Many operators of office buildings, hospitals, hotels, large condos, and other buildings are aware of the need to save money and reduce their environmental impact, through better water management. The result has been the growth of ways to reduce water consumption, ranging from low-flow showerheads to landscaping that requires min- imal watering.
Building operators are also aware of a similar need to reduce energy costs, and here too there has been a growth in development and installation of cost-management measures. Saving energy has its environmental benefits as well, such as reducing greenhouse gas emissions.
However, organizations may be missing out on the opportunities that are literally flowing away from them, through the heat energy avail- able to them through the water that they dispose of.
Wastewater (or greywater) may have been heated through hot-water tanks, dishwashers, laundry facilities, industrial processes or simply from passing through a heated building. The heat in this water is a potentially valuable resource. Rather than being used, however, this heat is often wasted as water that is sent into the municipal sewer treatment system.
New developments in heat-recovery systems
Several trends are forcing building operators to take a closer look at greywater heat as a resource:
• Volatility of energy costs, and the need to protect the organization’s financial interests from sudden energy cost increases, by using alter- native sources of energy — such as heat that would otherwise be wasted.
• The need to manage greenhouse gas emissions, through reduced use of fossil fuels — and the interests of many companies in gaining LEED certification for their buildings. • Corporate leadership is looking more closely at energy manage- ment, partly through applying the international energy management standard ISO 50001.
At the same time, we see improvements in greywater heat recovery systems, that help building operators recover heat from the greywater produced by their buildings. Some of these improvements lie in:
• Smaller physical footprint, with less need for large spaces for changing out tubes after they become fouled. • Greater reliability, for more trouble-free operation. • Better information systems that can quantify the savings gained through greywater heat recovery.
For example, consider a relatively small hotel, with 100 rooms, that has its own laundry facility and restaurant. Documented evidence indicates that a well-designed heat recovery system, installed for the hotel’s greywater, would save approximately $232,000 over a period of 15 years.
For a larger building such as a large residential condominium tower or 300-room hotel, the savings during this time period would likely be over 500,000 dollars over that timeframe. This can be done through steps such as having the building’s greywater circulated next to the incoming cold water (such as from the municipal system) before it goes to the building’s central boiler.
Recovery of heat energy in greywater is particularly important in water-intensive industries, such as pulp and paper plants, dairies and breweries. However, heat recovery is also important in sectors that process sludges, including municipal wastewater treatment facilities (WWTFs). Many industrial processes also produce sludges, that may contain considerable heat because of industrial processes, and this heat represents a valuable resource that should not be wasted.
Another reason for increased use of heat recovery exchangers is due to recent energy cost increases and the introduction of the Pasteurization and the Thermophilic Anaerobic Thermophilic Process as part of sludge digestion in municipal WWTPs.
Heat exchangers can also be used to support cooling — which would include reducing the load on air conditioning systems, as well as helping cool computing equipment such as servers, as well as indus- trial applications.
Heat exchangers can also be used to recover energy (heating and cooling) from the huge flow of municipal sewage that flows below the street near the building.
Three approaches to system design
There are three main configurations for heat exchange systems.
: as the name implies, this technology includes one tube inserted inside another, with the hot liquid (or sludge) in the inner tube, so heat gets transferred to the cooler incoming liquid in the outer tube. This design is not very efficient due to the low heat transfer compared to the other technologies, and only low of solids could be used and only part of the heat transfer surface is in contact with the greywater when the flow does not cover all the inside pipe circumference. Another disadvantage of this design is that double- walled tubing is required for the inner tube, in some jurisdictions, to reduce danger of cross-contamination, and the air space between the two walls inhibits heat transfer.
: the hot liquid runs through a series of narrow spirals. This design requires small gaps of perhaps an inch for the tubes, lim- iting the amount of solids and sludges that can be pumped through, to avoid risk of plugging.
: This design uses rectangular channels, with hot and
How To Tap The Energy Savings
In Your Greywater
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