Combined Sewer Overflow

Management of combined sewer overflow is a significant problem in many urban areas where the older sewerage network carries both stormwater and untreated wastewater. When peak storm events occur, the capacity of the wastewater treatment plant is exceeded; in the past, this condition often led to a temporary bypass and discharge of the untreated CSO to receiving waters. Current regulations now prohibit that practice, and wetlands are being given strong consideration as a treatment alternative for the CSO discharge.

A wetland designed for CSO management faces essentially the same requirements as a stormwater wetland, and the FWS constructed wetland is the preferred concept for the same reasons cited previously. Because the CSO flow always contains some untreated wastewater, the level of pathogens and the mass of pollutants contained in the storm event may be higher than found in normal stormwater flow. The "first flush" with many stormwaters contains the bulk of pollutants, but that may not be the case with CSO discharges because of the wastewater component.

The design of the CSO wetland must commence with an analysis of the frequency and intensity of storm events and the capacity of the existing wastewater treatment facilities. This analysis will be used to determine the volume of excess CSO flow to be contained by the proposed wetland. Containment of the CSO from at least a 5-year or a 10-year storm event is a typical baseline wetland volume. The CSO wetland will act as a batch reactor, and water quality improvements will depend on the intensity and frequency of storm events. Assuming the wetland is sized for the CSO from a 10-year storm event, the flow from any lesser event will be completely contained, and any discharge would be composed of previously contained and treated water.

The hydraulic retention time (HRT) in the wetland must include consideration of precipitation on the wetland, seepage, and evapotranspiration, as well as the input CSO flow. The water quality expectations are usually established by the regulatory authorities. If significant seepage is allowed, then the CSO wetland will perform similarly to the rapid infiltration concept described in Chapter 8 of this book. When the HRT in the wetland has been established for various situations, it is possible to estimate the water quality improvements that will occur by using the design models in this chapter and in Chapter 8 (if seepage is permitted). If the wetland is located adjacent to the ultimate receiving water and the hydro-logical investigation indicates that the seepage will flow directly to the receiving surface water, then seepage can be very beneficial, particularly with respect to phosphorus removal.

In some cases, trash removal and some form of preliminary treatment are provided separately. If not, these functions should be the initial components in the CSO wetland, with trash racks or similar, and a deep basin for preliminary settling. The wetland component should be designed as a FWS marsh system with a "normal" operating depth of 2 ft (0.6 m). The use of Phragmites, Typha, or Scirpus would permit a temporary inundation of up to 3 ft (1 m) during peak storm events. The use of Phragmites should be avoided if the CSO wetland is planned for habitat and recreational benefits in addition to water quality improvement. The wetland component should have at least two parallel trains of two cells each to allow flexibility of management and maintenance.

TABLE 6.11

Water Quality Expectations for a Combined Sewer Overflow (CSO) Wetland at Portland, Oregon

Preliminary

TABLE 6.11

Water Quality Expectations for a Combined Sewer Overflow (CSO) Wetland at Portland, Oregon

Preliminary

Untreated

Treatment

Wetland

Wetland

Parameter

CSO

Effluenta

Seepage

Overflow

Volume (m3)

31,000

31,000

15,000

3000

BOD (mg/L)

100

85

2

10

TSS (mg/L)

100

70

2

10

TKN (mg/L)

7.0

6.1

3

2

Nitrate nitrogen (mg/L)

0.2

0.2

0.1

0.0

Total phosphorus

0.6

0.45

<0.05

0.17

(mg/L)

Fecal coliform

110,000

200

<20

10

(number/100 mL) a Disinfection included.

(number/100 mL) a Disinfection included.

Note: 1000 m3 = 0.26 Mgal; BOD, biochemical oxygen demand; TSS, total suspended solids; TKN, total Kjeldahl nitrogen.

Source: Reed, S.C. et al., Natural Systems for Waste Management and Treatment, 2nd ed., McGraw-Hill, New York, 1995. With permission.

Determining the elevation of the bottom of the wetland component is critical for successful performance, particularly in situations where a shallow fluctuating groundwater table exists and where seepage is to be permitted. It is desirable to have the bottom soils moist at all times, even during drought conditions, but allowing the groundwater to occupy a significant portion of the containment volume during wet weather should be avoided. Phragmites and to a lesser degree Typha are drought resistant and would permit location of the wetland bottom in a position that would avoid seasonal groundwater intrusion.

Designing the wetland for inclusion of habitat values complicates this procedure. In this case, the wetland can consist of marsh surfaces above the normal groundwater level and deeper pools that intersect the minimum groundwater level so some water is permanently available for birds and other wildlife.

The results of a feasibility study of a CSO constructed wetland, conducted for the City of Portland, Oregon, are summarized in Table 6.11. The wetland component was designed to contain the 10-year storm event that produced a total CSO flow of about 11.8 Mgal (45,000 m3) from the peak 7-hour flow. Because of land area limitations, it was decided to provide separate facilities for trash removal and preliminary treatment. The potential wetland area contained about 23 ac (9.3 ha), and a 2-ft (0.6-m) water depth in the wetland would contain about

15 Mgal (57,000 m3). The soil beneath and adjacent to the proposed wetland and the ultimate receiving water was a permeable sand. The water quality expectations for this system are given in Table 6.11. The data in Table 6.11 are intended as an example only and cannot be utilized for system design elsewhere. It is necessary to determine the CSO characteristics and site conditions for a wetland for every proposed system because of possibly unique local conditions.

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