Animal Waste, Water Quality and Human Health

238Animal Waste, Water Quality and Human Healthlog 10 and complete removal are recorded for VTAs, with lower/upper quartiles of0.53/0.81 log 10 (Figure 2). Often, VTAs are capable of absorbing all the runofffrom individual storms – that is, there is complete attenuation of microbial fluxduring certain events. For example, Mankin et al. (2006) report zero runoffduring 93% of individual runoff events from sites with a VTA:catchment arearatio (AR) of ≥0.5. The review identifies various factors, such AR, VTA slope,nature of vegetation cover and flow characteristics, as being critical in affectingattenuation rates – for example, Komor and Hansen (2003) report greater FCattenuation values over winter when the surface is covered by a mat of wiltedgrass (as compared to tall growing grass in summer). Various designrecommendations are made (e.g. use of a pre-treatment sedimentation basin tominimise solid accumulation at the front end of VTAs, thereby minimisingvegetation damage and the likelihood of channelized flow; and the design ofinlets and headlands to initiate and maintain sheet-flow), and these have beenformalised in a guidance document on VTAs (USDA 2006).Constructed farm wetlands (CFWs) (see: Table 6.2, Section C). Constructedwetlands (CWs) are widely used as a means of municipal wastewater treatment(e.g. Kivaisi 2001, Song 2005, Vymazal 2006) and represent a sustainable,low-energy/maintenance form of treatment, particularly suited to rural areas(Kadlec & Knight 1996). Essentially, their design spectrum ranges from freewater surface (FWS) systems that are either free of, or have differentproportions of, emergent macrophytes (typically reeds) to subsurface flow (SSF)systems which act as a percolating filter, with wastewater being appliedintermittently and seeping down through, or horizontally past, the plantrhizosphere and supporting media, such as gravel and sand (Oliver 2007).Components range from a simple pond to an engineered horizontal-flowreedbed. In practice, many multiple pond/wetland systems combine elements ofthis spectrum into a cascading system. However, there are few “design rules” orengineering loading calculations available for treating effluent parameters otherthan BOD. Observed reductions in pathogen concentrations are thought to beeffected by a combination of predation (e.g. by protozoa and crustaceans, such ascopepods (Song, 2008), die-off through UV disinfection and sedimentation, withHRT being an important factor affecting attenuation (e.g. Hodgson, Perkins et al.2004, Vymazal 2005). Edwards et al. (2005) reviewed the efficacy of CWs inremoving FIOs from municipal and domestic wastewaters. While attenuationrates are generally very good, they note that concentrations of FIOs in CWeffluents often periodically exceed acceptable levels, especially at times of highflow when HRT is reduced.CFWs, and analogous vegetative treatment systems (VTSs – for feedlot runoff),are used to treat flows of faeces-contaminated surface water before they are