Analisa Kimia Sampel Air Sungai : Penentuan DO dan BOD

DO dan BOD

1.      Tinjauan Pustaka

Oksigen  terlarut  adalah suatu hal yang sangat diperlukan oleh makhluk hidup dalam air tergantung dari kemampuan air untuk mempertahankan konsentrasi oksigen minimal yang dibutuhkan untuk kehidupannya. Konsentrasi oksigen terlarut  minimal untuk kehidupannya.

Oksigen terlarut dalam air dapat berasal dari proses fotosintesis tanaman air, dimana jumlahnya tidak tetap tergantung dari jumlah tanamannya, dan dari atsmosfer (udara) yang masuk kedalam air dengan kecepatan terbatas. Konsentrasi oksigen terlarut dalam keadaan jenuh bervariasi tergantung dari suhu dan tekanan atmosfer. Semakin tinggi suhu air, semakin rendah tingkat kejenuhan. Misalnya danau di pegunungan yang tinggi mungkin mengandung oksigen terlarut 20-40 % kurang daripada danau pada permukaan laut.

BOD atau Biochemical Oxygen Demand atau kebutuhan oksigen biologis merupakan jumlah oksigen terlarut yang dibutuhkan oleh organismee untuk mengoksidasi bahan-bahan buangan dalam air. Dengan kata lain, BOD menunjukkan kebutuhan oksigen oleh organismee untuk mengoksidasi bahan-bahan buangan yang terlarut dalam air.( Metclaf,Eddy. 2003. Waste Water Engineering Design).

Angka BOD adalah jumlah oksigen yang dibutuhkan oleh bakteri untuk menguraikan hampir semua zat organik yang terlarut dan sebagian zat-zat organik yang tersuspensi dalam air. (Alaert.G dan Sri Sumestri Santika,Msc. 1984. Metoda Penelitian Air).

BOD penting untuk mengetahui banyaknya zat anorganik yang terkandung dalam air limbah. Makin banyak zat organik, makin tinggi BOD-nya. Nilai BOD dipengaruhi oleh suhu, cahaya, matahari, pertumbuhan biologik, gerakan air dan kadar oksigen. .( Metclaf,Eddy. 2003. Waste Water Engineering Design).

2.             Prosedur Kerja

2.1              Penentuan Dissolve Oxygen

Metode Titrasi Winkler (Iodometri)

·         Prosedur Pengambilan

Botol Winkler yang digunakan untuk mengambil sampel harus bersih, dan telah dibilas dengan air suling terlebih dahulu, kemudian dilanjutkan  pengkondisian cairan yang akan digunakan untuk mengisi botol. Hal yang sama juga berlaku untuk alat-alat pengambilan sampel yang digunakan. Alat-alat ini harus bersih dan tidak mengandung sisa dari bekas sampel yang lama, khususnya tumbuhnya jamur dan lumut harus dicegah.  Pengambilan sampel  dilakukan di sungai dibawah permukaan air sekitar 5 m. kemudian di tempatkan dalam botol sampel/ botol winkler sampai penuh, kemudian di tutup.

Selama penentuan oksigen terlarut, baik untuk DO maupun BOD, diusahakan seminimal mungkin larutan sampai yang akan diperiksa tidak berkontak dengan udara bebas. Sampel dalam botol winkler kemudian ditentukan DO-nya dengan titrasi winkler.

·         Prosedur Pengukuran DO

Alat – alat :

1.      Botol Winkler yang volumenya telah diketahui  dengan ketelitian ± 0,1 ml lengkap dengan tutupnya

2.      Kotak inkubator

3.      Pipet ukur 10 ml

4.      Gelas arloji

5.      Pipet tetes

6.      Erlenmeyer 250

7.      Labu ukur 100 ml

Bahan :

1.      MnSO₄ . 2H₂O 20 g

2.      NaOH 26 g

3.      KI 7,5 g

4.      NaN₃ 0,5 g

5.      Indikator amilum

6.      Na₂S₂O₃ 3,1025 g

7.      K₂Cr₂O₇ 0,1205 g

8.      KI murni 2 g

9.      H₂SO₄ 4 N 40 ml

10.  Akuades

11.  Es batu

Pembuatan Reagen :

a.       Larutan Mangan Sulfat

Larutkan MnSO₄ . 2H₂O 20 g didalam 50 ml akuades pada labu takar

b.      Larutan alkali – iodide – azida

Larutkan secara terpisah 25 g NaOH, 7,5 g KI dan 0,5 g NaN₃. Campurkan dalam labu takar  dan diencerkan dengan akuades sampai 50 ml lalu didinginkan

c.       Larutan tiosulfat 0,025 N

Na₂S₂O₃ 3,1025 g dilarutkan dalam labu takar dengan akuades sampai 500 mL. Diawetkan dengan tambahan 0,25 g NaOH

-          Standarisasi larutan tiosulfat dengan titrasi K₂Cr₂O₇ 0,025 N untuk meningkatkan ketelitian

Dilarutkan 0,1205 gr dalam labu ukur 100 ml (K₂Cr₂O₇ sebelumnya harus dikeringkan pada suhu 105^oC selama 2 jam lalu didinginkan dalam desikator). Dipipet 20 ml larutan K₂Cr₂O₇ kedalam Erlenmeyer 250 ml diencerkan dengan aquades sampai 100 ml ditambah 2 gr KI murni (p.a) dan 10 ml  H₂SO₄ 4 N. dikocok dan disimpan dalam pada tempat gelap selama 5 menit. Dititrasi dengan larutan nantrium tiosulfat yang akan distandarkan bila warna kuning dalam larutan hamper hilang ditambah 2 ml indicator amilum. Diteruskan titrasi sampai warna biru yang baru muncul habis menjadi bening.

Cara kerja Penentuan DO:

1.      Dipipet  2 ml larutan mangan sulfat kedalam sampel  yang ada dalam botol winkler dimana penambahan dilakukan   di bawah permukaan cairan.

2.      Ditambah 2 ml larutan alkali-iodida-azida  kemudian botol ditutup kembali untuk mencegah udara terperangkap udara luar, kemudian dikocok dengan membalik-balikkan botol  beberapa kali.

3.      Dibiarkan  10 menit agar gumpalan mengendap.

Setelah pengendapan sempurna , maka bagian larutan yang jernih dikeluarkan dari botol dengan menggunakan pipet ; sebanyak  kurang lebih 100 ml dipindahkan dalam Erlenmeyer 500 ml.

4.      Ditambahkan 2 ml H₂SO₄  pekat  dikocok, dimasukkan kedalam Erlenmeyer 250 ml

5.      Dititrasi dengan larutan tiosulfat 0,025 N sampai timbul warna kuning pucat.

6.   Ditambah indicator kanji 1- 2 ml sehingga timbul warna biru. Titrasi dilanjutkan sampai warna biru hilang, dicatat volum titrasi dan volum contoh

Perhitungan

Penentuan nilai oksigen terlarut (DO)  dengan rumus:

DO(ppm) =  V Na₂s₂o₃ x N  Na₂s₂o₃ x 8 x 1000

                                      V sampel

2.2          Penentuan BOD

Pada penentuan BOD, digunakan Metode Titrasi Winkler (Iodometri) yang sama dengan metode pada penentuan DO. Perlakuan yang berbeda adalah pada perlakuan awal sebelum titrasi winkler, meliputi :

·           Prosedur pengambilan sampel

Botol Winkler yang digunakan untuk mengambil sampel harus bersih, dan telah dibilas dengan air suling terlebih dahulu, kemudian dilanjutkan  pengkondisian cairan yang akan digunakan untuk mengisi botol. Hal yang sama juga berlaku untuk alat-alat pengambilan sampel yang digunakan. Alat-alat ini harus bersih dan tidak mengandung sisa dari bekas sampel yang lama, khususnya tumbuhnya jamur dan lumut harus dicegah.  Pengambilan sampel  dilakukan di sungai dibawah permukaan air sekitar 5 m. kemudian di tempatkan dalam botol sampel/ botol winkler sampai penuh,  dikondisikan Ph pada 7,0 ± 0,1 dengan menggunakan asam atau basa  kemudian langsung di tutup.

Selama penentuan oksigen terlarut, baik untuk DO maupun BOD, diusahakan seminimal mungkin larutan sampai yang akan diperiksa tidak berkontak dengan udara bebas.

·           Prosedur pengenceran sampel

Oleh karena jumlah oksigen dalam botol terbatas, maksimum 9 mg O₂/L tersedia, maka oksigen terlarut pada akhir inkubasi antara 3 dan 6 mg O₂/L sehingga perlu diencerkan. Karena COD sampel belum diketahui, untuk menaksir pengenceran (Derajat Pengenceran/ P) yang cocok  disesuaikan dengan sumber atau asal sampel (Tabel 10.2 di lampiran) . Untuk air sungai yang tercemar zat organic, maka dipilih P = 0,25; 0,125 dan 0,0625.  Air buangan penduduk P = 0,015 dan 0,075. Dan air dari buangan industry P= 0,075; 0,004; 0,002; dan 0,001.

·           Prosedur pembawaan sampel BOD ke lab

Botol BOD ini disimpan dalam incubator (suhu 20^o C) selama kira-kira 1 jam saat di bawa ke lab.

·           Penyimpanan sampel selama 5 hari

Jika suhu awal sampel lebih dari 20^o C, maka setelah pendinginan 1 jam, volum larutan akan berkurang, sehingga ditambahkan kembali air pengencer sehingga di dalam botol tertutup tidak ada gelembung udara. Kemudian disimpan terus dalam incubator (suhu 20^o C) selama 5 hari.

·           Penentuan BOD dengan titrasi Winkler (sama seperti pada penentuan DO)

Perhitungan  untuk kadar  BOD:

BOD (ppm) = 5x (DO awal – DO akhir)

3.      Data Hasil Pengamatan

-          Penentuan DO Awal

Perkotaan

·         Permukaan (A)

DO(ppm) =  12 ml x 0,025 N x 8 x 1000  = 8,187 ppm

                                      293,11 ml

·         Kedalaman (C)

DO(ppm) =  11,2 ml  x 0,025 N x 8 x 1000  = 7,676 ppm

297 

-     Penentuan DO akhir

Perkotaan

·         Permukaan (A)

DO(ppm) =  9,1 ml x 0,025 N x 8 x 1000  = 6,289 ppm

                                      289,4 ml

·         Kedalaman (C)

DO(ppm) =  7,5 ml  x 0,025 N x 8 x 1000  = 5,117 ppm

                                293,16   ml

-          Penentuan BOD

Perkotaan

·         Permukaan (A)

BOD (ppm) = 5x ( 8,817 – 6,289 ) = 12,64 ppm

·         Kedalaman (C)

BOD (ppm) = 5x ( 7,676 – 5,117 ) = 12, 729 ppm

4.      Pembahasan

Berdasarkan data yang diperoleh, pada penentuan DO, untuk sampel air pada permukaan (A) memiliki nilai DO yang lebih tinggi daripada nilai DO sampel air kedalaman (C) karena pada permukaan air berinteraksi dengan oksigen dari udara secara langsung, sedangkan pada air kedalam tidak. Selain itu pada air kedalaman (C) juga dipengaruhi oleh  organisme seperti ikan , hewan air, dan tumbuhan air yang hidup di dalam air dan bergantung pada oksigen terlarut dalam air. Hal ini yang menyebabkan kadar DO di kedalaman makin kecil.

Pada penentuan BOD, air permukaan (A) akan memiliki kadar BOD lebih rendah dari pada BOD  air kedalaman (C). Rendahnya BOD di air kedalaman dikarenakan  ketergantungan organisme seperti  air ikan , hewan air dan tumbuhan air yaitu  pada oksigen dalam air akan semakin tinggi.

5. Kesimpulan

                        Dari data diatas dapat disimpulkan bahwa konsentrasi DO paling besar yaitu pada daerah permukaan karena kontak langsung dengan udara luar. Sedangkan untuk BOD konsentrasi paling tinggi pada daerah kedalaman tertentu karena semakin banyaknya mikroogranisme didaerah kedalaman sehingga kandungan oksigennya semakin sedikit.

Daftar pustaka:

Dr.Ir.G.Alaerts, Ir. Sri Sumestri Santika, MSc, 1987, METODE PENELITIAN AIR, Penenerbit: Usaha Nasional, Surabaya.

Salmin, 2005, OKSIGEN TERLARUT (DO) DAN KEBUTUHAN OKSIGEN BIOLOGI (BOD) SEBAGAI SALAH SATU INDIKATOR UNTUK MENENTUKAN KUALITAS PERAIRAN, Bidang Dinamika Laut, Pusat Penelitian Oseanografi-LIPI, Jakarta

Metclaf,Eddy. 2003. WASTE WATER ENGINEERING DESIGN, Mc Graw-Hill, New York.

Alaert.G dan Sri Sumestri Santika,Msc. 1984. METODA PENELITIAN AIR, Usaha Nasional, .Surabaya-Indonesia

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Turbidity, Color, Odor, and Taste in Domestic Water

Ronald E. Hermanson, P.E.

From the clouds to the tap, water contacts and carries many substances, including gases, minerals, and organic matter. Many of these are impurities that interfere with water use by humans. Some of these impurities are completely dissolved; others are solid, suspended particles in the water that cause objectionable cloudiness, color, odor, and taste.

Finely divided, solid particles that absorb or reflect light cause "cloudy water," or turbidity. These particles are generally undissolvable, inorganic mineral matter or organic matter picked up by water flow over and through the ground. Surface water from lakes, streams, and ponds usually has significant amounts of turbidity from surface water runoff, or from bottom deposits stirred up by water movement. Because the earth normally serves as an excellent filter, it is unusual to find significant amounts of turbidity in water from deep wells.

Water drinkers find turbidity objectionable primarily because the physical appearance of "dirty water" is less appealing than clear, sparkling water. Turbidity caused by inorganic minerals is undesirable because its abrasiveness can erode a plumbing system's pipes and fittings, and score its valve seats and washers. Turbidity caused by suspended organic matter is objectionable because it can stain sinks and fixtures, and discolor laundered fabrics.

Dissolved organic matter causes most water color. Surface water usually has some color, and it is sometimes found in well water. This often occurs in areas where swamps or bogs are common. The water picks up colored substances extracted from decaying organic matter. Dissolved organic matter also makes water unpleasant to drink, frequently contributes tastes and odors, and stains surfaces and materials. Even low amounts of organic matter may produce unpleasant "off" tastes and musty odors in drinking water, and foods and beverages prepared with the water.

Hydrogen sulfide gas in water, or sulfur water, produces a rotten egg odor, corrodes plumbing metals, and rapidly tarnishes silver. Even very low concentrations of hydrogen sulfide produce a strong, easily identifiable odor. Minerals dissolved in water at moderate levels add a taste pleasant to most palates; "flat" water appeals to few people. However, too high a mineral concentration gives the water an unpleasant soda or salty taste.

The U.S. Environmental Protection Agency and the Washington Administrative Code for Public Water Supplies has set limits for physical characteristics of water.

Drinking water should contain no impurity that would offend the senses of sight, taste, or smell. Under general use, do not exceed the following limits:

These characteristics are measured by laboratory tests. Although these tests do not directly measure the safety of the water, they are related to consumer acceptance. One unit of turbidity, 15 units of color, and a threshold odor number of 3 are levels at which these characteristics become objectionable to a considerable number of people.

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*Many individuals will accept greater concentrations.
**Milligrams per liter (parts per million).

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Water Treatment Equipment

Mechanical filters designed to remove solid particles from water are available in different types and sizes. These filters are fine screens that trap solid particles but allow water to pass.

One type uses specially graded sand or other granular synthetic material as filter media in tanks about the same size as household water softeners. This filter is usually sufficient to handle an entire household water supply. It works very well when suspended particles are relatively large or gelatinous. It is not effective when extremely fine particles cause turbidity. The filter must be backwashed periodically to clean its beds, and to flush accumulated matter.

A second type is the cartridge filter. This unit is usually smaller than the tank type. It is often installed to treat water in a single water line. Instead of loose media, this filter uses media formed into a semirigid cartridge. Although some are designed for mechanical cleaning and reuse, most are designed for replacement when they become clogged. This filter cartridge, available in several ratings based on particle size, can remove extremely tiny particles. However, the cartridge has a higher resistance to water flow and can become clogged quite rapidly.

Activated carbon is known for its ability to adsorb soluble organic compounds and certain gases that contribute tastes and odors to a water supply, such as chlorine and hydrogen sulfide. Activated carbon is widely used in granular form in tank-type filters and as finely divided powder in a cartridge. A granular filter must be backwashed periodically; a cartridge must be cleaned or replaced periodically.

If the water to be treated with granular activated carbon contains a high concentration of hydrogen sulfide, the carbon ultimately becomes saturated. Bed treatment with high dosages of household hypochlorite bleach to "burn off" the adsorbed impurities and extend bed life works well. However, the activated carbon bed must be replaced eventually. Chlorine removal consumes some activated carbon. Small amounts must be added to the bed as replacement. Chemical feed pumps may be used to add bleach. Chlorine bleach also oxidizes many impurities.

Reverse osmosis units force water against a semipermeable membrane. This membrane allows some water but few impurities, including dissolved minerals, to pass through. One flow of water enters the unit, but two streams exit. One stream is purified water. The second contains concentrated impurities.

Unfortunately, the large equipment needed in reverse osmosis to treat all domestic water is relatively expensive and generally not feasible. However, a small unit to treat only cooking and drinking water is available. This unit operates continuously and contains a treated-water reservoir. Filtration of the inflow to the unit prevents membrane clogging. The need for filtration depends upon the concentration of suspended particles in the water supply. A reverse osmosis unit, however, needs no regeneration or backwashing, and the membranes last a long time.

Treatment Methods and Equipment Application

Equipment design determines its application to turbidity, color, odor and taste problems. Mechanical filters effectively remove turbidity caused by suspended, solid particles from water not tainted by color, odor and taste. A large, tank-type unit efficiently filters an entire water supply after the water leaves the pressure tank. A cartridge filter in a water line to a specific tap solves the problem of incomplete removal of turbidity. A cartridge filter on specific taps without a tank-type unit is sufficient to purify water used only for cooking and drinking. Activated carbon filters have similar applications: a large, tank-type unit removes turbidity and many tastes and odors from the entire water supply; a cartridge filter purifies an individual water line.

Very high organic matter and rapid saturation of activated carbon may require chlorine oxidation. You can use a chemical feed pump to inject a solution of household bleach into the water pipe between the well pump and the pressure tank. Wire the chemical feed pump to operate simultaneously with the well pump to properly proportion the bleach and water. The pressure tank serves as a mixing vessel, and allows at least some time for organic matter oxidation. Sometimes it is necessary to install extra tanks for more contact time when the organic matter resists oxidation. The chemical feed should provide a chlorine residual of 3 to 5 ppm after it leaves the tank or tanks. An activated carbon filter in the water line will remove this chlorine, which is too high an amount for cooking and drinking, and any precipitated iron or other materials in the water. Water treatment by filtration and chlorination removes iron and disinfects the water.

Hydrogen sulfide is a special case. At very low concentrations, use an activated carbon filter and occasional bleach treatment to extend carbon life.

An iron-removal filter effectively removes low to moderate sulfur concentrations. A chlorination system with activated carbon filtration is the best solution for higher concentrations of hydrogen sulfide.

In all of these applications, the equipment should be installed ahead of a water softener to protect the softener against fouling, and to permit use of unsoftened water in some water lines.

Finally, a small reverse osmosis unit is the most effective if high concentrations of dissolved minerals cause objectionable water taste. This unit is subject to fouling when water contains organic matter, turbidity, iron, and similar contaminants. Prefiltration is the solution. If a water softener is used for hardness removal, the softened water fed to the reverse osmosis unit improves mineral removal performance and extends membrane life.

The best treatment method results from careful consideration of such factors as economics, water quality characteristics, water end-use, water temperature variances, and the inherent limitations of treatment technology. Consult local water treatment representatives before purchase and installation of any water treatment equipment.

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By

Ronald E. Hermanson, Ph.D., P.E., Washington State University Extension Agricultural Engineer, Pullman.

Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office. Trade names have been used to simplify information; no endorsement is intended.
Revised May 1991. Subject code 376. A. EB0994

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