Sewage or wastewater is a dilute mixture of various wastes from residential, commercial and industrial areas. This wastewater is treated to reduce its objectionable properties that can cause unfavorable changes to environment and man. The conventional wastewater treatment methods are used to reduce the amount of suspended or floatable materials and treatment of biodegradable organic matters present in it. But advanced wastewater treatment methods are mainly used to remove all nutrients, suspended solids, dissolved solids and toxic substances present in it. Suitable waste treatment method is adopted on the basis of the contaminant present in the wastewater.
TYPES OF ADVANCED WASTEWATER TREATMENT METHODS
For suspended solids:
- Chemical coagulation and clarification
For organic matter:
- Adsorption using activated carbon
- Biological oxidation
For the removal of Phosphorous:
- Chemical precipitation and clarification
- Chemical coagulation and clarification
All these methods are briefly explained as below:
WASTEWATER TREATMENT BY MICROSCREENING
The equipment used for microscreening process is a rotating drum either with a plastic filter fabric or woven metallic mesh having an opening size of 20 to 60µ attached on its periphery. The drum is fitted inside the wastewater flow channel and it continuously rotates at a speed of 4 rpm. The wastewater enters into the horizontal drum at its upstream end and rotates radially outward through the mesh or micro fabric and leaving behind the suspended solid material which has a diameter greater than the diameter of mesh.
The retained solids in the mesh and inside the drum are backwashed using a high pressure water jet and recycled into the sedimentation tank. When the screening process proceeds the pores get clogged by the accumulation of particles and this causes development of a pressure inside the drum. This pressure will further help to filter small suspended particles which have diameter less than the mesh opening.
WASTEWATER TREATMENT ULTRAFILTRATION
It is a bundle of thin membranes of organic polymer of about 0.005 to .01 inches thick that are casted together to act as a single tube for filtration. It has extremely thin separation layer with a pore size of 3 to 100 Å. This technique is widely used in conjunction with the activated sludge process. When the effluent passes through the ultrafiltration tube, the membranes filter the suspended materials in it. These suspended solids are further returned into the activated sludge reactor to concentrate the biological solid material in there.
TREATMENT BY CHEMICAL COAGULATION & CLARIFICATION
Chemical coagulation is the process of adding a chemical which can destabilize the colloidal and suspended particles in the wastewater. As a result of destabilization the size of particles will increase and they settle as floc due to flocculation and agglomeration. These settled flocs can be removed using a sedimentation tank and the supernatant will be the treated effluent. Typical coagulants used are natural and synthetic organic polymers, metal salts such as Alum, Ferric sulfate etc.
Biological oxidation is the process of converting carbon based organic material present in the wastewater into humus. This process is aided by either bacteria or fungi through their enzymatic catalysis. When the organic matter is degraded CO2 is produced and the different components present in the organic matter are assimilated by pants. This process can either be anaerobic or aerobic depending upon the type of bacteria present in the organic matter to decompose.
ADSORPTION USING ACTIVATED CARBON
Char is produced when organic materials are heated at a temperature of 700˚Ϲ. This char is activated by exposing them to oxidizing gases such as steam and CO2 at high temperatures. These gases develop a porous structure on the surface of char which in turn will be activated carbon. This can be used to adsorb the organic materials into its surface. The adsorption sites can be regenerated when it is saturated.
Phosphorus is presented in wastewater is generally seen in the form of organic phosphate, polyphosphate or orthophosphate. This reaction consisting of adding chemicals such as alum, ferric chloride into the wastewater and they coagulate the phosphorous presented in it. The coagulated material will precipitate out such as Aluminum phosphate and ferric and this can be clarified further.