Hybrid Constructed Wetland

Constructed wetlands are secondary treatment facilities for household (blackwater or greywater, in some cases also brownwater) and/or biodegredable municipal or industrial wastewater. Constructed wetlands are a treatment step of DEWATS systems and they can even be used as a tertiary treatment system for polishing after activated sludge or trickling filter plants (HOFFMANN et al. 2010). The plants grown in the wetland may be the used for composting or biogas production (see also composting small scale, composting large scale or anaerobic digestion). Effluents, if they correspond to the WHO guidelines (see also WHO 2006): Guidelines for the safe use of wastewater excreta and greywater Volume I, Volume II, Volume III and Volume IV) may be used for fertigation.

Basically, there are three different types of constructed wetlands (CWs). They are classified according to the water flow regime as:

• Free surface constructed wetlands
• Horizontal flow constructed wetlands
• Vertical flow constructed wetlands

These three types of CWs may be combined with each other in hybrid constructed wetlands in order to exploit the specific advantages of the different systems.

One of the main advantages of CWs is that they are natural systems and thus not require chemicals, energy or high-tech infrastructure. Moreover, they are suited to be combined with aquaculture or sustainable agriculture (irrigation).

Almost every type of constructed wetlands can be combined to fit different types of wastewater. However, hybrid systems are comprised most frequently of vertical flow and horizontal flow systems arranged in a staged manner. Horizontal flow systems cannot provide nitrification because of their limited oxygen transfer capacity. Vertical flow systems, on the other hand, do provide good conditions for nitrification but denitrification does not really occur in these systems. In hybrid systems (also sometimes called combined constructed wetland) the advantages of the horizontal and vertical flow systems can be combined to complement processes in each system to produce an effluent low in BOD, which is fully nitrified and partly denitrified and hence has much lower total-N outflow concentrations (VYMAZAL 2005). At present, hybrid constructed wetlands are in operation in many countries around the world and they are used especially when removal of ammonia-N and total-N is required (VYMAZAL 2010).

Many of these hybrid systems are based on the original hybrid systems developed by Seidel at the Max Planck Institute (Germany). The design consists of two stages of several parallel vertical flow beds (usually planted with Phragmites australis), followed by two or three horizontal beds (planted e.g. Typha or Carex). In these systems, the vertical flow beds are loaded with pre-treated wastewater for 1–2 days, and then allowed to dry out for 4–8 days. The thin crust of solids that forms on top of the vertical flow beds is mineralised during the rest period (VYMAZAL and KRÖPFELOVÀ 2011).

In the mid-1990s, Johansen and Brix introduced a horizontal-vertical flow hybrid system with a large horizontal flow bed placed first and a small vertical flow bed as the second stage. In this system nitrification takes place in the vertical flow stage at the end of the process sequence. If nitrate removal is needed, it is necessary to pump the effluent back to the front end of the system where denitrification can take place in the less aerobic horizontal flow bed using the raw feed as a source of carbon needed for denitrification (VYMAZAL and KRÖPFELOVÀ 2011).

The two-stage horizontal-vertical-flow constructed wetland system consist of three basic units (BRIX and JOHANSON 1999):

1. A mechanical pre-treatment step, usually a two or three-chamber settler, for removal of large particles and settleable materials.
2. A horizontal subsurface flow sand or gravel-based constructed reed bed for TSS and BOD removal (and denitrification if recirculation is applied).
3. An intermitted-loaded vertical flow constructed wetland for nitrification.

Hybrid systems are more expensive than non-hybrid systems (free surface-flow constructed wetlands, horizontal and vertical flow constructed wetlands). A larger area is required and they are more complicated and complex to operate, especially for adjustment and monitoring of the loads (HOFFMANN et al. 2010). However, compared to other intensive high-rate aerobic treatment options (e.g. activated sludge), constructed wetlands are natural systems, which work extensively. That means treatment may require more land and time, but you can safe costs because there is no or just a low energy need. Furthermore, there is no need for sophisticated equipment, expensive spare parts or chemicals (GAUSS 2008).

Basically, O&M is similar to horizontal flow constructed wetlands or vertical flow constructed wetlands. The difficulty is to ensure that the system is adjusted and the filters loaded correctly. That means it needs expert knowledge and skilled operators. It should be considered that a wetland system will require maintenance for the entire duration of its life.

The risk of mosquito breeding in warm countries is low as long as there is no open water surface (e.g. free surface constructed wetlands). It should be ensured that residents do not come in contact with the sludge/wastewater in the pre-treatment facility nor with the influent of the filter because of the risk if infection (TILLEY et al. 2008)

The effluents can generally meet the standards for pathogen levels for safe discharge to the environment without further treatment. In case of domestic wastewater, the situation could be different and for safety reasons disinfection (by tertiary treatment) might be necessary, depending on the intended reuse application (HOFFMANN et al. 2010).

The biggest health risk arises from settled wastewater in the pre-treatment facility; this should be considered during inspections and emptying. A proper emptying process (human powered or motorised) can decrease the health risks (TILLEY et al. 2008). After that, also sludge must be treated correctly, for example in drying beds or composting facilities.