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Water: Treatment

Progress in Water Quality

What has been the record of progress associated with the water quality benefits provided by POTWs?

For many years, EPA and the States have both regulated and helped fund the efforts of Publicly Owned Treatment Works (POTWs) to address water pollution problems with municipal wastewater. From 1970 to 1995, EPA provided $61.1 billion in Federal Construction Grants Program funds to help fund new or upgrade existing POTWs. Since 1988 EPA provided over $16.1 billion in support for state revolving loan funds for a wide range of water quality improvement projects.

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Figure 1: Annual Funding Provided by USEPA's Construction Grants and CWSRF Programs to local municipalities for improvements in water pollution control infrastructure from 1970 to 1999. Costs reported in current year dollars.

The States, local authorities and private sector have made an even greater investment in municipal wastewater infrastructure and operations. The total National capital investment in this area during this time-frame has been well over $200 billion with a comparable amount for O&M.

But what have been the water quality benefits associated with this national investment in municipal wastewater treatment? Such questions concerning the environmental benefits, as well as the cost-effectiveness, of this national investment continue to be raised by EPA, the States, Congress and by special interest, environmental, and business advocacy groups.

The benefits of these large investments are assessed in an extensively peer reviewed, 450+page technical report, Progress in Water Quality: An Evaluation of the National Investment in Municipal Wastewater Treatment (EPA-832-R-00-008; June'00). cover This report looks at changes in loadings of biochemical oxygen demand (BOD) from POTWs Nationwide before and after the Clean Water Act (CWA) was passed and changes in "Worst-Case" dissolved oxygen (DO) levels in waterways below these point sources over the same time period. A series of nine case studies of urban waterways document trends in water quality conditions and biological resources. The assessments compiled data to characterize long-term trends for population, upgrades to municipal wastewater facilities, effluent loading, water quality, environmental resources, and recreational uses. Field-validated models available for several of the case study sites allowed additional insights to be made into quantifying water quality improvements associated with different levels of wastewater treatment by POTWs.

Among other findings, the nationwide analysis revealed that between 1968 and 1996:

red arrow    The overall number of people served by POTWs increased from 140.1 million in 1968 to 189.7 million in 1996 (a 35% increase)

red arrow    The number of people served by POTWs with secondary or greater levels of wastewater treatment almost doubled from 85.9 million in 1968 to 164.8 million in 1996, while the number served by POTWs with less than secondary (raw or primary) dropped from 54.6 million in 1968 to 17.2 million in 1996, including ~5.1 million served by POTWs with CWA 301(h) waivers allowing less than secondary treated effluent to be discharged into deep, well-mixed ocean waters.

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Figure 2: Population served by POTWs in 1968 (before the CWA) and in 1996 (after the CWA) by treatment type.

red arrow    POTW effluent BOD5 and BODU discharged to the Nation's waterways decreased by about 45% and 23% respectively, (using design-based estimates) despite a significant increase (35%) in both the population served and influent loadings of BOD5 and BODU to POTWs.

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Figure 3: Influent and Effluent Loading of BOD to and from POTWs in 1968 (before the CWA) and in 1996 (after the CWA) by treatment type and associated BOD aggregate removal effiences.

red arrow    Nationwide aggregate removal efficiencies for BOD5 and BODU in 1968 were about 63% and 39% respectively; by 1996, these removal efficiencies rose to nearly 85% and 65% respectively. Without continued improvements in wastewater treatment infrastructure resulting in increased removals of pollutants, future population growth will erode away many of the CWA achievements in effluent loading reduction to the point that by the year 2016, BOD loadings rates could be similar to those experienced in the mid-1970s.

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Figure 4: Projections of design-based national effluent BODU loadings through 2025 using middle-level U.S. population projections.

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Figure 5: Percent changes in population served, influent BOD loading, and effluent BOD5 and BODU loading before and after the 1972 CWA (1968 to 1996).

redarrow    Where adequate water quality data are available to make before and after the CWA "Worst-Case" DO level comparisons over different spacial scales [i.e., river reaches (which average 10 miles in length), catalog units, and major river basins], it was found that:

  • 214 of the 311 (69%) reaches below POTW outfalls (with comparable data) showed improvements in worst-case DO; the top 25 improving reaches had worst-case DO increase by 4.1 to 7.2 mg/l the number of reaches characterized by worst-case DO <5.0 mg/l decreased from 167 to 97 (from 54% to 31%).
  • 167 of the 246 (68%) catalog units with reaches below POTW outfalls (with comparable data) showed improvement in worst-case DO; 53 catalog units improved by >2 mg/l; the number of catalog units characterized by worst-case DO <5.0 mg/l was reduced from 115 to 65 (from 47% to 26%).
  • 8 out of 11 (73%) major river basins with comparable data had statistically significant improvements in worst-case DO after the CWA; none of the major river basins had any statistically significant degradation in worst-case DO.
  • Results of this analysis show that there were significant after-CWA improvements in worst-case summer DO conditions in two-thirds of the hydrologic units at all three spatial data aggregation sales. However, this analysis only relates to those waters receiving discharges from point sources.
  • On a Nationwide basis, current POTW effluent loadings of BOD5 account for only about 38% of total point source loadings and only 21% of total loadings from all sources.

redarrow    Case study assessments for nine urban waterways with historically documented water pollution problems (the Connecticut River, Hudson-Raritan Estuary, Delaware Estuary, Potomac Estuary, James Estuary, Chattahoochee River, Ohio River, Upper Mississippi River, and Williamette River), including four sites with field-validated water quality models, demonstrated that:

  • Tremendous progress has been made for each waterway in improving water quality, restoring valuable fisheries and other biological resources (e.g. gamefish, migratory fish, waterfowl, fish-eating birds, opened shellfish beds, submerged aquatic vegetation for fish habitat etc.) and creating recreational opportunities (e.g. swimming, angling, hunting, boating, bird-watching etc.).
  • Before the CWA, during the 10-year period from 1961-1970, worst-case DO levels were in the range of 1-to-4 mg/l for most of the case study sites; after the CWA the DO levels had improved to the range of 5-to-8 mg/l during 1986-1995.

redarrow    As new monitoring data are collected, it is crucial for the success of future performance measure evaluations of pollution control policies that the data be submitted, with appropriate QA/QC safeguards, to accessible databases.

Refer to the full report, Progress in Water Quality: An Evaluation of the National Investment in Municipal Wastewater Treatment (EPA-832-R-00-008; June 2000) for further details on the record of progress associated with the water quality improvement benefits provided by POTWs.

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