Knowledge of wastewater treatment

Objectives of WWT

Reduce organic content (reduction of BOD), Removal/reduction of nutrients i.e, N,P and Removal/inactivation of pathogenic microbes.

What is Wastewater?

Dairy and industrial waste–slaughterhouse waste, dairy waste, tannery wastewater, etc.

Domestic waste: human and animal excreta and waters used for washing, bathing, and cooking.

100-500 g wet weight of feces and 1-1.3 L of urine/capita/day and 15-20 g BOD/day is contributed by each person

Wastewater Contaminants

Suspended solids

Priority pollutants: metalloids (As, Se) and metals (Cd, Hg), benzene compounds, and chlorinated compounds

Microorganisms: pathogenic and nonpathogenic

Organics: refractory and biodegradable

Nutrients: Phosphorus Nitrogen (ammonia, nitrites, nitrates)

Objective 1 – Reduce organic content (reduction of BOD)

The amount of organic carbon present determines: the amount of O2 needed for biological treatment ,the size of waste treatment facility needed and the efficiency of the treatment process.

There are three methods to determine carbon present:

biochemical oxygen demand, total organic carbon and chemical oxygen demand

Biochemical Oxygen Demand (BOD)

Definition: The amount of dissolved oxygen utilized by microbes for the biochemical oxidation of organic (carbonaceous BOD) and inorganic (autotrophic or nitrogenous BOD)

The BOD test was developed in 1930’s. This is a five day test that measures the amount of O2 consumed in a wastewater sample by a mixed population of heterotrophic bacteria in the dark at 20oC

BOD of wastewater is typically 110-440 mg/L and must be reduced to 20 mg/L for discharge

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BOD = (Di – Df) /P

where:

Di = initial dissolved O2 concentration

Df = final or 5-day dissolved O2 concentration

P = volumetric fraction of wastewater

Example: 5 ml wastewater is added to a 300 ml BOD flask

P = 5/300 = 0.0167 Di = 8 mg/L Df = 2 mg/L

BOD = (8 – 2)/ 0.0167 = 359 mg/L

Oxidation is usually 60-70% complete after 5 days