The Federal Role in Secondary Treatment After the Clean Water

Enactment of the 1972 Amendments to the Federal Water Pollution Control Act, now popularly known as the Clean Water Act (CWA), by the 92nd U.S. Congress redirected national policy for water pollution control onto a new path. Sparked by publication of Rachel Carson's Silent Spring in 1962 (Carson, 1962), national publicity about environmental issues during the 1960s led to public awareness of the existence of nationwide air and water pollution problems and political demands by the "Green Movement" for governmental action to address pollution problems (Zwick and Benstock, 1971; Jobin, 1998).

On October 18, 1972, a new era for POTWs began when the 1972 Amendments to the Federal Water Pollution Control Act (PL 92-500) were unanimously passed by the U.S. Congress and, despite a veto by President Richard M. Nixon, who thought that the $24 billion investment over 5 years was "excessive and needless overspending," the act became law (Knopman and Smith, 1993). The act established a new national policy that firmly rejected the historically accepted use of rivers, lakes, and harbors as receptacles for inadequately treated wastewater. Congress's objective was clear. They wanted to "restore and maintain the chemical, physical and biological integrity of the nation's waters" and to attain "fishable and swimmable" waters throughout the nation. With PL 92-500, the federal government took control of directing and defining the nation's water pollution control programs. This commitment led to the completion of the urban water cycle in many communities across the United States.

Congress recognized that success or failure of PL 92-500's lofty objectives hinged on a combination of money, compliance, and enforcement. Consequently, the basic framework of the act included the following.

• Establishment of the National Pollutant Discharge Elimination System (NPDES), a program that requires that every point source discharger of pollutants obtain a permit and meet all the applicable requirements specified in regulations issued under sections 301 and 304 of the act. These permits are enforceable in both federal and state courts, with substantial penalties for noncompliance.

• Development of technology-based effluent limits, which serve as minimum treatment standards to be met by dischargers.

• An ability to impose more stringent water quality-based effluent limits where technology-based limits are inadequate to meet state water quality standards or objectives.

• Creation of a financial assistance program to build and upgrade POTWs. PL 92500 authorized $5.0 billion in federal spending for fiscal year 1973, $6.0 billion for fiscal year 1974, and $7.0 billion for fiscal year 1975. In contrast, the year before the act was passed, a total of $1.25 billion (federal dollars) was spent. Under the Construction Grants program, the federal share was 75 percent of cost from fiscal years l973 to l983 and 55 percent thereafter. Additional funds were made available for projects using innovative and alternative treatment processes.

The story of the Clean Water Act and its evolution from 1972 to the present day is richly complicated. The purpose of this section is not to summarize all aspects of this landmark act. Rather, the objective is to focus on the role it played in implementing secondary treatment in the nation's POTWs. Other sources, such as The Clean Water Act, 25th Anniversary Edition, published by the Water Environment Federation (WEF, 1997), should be consulted for a complete overview of the act. Figure 2-5 summarizes the key amendments and regulations that occurred from 1972 to 1996.

Federal Water Pollution Control Act Amendments of 1972

The Federal Water Pollution Control Act of 1972 (later to be renamed the Clean Water Act) contained the first statutory requirement for a minimum of secondary treatment by all POTWs. The act also established the National Pollutant Discharge Elimination System (NPDES), under which every discharger of pollutants was required to obtain a permit. Under the permit each POTW is to discharge only effluent that had received secondary treatment. EPA defined secondary treatment in a regulation as attaining an effluent quality of at least 30 mg/L BOD5, 30 mg/L TSS, and 85% removal of these pollutants, in a period of 30 consecutive days.

Clean Water Act Amendments of 1977

The Clean Water Act Amendments of 1977 created the 301(h) program, which waived the secondary treatment requirement for POTWs discharging to a marine environment if they could show that the receiving waters would not be adversely affected. Extensive requirements had to be met before such a waiver could be issued.

Clean Water Act Amendments of 1981, PL 97117

The Clean Water Act Amendments of 1981 amended the Clean Water Act to the effect that "such biological treatment facilities as oxidation ponds, lagoons and ditches and trickling filters shall be deemed the equivalent of secondary treatment." EPA is directed to provide guidance on design criteria for such facilities, taking into account pollutant removal efficiencies and assuring that water quality will not be adversely affected (Sec. 304(d)(4)). Regulations to this effect were published in final on September 20, 1984. Also, a notice was issued to solicit public comments on "problems related to meeting the percent removal requirements and on five options EPA was considering for amending the percent removal requirements.

Secondary Treatment Regulations, January 27, 1989

This secondary treatment regulation allows adjustments for dry weather periods for POTWs serving combined sewers.

National Municipal Policy, January 30, 1984

The EPA National Municipal Policy was published on January 30, 1984. It was designed to ensure that all POTWs met the compliance deadlines for secondary or greater treatment of discharges. The key to the policy is that it provides for POTWs that had not complied by the July 1,1988, deadline to be put on enforceable schedules. The policy has been outstandingly successful and has resulted in significant increases in compliance.

Secondary Treatment Regulations, June 3, 1985

The secondary treatment regulation published in final on June 3, 1985, revised the previous regulations published in Title 40, Part 133, of the Code of Federal Regulations. Specifically, on a 30-day average, the achievement of not less than 85% removal of BODs, CBODs and suspended solids for conventional secondary treatment processes was required. However, for those treatment processes designated by the Congress as being equivalent to secondary treatment (such biological treatment facilities as oxidation ponds, lagoons, and ditches, and trickling filters), at least 65% pollution removal was required, provided that water quality was not adversely affected. Waste stabilization ponds were given separate suspended solids limits. Special consideration was provided for various influent conditions and concentration limits.

Figure 2-5 Timeline of federal water pollution control acts, 1972-1996.

The Water Pollution Control Act Amendments of 1972 (PL 92-500) and Secondary Treatment Information (38 FR 22298-22299), Published in Final on August 17, 1973 After debating the merits of secondary treatment for the better part of two decades, Congress finally put the issue to rest in the Federal Water Pollution Control Act Amendments of 1972. Section 301 required POTWs to achieve effluent limitations based on secondary treatment.

A simple, aggressive schedule was set to meet this requirement. By July 1, 1977, all existing POTWs and all facilities approved for construction before June 30, 1974, must incorporate secondary treatment. Then, by July 1, 1983, POTWs must meet an additional level of treatment described in the act as "best practicable wastewater treatment."

While developing the 1972 Amendments, Congress understood that the term "secondary treatment" needed to be carefully researched and clearly articulated before regulations could be drafted. At the time, several "working" definitions existed, including one offered by Congressman Vanik in the House debate on the amendment. He defined secondary treatment as a process that removes 80 to 90 percent of all harmful wastes from POTW influent.

Section 304(d)(1) directed USEPA to investigate and publish in the Federal Register "information, in terms of amounts of constituents and chemical, physical and biological characteristics of pollutants, on the degree of effluent reduction attainable through the application of secondary treatment." USEPA assembled a work group the next year to accomplish this task and invited outside commentators and contractors to participate.

Early on, the group decided that the effluent limitations to be used to define secondary treatment needed to include concentrations of key parameters as well as percent reduction limits. Also weighing in on the minds of the group was a congressional and public concern that, if percent removal targets were set too high, incremental environmental benefits would not be worth the cost. Consequently, economic considerations became an important part of the decision-making process. Figure 2-6 is an example of how costs were analyzed in relation to percent removal targets for BOD5. The graph shows that costs rise rapidly beyond the 85 to 88 percent removal level. Analyses such as these helped the work group put technical capabilities in a practical (i.e., economical) context.

In April 1973, USEPA published a proposed regulation based on the group's report. After comments were addressed, the Agency issued its final regulation on August 17, 1973. It defined secondary treatment effluent concentration limits for the following parameters:

• Five-Day Biochemical Oxygen Demand (BOD) Average value for BOD5 in effluent samples collected in a period of 30 consecutive days shall not exceed 30 milligrams per liter (mg/L). The average value for BOD5 in effluent samples collected in a period of 7 consecutive days shall not exceed 45 mg/L.

• Total Suspended Solids (TSS). Average value for TSS in effluent samples collected in a period of 30 consecutive days shall not exceed 30 milligrams per liter (mg/L). The average value for TSS in effluent samples collected in 7 consecutive days shall not exceed 45 mg/L.

Figure 2-6 Cost versus BOD5 removal efficiency of a new 1 million gallon per day POTW. Source: USEPA, 1973.

• Fecal Coliform Bacteria. Geometric mean of fecal coliform bacteria values for effluent samples collected in a period of 30 consecutive days shall not exceed 200 per milliliter (mL). The geometric mean of fecal coliform bacteria values for effluent samples collected in a period of 7 consecutive days shall not exceed 400 per milliliter (mL).

• pH. Effluent values for pH shall remain within the limits of 6.0 and 9.0.

Also included were percent removal limits for BOD5 and TSS. Specifically, average values for BOD5 and TSS in effluent samples collected in 30 consecutive days may not exceed 15 percent of the mean of influent samples collected at approximately the same times during the same period (85 percent removal).

The BOD and TSS limits were chosen based on an assumption that the wastewater entering a POTW (influent) contains about 200 mg/L of BOD5 and TSS. Knowing this assumption did not hold true in every case, USEPA made a couple of allowances. Specifically, the Agency allowed a POTW to have higher BOD5 and TSS concentrations in its effluent if the facility received more than 10 percent of its design flow from industrial facilities for which less stringent effluent limitations had been promulgated.

Special consideration was also given, on a case-by-case basis, to treatment works served by combined storm and sanitary sewer systems where increased flows during wet weather prevented the attainment of the defined minimum level of secondary treatment. Of chief concern was the 85 percent removal requirement. In stormy weather, storm water runoff dilutes the normal volume of influent, lowering BOD5 and TSS concentrations. Expecting to reduce already reduced concentrations by 85 percent was beyond the means of many facilities.

Two subsequent amendments to the secondary treatment information were promulgated on July 26, 1976 (41 FR 30788) and October 7, 1977 (42 FR 5665). These changes provided for:

• Deletion of the fecal coliform bacteria limitations and clarification of the pH requirement.

• Special consideration for the TSS effluent limitations applicable to waste stabilization ponds with wastewater flows of less than 2 million gallons per day (mgd).

Publishing the regulation defining the minimum level of secondary treatment to be implemented by POTW facilities by 1977 was a major accomplishment for USEPA. On the horizon, however, loomed the prospect of developing a second, more stringent, level of requirements for implementation by July 1, 1983. Congress fortunately realized that this second set of requirements, or best practicable treatment, might not be needed. Section 315(b) of PL 92-500 established a national study commission to help them make this determination. Composed of five Senators, five Representatives, and five members of the public, the commission was given 3 years to accomplish this task. In the end, the group issued several general recommendations, one of which was that the secondary treatment effluent limits developed for the 1977 deadline not be changed for the 1983 deadline. Essentially, the commission determined that secondary treatment was the best practicable treatment for POTWs. Thus, the headaches associated with setting a second level of requirements were avoided.

The Clean Water Act Amendments of 1977 (PL 95-217) The tight timetable Congress established for implementing secondary treatment proved to be unrealistic for many municipalities. In fact, only about 30 percent of major POTWs (those processing 1 million—or more—gallons of wastewater per day) were in compliance when the July 1, 1977, deadline rolled around. In many cases, upgrade schedules were slowed due to delays in receiving federal funds. The Clean Water Act Amendments of 1977 (PL 95-217) responded to this situation by allowing time extensions for municipalities encountering funding problems. Time extensions aside, probably the most significant aspect of PL 95-217 in terms of secondary treatment was the fact that Congress backed off from PL 92-500's original objective of having all POTWs implement secondary treatment as a minimum technology-based standard. Municipalities discharging into ocean waters had been arguing that the benefits associated with their upgrading to secondary treatment were not worth the cost. The vastness of the marine environment, they said, effectively dilutes and incorporates wastes into the water and sea bottom without harming either the indigenous biota or users or the environment.

Congress agreed and added Section 301(h) to the Clean Water Act, allowing marine dischargers to apply for a waiver of secondary treatment requirements. USEPA would subsequently review the application and issue modified NPDES permits to POTWs that met certain environmental criteria and received state concurrence. These criteria included:

• Existence of and compliance with water quality standards.

• Protection and propagation of a balanced indigenous population of fish, shellfish, and wildlife.

• Allowance of recreational activities.

• Establishment of a monitoring program.

• Satisfactory toxics control programs, including an approved industrial pretreat-ment program.

• No additional treatment requirements for other sources.

• Acceptable discharge volume and pollutant limits.

• Protection of public water supplies (desalinization plants).

Municipal Wastewater Treatment Construction Grants Amendments of 1981 (PL 97-117) and Secondary Treatment Regulations (49 FR 3698637014), Published in Final on September 20, 1984 When the decade of the 1980s dawned, the goal of implementing secondary treatment in the nation's POTWs seemed a long way off. About half of the 20,000 municipal discharges, including more than 100 larger cities, were still not in compliance with the 1977 deadline. Construction projects were bogged down with funding problems, complicated regulatory procedures, and lack of staff at state and federal agencies. To address these and other problems, Congress passed the Construction Grants Amendments of 1981. Section 301(i) recognized that funding issues were still holding up secondary treatment compliance and therefore extended the implementation deadline to July 1, 1988, on a case-by-case basis.

PL 97-117 and its companion regulations also addressed another concern involving USEPA's definition of secondary treatment effluent requirements. In theory, the requirements were not intended to favor one treatment process over another, yet they did. As it turned out, activated sludge facilities were the only ones that could consistently meet the requirement of 85 percent removal of BOD5 and TSS limits. This situation caused an immediate problem for the many smaller communities that had invested in trickling filters, waste stabilization ponds, and other types of biological wastewater treatment. Even when their facilities performed as designed, they were in noncompliance according to USEPA's standards for secondary treatment.

Upgrading or replacing these facilities was an expensive proposition. Many questioned whether environmental benefits gained would be worth the cost. Congress agreed and PL 97-117 and its companion regulations included the following:

• Introduced the concept of "equivalent of secondary treatment" to describe facilities that use a trickling filter or waste stabilization pond as a principal treatment process and that were not meeting the secondary treatment requirements as promulgated by USEPA in 1973.

• Lowered the minimum level of effluent quality to be achieved by those facilities during a 30-day period as an average value not to exceed 45 mg/L for BOD5 and TSS, an average 7-day value for BOD5 and TSS of not to exceed 65 mg/L, and a percentage removal of those constituents of not less than 65 percent (30-day average).

• Required that NPDES permit adjustments for "equivalent to secondary treatment" facilities reflect the performance or design capabilities of the facility and ensure that water quality is not adversely affected.

National Municipal Policy (49 FR 3832-3833), Published on January 30, 1984 Continually pushing back the deadline for implementation of secondary treatment in POTWs created confusion. The 1972 Amendments had set the original deadline for compliance for l977. For some municipalities, it was extended to 1983 by PL 95-217 and then to 1988 by PL 97-117. The USEPA National Municipal Policy, published in the Federal Register on January 30, 1984, was designed to eliminate this confusion and ensure that all POTWs would comply with the statutory requirements and compliance deadlines in the Clean Water Act. It also established that where there were extraordinary circumstances that precluded compliance by the July 1, l988, deadline, POTWs would be put on enforceable schedules designed to achieve timely compliance. The policy described EPA's intentions to focus its efforts on:

• POTWs that previously received federal funding assistance and are not in compliance.

• Minor POTWs (less than 1 mgd capacity) that are contributing significantly to impairment of water quality.

This municipal treatment policy has been outstandingly successful, with over 90 percent compliance achieved to date for major POTWs (1 mgd or over).

Secondary Treatment Regulations, Published in Final on June 3, 1985

The secondary treatment requirement of 85 percent removal of BOD5 and TSS continued to present problems for POTWs receiving diluted influent wastewater. Whether it was a secondary treatment facility (85 percent removal) or an equivalent of secondary treatment facility (65 percent removal), to stay in compliance a facility had to install advanced technology, even if it consistently met its concentration limits. Recognizing this problem, USEPA, on November 16, 1983, published a Federal Register notice soliciting public comment on a number of options for amending the percent removal requirements.

Based on the public comments received, the Agency proposed and then finalized a revised Secondary Treatment Regulation. Published in final on June 3, 1985, it authorized USEPA to lower the percent removal requirement, or substitute a mass limit for the percent removal requirement, for certain POTWs. The Agency would make this determination on a case-by-case basis based on the removal capability of the treatment plant, the influent wastewater concentration, and the infiltration and inflow situation.

Treatment plants could apply for a permit adjustment in its percent removal limit only if:

• The treatment plant is meeting or will consistently meet its other permit effluent concentration limitations, but its percent removal requirements cannot be met due to less concentrated influent wastewater for separated sewers.

• To meet the percent removal requirement, the treatment works would have to meet significantly more stringent concentration-based limitations.

• The less concentrated influent wastewater to the treatment works was not a result of excessive infiltration and inflow.

The concentration limits in the permit would remain unchanged, and in no case was a permit to be adjusted if the permitting authority determined that adverse water quality impacts would result from a change in permit limits.

Amendment to the Secondary Treatment Regulation, Published in Final on January27,1989, in the Federal Register The Secondary Treatment Regulation, published in June 1985, addressed the problem POTWs with separate sewers had in meeting percent reduction standards due to the dilution of influent wastewater by wet weather conditions. The city of New York also had a problem. Its combined sewer system delivered diluted influent to city POTWs, even during dry weather. Consequently, the city petitioned to be eligible for adjustments of percent removal requirements too, arguing that nonexcessive infiltration can dilute the influent waste-water of treatment works served by combined sewers just as it does for treatment works served by separate sewers. USEPA agreed with this position and published an amendment to the regulation on January 27, 1989, to allow for percent removal adjustments during dry weather periods for POTWs with combined sewers. To obtain this adjustment, the treatment works had to satisfy three conditions:

• It must consistently meet its permit effluent concentration limitations, but the percent removal requirements cannot be met due to less concentrated influent wastewater.

• Significantly more stringent effluent concentration than those required by the concentration-based standards must be met to comply with the percent removal requirements.

• The less concentrated influent wastewater must not result from either excessive infiltration or clear water industrial discharges to the system.

Regarding the last condition, the regulation established that, if the average dry weather baseflow (i.e., the total of the wastewater flow plus infiltration) in a combined sewer system is less than 120 gallons per day per capita (gpcd) threshold value, infiltration is assumed to be nonexcessive. However, sewer systems with average dry weather flows greater than 120 gpcd might also have nonexcessive infiltration if this is demonstrated on a case-by-case basis. An applicant, therefore, has an opportunity to demonstrate that its combined sewer system is not subject to excessive infiltration even if the average total dry weather baseflow exceeds the 120 gpcd threshold value.

D. NATIONWIDE TRENDS IN BOD LOADING FROM POTWs

From 1940 to the present day, the combination of advancing wastewater treatment technology, increased public concern, various state wastewater treatment regulations, and, finally, the 1972 CWA secondary treatment mandate resulted in an increased number of POTWs with at least secondary and, in many cases, greater than secondary levels of treatment. Table 2-2 presents descriptions of the six major types of treatment found at POTWs, along with their corresponding design-based BOD5 removal efficiency1 (expressed as percent removal).

The total population in the United States grew rapidly in the latter half of the twentieth century, increasing from around 140 million people in 1940 to about 270 million in 1996 (see Figure 2-2). This population growth meant POTWs not only had to upgrade their treatment processes to increase pollutant removal efficiency, but they had to accomplish it while dealing with increasing influent wastewater loads. This section examines trends concerning the nation's expansion and upgrades of POTWs and analyzes how increased use of secondary and greater than secondary treatment after the 1972 CWA affected the rate of effluent BOD loading to the nation's waterways. Specifically examined are the following:

• The inventory of POTWs in the United States.

• The number of people served by those POTWs and the amount of wastewater flow they generated.

• The rate of BOD entering POTWs (influent loading).

• The rate of BOD discharged by POTWs into receiving waterways (effluent loading).

• BOD removal efficiency of POTWs.

• Projections of effluent BOD loading into the twenty-first century.

The information sources for this study include municipal wastewater inventories published by the U.S. Public Health Service from 1940 through 1968 (USPHS, 1951; NCWQ, 1976; USEPA, 1974) and USEPA's Clean Water Needs Surveys (CWNS) conducted from 1973 through 1996 (USEPA, 1976, 1978, 1980, 1982, 1984, 1986, 1989, 1993, 1997). Many of these sources categorize their information by the six types of wastewater treatment described in Table 2-2. Some sources, however, combine primary and advanced primary data and report it simply as "less than secondary" treatment data. Similarly, data for advanced secondary and advanced wastewater treatment are combined and reported as "greater than secondary" treatment data. To

'Designed-based BOD5 removal efficiencies are minimum requirements typically assigned by NPDES permits according to the treatment process and treatment plant design assumptions (Metcalf and Eddy, 1991). Generally, they represent conservative estimates of BOD5 removal efficiencies. Many modern POTWs report a higher rate of BOD5 removal than their permitted rate. This study, however, focuses on designed-based BOD5 removal efficiencies because it is assumed that these conservative rates would provide a more effective and consistent comparison of BOD5 removal over the entire historical period of record used in the analysis.

keep the categories consistent, this convention was followed in the analyses presented in this section.

Types of BOD Reported in this Trends Analysis

BOD5 is the most widely used measurement of BOD. In spite of its popularity, there are important limitations of this measurement. The subscript "5" refers to the laboratory incubation period of 5 days at 20° C. Many biochemical reactions that determine the ultimate consumption of DO in both wastewater and natural waters are not completed within the 5-day limit, however. Therefore, an estimate of "ultimate" BOD (BODu) of a sample requires consideration of all the biochemical processes that consume DO over a longer time scale. Figure 2-7 presents the relationships among the components of BODu.

Familiar to most environmental engineers is the oxygen demand associated with the bacterial decomposition of carbonaceous organic matter under aerobic conditions. Through respiration, organic matter is broken down and oxygen is consumed. Parameters in Figure 2-7 relating to carbonaceous BOD are:

• CBOD5: BOD at 5 days that includes only the carbonaceous component of oxygen consumption.

• CBOD: BOD at an unspecified time that includes only the carbonaceous component of oxygen consumption.

• CBODu: Ultimate BOD of carbonaceous component of oxygen consumption at completion of decomposition process.

Along with the decomposition of carbonaceous matter is an additional oxygen demand associated with nitrification, the process that converts ammonia to nitrate. Nitrogen in wastewater generally appears as organic nitrogen compounds (urea, proteins, etc.) and ammonia. Over time, the nitrogen compounds are hydrolyzed and are converted to ammonia. Autotrophic bacteria of the genus Nitrosomonas convert the ammonia to nitrite, using oxygen in the process. Nitrite, in turn, is converted to nitrate by bacteria of the genus Nitrobacter, consuming additional oxygen in the process. Parameters in Figure 2-7 relating to nitrogenous BOD are:

• NBOD: BOD at an unspecified time that includes only the nitrogenous component of oxygen consumption from nitrification.

• NBODu: Ultimate BOD of the nitrogenous component of oxygen consumption at completion of nitrification process.

In Figure 2-7, carbonaceous and nitrogenous BOD components combine to yield the following parameters:

• BOD5: BOD at 5 days that includes the carbonaceous and nitrogenous components of oxygen consumption.

• Total BOD: BOD at an unspecified time that includes the carbonaceous and nitrogenous components of oxygen consumption.

TABLE 2-2 Six Levels of Municipal Wastewater Treatment

Treatment Type

Design BOD5 Removal Efficiency (percent)

Description

Primary

Advanced Primary Secondary

Activated sludge

Waste stabilization

0 Wastewater is collected and discharged to surface waters without treatment, or removal, of pollutants from the influent stream.

35 Incorporates physical processes of gravitational settling to separate settleable and floatable solids material from the raw wastewater. The removal of settleable solids results in the removal of pollutants associated with solid particles such as organic matter, nutrients, toxic chemicals, heavy metals, and pathogens. Other physical processes such as fine screens and filters can also be used.

50 Enhancement of the primary clarification process using chemical coagulants such as metal salts and organic polyelectrolytes.

85 Biological processes are added to break down organic matter in the primary effluent by oxidation and production of bacterial biomass. Biological waste treatment systems, based on bacterial decomposition of organic matter, can be classified as activated sludge, waste stabilization ponds (suspended bacterial growth), and trickling filters (attached bacterial growth). 84 to 89 percent removal of TSS and 30 mg/L effluent concentration for BOD5 and TSS.

Involves the use of bacteria to decompose suspended solids in the sewage so that they can be settled out. Oxygen to speed the bacteriological process is generaed by mechanical aeration or by the infusion of additional oxygen. The solids produced (sludge) by the biological action are settled out and removed, except for a portion of the bacteria-rich sludge that is returned to the head of the secondary treatment process to activate the biological processes to treat sewage. This is the standard method of treatment for medium and large cities.

Pools in which mechanical aeration is used to supply oxygen to the bacteria. In other processes, oxygen is supplied by natural surface aeration or by algal photosynthesis with no mechanical aeration.

TABLE 2-2 Continued

Treatment Type

Design BOD5 Removal Efficiency (percent)

Description

Trickling filters

Advanced Secondary

0 0

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