United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-86/075 Jan. 1987
oEPA Project Summary
Dewatering Wastewater
Treatment Sludge
by Clathrate Freezing:
A Bench-Scale
Feasibility Study
Benjamin Molayem and Tevfik Bardakci
Preliminary investigations were con-
ducted on the technical feasibility of
clathrate freezing applied to the dewater-
ing of municipal wastewater sludge. This
project has successfully demonstrated the
fundamental feasibility of the process,
although contact times of 15 to 20 min-
utes were necessary to achieve high
dewatering. Also, other clathrating agents
should be examined. Considerable work
would still be required, therefore, for bring-
ing the process to full-scale commer-
cialization. The work was done under a
cooperative agreement with the Metro-
politan Sewer District of Greater Cincin-
nati, Ohio, subcontracting to Benmol Cor-
poration, who initially suggested the
process.
This Project Summary was prepared by
EPA's Water Engineering Research Labor-
atory, Cincinnati, OH, to announce the
major findings of a project that is
documented in a separate report of the
same title (see Project Report ordering
information at back}.
The Process
The clathrate dewatering process would
use a liquid refrigerant to form a complex
with the water in sludge. The complex, or
clathrate as it is termed, freezes to form
a solid crystal. In the clathrate crystal,
molecules of water surround a molecule of
clathrating agent. The water and clath-
rating agent are likely held together by a
combination of intermolecular attractions
and crystal geometry that permits the
trapping of one molecule within the voids
of another's crystal lattice. The clathrate
freezes at temperatures above the freez-
ing point of water. Therefore, if sludge is
immersed in an excess of clathrating agent
at a temperature between the freezing
points of the clathrating agent and water,
clathrate crystals could form without the
formation of ice crystals. It is proposed
that water inside sludge particles and
cellular material will diffuse outward and
form clathrate crystals in the bulk liquid
phase, producing a suspension of totally
dehydrated sludge particles, clathrate
crystals, and the liquid clathrating agent.
It should then be possible to separate the
clathrate crystals from the sludge solids
by virtue of their density difference, and
thus effect a water removal and dewater-
ing of the sludge. Upon melting of the
clathrate, water can be separated and
clathrating agent recycled to the process.
Easily liquified compounds that form clath-
rates with water include propane, many
Freons, other hydrocarbons and gases,
and halogens.
Project Objectives and Methods
The objectives of this initial study were
to concentrate on the three following con-
cerns relating to technical feasibility:
(1) Extent of clathrate formation;
(2) Ease of achieving phase separations
required for the process; and
(3) Extent to which water is removed
from the bulk of the sludge solids.
In the preliminary study of technical
feasibility, Freon-11 was used as the clath-
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rating agent. Although liquid propane
would be preferred for a commercial pro-
cess due to its lower cost, working with
Freon-11 was much easier. Propane studies
would require higher pressures, and
propane is flammable. With Freon-11
experiments can be conducted nearer to
normal laboratory conditions with con-
siderably less complicated techniques.
The feasibility was investigated in a
simple apparatus where sludge and liquid
Freon-11 were placed in a 1-liter test-tube
shaped vessel, stirred with a laboratory
mixer, and allowed to settle. The test-tube
shaped vessel was immersed in a 1.5°C
water bath. Primary, secondary, and mixed
sludges were tested. The mixed sludge
had a 60/40 volume ratio of primary-to-
secondary sludge, typical of that processed
at the Mill Creek Treatment Plant in
Cincinnati.
Results
Average water removals by clathrating
varied between 48 and 97.9 weight
percent (9.25-52.6% dry solids in the
product), depending upon contact time
and sludge type. Sludge was mixed with
liquid Freon-11 for both 10 minutes at a
time and also for as much as 20 minutes
with the phases separating but remaining
in contact for as long as 2 hours. Single
contacts and two-stage contacts (two
repeated contacts of the sludge with pure
Freon-11) were used. To obtain the higher
degrees of dewatering, the longer and/or
two-stage contacts were necessary. Clath-
rate crystals obtained were of two types,
a rather soft grayish crystal with a molar
ratio (water to Freon-11) of about 17 to 1
and a hard white crystal with a molar ratio
of about 8 to 1. Procedures could be
adjusted to obtain the latter kind of crystal
which separated well under gravity. In
summary, high water removals with good
phase separations appeared to be possible.
Although basic technical feasibility was
shown, considerable work would still be
required to develop the process to the
point of commercialization. First, the
conditions favorable for high water
removals need to be more accurately
defined. Next, studies would be desirable
with less expensive propane rather than
Freon-11, and phase separation devices
used in full-scale processes, such as
hydroclones, should be evaluated. A
careful economic analysis would also I
desirable. Even though the process doi
show possibilities, it is fairly complex wi
considerable development work still r
quired. For the present, additional EF
funding is not planned, but continuf
development by Benmol Corporation mi
take place.
The full report was submitted in fulfi
ment of Cooperative Agreement No. C
810549 by the Metropolitan Sewer Distri
of Greater Cincinnati, under the sponsc
ship of the U.S. Environmental Protecti<
Agency.
Benjamin Molayem and Tevfik Bardakci are with BENMOL Corporation.
Alexandria, VA 22314.
Harry E. Bostian is the EPA Project Officer (see below).
The complete report, entitled "Dewatering Wastewater Treatment Sludge by
Clathrate Freezing: A Bench-Scale Feasibility Study," (Order No. PB 86-239
779/AS; Cost: $11.95, subject to change) will be available only from.
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES P
EPA
PERMIT No G-3E
Official Business
Penalty for Private Use $300
EPA/600/S2-86/075
0063240
LOU W T
60604
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