Page 70 - North American Clean Energy July August 2015

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City Sewage Energy Recovery

by Erwin Schwartz, B.Eng.

MUNICIPAL WASTEWATER HEAT RECOVERY is a system in which a heat exchanger transfers
Ø Nom. Required Min low #
Total Volume min
Recovered heat energy from the sewer waste water to a solution of water and antifreeze (ethylene
hose sectional average required Residences
capacity recommended energy
glycol). A heat pump then uses the heat of this solution to heat buildings, preheat domestic
area. low (USGPM) *. of tanks (USGal)
per year hot water, and heat garages in the winter and also cool the space in the summer.
(ft2 ) (USGPM) served pumps (GJ)

to heat
Challenges
(tonnes)
he irst challenge of wastewater heat recovery concerns minimizing the energy loss
18 ''
1.77
1000
500
500
150
5000
9704
when wastewater sewage is carried across long distances. Heat exchangers and heat
24 ''
3.14
1778
889
889
267
8889
17252
pumps are normally located in the building and can be as far as 100 meters (328 ft) from
30 "
4.91
2778
1389
1389
417
13889
26957
the municipal sewer pipe. Also, these sewer pipes are normally buried 6 meters (20ft)
3'
7.07
4000
2000
2000
600
20000
38818
below street-level. To maximize eiciency, the pumping system should have very speciic
4'
12.57
7111
3556
3556
1067
35556
69009
characteristics:
5'
19.63
11111
5556
5556
1667
55556
107827
• Able to aspirate air until wastewater lows;
6'
28.27
16000
8000
8000
2400
80000
155270
7'
38.48
21778
10889
10889
3267
108889
211340
• Self-priming, as the sewage pipe is lower and often at a distance;
8'
50.27
28444
14222
14222
4276
142222
276036
• Capable of pumping viscous wastewater with dirt and particles; and
• Able to break down the mixture of air and wastewater, and to eliminate the air.
Table 1. Results of the analyses

Because of these issues, a self-priming, positive displacement pump, which aspirates a
mixture of water and air is used. Air reduces the eiciency of any exchanger. Pumps can be
added with automatic vents and loater valves which ventilate and later eliminate the air. and 3 million kWh per year. he savings potential, again, depends on the sewage low rate,
One problem with this set-up is a faulty loat valve may discharge sewage into areas next to amount of hot water, and the associated heating needs. In a metropolis area, the potential

the building.
for energy saving is endless.
hese systems often use a 4" supply pipe connected to the 18" municipal sewage pipe, Municipal wastewater heat recovery systems have excellent potential. Data on sewage
and a second challenge is to determine the ideal location to connect this supply line to the lows in Baie D’urfe is provided by the city. Despite low changes, the amount of heat
18" sewage pipe. If connected atop the 18" sewage pipe, too much air is drawn into the available for recovery would be enough to heat the domestic hot water in a large number of

supply line making ventilation more diicult. If it is connected to the bottom of the 18" residences.
sewage pipe, too much mud and dirt is drawn in which clutters the pump and calciies the he number of homes served, the corresponding heat pumps’ heat recovery capacities,
exchanger. hrough rigorous testing, the ideal place was found. he information on these and the annual energy recovered (according to sanitary pipe diameters) are presented in
tests is kept conidential but if the city sewage pipe is a clock, and the bottom of the pipe is Table 1. A residence represents an average single-family home with two children and two

1800 hrs, the overall range of ideal connection is from 1530 hrs and 1730 hrs.
adults and typical proile of domestic hot water consumption was based on he American
his concept works, and as the wastewater is pumped, the heat exchanger doesn’t Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) data. A
become scaled up or blocked and the system provides suicient heat to charge the transient thermodynamic analysis was made for each of the situations with pipe sizing
reservoir. It is necessary for the supply pumps to operate for ten minutes at startup and and difering bitrates. he required average low is suiciently abundant to provide heat

allow the unit to reach a steady state with the heat exchanger at 2500 kg steel mass.
at optimal temperatures and allows the pump to be switched of when low is below the
A third problem is the irregular low of wastewater. Wastewater low depends on the required minimum.
activities of the occupants in the buildings on the street and the frequency in which they he data provided by the cities of Montreal and Baie D’urfe suggests the low is
use their plumbing ixtures obviously varies from hour-to-hour. he recovery system must abundant throughout, and a 150-ton heat pump can serve 500 residences. Because average

be well designed and adjusted to compensate for this. he system shuts of when there is low suiciently exceeds the low required, this excess heat recovered can be stored. Storage
not enough low of city sewage or the heating is not required. It is controlled via a local tanks with recommended volumes are also listed in Table 1. Appropriately, in residential
control panel with preset parameters or remotely with an iPhone from anywhere in the areas, sewer low rates increase during morning and evening with increased domestic water

world.
consumption by the occupants making energy available when it is required.
Municipal sewage pipes are often reference-combined storm and sewage drainage. When Current residential systems include a 10 ton heat pump and a heat exchanger, which
it rains, storm water low adds to the volume of wastewater making it more stable and serves 4 showers and a garage heater. he recovery system has two reservoirs with a
increasing the potential for energy recovery. For northern areas, snow melt in the spring combined volume of 120 gallons. his system allows the heat pump to maintain the hot

also contributes.
water tanks at all times.
A inal challenge which afects the performance of these systems is the irregularity in As storm lows were not included in the data provided, the actual values were higher
the use of domestic hot water. he heat recovered from domestic hot water should be used than reported, and beneit the energy recovery system. A transient thermodynamic
immediately, because the capacity of reservoirs to store heat it is limited. In northern analysis with time steps and the heat pump requirements was completed for a residential

climates, this heat can be used immediately in the winter by heating garage spaces, building with 10 condominiums, a high rise with 500 hotel rooms, and oice spaces with
especially when the garage door is opened.
4 showers and fan coil in small garage. his study shows there is great potential for these
he variability of both of these heating loads, and at the rate sewer low must be systems and with the support of the Quebec Government and the City Sewage Authorities,
taken into account when designing the recovery system, as there is a large potential for the irst-ever, direct city sewage energy recovery system was demonstrated successfully.

energy savings. Wastewater lows from 140 and 240 GPM have been measured, and if
the low averages 7-8 hours per day, with continuous heat consumption, it is estimated Erwin Schwartz B.Eng. is the President at DDI Heat Exchangers
the savings could be upwards of 1 million kWh per year. In larger areas with 150 and 300
GPM low rates maintained at all hours of the day, the annual savings can be between 2
DDI Heat Exchangers | www.ddi-heat-exchangers.com | www.energyrecoverysolution.com

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