be shorted and daily hydraulic loads
must be reduced.”
High HLRs increased the water movement through the sand grains leading to a greater
distance between media and bacteria. Therefore the contact time between media and bacteria
shortened. This leads to a reduction in the likelihood that bacteria can be adsorbed by sand
grains. Stevik et al. (2004) stated that high HLR also diminishes the utilization of sand
surface area for bacteria removal especially in regard to the contact time between bacteria and
media. According to the results presented herein, filter columns operated intermittently with
HLRs higher than 0.1 m/h will show non-constant removal efficiencies and high effluent
bacteria concentrations after re-start of filter process. This is consistent to what Langenbach
(2010) reported pointed out that intermittent operation will be only effective at HLRs just
below 0.03 m/h. Therefore, a series of measurements with HLRs approximately 0.03 m/h
were conducted in order to establish filter efficiency.
4.3.2. Effect of lower HLR (0.03 m/h)
The results obtained in this second experiment show that at HLR 0.03 m/h, a
relatively constant and high bacteria removal is achieved, confirming the previous
experiments by Orb (2009) and Langenbach (2010). In filter column F4, bacteria removal
gradually increases with decreasing HLR, achieving 7 log-reductions of total coliforms and
6.4 log-reductions of E. coli. A similar pattern was also attained by filter column F1 as an
increasing bacteria removal was achieved when the filter was operated at lower HLR.
Bacteria removal increased dramatically achieving 6 log-reductions of total coliforms and 5.8
log-reductions of E. coli when a lower HLR of 0.03 to 0.01 m/h was introduced in the filter.
Nevertheless, at one time the HLR increased from 0.01 m/h to 0.05 and 0.06 m/h since there
was a technical error. This made the bacteria removal decrease significantly reaching
respectively 2.9 and 2.3 log-reductions of both total coliforms and E. coli. The pattern
obtained in these results are consistent to that reported by Stevik et al. (2004), Ausland et al.
11
(2002), Torrens et al. (2009), Abidi et al. (2009), and Vymazal (2005) who found out that the
increase of the HLR significantly decreased the bacteria removal. Huisman & Wood (p. 20 –
22, 1974) recommended as well, that the slow sand filter should be operated at a HLR as
constant as possible. Therefore, slow sand filters should not be operated in a significantly
different HLR during their operation.
12
Chapter V
Conclusions and Suggestions
5.1. Conclusions
The results of these experiments were summarized in terms of the effectiveness of
bacteria removal and the effect of operating conditions. From the results obtained it can be
concluded as follows:
1. SSF is a reliable process to improve microbiological quality of water.
2. A smaller grain size leads to an increase in bacteria removal.
3. A higher bacteria removal was achieved by the filter columns filled with the Lava
sand (angular shape) compared to that with Rhine sand (spherical shape).
4. This high number of bacteria in the influent tended to improve the adsorption
mechanisms that enhance bacteria removal.
5. The efficiency of bacteria removal is negatively affected by intermittent operation
mode under a HLR of 0.1 to 0.2 m/h. This can be proofed by a low and non-constant
bacteria removal after both < 24 hours and > 24 hours interval without feeding. But
under the same operation condition with HLR of 0.03 m/h, a relatively constant and
high removal of bacteria could be achieved.
6. All mechanisms i.e. straining, adsorption, and purification mechanism seemed to
work within the sand bed.
5.2. Suggestions
To improve the findings of this present study, some suggestions are as follows:
1. The modification of experimental setup should be carried out, i.e. using a pump in the
filter outlet in order to get a more precise and constant filtration rate. Within this
constant filtration rate, the results would become more reliable to be used as a basis to
design a pilot plant of slow sand filtration.
2. To ascertain the same concentration of bacteria in the influent, the method of
microbiological culture should be done before the experiments are carried out. This
method aims to multiply microbiological organisms by growing them up in
predetermined culture media under the controlled laboratory conditions. By doing so,
the precise measurement of filter efficiency on bacteria removal could be attained.
13
3. The further experiment relating to sand properties should be conducted in order to get
a trustworthy analysis in particle shape.
4. The subsequent experiments in terms of the development of biofilm or biolayer on the
sand surface or within the sand bed should be carried out to assure the effect of
intermittent operation to the development of biofilm or biolayer.
5. The following study should be conducted to know the clogging time or the running
period of sand filter.
6. The research of slow sand filtration using local material regarding bacteria removal
and turbidity should be carried out to acquire the real implementation of this study in
the case area (Gunung Kidul).
7. The chemical properties of both sand filter and raw water should be considered in the
subsequent study.
8. Filter column F4 (Lava sand, d10 = 0.07 mm and Cu = 4.2) performed the best bacteria
removal in this experiment achieving 4.7 log-removal of total coliforms and 5.0 log-
removal of E. coli. However, with this filter configuration, an excellent performance
can be achieved under specific condition i.e. a HLR 0.03 m/h. According to this
condition, it would be not anymore economically feasible due to the need of larger
area. It is probably good as a decentralized SSF or a household SSF.
9. Filter column F5 (Rhine sand, d10 = 0.2 mm and Cu = 2.1) with the configuration in
the range of recommendation and HLR 0.1 – 0.2 m/h, performed a better bacteria
removal than filter columns F1, F2, and F3. It achieved 2.7 log-removal of both total
coliforms and E.coli. This filter characteristic or configuration is very good to be
applied as a centralized SSF.
14
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16
EVALUASI PENGHILANGAN BACTERIA DENGAN SARINGAN PASIR LAMBAT:
PENGARUH DARI PASIR RHINE DAN LAVA DAN METODE OPERASI
Ringkasan Thesis
untuk memenuhi sebagai persyaratan mencapai derajat sarjana S-2
Magister Sistem Teknik
Program Studi Teknik Mesin
Fakultas Teknik
Oleh:
Ekha Yogafanny
09/305580/PTK/06802
Kepada
PROGRAM PASCA SARJANA
UNIVERSITAS GADJAH MADA
YOGYAKARTA
2011
Daftar Isi
Bab I ..................................................................................................................................... 2
Pendahuluan......................................................................................................................... 2
Bab II.................................................................................................................................... 4
Tinjauan Pustaka .................................................................................................................. 4
Bab III .................................................................................................................................. 6
Metodologi Penelitian .......................................................................................................... 6
Bab IV .................................................................................................................................. 8
Hasil dan Diskusi ................................................................................................................. 8
4.1. Pengaruh media filter pada penghapusan bakteri............................................... 8
4.1.1. Efek distribusi ukuran butir ............................................................................ 8
4.1.2. Efek dari jenis pasir ....................................................................................... 9
4.2. Pengaruh kualitas air influen (bakteri konsentrasi) ........................................... 10
4.3. Pengaruh mode operasi intermiten pada penghapusan bakteri .......................... 11
4.3.1. Pengaruh HLR tinggi (0,1 – 0,2 m/jam) ........................................................ 11
4.3.2. Pengaruh HLR rendah (0,03 m/jam).............................................................. 13
Bab V..................................................................................................................................... 14
Kesimpulan dan Saran.......................................................................................................... 14
5.1. Kesimpulan ....................................................................................................... 14
5.2. Saran ................................................................................................................. 14
Daftar Pustaka....................................................................................................................... 16
Bab I
Pendahuluan
Air merupakan sumber daya yang paling penting bagi setiap manusia di dunia, terutama
untuk air minum dan sanitasi. Saat ini, masih banyak orang masih tidak memiliki akses air minum
dan sanitasi yang baik, aman, dan berkelanjutan (WHO & UNICEF, 2010). Indonesia adalah
salah satu negara di Asia Tenggara yang memiliki beberapa daerah di mana air tidak dapat diakses
dengan mudah oleh masyarakat disekitarnya. Salah satu daerah tersebut adalah Kabupaten
Gunung Kidul, Provinsi D.I. Yogyakarta (DIY). Wilayah ini terletak di bagian selatan Provinsi
DIY. Sebagian besar wilayah Kabupaten Gunung Kidul terletak di zona dengan bentuk lahan
karst. Kawasan karst Gunung Kidul menempati 65% area karst Gunung Sewu (Haryono & Day,
2004). Karakter khas suatu bentuk lahan karst dimana air permukaan mudah masuk kedalam
sistem sungai bawah tanah karena proses pelarutan yang tinggi menyebabkan masyarakat yang
tinggal di daerah ini mengalami kekeringan dan kelangkaan air, terutama selama musim kemarau.
Situasi ini dapat diatasi dengan memompa air dari bawah tanah ke permukaan (Nestmann et al.,
2011). Air yang telah dipompa ke atas permukaan tidak dapat digunakan seketika karena kualitas
airnya yang kurang baik. Oleh karena itu, air tersebut harus diolah terlebih dahulu sebelum
didistribusikan. Berdasarkan penelitian yang dilakukan oleh Silva (2010), saringan pasir lambat
(SPL) merupakan teknologi yang paling tepat untuk diaplikasikan di wilayah pedesaam seperti di
Gunung Kidul ini mengingat beberapa faktor seperti biaya pembuatan, biaya operational, dan
kemudahan pengoperasiannya.
Tujuan dari penelitian ini adalah untuk mengevaluasi kinerja SPL dalam penghilangan
bakteri dalam air dengan mempertimbangkan beberapa variabel seperti media saringan (jenis
pasir dan distribusi ukuran butir) dan kondisi operasi (modus intermiten dan Hydraulic loading rate
(HLR)). Selanjutnya, untuk mencapai tujuan utama dari penelitian ini, tujuan khusus dapat
didefinisikan sebagai berikut:
1. Untuk memastikan apakah SPL dapat juga digunakan untuk menghapus (mengurangi)
bakteri dalam air baku.
2. Untuk menganalisis pengaruh media saringan (jenis pasir dan distribusi ukuran butir) dan
kondisi operasi (modus intermiten dan HLR) pada kinerja saringan dalam hal penghapusan
bakteri
3. Untuk mengidentifikasi mekanisme yang mungkin mempengaruhi proses penghapusan
bakteri.
Bab II
Tinjauan Pustaka
Saringan pasir lambat, suatu metode untuk mendapatkan air bersih, telah dikenal sejak
tahun 1804. Hal itu pertama kali dirancang dan dibangun oleh John Gibb di Skotlandia dan
dia berhasil menjual air yang diolah untuk masyarakat umum dengan harga per galon yang
sangat murah. Selama tahun 1860-an dan 1870-an sampai 1885, penelitian dikembangkan
untuk pengamatan kualitas air (microbiologi) dengan SPL (Huisman & Wood, hlm 15, 1974).
Selama 1970-an hingga 1990-an, para peneliti menunjukkan bahwa SPL memiliki efisiensi
yang tinggi untuk mengontrol kontaminasi mikroba (Logsdon et al, 2002.).
Saringan pasir lambat adalah proses pengolahan air yang memungkinkan air baku untuk
meresap dari permukaan melalui media pasir halus dan kemudian mengalir ke bawah. Ukuran
butir efektif (d10) yang digunakan dalam sistem ini lebih baik daripada d10 yang digunakan
dalam saringan pasir cepat (SPC). Pada SPL, d10 yang digunakan adalah pada kisaran 0,15-
0,30 mm dan koefisien keseragaman (Cu) adalah kurang dari 5 namun disarankan lebih baik
kurang dari 3 ( Visscher, 1990). Material padat, material koloid, dan juga bakteri dari air
baku yang terakumulasi pada lapisan paling atas menjadi media bagi bakteri untuk
berkembangbiak dan memainkan peran penting dalam penjernihan air dengan SPL. Ini adalah
salah satu alasan mengapa SPL disebut juga sebagai saringan "biologis" (Huisman & Wood,
hlm 18, 1974).
Dalam SPL continues (SPLc), proses dimulai dari air supernatan yang dibiarkan
selama beberapa jam sehingga beberapa partikel pengotor teraglomerasi dan terendapkan
diatas ataupun dalam saringan pasir tersebut. Air supernatan harus dipertahankan sekitar 0,3-
1,5 m di atas permukaan pasir agar lapisan Schmutzdecke (Ind. lapisan kotor) di lapisan pasir
atas tidak terganggu. Proses penyaringan air tersebut dilakukan secara terus menerus
sepanjang hari, minggu, bulan, bahkan tahun hingga saringan pasir tersumbat (Langenbach, p
11, 2010). Selama operasi, padatan tersuspensi dan mikroorganisme seperti bakteri, parasit,
dan virus tertahan dan terakumulasi di permukaan pasir. Akumulasi ini berisi microorganisme
yang sangat aktif yang dapat disebut pula lapisan Schmutzdecke atau biolayer. Biolayer ini
membutuhkan sekitar beberapa minggu atau bulan untuk dapat terbentuk. Lapisan ini diakui
memainkan peran penting dalam pengurangan material pengotor, kekeruhan, dan bakteri
patogen dalam air baku, sehingga air yang dihasilkan dari proses pengolahan air dengan SPL
ini memiliki kualitas yang sangat baik (Huisman & Wood, hal 21, 1974). SPLc secara
permanen dan terus-menerus tergenangi oleh air supernatan. Akibatnya total luas permukaan
pasir (sand surface area) jauh lebih kecil dan suplai oksigen sangat tergantung pada oksigen
terlarut di dalam air baku tersebut.
Saringan pasir lambat Intermiten (SPLi) merupakan modifikasi dari operasi normal
SPLc dan umumnya digunakan untuk pengolahan sekunder dan tersier air limbah. Ini berbeda
dengan SPLc terutama dalam hal interval pengolahan air baku (pengaliran air baku kedalam
saringan pasir lambat). SPLc tidak memiliki interval karena pengolahan dan pengaliran air
baku dilakukan secara terus menerus, sementara SPLi memiliki interval. SPLi tidak perlu
aliran air baku yang terus menerus ke saringan pasirnya. SPLi dioperasikan dalam 1 - 12
siklus pengaliran dan pengeringan air. Pengolahan air dalam SPLi dimulai dengan pengaliran
air baku kedalam saringan pasir dan kemudian air terserap dan tersaring oleh saringan pasir
tersebut. Setelah semua air tersebut keluar dan permukaan saringan pasir itu mengering,
difusi oksigen ke dalam lapisan tipis (biolayer) yang menutupi butiran pasir terjadi.
Dalam saringan pasir, proses pengolahan secara fisik, kimia, dan biologis terjadi
dalam beberapa cara. Proses tersebut kemudian didefinisikan sebagai transportasi,
pengikatan, dan mekanisme pemurnian. Mekanisme partikel yang diangkut dan dibawa oleh
air baku yang kemudian kontak dengan pasir (transportasi) terdiri dari penyaringan,
sedimentasi, gaya inersia dan sentrifugal, difusi, daya tarik massa, dan daya tarik elektrostatik
dan elektrokinetik (Huisman & Wood, hlm 27, 1974 ). Huisman & Wood (p.30, 1974)
menyebutkan bahwa mekanisme pengikatan terjadi ketika partikel dari air baku mendapatkan
kontak dengan butiran pasir. Mekanisme ini terdiri dari proses tarik-menarik elektrostatik,
Van der Waals, dan adhesi. Dua proses penting dalam mekanisme pemurnian adalah oksidasi
kimia dan mikrobiologi, namun proses biologis dari kehidupan organisme hewan dan
tumbuhan mungkin memainkan bagian penting dalam proses pemurnian. Lapisan
Schmutzdecke atau biolayer terdiri dari bakteri yang berasal dari air baku.
Bab III
Metodologi Penelitian
Penelitian dilakukan dalam skala laboratorium. Alat SPLi dalam percobaan ini terdiri
dari 5 kolom dengan diameter 5,2 cm dan tinggi 120 cm yang berisi lapisan pasir 50 cm dan
didukung oleh lapisan kerikil 5 cm diatas dan 5 cm dibawah saringan pasir. Setiap kolom
memiliki katup di outlet yang terhubung dengan selang dan penjepit. Penjepit digunakan di
setiap kolom untuk dapat mengendalikan kecepatan laju filtrasi di outlet. Penghilangan
bakteri dianalisis berdasarkan tiga konfigurasi pasir dan dua jenis pasir yang berbeda dengan
mode intermiten. Lapisan kerikil (5 cm) ditempatkan di bagian bawah setiap kolom untuk
mendukung saringan pasir (50 cm) dan untuk menghindari pasir mengalir keluar dari filter.
Lapisan kerikil (5 cm) ditempatkan pula pada permukaan saringan pasir untuk mengurangi
atau menghindari gangguan terhadap biolayer, yang mungkin terbentuk pada lapisan
permukaan pasir, saat air baku dimasukan ke dalam kolom. Ketiga konfigurasi dari yang
digunakan dalam saringan pasir ini adalah C1 (d10 = 0,13 mm, Cu = 3,7), C2 (d10 = 0,07 mm,
Cu = 4,2), dan konfigurasi filter kontrol (d10 = 0,2 mm, Cu = 2,1). Kolom saringan F1 dan F2
mengikuti konfigurasi C1, kolom saringan F3 dan F4 mengikuti konfigurasi C2, dan kolom
saringan F5 diisi dengan pasir yang diperoleh langsung dari alam (Rhine river). Kolom
saringan F1, F3, dan F5 diisi dengan pasir Rhine sementara kolom saringan F2 dan F4 diisi
dengan pasir Lava.
Pada percobaan awal, semua kolom saringan pasir dioperasikan dengan HLR 0,1 –
0,2 m/jam dan pada percobaan akhir, semua kolom dioperasikan dengan HLR 0,03 m/jam.
Selama konstruksi kolom, karakteristik pasir yaitu porositas, permeabilitas, dan area
permukaan pasir dihitung dengan beberapa persamaan sebagai berikut,
dimana n adalah porositas, Vvoid atau Vv adalah volume atau ruang pori, dan Vtotal atau V
adalah volume total sampel, γd adalah satuan berat kering, Gs adalah gravitasi spesifik, γw
adalah satuan berat air, Ws adalah kering berat sampel, D adalah diameter dari kolom filter,
dan H adalah tinggi tempat tidur pasir. Untuk permeabilitas, a adalah luas penampang dari
kolom, A adalah luas penampang spesimen, L adalah panjang spesimen, ho adalah ketinggian
di atas datum air dalam kolom di awal percobaan (t = 0), dan HF adalah ketinggian di atas
datum air dalam kolom pada waktu t. Untuk luas permukaan pasir, As adalah luas permukaan
pasir spesifik (m2/m3), ds adalah diameter butir tertentu, dan p adalah porositas, A adalah
luas permukaan pasir total (m2), d adalah diameter bagian dari kolom filter, dan l adalah
kedalaman pasir.
Mode operasi intermiten diterapkan dalam percobaan ini dengan memasukan air baku
ke dalam kolom sekali dalam sehari dan lima hari dalam seminggu. Dengan demikian,
periode jeda waktu atau periode kering tanpa air supernatant dapat dicapai. Air baku
dimasukan ke dalam masing-masing kolom sebanyak 1 liter pada pagi hari dan kemudian air
mengalir keluar dari kolom melalui saringan pasir hingga air supernatan turun ke permukaan
lapisan atas kerikil. Perhitungan laju filtrasi / HLR dilakukan dengan mengambil jumlah
filtrat air (ml) yang dihasilkan dari penyaringan air selama 20 menit. Tes microbiologi
mengunakan colilert-18 untuk mengukur konsentrasi bakteri (total coliform dan E. coli)
dalam air dilakukan pada air baku (influen) dan air hasil pengolahan (efluen). Pengukuran
dengan colilert-18 dilakukan setiap sore sehingga hasilnya dapat diperoleh pada hari
berikutnya di pagi hari. Air influen dibuat dengan mencampur air limbah dan air keran
dengan pengenceran 1:10. Nilai colony forming unit (CFU) per 100 ml dihitung dari jumlah
kotak positif dalam Quanti-Tray/2000 kemudian dikalikan dengan faktor pengenceran (jika
ada). Konsentrasi bakteri kemudian ditransformasikan ke dalam log10-unit menggunakan
persamaan di bawah ini.
Log pengurangan (log reduction) = log (cfu/100ml) influen - log (cfu/100ml) efluen
Bab IV
Hasil dan Diskusi
4.1. Pengaruh media filter pada penghapusan bakteri
4.1.1. Efek distribusi ukuran butir
Kinerja SPLi dalam menghilangkan bakteri cukup baik mencapai 1,6 – 4,7 log-unit
atau 97,7 – 99,998% pengurangan total coliform dan 1,6 – 5 log-unit atau 97,6 – 99,999%
pengurangan E. coli. Kinerja terbaik dengan hasil yang konsisten dicapai oleh saringan kolom
F4 yang terdiri dari pasir Lava dan memiliki konfigurasi C2 (d10 = 0,07 mm dan Cu = 4,2).
Ausland dkk.(2002), Langenbach dkk.(2009), dan Bellamy dkk. (1985) menemukan bahwa
penurunan ukuran butir (d10) menyebabkan peningkatan kinerja SPL dalam penghilangan
bakteria. Dari hasil yang diperoleh dalam penelitian ini, kedua parameter bakteri mengikuti
pola yang sama, yaitu penghilangan bakteri tertinggi dicapai oleh filter dengan ukuran butir
yang lebih kecil atau halus. Perbedaan kapasitas penghilangan bakteri menjadi jelas ketika
konfigurasi C1 dan C2 dibandingkan seperti yang dapat dilihat pada Gambar 4.1.
5.0
4.0
3.0
2.0
1.0
0.0
F1(d10 :
F2 (d10 :
F3 (d10 :
F4 (d10 :
F5 (d10 :
0.13, Cu: 3.7) 0.13, Cu: 3.7) 0.07, Cu: 4.2) 0.07, Cu: 4.2) 0.2, Cu: 2.1)
Gambar 4.1. Penghilangan total coliform pada lima kolom saringan
Dalam saringan dengan pasir Lava (F2 dan F4), penghilangan bakteri tertinggi baik
total coliform maupun E. coli dicapai oleh kolom saringan F4 yang memiliki pasir dengan d10
sebesar 0,07 mm dan Cu sebesar 4,2. Kecenderungan ini juga ditemukan pada saringan
dengan pasir Rhine sebagai media (F1 dan F3). Kolom saringan F3 (d10 = 0,07 mm dan Cu =
4,2) memiliki kemampuan penghilangan bakteria yang lebih baik dibandingkan dengan
kolom saringan F1. Kemudian, ketika membandingkan kolom saringan F1, F3, dan F5 yang
memiliki jenis pasir yang sama (pasri Rhine), kolom saringan F5 (d10 = 0,2 mm dan Cu = 2.1)
mencapai hasil yang lebih baik (mencapai penghilangan total coliform dan E. coli sebesar 2,7
log-unit) dibandingkan kolom saringan F1 dan F3. Hal itu memang sudah seharusnya dicapai
oleh kolom saringan F5 karena karakteristik media yang digunakan dalam saringan tersebut
berada dalam kisaran nilai yang direkomendasikan (d10 = 0,15 – 0,30 mm dan Cu = 24 jam), dan satu minggu (> 24 jam) menghasilkan penghilangan bakteri yang lebih
rendah dalam operasi hari pertama saat saringan mulai dioperasikan kembali setelah akhir
pekan. Ini berarti bahwa kemampuan filter pada penghilangan bakteri setelah mengalami
periode pengeringan menurun. Kecenderungan yang sama dijelaskan oleh Bouwer dkk.
(1984) bahwa konsentrasi bakteri selalu tinggi pada efluen setelah mengalami periode
pengeringan. Dikutip dari Auset dkk. (2002), "pengurangan bakteri menurun setelah interval
pengeringan saringan dan semakin panjang periode pengeringan tersebut maka semakin kecil
kemampuan saringan dalam pengurangan bakteri. Waktu yang dibutuhkan untuk
memulihkan kemampuan saringan tersebut adalah beberapa jam untuk pengeringan satu hari
dan lebih dari dua hari untuk waktu pengeringan yang lebih lama. Karena lama periode
pengeringan sangat sensitif dalam modus intermiten, maka untuk meningkatkan kemampuan
saringan dalam pengurangan bakteri, periode pengeringan harus dibuat seminimal mungkin
dan HLR harus diturunkan.”
HLR yang tinggi akan meningkatkan pergerakan air melalui butiran pasir yang
membuat jarak anatar media dan bakteri menjadi lebih besar dan waktu kontak antara media
dan bakteri tersebut menjadi lebih singkat. Hal ini menyebabkan penurunan kemungkinan
dari bakteri untuk terserap oleh media pasir. Stevik dkk. (2004) menyatakan bahwa tingginya
HLR juga mengurangi pemanfaatan area permukaan pasir untuk mengikat dan
menghilangkan bakteri dalam air baku. Menurut hasil yang diperoleh dari penelitian kali ini,
saringan pasir lambat yang dioperasikan secara intermiten dengan HLR yang lebih tinggi dari
0,1 m/jam akan menghasilkan efisiensi pengurangan bakteri yang tidak konstan dan relatif
rendah. Hal ini konsisten dengan penelitian yang dilakukan oleh Langenbach (2010) bahwa
saringan yang akan dioperasikan secara intermiten hanya efektif pada HLR di bawah atau
sama dengan 0,03 m/jam. Oleh karena itu, serangkaian pengukuran dengan HLR sekitar 0,03
m/jam dilakukan dalam rangka meningkatkan efisiensi saringan.
4.3.2. Pengaruh HLR rendah (0,03 m/jam)
Hasil yang diperoleh dalam percobaan kedua ini menunjukkan bahwa pada HLR 0,03
m/jam, penghilangan bakteri yang tercapai relatif konstan dan tinggi, membuktikan hasil dari
percobaan yang dilakukan sebelumnya oleh Orb (2009) dan Langenbach (2010). Dalam
kolom saringan F4, penghilangan bakteri secara bertahap meningkat dengan menurunnya
HLR, mencapai 7 log-unit pengurangan total coliform dan 6,4 log-unit pengurangan E. coli.
Pola yang sama juga dicapai oleh kolom saringan F1 dimana penghilangan bakteri meningkat
ketika saringan dioperasikan pada HLR yang rendah. Penghilangan Bakteri meningkat drastis
mencapai 6 log-unit pengurangan total coliform dan 5,8 log-unit pengurangan E. coli ketika
HLR berada pada rentang 0,03 – 0,01 m/jam dioperasikan pada saringan. Namun demikian,
pada saat HLR meningkat dari 0,01 m/jam menjadi 0,05 dan 0,06 m/jam karena ada
kesalahan teknis, kemampuan penghilangan bakteri menurun secara signifikan masing-
masing mencapai 2,9 dan 2,3 log-unit pengurangan dari total coliform dan E. coli. Pola yang
dicapai pada hasil ini konsisten dengan hasil yang dilaporkan oleh Stevik dkk. (2004),
Ausland dkk. (2002), Torrens dkk. (2009), Abidi dkk. (2009), dan Vymazal (2005) yang
menemukan bahwa peningkatan HLR secara signifikan dapat menurunkan kemampuan
saringan dalam penghilangan bakteri. Huisman & Wood (hal. 20 – 22, 1974)
merekomendasikan juga bahwa filter pasir lambat harus dioperasikan dengan HLR sekonstan
mungkin. Oleh karena itu, filter pasir lambat tidak boleh dioperasikan dengan HLR yang
berubah-ubah secara signifikan selama operasi pernyaringan air.
Bab V
Kesimpulan dan Saran
5.1. Kesimpulan
Hasil percobaan mengenai efektifitas saringan pasir lambat dalam penghilangan
bakteri dapat disimpulkan berdasarkan media saringan dan modus operasi (intermiten).
Kesimpulan tersebut dapat diuraikan sebagai berikut:
1. Saringan pasir lambat adalah proses yang dapat diandalkan untuk meningkatkan kualitas
air (microbiologi).
2. Ukuran butir pasir yang kecil akan menghasilkan peningkatan dalam penghilangan
bakteri.
3. Penghilangan bakteri yang lebih tinggi dicapai oleh kolom saringan yang diisi oleh pasir
Lava (bentuk sudut) dibandingkan dengan pasir Rhine (bentuk bulat).
4. Tingginya jumlah bakteri dalam influen/air baku cenderung untuk meningkatkan
mekanisme adsorpsi yang kemudian akan meningkatkan penghilangan bakteri.
5. Modus intermiten pada saringan pasir lambat dengan HLR 0,1 sampai 0,2 m/jam
mempunyai efek yang negatif terhadap efisiensi penghilangan bakteri. Hal ini dapat
dibuktikan dengan penghilangan bakteri yang rendah dan tidak konstan setelah < 24 jam
maupun > 24 jam periode pengeringan. Tetapi dengan kondisi saringan yang sama
namun dioperasikan dengan HLR 0,03 m/jam, pengurangan bakteri yang relatif konstan
dan tinggi dapat dicapai.
5.2. Saran
Untuk meningkatkan kualitas penelitian ini, ada beberapa usulan yang dapat
dilakukan, antara lain sebagai berikut:
1. Modifikasi rancangan eksperimen harus dilakukan, yaitu dengan menggunakan pompa di
outlet kolom saringan untuk mendapatkan laju filtrasi lebih konstan dan akurat.
2. Untuk memastikan konsentrasi bakteri yang sama pada influen setiap saat operasi
penyaringan, metode kultur mikrobiologi harus dilakukan sebelum eksperimen. Metode
ini bertujuan untuk melipatgandakan organisme mikrobiologis dengan
mengembangbiakan mereka dalam media kultur yang telah ditentukan di bawah kontrol
laboratorium. Dengan demikian, pengukuran yang lebih tepat dapat tercapai.
3. Percobaan lebih lanjut berkaitan dengan karakteristik media pasir harus dilakukan dalam
rangka untuk mendapatkan analisis yang lebih detail.
4. Penelitian berikutnya mengenai perkembangan biofilm atau biolayer pada permukaan
pasir atau dalam pasir dapat dilakukan untuk memastikan efek operasi intermiten untuk
perkembangan biofilm atau biolayer.
5. Penelitian mengenai saringan pasir lambat menggunakan material lokal dalam
penghilangan bakteri dan kekeruhan harus dilakukan untuk memperoleh implementasi
nyata dari studi ini di Gunung Kidul.
6. Sifat-sifat kimia dari saringan pasir dan air baku harus dipertimbangkan dalam studi
berikutnya.
7. Kolom saringan F4 (pasir Lava, d10 = 0,07 mm dan Cu = 4,2) menghasilkan pengurangan
bakteri terbaik dalam percobaan ini mencapai 4,7 log-unit penghapusan dari total
coliform dan 5,0 log-unit penghapusan dari E. coli. Namun, dengan konfigurasi filter
tersebut, kinerja maksimal saringan dapat dicapai pada kondisi tertentu yaitu HLR 0,03
m/jam. Dengan kondisi demikian, saringan ini akan tidak lagi ekonomis mengingat
kebutuhan area yang lebih besar jika dioperasikan pada saringan air pusat. Oleh karena
itu, saringan dengan konfigurasi pasir seperti ini cocok untuk diterapkan dalam skala
kecil atau rumah tangga.
8. Kolom saringan F5 (pasir Rhine, d10 = 0,2 mm dan Cu = 2,1) dengan konfigurasi pasir
yang berada pada kisaran yang direkomendasi untuk SPL dan HLR 0,1 – 0,2 m/jam,
dapat mencapai pengurangan bakteri yang lebih baik daripada kolom saringan F1, F2,
dan F3. Karakteristik media pada saringan ini sangat cocok untuk diterapkan pada
saringan air pusat.
Daftar Pustaka
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Auset, M., & Keller, A.A. 2005. Intermittent filtration of bacteria and colloids in porous media. Water
Resources Research, Vol. 41, W09408, doi:10.1029/2004WR003611.
Auset, M., Brissaud, F., Drakidès, C., & Lazarova, V. 2002. Clogging Management and Removal of
Enteric bacteria from secondary effluents by infiltration percolation. IWA Regional
Symposium on Water Recycling in the Mediterranean Region, Iraklio, 26-29. 2. 147-150.
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incorporati
September 2011
R 604.6 Yog e c.1 09.2011
604.6
Thesis
English
Magister Teknik Sistem FT UGM
2011
Yogyakarta
x, 108 hlm.; ilus.; 29 cm.
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