Biological Methods of Waste Treatment

PRINCIPLES OF BIOLOGICAL METHODS OF WASTE TREATMENT

1. INTRODUCTION

In general biological treatment of wastewater involves (1) the conversion of the dissolved and colloidal organic matter present in wastewater to biological cell tissue and to end products, and (2) the subsequent removal of the cell tissue, usually by gravity settling in secondary settling tanks. Thus from a practical standpoint, the major concerns in biological treatment of sewage are with the creation of the optimum environmental and physical conditions to bring about the rapid and effective conversion of organic matter to cell tissue and its subsequent removal.

2. CLASSIFICATION OF BIOLOGICAL TREATMENT PROCESS

Since only 0.3 to 0.7% of solids are present in the sewage, if these solids are removed the water can be reclaimed and reused. The purpose of the sewage treatment is to remove the solids present in the sewage.

The solids present in the sewage are of two types viz.,
1) Organic solids, and
2) Inorganic solids.
Organic solids are the substances derived from living things like products from plant and animal. Examples of organic solids are carbohydrate, protein, and fat. The organic solids undergo decomposition by the microorganisms. Inorganic solids are inert materials and they do not undergo decomposition by the microorganisms. Inorganic solids are inert materials and they do not undergo decomposition. Examples of inorganic solids are grit, salt etc,
Microorganisms are unicellular microscopic living things. They multiple by cell division. They require oxygen for their respiration. They decompose the organic matter and convert them into cells. Examples of microorganisms are Bacteria, fungi, Virus etc. There are two types of microorganisms. They are:
1) Aerobic bacteria, and
2) Anaerobic bacteria.

Aerobic bacteria use dissolved oxygen (DO) from the water bodies for their respiration. They oxidize organic matter under aerobic conditions. The end products of the decomposition are water, CO2 and Cell tissues. Anaerobic bacteria use oxygen derived from chemical substances for their respiration. They multiply in absence of DO in the water bodies. They oxidize the organic matter under septic conditions. The end products include fowl smelling gases like H2S CH4 etc.
In the biological treatment, the microorganisms present in the wastewater are exploited by providing the favourable environment with food, DO, pH, temperature etc., to decompose the complex organic solids into simpler products like water, gases, nutrients and biological cells. The organic solids present in the wastewater serve as food for the aerobic microorganisms. The only thing to be provided is the DO, which is essential for the respiration of the aerobic organisms. In the biological treatment processes the DO is supplied either through nature means or by mechanical means by agitation.
Anaerobic organisms can multiply in the absence of DO and do decomposition, but the end products are undesirable fowl smelling gases. Hence anaerobic decomposition process is not generally preferred. However, anaerobic treatments are also adopted in certain situations because of certain specific advantages. Examples of anaerobic treatment processes are Septic tanks, UASB, Anaerobic Sludge digesters.

The biological conversion of organic matter can be accomplished both aerobically (in the presence of oxygen) and anaerobically (in the absence of oxygen). As stated earner As such based on the oxygen dependence or otherwise of the microorganisms responsible for the conversion of the organic matter biological treatment processes may be classified as (1) aerobic processes, (2) anaerobic processes; (3) anoxic processes and (4) facultative processes.

(1) Aerobic process. Aerobic processes are biological treatment processes that occur in the presence of oxygen.

(2) Anaerobuic processes. Anerobic processes are biological treatment processes that occur in the absense of oxygen.

(3) Anoxic processes. Anoxic processes are biological treatment processes in which nitrate nitrogen is converted biologically to introgen gas in the absense of oxygen. These processes are also known as anoxic denitrification process or anaerobic denitrification processes.

4) Facultative processes. Facultative processes are biological treatment processes in which the micro-organisms are indifferent (neutral) to the presence of dissolved oxygen. These processes are also known as aerobic-anoxic processes, or aerobic-anaerobic processes.

The biological treatment processes which involved aerobic conversion are usually preferred because aerobic bacteria are about three times more active than anaerobic bacteria at normal temperature and hence the rate of aerobic conversion is significantly more rapid than that of anaerobic conversion. Moreover, aerobic conversion does not produce bad smells and gases as are produced by anaerobic conversion.

The micro-organisms responsible for the conversion of organic matter can be maintained in suspension or attached to a fixed or moving medium and hence the above indicated biological treatment processes may be further subdivided as (a) suspended growth processes; (b) attached growth processes; and (c) combined processes.

(a) Suspended-growth processes. Suspended-growth processes are the biological treatment processes in which the micro-organisms responsible for the conversion of the organic matter or other constituents present in sewage to gases and cell tissue are maintained in suspension within the liquid.

(b) Attached-growth processes. Attached - growth processes are the biological treatment process in which the micro-organisms responsible for the conversion of the organic matter or other constituents present in sewage to gases and cell tissue are attached to some inert medium, such as rocks, slag or specially designed ceramic or plastic materials. Attached growth processes are also known as fixed film processes.

(c) Combined processes. Combined processes are the biological treatment processes in which the micro-oganisms responsible for the conversion of the organic matter or other constituents present in sewage to gases and cell tissue are maintained in suspension within the liquid as well as are attached to some inert medium.

The activated sludge process is the most extensively used aerobic suspended-growth biological treatment process. The other aerobic suspended-growth biological treatment processes which are also commonly used include oxidation ditch, aerated lagoons, and aerobic algal ponds (or aerobic stabilization ponds).

The trickling filter process, and its variations, is the most commonly used aerobic attached-growth biologicl treatment process. In a trickling filter the medium to which the micro-organisms are attached is fixed. The rotating biological contactor or rotating biological disc process is a recent variant of trickling filter process in which the medium to which the micro-organisms are attached is moving.

The aerobic combined biological treatment processes include in sequence trickling filter and activated sludge processes, and activated sludge and trickling filter processes.

The anaerobic suspended-growth biological treatment processes include anaerobic digestion process and anaerobic contact process.

The anaerobic attached-growth biological treatment process include anaerobic lagoons (ponds) and anaerobic filters.

The facultative (or aerobic-anoxic, or aerobic-anaerobic) suspended-growth and attached-growth biological treatment process include single-stage nitrification-denitrification process and nitrification-denitrification process respectively.

The facultative (or aerobic-anoxic, or aerobic-anaerobic) combined biological treatment processes include facultative lagoons (ponds), anaerobic facultative lagoons, anaeobic-facultative-aerobic lagoons, and maturation or tertiary ponds.
The most popular aerobic processes namely activated sludge and tricklity filter process have been discussed hereunder.

3. ACTIVATED SLUDGE PROCESS

The activated sludge process of wastewater treatment is based on providing intimate contact between the wastewater and activated sludge. The activated sludge is the sludge which is obtained by settling sewage in presence of abundant oxygen so as to be enriched with aerobic micro-organisms. Thus activated sludge is biologically active and it contains a large number of aerobic bacteria and other micro-organisms which have an unusual property to oxidize the organic matter.

In the activated sludge process wastewater enters an aeration tank where the organisms and sewage are mixed together with a large quantity of air. Under these conditions three more-or-less distinct activities occur. First, the organisms oxidize a portion of the organic matter present in the sewage to carbon dioxide and water, and other end products to obtain energy for cell maintenance and synthesis of new cell tissue, Second, synthesize the other portion of the organic matter and convert it into new microbial cell tissue using part of the energy released during oxidation. fInally, when the organic matter is used, the new microbial cells begin to consume their own cell tissue to obtain energy for cell maintenance. This third process is called endogenous respiration. These three activities are defined by the following generalized chemical reactions:

Oxidation
Organic matter + O2 + bacteria CO2 + H2O + NH3
(COHNS)* + other end products + energy …

Synthesis
Organic matter + O2 + bacteria + energy New microbial cell tissue
(COHNS)* (C5H7NO2)**…

Endogenous respiration
New microbial + 502 5CO2 + NH3 + 2H20 + energy cell tissue
(C5H7NO2)**…
As a result of intense mixing of activated sludge with sewage in the presence of ample quantity of oxygen the following effects take place:
(i) Organic matter present in sewage is oxidized; and
(ii) suspended and colloidal matter coagulate and form flocculent masses which are readily settleable.

The mixture of returned activated sludge and sewage in the aeration tank is referred to as mixed liquor. The mixed liquor then enters a settling tank where the flocculent masses enriched with micro-organisms settle and are removed from the sewage. Thus new activated sludge is continuously being produced in this process. A portion of the settled sludge enriched with micro-organisms or activated sludge is then recycled to the head end of the aeration tank to be mixed again with sewage. Once the required concentration of micro-organisms in the mixed liquor has been reached so as to maintain proper food/micro-organisms (F/M) ration for optimum ---------------------------------------------------------------------------------------------------------------------
* The term COHNS represents the elements carbon, oxygen, hudrogen, nitrogen and sulphur and its is used to represent the organic matter.

** The term C5H7NO2 is used to represent cell tissue.

operation, its further increase is prevented by regulating the quantity of sludge recycled. The excess sludge produced each day (termed as waste activated sludge) is disposed of together with the sludge from the primary settling tanks.

Aerobic and facultative bacteria are the predominant micro-organisms which carry out the above reactions of organic matter i.e., oxidation and synthesis. Nutrients should be present in sufficient quantity in the sewage or they may be added as required for the reactions to proceed satisfactorily. A generally recommended ration of BOD5:N:P is 100 : 5 :1.

The effluent obtained from a properly operated activated sludge plant is clear sparkling and it is of high quality, usually having a lower BOD than that from a trickling filter. Typical BOD5 and suspended solids concentrations in the effluent obtained from an activated sludge plant vary from 10 to 20 mg/L for both constituents. In general the percentage of removal of suspended solids and BOD in an activated sludge plant may be as high as 90 to 95 percent.

The main advantage of activated sludge process is that it offers secondary treatment and an effluent of high quality with a minimum land area requirement. However, in this process a rather close degree of control is necessary in its operation to ensure that (I) an ample supply of oxygen is present; (ii) there is intimate and continuous mixing of the sewage and the activated sludge; and (iii) the ration of the volume of activated sludge added to the volume of sewage being treated is kept practically constant. Moreover, there is a problem of obtaining activated sludge at the start of a new plant. Hence, when a new plant is put in operation, a period of about 4 weeks is required to form a suitable activated sludge, and during this period almost all the sludge from the secondary settling tank will be returned through the aeration tank. However, a new plant may also sometimes be seeded with activated sludge from another plant, so as to quickly start the process in the new plant.

4.0 TRICKLING FILTERS.
Trickling filters are also known as percolating filters or sprinkling filters. The concept of a trickling filter came up from the use of contact bed filters. The modern trickling filter consists of a bed of highly permeable media of coarse, rough and hard material. The micro-organisms responsible for the conversion of the organic matter or other constituents present in the sewage to gases and cell tissue, grow attached to some inert medium, such as rocks, slag or specially designed ceramic, plastic material or synthetic materials under aerobic conditions are classified as aerobic attached growth processes (or fixed film processes). The aerobic attached growth processes include different types of sewage filters. The filter units used for biological treatment of sewage consist of open beds of coarse aggregates over which effluent from primary settling tank is applied or sprinkled intermittently. In sewage filtration there is very little mechanical action of filtration, since only the coarsest particles are arrested. The filtering media in sewage filters provide contact surface essential for the growth of aerobic bacteria which help in the oxidation of the fine suspended, colloidal and dissolved organic matter present in the sewage and thus BOD of the sewage is satisfied, and they also help to achieve nitrification (the conversion of nitrogen in the form of ammonia to nitrate). These bacteria derive the necessary air from the atmosphere. The finer the filtering material, the larger is the total superficial area giving lodgment to the aerobic bacteria, but the possibility of clogging and reduction in the voids which supply the air are the factors which limit the size of the filtering media. The effluent from the filtering units, containing stabilized new compounds, is settled out in secondary settling tanks (or secondary clarifiers). Sewage is applied to the surface of the bed intermittently by sprinkling through rotary distributors and trickles downward through the bed to underdrains, where it is collected and discharged through an outlet channel, and conveyed to a conduit leading to a settling tank. A gelatinous microbial or biological film or slime layer is formed around the particles of the filtering media. The colour of this film is blackish, greenish and yellowish. The film mainly consists of aerobic bacteria and besides bacteria and besides bacteria it may consist of fungi, algae, lichens, protozoa, etc. for the existence of this film, oxygen is supplied by the provision of suitable ventilation facilities in the body of the filter and also by the intermittent working of the filter. The fine suspended, colloidal, and dissolved organic matter present in the sewage collect on this film, where biochemical oxidation of the organic matter accomplished by aerobic bacteria. A new filter when first put into use will usually be quite ineffective for about two weeks until a satisfactory gelatinous film has formed on the particles in the bed. The film eventually becomes quite thick with accumulated organic matter and will slough off (or unload) from time to time and be discharged with effluent. The effluent from trickling filters therefore require sedimentation to remove the solids that pass the filter. Thus trickling filter is always followed by a secondary settling tank to remove from the filter effluent the organic solids produced in the filtration process.

The trackling filter is always preceded by primary sedimentation along with skimming devices to remove scum. This will prevent the clogging of the filter with the larger suspended solids present in the sewage. Many trickling filter installations are designed so that a portion of the effluent can be recirculated back through the filter. Recirculation provides a longer effective contact time in the filter and odours are reduced since the waste is diluted in the recircultion. In proportion which is recirculated.

The trickling filter serves both to oxidize and bioflocculate the organic matter in sewage and their efficiency is assessed on the total reduction BOD effected through the filter and the subsequent settling tank, since the effluent quality is reckoned after the settlement of the bioflocculated solids.

Trickling filter are used for the biological treatment of domestic sewage and those industrial wastes which are amenable to aerobic processes. They find use for complete treatment of moderately strong wastes and as roughing filters for very strong wastes prior to activated sludge units. Trickling filters possess a unique capacity to handle shock loads and provide dependable performance with a minimum of supervision.

As sewage trickles through the filter media a microbial or biological film or slime layer is formed on the filter media. The organic matter present in the sewage is adsorbed onto the biological slime layer. In the outer portion of the biological slime layer the organic matter present in the sewage is degraded by aerobic micro-organisms. As the organic matter abstracted from the flowing sewage is synthesized into new microbial cells the biological slime layer grows in thickness. As the thickness of the slime layer increases the diffused oxygen is consumed before it can penetrate the full depth of the slime layer, and hence in the lower portion of the slime later near the surface of the filter media an anaerobic environment is established. Thus the grown slime layer which usually has a total thickness between 0.1 and 2.0 mm consists of both aerobic and anaerobic layer. The thickness of the aerobic portion of the slime layer is limited by depth of penetration of oxygen into microbial layer which depends on the coefficient of diffusivity of oxygen in the film, the concentration of oxygen at the solid-liquid interface and the overall oxygen utilization rate of micro-organisms present in the slime layer.
As the slime layer increases in thickness, the adsorbed organic matter is metabolized before it can reach the micro-organisms near the surface of the filter media. As a result of having no external organic source available for cell carbon, the micro-organisms near the surface of the filter media enter into an endogenous phase of growth and lose their ability to cling to the surface of the filter media. Eventually there is scouring of the slime layer due to flowing liquid and a new slime layer begins to grow on the filter media. This phenomenon of scouring of the slime layer is called sloughing or unloading of the filter and is primarily a function of the organic and hydraulic loading on the filter. The organic loading accounts for the rate of metabolism in the slime layer, and the hydraulic loading accounts for the rate of metabolism in the slime layer, and the hydraulic loading accounts for the rate of metabolism in the slime layer, and the hydraulic loading accounts for shear velocities. Filter sloughing aids ventilation by keeping the filter media open. It also continuously renews the bio-mass, maintaining it active which is essential for efficient functioning of the filter.

On the basis of hydraulic and organic loading rates trickling filters are classified as (i) low-rate filters; (ii) intermediate-rate filters; (iii) high-rate filters; and (iv) super-rate filters.


CONCLUSION:

By these methods we can do the biological treatment of Waste Water.
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