Design Requirements

Sludge application to these reed beds is similar to the freezing process previously described, in that sequential layers of sludge are applied during the operational season. The solids content of the sludge can range up to 4%, but 1.5 to 2% is preferred (Banks and Davis, 1983a). Solids content greater than 4% will not allow uniform distribution of the sludge on the densely vegetated bed. The annual loading rate is a function of the solids content and whether the sludge has been digested anaerobically or aerobically. Aerobically digested sludges impose less stress on the plants and can be applied at slightly higher rates. At 2% solids, anaerobically digested sludges can be applied at a hydraulic loading of about 25 gal/ft2-yr (1 m3/m2-yr) and aerobically digested sludges at 50 gal/ft2-yr (2 m3/m2-yr). The corresponding solids loadings would be 4.2 lb/ft2-yr (20 kg/m2-yr) for anaerobic sludges and 8.3 lb/ft2-yr (40 kg/m2-yr) for aerobic sludges. For each 1% increase in solids content (up to 4%), the hydraulic loading should be reduced by about 10% (for example, for aerobic sludge at 4% solids, the hydraulic loading is 1.6 m3/m2-yr). For comparison, the recommended solids loading on conventional sand beds would be about 16.4 lb/ft2-yr (80 kg/m2-yr) for typical activated sludges. This suggests that the total surface area required for these reed beds will be larger than for conventional sand beds.

The typical operational cycle allows a sludge application every 10 d during the warm months and every 20 to 24 d during the winter. This schedule allows 28 sludge applications per year; for 2% solids aerobic sludges, each layer of sludge would be about 4 in. (10.7 cm). It is recommended that during the first year of operation the loadings be limited to one half the design values to limit stress on the developing plants.

An annual harvest of the Phragmites plants is typically recommended. This usually occurs during the winter months, after the top of the sludge has frozen. Electrical or gasoline-powered hedge clippers can be used. The plant stems are cut at a point that will still be above the top of the sludge layers expected during the remainder of the winter. This allows the continued transfer of air to the roots and rhizomes. In the spring, the new growth will push up through the accumulated sludge layers without trouble. The harvest produces about 25 ton/ac, dry solids 2.5 ton/ac (56 mt, wet weight per hectare). The major purpose of the harvest is to physically remove this annual plant production and thereby allow the maximum sludge accumulation on the bed. The harvested material can be composted or burned.

Sludge applications on a bed are stopped about 6 months before the time selected for cleaning. This allows additional undisturbed residence time for the pathogen content of the upper layer to be reduced. Typically, sludge application is stopped in early spring, and the bed is cleaned out in late fall. The cleaning operation removes all of the accumulated sludge in addition to the upper portion of the sand layer. New sand is then placed to restore the original depth. New plant growth occurs from the roots and rhizomes that are present in the gravel layer.

The number of separate reed beds at a facility will depend on the frequency of sludge wasting and the volume wasted during each event. Typically, the winter period controls the design because of the less frequent sludge applications (21 to 24 d of resting) permitted. For example, assume that a facility wastes aerobi-cally digested sludge on a daily basis at a rate of 10 m3/d (2% solids). The minimum total bed area required is (10 m3/d)(365 d/yr)/(2 m3/m2-yr) = 1825 m2. Try 12 beds, each 152 m2 in area; assume that each is loaded for 2 d in sequence to produce a 24-d resting cycle during the winter months. The unit loading is then (10 m3/d)(2 d)/(152 m2) = 0.13 m = 13 cm. This is close to the recommended 10.7-cm layer depth for a single application; therefore, in this case, a minimum of 12 cells would be acceptable.

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