Principles of Unit
operations and processes in water and wastewater treatment and disposal
Water and waste water
differ only in the way that solids concentration will be more in waste water
and which will be there in water too if the same is taken from the river water
which makes the treatment process to both the same except that filtration is on
straining the finer particles in water treatment and is towards biological
action of bacteria on the trickling filter to remove organic solids.
Unit operations are the
physical operations to remove the impurities present in the water and waste
water where as the unit processes are the chemical and biological conversion on
the status of the impurities that they will be easily removed by chemical or
biological processes, both are applied especially to make the fine colloidal
particles to coalesce and grow in size to be removed from the water or waste
Screens are in use from
the intake structure where they prevent the floating matter to enter into the
pumping units, and fine and coarse screens are in use to treat waste water to
prevent the entry of floating wastes and coarse solids into the treatment.
Sedimentation is simply
detaining water for a sufficient time mostly in stagnant or relatively stagnant
position to make the flow velocity of water less than the settling velocity of
the solid particles that they without being driven by horizontal force settles
down by gravity. The efficiency of the process depends on the detention time,
how long the waste water remains within the sedimentation tank. When applied to
individual units we need not reduce the flow velocity but make it stagnant as
fill and draw type that the efficiency will be more. In the continuous flow
type the flow velocity is reduced to the level of the minimum velocity which
will not carry the particles with it according to stokes law that the vertical
velocity, which is the settling velocity of the particle will be more than the
horizontal drag velocity and the particle settles down. Mostly the tanks will
be rectangular and we also have circular tanks where the flow will be from
centre to periphery. Whatever may be the shape of the tank, it is the surface
area which makes the travel of particles independent of others which makes the
settling efficient that the depth has to be considered taking into effect the
sludge accumulation and to prevent the reentry of particles back to flow.
Coagulants are added to
the water that the flocculent particles grow bigger in size which is by
chemical reaction by rapid mixing and slow mixing and the coalescent particles
which grew in size gets removed by settling. The coagulant we add changes the
quality of water and the sludge volume too, and some of the coagulants add to
bulking of sludge where the removal of moisture is difficult. Lime water
instead of lime reduces the volume of sludge which is to all the solid
coagulants. Liquid coagulants have more influence readily on coagulant
particles than the solid coagulants which itself will take time to dissolve and
react with the particles.
Filtration is to the
removal of fine particle sand dissolved solids where the fine sand layer and
coarse sand layer below serves as the media to remove colloidal solids and the
water remains completely free of solids. In trickling filters the waste water
that trickles down gets oxidized that the organic matter grows in size and
retained over the sand medium and bacteria assimilate on the organic matter to
form layer on the surface which grows thicker and thicker to give more
bacterial mass to act upon the organic solids. The bottom most layer becomes
deprived of oxygen in due course of time that it sloughs and the same reaches
the secondary settling tank where the same gets settled for its increased
density. The activated sludge process is by induced aeration where the reactor
is completely mixed with sludge and air, that there will be complete oxidation
after which the waste water enters the secondary settling tank.
When all solids are
removed the minerals present in the water and waste water is removed by reverse
osmosis, demineralization process which is basically ion exchange process.
Odour and colour
present in water and waste water are removed by aeration and adsorption process.
The odour and colour causing elements are adsorbed and aerated that the water
is free from impurities for use and waste water for reuse and recycling. Toxic
chemicals and metals too get adsorbed with suitable media for adsorption.
The waste water if not
intended for recycling or reuse, they can be altered with low cost biological
treatment as septic tank, cess pools, oxidation ponds and ditches which are
long time process and adds nutrient value to the soil and can be used for
agriculture and this leads to lot of troubles as soil sickness, fly nuisance
and skin diseases as the sewage directly comes into contact with the plants.
The unit operations and
processes can be applied in individual units of houses, colonies and industries
that it gives fewer problems to the environment and handled with more
efficiency. The entire process of sedimentation, filtration and hardness
removal can be done at home, for removal of hardness we need not go for reverse
osmosis which is much expensive on installation and maintenance but the simple
lime soda process or boiling serve the purpose of both disinfection and
hardness removal as the water from the top stratum of aquifer will not be
saline in nature with chlorides and sulphates of calcium and magnesium as is seen
common with river water discharged with domestic and industrial wastes.
the lean concepts to treat water and waste water and which will lead to a
compact cost effective unit which is holistic and beneficial in the long run
for which the perspectives must be in line with the nature, Nature has answers
to all in a much simple way and we think we are superior to nature which makes
us live a life with complications. We cannot put the burden on government and
make them work for us which are proving futile neither we nor the government
show the inclusion for which we experience inefficiency all through.