Water is a precious commodity that once available almostfree of cost, hence water is a vital substance and ofgreat importance in all natural and anthropogenic activities. It regeneratesseas, rivers, lakes and forests, therefore it is becoming part of the identityof environments and landscapes and of paramount importance for the developmentof ecosystems and human life (Jayalakshmiet al., 2017).Most of the earth water is seawater. The world consists of a major portion (about 71%) ofwater, but fresh water contributes to only a minor fraction of 2.5%. Availability of fresh water, the nature’s gift controls the major partof the world economy.
However,more than 60 billion m3 a year of freshwater is needed for a person(cope) with the annual global population growth of 80 million people. In Asiaalone, urban populations can be expected to double in the year 2000–2030(Martine, 2007). About2.5% of the water is fresh water that does not contain significant levels ofdissolved minerals or salt and two third of that is frozen in ice caps andglaciers. Only 0.01% of the total water of the planet are accessible forconsumption.
Fresh water is a finite resourceessential for use in agriculture, industry, propagation of wildlife & fisheriesand for human existence. Water recycling provides a great opportunity toconserve one of our natural resources that is essential for the survival ofmankind. Water is of fundamental importance for life on earth.The synthesis and structure of cell constituents and transport of nutrientsinto the cells as well as body metabolism depend on water. The contaminationspresent in water disturbs the spontaneity of the mechanism and result in longor short-term diseases (Sharma and Bhattacharya, 2017).Contaminated water and poor treatmentare linked to transmission of various diseases such as diarrhea, typhoid,dysentery, cholera, hepatitis A etc.
Unsuitable managed water and sanitationservices expose individuals to preventable health risks. Globally, 15% or moreof patients developed an infection during a hospital stay for long time, withthe proportion much greater in low-economic countries. Diarrhea is the mostwidely known disease linked to contaminated food and water, but there are otherhazards.
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Almost 240 million people are affected by schistosomiasis an acute andchronic parasitic disease caused by Schistoma(WHO 2017).Contaminants are man-made by-products of industry, and agriculture,including heavy metals like mercury (Hg), copper (Cu), chromium (Cr), lead (Pb),and hazardous chemicals, dyes and compounds like insecticides and fertilizers(Sharma and Bhattacharya, 2017). InIndia the majority population still lives in villages and small towns. Theserural/tribal communities do not have access to public water supplies.
Thispopulation obtains their water supply from unprotected sources such as open dugwells or small Streams and ponds which are polluted. The treatment of water inthese areas has a unique problem. Therefore, there is an urgent need fordevelopment and widespread promotion of simple treatment techniques inrural/tribal areas.
The most water polluting sources are the citysewage and industrial waste discharged into the rivers. The facilities to treatwastewater are not adequate in any city of the India. Discharged wastewater possessescolor, biochemical oxygen demand (BOD), chemical oxygen demand (COD), pH,temperature, turbidity and toxic chemicals, etc. The direct discharge of thiswastewater into the water bodies like lakes, rivers, etc. pollutes the waterand affects the flora and fauna, thereforeresponsible for destroying the beauty of the lakes and rivers.The wastewater produced fromdifferent kinds of industries normally contains very fine suspended solids,dissolved solids, inorganic and organic particles, metals and other impurities.
Due to very small size of the particles and the presence of surface charge, thetask to bring these particles closer to make heavier mass for settling and filtrationbecomes challenging (Bratby, 2006). Hence, removal of these colloidal particlesfrom the wastewater becomes a serious challenge for the industries (Divakaran et al., 2001; Nasser and James, 2006).
Various traditional and advancedtechniques are utilized to remove the pollutants from wastewater; such as ionexchange, membrane filtration, solvent extraction, adsorption and coagulation-flocculation,methods etc. (Radoiu et al., 2004).Among those methods, coagulation-flocculation is one of the most widely used forliquid-solid separation for the removal of suspended and dissolved solids (DS),colloids and organic matter present in wastewater (Renault et al., 2009).
The modern use of coagulants for watertreatment used more than ~100 years ago, when ferric chloride (FeCl3)and aluminum sulfate (alum) were used as coagulants in full-scale watertreatment works. The coagulation mechanism was firstly explained by theSchultz– Hardy rule and the Smoluchowski’s particle collision function, whichform the theoretic basis of coagulant demand and changes in particle number in theflocculation process (Jiang 2015).It is an easy and effectivemethod for removal of pollutant form wastewater, and it has been extensivelyused for the treatment of various types of wastewater such as pulp and paperindustry, palm oil mill effluent, textile wastewater, sanitary landfillleachates, automobile effluent (Ahmad etal., 2005). The coagulant selection is difficult at first sight sincedifferent coagulants show many similar results. Thus, a decision has to be madeaccording to different criteria and not only evaluating the pollutant removalefficiency. Flocculation is a physical andchemical process, hence it is used for the removal of the small sediments andmaterial from water which makes it a colloidal solution.
It can be done throughagitation or by flocculating agents.In this process, addition ofcoagulants and flocculants in polluted waste water, finely dispersed and destabilized particles are aggregated or clump together to form large aggregate particles of such a size, known as flocs, which settle after a certain time,known as sedimentation time (unit-Hr or min etc.) and cause clarification ofthe system (Sharma et al., 2006). A negative (-) chargeprevents them to coagulate; as like two same magnetic poles (N-N or S-S) repeleach other. They are very stable in colloidal systems. If we are able toneutralize these charges, then they would be build up into coarse formations.
For resolving this problem we add cationic coagulant. Coagulation-flocculation is the conventional treatment method, where thecationic inorganic metal salts are commonly used as coagulants and long chainsanionic or non-ionic polymers are usually employed as flocculants (Chong 2012).In coagulation-flocculation process, many factors can influence itsefficiency, such as the type and dosage of coagulant-flocculant, pH (Najafi andMovahed, 2009), mixing speed, temperature and retention time etc.Optimization assays were conducted by the standard jar test method. The effectof pH, coagulant dosage and mucilage dosage on the percent removal of chemicaloxygen demand (COD), turbidity and color were analyzed (Freitasa et al., 2015). Inorganiccoagulants and synthetic or natural polymers are added to wastewater in orderto destabilize colloidal material and cause the aggregation of small particlesinto larger, more easily removed flocks.
In general, inorganic coagulants arewidely used as coagulants in wastewater treatment because of their performance,availability and cost-effectiveness (Shak and Wu, 2014) However, severaldrawbacks from using inorganic coagulants are frequently reported. For example,the overdosing of alum in water or in wastewater treatment may lead to a highresidual aluminum concentration. The prolonged exposure to water with highresidual aluminum content is linked to serious health issues, such as thedevelopment of Alzheimer’s disease and senile dementia (Bodlund et al., 2014; Shak and Wu, 2014)Themain process variables that are commonly measured to justify the flocculationefficiency include settling rate of flocs, sediment volume (sludge volume index,SVI), percent solids settled, turbidity or supernatant clarity, percentage of pollutantremoval or water recovery depending on the industrial application (Bohuslav,2010). All these output variables are actually manifestations of the floc oraggregate size distribution and the shape and structure of flocs producedduring the flocculation process.
Bigger, stronger and denser flocs arepreferable for good sedimentation, easy filtration and high clarification.
