Sludge Conditioning with Incinerator Ash
J. E. SMITH, JR., Research Sanitary Engineer
S. W. HATHAWAY, Research Chemist
J. B. FARRELL, Research Chemical Engineer
R. B. DEAN, Chief
Ultimate Disposal Research Program
Advanced Waste Treatment Research Laboratory
National Environmental Research Center, EPA
Cincinnati, Ohio
INTRODUCTION
Sewage sludge handling and disposal can account for as much as 50 percent of the total
cost of treating sewage. A substantial portion of this cost is incurred in a filtration step
which converts a low solids liquid sludge into a filter cake which can be disposed of by
incineration or landfill. Biological sludges generated by secondary treatment such as the
activated sludge or trickling filter processes are low in solids and are exceptionally difficult
to dewater. The use of secondary treatment is increasing at a rapid rate; consequently, the
prospect is for generation of more sludges which will be more difficult to dewater.
Traditionally, sludges have been conditioned for vacuum filtration by the addition of
either ferric chloride alone or combined with lime. In recent years, polymeric sludge
conditioning agents have been developed. These materials have been adopted by many
treatment plants, where they have improved the filter yield and reduced overall operating
cost. However, these polymers are expensive chemicals and do not work on all sludges.
Some sludges, such as waste activated sludge, do not respond well to polymeric
conditioners, nor to inorganic chemical conditioners. Mixing them with primary sludge
makes it possible to dewater them, but increases the unit cost of conditioning in proportion
to the amount of waste activated sludge present. Continued need to develop better ways of
dewatering sludge is thus evident.
In Germany, addition of from one to five parts of sludge incinerator ash for every part
sludge solids has successfully aided sludge dewatering by both rotary vacuum filtration and
pressure filtration (1,2). Vater at Stuttgart reported vacuum filter yields as high as
8.5#/ft2/hr when using 2.5 parts ash for each part sludge solids (3). The Beloit-Passavant
"Sludge All" process employs ash as a conditioning agent in the filter pressing of sludge (4).
Busse patented a process using ash as a filter aid (5) which states that for the process to be
effective, the ash particle size must be below 0.1 mm.
Most published work in the U.S. has only been done on a laboratory scale, with the
buchner Funnel test, using fly ash from coal combustion (6,7). A plant scale pressure
filtration study is now getting underway at Cedar Rapids, Iowa (8). The design of this plant
is based on unpublished pilot scale pressure and rotary vacuum filtration studies using fly
ash. All of the rotary vacuum filter work was done with fly ash in combination with
chemical coagulants. No work was done with fly ash alone, or at levels of addition greater
than 2 parts ash per part sludge solids. Tenney (9) in working with fly ash found that suitable
dosage levels for successful sludge conditioning were from 1 to 1.5 parts ash for each part
sludge solids for digested sludge, and from 5 to 7 parts ash for dilute activated sludge.
LaRocca and Eckenfelder (10) used fly ash to successfully dewater chemical sludges.
Tenney (9) noted that fly ash also helps to improve filtrate quality, by lowering the
phosphorus and COD content. Fly ash is said to give the cake a more rigid structure;
however, the cake may be higher in moisture on an ash-free basis (11).
Most of the work on ash-conditioning of sludge has been directed toward finding a
beneficial use for fly ash in the wastewater treatment industry, since it is an abundant waste
product. Little has been done in this country with the ash from sludge incineration. For
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