After reading this article you will learn about:- 1. Introduction to Waste Water Treatment through Wetlands 2. Concept of Waste Water Management through Wetlands 3. Types of Wetland 4. Sizing of Constructed Wetlands 5. Recalcitrant Waste Waters.
Introduction to Waste Water Treatment through Wetlands:
Wetlands are soils which are more or less water saturated and constructed wetlands are a copy of natural marshes. They all are based on the same principles; to feed basins or channels that contain a soil (Sand, Gravel or Natural soil) in which wetland plants or macrophytes can grow.
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The use of natural eco-systems for waste water treatment is an important and emerging aspect of environmental management using constructed wetlands. There is a large and growing need to better treat many municipal, industrial, agricultural waste waters, especially those which are more difficult to handle because of their nature (saline, highly contaminated, alkaline, acidic, non-point source etc.), but conventional (mechanical) waste water treatment facilities are expensive and capital is in short supply.
Concept of Waste Water Management through Wetlands:
Natural treatment systems such as constructed wetlands (CWs) are more economical to build and operate. This is a new approach for decreasing environmental pollution, based on the purification of waste waters with vegetation’s planted in them.
The CW with a horizontal subsurface flow of waste water is composed of one or more beds, filled with substrate, enabling the growth of selected plant species and micro-organisms. The bottom of the bed is fortified with an impermeable layer.
The substrate is directly involved in eliminating pollutants with physicochemical inter-actions such as filtration and sedimentation of suspended solids, Alteration of pathogenic organisms, sorption of organic matter, Nitrogen, Phosphorus and heavy metals.
Indirectly, the substrate acts as a support for root systems and as a surface area for the adhesion of microorganisms. Micro-organisms use organic matter as the energy rich substrate and transform it into nutrients and energy.
Vegetation plays an important role since the diffusion of oxygen from roots create conditions needed for the development of micro-organisms which participate in the aerobic decomposition of organic matter. The extensive root systems serve as large surface areas for the development of micro-organisms and enable filtration as well as adsorption of sediment matter.
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However, these constructed wetlands normally only remove part of any nutrients (Nitrogen and Phosphorus compounds) in waste waters being treated in them. What needed was a new type of constructed wetland which remove ‘nutrients as well as it did other contaminants, which operated better especially with difficult waste waters.
The answers may be found with engineered wetlands, a type of constructed wetland in which process conditions and/or operations are re-designed, modified, manipulated and/or controlled, in contrast to the more passive operation of ordinary constructed wetlands.
Constructed wetland systems may be engineered in many ways, for example, influent (streams may be varied in flow rate or periodically turned off), effluents from various points in the wetland system may be recycled to other points, ordinary substrate e.g., the ability to permanently adsorb certain pollutants from waste water passing through them), things may be added and certain points may be replaced with special area having specific qualities (e.g., Heat, Chemicals, Air) wetland vegetation may be selected for its phytoremediation property or other options may be considered. This paper deals with engineered wetland eco-technology, its technical and economic advantages over current conventional wetland waste water treatment technologies and some example thereof.
Types of Wetland used for Waste Water Treatment:
Three types of constructed wetlands may be defined:
1. Pond Wetlands:
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These are simple shallow pools, usually vegetated around the peripheries and having some portion of their surface consisting of open water. They are most commonly used in conjunction with other type of wetlands. These provide quiescent areas where sediments and some of the suspended solids in a waste water can settle out.
2. Free Water Surface (FWS) Wetlands:
Here water flows over the surface and the submerged portions of wetland plants, soil and detritus act as substrates for micro-organisms and these are responsible for much of the pollution removal.
3. Sub Surface Flow (SSF) Wetlands:
Here pollutant removal is via substrate and vegetation root systems. Although wetland vegetation is apparent in them, their surfaces are largely dry. Generally SSF wetlands consist of one or more vegetated beds of rock, gravel, aggregate, soil or some type of engineered growth substrate.
SSF wetlands are usually smaller in area than FWS ones for the same level of pollutant removal, and can tolerate higher loadings. They are, however, generally more expensive, costing up to several times the cost of a FWS wetland of equivalent size.
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There are two types of SSF wetlands:
(i) Those which are horizontally fed and in which the waste water flow horizontally through the substrate, and
(ii) Those which are fed by applying waste water to their surfaces.
Sizing of Constructed Wetlands for Waste Water Treatment:
The first step in designing a constructed wetland system is to size the wetlands. One way to size constructed wetlands is to consider them to be plug flow reactors, in which linear first order reaction kinetics are involved.
The equation for the purpose:
C/Ci = exp (-KvT) … (i)
Here, C is concentration of a pollutant, i represents the inlet, T the residence time of a volume of a waste water in the wetland and Kv volumetric rate constant for its degradations (removal) in a wetland. The hydraulic residence time, T is given by:
T = EAh/Q … (ii)
where E is the porosity of the wetland; A is the wetland surface area; h is the average water depth in the wetland, and Q is the flow rate of waste water through the wetland. Wetland sizing may be carried out for desired levels of removals at a particular temperature for any of biological oxygen demand (BOD), chemical oxygen demand (COD), suspended solids (TSS), ammonia nitrogen (NH4-N), organic nitrogen (org-N), nitrate nitrogen (NO3-N), total Nitrogen (TN) total phosphate (TP), Pathogens (faecal coiform or FC) or other contaminants. The Kv for particular contaminant can be contained by carrying out a treatability test or for some common ones obtained from published value.
In some circumstances, the background level of a particular pollutant (designated C”) in a wetland may be high enough to merit consideration. For example, the background level of BOD in a typical constructed wetland can vary due to a variety of factors from 2 to 30 mg/L as a result of ordinary biological activity in it while this would have little effect where inlet BOD concentrations were in the hundreds or thousands of mg/L, it would be a factor for lower inlet concentrations and/or if target outlet concentration were set at, say 20 or 25 mg/L.
To allow for background levels of contaminants, equation (1) can be modified as follows:
(C – C*)/ (Ct – C*) = exp (- KvT) … (iii)
The degradations of many pollutants in constructed wetlands are low at lower temperature effect, the rate constants must be adjusted using Arrhenius equation.
KT – K20 Q (T-20) …(iv)
where KT is the first order rate constant (either volumetric or areal) at a temperature of T degree Celsius, K20 is its value at 20°C and Q is Arrhenius co-efficient.
Recalcitrant Waste Waters:
Constructed wetlands have been successfully used to treat all sorts of municipal, industrial and agricultural waste water.
However it is really difficult to achieve good pollutant removals when some of the following conditions apply for the waste water being treated:
1. Very high BOD, COD, ammonia and/or other pollutant levels,
2. Very low BOD and/or nutrient levels,
3. High or low pH,
4. Very cold waste water,
5. Highly intermittent flows and/or pollutant levels,
6. Salinity, and
7. Saturated with certain species.
Examples of such difficult to treat waste waters include some leachates from landfills, land farms and industrial waste; and mine drainage (AMD) from mine tailing piles, certain industrial process waters (e.g., high BOD, high phosphate stream from food processing plants), contaminated ground water and surface water flowing through sites requiring remediation and a variety of liquid sludge’s (e.g., 2-4% solids) from the activated sludge units of conventional waste water treatment plants.
Recalcitrant waste waters may be toxic, odorous, corrosive or just too strong to treat by ordinary methods. There are a variety of conventional and natural waste water treatment technologies (and combination thereof) that can be used to treat them. One of these is engineered wetland systems.