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Notes: Raw = Effluent concentration of 396.5 mg/L BODu equivalent to raw (untreated) influent concentration; Less than secondary = Primary and advanced primary wastewater treatment; Greater than secondary = Advanced secondary and advanced wastewater treatment.

Notes: Raw = Effluent concentration of 396.5 mg/L BODu equivalent to raw (untreated) influent concentration; Less than secondary = Primary and advanced primary wastewater treatment; Greater than secondary = Advanced secondary and advanced wastewater treatment.

Effluent BOD loading by POTWs was significantly reduced between 1968 and 1996. In 1968, 4 years before the 1972 CWA, effluent CBOD5 and BODu loadings were 6,932 and 21,281 mt/day, respectively. By 1996, CBOD5 and BODu loadings were reduced to 3,812 and 16,325 mt/day, respectively. This represents a 45 percent decline in CBOD5 and a 23 percent decline in BODu between 1968 and 1996. Notably, these declines were achieved even though influent CBOD5 and BODu loading to POTWs each increased by 35 percent during the same time period!

The proportion of effluent CBOD5 loading attributable to raw and less than secondary wastewater treatment was reduced from about 94 percent in 1940 to 35 percent in 1996 [see Figure 2-13(a)]. The proportion of effluent BODu loading attributable to raw and less than secondary wastewater treatment was reduced from about 84 percent in 1940 to 20 percent in 1996 [see Figure 2-13(b)].

Trends in BOD Removal Efficiency

The rate of effluent BOD loading from a POTW is determined by two main factors: the rate of influent BOD loading and the BOD removal efficiency of the facility. Influent BOD loading, in turn, is determined by the number of people connected to the system and the rate at which they generate and export BOD in their wastewater flow. The analysis above indicates that tremendous progress was achieved between 1968 and 1996 in reducing effluent BOD loading from POTWs into the nation's waterways. Notably, this reduction occurred at the same time the number of people served by POTWs was increasing rapidly. Figures 2-14 and 2-15 present influent and effluent loadings and removal efficiencies for CBOD5 and BODu, respectively. Key observations from Figures 2-14 and 2-15 include the following:

BOD removal efficiency nationwide significantly increased between 1940 and 1996. In 1940, the aggregate national removal efficiency stood at about 33 percent for CBOD5 and 20 percent for BODu. By 1968, removal efficiencies had increased to 63 percent for CBOD5 and 39 percent for BODu. By 1996, they had further increased to nearly 85 percent for CBOD5 and 65 percent for BODu! The BOD removal efficiency increased substantially between 1972 and 1978, the 6-year period after the passage of the CWA (from 64 to 74 percent for CBOD5 and from 41 to 52 percent for BODu). Between 1978 and 1996, removal

efficiency increased an additional 11 percent for CBOD5 and 13 percent for BODu. Those larger increases in BODu removal efficiency reflect the ever-increasing role of greater than secondary POTWs over this time period.

Figure 2-16, a three-dimensional graph of the population data presented earlier in Table 2-5 and Figure 2-9, is useful for visualizing the trends in population served by POTW treatment type. The population served by secondary treatment facilities declined sharply between 1968 (85.6 million) and 1978 (56.3 million) and then leveled off at about 82 million in the 1990s. In contrast, the number of people served by greater than secondary treatment surged between 1968 and 1978 (0.3 to 49.1 million) and then increased steadily to about 82.9 million in 1996. Unlike secondary treatment, advanced wastewater treatment enhances biological processes to incorporate nitrification (ammonia removal) and denitrification (nitrate removal), thus reducing the NBOD fraction of effluent BODu loading.

### Future Trends in BOD Effluent Loading

The data presented in the previous sections indicate that the increase in BOD removal efficiency between 1940 and 1996 resulted in significant reductions in BOD effluent loading to the nation's waterways even though the number of people served by POTWs greatly increased. Given that the population served by POTWs is projected to continue to increase well into the twenty-first century, will the trend of effluent

BOD loading reductions also continue into the future? A preliminary examination of estimated influent and effluent BOD loadings based on USEPA projections of POTW facility inventories and population served for the year 2016 indicates that the answer might be "no."

Table 2-12 presents a summary of the population served, wastewater flow, influent and effluent BOD loading rates, and BOD removal efficiencies for 1996 and corresponding projections for 2016 and 2025. Figure 2-17 is a column chart that extends the influent and effluent BODu loading totals and POTW removal efficiencies originally presented in Figure 2-15 into the twenty-first century by adding columns for the years 2016 and 2025 to the chart. These projections are based on the following assumptions:

• USEPA Clean Water Needs Survey (USEPA, 1997) estimates that 275 million people will be served by POTWs in the year 2016. This figure is based on middle-level population projections from the Census Bureau (USBC, 1996) and the assumption that 88 percent of the population will be served by POTWs in 2016. Assuming that 88 percent of the population projected for 2025 is also served by POTWs, about 295 million people will be served by POTWs.

• Design-based BODu removal efficiency will increase from a nationwide average of 65 percent in 1996 to 71 percent by 2016 based on projections of population served by the different categories of POTWs. This removal efficiency is assumed to remain at that level through 2025.

• Influent wastewater flow will remain a constant 165 gpcd and influent BODu concentration will remain a constant 396.5 mg/L for the projection period from 1996 to 2025.

TABLE 2-12 1996 Estimates and 2016 and 2025 Projections of POTW Infrastructure and Influent and Effluent Loading by Treatment Type

Total*

Less than Secondary

Secondary

Greater than Secondary

### On-Site

Inventory of POTWs Population of U.S. (millions) Population served (millions) Percent of population served Influent wastewater flow (mgd) Unit flow (gpcd)

1996 Estimates

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