JURNAL TEKNOLOGI, Edisi No.4 Tahun XX, Desember 2006, 287-291 ISSN 0215-1685
Activation of Raw Water Pre-Treatment Facility in PLTU Ombilin
Misri Gozan, Emil Mahfuzi, Lili Hambali dan Asep Handaya Saputra
Chemical Engineering Department, Faculty of Technology, University of Indonesia
Kampus Universitas Indonesia, Depok 16424, Indonesia
e-mail: mgozan@che.ui.edu
Abstrak
Pada sebuah Pembangkit Listrik Tenaga Uap (PLTU), seperti di PLTU Ombilin, Sumatera Barat, air
digunakan terutama untuk air umpan boiler, untuk pendingin, pemadam kebakaran, service water, dan
air minum. PLTU Ombilin menggunakan Sungai Ombilin sebagai sumber air baku untuk memenuhi
semua kebutuhannya akan air. Sebelum dapat digunakan untuk memenuhi semua keperluan tersebut, air
baku harus diolah terlebih dahulu melalui berbagai tahapan untuk menghilangkan berbagai pengotor
yang secara alami terkandung di dalamnya. Penelitian ini bertujuan untuk merancang unit pengolahan
awal air baku untuk utilitas di PLTU Ombilin dan membandingkannya dengan unit pengolahan yang
sudah ada dan beroperasi. Untuk keperluan perancangan digunakan data laju alir air yang diolah yang
diperoleh dari lapangan yaitu sebesar 1.160 m3/jam. Perancangan ditujukan untuk menyisihkan TSS,
patogen, dan kekeruhan yaitu dengan menggunakan prinsip-prinsip koagulasi, flokulasi, sedimentasi,
disinfeksi, dan membran ultrafiltrasi. Hasil perancangan ini berupa rangkaian proses yang tersusun dari
static mixer, flokulator, clarifier, dan membran ultrafiltrasi, serta dengan menggunakan senyawa kimia
meliputi alum, kapur, NaOCl, dan polielektrolit. Static mixer yang digunakan memiliki diameter pipa
sebesar 16 in. Flokulator dirancang berupa saluran berpenampang (2x2) m2 dengan panjang 100 m.
Clarifier berupa unit aliran horizontal, dengan permukaan (40 x 20) m2 dan kedalaman 5,8 m. Clarifier
ini memiliki laju beban permukaan 35 m3/m2.d, laju beban weir 250 m3/m.d dengan panjang weir 111,36
m. Membran ultrafiltrasi hanya mengolah 80% air umpan, dengan fluks 50 lmh, dan luas permukaan
yang dibutuhkan 13.290 m2. Dari perbandingan hasil perancangan dan unit pengolahan yang sudah ada,
didapatkan rekomendasi bagi unit pengolahan yang ada untuk tidak menggunakan screen, memodifikasi
flokulator dan clarifier, serta mengganti saringan pasir dengan membran ultrafiltrasi.
Kata kunci: Pengolahan air, filtrasi, sedimentasi, koagulasi dan flokulasi umpan boiler.
Abstract
In a power supply (PLTU) located in West Sumatera Barat, water is massively utilized for boiler feed,
chiller fire fighters, service water, and drinking water. This need is supplied by a river nearby as the only
source. This raw water is subject to a pre-treatment unit to remove the contaminants. This research was
aimed to design a raw water pre treatment unit for utility in the power supply and to compare with the
existing and operating treatment unit. The feed flow rate is 1,160 m3/hour. The design objective was to
remove TSS, pathogenic bacteria, and turbidity using coagulation, flocculation, sedimentation,
disinfectant, and ultra filtration membrane. Static mixer was used with pipe diameter 16 in. Flocculator
has tubular size of (2x2) m2 with 100 m length. Clarifier was a horizontal flow type with surface of
(40x20) m2 and depth 5,8 m. This clarifier has surface loading flow rate of 35 m3/m2.d, weir loading 250
m3/m.d with weir length of 111.4 m. Ultra-filtration membrane treats only 80% of feed water (50 lmh
flux) and need surface area of 13,290 m2. From the comparison we found that we do not the screening.
Furthermore, we should modify the flocculator and clarifier, and replace the sand filtration unit with
ultra-filtration membrane.
Keywords: Water treatment, filtration, sedimentation, coagulation, flocculation and boiler feed.
1. Introduction
A power supply in Sawahlunto, West Sumatera takes water from Ombilin river and treats the water for several purposes. Water fed to boiler should be free of
mineral. Therefore, raw water undergoes pre treatments and finally goes to
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M. Gozan, E. Mahfuzi, L. Hambali dan A. H. Saputra
demineralisation plant. Water goes through
belt screen, static mixer, floculator,
clarifier, and sand filter. Petreatment plays
an important role prior to demineralisation
to remove most of the organic and
inorganic.
In the pretreatment stage, Aluminum
sulphate (Al2(SO4)3), lime (Ca(OH)2),
NaOCl, and a poly-electrolite are used to
remove the suspended solid. Function of
Alum as coagulant, lime as pH regulator,
NaOCl as disinfektan, and polielektrolit as
(coagulant aid) or flokulan. To re-activate
raw water pre-treatment need a study again
to the existing unit
2. Metodology
The objectives of this research is to
obtain first design stage of raw Water
pretreatment for Ombilin Steam Power
Plant for utility by several process stage,
include rapid mixing, low mixing and ultra
filtration membrane. The equipments are
designed for rapid mixing, sedimentation
tank and ultra filtration for fine strain.
Raw water has strict requirements
criteria, even more strict than requirement
of domestic water. Boiler fed water must be
free of mineral ingredients in raw water,
therefore fed water must be proceed in
deminerasation to remove mineral
ingredients. Demineralisation can be run in
several methods, one of them is ion
exchange. The design is performed base on
datas which has taken on site (the existing
water pretreatment).others, asumption datas
are used in calculation.
3. Result and discussion
Water pre-treatmentis located in
Figure 1.
Pretreatment Process Schematics
The raw water from river is add with
chemical coumpound which consist of
alum, lime, natrium hipoklorit (NaOCl).
The coumpound is feed to a static mixer.
Raw Water pretreatment in Steam
power plant Ombilin produce water to
fulfill need of demineralisation and other
neccesities such as service water, fire
fighting water and cooling water. Most of
ouput water of Clarifier (80%) need further
treatment by using sand filter, others is used
for system service, fire fighting and cooling
water system. Sand filter produce water
where fulfil requirement water feed for
demineralisation. Quality of water output
clarifier and output of sand filter are listed
in Table 1.
Tabel 1.
Quality of Water Output Clarifier
Tabel 2.
Quality of Water Output of Sand Filter
Ombilin Steam Power Plant which used
principle of coagulation, floculatin,
sedimentation, disinfectan, and filtration.
Equipment used include static mixer,
screen, pulsator clarifier and sand filter.
Schematic process of water pretreament is
shown in Figure 1
288 JURNAL TEKNOLOGI, Edisi No.4 Tahun XX, Desember 2006, 287-291
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Activation of Raw Water Pre-Treatment Facility in PLTU Ombilin
The design is summarized on table 3.
Table 3.
Summary of Design Result
There are several differences between
design result with existing plant, the
differences are the usage of screen, clarifier
design, and the difference of the usage of
sand filter and ultra filtration membrane.
Flocculator channel
The result of design shown that
flocculator channel 100 meter in length and
(2 x 2) m2 cross section. Meanwhile on
site, flocculator channel about 30 meter in
length with (1 x 1) m2 in cross section. It
cause imperfection in floculation because
detention is not sufficient
To improve performance of flocculation
of operated water treatment units, floculator
channel should be modified. It could be
modified as calculation result change, so
that floc can be obtained and easy to form
sediment.
Clarifier Design
Clarifier design as a result of
calculation differ with existing clarifier on
site. It is occured because basic operation is
different. The design of clarifier use
horizontal flow meanwhile exist plant
special type of flow is clarifier pulsator
vertical flow.
Pulsator system use fan to vacuum the
vacuum chamber on surface of water. As
fan cause chamber vacuum, the surface of
water would flow up in vertical direction in
the chamber. a moment, i.e. 40 seconds,
then the vacuum would be released so that
the surface of water would down
immediately. 20 seconds then vacum
chamber would be vacuum again and the
process will be done countinuosly base on
certain time periods. It is combined with
spread jet channel that exist in clarifier to
form sludge as a result of pulsation system,
they spread on the clarifier. By using this
method, sedimentation process would be
faster because flocs would not only form
sediment because of gravitation but also
force on pulsation process.
4. Summary
Water Pre-treatment Plant for the utility
use coagulant principle, flocculation,
sedimentation, and ultra filtration
membrane need following equipments:
Mixer Static is to mix chemical
compound, include alum as coagulant, lime
as pH regulator, and NaOCl as disinfectant.
The operated Water pretreatment plant
in PLTU Ombilin needs some modification.
JURNAL TEKNOLOGI, Edisi No.4 Tahun XX, Desember 2006, 287-291 289
M. Gozan, E. Mahfuzi, L. Hambali dan A. H. Saputra
Floculator channel need to modify to
produce sufficient detention time in order to
form better flocs. Pulsator clarifier should
be replaced with horizontal flow clarifier.
Sand filter should be replaced with ultra
filtration where produce water with better
quality.
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