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<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

5.1 Wastewater characteristics (Topic a)

It is not an easy task to report what may be Regional wastewater characteristics. In developed countries, where sewage collection and treatment serve the vast majority of communities and people, it is rather trivial to identify per capita average and limits (e.g, using standard deviation) for sewage flows and analogous figures for BOD, COD, SS, N, P, etc. In contrast data normally collected in the Region quite often does not show the real situation because they reflect only what is happening in the sewered sections, which is not the prevailing situation, of all the various sewage collecting basins.

Nevertheless data collected from communities with homes and businesses utilising piped water and sewers in Central America (also valid for South America), have the following typical pollutant composition of domestic sewage:

Total Suspended Solids (TSS) 200-300 mg/L
5-day Biochemical Oxidation Demand (BOD) 200-250 mg/L
Chemical Oxidation Demand (COD) 350-450 mg/L
Total Nitrogen as N 25-60 mg/L
Total Phosphorus as P 5-10 mg/L
Oil and Grease 80-120 mg/L

In unsewered areas, but served by septic tanks, the typical pollutant composition of septage in communities of Central America (also valid for South America) is as follows:

TSS 10,000-25,000 mg/L
5-day BOD 3,000-5,000 mg/L
COD 25,000-40,000 mg/L
Total Nitrogen as N 200-700 mg/L
Total Phosphorus as P 100-300 mg/L
Oil and Grease 2500-7500 mg/L

There are many examples in the countries of the Region where areas which are reasonably sewered co-exist along side others which are poorly served and, even worse, with nearby slums and squattered areas whose people convey their sewage downstream using their own resources and means, i.e., at no help provided by the sewerage utilities. This is especially the case in the Regionís largest cities where these three typical areas, namely the adequately served, the poorly served and the non-served are in most cases adjoining each other. In turn this makes things even more difficult, if not nearly impossible, for a valid assessment of overall wastewater characteristics.

In short it might be most accurately said that in the Regionís wealthy areas wastewater characteristics are similar to the ones encountered in the First World cities. An exception occurs when the solid waste systems and not sewers receive food debris. In this case, which occurs quite often, sewage tends to be weaker (low organic concentration). In the low-income served areas the sewage tends to be even weaker again given the reduced food consumption.

As a general rule, which is frequently valid for the Region and non-developed countries as a whole, domestic sewage originating mostly from households, public facilities, and businesses, is still the most significant contributor to water body pollution. Even so, in some specific areas of the Region industrial wastes discharged into sewers or channels significantly worsen wastewater characteristics beyond domestic sewage contributions. This happens not only in the most industrialized cities of the Region but also in specific areas close to oil refineries, mining exploitation sites, petrochemical and industrial complexes, etc. In the Regionís agriculture areas the overuse of pesticides and fertilizers have dramatically worsened the quality of wastewaters draining into downstream channels.

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