We live in a "flush without fear" society, in which most of us just flush away and presume that "they" will take care of it somewhere downstream. Of course, for most of us, our downstream is someone else's upstream! Nevertheless, for the most part, this is a reasonable presumption, since modern sewage treatment systems have been perfected over the past 100 years, and flush toilets have been around since Thomas Crapper first popularized them in London in the 19th century.104 However, a basic principle of sustainable design is that there is no "away" place to dispose of waste, since everything is interconnected.
Urban sanitation systems, with secondary and even tertiary sewage treatment, still burden many rivers and coastal waters with excessive amounts of sewage sludge and industrial waste, even in modern societies. In older cities, combined sewer overflows during rainy periods still pollute local rivers. Therefore, green building designers are increasingly considering treating sewage onsite, rather than sending it offsite. Engineers are developing new ways to treat sewage in confined spaces, such as building basements, using both aerobic and anaerobic digestion processes as well as a final "polish" with ultraviolet radiation to kill any remaining microorganisms.
In one project the savings from not paying fees to connect a project to the local sewer system paid for the capital cost of having a third-party firm design and construct a system to serve 1,000 people per day in a 16-story building. The same firm will operate the system and charge the project the same amount it would have paid to the local sewer utility. At no cost to itself, the project will recycle five million gallons per year of sewage for use in flushing toilets and irrigation.105
Other systems use land disposal by filtering waste through constructed wetlands or through treatment in a series of ponds or tanks in more confined spaces such as greenhouses, a system first pioneered in the 1970s by John and Nancy Todd of the New Alchemy Institute. In a "living system" greenhouse, the nutrients in sewage feed algae and water plants that in turn nourish fish. The final effluent is quite clean and can be reused in various ways. Using treated wastewater is nothing new. Many cities use reclaimed wastewater for irrigating golf courses and other spaces such as median strips and parks. Properly applied, it is a beneficial way to save water and reuse the nutrients present in wastewater.
For some green building projects, there may be a significant financial benefit from onsite sewage treatment, namely, that it helps reduce water use dramatically and often saves considerable money upfront that would be paid for a municipal sewer connection. It can also reduce utility costs, since water supply charges in many places also include significant charges for sewage treatment. These savings can be enough to pay for the cost of constructing and maintaining an onsite sewage treatment system.
And, of course, why not use treated sewage for flushing toilets, since "black water" is about 98.5% water anyway? That helps close the water cycle, an essential element in a truly sustainable building. Other uses for treated sewage in a building can include washing of roadways and sidewalks, landscape irrigation and cooling tower "makeup" water (to replace water lost by evaporation and back-flushing).
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