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Hi mugee!

”Today, we will see all about a microbiological wastewater treatment
plant”

The biological process of wastewater is a secondary treatment
involving the components of removing, stabilizing and rendering
harmless very fine suspended matter, colloids and dissolved solids
of the sewage, that come from the sedimentation tank, where most of
the matter in suspension has been removed. In some cases, effluent
from sedimentation tank may be good enough for disposal if the
dilution is great. However, in most cases, oxidation of the organic
putrescible matter is necessary.

**Principle of action**

The primary principle of action on which the biological process is
based is the availability of a large sewage surface fed by the
oxygen from air, where certain type of bacteria, the aerobics, live
and use that oxygen to oxidize putrescible matter in the sewage to
stable and inoffensive sulfates, nitrates and other compounds.

The sewage filtration, which is the vehicle used for process, can
at best cause only the coarser particles of suspended matter to be
removed by mechanical straining. This action is only minor and of a
secondary nature. The major action takes place at the surface,
where the aerobic bacteria oxidizes the finer organic particles of
sewage abounding large surface areas, forming a bacterial film. is
formed. The film adsorbs more of the finer matter which is then
worked upon by the organisms present after which it is released as
a coagulated suspended matter, rather heavy and capable of settling
readily.

It should be noted that this bacterial film also contains, in
addition to the aerobic bacteria, other organisms as protozoa,
algae, besides certain species of worms. But their action is
somewhat uncertain and the biological action is considered to be
mainly due to the aerobic bacteria.

**Microbiology of wastewater treatment**

Most wastewaters have putrifying (rotting in due course) organic
matter. Biological wastewater treatment systems are to covert the
organic matter into easily manageable end products, such as carbon
dioxide, methane and humus, which can be utilized or disposed off
without affecting the environment. The microorganisms use the
organic matter as food to provide energy and carbon for cellular
synthesis.

Industrial fermentation uses aseptic techniques to maintain pure
cultures and the environment is controlled. Biological wastewater
treatment systems are only partially controlled. The wastewater
(substrate or food) characteristics may change from time to time,
there are changes in temperature and there is always a
heterogeneous inoculum of microorganisms from soil and air. This
results in a variety of microorganisms participating in the
reaction. The fittest survive and dominate the population. When the
compounds in wastewater are metabolized, intermediate compounds
serve as food for other microorganisms.

The population of individual microorganisms and the community
structure also changes from time to time reflecting the changes in
environmental conditions. It is possible to zero in on groups of
microorganisms participating in the process, based on their overall
biochemical reactions.

Today, we will continue from where we left on the microbiological
wastewater treatment plant. I will compare the construction, uses,
merits and demerits of different components of the microbiological
wastewater treatment plant with reference to each other.

**Intermittent sand filters**

The treatment involved in the case of intermittent sand filters
applies the sewage, that has already undergone preliminary
treatment, onto the filter beds of sand at regular intervals. By
this, air can enter the interstices of the bed between the dose of
sewage to supply the required aerobic bacteria.

**Construction**

The filter consists of a layer of clean, sharp sand, with an
effective size 0.2 - 0.5 mm and of uniformly coefficient 2 - 5, 75
to 105 cm deep having underdrains, surrounded by gravel to carry
off the effluent. The sewage is applied by means of a dosing tank
and siphon; it then flows into troughs laid on the filter bed. The
troughs have side openings, which allow the sewage to flow on the
sand. To prevent any displacement of sand, blocks may also be used
underneath the sewage streams. After an interval of 24 hours,
sewage is now applied over a second bed while the first bed rests.
Usually, three to four beds may thus be working in rotation. During
the resting period, the dried sludge accumulating on the sand
surface is the resting period; the dried sludge accumulating on the
sand surface is scraped off. The organic loading of the filter bed
is not heavy, only 0.825 to 1.1 million liters per hectare per day.

**Uses**

It is found that the effluent from an intermittent sand filter is
usually better in quality than that resulting from any other type
of treatment and can even be disposed off without dilution.
However, because of the large land area required, filters of this
type are now seldom constructed in cities. They are primarily
suited for institutions, hospitals and other small installations.

**Contact beds**

In this type, the sewage applied on the contact material is allowed
to stand undisturbed for some time before, being emptied and an
interval is allowed before recharging the bed. During the 'contact
period', when the filter is standing full, the fine suspended
particles of sewage are deposited on the contact material and
worked over by the anaerobic organisms. During the 'empty period'
that follows next, the deposited matter is oxidized by the aerobic
bacteria. It is then washed off the contact material and carried
out with the effluent on the next emptying of the tank.

**Construction**

A contact bed is a watertight tank with masonry walls and very much
similar in construction to an intermittent sand-filter. The contact
material is made of broken stone called ballast and of 2.5 - 7.5 cm
gauge. The tank is filled with the sewage over a period of an hour;
allowed to stand full over a period of two hours, then emptied
through underdrains. This process takes another hour. The tank is
now left empty ffor 3 to 4 hours before admitting the next charge.
(Thus with a total working period in a shift of 8 hours, the
contact bed can be worked in three shifts daily). The organic
loading in this case is about the same i.e., 1.1 million liters per
hectare per day.

**Uses**

The contact beds method is now only of historical interest and not
commonly used. This is mainly because of the loss of efficiency
brought about by the exclusion of air when the tank is standing
full. For an efficient biological action, it is imperative That the
aeration should be through the mass of sewage. It has therefore,
been superseded by more efficient biological methods, as in the
case of trickling filters and activated sludge plants.

However, the contact beds have some merit when compared to the
trickling filters as:

A.Lesser operating head required
B.Freedom from filter (psychoda) flies
C.Lesser nuisance due to odor

**Activated sludge**

When wastewater is aerated sufficiently, its organic matter reduces
and a flocculant sludge (consisting of various microorganisms) is
formed. In order to improve the process, the flocculant activated
sludge is retained in the system as inoculum. This is achieved by
settling the wastewater and recirculating the microbial mass. A
part of this sludge is wasted periodically as synthesis of new
cells continues. The organisms involved are aerobic
chemoheterotrophic, i.e., those which utilize organic compounds as
source for carbon (for cellular synthesis) and energy (by using
oxygen as electron acceptor).


1.Phase i: initially, the macromolecules are hydrolyzed or broken
down into their monomer compounds. These reactions are usually
carried out extracellularly. Once their size is reduced they are
transported into the cell.
2.Phase ii: later, the small molecules produced in phase i are
partially degraded, releasing 1/3rd of their total energy to the
cell. In the process a number of different products are formed
which serve as precursors of both anabolic and catabolic routes of
phase iii.
3.Phase iii: the catabolic route oxidizes the compounds and produces
carbon dioxide and energy. The anabolic route (which requires
energy) results in synthesis of new cellular material. Many
microorganisms participate in the above reactions. Both the lower
and higher protists have significant roles to play. Generally, the
organisms in activated sludge culture may be divided into four
major classes (these are not distinct groups and any particular
organism may display more than one such behavior):

i. Floc-forming organisms: these help to separate the microbial
sludge
from the treated wastewater. Zooglea ramigera and a variety of
other organisms flocculate. Flocculation is understood to be caused
by the extracellular polyelectrolytes excreted by these
microorganisms. Saprophytes: the saprophytes are micro-organisms
that degrade the organic matter. These are mostly gram-negative
bacilli such as pseudomonas, flavobacterium, alcaligenes and the
floc formers.

ii. Predators: the main predators are protozoa which thrive on
bacteria. It has been found that the protozoa can be upto 5% of the
mass of biological solids in the systems. Ciliates are usually the
dominant protozoa. They are either attached to or crawl over the
surface of sludge flocs. Rotifers are the secondary predators. When
rotifers occur in plenty, we can be sure of a well stabilized
waste, since rotifiers perish in highly polluted waters.

iii. Nuisance organisms: nuisance organisms interfere with the
smooth
functioning of the system, when present in large quantities. Most
problems arise due to sludge settling (due to presence of
filamentous forms which reduce the specific gravity of the sludge).
The bacterium sphaerotilus natans and the fungus geotrichium are
often responsible for this situation.

**Trickling filter**

Trickling filters have biomass growth attached to a solid surface
over which the wastewater flows in thin sheets, supplying nutrients
to the microbial community. The biochemical reactions are similar
to those in an activated sludge, which have a rich mixture of:

Eucaryotic Procaryotic organisms

Trickling filters contain these and also higher life forms like:
Nematodes Rotifers Snails Sludge worms Insect larvae Filter flies
(psychoda)

The complex food chain prevailing in this allows complete oxidation
of organic matter and lower quantity of surplus organisms (sludge).
The microbial film grows in thickness, due to increased hydraulic
shearing and development of an anaerobic layer next to the solid
medium. The anaerobic reactions solubilize the anchoring
microorganism. Algae can also flourish on the upper surface.
However, they do not play significant role in waste stabilization.

Also called percolating filters, the trickling filters are similar
to contact beds in construction, but allow constant aeration and
the action is continuous. The name is a misnomer since the
biological unit neither filters nor it trickles. The main function
of a trickling filter is to remove unstable, organic materials in
the form of dissolved and finely-divided organic solids and to
oxidize these solids biologically to form more stable materials.
The biological process involved in the filter is due to the growth
of a microbial film on the surface of the filter medium. The film
is made up of zoogleal slime, viscous jelly-like substance
containing bacteria and other biota. Under favorable environmental
conditions, the slime adsorbs and utilizes suspended, colloidal and
dissolved organic matter from the sewage. Although classified as an
aerobic treatment device, the microbial film is aerobic to a small
depth of 0.1 - 0.2 mm. While at the bottom, a larger depth is
anaerobic. When the sewage is flowing over the film, the soluble
organic matter is rapidly metabolized with the colloidal organics
adsorbed onto the surface. As the biota die, they are discharged
from the filter with more or less partly decomposed organic matter.
This sloughing off of material may occur periodically as in a
standard rate filter or continuously as in a high rate filter.

The essential features necessary to the process are:

1. Sufficient surface area must be provided for biologicalgrowth.
2. Free oxygen must be available at the surface to replenish the
dissolved oxygen extracted from the liquidlayer.
3. Sewage, and in particular industrial wastes must be amenable to
biological treatment.

**Construction**

A trickling filter consists of a bed of crushed stone or other
non-disintegrable contact material viz., granite, limestone etc.,
25 cm and 75 cm in size, with the filter depth usually between 2
and 3 m. The larger stones 8 cm - 10 cm. in size are placed in a
layer 15 cm - 20 cm thick at the bottom of the bed, while the
smaller size stones 2.5 cm size make up the filter bed. The Inside
walls of brick masonry may be honey combed (with the idea of
securing better aeration of the beds) and provided with airinlets.
In such a filter, air must circulate freely so as to maintain the
zooleal flora, which thrives over the stones in the presence of
oxygen. The sewage from the sedimentation tank is applied either
intermittently through fixed sprays located at the surface of the
bed or by what is more favored, i.e., applying sewage continuously
through rotary distributors. A rotary distributor consists of two
or more arms which are turned in a horizontal plane through the jet
action, or sometimes when it is insufficient, moved by the
electrical power. The spray nozzles are circular holes 9 mm - 13
mm, and spaced in such a manner that the distribution of applied
sewage is more or less in direct proportion to the area of the bed
covered by each part of the distributor.

The floor of the trickling filter is made of concrete laid to a
slope of 1 in 200. It has a system of underdrains, half-round or
v-shaped channels cast into it and making a false bottom with
perforated cover to support the coarse media above. The
underdrainage system keeps the filter self-cleansing and also
assists in the ventilation of beds.

**Merits and demerits**

The advantages of trickling filters are:

1. They are self-cleaning. Rate of filter loading is much higher.
2. No diminishing of capacity even if overdosed, they can recoup
after
rest.
3. They are cheap and simple in operation.
4. Mechanical wear and tear is very small.

The disadvantages are:

1. High head loss through the filter, making automatic dosing of
filters as necessary.
2. Odor and fly nuisance due to psychoda which may be carried away
into human habitation and may prove a serious nuisance to man. The
latter may be overcome by flooding the filter or by the use of DDT
or other insecticides.
3. Large land area is required. Cost of construction is relatively
higher.
4. They require preliminary treatment and, therefore, cannot treat
raw
sewage as such.

I hope this comparitive study of the different components of the
microbiological wastewater treatment plant provides the right
guidance for your plant.

For more:...
http://www.all-about-wastewater-treatment.com/inst1.html

"Everything you really need to know about
Wastewater treatment, in one place!"

From the Desk of Richard Runion and Team

Richard Runion
The Water Patriot
6423, Woodbine Court,
St. Louis, Missouri,
63109, USA
http://www.all-about-wastewater-treatment.com/inst1.html

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