Removal of Microorganisms from Domestic Greywater Using a Recycling Vertical Flow Constructed Wetland (RVFCW)

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ABSTRACT
The reuse of greywater (GW) is an attractive addition to water management options. One of the major concerns limiting GW reuse is the possible presence of harmful microorganisms. In addition to total coliform organisms, enteric organisms (Salmonella, Shigella and poliovirus Type 1 Cryptosporidium, Giardia) and opportunistic pathogens ( Staphylococcus aureus and Pseudomonas aeruginosa) may be present in GW. Recently, researchers at Ben Gurion University have developed a new small-scale, decentralized greywater treatment system, the Recycling Vertical Flow Constructed Wetland. The population dynamics of E. coli, Staph. aureus and Ps. aeruginosa in three different synthetic GWs formulated to reflect the variable composition of domestic GW were examined in this treatment system. E. coli, Staph. aureus, and Ps. spp. were present in samples from all of the systems at the beginning of each treatment cycle. Control studies indicated that the concentration of E. coli changed by less than 2% in untreated samples of GW over 3 days. In the RVFCW, E. coli was reduced to concentrations below detection (<1 CFU (100ml)-1 ) at the end of the treatment cycle. (2-3 d) and therefore, consistently resulted in E. coli levels that meet the USEPA standards for water reuse (USEPA, 2004) Removal of Staph aureus varied depending on the formulation of GW while there was no significant change in the number of Ps. spp. in any of the GW formulations. Thus, both of these opportunistic pathogens could be found in treated effluent in the absence of E. coli. These results underscore the need for extensive microbiological monitoring of greywater systems as well as the need for the development of greywater specific standards.

INTRODUCTION
With shrinking global freshwater resources, there is a need to shift the fundamental paradigm of water management to water reuse. Interest in the reuse of freshwater for non-potable uses is not limited to countries with dry climates such as those in the Mediterranean region, the Middle East, and Northern Africa. European countries with high population densities such as Belgium, France, UK, and Germany have all increased their use of recycled water (Lazarova et al. 2003). Household greywater production is estimated to produce daily per capita volumes as high as 90 – 140 L (Carr et al. 2004). Thus, GW reuse represents the largest potential source of water savings in domestic residence (Eriksson, Auffarth, Henze, and Ledin, 2002).

Greywater (GW) is usually defined as all of the wastewater produced in a household except toilet wastes (blackwater). Typically, this includes wastes from bathroom sinks, baths, and showers and may also include wastes from laundry facilities and dishwashers. Some definitions include wastes from kitchen sinks although there is no consensus (Queensland Government, 2003). Traditionally, in centralized wastewater systems, greywater and blackwater from domestic sources are treated as a single combined waste stream. In areas with centralized wastewater treatment, GW is simply added to domestic sewage, treated at a central facility, and then discharged at a site removed from where is was generated. Similarly, in areas with traditional decentralized systems such as on lot septic systems, greywater is composited with domestic sewage into a single waste stream. Greywater, however, differs significantly from blackwater.
While GW can have an organic load similar in concentration to blackwater the composition of the organic fraction is different (Jefferson, Judd, and Diaper, 2001). A larger fraction of the organic load in GW is poorly biodegradable (soaps, greases, etc.) giving GW a higer COD:BOD ratio than domestic sewage (COD:BOD ratio as high as 4:1) (Sayers, 1998; Brandes, 1978; Christova-Boal , 1996). GW has a significantly lower concentration of inorganic nutrients (N and P) which can limit biological treatment of this material (Jefferson, et al., 2001). Finally, both the quantity of GW generated per day and the composition of the GW fluctuates greatly depending on the geographical location, demographics and level of occupancy of the household (Jefferson, Judd, and Diaper, 2001; Olsson, Karlgren, and Tullander, 1968). Greywater composition has been found to vary with the number and age of residents, frequency of bathing and laundry, and, in cases where kitchen waste is included as a component of greywater, the eating habits of those living in the household.

The bulk of the small systems being proposed for the treatment of greywater fall into one of three categories – physical, biological, or natural systems(Jefferson, Judd, and Diaper, 2001; Jefferson, Laine, Persons, Stevenson, andd Judd, 1999) Physical treatment systems usually involve some type of coarse filtration followed by disinfection of the filtrate (Costner, 1990). BOD removal between 60 and 80% and turbidity removal between 30 and 99% (NTU reduction) have been reported for physical systems. Biological treatment systems include rotating biological contactors and membrane bioreactors. These systems are not usually capable of treating GW to an acceptable level for reuse without the addition of a physical system to retain the biomass. While these combined systems can produce an excellent quality effluent, they are complex systems requiring routine maintenance and tend to be too expensive for use by a single household. Natural treatment systems which depend on soil infiltration and/or nutrient uptake by plants to remove contaminants. These systems have been reported to have excellent BOD removal (95 – 100%). Removal of bacteria (coliforms) using natural systems in highly variable ranging from 2 to 6 log reductions (Fittschen and Niemcynowicz, 1997; Gauther , 2000).

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