**1. Introduction**

Many parts of the world suffer of water scarcity and insufficient water quality demanded by population and environment. Water stress effects can vary dramatically from one region to another; it can damage not only public health, economic development, and global commerce, but also can increase migrations and spark conflict.

According to Food and Agriculture Organization (FAO), in the case of demand side, around 70% of the world's freshwater is used for agriculture, 19% for industrial uses and 11% for domestic uses [1]. In the case of supply side, the water sources include groundwater and surface water such as rivers lakes and reservoirs. Indeed, many causes can trace to the water stress including the increase of human withdrawals from surface or groundwater and the increased need of agricultural irrigation. The population growth who lead to economic development, change of lifestyle and consumption patterns, and the increase of agriculture production impact dramatically the planet's reserves, also the climate disruption as rising temperatures, floods and drought who can decrease water supply. Another important approach of water scarcity is identified by discharge of polluted wastewater generated from industry, agriculture and household activities causing deterioration of freshwater quality and infiltration of contaminant compounds in the groundwater [2, 3]. Around 80% of wastewater in the world is evacuated, largely untreated, in the lakes, rivers, oceans and environment [4]. In face of this challenge, many countries seek an alternative to protect water resources and to improve international collaboration on water practices, as well as to implement innovative and sustainable technologies in order to ameliorate the water quality by suggesting significant processes to reduce hazardous organic pollutants from polluted wastewater. Several approaches have been investigated to eliminate the contaminants from domestic sewage using physical, chemical and biological processes. Physical processes include evaporation, distillation, combustion, centrifugation, filtration, pyrolysis and membrane separation [5–7]. While chemical processes include coagulation-flocculation, electrocoagulation, oxidation and advanced oxidation and ion exchange [8–10]. Infiltration-percolation, aerobic, anaerobic and constructed wetland are included in biological processes [11–13].

The limitations of physical and chemical processes like expensiveness, high power consumption and production of toxic products, as well as the risk of secondary pollution, can affect the performance of these techniques for the treatment of sewage [14, 15]. Thus, biological processes are widely considered among the efficient techniques and have become the center of interest of several researchers in the pollutants removal of sewage. Nowadays, constructed wetlands have been attracting increasing attention compared to other treatment techniques. Constructed wetlands are a costeffective and sustainable approach to treat not only sewage but also agriculture and industrial wastewater as olive mill wastewater, tannery wastewater, winery wastewater and petrochemical industry wastewater [16–19]. Constructed wetlands system offers many benefits, such as less expensive to construct, low operational and maintenance expenses, tolerate fluctuation in water flow, eliminate odors associated with wastewater, allow a high process stability during wastewater treatment and facilitate wastewater reuse and recycling.

Constructed wetlands are a natural wastewater treatment system that use natural wetland vegetation (aquatic plants), substrate (sand, soil, gravel, zeolite, pozzolan) and microorganisms for the removal of conventional pollution parameters such as metals, organic matter and nutrients (nitrogen and phosphorus) from polluted water [17, 20–23]. Aquatic plants are considered as a central of sewage treatment and they play a major role in the adsorbion, precipitation and degradation of organic compounds, as well as concentration of heavy metal from contaminated water. While substrates are also beneficial to the efficiency of the constructed wetlands, they provide storage for many contaminants by the adsorption process and they report support to many microorganisms responsible to degradation of organic matter in the favorable conditions by biological process. On the other hand, different designs and

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

structures of constructed wetlands are investigated in the literature. Overall, we can distinguish three types of constructed wetlands (i) surface flow constructed wetlands (SF CWs), (ii) horizontal subsurface flow constructed wetlands (HSF CWs) and (iii) vertical flow constructed wetlands (VF CWs). The selection of the effective constructed wetlands depends on several parameters such as load and type of pollutants, material of substrates, local available vegetation, climatic conditions, nature of polluted water, and hydraulic loading rate (HLR) and hydraulic retention time (HRT).

In this chapter, the treatment of sewage using constructed wetlands as approach to improve the quantitative and qualitative management of groundwater resources will be discussed. For this purpose, selection of favorable constructed wetlands design, optimization of operation parameters and factors affecting remediation processes such as substrates, aquatic plants type and availability of microorganisms can improve the performance of sewage treatment. Finally, the recommendations and future possibilities in this field will also be discussed.

#### **2. Characteristics and composition of sewage**

Sewage is classified mainly into two categories, namely, domestic and industrial wastewater. Domestic wastewater is derived from human activities in households such as bath, laundry, dishwashing, garbage disposal and human waste, mainly feces and urine [24]. The variation in the characteristics of domestic wastewater depends on several factors such as the water use, quality and type of water supply, nature and condition of sewerage system and population habits. Compared to industrial wastewater, domestic wastewater usually contains low content of pollutants but even small amounts of contaminants can be responsible of environment pollution. Industrial wastewater is generated from various industrial processes, namely, the water released from battery, chemical and pharmaceutical manufacturing, agricultural and mine activities, paper and fiber plants, refining and petrochemical operations and other industrial activities [25]. The volume, flow and load of wastewater is closely linked to the type of industries and industrial establishment.

Generally, the characteristics and composition of sewage depend on the nature of wastewater discharged and its source. Sewage contains a high content of organic compounds, which may be in dissolved, suspension and colloidal state. These compounds may be toxic, resist to biological degradation, can damage sewers and other structures, and affect the condition operation of the wastewater treatment plant. Sewage also may contains some heavy metals provided through industrial discharges. These compounds limit the treatment by biological process and their disposal in stream and land affect the human and aquatic life. On the other hand, various types of microorganisms may be identified in sewage. Some of these are pathogens and are harmful to the human and animal life.

Many heavy metals are identified in sewage, such as Lead (Pb), Zinc (Zn), Mercury (Hg), Nickel (Ni), Cadmium (Cd), Copper (Cu), Chromium (Cr) and Arsenic (As) released by paint and dye manufacturing, textile, pharmaceutical, paper and fine chemical industries. These metals are non-biodegradable and can be carcinogenic, their easily adsorption to suspended particles in water threat dramatically ecosystems and their infiltration in the groundwater can present a risk for living organisms. Solids contained in sewage may be classified into dissolved solids, suspended solids and colloidal solids. Suspended solids and dissolved colored material and reduce water clarity by creating an opaque, hazy or muddy appearance. Their remove can be



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

assured by sedimentation if their size comparatively large or by filtration process. Generally, solids are an important factor for sewage treatment processes. Sewage also contains high load of organic pollutants, namely, phenolic compounds, dioxins and dibenzofurans, polycyclic aromatic hydrocarbons, and organochlorine pesticides [26]. Organic pollutants are highly resistant to environmental degradation via physical, chemical and biological processes. They have long half-lives in soil, water, and air and their toxicity can pose a threat to human health and environment. High content of nitrogen and phosphorus are also one of the major contaminants present in sewage. The principal nitrogen elements in sewage are proteins, amines, amino acids, and urea. Indeed, the ammonia nitrogen in sewage results from the bio-degradation of organic matter present in the water. In domestic wastewater, human urine contains approximately more than 95% of total nitrogen, 90% of total phosphorus and 50% of the total COD [27]. Phosphorus is contributing to sewage from laundry detergent, human waste and other household cleaning products with 0.30, 0.60 and 0.10 kg of phosphorus per capita/year, respectively [28]. It considered as an essential element for the biological method. With an adequate concentration, phosphorus can support aerobic biological wastewater treatment. **Table 1** summarizes the concentration of major pollutants resulting from different industrial sector.

### **3. Constructed wetlands as sewage management pattern**

#### **3.1 Free water surface constructed wetlands**

Free water surface constructed wetlands (FWS CWs) are defined as wetland systems comprise shallow basins or channels, with a sealed bottom to prevent wastewater to infiltrate in the groundwater. FWS CWs are characterized by the presence of dense aquatic plants covering more than 50% of surface, emerged in 20–40 cm of water into 20–30 cm of rooting soil [42]. In FWS CWs, water flows through over vegetated soil surface from an inlet to an outlet point (**Figure 1**). This operation allows the physical, chemical and biological processes to take place in order to remove and degrade the various contaminants. However, in some cases, standing water may increase the possibility of mosquito breeding, or water may completely lost by evapotranspiration especially in the hot region, which affect the treatment process.

Based on the literature, FWS CWs are efficient in removal of organic pollutants through bio-degradation and settling of colloidal particles. Suspended solids removal is assured by sedimentation, filtration and aggregation mechanisms in function of particles size and structure [43]. The smaller and lighter particles may settle out through the dense wetland plants, while largest and heaviest particles may settle out in

**Figure 1.** *Free water surface constructed wetlands (FWS CWs).*

the inlet open water zone. Indeed, the wetland plant tissue play a major role in suspended solids removal by reducing wind speed which supports sedimentation of suspended solids and prevents re-suspension [44]. On the other hand, many studies indicated that the high suspended solid concentrations can cause clogging of the soil and effects negatively the treatment process. To avoid the excess accumulation of solids, FWS CWs facilities should be coupled with a pretreatment stage, namely septic tank, lagoons, settling basins or compost filter [45].

Nitrogen is generally removed from wastewater through nitrification/denitrification mechanisms. Before nitrification step, organic-nitrogen is converted to ammonia by hydrolysis process, ammonia is oxidized to nitrate by nitrifying bacteria under aerobic conditions. Nitrate is reduced to free nitrogen or nitrous oxide under anaerobic conditions by denitrification process. Many parameters may influence nitrification-denitrification processes such as carbon source, temperature, pH, dissolved oxygen availability and nitrite accumulation. In CWs, nitrogen is effectively removed primarily by nitrification/denitrification, and ammonia volatilization under higher pH values due to algal grow. However, the presence of the ammonia in the atmosphere can pollute aquatic and terrestrial environments [46]. Indeed, in FWS CWs, the growth of algae is very limited, due to the presence of emergent wetland plants, which cover completely the surface water and as consequence limit algal photosynthesis by preventing light to penetrate into the water column. Denitrification can be increased by availability of carbon provided from decaying plant biomass. The supply of dissolved oxygen in FWS CWs is assured by the air-water interface and the plant roots, which release oxygen into the environment media, creating the favorable conditions for nitrification process. While in the soil layer below the water, the dissolved oxygen is practically non-existent. A viable solution to this behavior is to report an extended aeration to achieve nitrification process and provide biological treatment for the removal of bio-degradation pollutants. Air may be supplied by diffusion or mechanical in required conditions to maintain the aerobic biological process.

FWS CWs are efficient in removal of organic pollutants trough aerobic and anaerobic processes. The aerobic process is carried out under redox conditions in the water columns, while anaerobic process is realized by fermentation or biomethanation in the litter layer near the bed bottom. The decomposition performance of organic pollutants is assessed by the balance between organic load and available oxygen. Other pathways for organic matter removal are identified, namely, photochemical reactions, uptake by plants and metabolization [47, 48]. Removal of phosphorus in CWs is assured by adsorption, complexation-precipitation and plant uptake if the biomass is harvested, otherwise, the phosphorus releases in the system and its concentration increase in wastewater. The complexation-precipitation mechanism is highly dependent on the origin and chemical composition of the substrate used. Soil rich in reactive minerals such as Fe, Al or calcareous materials that favorite Ca phosphate precipitation are very significant in phosphorus removal. In FWS CWs the elimination of phosphorus is relatively with a slow rate, due to little contact between water column and the soil containing mineral elements for precipitation.

Based on the literature studies, CWs have been generally designed to treat municipal and domestic wastewater. Recently, CWs are strongly applied to other type of wastewater as industrial wastewater. Among effluents treated by FWS CW are petrochemical wastewater, pulp mill wastewater, tannery wastewater, aquaculture wastewater, distillery wastewater, brewery wastewater, meat processing wastewater, abattoir wastewater, seafood wastewater and olive mill wastewater. Goulet and

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

Sérodes [49] reported that the treatment of abattoir wastewater using FWS CWs planted with *Typha sp*. and consisted of storage tank (750 m<sup>3</sup> ) and two cells a total surface area of 1420 m<sup>2</sup> allowed removal efficiencies amounted to 95%, 85%, 66%, 54% and 74% for TSS, BOD5, TKN, NH4 + -N and TP respectively. Two FWS CWs occupied an area of 45.5 m<sup>2</sup> for each, planted with *Phragmites australis* and filled with gravel were used to treat diluted olive mill wastewater. The results showed that the bed without recirculation allowed a reduction of 80%, 83%, 80%, 78% and 74% for TSS, COD, total phosphorus, NH4 + -N and phenols respectively. While the reduction using bed with recirculation acceded 90%, 98%, 85%, 55% and 87% respectively [50]. In another study, the treatment of petrochemical wastewater by 15.5 m<sup>2</sup> FWS CW planted with *P. australis*,*Typha angustifolia* and *Typha latifolia*, and HLR ranged between 1.3 and 1.6 cm d<sup>1</sup> was investigated. The average annual removal of COD, BOD5, TN and total phosphorus from pretreated wastewater reached 54%, 59%, 22% and 43%, respectively [51]. Allen et al. [52] presented the results from a full-scale FWS CW (8.19 ha) designed to treat increasing amounts of pre-treated domestic wastewater. The system received an annual average loading rate of 947 kg/year BOD5, 19,644 kg/year TN, 31039 kg/year NH4-N, 18140 kg/year TKN and 807 kg/year total phosphorus and removal rate of 8%, 72%, 73%, 78% and 246% respectively. The study of Bydalek et al. [53] aimed to assess microplastics fate in FWS CW following an oxidation process. The FWS CW has an operational surface of approximately 8000 m<sup>2</sup> and planted with *Schoenoplectus lacustris* and *Typha angustifolia*. The results showed that over 95% of microplastics were retained within the first 20% of the FWS CW and allowed an aerial removal rate exceed 4000 microplastic m<sup>2</sup> d<sup>1</sup> .
