Type: Process Essays
Sample donated: Eloise Diaz
Last updated: May 24, 2019
As thepopulations of many cities grew significantly larger during the late 19thCentury due to industrial expansion, sewer systems were installed to transportdomestic wastewater (from toilets, washing, etc.
) and industrial wastewater torivers or other surface waters for disposal with little or no treatment.Primary wastewater treatment, that employed only sedimentation basins, removedlarge debris and readily settle able solids; however, the majority of theorganic material was not removed because it was either dissolved or of lowdensity so that it settled slowly. Thus, as human populations increased, so didthe loading of organics to the nearby surface waters. The increased organicloading stimulated microbial decomposition that utilized dissolved oxygen (DO)in the surface water.
This consumption of DO and attendant DO depletion in manycases led to the development of anaerobic conditions that could not supportdesired aquatic life, such as fish, and also caused aesthetic water qualityproblems. Advanced (secondary) wastewater treatment was then introduced tobiologically remove the organic matter to alleviate this problem. The depletionof dissolved oxygen thus became a primary water quality concern after moreimportant priorities such as disinfection (pathogen destruction) wereaddressed. During the period from 1950 to 1970 many industrialized nationspassed legislation that aimed to reduce surface water pollution. As a result,wastewater treatment facilities were issued permits which established maximumallowable levels of oxygen demanding wastes (and other contaminants such assuspended solids) in their effluents. Since the BOD(Biochemical Oxygen Demand) test is the backbone of water pollution regulationand the process design basis of sewage treatment plants, its understanding andcorrect application is essential. The test was developed around 1910 inEngland, in order to predict the ‘oxygen sag’ when sewage is discharged intothe river and how long it would take the river to ‘self purify’. ‘Oxygen sag’is the lowering of dissolved oxygen in the river over a certain distance, down streamsof the discharge of sewage and ‘self purification’ is achieved when thedissolved oxygen level is again the same, as it was before the sewage wasdischarged in the river.
The biologicalmeasurement “Biochemical Oxygen Demand” (BOD) was selected in 1908 as anindicator of the organic pollution of rivers by the U.K. Royal Commission onRiver Pollution. The traditional five day period to estimate the BOD5 parameter was chosen for this testbecause this is supposedly the longest time that river water takes to travelfrom its source to its estuary in the U.K. (Great Britain). Royal commission onsewage disposal, 1908).
Thereafter, this parameter was adopted by the AmericanPublic Health Association Standard Methods Committee in 1936 as a referenceindicator to evaluate the biodegradation of chemicals and hazardous substances.