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Improving the Efficacy of Wastewater-Polishing Reed Beds

Thaddeus K. Graczyk

20 pages
IWA Publishing
Available as an eBook only.

Microbiological Contaminants in Treated Wastewater Demand for high quality drinking and recreational waters rises exponentially due to global demographic growth in the human population, reinforcing an urgent need for microbiologically safe reclaimed waters (Graczyk and Lucy, 2007).  

Wastewater discharges are worldwide risk factors for the introduction of human pathogens into surface waters used as drinking and recreational resources. Cryptosporidium parvum, Giardia duodenalis, and human-virulent microsporidia, (i.e., Encephalitozoon intestinalis, E. hellem, E. cuniculi, and Enterocytozoon bieneusi) are waterborne enteropathogens inflicting considerable morbidity in healthy people and mortality (e.g., Cryptosporidium and microspora) in immunodeficient individuals (Savioli, et al., 2006; Weber and Bryan, 1994).  Their transmissive stages, i.e., oocysts, cysts, and spores, respectively, are resistant to environmental stressors and are therefore long-lasting and relatively ubiquitous in the environment (Graczyk, et al., 1997; Matchis, et al., 2005; Wolfe,1992). These pathogens are category B biodefense agents on the NIH list. Microsporidian spores are on Contaminant Candidate List of the U.S. EPA (Nwachcuku and Gerba, 2004) because spore identification, removal, and inactivation in drinking water are technologically challenging. Surface water is not routinely monitored for these pathogens, despite evidence demonstrating environmental contamination derived from wastewater discharges (Graczyk and Lucy, 2007). Environmentally, all these pathogens have a broad zoonotic reservoir (Graczyk et al., 1997; Matchis et al., 2005; Savioli, et al., 2006). 

Constructed wetlands of either vertical or horizontal flow are increasingly used worldwide for secondary or tertiary treatment of municipal wastewater due to minimum electric requirements and low maintenance costs (Davidson et al., 2005; Reinoso et al., 2008). The wetland concept has become an attractive wastewater treatment alternative to conventional tertiary treatment processes for: a) municipal wastewater; b) on-site domestic wastewater treatment; and c) concentrated animal feeding operations (CAFO) (Karpiscak et al., 2001). In wetlands, human-pathogenic microorganisms are physically removed and biodegraded by sedimentation (Dai and Boll, 2006; Karim et al., 2004), filtration and evapotranspiration-driven attachment to plant roots (Gerba et al., 1999; Dorsch and Veal, 2001; Weaver et al., 2003), natural die-off (Nokes et al., 2003), UV radiation, straining and sorption by the biofilm (Quinonez-Diaz et al., 2001), and protozoan predation (Stott et al., 2001). It is thought that performance of wetlands in removing human pathogens is superior to that of secondary wastewater treatment, i.e., conventional sewage sludge activation (Ulrich et al., 2005). Horizontal wetlands usually discharge to surface waters that are frequently used for recreation or drinking water production (Davidson et al., 2005). 

In general, wastewater can be injected under the wetland surface for plug flow hydraulics (Weaver et al., 2003), or be delivered to the wetland surface for free-surface flow.  Because the wastewater resides in wetlands for certain time, these areas can act as endemic sites supporting both propagation and transmission of human zoonotic pathogens (Graczyk et al., 2007). Sizing reed-bed systems for a residence time of 5 days has become a standard practice (Davidson et al., 2005; Quinonez-Diaz et al., 2001; Thurston et al., 2001), leaving plenty of time for propagation and spreading of wastewater-derived pathogens in wetland habitats via a wide variety of wildlife (Graczyk and Lucy, 2007; Graczyk et al., 2007). In addition, any temporal or permanent malfunctioning caused by clogged inlet pipes can cause hydraulic short circuits that bypass part of the filtration area in wetlands.  

The purposes of the project were to: a) determine species of microbiological contaminants entering, residing, and leaving constructed horizontal wetlands used for tertiary treatment of municipal wastewater; and b) determine removal efficacy of Cryptosporidium oocysts, G. duodenalis cysts, and human-virulent microsporidian spore species by wetlands from secondary-treated wastewater.