This article provides notes on aquatic macrophyte treatment system (AMATS).
Regardless of the type of aquatic macrophyte based treatment system, whether a natural wetland or an artificially constructed wetland system with a monoculture or poly-culture using either floating or emergent plants, the processes thought to operate are essentially the same.
In addition to the direct uptake and accumulation of contaminants, pollutant removal may be achieved by a complex range of chemical and physical reactions, occurring at the water-sediment, root-sediment and plant water interfaces.
Aquatic macrophytes, particularly floating species or reeds are capable of very high rates of growth and such growth rates are associated with high levels of nutrient uptake and demand, particularly for nitrogen and phosphorus.
In order to maximise removal of nitrogen and phosphorus via direct uptake, frequent harvesting may be required to remove accumulated nutrients, encourage new growth and prevent releases from senescent plant material.
In addition, direct uptake and accumulation by plants can also significantly reduce the concentration of other contaminants of wastewater viz., metals (e.g., Cu, Zu, Pb, Hg, Ni, Cr, Cd etc.), partly as a result of both active and passive plant uptake and accumulation. Decomposing bacteria associated with plant surfaces will also utilise organic carbon present in wastewater as an energy source.
This process helps in rapid reduction of BOD of wastewater. Pathogenic viruses and bacteria present in sewage effluent are removed chiefly by adsorption or sedimented as soil particles followed by antimicrobial action of the soil- root-micro flora.
Although the basic biological processes responsible for the removal of pollutants by AMATS are well established, there has been little standardisation of their design or methods of construction. On the whole based largely on the root zone concept, a body of general design criteria and principles for the construction of reed bed systems is beginning to emerge.
The important factors that determines the efficiency and pollutant removal potentialities of this system include the following:
1. Choice of plant species;
3. Area of reed bed/macrophytes bed;
4. The nature, loading and distribution of effluent.
Macrophytes are required to fulfill four major functions in order to be used in AMATS:
1. Filter solids out of suspension;
2. Provide surfaces for bacterial growth;
3. Translocate oxygen into the root zone, thereby increasing the efficiency of bacterial degradation or transformation of pollutants;
4. Maintaining the hydraulic permeability of the substrate.
The choice of substrate for this system is very critical. In addition to gravel, river sands and pulverized fuel ash, a wide range of soils have been used with varying degrees of success. The substrate must provide a suitable medium for successful plant growth, and allow even infiltration and movement of wastewater.
The area of reed bed or macrophyte bed can be computed by the following formulae:
Ah= KQd(InC0 – InC1).
where Ah = estimated area of reed bed/macrophyte bed required;
K = constant (5.2)
Qd = the average flow rate of wastewater (m3d-1)
C0 = the average BOD5 of the influent (mg1-1)
C1 = the average BOD5 of the effluent (mg 1-1)
Although AMATS have been used to treat screened primary sewage effluent, their long-term efficiency is improved if the effluent is pretreated by storage in a settlement tank or pond for 24 hrs. at least prior to discharge into the active macrophyte bed or into the treatment lagoon.
During storage of sewage effluent, the BOD may be reduced by 30-40% as suspended particles settle. The removal of part of the suspended solids will help to prevent the treatment system from prematurely silting up.
The major difference between conventional methods of wastewater treatment and AMATS is that in conventional systems wastewater is treated rapidly in a highly managed and energy intensive way, whereas AMATS rely on a slow flow of effluent through the system giving long retention times. Thus, the discharge and flow of wastewater through AMATS must be regulated so that retention times are sufficiently long for pollutant removal to be efficient.
Several aquatic macropytes are morphologically and physiologically well adapted for absorption of nutrients from wastewater. Among them the most important macrophytes are Eichhornia crassipes, Lemna minor, Lemna gibba, Spirodela polyrbiza, S. punctata, Azolla filiculoides, Najas flexilis, Egeria densa, Potamogeton crispus, Scirpus lacustris and Phragmites australis. Over past three decades several studies were made by using different macrophytes in municipal and domestic wastewater treatment.
In India, a good number of aquatic macrophytes were reported to grow in polluted waters. The potential uses of these aquatic macrophytes in wastewater treatment systems are manifold. In addition, this is a low cost method and very simple designed treatment system as can easily be constructed for wastewater treatment.
Generally domestic and municipal wastewater are characterised by high total dissolved solids (TDS), biological oxygen demand (BOD), chemical oxygen demand (COD), nitrate nitrogen and phosphate contents. Thus pollutant load can easily be reduced to a great extent by growing a number of aquatic macrophytes in artificial tank.
In an in vitro study made by Ghosh and Santra (1994), it was reported that several macrophytes have differential ability for removal of pollutants. Twelve macrophytes were tested for their cleaning efficiencies using raw and partially treated wastewater. The plant’s efficiency for removal of COD from raw wastewater can be arranged in the sequence as Phragmites, Typha, Monochoria, Cyperus, Alternaria, and Ludwigia.
However, performances of these plants with respect to removal of nitrate nitrogen is satisfactory. Alternanthera philoxeroides and Typha domingensis proved to be better than other species in all respects. In case of primary treated wastewater, Salvinia provided better option than others in almost all respect followed by Lemna sp.
Plants like species of Typha and Nymphoides were proved to be better than all other plants tested for wastewater treatment following secondary treatment.
The best performed cleaning efficiency of different plants using different types of wastewater are depicted in Figs. 13.4, 13.5, 13.6, 13.7 and 13.8 respectively. The pollutant removal rate is very much time dependent and the relative rates are widely differing.
Considering all these facts a model macrophyte based wastewater treatment facilities were also suggested (Fig. 13.8). Thus the use of tropical aquatic macrophytes in the field of wastewater treatment is very much promising and intuitively correct approach towards reducing pollution levels from wastewater and gaining wealth for economic benefit of the society.
The uncontrolled dumping, land application and accidental spills of toxic environmental pollutants pose a continued world-wide environmental threat, in particular to aquatic environment. Bio-accumulative contaminants are rapidly absorbed out of water borne ambient environments, and concentrated in the tissues of living aquatic organisms at concentrations that can range from thousands to millions of times greater than levels in the ambient environment.
These absorbed levels are high enough to cause dis-function in the organisms and potential harmful effects to humans. Various bioremediation processes thus could be used in next millennium to filter, concentrate and remove bio-accumulative contaminants from polluted aquatic systems.