Global water resources today is extremely polluted due to a number of reasons.
The pollutants may be various inorganic salts, metals, pesticides, oil & grease, aromatics and also a variety of pathogenic organisms.
Most of the surface fresh water bodies, ground water, and coastal marine water gets polluted over the years. The major water pollutants and their sources are depicted in Table 13.7.
In rural and suburban areas, the sewage from each home is usually discharged into a septic tank. But in urban areas, most waterborne wastes from home, business, factories, and storm runoff flow through a network of sewer pipes to waste water treatment plants.
When sewage reaches a treatment plant, it can undergo up to three levels of purifications, depending on the type of plant and the degree of purity desired. Primary sewage treatment is a mechanical process that uses screens to filter out debris such as sticks, stones and rags; suspended solids settle out as shidge in a set-ling tank.
Secondary sewage treatment is a biological process in which aerobic bacteria are used to remove up to 90% of biodegradable, oxygen-demanding organic wastes.
Some plants use trickling filters in which aerobic bacteria degrade sewage as it seeps through a bed of crushed stones covered with bacteria and protozoa. Others use an activated sludge process, in which the sewage is pumped into a large tank and mixed for several hours with bacteria rich sludge and air bubbles to facilitate degradation by microbes.
The water then goes to sedimentation tank, while sludge’s were collected for aerobic/anaerobic digestion. Advanced sewage treatment is a series of specialised chemical and physical processes that remove specific pollutants left in the treated waste water after primary and secondary treatment.
Effect of Water Pollutants on Plants:
Depending on the nature of water pollutants, the effect on plankton population and diversities, macrophyte diversity varies over the years. Excess accumulation of nutrient like nitrate and phosphate causes eutrophication of water bodies, thus there may be bloom of selected planktons in water bodies.
The dominant one is Microcystis aeruginosa a colonial blue green algae. In addition, organic waste decomposition may also leads to eutrophication, where flagellates or green algae may form bloom. But certain pollutants like phenolics or acidic or alkaline materials may damage both planktons and macrophytes.
Other parameter like suspended particulates of water also play a crucial role in submerged vegetation growth. By and large macrophytes played a very crucial role in polluted water transformation. Many tolerant species showed unique properties for wastewater treatment.
Domestic and Municipal Wastewater Treatment: Biological Options:
It is well known that the domestic and municipal wastewater was treated by convention sewage treatment plant having a number of step-wise treatment facilities. This facility is usually helpful for removal of total suspended solid (TSS). BOD (Biological Oxygen Demands), COD (Chemical Oxygen Demand), and also nutrients like phosphate, nitrate etc. Though this treatment process is faster, yet it is a much energy consuming system.
As such low cost biological treatment facilities which emerges as a potent alternative method for wastewater treatment appears to be more popular day by day. In addition, persistent toxic environmental contaminants such as pesticides, aromatic hydrocarbons and metals were also removed from waste water through biological treatment processes.
In recent years, effective treatment is achieved by the construction or management of wetland so that environmental conditions favour rapid degradation and cleaning of effluent. In the early 1960s NASA began actively researching into the use of aquatic plant systems for the treatment of wastewater. Interest was initially centred on the use of aquatic macrophytes namely Eichhornia crassipes (water hyacinth) based treatment system.
The system has now been in successful operation for well over a decade. Subsequently more efficient read bed system was used as an effective substrate-plant microbial filter. At present over two dozens of countries have active research programmes evaluating the use of their own macrophytes in water pollution control.
Although the macrophyte based root zone method can be very effective, reductions of total suspended solid (TSS), BOD, total-N and total-P, yet its long-term application in a waste treatment system appears to have a number of problems viz., surface runoff of effluent, development of preferential drainage routes and poor penetrations of the soil by wastewater.
With careful design, placement of inlet and outlet channels and choice of planting substrate, these problems may be minimised. On the whole, artificial wetland systems have been seen as an economically attractive, energy efficient way of providing high standards of wastewater treatment.
In developing countries, they have the additional advantage of representing a “low technology” solution to the treatment of sewage produced by both large and small dispersed populations. Unfortunately the performance of many of these systems has not labeled up to early expectations perhaps due to faulty design and operation of waste-treatment system.