Flocculation and precipitation has often been proposed as a low cost and simple to implement process for municipal solid waste landfill leachate treatment. It is also a natural first thought for anyone new to leachate treatment that the combination of these two processes might be a very effective treatment combination, as they can be for certain other types of contaminated water.
The purpose of flocculation is to form flocs of particles that settle quickly. Generally , flocculation follows coagulation to get rid of colloidal (floc) particles quickly through rapid settlement.
These particles have dimensions in the region of one nm-1 m, and are distinguished by a abnormally large surface area. As an effect, they are very susceptible to surface forces. During coagulation, colloidal particles are destabilized to improve their ability to merge into bigger particles and then this speeds up their removal by gravity. Destabilization is helped by way of chemical reagents ( coagulants ) which are chosen to be minimize repulsive forces thru neutralization of electric charges present in colloidal particles ; this occurs by way of bonding or adsorption mechanisms.
All these points it is necessary to consider concern hydrophobic colloidal particles, for which stabilization derives from negative electrical charges. The more common coagulants are Aluminium Al), and Iron (Fe ( III )) salts, which are identified by multivalent ions with opposite charges.
These salts have an acid behaviour and, therefore, change the physico-chemical traits ( pH, alkalinity ) of wastewater. Their potency relies on the alkalinity of wastewater. Polymeric organic compounds ( polyelectrolytes ) are also often used as coagulants due to their capacity for charge neutralization ( cationic polyelectrolytes ) and to extend bridging between particles. The merger of destabilized colloidal particles is augmented by controlled stirring, and is further helped by addition of categorical chemicals ( 'flocculating agents' ). Among these, turned-on silica or clay ( inorganic flocculants ) and polyacetate ( organic flocculants ) are principally made use of.
Likewise , Al and Fe salts also behave as flocculants, since their low solubility permits rainfall with floe merger and concomitant capture of colloidal particles by electrostatic action or adsorption. Coagulation / flocculation is in a position to reduce colloidal suspension which is partly in charge of turbidity and color.
Also, organic substances, principally those with the larger range dimensions ( about one nm ), are concerned in the flocculation process, because they are adsorbed in the flocs and successively removed thru gravity settling.
Commonly, the term 'precipitation' is used to describe the phase that straight away follows flocculation, and, also, to the formation of insoluble compounds got by adding reagents which shift the chemical equilibrium towards the insoluble form of the compound or the elements which need to be removed.
Precipitation is principally applied to metals removal ( particularly heavy metals ), with metal hydroxide or metal sulphide formation, or phosphorus removal by formation of insoluble compounds with cationic metals, including Al or Fe coagulants. Many experimental studies utilizing coagulation / flocculation for the removal of organic substances from raw leachate have been conducted, essentially in the 1970s.
Salts of Aluminium and Iron together with lime were principally made use of as precipitation agents. Results were adverse, as COD removal potency lower than 40% was noted. The reason behind these low efficiencies can be ascribed to the incapacity of the method to get rid of substances aside from molecules of large dimensions and high molecular weight.
It was concluded that, higher treatment potency is possible but just for 'old leachate' ( with low BOD / COD ratios ) or for biologically pre-treated leachate. In fact it is most often needed for 'young leachate' (acetogenic leachate) which is distinguished by high levels of volatile trans-acids, i.e. small dimensions and only a little in the way of precipitable molecules, so that the removal involves only a minor fragment of the total of organic compounds in raw leachate.
Researchers also cite many other drawbacks like the rise of salt content and the low potency of ammoniacal compound removal. This last is almost always the final concern which rules out the use of this process in the minds of most leachate treatment experts.
OK. So we have told you the disdvantages. What are the alternatives? To find out how leachate treatment can be achieved without the problems cited here visit the leachate web site for all you need to know about garbage juice. If you still need a suitable reagent for this method go to the flocculant supply web site.
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