Black Water Treatment and Recycling
Sustainable strategy to reduce water consumption.
Approximately 25 percent of in-building residential water use is for toilet flushing. The resulting wastewater is often referred to as blackwater, in order to contrast it with "greywater" (representing 60 percent of in-building residential water use) that is comprised of shower/tub, lavatory, laundry, and kitchen sink drainage. The greywater, in turn, is often fractionated into "light" and "dark" components, reflecting the degree of contaminant loading, with the latter being kitchen sink drainage. The perception is that light greywater is inherently less contaminated and safer to handle than mixed wastewater. This is not the case, as many studies have shown that greywater contains largely the same contaminants as mixed wastewater, including microorganisms that can cause disease. while the treatment technologies used for treating mixed wastewater and greywater are identical, treating light greywater alone can be problematic due to insufficient micro-nutrients and other issues.
Many technologies exist that are capable of treating wastewater to a water quality standard that is safe to handle and use for a wide variety of non-potable water applications (i.e. applications that do not involve ingesting the water). That being said, there are communities, such as Singapore, who treat their wastewater and then blend it with surface water for potable use. The present challenges aren't as much technology as the regulatory and management frameworks that are required for the ongoing monitoring, operation and maintenance of the treatment systems. As a consequence, the majority of water reuse applications are either at a large community-wide scale, where water is treated at a large central facility and returned to the community in a pipeline for non-potable use, or it is treated at a household single-family residence scale where the risks and consequences of system failure are localized.
The establishment of parcel-level in-building water reuse systems, and cluster developments, is growing. Aside from becoming a necessity in areas with diminishing water supplies, there are a large number of social and economic benefits for water reuse applications that go beyond the saving the cost of a volume of water. Taking a commercial office building for example, approximately 85 percent of the water use in the building could be satisfied by using non-potable water. Eliminating 85 percent of the water supply and wastewater treatment and disposal demands reduces the municpal costs associated with those services, as well as freeing up capacity within the existing urban infrastructure for future growth and enabling development densities to be increased with the same infrastructure. The reduced water demands and effluent dicharges also greatly benefit the environment. Faced with having to treat their own wastewater, there is more awareness of what is being disposed of to drains and down toilets, causing individual habits to change. Most of the chemicals that are discharged to sewer are not treated, but pass through to the environment. Making people aware of the consequences of what they dispose to sewer will change habits and control the contaminants at the source.