City Sewage Energy Recovery

Municipal wastewater heat recovery is a system in which a heat exchanger transfers heat energy from the sewer waste water to a solution of water and antifreeze (ethylene glycol). A heat pump then uses the heat of this solution to heat buildings, preheat domestic hot water, and heat garages in the winter and also cool the space in the summer.

Challenges

The first challenge of wastewater heat recovery concerns minimizing the energy loss when wastewater sewage is carried across long distances. Heat exchangers and heat pumps are normally located in the building and can be as far as 100 meters (328 ft) from the municipal sewer pipe. Also, these sewer pipes are normally buried 6 meters (20ft) below street-level. To maximize efficiency, the pumping system should have very specific characteristics:

Able to aspirate air until wastewater flows;
Self-priming, as the sewage pipe is lower and often at a distance;
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.

Because of these issues, a self-priming, positive displacement pump, which aspirates a mixture of water and air is used. Air reduces the efficiency of any exchanger. Pumps can be added with automatic vents and floater valves which ventilate and later eliminate the air. One problem with this set-up is a faulty float valve may discharge sewage into areas next to the building.

These systems often use a 4" supply pipe connected to the 18” municipal sewage pipe, and a second challenge is to determine the ideal location to connect this supply line to the 18” sewage pipe. If connected atop the 18’' sewage pipe, too much air is drawn into the supply line making ventilation more difficult. If it is connected to the bottom of the 18" sewage pipe, too much mud and dirt is drawn in which clutters the pump and calcifies the exchanger. Through rigorous testing, the ideal place was found. The information on these tests is kept confidential but if the city sewage pipe is a clock, and the bottom of the pipe is 1800 hrs, the overall range of ideal connection is from 1530 hrs and 1730 hrs.

This concept works, and as the wastewater is pumped, the heat exchanger does not become scaled up or blocked and the system provides sufficient heat to charge the reservoir. It is necessary for the supply pumps to operate for ten minutes at startup and allow the unit to reach a steady state with the heat exchanger at 2500 kg steel mass.

A third problem is the irregular flow of wastewater. Wastewater flow depends on the activities of the occupants in the buildings on the street and the frequency in which they use their plumbing fixtures obviously varies from hour-to-hour. The recovery system must be well designed and adjusted to compensate for this. The system shuts off when there is not enough flow of city sewage or the heating is not required. It is controlled via a local control panel with preset parameters or remotely with an iPhone from anywhere in the world.

Municipal sewage pipes are often reference-combined storm and sewage drainage. When it rains, storm water flow adds to the volume of wastewater making it more stable and increasing the potential for energy recovery. For northern areas, snow melt in the spring also contributes.

A final challenge which affects the performance of these systems is the irregularity in the use of domestic hot water. The heat recovered from domestic hot water should be used immediately, because the capacity of reservoirs to store heat it is limited. In northern climates, this heat can be used immediately in the winter by heating garage spaces, especially when the garage door is opened.

The variability of both of these heating loads, and at the rate sewer flow must be taken into account when designing the recovery system, as there is a large potential for energy savings. Wastewater flows from 140 and 240 GPM have been measured, and if the flow averages 7-8 hours per day, with continuous heat consumption, it is estimated the savings could be upwards of 1 million kWh per year. In larger areas with 150 and 300 GPM flow rates maintained at all hours of the day, the annual savings can be between 2 and 3 million kWh per year. The savings potential, again, depends on the sewage flow rate, amount of hot water, and the associated heating needs. In a metropolis area, the potential for energy saving is endless.

Municipal wastewater heat recovery systems have excellent potential. Data on sewage flows in Baie D’urfe is provided by the city. Despite flow changes, the amount of heat available for recovery would be enough to heat the domestic hot water in a large number of residences.

The number of homes served, the corresponding heat pumps’ heat recovery capacities, and the annual energy recovered (according to sanitary pipe diameters) are presented in Table 1. A residence represents an average single-family home with two children and two adults and typical profile of domestic hot water consumption was based on The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) data. A transient thermodynamic analysis was made for each of the situations with pipe sizing and differing bitrates. The required average flow is sufficiently abundant to provide heat at optimal temperatures and allows the pump to be switched off when flow is below the required minimum.

Ø hose	Nom. sectional area. (ft² )	Required average flow (USGPM)	Min flow required (USGPM)	# Residences *. served	Total capacity of pumps to heat (tonnes)	Volume min recommended tanks (USGal)	Recovered energy per year (GJ)
18 ''	1.77	1000	500	500	150	5000	9704
24 ''	3.14	1778	889	889	267	8889	17252
30 "	4.91	2778	1389	1389	417	13889	26957
3'	7.07	4000	2000	2000	600	20000	38818
4'	12.57	7111	3556	3556	1067	35556	69009
5'	19.63	11111	5556	5556	1667	55556	107827
6'	28.27	16000	8000	8000	2400	80000	155270
7'	38.48	21778	10889	10889	3267	108889	211340
8'	50.27	28444	14222	14222	4276	142222	276036

Table 1. Results of the analyses

The data provided by the cities of Montreal and Baie D’urfe suggests the flow is abundant throughout, and a 150-ton heat pump can serve 500 residences. Because average flow sufficiently exceeds the flow required, this excess heat recovered can be stored. Storage tanks with recommended volumes are also listed in Table 1. Appropriately, in residential areas, sewer flow rates increase during morning and evening with increased domestic water consumption by the occupants making energy available when it is required.

Current residential systems include a 10 ton heat pump and a heat exchanger, which serves 4 showers and a garage heater. The recovery system has two reservoirs with a combined volume of 120 gallons. This system allows the heat pump to maintain the hot water tanks at all times.

As storm flows were not included in the data provided, the actual values were higher than reported, and benefit the energy recovery system. A transient thermodynamic analysis with time steps and the heat pump requirements was completed for a residential building with 10 condominiums, a high rise with 500 hotel rooms, and office spaces with 4 showers and fan coil in small garage. This study shows there is great potential for these systems and with the support of the Quebec Government and the City Sewage Authorities, the first-ever, direct city sewage energy recovery system was demonstrated successfully.

Erwin Schwartz B.Eng. is the President at DDI Heat Exchangers

DDI Heat Exchangers | www.ddi-heat-exchangers.com | www.energyrecoverysolution.com

Volume: July/August 2015