Waste Water Treatment
Waste Water Treatment Techniques
There are various Physical, Chemical and Biological Techniques of conventional wastewater treatment.
Chemical Treatment : Coagulation
Physical Treatment : Flocculation, Sedimentation, Filteration etc
Various steps involved in wastewater treatment are summarized in the flow chart and will be discussed in this tutorial.
Coagulation and Flocculation
Suspended particles are sometime too small to settle in reasonable time period and cannot be removed by simple filtration
Coagulation alter the size of these particles in such a way to allow them to adhere to each other, thus can grow in size and settle easily (0.001 to 1µm)
Most naturally occurring particles in water are negatively charged. Since like charges repel, these small particles, or colloids, will remain suspended almost indefinitely.
Chemicals reduce colloidal surface charge and form precipitates that enhance the clustering process and sedimentation
Flocculation is the slow stirring or gentle agitation to aggregate the destabilized particles and form a rapid settling floc.
Alum – Al2(SO4)3.18H20 is generally used as a coagulant.
Al+3 is formed in water, most of Al+3 react with alkalinity (bicarbonate) to for insoluble Al(OH)3 which adsorbs positive ions from solution and forms precipitate of aluminium hydroxide and adsorbed sulfates
If less bicarbonate, pH raised by Ca(OH)2, Na2CO3
Other coagulants are FeCl3/FeSO4
Prior to entering a flocculation tank, the water flows through a rapid mixing basin which has rotating paddles. Detention time of one-half minutes
Flocculation is gentle mixing to speed the agglomeration of colloidal materials. Time 1.5 hr. Precipitating Al(OH)3 attracts colloidal particles forming visible flocs.
The water enters a small tank or section of a tank in which paddles are turning slowly. Their movement causes the small particles to collide and stick together (fast or vigorous mixing would separate combined particles).
The gentle mixing combined with the reduction in repulsive surface charges allow the particles to join rapidly. Mixing also prevent settling of flocs in this chamber
Sedimentation and Filtration
Its is a large circular/rectangular concrete tank designed to hold water for long time so that all suspended solids can settle out
Detention time (DT) 1-10 hours (Time duration for which wastewater is stored or stagnated in sedimentation tank for effective sedimentation). Longer DT, more expensive the system becomes and gives better performance.
Solids that collect at bottom are removed manually by shutting down tank and washing sludge or mechanically by using bottom scraper
Effluent from the tank is then filtered
Rapid Sand Filters – layer of sieved sand on top and bed of graded gravels; Opening b/w grain of sands often greater than size of flocs. Clogging removed by shutting down and cleaned by forcing water backward through the sand for 10-15 min
- Clarified supernatant leaving the top of the sedimentation tank (overflow).
- Concentrated sludge leaving the bottom of the sedimentation tank (underflow).
Disinfection is done to kill pathogens. Chlorination using Cl2 gas, sodium hypochlorite (NaOCl), Calcium hypochlorite Ca(OCl)2
There are long drain like channels in which disinfection is done.Chlorine mostly used as cheap and available. It is strong oxidizing power destroys enzymatic process of cell life. Good for bacteria but less effective for protozoan cyst, Giardia lamblia, Cryptosporidium or viruses