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Biological Oxygen Demand (BOD)

BOD: Oxygen is removed from water when organic matter is consumed by bacteria.
Low oxygen conditions may kill fish and other organisms.

Sources of organic matter
Natural inputs-- bogs, swamps, leaf fall, and vegetation aligning waterways.
Human inputs-- pulp and paper mills, meat-packing plants, food processing industries, and wastewater treatment plants.
Nonpoint inputs-- runoff from urban areas, agricultural areas, and feedlots.

Wastewater Treatment Objectives

Wastewater treatment systems take human and industrial liquid wastes and make them safe enough (from the public health perspective) to return to the aquatic or terrestrial environment.
In some cases, wastewater can be clean enough for reuse for particular purposes.
Wastewater treatment systems use the same processes of purification that would occur in a natural aquatic system only they do it faster and in a controlled situation.

Sewage or Wastewater Treatment

Sewage or wastewater is composed of sewage or wastewater from:
Domestic used water and toilet wastes
Rainwater
Industrial effluent (Toxic industrial water is pretreated)
Livestock wastes
** microbes degrade organic compounds
** elimination of pathogens occurs

Wastewater Treatment
Types of treatment systems include: Septic Tanks or Wastewater Treatment Plants (WWTPs).
Septic Tanks typically treat small volumes of waste (e.g., from a single household, small commercial/industral)
WWTPs typically treat larger volumes of municipal or industrial waste.

Decentralized Alternatives
In rural areas or in particular urban communities in the U.S., human wastewater will be treated through individual septic tank systems (pumped or leachfield varieties)
Wastewater is filtered, microorganisms killed and chemicals adsorbed and/or diluted in its passage through the soils and rocks of the leachfield
In developing countries, urban wastewater is seldom treated and instead flows raw through collectors to receiving water bodies (like in the US 100 years ago)
The solution for many developing nations is centralized oxidation lagoon systems (but this needs space) or the use of individual ventilated pit-latrines, especially for shanty towns and rural villages

Sewage Treatment
Wastewater or sewage treatment is a multistep process:
1. Primary Treatment (Physical Process)
Removal of large objects using grates and screens
Settling to remove suspended solids (primary sludge)
flocculating chemicals are added to enhance sedimentation
Secondary Treatment (Microbial Process)
Supernatant or primary effluent contains high levels of dissolved organic load (Biological Oxygen Demand)
Aeration to stimulate aerobic degradation
activated sludge reactor
trickling filter reactor

Anaerobic Digestion of Sludge
Sludges from the primary and secondary treatment settling tanks are pumped into an anaerobic digester
Sludges contain cellulose, proteins, lipid and other insoluble polymers
Anaerobic bacteria digest the sludge to methane and carbon dioxide

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