**3.2 Horizontal sub-surface flow constructed wetlands**

Horizontal sub-surface flow constructed wetland (HSF CW) systems are filled with gravel or soil and usually planted with common reeds [54]. The depth of the substrate varies between 30 and 80 cm and it is most often selected based on many parameters such as low patterns, effluent charge and aquatic plant types as a nutrient source required for removal process [55]. Generally, the media mostly used include gravel, sand, soil and compost. In HSF CWs, the wastewater is fed in at the inlet and flows horizontally along bed media below substrate surface through its pores and plant roots. The treated wastewater is collected in outlet zone from the opposite side of the system before leaving level control arrangement at the outlet, thus keeping a large contact between water and substrate (**Figure 2**). Consequently, the availability

**Figure 2.** *Horizontal sub-surface flow constructed wetlands (HSF CW).*

of oxygen in the substrate for organic matter oxidation is limited despite the creation of some aerobic zones by plants by transportation of oxygen from the atmosphere to the roots through the plant stems. HSF CWs included many processes and mechanisms for removal of contaminant elements and compounds from sewage such as sedimentation, filtration, and aerobic and anaerobic microbial processes for organic matter remove, nitrification/denitrification for nitrogen elements remove, and sorption and precipitation for phosphorus remove.

Organic compounds in HSF CWs are very effectively degraded by aerobic and anaerobic microbial processes as well as by filtration and sedimentation. The aerobic process is carried out in plant roots and surface waters where oxygen can be supplied from atmosphere, while anaerobic process is occurred in the soils with microorganisms adapted to each condition. The aerobic microorganisms consume oxygen to degrade organic contaminants to produce biomass for microorganism. The methane is produced through the degradation of organic matter by anaerobic bacteria. The presence of plant roots favors the development of biofilm, which increase the organic matter decomposition. The amount decomposed of organics is related dramatically to the availability of dissolved oxygen in HSF CWs. On the other hand, plant biomass are capable to storage organic carbon thus making constructed wetlands as an alternative approach to remove organic matter. Suspended solids are removed in HSF CWs by gravity sedimentation, flocculation/settling of colloidal particulates and adsorption. Gravity sedimentation occurred when particle sediment independently without contact with other particles. While, flocculation/settling involves the interacting of particles in order to form large flocculants result from charge imbalances on the surface of particles. The settling rapidly take place when a new particle as larger flocs are formed. Filtration and adsorption onto gravel and plant media play an important part in suspended solids removal. The surface of gravel, stem, and root plants are coated by biomass film, which can absorb colloidal and soluble matter. The solids adsorbed may be metabolized, and then converted to biomass or gases.

As viewed in the literature for nitrogen removal, regardless of removal pathways including plant uptake, ammonification, plant root and substrate adsorption, volatilization in forms of ammonia, fixation by converting nitrogen gas to organic nitrogen, and transformation into nitrogen gas, the nitrification-denitrification is usually considered as the principal mechanism to reduce nitrogen amount in constructed wetlands. Nitrification is carried out by converting ammonia and ammonium to nitrite and then nitrite to nitrate. Many parameters influence nitrification process such as temperature, pH, alkalinity, dissolved oxygen concentration, retention time, and organic load. According to Vymazal et al. [55], the optimum temperature for nitrification is generally considered from 30°C to 40 °C, pH values range from 6.6 to 8.8, and dissolved oxygen and alkalinity amounts reach 4.3 mg of oxygen and 8.8 mg/mg of ammonia oxidized respectively. Denitrification process is the process in which microorganisms reduce nitrate to nitrite and nitrite to nitrogen gas. The nitrogen gas produced is in the form of nitric oxide (NO), nitrous oxide (N2O) or nitrogen gas (N2). Environmental conditions that affect the efficiency of denitrification include nitrate concentration, anoxic conditions, presence of organic matter as well as pH, temperature, alkalinity and the effects of trace metals. Denitrification rate decreases as dissolved oxygen increases and can occur between 5 and 30°C, and under optimum pH values between 7.0 and 8.5. In HSF CWs the denitrification process is favorable due to availability of carbon source contained in raw sewage. However, the nitrification process is limited by the absence of oxygen in these systems. The alimentation process of HSF CWs maintain filtration bed submerged continually by water,

#### *Constructed Wetlands Process for Treating Sewage to Improve the Quantitative… DOI: http://dx.doi.org/10.5772/intechopen.110630*

consequently the presence of oxygen is limited and, therefore, the removal of nitrogen is less effective. The major phosphorus removal mechanisms in wastewater wetlands are physical/chemical (adsorption, absorption, precipitation) and biological processes. The main mean for phosphorus removal in constructed wetland is plant absorption. The absorption is carried out through leaves and root plants, and is increased at the beginning of the plants growing season. Many aquatic plants were showed their efficiency in the storage of phosphorus such as *Iris pseudacorus*, *Panicum virgatum*, *Canna sp*., *Oenanthe javanica*, *Myriophyllum aquaticum*, etc. [56, 57]. However, the die of plant parts and the beginning of their decay generates the release of plant matter above ground in the water and the secretion of decaying roots into the soil and, therefore, the contamination of groundwater. Adsorption and precipitation is another process to removal phosphorus amounts. Different cations present in the substrate such as Fe3+, Ca2+, Al3+ can be combined with orthophosphate and forme insoluble phosphates. In addition, phosphates charged negatively can react by anions-cations exchange with substrate elements. Various substrates such as steel slag and oyster shell approved their efficiency to removal phosphorus from sewage [58]. Although the removal of phosphorus can takes place by microorganisms absorption due to their availability and quick multiplication, microorganisms are unable to storage a large contents of phosphorus.

Several studies applied HSF Cws as a promote technology to treat industrial wastewater. Chapple et al. [59] study focused on the reduction of dissolved hydrocarbons (Diesel Range Organics-DRO- C10-C40) from oil refinery using four pilot wetlands. Two filled with soil and others filled with gravel (300 m<sup>2</sup> each), and planted with *P. australis*. The results showed that with the mean inflow DRO concentration of 410 mg/L, all beds presented an efficiency removal, which exceed 99%. Choudhary et al. [60] studied 5.25 m2 HSF CW filled with gravel and planted with *Canna indica* to treat chlorinated resin and fatty acids (RFAs) from a paper mill wastewater. The system achieved 92% removal of 9,10,12,13-tetrachlorostearic acid and 96% removal of 9,10-dichlorostearic acid. The authors concluded that at hydraulic retention time of 5.9 days, the most probable mechanism for this removal is microbial decomposition in the plant roots as well as adsorbtion/absorption. To treat a primary treated sewage, Shukla et al. [61] study three HF CWs (35 m<sup>2</sup> each) filled sequentially and supplied with gravel media. CW1 (unplanted), CW2 (planted with *Typha latifolia*) and CW3 (planted with *Typha latifolia* and *Commelina benghalensis*). The CWs were aerated and operated in continuous mode at an average hydraulic loading rate of 250 L/h with different hydraulic retention time (HRT) 12, 24, 36 and 48 h. The authors reported that among the constructed wetlands used, CW3 was the best performer reducing 79, 77, 79, 79 and 78% of BOD, COD, N-NO3, N-NH4 and phosphate respectively in 48 HRT.

#### **3.3 Vertical sub-surface flow constructed wetlands**

Vertical sub-surface flow constructed wetlands (VSF CWs) are a flat bed filled by graded gravel topped with sand layers and planted with macrophytes usually is common reeds. Wastewater is poured onto the bed surface from above. The water is draining vertically down by gravity through the porous media to the bottom of the bed where is collected by a drainage pipe (**Figure 3**). With this mode of operation, the bed inside is aerated by pushing out the trapped air, thus increasing aeration. On the other hand, the aeration may be enhanced by insertion of aeration pipes, and employing a wet-dry cycle of operation. This way of feeding plays an important part

**Figure 3.** *Vertical sub-surface flow constructed wetlands (VSF CW).*

in the provide of aerobic conditions, since increase of oxygen transfer within the filter enhance the oxidation of ammonia nitrogen by nitrification and decomposition of organic contaminants. The crucial difference between HSF CWs and VSF CWs is not only the direction of the flow path, but also the availability of the oxygen in the bed.

In VSF CWs, the mechanisms removal of pollutants follow the same approach applied in HSF CWs. The main chemical-physical and biological processes mentioned are filtration, sedimentation, sorption, chemical oxidation, evaporation as well as aerobic/anaerobic degradation, plants adsorption, phytodegradation and phytoevaporation. In addition, due to better aeration of the bed, VSF CWS are very effective in the removal of organic contaminants (COD, BOD5) and ammonia nitrogen. Concerning phosphorus, the removal rate remains limited. For this purpose, different filter materials were investigated in order to improve the efficiency of phosphorus removal such as fragmented Moleanos limestone, bauxite, and zeolite, a mixture of river sand and dolomite and wollastonite [62, 63].

VSF CWS are applied for treatment of sewage from different sources, namely, olive mill wastewater [17, 64], laundry wastewater [65], aquaculture wastewater [66], textile wastewater [67] and olive pomace leachate [68].

Achak et al. [17] showed the potential application of experimental system composed of sand filter and VSF CW to achieve nutrient and COD removal from olive mill wastewater. VSF CW consists of a tank (1 m<sup>3</sup> ) filled with gravel and soil, and planted with a mixture of aquatic plants (*Phragmites australis*,*Typha latifolia* and *Arundo donax*). The presence of aquatic plants was more efficient in removing of nutrients and organic load. The average elimination of experimental system in terms of flow was 62.48% for NTK, 90.43% for NH4 + , 77.25% for NO3 , 98.51% for PO4 <sup>3</sup>, 97.53% for PT and 99.05% for COD. The same wastewater was treated by Herouvim et al. [64] using three identical series with four pilot scale VSF CWs filled with various porous media such as gravel, sand and cobble, with several sizes. Two series of pilot scale units were planted with common red and the third is as control unit (unplanted). The authors concluded that COD, phenol and TKN removal seems to be significantly higher in the planted series, while orthophosphate removal shows no significant differences among the three series. The purpose of the study reported by Sotiropoulou et al. [65] aims to indicate the significant improvement in the overall quality of laundry wastewater due to the use of the VSF CW. The VSF unit had a length of 64 cm and a diameter of 20 cm, filled with sand/gravel of different gradations, and planted

#### *Constructed Wetlands Process for Treating Sewage to Improve the Quantitative… DOI: http://dx.doi.org/10.5772/intechopen.110630*

with *Zantedeschia aethiopica*. Results showed an important decrease of microfibers concentration, COD and turbidity of 98, 93 and 94% respectively from laundry wastewater when a hydraulic load retention of 63.7 mm/d was applied. The treatment of aquaculture wastewater by a system composed of an intermittent sand and VF CWs with a total surface area of 9.5 m<sup>2</sup> was studied by Behrends et al. [66]. At 5.5 days of hydraulic retention time, the inflow concentrations of BOD5, COD, TN, ammonium and TP of 771 mg/L, 3609 mg/L, 67.4 mg/L, 0.22 mg/L and 40.5 mg/L respectively were amounted to rate removal of 99.5% for BOD5, 99.1% for COD, 95.5% for TN, 85.9% for ammonia and 84.4% for TP. Davies et al. [67] used VF CWs to remove an azo dye acid orange (AO7) from textile wastewater. VF CWs planted with *Phragmites australis* received an organic loading rates varied between 21 and 105 g COD/m2 d and reduced to 11–67 g COD/m2 d. The rates removal of COD, total oxygen carbon and AO7 amounted to 64%, 71% and 74%, respectively. The authors concluded that *Phragmites australis* not simply had a principal role in AO7 removal but also can degrade aromatic amines released during AO7 degradation. The combination of electrochemical oxidation for 360 min at 20 A and VSF CWs planted with *Phragmites australis* at retention time of 3 days and hydraulic organic loading between 5 and 15 g COD/m d was studied by Grafias et al. [68] for treating olive pomace leachate fed intermittently at organic loadings between 5 and 15 g COD/m d and a residence time of 3 d. The treatment by VF CWs followed by electrochemical oxidation enhance rates removal from 86 to 95% for COD and from 77 to 94% for color. However, the reverse approach yielded only 81% COD and 58% color removals.

### **4. Conclusion and recommendations**

Constructed wetland is a natural wastewater system that have been proven sustainable, eco-friendly and low-cost technology compared with many other wastewater treatment technologies. Constructed wetlands consist on aquatic plants, soils, bed media, water, and microorganisms and encompass many processes for eliminating various wastewater pollutants such as biological process (microbial oxidation), physical process (sedimentation, precipitation, adsorption, filtration, and absorption) as well as chemical process (ion exchange and chelation). Many different designs and structures of constructed wetlands are experimented to remove the maximum contaminants from different wastewater sources including FWS CWs, HSF CWs and VSF CWs. Each system as it has many advantages it also has disadvantages. FWS CWs are characterized by low operating costs, can be built with locally available materials, do not use chemical elements for treatment process and provide a high reduction of BOD and suspended solids. However, FWS CWs require expertise for system construction and a large land area for setup; also, they are not tolerant to cold climates. Although HSF CWs necessity low operation and maintenance cost, and allow a high reduction of BOD and suspended solids, but require large land area, allow a low remove of nitrogen and phosphorus (absence of oxygen), and can present a clogging risk. Contrary to FWS CWs and HSF CWs, VSF CWs are a high oxygen transfer capacity, require less land area, report a good nitrification (presence of oxygen) and present less clogging. Nevertheless, VSF CWs require frequent maintenance and pre-treatment of wastewater to prevent clogging.

The increase of water treatment efficiency is mainly based on the optimization of all operation parameters factors affecting processes application such as vegetation,

#### *Sewage Management*

wastewater sources, substrate media, feeding mode, and hydraulic loading rate (HLR) and hydraulic retention time (HRT).


*Constructed Wetlands Process for Treating Sewage to Improve the Quantitative… DOI: http://dx.doi.org/10.5772/intechopen.110630*
