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<International Source Book On Environmentally Sound Technologies
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4.1.3 Wastewater biochemical characteristics


Higher contaminant concentrations are associated with the low flow conditions described above, and lower contaminant concentrations are often associated with affluent areas and areas with combined sewers. Typical medium contaminant concentrations are shown in Table 4.1.

Industries which discharge a significant fraction of the total wastewater flow are often required to pre-treat before disposal to municipal sewers to bring their discharges down to typical sewage concentrations. Food processing industries can be a significant source of high seasonal solids, biochemical oxygen demand (BOD), nitrogen and phosphorus loading to a plant, resulting in seasonal concentration changes, especially in agricultural areas with large numbers of food processing industries.

In combined sewer systems, the "first flush" effect is often observed during a storm event in an area with relatively flat sewers following a dry period. The scouring of solids and associated contaminants deposited in the pipes results in a high solids and contaminant load to the treatment plant soon after the beginning of the storm. In such cases, the solids concentration may increase slightly as the flows increase rather than being diluted by the stormwater as would be expected. The effect typically lasts for only a few hours, and is followed by the expected dilution if the storm persists. The parameters most affected are solids concentration and biochemical oxygen demand (BOD5), but toxic inorganic and organic contaminants adsorbed to the solids will also exhibit a concentration peak. Strategies to control the impact of combined sewer overflows often involve treating this "first flush" by capturing the initial flow in a reservoir, and treating the remainder through a vortex style stormwater solids separator. Once the storm is over, the stored liquid is pumped back into the sewer to be treated at the treatment plant.

Combined sewer systems produce higher total solids per capita loading than separate sewers because of a higher amount of inorganic solids in the waste stream. Stormwater runoff can be as high as 100 mg/L suspended solids. Typically, inorganic solids comprise 25 percent of total suspended solids entering a treatment plant in separate sewers.

The organic fraction of total suspended solids is affected by the high cellulose content from toilet paper. The organic fraction in sludge from wastewater treatment plants is thus typically less than 50 percent readily digestible. The use of garburators in a community can increase the per capita solids load by 30 percent or more.

Septage discharged into the wastewater plant can be very high in solids, 2000 to 100,000 mg/L including gravel and cobbles. The organic material is partially digested, thus septage solids tend to be higher in the inorganic fraction than the rest of the waste stream.

Table 4.1: Typical Composition of Untreated Domestic Wastewater - medium strength
Contaminants Unit   Concentration Medium
Solids, total (TS) mg/L



Dissolved, total (TDS) mg/L 500
Fixed mg/L 300
Volatile mg/L 200
Suspended solids (SS) mg/L 220
Fixed mg/L 55
Volatile mg/L 165
Settleable solids mL/L 10
Biochemical oxygen demand,:    
5 day, 20oC (BOD5, 20oC) mg/L 220
Total organic carbon (TOC) mg/L 160
Chemical oxygen demand (COD) mg/L 500
Nitrogen (total as N) mg/L 40
Organic mg/L 15
Free ammonia mg/L 25
Nitrites mg/L 0
Phosphorus (total as P) mg/L 8
Organic mg/L 3
Inorganic mg/L 5
Chloridesa mg/L 50
Sulfatea mg/L 30
Alkalinity (as CaCO3) mg/L 100
Grease mg/L 100
Total coliform CFU/l00 mL 107-108
Volatile organic compounds (VOCs)  mg/L 100-400
aValues should be increased by amount present in domestic water supply.
Source: Metcalfe & Eddy, Inc. (Tchobanoglous, George, Burton, Frank) Wastewater Engineering: Treatment, Disposal & Reuse. Third Edition. McGraw Hill, 1991.


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