United States
Environmental Protection
Agency
Municipal Environmental Research ^i i/
Laboratory N P -"
Cincinnati OH 45268 ^:
Research and Development
EPA-600/S2-84-102 Sept 1984
Project Summary
Monitoring Integrated Energy
Systems at a Wastewater
Treatment Plant in Maine
David R. Fuller, Douglas A. Wilke, Patric L. Thomas, and Anthony J. Lisa
Performance was monitored for sev-
eral alternative energy systems installed
in the municipal wastewater treatment
plant in Wilton, Maine. These systems
include active and passive .solar, effluent
heat recovery, digester gi is generation,
air-to-air heat recovery, i nd electricity
generation using digester gas.
To accomplish the monitoring, an
instrumentation system was installed
and data were collected f r jm May 1979
to March 1981. This instrumentation
system includes solar pyranometers,
hydronic BTU computers, electrical and
gas meters, a weather station, and
numerous temperature transmitters.
Data for the solar and di (ester system
are available in both dig!' al and analog
forms.
The data analysis resul ts and subse-
quent engineering evaluation of the
design concepts led to the conclusions
that (1) effluent heat rec >very through
the use of a heat pump a id ventilation
air heat recovery are cosi effective; (2)
the standard procedure for designing
active solar systems based on instanta-
neous efficiencies can lea< I to significant
overestimates of project* d system per-
formance; and (3) the use of solar
thermal energy collection to supple-
ment anaerobic digester heating is not
cost effective.
This Project Summary was developed
by EPA's Municipal Environmental Re-
search Laboratory. Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separata report of the same title (see
Project Report ordering information at
back).
Introduction
The municipal wastewater treatment
plant at Wilton, Maine, was designed to
incorporate energy conservation and al-
ternative energy features such as active
and passive solar space and process
heating, effluent-heat recovery, digester
gas generation and use, ventilation air
heat recovery, and electricity generation
using digester gas. Designed in 1975, the
plant became operational in September
1978. A grant from the U.S. Environ-
mental Protection Agency (EPA) funded
the monitoring of the plant's energy
systems.
To accomplish the monitoring, an in-
strumentation system was installed and
data were collected from May 1979 to
March 1981. This instrumentation sys-
tem includes solar pyranometers, hy-
dronic BTU computers, electrical and gas
meters, a weather station, and numerous
temperature transmitters. Data for the
solar and digester systems are available
in both digital and analog forms.
The primary objectives of the project
were:
1. To determine the effects of weather
performance of the systems.
2. To establish the amount of energy
used and produced by each system
and its contribution to the various
process and building requirements.
3. To determine the cost effectiveness
for each system.
4. To review possible design and oper-
ational problems for improved per-
formance.
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Description of Treatment Plant
The Wilton Wastewater Treatment
Plant was designed to provide secondary
treatment of the wastewater generated
by the town's population of 4,200, with
the added requirement that there be zero
discharge to the receiving stream at
stream flows less than 0.12 mVday, the
7-day, 10-year drought flow (7/Q/10).
Wastewater is lifted into the plant by
screw pumps. Preliminary treatment con-
sists of grit removal, comminution, flow
measuring, sampling, and screening.
Secondary biological treatment consists
of rotating biological contactors (RBC).
Final clarification takes place in two
peripheral feed clarifiers. The effluent is
then disinfected and discharged. To meet
the 7/Q/10 steamflow limitations, a 4.5-
ha spray irrigation plot is used directly
north of the treatment facility. This system
is designed for 10 continuous days of
operation during drought conditions.
Sludge consisting of screenings from the
preliminary screens and sludge from the
final clarifiers is combined and mixed in a
raw-sludge holding tank; then it is
pumped to anaerobic sludge digesters for
stabilization. High-rate, two-stage meso-
philic digestion is used. Following diges-
tion, the sludge is pumped to the de-
watering area, dewatered, conveyed to
the disposal vehicle, and trucked to a local
farm for spreading on fields.
Architecture is a key element in any
project where energy conservation and
the use of alternative energy sources are
prime objectives. Because of Wilton's
cold climate, the entire plant is enclosed
in two structures with all the processes
placed close together to keep the struc-
tures as small as possible and to reduce
hydraulic runs while providing for future
expansion. Gravity is used to avoid un-
necessary pumping. The building is
shaped to hold snow on the roof and
collect drifts of snow against walls for
increased natural insulation during the
colder months. Projections past the wall
lines adjacent to glazed surfaces reduce
surface wind velocity and cut heat loss, as
do recessed windows and doors. Basic
concrete materials were chosen for walls
and roofs because of heat retention
potential and low maintenance factors.
Insulated glass is used in all windows.
The building's interior spaces are parti-
tioned into separate areas requiring dif-
ferent temperatures and different air
changes for maximum control of heat
loss. Partitions and doors between spaces
with a temperature differential of more
than 6°C are insulated. Wherever possi-
ble, translucent fiber glass partitions are
used so that lighting from adjacent spaces
can be shared. Natural ventilation and air
flow are controlled by louvers and win-
dows. To minimize the exterior surface,
the building is built into a hillside with
little exposure to the north. Shrubs and
trees provide wind breaks. Reflection
from snow in front of the building and
from an earth mound to the west supple-
ment solar energy collection. The enclos-
ing structures face south to achieve maxi-
mum value from the sun's direct energy
through both passive and hydronic solar
energy collection devices. Insulated trans-
lucent fiber glass panels face south at a
60° tilt for passive collection of solar
energy. The panels cover 89 m2 and have
a light transmission factor of 45%anda U
factor of 0.29.
Flat-plate hydronic solar collector pan-
els covering 130 m2 are set at a 60° slope
from the south roof of the treatment
plant. The collector consists of an extrud-
ed aluminum plate and frame, copper
tubing to transport the collector fluid, and
two panes of low-iron-content tempered
glass. The backs of the collectors are
insulated with 114 mm of rigid polyiso-
cyanurate foam board insulation. An
ethylene glycol solution is pumped
through these panels and heated to 50°
to 60°C by the sun.
Although solar energy is used at Wilton
for space heating and domestic hot water,
its primary purpose is to provide heat for
the anaerobic digesters. The methane
gas produced in the digestion process can
be stored and used not only in the
methane boiler for heating purposes, but
to run the plant's emergency generator.
This application of solar energy attempts
to overcome two-of the main constraints
on its widespread acceptability—namely,
its traditional seasonal use and the
difficulty of storing solar energy. By using
solar energy to heat the digesters, the
solar equipment is used year-round,
significantly decreasing the payback per-
iod. By producing methane as part of the
anaerobic sludge digestion process, solar
energy is effectively stored in the form of
a compressible, combustible gas, which
is a much more efficient storage medium
than the usual hot air or water.
A sophisticated heating and ventilating
design had to be achieved to accomplish
the energy goals. Because the main
design component is solar energy gath-
ered by flat-plate collectors in a cold
climate, a basic constraint on the heating
system design was the use of relatively
low-temperature hot water (50°C) instead
of the conventional 90°C water. Energy
conservation practices become extremely
important when dealing with such low
temperatures. The thermal zoning of
rooms to allow for individual room tem-
perature control is very important. The
office, locker room, and laboratory, which
are clustered at the northeast end of the
building, can be maintained at 20°C for
the operator's comfort. The rest of the
areas in the plant will experience sea-
sonal and diurnal temperature fluctua-
tions, dropping as low as 7°C in winter
and perhaps approaching 30°C on warm,
sunny days in the summer. The operators
are normally not in these areas for any
extended period of time. Most of these
areas contain processes that are not
normally housed. The operator is very
important to the success of the energy
conservation effort since he must see to it
that temperature controls are properly
set, the doors are kept closed, and tem-
peratures are set back at night.
The heating system is designed for
cascading the heating loads. The digest-
ers can be heated with 49°C water,
building space heating can be accom-
plished with 38°C water, and ventilation
units use 32°C water. The water that
heats the digesters can in turn be used to
supply heat to the building's heating
system and ventilation units without
requiring supplemental heat between
steps. The system maintains the flexibility
of supplying heat directly to one specific
load as required.
Ventilation is a major problem in
energy-efficient heating design for waste-
water treatment plants, since many of the
process areas require many air changes.
To avoid throwing heat away, the plant
makeup is warmed by heat exchange
with the exhaust air before the exhaust
air is vented.
Solar energy is the prime source of
heat, both for the sludge digestion pro-
cess and the building. A secondary source
of heat is the methane gas produced in
the anaerobic digesters, and its availability
intimately depends on the success of
solar heating. Another source of heat is
an electric heat pump that uses the heat
energy available in the plant's effluent.
This heat pump is approximately three
times as efficient as electric resistance
heat, and its use in wastewater treatment
facilities can be very cost effective.
The interrelationships among the heat-
ing sources and heating loads are de-
picted m Figure 1. Heat from the solar
collectors can either be transmitted direct-
lytothe heat distribution system, or itcan
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be sent to the solar energy storage tank,
which is a 7.5-m3 water tank. Which
route it takes depends on the relative
temperatures of the solar collector loop
and the solar storage tank, and on the
needs of the system for heat.
Fueled by digester gas produced in the
anaerobic digesters, the methane boiler
comes on only when there is a call for
heat that cannot be satisfied either by
direct input of solar energy or by hot
water stored in the solar storage tank.
The heat pump operates only when solar
energy is unavailable (either directly or
indirectly) and when methane is unavail-
able.
Backing up the three primary heat
sources are some secondary sources that
can be significant. Methane is used as the
prime fuel for the plant's 55-kW electric
generator in addition to being used to fuel
the methane boiler. Propane is stored
onsite in case of prolonged power out-
ages when the supply of methane is
exhausted, but it is not normally used as a
fuel for the generator. When the gener-
ator is operating, it gives off a great deal of
heat and is cooled by water from the solar
storage tank, thus becoming a usable
source of heat.
The solar storage tank can be con-
sidered a heat source, since it is used for
short-term storage of excess energy that
is not directly usable. By using the
storage capacity, the operator does not
waste potentially usable energy from the
solar collectors or the electric generator.
Heat from the solar storage tank can
either be put into the heat distribution
system or go directly to heating domestic
hot water. The hot water heater can be
heated electrically, but such a measure is
unlikely to be required often.
Passive solar energy is provided by
translucent fiber glass panels for direct
heating and lighting. Finally, building
ventilation air is conditioned by exhaust
air through the use of a heat exchanger.
To accomplish the objectives of the
digester gas system, two levels of storage
are used. Short-duration storage is ac-
complished by compressing the gas to
0.138 MPa (absolute pressure), a condi-
tioning storage mode used to ensure
steady flow of methane for digester
mixing and to prevent too rapid cycling of
the high-pressure methane storage com-
pressor. Longer-duration storage is ac-
complished through the high-pressure
methane storage system, in which meth-
ane is compressed to 1.38 MPa (absolute
jressure). The boiler and generator are
supplied from the high-pressure gas
storage tank.
Energy Monitoring
Objective and Description
The objectives of the energy systems
monitoring program were to predict,
verify, and summarize the performance of
the building thermal systems on a totally
integrated basis that is not available
through predictive analysis of individual
components. These objectives are tied
very closely to the basic energy-conserv-
ing design of the plant. No subsystem has
a unique design by itself, but the inte-
grated subsystems pose complex prob-
lems in the interaction of process vari-
ables. Such problems are especially prev-
alent in the interaction among energy
sources (i.e., solar input, methane boiler,
and heat pump) and energy users (i.e.,
digesters, building heating, and ventila-
tion).
The integration of these subsystems is
unique, and the design was partly based
on data provided by manufacturers of
solar equipment, emergency generators,
boilers, and digesters. These manufac-
turers have detailed knowledge of their
equipment but limited knowledge, of the
total system. Monitoring of these sub-
systems was conducted to determine
how the components interface.
Results
The monitoring system was designed
and installed based on anticipated results.
Generally, the equipment had sufficient
sensitivity and range to provide valid
results. With the available data, each
major component of the heating system
was analyzed and evaluated for its energy
and cost effectiveness.
Active Solar System
The active solar system data collected
between June 1979 and April 1980 are
summarized as follows:
1. Recorded clear-day insolation lev-
els were consistently above Ameri-
can Society of Heating, Refrigera-
tion, and Air Conditioning Engi-
neers (ASHRAE) estimates with an
average difference of 13.8%.
2. The recorded percent of possible
sunshine was 60% versus the 52%
average predicted. This figure is
consistent with the unusually low
precipitation levels experienced.
3. The average incident solar radiation
was 37.3% above that estimated.
4. The total solar energy collected was
122 gigajoules (GJ), which was
64% of that estimated.
5. The overall solar system efficiency
(the net energy collected divided by
the total incident available) was
23%.
An overall efficiency of 23% is signifi-
cantly lower than that anticipated. A great
deal of effort was spent in investigating
the reasons, which were presumed to be
one or more of the following:
1. Data or instrumentation error
2. Collector heat loss factor:
a. Inadequate thermal insola-
tion
b. Possible convective losses be-
tween the absorber plate and
the rigid insulation
3. Collector heat transfer losses:
a. Air within the fluid loop
b. Effect of the glycol solution
4. Control sequencing and response
5. Collector response sensitivity
6. Collector efficiency losses because
of dirt accumulated during construc-
tion.
Though all of these factors but the first
certainly contribute to the solar system
performance, the overriding cause ap-
pears to be the combination of all of them
coupled with the lack of an accurate
calculation procedure to simulate this
interaction.
Obviously, significant differences exist
among instantaneous collector efficien-
cies. The latter are useful in comparing
various types of collectors under similar
steady-state conditions, but they tend to
create a misleading picture of the effi-
ciency of water heating systems operating
over long periods.
In the month of March, for example,
approximately one-third of the incident
radiation was of too low an intensity to
collect. In many cases, the collector plate
temperature never reached a usable or
threshold level, though considerable in-
solation was available. An illustration of
these losses in long-term efficiency ap-
pears in Figure 2 for a clear day. The
corresponding losses for a cloudy day are
obviously greater, particularly if the solar
radiation intensity equals the critical or
threshold intensity.
The dominant factor in establishing the
threshold intensity level is the temper-
ature difference between the absorber
plate and the ambient air. If this difference
is minimized by either a warmer climate
or a cooler collector fluid (as in the case of
a heat-pump assisted system) or both,
then this threshold level would be re-
duced and the long-term system efficien-
cy would be improved. For example, in the
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case of a heat pump and solar system
with an average collector fluid temper-
ature of 10°C and an ambient temper-
ature of 24°C, the threshold intensity
level would actually be negative, enabling
the system to collect heat within solar
radiation.
As indicated, the causes for the lower
overall efficiency were investigated based
on the assumption that the estimating
procedures were accurate. Numerous
alternative procedures were studied with
similar inflated results. In the course of
the investigation, several publications
were discovered that included test data
for actual solar systems monitored for a
1 -year period. The results of these studies
are similar to the findings at the Wilton
plant.
Passive Solar Systems
The passive solar array is built into the
wall at two levels. Solar radiation passes
through the array, providing both heating
and lighting in the clarifier room. Each
bank of panels has an overhang that
tends to shade them during the summer
months when heating is not required.
Operable windows cool the space when
the temperatures become excessive. The
energy collected during these times is
lost and thus is not considered useful.
Increasing the transmissivity and/or
decreasing the U factor would increase
the net quantity of collected solar radia-
tion. More would be provided as useful
during the winter, but more would have to
be dumped during the summer. For the
panels, varying the ratio of transmissivity
to U factor and estimating the net energy
collected would provide an indication of
what a change in that ratio would do to
the system. Other factors such as over-
hangs, the blocking effect of the internal
panel spacers, and variations in trans-
missivity with incident angle also need to
be considered.
For this installation, the variation in
overall performance appears adequate
with transmissivity increasing during the
winter and decreasing during the sum-
mer. The overhang is responsible for a
daily average decrease in transmissivity
of up to 14%.
Heat Pump
The heat pump operating time was
greater than anticipated during this per-
iod. The coefficient of performance (COP)
was a quite acceptable 2.9 during the
heating season. The generation of the
heating energy gave a net cost savings,
and the payback period was reasonable
(11.4 years). Had the heat pump operat-
ing time equaled the projected operating
time, the payback period would have been
closer to 25 years, which is still reason-
able.
The actual and projected hours of
operation are based on present operating
conditions. As the plant flow increases
and digester gas production increases,
the heat pump may be used less, with an
increasing payback period as a result of
less operating time.
To date, the operation and maintenance
problems encountered have been rela-
tively small, but considerable time has
been spent in cleaning the effluent
strainers. Records should be kept for
several years to determine realistic oper-
ating and maintenance costs for the
system.
Generator Heat Recovery
The generator heat recovery loop oper-
ated approximately one half-hour per
week during the generator exercise per-
iod. During part of that time, the recovery
loop was not used because the storage
tank temperature was warmer than al-
lowed by the maximum generator-cooling
heat exchanger. Town water was then
used to cool the generator. This condition
arises regularly during the summer,
when there is a limited heating demand
and the active solar system is able to
provide most of the heating required. The
generator could be exercised for longer
periods of time during the heating season,
when digester gas is available.
The generator heat recovery loop has
the highest energy output to input ratio,
but the payback period is the worst. The
reason is the low periods of use being
experienced. Increased usage would in-
crease cost effectiveness.
Digester Gas Generation
The digester gas generation system is
not cost effective with regard to recovery.
But process requirements must also be
considered when evaluating this system.
Reflected Solar Radiation
The attempt to measure the magnitude
of the ground-reflected solar radiation
component met with limited success. The
reflected component for peak nonclear
days was within the experimental ac-
curacy of the equipment being used. The
average daily reflected component con-
sists of both reflected and some diffuse
solar insolation. Thus no meaningful data
on the magnitude of the reflected com-
ponent were gathered. Part of the reason
wasthe minimal snow cover experienced
during the winter of 1979-80.
Electrical Usage
Even with the heat pump operating
more and the generator operating less
than anticipated, the total electrical usage
has been 12.5% less than projected.
Conclusions
The analysis of the monitoring data and
the engineering evaluation of the energy
systems at the Wilton, Maine, wastewater
treatment plant have led to the following
conclusions:
1. Heat recovery from wastewater
treatment plant effluent through
the use of a water-to-water heat
pump is cost effective under rela-
tively severe temperature condi-
tions. Operational problems can be
minimized by properly designing
the effluent sump from which the
heat pump draws to provide suf-
ficient capacity at minimum plant
effluent flows.
2. Heat recovery from ventilation air is
cost effective.
3. Heat recovery from the generator
cooling loop may be cost effective if
increased use of the generator is
warranted.
4. Passive solar heating is not cost
effective as analyzed for the Wilton
plant, but it can be made cost
effective with design modifications.
Passive solar heating of treatment
plant structures can be cost effec-
tive in occupied areas such as the
office and laboratory if good energy
conservation principles are followed
in the design of such areas. Passive
solar heating is less cost effective
when applied to process areas
exposed to water surfaces. In the
latter case, room thermostats must
be set at 5°C, a temperature that
can largely be maintained by the
water passing through these areas.
Combined with task heating (if
necessary) and proper energy con-
servation design, passive solar heat-
ing and lighting can be cost effec-
tive in process areas.
5. When solar system instantaneous
efficiencies are applied to clear-day
insolation levels and the mean
expected percent of sunshine, the
result may be an overly optimistic
evaluation of the actual long-term
performance. These losses are
lated to the system threshold
solation intensity and the random
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weather patterns. Thus accurate
estimate of long-term efficiences
would require computer simulation
using site-specific, averaged, hourly
weather data and system perform-
ance criteria. Moreover, since the
threshold intensity level is predomi-
nantly affected by the difference in
the average collector fluid and
ambient temperatures, the long-
term efficiency may be significantly
reduced in northern climates.
6. Collecting solar thermal energy to
produce supplemental heat for an-
aerobic sludge digesters is probably
not cost effective under currently
accepted economic projections, re-
gardless of the size or location of
the facility.
7. Instrumentation and controls should
be simplified as much as possible.
The theoretical advantages of inte-
grated energy systems can be offset
by complicated, trouble-prone instru-
mentation.
The full report ws submitted in fulfill-
ment of Contract No. 68-03-2587 by
Wright-Pierce Engineers and Architects
under the co-sponsorship of the U.S.
Environmental Protection Agency and
the U.S. Department of Energy.
*USGPO: 1984-759-102-10667
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DavidR. Fuller, Douglas A. Wilke. PatricL Thomas, and Anthony J. Lisa are with
Wright-Pierce Engineers and Architects, Topsham, ME 04086.
R. V. Villiers was the EPA Project Officer (see below).
The complete report, entitled "Monitoring Integrated Energy Systems at a
Wastewater Treatment Plant in Maine," (Order No. PB 84-197 292; Cost:
$14.50, 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:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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