Total water quantity on our earth is constant and is recycled in atmosphere, earth’s surface, below the surface and oceans by nature. Population explosion, rapid industrialisation and consequent urbanisation have increased the demand for required quality of water for different end users.
Man has been solely responsible for over contaminating the sources of water by neglecting the pollution control of water over decades. It is therefore, man’s responsibility now to swing into action to rectify the situation by purification and recycle of water to save our water bodies from further contamination.
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Ever increasing efforts of men were to obtain required quality and quantity of water by attempting only purification which further shifted the pollutants from a source to another which aggravated the situation.
The essential components of the systems are:
(i) The reed type of wetland plants.
(ii) The soil bed.
(iii) Micro-organisms of different type.
The engineered Rootzone bed is shown in Fig. 4.25. As seen from this figure, the bottom and side bunds of system are compacted and made impermeable using either plastic liner or certain clays. The bottom has a slope in one direction which depending upon the hydraulic load and organic load is varied between 0.5 to 0.2 per cent.
There is an under-drain pipe system at the bottom with a provision to vary the water level in the bed as required. The soil bed is especially prepared to achieve the hydraulic permeability required for a particular effluent as well as for required output quality. The addition of certain chemicals in powdered or granular form is determined to take care of the constituents in the effluent and also for pH correction, if required.
Since it is a matter of details of the technology which is a proprietary know-how. There are naturally occurring bacterial strains of aerobic as well as anaerobic type numbering more than 2500 to 3000 and hence the treatment is extensive. The area where roots are spread is called the Rootzone. In this zone also, there are two types of areas- (i) near the roots and (ii) away from the roots.
The water is fed through the stone aggregate channel which normally takes care of uniform flow distribution over the entire cross section of the reed bed. Due to the slope deliberately provided in one direction, the effluent percolates through the porous soil bed towards the other end horizontally and comes out through a perforated under-drain pipe system provided at the outlet end of the reed bed. With the proper residence time which could range from few hours to few days, the required extent of treatment is achieved.
As the effluent is percolating the plants which have been provided by nature, a special property of absorbing oxygen from atmosphere and passing it down to its roots provide the required oxygen to the soil bed area adjacent to the roots. This oxygen is utilised by the aerobic type of bacteria to biodegrade the organic matter from the effluent to CO2, N2 and H2O and elemental sulphur thus leaving practically no sludge behind.
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The soil bed area where roots do not reach has anaerobic bacterial population and also biodegrades the organic matter reducing the COD and BOD of the effluent as it flows furtherdown through the bed. During the percolation, there is a very fine filtration which arrest the suspended organic matter along with some inorganic colloidal matter.
The organic particles are slowly biodegraded but small quantity of inorganic particles may remain in the bed but the porosity reduced by such particles getting trapped is compensated by increased porosity caused by crevices created by decay of old roots. The special chemicals added to the bed react with some of the products of bio-reaction to neutralise the effects by precipitation and adjusting the pH.
Types of Constructed Wet Lands:
There are two principal types of constructed wet lands:
(i) Free water surface systems with emergent plants.
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(ii) Subsurface closed systems with emergent plants.
(i) Free water surface system consists of basins or channels with subsurface barrier of clay or impervious membrane to prevent seepage to soil or other suitable medium is present to support the emergent vegetation and water flowing through at a shallow depth. Cattails and bullrushes are commonly used in free water surface wet lands.
The shallow water depth, low flow velocity and presence of plant stalks and litter regulate flows specially in long narrow channels, ensuring good flow conditions. This system may produce H2S odour if wastewater contains more than 50 mg/l of sulphate due to presence of sulphate reducing bacteria. Due to surface flow, mosquito problems may occur when the system is overloaded and anaerobic conditions develop.
(ii) Subsurface flow system lined or unlined trenches or basins are filled with permeable packing medium and placed with emergent plants. Rootzone method and rock reed filter are two main categories of subsurface flow systems. To grow plants, a soil medium in Rootzone method, rock and sand are used in a rock-reed filter system.
Types of Plants Used in Wet Land Systems:
(i) Cattail
(ii) Reed-phragmites
(iii) Bullrush
(iv) Sedge and
(v) Various plants in grass family
(vi) Phalaris
(vii) Spartina
(viii) Carex are some of the plant types used in wet land systems.
The most efficient and hardy plant was found to be phragmitis australis specially in subsurface Rootzone systems. These plants grow to the height of about 3.5 to 4 mts. Their stems are hollow, the root density is very high and the spread is horizontal after 30 to 40 cm depth.
Types of Micro-Organisms Acting in Wet Land:
The micro-organisms developed differently are difficult to classify as plants or animals. Even the criteria for life has to be modified. Most micro-organisms are microscopic, differentiation of tissues and live in interrelated groups.
Bacteria for example include many size and shapes of unicellular micro-organisms and are present in almost all natural environments in large numbers. Like plants they have rigid cell walls, but like animals are mobile and require nutrients. They are not photosynthetic, bacteria, modulus, yeasts, viruses and algae are assigned to the vegetable kingdom.
To chemically change an organic material micro-organisms will utilise an enzymatic or biological chemical process to bio-convert a targeted substrate into carbon dioxide, biomass and water. This results in reduction of BOD5 and COD of the effluent that comes in contact with the micro-organisms which mainly are aerobic or anaerobic in nature.
Presence of fungi and yeasts also helps in treatment particularly in colour removal. The bacterial activity is maximum at pH 6 to 8 but bioconversion takes place from 4 to 10 pH in Rootzone systems.
The resistance to heavy metal toxicity is developed in bacteria by:
(i) Increasing impermeability of cell.
(ii) Biochemically transforming the metal.
Thus biodiversity is the key to the Rootzone treatment.
Process of Rootzone Treatment:
The process of Rootzone is very simple to explain. Raw effluent after removing grit or floating material is passed horizontally or vertically through the bed of soil having an impervious bottom. The effluent percolated through the bed which has all the roots of the wetland plants spread very thickly. Nearly 2500 types of bacteria which harbour around roots get oxygen from the weak membranes of the roots and aerobically oxidise the organic matter from the effluent. Table 4.16 shows the comparison of conventional aeration process with rootzone.
The nature gift, that they can absorb oxygen through their leaves can pass it down to roots through their steams which are hollow, is utilised as a bio pump. Away from the roots anaerobic digestion also takes place. The filtering action of the soil bed, the action with fungi etc. and chemical action with certain existing or added inorganic chemicals help in finally obtaining a very clear and clean water with BOD5 as low as 10 ppm.
The treated water gets accumulated in the outlet channel and with the help of suitable piping network is taken out and pumped to the location of refuse or to the tertiary treatment to remove undesired inorganics to the extent required for reuse. Technologies like RO, ED, Ion exchange, demineralising, nanofiltration, ultrafiltration etc. are employed depending upon the treatment level required for reuse.
However, a portion of water is lost through evapotranspiration and hence 100 per cent recovery may not be possible. The system of plants regenerates itself as the old plants die and form useful humus. Hence the system becomes maintenance free and can run upto 50 to 60 years without any loss of efficiency. Experience with plantation density to start with is 1 or 2/m2.
Advantages of Rootzone System for Treatment of Effluents:
(i) Capital cost involved is comparable or lower depending upon the topography of site and type of soil available.
(ii) Construction of bed is simple and involves no mechanical and electrical equipment.
(iii) System requires little or no maintenance.
(iv) It is a robust process and is able to withstand wide variation of operating conditions.
(v) It gives a consistent quality of treated water.
(vi) It has a potential for bird sanctuary and wildlife habitat.
(vii) Regardless of application (municipal wastewater, mine drainage or others) the system has a potential to develop into a beautiful landscape.
(viii) It can be utilised for wide range of organic and inorganic contents in the effluent.
(ix) It can be installed economically for capacities as low as 1 m3/day to 10,000 m3/day and for COD input as high as 50000 to 60000 mg/l.
(x) Lasts for more than 50 to 60 years without any major maintenance.
(xi) It can treat effluents of wide range of contaminants ranging from sewage to distillery spentwash.
(xii) There is no production of sludge in the system.
(xiii) It does not require skilled operators.
Limitations of Rootzone System:
(i) Highly advanced treatment requires relatively larger area.
(ii) Biological and hydrological complexity and lack of full knowledge of nature’s treatment know-how makes it difficult to design for unknown effluents.
(iii) Possible problems relating to steep topography, shallow soils, high water table and susceptibility to floods.
Major Applications of Rootzone System:
(i) Sewage.
(ii) Dairy mill waste.
(iii) Sugar mill waste.
(iv) Food processing/fruit processing.
(v) Petroleum refineries (oil + grease).
(vi) Chemical industries.
(vii) Textile industry.
(viii) Pulp and paper.
(ix) Breweries and distilleries.
(x) Steel plants.
(xi) Coal mines.
(xii) Agrobased industry.
(xiii) Meat and fish processing industry.
(xiv) Auto service station’s effluent.
Design and Construction Details for Rootzone Technology:
Characteristics of the wastewater to be treated as well as desired treatment keeping in mind the reuse of treated water need to be taken into consideration in the early stages of designing a reed bed system such as BOD5, COD, suspended solids, nitrogen compounds, phosphorus compounds, heavy metals and pathogenic bacteria/viruses.
Sizing of Beds:
This is done on two considerations (i) Hydraulic flow capacity (ii) Organic load to be degraded and higher of the two area values is selected.
Areas calculated by these two formulae are considered and higher area is taken for designing.
Soil and Growth Medium:
Proper selection of soil considering permeability, inorganics and organic contents is most important and determination of proper admixtures and their proportions is complicated and propriety information to be given here.
Plants and Plantation:
Being very hardy and robust and having high root density for most of the treatment plants of subsurface nature, phragmites australis has been very useful.
For quick growth, clumps of the plants are planted with a plant density of 1 to 2 per m2, though propagation through seed and seedlings is also practised. Multiplication using Tissue Culture method has been found most scientific as the health of the plant is uniform.
Under-Drain System:
Plastic pipes not less than 100 mm dia with or without perforations are used in the under-drain system. Depending upon the length of the bed, two or more intermediate drains are provided in the bed. A sump or manhole is provided in the outlet channel or outside where all drains are connected. By increasing or lowering the outlet level of the pipes in the outlet manhole, the water level in different zones can be altered during the operation and primary growth period.
In a vertical flow system, cut channels with serrations are installed on the top of the bed and effluent is fed by gravity/overflow combination for uniform distribution. Using a pressurised flow and spraying the effluent over the entire bed also practised.