The major advantage of the reed bed concept is the ease of operation and maintenance and the very high final solids content (suitable for landfill disposal). This significantly reduces the cost for sludge removal and transport. A 6- to 7-yr cleaning cycle for the beds seems to be a reasonable assumption. One disadvantage is the requirement for an annual harvest of the vegetation and disposal of that material; however, over a 7-yr cycle, the total mass of sludge residue and vegetation requiring disposal will be less than the sludge requiring disposal from sand drying beds or other forms of mechanical dewatering.
A community near Pittsburgh, Pennsylvania (see Example 9.2), produces 3000 m3 of sludge (at 3.5% solids) per year. Compare reed beds for dewatering with a combination reed-freezing bed system.
Assume a 4-month freezing season, a design loading for reeds of 2.0 m3/m2, and a design depth for freezing of 70 cm (satisfactory value; Example 9.2 indicates a maximum potential depth of 70 cm as feasible). Use 12 beds:
1. Calculate bed area if reed dewatering is used alone:
3000 m3 2
Individual bed =-= 125 m
The schedule allows 28 sludge applications per year to the reed beds. Then, 3000 m3/12 beds/28 applications/125 m2/bed = 0.07 m/application = 7 cm.
2. 21 warm-weather applications = 21 x 7 cm = 147 cm.
7 winter applications using reed bed criteria = 7 x 7 = 49 cm.
3. Freeze-thaw criteria allow a total winter application of 70 cm; therefore, an additional 21 cm or three additional applications are allowed, for a total of 10, and an annual total of 31. At 31 annual applications, the allowable loading is 2.17 m3/m2-yr, and the required bed area is 3000 m3/2.17 m3/m2-yr = 1382 m2, so each of the individual beds can be reduced in area to 115 m2. This savings in area might be very significant in climates colder than in New Jersey.
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