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
3

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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
                                   4

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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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