24 Modified and Combined Systems • High-solids treatment wetlands with frequently cleaned forebays • Operating wetlands in a load-and-rest operating regime to address bed clogging or improve treatment efficiency There are situations where these wetland modifications make sense from a process design and economic standpoint, as discussed in this chapter. However, the system will invariably require more operator attention, and there is often a reduction in system reliability. For instance, if a treatment wetland requires an air blower to achieve regulatory compliance, system reliability is essentially governed by the mechanical blower, not the wetland ecosystem. The same is true for different operating. | 24 Modified and Combined Systems Treatment wetlands can be designed to be au natural or complex. Examples of designs along a full spectrum from almost zero fossil-fuel input to highly engineered and managed systems can be found around the world. In this book we term these design and operation strategies as Type A and Type B wetlands. Type A systems are at the most natural low-energy end of the scale. Type B wetlands are higher-energy and more highly engineered systems. There are good reasons to slide on this scale of design and management complexity depending upon project goals. In this chapter we describe some of the rationale in selecting the appropriate level of management intensity. Type A treatment wetlands clearly cost less to operate than Type B systems. Type A systems may also cost less to construct even though they typically occupy a larger footprint area than Type B systems. They are the preferred solution when a number of factors are not limiting Relatively inexpensive land area is abundant. Land is relatively level. There is insufficient treatment potential with a Type A approach. There is insignificant potential for unauthorized human interaction with the wastewater in the wetland. Habitat and aesthetic benefits outweigh the potential for nuisance conditions related to mosquitoes or other wildlife. Type A wetlands are typically constructed free water surface FWS marshes and relatively low-tech constant-flow horizontal subsurface flow HSSF wetlands. The Type B treatment wetland-management strategy is preferred when one or more of the factors in the preceding list are limiting the design. Specific examples of Type B treatment wetlands include HSSF wetlands augmented with reactive media intended for phosphorus or metals removal Addition of aerators for treatment improvement Operation of vertical flow VF wetlands in a fill-and-drain mode for improvement of nitrification and denitrification Combinations of VF HSSF or FWS wetlands to achieve specific process
