It seems, however that in systems w

It seems, however that in systems with free water surface fixation may take place intensively via fixation by blue-green algae (cyanobacteria).
Plant uptake is the major removal mechanism in constructed wetlands with free-floating macrophytes. The potential of emergent plants is quite low especially in constructed wetlands for the treatment of municipal or domestic sewage. It seems, however, that in tropical regions where seasonal translocations are minimal, and multiple harvest is possible, harvesting of emergent plants could play a significant removal route especially for lightly loaded systems.
Ammonium adsorption is limited to constructed wetlands with sub-surface flow where the contact between substrate and wastewater is efficient. In addition, substrates used for constructed wetlands usually do not provide large amounts of sorption sites. The clayey soils which are most effective in ammonia sorption are usually not used for constructed wetlands at present.
Organic nitrogen burial as removal mechanisms is restricted to constructed wetlands with emergent veg- etation and free water surface where the peat/litter layer plays an important role in the removal of nutrients.
The removal of total nitrogen in constructed wetlands with free-floating plants, free water surface CWs with emergent vegetation and sub-surface flow CWs is summarized in Table 3. The removal efficiency is similar in all systems with a slightly higher removal found for FFP CWs as a result of multiple harvesting. Removal of total nitrogen in studied types of constructed wetlands varied between 40 and 50% with removed load ranging between 250 and 630 g N m− 2 yr− 1 depending on CWs type and inflow loading. It is also important to note that SSF systems have much higher inflow concentrations due to the fact that these systems are commonly used as secondary treatment stage while FFP and FWS systems are commonly used as tertiary stage. Therefore, outflow TN concentrations are higher in SSF systems. Also, inflow TN concentrations in VSSF CWs are higher than in HSSF because HSSF wetlands very frequently treat diluted wastewaters from combined sewer systems. For
loadings, the same pattern has been found. When com- paring inflow loading of constructed wetlands (Table 3) and aboveground standing stocks for emergent macrophytes (Section 2.1.7), it is obvious that the amount of nitrogen removed via harvesting is quite low and usually does not exceed 10% of the inflow load for secondary treatment systems. When inflow loading is low (cca b 100–200 g N m− 2 yr− 1) as in the case of tertiary
It is also important to take into account the fact that standing stock is limited and does not increase with increasing loading rate in constructed wetlands for wastewater treatment (Fig. 3).
VSSF CWs remove more ammonia-N than FWS and HSSF wetlands due to the high oxygenation of VSSFbeds (Table 4). On the other hand, the potential to removenitrate concentration in the outflow is substantial. Both FWS and HSSF wetlands remove nitrate. The results presented in Tables 3 and 4 clearly indicate that single- stage constructed wetlands are not able to remove substantial amounts of total nitrogen unless it is achieved at the expense of a large treatment area and, therefore, hybrid systems may be a better solution when total nitrogen is the main target value.

2.3. Removal of nitrogen in hybrid constructed wetlands

Various types of constructed wetlands may be combined in order to achieve higher treatment effect, especially for nitrogen. However, hybrid systems com- prise most frequently VSSF (VF) and HSSF (HF) sys- tems arranged in a staged manner (Fig. 4). However, there has been a growing interest in achieving fully nitrified effluents. HF systems cannot do this because of their limited oxygen transfer capacity. VF systems, on the other hand do provide a good conditions for ni- trification but no denitrification occurs in these systems. Therefore, there has been a growing interest in hybrid systems (also sometimes called combined systems). In combined systems, the advantages of the HF and VF systems can be combined to complement each other. It is possible to produce an effluent low in BOD, which is fully nitrified and partly denitrified and hence has a much lower total-N concentrations (Cooper, 1999).