Efficiency
There are many different types of efficiency including removal, gain, and energy transfer. A simple formula for finding percent efficiency is:
More on Efficiency
Hello, I’m Jay Blevins from Mountain Empire Community College. Efficiency can
be stated as the act of being adequate in performance with a minimum of waste
or effort. There are many different types of efficiencies that play a role in
water and wastewater treatment. At a sewage treatment plant or a water treatment
plant, efficiency often involves the removal of a substance. For example, if
a 200-milligram per liter of B.O.D. (influent) in a sewage treatment plant is
reduced to 10 milligrams per liter of B.O.D. (effluent), what is the efficiency?
The efficiency is found by subtracting the amount reduced to- 10 milligrams,
from the initial amount- 200 milligrams to find the amount reduced- 190 milligrams
per liter. Since 190 milligrams per liter were removed, the efficiency is190
divided by 200, which yields an efficiency of 95%.
The efficiency of removal works in other processes in addition to water and
sewage treatment. The second type of efficiency involves being able to add a
substance. Oxygen, for example, must be added to water during wastewater treatment.
Nature adds oxygen to water by the air to surface ratio, atmospheric pressure,
and temperature. In a sewage treatment plant, an
operator changes the rate of oxygen being added to the water by increasing the
surface area between the air and water.
The surface area can be increased in several ways. Large bubbles having less
surface area for the mass of air are less efficient than small bubbles at the
transfer of oxygen because the surface area is actually less for the mass of
oxygen. Most treatment plants use the larger bubbles (greater than 1/8") because
less maintenance is required on the aerator nozzle. The air, in order to be
pumped, must be compressed and this requires energy. When the air is released,
the energy of compression is lost within the system. Pumping air is one of the
most inefficient ways of putting oxygen into water. Even so, most sewage treatment
plants pump air.
The second method for increasing the amount of oxygen in water is achieved by
the use of step aerators. Since water can be pumped and is non-compressible,
the energy of compression is not lost in the system. Operators can pump water
over a set of baffles, or steps, and increase the surface area of the water,
thus transferring oxygen at a much cheaper rate. The limiting factor is being
able to supply enough space for the reaction to occur. At the Blountville, Tennessee
sewer plant, water is pumped over baffles and the average electricity cost per
ton of B.O.D. removal is $89. In contrast, the CNW plant at Coeburn, Virginia
pumps air and the average cost per ton of B.O.D. removal for electricity is
$350. (Note- Blountville does not thicken sludge. The plant mixes the sludge
with lawn clippings, wood chips, leaves, etc. to make compost for land application.)
Efficiency is sometimes measured against a standard. Often the standard is the
most that can be achieved given a set of circumstances. For example, if a plant
requires $50 worth of energy to remove a ton of sewage from the water, then
that number establishes the efficiency for the energy required to remove the
ton of sewage. When all the sewage is removed at a cost of $50, the system is
100% efficient. If it takes a $100 worth of energy a ton to remove the same
ton of B.O.D., what would the efficiency be? The efficiency would be 50%. How
does efficiency work? 100 % efficiency is the standard by which all processes
are rated. Efficiency ratings are useful for progress monitoring. In other words,
efficiency compares work required with work accomplished in a meaningful way.
For improvements to occur in any system, a set standard is required to enable
goals to be set, maintained, and even broken. Efficiency deals with the utilization
of the energy involved within a system.
Efficiency can also be an inverse function of the process in the situation of
transferring energy. To illustrate, look at the process of raising animals.
In order for a farmer to make a profit from selling livestock, the output must
be greater than the input. A pig farmer would determine efficiency by the pounds
produced in meat verses pounds in grain fed. To improve efficiency, a farmer
would only breed pigs, which consistently yielded an increase in weight with
the least amount of food. This process optimizes the food to organism ratio.
Organisms differ in efficiencies. Pigs are only about 35% efficient at converting
food to weight, while fish have been determined to be about 75% efficient in
food to weight gain. Some of the reasons for the difference can be attributed
to the fact that pigs and other warm blooded animals must keep themselves warm
(sustain a metabolism) while fish are dependent upon the surrounding water temperature
to maintain body temperature. Since less energy is spent controlling body temperature,
fish are more efficient at increasing weight for the same amount of food.
So far this lesson has covered the efficiency of removal, efficiency of transfer,
and the efficiency of gain, of which the example is weight. The concept of efficiency
is important when dealing with microorganisms. Microorganisms eat organic matter
in the water. Matching the food to microorganism ratio optimizes the growth
of the microorganisms and removal of B.O.D. Other conditions, such as pH, temperature,
and D.O., affect microorganisms and thus have an impact on efficiency. Conditions
can be changed to favor the growth of one type of microorganism over another
to produce a desired effect such as ammonia treatment by raising the pH and
lowering the D.O. Water temperature is harder to control because heating requires
energy and is not cost
effective.
Efficiency, power, and energy are interrelated. The energy produced divided
by the energy supplied defines the efficiency. For example, fuel oil is burned
in a furnace to produce heat. If all the fuel oil consumed provided useable
heat, the furnace would be 100% efficient. Some of the heat is lost through
the chimney and all the fuel is not burned within the furnace. A furnace is
only 80 to 85% efficient. Thus energy is related to efficiency by the utilization
there of. Energy is the ability to do work and can be either potential or kinetic.
Power is the way or system in which the energy is harnessed. There are five
basic power systems: electrical, mechanical, heat, light, and fluid. A car with
a 200 horsepower motor has twice the power of a car with a 100 horsepower motor.
The car with more power is faster or in other words performs the same job at
a faster rate. As the power and rate increase, the effective utilization of
energy decreases. To illustrate this principle, water is transferred from inside
the college to the greenhouse below. If the water is allowed to flow through
a 3/4-inch line all day, gravitational pull would be the only energy required
to accomplish the task. However, if the same amount of water was supplied in
5 minutes through the same size line and over the same distance, a 5000 horsepower
pump would be required to accomplish the task. The distance of pipe, the amount
of water, and the size of the pipe remained constant in both situations. When
the allotted time was decreased, the amount of power required was increased,
and the efficiency of energy utilization decreased. Thus the faster a job is
performed, the less efficient a system becomes in the transfer of energy and
the higher the monetary cost.
Efficiency requires balance in many different problems. All components must
match for a system to be efficient in energy transfer. For example, pumps have
to match hydraulic systems and even hammers have to match nails. A small hammer
is ineffective at driving a large nail and vice versa. The hammer must match
the nail in order to provide the most efficient transfer of
energy. In addition among the concepts, vector forces contribute to efficiency.
The direction of force a hammer is applied can organize the power and energy
is such a way that either drives the nail in or glances off. Efficiency is an
important concept to an operator and is often implemented in the water waste/water
field.
