2. LITERATURE REVIEW
2.4. Methods of Landfill Leachate Treatment
2.4.2. B IOLOGICAL T REATMENT M ETHOD
2.4.2.1. Wetlands
Definition of wetlands
Wetlands can be defined as a land in which water covers the soil or is at ground level, all year or for a longer period of time to maintain a saturated soil condition and allow the growth of micro-organisms and related vegetation (Vymazal, 2010).
Constructed wetlands (CW)
Constructed wetlands are engineered systems that has been designed and constructed to make use of the natural processes which involves wetland vegetation, soils and their associated microbiology to treat the wastewater. Constructed wetlands have been used for treatment of different wastewater such as landfill leachate, storm-water runoff and agricultural runoff (Kadlle
& Knight, 1996). Due to the high cost of treatment of wastewater CWs are regarded as the most economical and technically feasible approach to treat wastewater. Constructed Wetlands can be divided into two general types, which are Free Water Surface (FWS) wetlands and Vegetative Submerged Bed (VSB) wetlands.
Advantages and disadvantages of CW
Detail advantages and disadvantages of the use of CW are listed below:
Advantages
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The operation, maintenance cost are low (Reed et al, 1995)
It can be integrated with other treatment method that already exists
Wastewater with low organic loads can easily be treated with CW as opposed to activated sludge treatment method.
CW has an aesthetic appeal and allows for the wetland organisms to habitat in them (Bricken, 2003)
There are some limitations associated with CW treatment system. According to Wood, 1999 and Reed et al, 1995 the limitations includes:
Requires large area of land like four to ten times for standard treatment but if a zero discharge is required the area of land required would increase up to 100 times.
The removal of nutrient such as phosphate and total nitrogen is limited.
The efficiency of the treatment is decreased during winter period because of the cold temperature (Bricken, 2003).
Types of constructed wetlands
As stated in section 2.4.2.1 there are two general types of CW which includes the free water surface wetland (FWS) and subsurface flow wetland (SSF).
Free water surface (FWS) constructed wetlands
The FWS constructed wetland is constructed to resemble a natural wetland including its operation. It consists of dense vegetation of plant of different species (IWA, 2001). The depths of the water in the CW are typically shallow of about 0.3meters but can range up to 0.8meters depending on the use of the wetland system. As stated already, the FWS follows a natural wetland where the wastewater slowly flows and spread throughout the vegetated surface area thus allowing the solid particles in the wastewater to settle and the vegetation absorbs the nutrients from the wastewater (Bricken, 2003) thereby destroying the pathogens. On the outlet the treated effluent is allowed to flow out as shown in Figure 2.4-1. In the FWS the water is allowed to flow from above ground, thereby exposing the water to the atmosphere and direct sunlight. As the wastewater flows slowly through the wetland it undergoes a physical, chemical and biological process whereby the solid are filtered, the organic degrade and nutrients are removed from the wastewater.
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Figure 2.4-1: Free-water surface constructed wetland (Source: Sandec/Eawag, 2009)
Advantages
Pleasing aesthetically and provides habitation for animals.
Building and repairs can be done using readily available materials locally.
Electrical energy not required for operation.
If operated correctly no real problems with odours and flies.
There is high reduction of BOD and solids.
Labour intensive hence can provide short-term employment during construction phase.
According to Sandec/ Eawag, 2009 there are some limitations associated with FWS which are listed below:
Requires large area of land
For effective use long start up time required
Design and supervision of experts is required.
Subsurface flow wetland (SSF)
The treatment of wastewater in a SSF is done by sending water through a permeable shallow medium. The submerged bed constructed wetland can be categorized into two types namely vertical flow constructed wetland and horizontal flow constructed wetland. SSF wetland has a bed depth between 0.6meter and 1.0meter. The SSF wetland does not allow any free standing water (Bricken, 2003) and this is normally achieved by slopping the bed. The main difference between VF and HF is the aerobic conditions.
24 Vertical flow (VF) constructed wetland
VF constructed wetland consist of a filter bed that is planted with aquatic plants. A mechanical dosing system is used to pour the wastewater from above onto the wetland surface as shown in Figure 2.4-2 thereby allowing the water to flow vertically down through the filter bed (Sandec/Eawag, 2009).
The wastewater is dosed four to ten times a day, the dosing of the wastewater should be timed so that the wastewater that was dosed previously could have time to percolate through the bed filter and allowing the oxygen to diffuse through the bed media filling up the voids space.
VF is normally design to treat wastewater that has already undergone primary treatment. The VF consists of a drainage that has a limit of 20cm in depth followed by sand and gravel to allow the effluent to settle. The bed filter media act as solid removal and a fixed surface upon which the bacteria can be attached to and also a base for the growth of vegetation.
Process
The wastewater percolates through the unsaturated media downwards where the solids are filtered by the gravel matrix. The nutrient and organic material in the wastewater are absorbed and allowed to degrade by the microbial population which is attached to the surface of the bed filter. During the gap of dosing of wastewater the organisms in the bed filter are forced to starve thereby decreasing the excessive growth of the biomass and increasing the porosity in the media.
Advantage
No mosquito problems as water always flows
Lesser space is required compared to FWS wetland
BOD, suspended solids and pathogens are reduced greatly
Disadvantages
To prevent clogging pre-treatment of the wastewater is required
Requires constant source of electrical energy
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Figure 2.4-2: FWS Wetland systems (Source: Sandec/Eawag, 2009)
Horizontal Subsurface Flow Constructed Wetland (HSF)
The HF wetland system is a system whereby wastewater flows from the inlet point through the filter material (Permeable material) filtering out the particles. The outlet point in the system is under the bed surface as shown in Figure 2.4-3. In the HF system physical and chemical processes takes please thereby cleaning the wastewater. The HF wetland performs a good nitrogen removal which is mostly done by nitrification and denitrification (Cooper et al, 1996).
The ammonia in the wastewater is converted to nitrates by the nitrifying bacteria in the aerobic zone and then converted into nitrogen gas by denitrification bacteria in the anaerobic zone.
According to IWA, 2001 the presence of carbon source in the HF improves the nitrate removal efficiency by 30%-80%, carbon sources such as grass and wetland plants.
Advantage
No mosquito problems as water always flows
Lesser space is required compared to FWS wetland
BOD, suspended solids and pathogens are reduced greatly
Can be built and repaired with locally available materials.
Construction can provide short-term employment to local labourers.
Disadvantages
To prevent clogging pre-treatment of the wastewater is required
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Figure 2.4-3: HSF Wetland systems (Source: Sandec/Eawag, 2009)
Ammonia ( ), ammonium ( ), Nitrite ( ), Nitrate ( ), Nitrous Oxide ( ) and dissolved Nitrogen element or dinitrogen gas (N2) are the most important form of inorganic nitrogen in constructed wetland. Some mechanism can be used to remove the organic nitrogen which is in the suspended solids.