Investigation of Biochemical Oxygen demand (BOD) Concentration in Effluent from a Sewage Treatment Works

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Investigation of Biochemical Oxygen demand (BOD) Concentration in Effluent from a Sewage Treatment Works

NAME:                                          Leo Chiedu

Introduction

Nature has a way of bringing everything to its normal position, plants give off oxygen during photosynthesis, humans and animals use up this oxygen for survival and in turn give off carbon dioxide (CO2), which is used up by plants. This relationship shows how nature has created a balance between plants and animals.

In natural water all living organism depend on the oxygen or dissolved oxygen in water for sustainability. Dissolved oxygen (DO) I water helps maintain a healthy river, lake and ocean and thus supporting different kinds of aquatic organism. A symbiotic system exits whereby all aquatic organisms depend on themselves for survival.

Most of the dissolved oxygen (DO) in water comes from photosynthesis and atmosphere, about 8 – 10mg/l is needed to maintain 100% saturation in water. However, the level of dissolved oxygen in water varies, the amount or concentration of oxygen (O2) rises from morning through o evening, this is as a result of serious activity by plants in water. At this time photosynthesis is taking place and so oxygen (O2) is given off, whereas at night, the level of oxygen drops because photosynthesis has stopped. Nonetheless, plants and animals continue to consume oxygen. The level of oxygen drops, this drop may bring down the level of to about 4mg/l and this is the minimum amount that is required to sustain the living organisms in water.

Man and nature contribute significantly to the level of dissolved oxygen (DO) in water both in a positive and negative way.

Nature

The levels of dissolved oxygen in water can greatly be affected by weather conditions, these are; temperature, pressure, erosion, sedimentation and ice cover. These factors affect the solubility of oxygen (O2) in water. An increase in temperature reduces the amount of dissolved oxygen. As the temperature increases the saturation concentration decreases (Gray N.F 1999). Table (1) shows the relationship between dissolved oxygen in water and temperature at 1 atmosphere.

Table  (1) source; Gray N.F 1999 pg 67

From table (1) above it can be observed that as water gets warmer, there is a reduction in dissolved oxygen (DO), this is because the oxygen molecule becomes energised and diffuses to the water surface thereby leaving fewer dissolved oxygen in water.

Dissolved minerals in water could be as a result erosion, sedimentation and weathering. Dissolved salt in water reduces the concentration of dissolved oxygen in water; water being a universal solvent dissolves salt. Oxygen is used up to form other compounds as shown below;

SiO2(s) + 2H2O         Si(OH)4                                                (1)

Fe2O3(s) + 3H2O         Fe 2O3.3H2O(s)                                        (2)

CaO(s) +H2O         Ca(OH)(aq)                                                (3)

CaCO3(s) + CO2 + H2O                     Ca2+(aq) + 2HCO3-                (4)

(Equations from Harrison R.M et al 1996)

When these solid minerals dissolve in water, oxygen is used up to another compound.

Man

The quest by man for a better living has in a way impinged on the quality of water. To improve agricultural yield, fertilizer is added to plant root for growth, however, plants take up few amounts and the rest is washed into river and or lake. Nitrogen and phosphorus are the major constituents of fertilizer, if in excess, both elements in a compound form causes eutrophication thereby reducing the amount of dissolved oxygen in water.

In developed countries treated sewage is disposed off into river, this waste contains microorganisms, detergents, and other waste product. If untreated or regulated these causes severe reduction of dissolved oxygen in a water body. Sewage contribute large amount of nutrients into the river, plants and microorganisms use up these nutrients, rivers or lake that contains essential mineral nutrient may support heavy growth of algae (Manaham S.E 1993).

Biochemical Oxygen Demand (BOD)

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In water dissolved oxygen (DO) place an important role in maintaining a balance as enumerated above, for instance, when oil or for that matter any substance that is a ‘’stranger’ is introduced into a water body, some biodegradation will take place to break down the hydrocarbon molecules, usually oxygen is the fuel that is used up or consumed to carry out this breakdown of the hydrocarbon molecules. Biodegradation can be chemical reaction or biological that is caused by living organisms like; bacteria, fungi, sulphate reducing bacteria etc.

The amount of oxygen required to completely breakdown the hydrocarbon molecules by chemical ...

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