Performance

It is estimated that 75 to 80% of the volatile solids (VSS) in the sludge will be reduced during the long detention time on the bed. As a result of this reduction and the moisture loss, a 10-ft-deep (3-m) annual application will be reduced to

2.4 to 4 in. (6 to 10 cm) of residual sludge. The useful life of the bed is therefore 6 to 10 yr between cleaning cycles. With one exception, all the reed bed systems in the United States are located where some freezing weather occurs each winter. The exception is the reed bed system at Fort Campbell, Kentucky. Observations at these systems indicate that the volume reduction experienced at Fort Campbell is significantly less than that experienced at systems in colder climates. The reason is believed to be the freezing and thawing of the sludge that occurs in the colder climates, which results in much more effective drainage of water from the accumulated sludge layers. This suggests that reed beds in cold climates should follow the criteria described in a previous section for freezing rather than the arbitrary 21-d cycle for winter sludge applications. This should result in a more effective process and, in colder climates, more frequent sludge application.

The loss of volatile solids during the long detention time on these reed beds raises the concern that the metals concentration of the residual sludges could increase to the point where beneficial uses of the material or normal disposal options are limited. Table 9.10 summarizes data from the reed bed system serving the community of Beverly, New Jersey. The reed bed system in Beverly has been in operation for 7 yr; therefore, the average age of the accumulated sludge was

3.5 yr. The applied sludges sampled from 1990 to 1992 are believed to be representative of the entire period. The tabulated data on accumulated sludge represents a core sample of the entire 7-yr sludge accumulation on the bed. The total volatile solids experienced a 71% reduction, and the total solids demonstrate a 251% increase due to the effective dewatering. All of the metals concentrations show an increase. If beneficial use of the removed sludge is a project goal, it is suggested that the critical metals in the accumulated sludge be measured on an annual basis. These data will provide the basis for following the trend of increasing concentration and can be used to decide when to remove the sludge from the bed prior to developing unacceptable metal concentrations.

TABLE 9.10

Comparison of Applied vs. Accumulated Sludge

TABLE 9.10

Comparison of Applied vs. Accumulated Sludge

Applied

Accumulated

Parameter

Sludges3

Sludgeb

Total solids (%)

7.1

17.8

Volatile solids (%)

81.14

56

PH

5.3

6

Arsenic (mg/kg)

0.64

1

Cadmium (mg/kg)

6

8.3

Chromium (mg/kg)

16.3

62.3

CoPPer (mg/kg)

996.5

2120

Lead (mg/kg)

510

1130

Mercury (mg/kg)

10.2

28.3

Nickel (mg/kg)

29.8

45.7

Zinc (mg/kg)

4150

6400

a Digested primary sludges applied to the bed from 1990 to 1992. b Accumulated dewatered sludge on the bed March 12, 1992.

a Digested primary sludges applied to the bed from 1990 to 1992. b Accumulated dewatered sludge on the bed March 12, 1992.

Source: Costic & Associates, Engineers Report: Washington Township Utilities Authority Sludge Treatment Facility, Costic & Associates, Long Valley, NJ, 1983. With permission.

Another issue of concern in some states is the use of Phragmites on these systems. The Phragmites plant has little habitat value and has been known to crowd out more beneficial vegetation species in marshes. The risk of seeds or other plant material escaping from the operational reed bed and infesting a natural marsh is negligible; however, when the sludge is cleaned out of the bed, some root and rhizome material may also be removed with the sludge. The final sludge disposal site may have to be considered if regrowth of the Phragmites at that site would pose a problem. Disposal in landfills or utilization in normal agricultural applications should not create problems. If it is absolutely necessary, the removed sludges can be screened and the root and rhizome stock separated. It also should be possible to stockpile the removed sludge and cover it with dark plastic for several additional months to kill the rhizome material.

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