United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-86/084 Dec. 1 986
Project Summary
Enzyme Addition to the
Anaerobic Digestion of
Municipal Wastewater
Primary Sludge
Gregory M. Higgins and Joseph T. Swartzbaugh
This study was conducted to evaluate
the effects of adding enzymes to a munic-
ipal wastewater (MWW) sludge anaerobic
digester. The primary objective was to
examine the impact of using enzymes to
enhance the degradation of the cellulosic
and the oil- and grease-rich sludge frac-
tions. This project was conducted under
a cooperative agreement to the City of
Xenia, Ohio, with subcontracting to
Systech Corporation, Xenia, Ohio.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, to announce
the major results of a project that is
documented in a separate report of the
same title (see Project Report ordering
information on the back page).
Introduction
One of the most commonly employed
treatment methods for municipal waste-
water (MWW) solids is anaerobic digestion
followed by one of a number of ultimate
disposal options. This process is particu-
larly advantageous in that it produces a
sludge suitable for land application, effi-
ciently reduces pathogenic organisms,
produces a potentially useful by-product
fuel gas, and is generally accompanied by
low operating costs. Despite these advan-
tages, currently used anaerobic digestion
technology has several operational and
technical limitations. One of the most
significant of these is the level of volume
reduction achieved. Although anaerobic
digestion reduces sludge volume and
mass, the constantly growing cost and
number of federal and state regulations
governing the disposal of MWW sludges
requires that current technology be opti-
mized to produce the smallest amounts of
sludge possible.
The objective of this program was to
evaluate the application of enzyme aug-
mentation to current MWW anaerobic
digestion practices. This concept was
viewed as a method to enhance the sludge
volume reduction and fuel gas production.
Although commercial enzyme augmenta-
tion products are available for use in MWW
treatment systems, an extensive search of
the relevant experimental literature con-
ducted during this project indicated very
few useful data regarding their use.
Cellulase Enzyme Addition
The application of cellulase enzymes to
enhance the anaerobic digestion of cellu-
losic component of MWW sludge was
evaluated in a series of laboratory-scale
studies. Three modes of applying the en-
zymes were examined — as a pretreat-
ment, post-treatment, and simultaneous
treatment option with respect to anaerobic
digestion. The pretreatment option incor-
porated a two-phase digestion process
and was by far the most technically suc-
cessful of the options examined. However,
the advantages of this option were largely
due to the two-phase process used to
accommodate the pH requirements of the
enzyme rather than to the enzyme addi-
tions. Compared with a single-phase di-
gestion system, increases of 45 and 44
percent were observed for total solids
reduction and methane production as a
result of the two-phase digestion process
alone. Addition of enzyme to the two-
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phase system further increased solids
reduction and methane production to 60
and 66 percent.
Experimental data scaled to the size of
the Xenia facility were compared with
typical operations at a 15-day hydraulic-
retention-time, single-phase digester sys-
tem modeled on the Xenia facility. Results
indicated that a net yearly savings of
$11,133, equivalent to a 7-mo payback
period for additional equipment, could be
achieved by using the two-phase digestion
process without enzyme addition. To ac-
complish this result, the facility would
need a minimal increase in tankage capac-
ity. Based on the current cost of commer-
cially available enzyme, an additional
yearly savings of $1,525 would result from
adding enzyme to the two-phase system.
This savings could increase if enzyme
costs could be reduced.
Lipase Enzyme Addition
The use of lipase enzymes to increase
the rate of decomposition of the oil and
grease component of MWW sludge solids
was evaluated in a similar series of labora-
tory experiments conducted with direct
enzyme addition to a single-phase anaer-
obic digestion system operated at a 15-day
hydraulic retention time. Compared with
control conditions, increases of 2 and 4
percent were observed for total solids
reduction and methane production as a
result of lipase enzyme addition. Based on
current enzyme costs, a net yearly savings
of $701 would result from the use of
lipase enzymes. No additional tankage
capacity would be required for this pro-
cess. Further increases could potentially
be achieved by operating the system at a
longer retention time.
Conclusions
The overall results of the project show
little advantage for enzyme addition to
anaerobic digestion. The major improve-
ments seen were due to the staging of the
digestion process, which was done in
order to accommodate the pH require-
ments of the cellulase enzyme. The con-
clusions are, therefore, as follows:
(1) Two-phase digestion used to accom-
modate cellulase addition in this
study shows promise for increasing
the efficiency for anaerobic diges-
tion.
(2) The addition of cellulase enzyrr
shows a moderate improvement c
the two-phase digestion proci
without enzyme addition.
(3) The addition of lipase enzyme un
the condition studied shows onl
minimal improvement in anaero
digestion.
The full report was submitted in full
ment of Cooperative Agreement No. (
810279 by the City of Xenia, Ohio, unc
the sponsorship of the U.S. Environrm
tal Protection Agency.
Gregory M. Higgins and Joseph T. Swartzbaugh are with SYSTECH Corporation,
Xenia, OH 45385-9354 (J. T. Swartzbaugh is presently with PEER Consultants,
Inc., Dayton, OH).
B. Vincent Salotto was the EPA Project Officer (see below for present contact).
The complete report, entitled "Enzyme Addition to the Anaerobic Digestion of
Municipal Wastewater Primary Sludge," (Order No. PB87-101 788/AS; Cost:
$16.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
For further information, Harry E. Bostian, 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
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