Geoscience Reference
In-Depth Information
24.14.4.3 Biosolids Withdrawal to Drying Beds
Pumping digested biosolids to drying beds is one method among many for dewatering biosolids,
thus making the dried biosolids useful as a soil conditioner. Depending on the climate of a region,
the drying bed depth may range from 8 to 18 in. The area covered by these drying beds may be
substantial. For this reason, the use of drying beds is more common for smaller plants than for larger
plants. When calculating biosolids withdrawal to drying beds, use Equation 24.161:
Biosolids withdrawn (ft
3
) = 0.785 × (Diameter)
2
× Drawdown (ft)
(24.161)
■
EXAMPLE 24.129
Problem:
Biosolids are withdrawn from a digester that has a diameter of 40 ft. If the biosolids are
drawn down 2 ft, how many ft
3
will be sent to the drying beds?
Solution:
Biosolids withdrawn (ft
3
) = 0.785 × (Diameter)
2
× Drawdown (ft)
= 0.785 × (40 ft × 40 ft) × 2 ft
= 2512 ft
3
withdrawn
24.14.5 b
iosolids
d
isposal
C
alCulations
In the disposal of biosolids, land application, in one form or another, has become not only a necessity
(because of the banning of ocean dumping in the United States in 1992 and the shortage of landfill
space since then) but also quite popular as a beneficial reuse practice. Beneficial reuse means that
the biosolids are disposed of in an environmentally sound manner by recycling nutrients and soil
conditions. The application of biosolids is occurring throughout the United States on agricultural
and forest lands. For use in land applications, the biosolids must meet certain conditions. Biosolids
must comply with state and federal biosolids management and disposal regulations and must also
be free of materials dangerous to human health (e.g., toxins, pathogenic organisms) or dangerous to
the environment (e.g., pesticides, heavy metals). Biosolids can be land applied by direct injection,
by application, by incorporation (plowing in), or by composting.
24.14.5.1 Land Application
Land application of biosolids requires precise control to avoid problems. Use of process control
calculations is part of the overall process control process. Calculations include determining disposal
cost, plant available nitrogen (PAN), application rates (dry tons and wet tons/acre), metals load-
ing rates, maximum allowable applications based on metals loading, and site life based on metals
loading.
24.14.5.1.1 Disposal Cost
The cost of disposal of biosolids can be determined by using Equation 24.162:
Cost = Wet tons biosolids produced per year × % Solids × Cost per dry ton
(24.162)
■
EXAMPLE 24.130
Problem:
The treatment system produces 1925 wet tons of biosolids for disposal each year. The
biosolids are comprised of 18% solids. A contractor disposes of the biosolids for $28 per dry ton.
What is the annual cost for biosolids disposal?
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