microbiological wastewater treatment

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.

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