SLUDGE DRYING BED DESIGN REVIEW
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SLUDGE DRYING BED DESIGN REVIEW
Contract Number 68-03-1821
Work Assignment Number 2-14
September 26, 1986
Submitted to:
United States Environmental Protection Agency
Cincinnati, Ohio
Project Officer: Jon Bender
Submitted by:
James M. Montgomery, Consulting Engineers, Inc*
Pasadena, California
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INTRODUCTION
The sludge dewatering system most commonly used in small wastewater
treatment facilities is the sand drying bed. Insufficient drying bed capacity can
severely limit the capability to remove excess activated sludge from the
biological treatment processes which, in turn, can limit the treatment process
performance. Insufficient dewatering system capacity can result from
inadequate estimates of sludge production from the wastewater treatment
processes, inadequacies of sludge treatment processes and/or inadequacies in the
design of the sludge dewaterlng system.
Inadequate design of sand drying bed systems is potentially one of the reasons
that wastewater treatment plants experience sludge handling limitations. A
survey of explicit sludge drying bed design criteria was therefore conducted to
determine if sludge drying bed design Is a potential problem. This report
presents the results of the survey of design textbooks and manuals, published
state guidelines, and technical literature for sand drying bed. design criteria. The
survey was limited to explicit design criteria for drying bed capacity which
typically may be employed for the design of small wastewater treatment works.
DESIGN CRITERIA
Design criteria for sludge drying beds are most often presented as explicit values
expressed as per capita population equivalents. Such units of expression are
typically square feet of bed area per capita. Alternate units of expression for
drying bed design criteria that are encountered are pounds of solids per square
foot of bed area or depth of sludge applied per year.
Necessary distinctions which are made to qualify the available design criteria
are the type of sludge to be dewatered and whether the beds are exposed or
covered. Exposed and covered drying conditions can be distinguished through
separate design. criteria or by percent reductions in the required exposed—bed are
to account for the more favorable drying conditions of an enclosed installation.
Drying bed design criteria as recommended by state agencies are presented. in
Table 1. The open—bed capacities ranged from 1 to 2.5 sq ft/capita. Alternate
units of expression were utilized by the States of Arizona (lbs/sq ft/yr) and
Wyoming (ft/yr). A lack of continuity in the units of expression and the type of
sludge to be dewatered, when noted, preclude detailed comparison of the
reported values other than to acknowledge that geieral agreement exists.
The explicit drying bed design criteria obtained from design textbooks, manuals
and the literature are presented as Table 2. Table 2 was prepared to illustrate
the typical design criteria that are readily available to the practicing engineer.
More thorough presentations covering a wider variety of sludge types and
environmental conditions can be found elsewhere (U.S. EPA, 1979).
Most of the texts reviewed presented drying bed design criteria which were
citations from WPCF MOP No. 8 (1959). The design criteria from this original
work were calculated on the assumption of a per capita raw sludge production
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rate, and were confirmed to be appropriate in a 1957 study of the performance
of existing plants. A range of values was originally presented because the sludge
drying process is subject to several factors including rainfall, temperature,
sludge drainage rate and relative humidity.
In addition to the original work of WPCF (1959), the tore extensive presentation
of the U.S. EPA (U.S. EPA, 1979) (U.S. EPA, 1978, 1982) cited Imhoff, et al.
(1956), and an early edition of Ten States Standards (1971) when specifying
values for particular regions (latitudes). Imhoff made his predictions from an
assumed amount of digested sludge per capita and an assumed length of drying
season and number of draws on the bed per drying season. The loading values
ranged from 0.75 to 3 sq ft/capita, or 15 to 27.5 lb dry solids/sq ft/yr for open
beds, to 0.75 to 1.5 sq ft/capita for enclosed beds.
Different types of sludge are represented by the range of values reported in
Table 2. Digested primary plus chemical precipitate sludge requires nearly
double the surface area than does primary plus digested activated sludge. This is
because these sludge mixtures must lose most of the water by evaporation
(Imhoff, et at., 1956).
The addition of polymer can often increase the sludge loading by 20 to
50 percent. This is because more water drains during the filling of the bed.
When treated with polymer, anaerobically digested sludge can float, leaving
several inches of free water underneath which quickly drains. Also, the polymer
flocculant will release large amounts of free water while forming large floc, thus
requiring less water to be evaporated. Po l ymer may also capture fine particles
which could otherwise blind the drainage media (Beardsley, 1916).
Some rational equations do exist for the design of sludge drying beds. These are
necessarily complex formulas which require the use of such empirical factors as
evaporation rate, rainfall, and rate of drainage through the drying bed drainage
system. Two of the most recent models are those developed by Rolan (1980) and
Walski (1976). The Rolan model can be used to design and determine optimum
operating parameters for drying beds. The major difference between these
models and the typical explicit values is that the models account for
environmental factors as well as operational factors involved in operating the
sludge drying beds, which are lacking in explicit per capita values.
DISCUSSION
The most accurate methods available for design oE sludge drying beds are the use
of rational equations. These equations take into account many factors not
considered by per capita design values. The use of these equations in practice
may be limited by their complexity and the need for empirical factors which may
not make them particularly suitable for the design of small wastewater
treatment pLants.
Per capita design criteria are of limited value In design as they cannot account
for variations in sludge quantities produced within a plant of given capacity.
This is because per capita values are a function of the total wastewater flow and
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reflect the “typical” amounts of solids carried into- or generated within the liquid
treatment process train.
It must also be kept in mind that sand drying beds are primarily used to dewater
digested sludge solids. Sludge volume reduction by digestion is, perhaps, the
most critical step in a sludge treatment and disposal scheme and inadequacies in
the digestion step can result in overloading of an otherwise adequately designed
sludge drying bet.
The best approach to the design of sludge drying beds is to use solids loading
criteria. These criteria best take into account the actual amount of sludge to be
dewatered. However, the use of solids loading criteria are dependent upon on
accurate estimations of excess activated sludge solids and the degree of sludge
treatment or volume reduction achievable by intermediate operations and
processes. Empirical drying bed loading rates from treatment facilities nearby
to the facilitiy being designed should be used, if possible, in developing design
criteria which are sensitive to local environmental factors. The use of solids
loading criteria based on local empirical data will help to reduce the potential
for the design of undersized sludge drying beds.
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TABLE 1
STATE DESIGN GUIDELINES FOIt SLUDGE DRYING BEDS
Source
Open
(ft 2 /cap)
Covered
(1 t 2 /cap)
Open
(lb/f t 2 /yr)
Covered
(lb/f t 2 /yr)
Other
Comments
Maryland
1.75—2.5
I.2 —l.5
1°+A.S.
Utab
1.75
Zbedsminimum
Iowa
2
1 ft 2 /cap if used as backup
Wyomrng
4 ft/yr
Kansas
1
Arizona
15
10
10 + 20 digested
0.
2 digested
Missouri
2
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TABLE 2
SLUDGE DRYING BED SIZING
Sludge
Source Source
(pen
(ft leap)
Covered
(f 1 2 /cap)
O en
( lb/1t 2 /yr)
Coverd
( lb/ItZIyr) Comments
Vesiland (1979) primary digested I — 1.5 0.75 — 1
10-25 1 1—40
primarypius 1.75—2.5 1.25—1.5
activated dig.
primary plus chem. 2 — 2.5 1.25 — 1.5
precip. digested
Clark, et al. (1977) 1 - 2 NomInal 25’ a 100’ of 6 to
9 coarse sand or graded
20 (Northern climates) gravel bed. 18 of wall
40 (Southern climates) above surface. Pump In 8
to 10. of sludge. Pipes
spaced at 20’.
Metcalf and Eddy (1979) prImary 1 - 1.5 24 - 40 Bed 20’ a 20’-lOO’. Piping
spaced 8’ to 20’. 15 to
10 + act, dIg. 1.75 — 3 12 — 20 II ’ wails (partitions). 9
to 12 sand. 8 to 12 sludge
10 + chem. preclp. 2 - 2.5 20 - 32
digested
Greet Lakes (1971) 2 when bed Is primary method of dewatering Gravel 12 thick extending
1 when bed Is back-up 6 above top of drain pipes.
Pipe. 4 spaced less than
20’. 6—9 of sand. Walls
151 to 18 above surface
extending 6 below surface
Imhoff (1956) 1° 1.3 18.25 Up to about 4’ of sludge
per year. This I. 8 sludge
Activated 2.25 layers with about 6 draws
per year.
Beardaley (1976) UsIng polymer, a bed can often be loaded
with 20% to 25% more sludge
WPCF MOP NO. 20 (1985) w/o Chemicals 10-25 12-40
w/Chem lca ls 40-60
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BIBLIOGRAPHY
Beardsley, J.A. “Sludge Drying Beds Are Practical.” Water and Sewage Wcrk ,
48, 7, 82 (1976)
Clark, LW., Viessinan, W., Jr. and Haxiner, M.3. Water Supply and Pollution
Control . Third Edition, LE.P. (1977)
Great Lakes - Upper Mississippi River Board of State Sanitary Engineers,
Recommended Standards for Sewage Works , Health Education Service, Inc.,
Albany, NY (1978).
Imhoff, K. and Fair, G.M. Sewage Treatment . Second Edition, John Wiley and
Sons, Inc. (1956)
Metcalf and Eddy, Inc. Wastewater Engineering . McGraw-Hill (1979)
Rolan, A.T. “Determination of Design Loading for Sand Drying Beds.” J.N.C.
Sect., Am. Water Works Assoc., N.C.W.P.C.A . L5, 25 (1980)
U.S. EPA Process Design Manaual for Dewatering Municipal Wastewater Sludges .
EPA 625/1-82-014 (1982)
U.S. EPA Sludge Dewatering and Drying on Sand Beds . EPA 600/2-78-141 (1978)
Vesilind, P.A. Treatment and Disposal of Wastewater Sludges . Ann Arbor
Science (1979)
Waiski, T.M. “Mathematical Model Simplifies Design of Sludge Drying Beds.”
JWPCP , 50, 1965 (1978)
WPCF. Sludge Dewatering . Manual of Practice No. 20. Water Pollution Control
Federation (1983)
WPCF. Wastewater Treatment Plant Design . Manual of Practice No. 8. Water
Poilution Control Federation. (1959)
B-i
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