Aquatic Macrophyte Treatment System (AMATS)

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;

2. Substrate;

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:

A_h= KQ_d(InC₀ – InC₁).

where A_h = estimated area of reed bed/macro­phyte bed required;

K = constant (5.2)

Q_d = 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.