Greywater is the domestic wastewater from all sources except the toilets or sewer. As per the U.S. EPA wastewater sources can include water from:
- Washing machines
- Kitchen sinks
Figure 1 – Residential Greywater System
Although both greywater and blackwater are types of wastewater, the primary difference between greywater and blackwater is that blackwater includes urine and fecal matter and is thus considered sewage water. Black water is extremely difficult to filter at home to obtain clean water for reuse purposes as this sewage water contains harmful bacteria and disease-causing pathogens, since greywater does not contain fecal matter, it can be filtered and cleaned for reuse in garden or irrigation purposes. Due to the low levels of contaminating pathogens and nitrogen, reuse and recycling of greywater is receiving more and more attention. Greywater recycling is an economical method to help consumers reuse wastewater generated at home and thus facilitate water conservations.
This episode provides an overview to Residential Greywater Treatment Technologies for beneficial reuse thus demonstrating its significance as a sustainable solution to the emerging water crisis in UAE. It is the second part of a series titled “Residential Greywater Series” by Sani Water. Sani Water has been at the forefront of providing their customers with the latest products and advanced water treatment technologies available in the drinking water industry. With “Residential Greywater Series”, Sani Water aims to educate and inform their customers about the fundamentals of residential greywater, its environmental reuse benefits, and some of the most popular methods of residential greywater reuse available in the market today.
IMPORTANCE OF GREYWATER TREATMENT
The reclaimed or treated greywater should fulfill four primary criteria – hygienic safety, aesthetics, environmental tolerance, and economic feasibility. Although greywater is not odorous immediately after discharge, prolonged storage of greywater in tanks will result in development of anaerobic conditions. Once it reaches the septic state, greywater forms sludge that either sinks or floats depending on its gas content and density. Septic greywater can be as foul smelling as any sewage and will also contain anaerobic bacteria, some of which could include human pathogens. Consequently, the key to successful greywater treatment lies in its immediate processing and reuse before it reaches the anaerobic state. The simplest, most appropriate treatment consists of directly introducing freshly generated greywater. There are four reasons why greywater may need to be treated for the following purposes:
- Removal of substances that may be harmful to human health.
- Removal of substances that may be harmful to plants and soil.
- Removal of substances that may be harmful to the environment.
- Removal of substances that may clog the irrigation system.
TYPES OF TREATMENT TECHNOLOGIES
Greywater reuse treatment technologies can range from low-cost methods, such as primary treatment methods that coarsely screen oils/grease and solids from the greywater before irrigation, to more expensive secondary treatment systems that treat and disinfect the greywater to a high standard before using it for irrigation spray systems, or for toilet flushing. The choice of system depends on a number of factors including:
- The owners’ willingness to operate and maintain the facility;
- The source of greywater to be recycled;
- The purpose of the greywater reuse (whether for subsurface irrigation or sprinkler irrigation or for toilet flushing or waterfalls).
There are two main types of treatment systems – Primary Diversion Systems and Secondary Treatment Systems.
Primary Diversion Systems: Primary diversion methods use coarse screen filters or sedimentation to remove oils/grease and solids prior to discharge to the land application areas. These systems are likely to be considered the most economically attractive for greywater use because maintenance can usually be carried out by the homeowner, and they generally do not rely heavily on electricity or chemicals to operate. These include the gravity diversion system and the pump diversion system.
Figure 2. Gravity System with potable tanks
Secondary Treatment Systems: Secondary treatment systems further treat the greywater to remove more of the oils/ grease, solids and organic material. This allows secondary treated greywater to be irrigated via micro-drip or surface irrigation methods. These systems are generally more expensive due to the initial establishment costs associated with the continuing treatment needs and ongoing maintenance costs. However, the treatment level enables a much more conventional surface irrigation system and presents less of a health risk in case of human contact. There are many types of secondary treatment systems, including: slow sand filter, activated sludge, constructed wetland, trickling filter and rotating biological contactor.
Figure 3. Greywater collection, treatment and reuse for toilet flushing and outdoors
- Li, F., Wichmann, K., & Otterpohl, R. (2009). Review of the technological approaches for grey water treatment and reuses. Science of the total environment, 407(11), 3439-3449.
- World Health Organization. (2006). Overview of greywater management health considerations (No. WHO-EM/CEH/125/E).