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FIXED FILM SEWAGE TREATMENT SYSTEMS

iii.            Trickling Filters: It is simple and relatively inexpensive device consisting of 8-12 feet deep bed of stones at the surface of which the liquid sewage is sprayed either by a series of sprinklers or by a rotating sprinkler. The spraying is done intermittently to permit the proper aeration in between the stones to maintain the aerobic condition. The stones become coated with a film of microorganisms that oxidize the organic matter in wastewater as it seeps slowly through the bed. Thus, at the bottom of the filter, relatively clear water is collected that is drained off. Sewage may be passed through two or more trickling filters or recirculated many times through the same filter. The drawback of this system is that nutrient overload may lead to excess microbial slime, reducing the aeration and percolation rates and it becomes necessary to renew the filter bed (Fig. 17.3).

Fig. 17.3 Trickling filter device.

iii.            Rotating Biological Contractors (Discs): This system consists of a set of 2-3 m diameter rotating discs (1-2 rpm) made up of wood, metal or plastic that are oriented vertically to the direction of flow of wastewater and are partially submerged in the water. The microbial film adhering to the disc decomposes the organic matter in the wastewater aerobically where the rotation of discs provides aeration. The microbial film grows to a thickness that prevents the nutrients to reach the inner microbial cells leading to their death and detachment that can removed by settling. It has also high BOD removal efficiency, requiring less space and energy for rotating the discs but requires higher initial investment (Fig.17.4).

Fig. 17.4 Rotating biological contractor unit.

(b) ANAEROBIC DECOMPOSITION

These digestors, though, are slower but they save energy. (Large-sca1e anaerobic digestors are primarily used for processing of sewage sludge and for the treatment of some very high industrial effluents having very high BOD).

(i)                 Septic Tank: This type of disposal system is simplest and is widely used in rural areas by individual families where public sewers are not available. It is made up of metal or concrete chamber below the ground level to which all the domestic wastes flow. Here, the wastewater is retained long enough to permit limited anaerobic digestion of various organic materials by anaerobic bacteria to produce simpler chemicals and gases, like H2S, CH4, CO2 and H2 and the residual solids settle to the bottom of the tank. This water has still high BOD and is odourous. The clarified effluent from the tank is disposed on under the soil surface (leaching field) through perforated pipes where dissolved organic matter undergoes oxidative biodegradation. The process does not eliminate pathogenic microorganisms. The undecomposed solid, called called sludge, is pumped out periodically to prevent clogging of tank and the drain field (leaching ground). After dying, the sludge can be used as manure (Fig.17.5).

 

Fig.17.5 Septic tank device

(i)                 Inhoff Tank: It is an improved design of septic tank with a a separate settling compartment from where the stabilized solids are removed (from the bottom of the tank) (Fig.17.6).

(ii)               Tertiary Treatment: This is mainly the chemical treatment to remove the turbidity due to presence of nutrients and, being expensive, generally applied when the wastewater is to be used for drinking purpose and rarely for the disposal purpose. The inorganic salts in water after secondary treatment may stimulate growth of aquatic plant life, e.g. algae. Some of these salts can be removed by chemical treatments, e.g.
addition of alum that induces the precipitation of phosphate salts, which can be settled out. Activated carbon filters are used to remove non-biodegradable organic pollutants from secondary treated industrial effluents. NH4+ can be removed at high pH by volatilization as NH3 (but it can return through rainfall again). The pathogens can be killed by treatment of water by chlorine (2-5 ppm hypochlorite or Cl2 gas)
that is most commonly practised, UV radiation or O3. Chlorination by adding hypochlorous acid (HOCI) can also remove NH3

Fig.17.6 Section of an Imhoff tank.

Hypochlorite is used as Ca(OCl)2 or NaOCI. The Cl2 gas reacts with H2O to produce hypochlorous acid and hypochloric acid that are the actual disinfectants as they are strong oxidizing agents. O3 as disinfectant doer not result in the synthesis of the carcinogens, like trichloromethane compounds, as produced in chlorination, but being expensive is used in USA and Europe.

Toxic products associated with microbial biopolymer matrix can be removed along with the microbial biomass.

Processes similar to microbial mining are being considered to reduce the heavy metals. Many organisms produce acids, e.g., (H2S04) produced by Thiobacillus, that solubilize the heavy metals, that can be leached from the sludge. These heavy metals can be chemically precipitated from the leachate and they are either reprocessed for use or permanently immobitized.

(i)                 Sludge Treatment: The settled or screened out solid waste from the primary treatment (sludge) and the activated sludge (solid waste containing large amount of microbial biomass is digested in a tank: mainly anaerobically to reduce its odour, volume and pathogens.

Under anaerobic condition, organism, like Desulfovibrio desulfuricons reduce SO42- to S2- (H2S). Thiobacillus denitrificans NO3 to N2 (denitrification), where SO42- and NO3 act as acceptor of electrons instead of O2 in anaerobic respiration. The methanogenic bacteria produce CH4. Thus, soluble substances and various gases, like about 70% CH4, 25% CO2, H2 and N2 are produced. This gas mixture can be used as fuel and the sludge obtained after digestion can be used as manure after drying in sun. The sludge can also be landfilled or aerobically composted. Because of noxious odour, these digestors should be constructed at a sufficient distance from the residential areas (Fig. 17.7).

Fig. 17.7 Anaerobic digestor for sewage sludge.

Fig. 17.8 Sewage treatment system.

2. Biological Filters to Clean the Air

Biofilters are used for the treatment of gases and they are made up of some solid support (that is suitable for the growth of microorganisms, e.g. peat) or a liquid-gas system, where, the liquid and gas phases are separated by a membrane containing appropriate microorganisms. The polluted gas is passed through the biofilter where various microorganisms convert the hazardous gaseous pollutants into non-hazardous compounds.

 

3. Solid Waste Management

The various kinds of solid wastes, like municipal waste and sludge, agricultural, mining, industrial wastes, may be biodegradable (paper, food wastes, sewage wastes, etc.) or non-biodegradable glass, plastic, many insecticides, etc) and their disposal is problematic. The reduction in the use of various resources (e.g minimum packaging, avoiding disposable items, plastic bags, etc.), reusing some of the products (e.g. fly ash or the fine particulate matter produced mainly from thermal power plants can be used for the construction of roads and bricks; glass and plastic bottles, etc.) and recycling of many items (e.g. paper, plastic, glass. metal items, etc.) may not only help in the reduction of pollution but also in reduced consumption of energy. Incineration, the burning of combustible municipal waste in a furnace in the presence of O2 at very high temperature (815oC for 1 hour) producing CO2 and H2O, though reduces the volume of the waste to 90% but generates CO2 and other gases and fly ash in the air. Some of the safer technologies used for the management of solid wastes are:

(i)                 Landfill Technology: Landfills are gigantic anaerobic bioreactors used for the management of solid wastes that can be decomposable or non-decomposable. This is simple and cheaper way to handle solid wastes. The solid wastes are dumped into the low lying and low value lands. Everyday’s waste is compressed and covered with a layer of soil. Thus, a large heap of wastes is made. The indigenous microorganisms in the waste break down the complex organic matter into simpler substances anaerobically. Finally the methanogenic bacteria generate CH4 that commences after several months to years of waste disposal and reaching to a peak value, gradually declines after several (5-10) years. Thus, the organic content undergoes slow anaerobic decomposition over a period of 30-50 years. This CH4 can be used as fuel by inserting a pipeline into the site. The landfill sites should be airtight (to prevent the escape of CH4 that is a greenhouse gas and other gases into the air) and water tight (to prevent the leaching of toxic substances into underground water) to protect the environment and this technology, if improperly managed, can be unsightly, smelly and unhygienic. Toxic wastes (like heavy metals, insecticides, etc.) or toxic products generated by anaerobic decomposition can create problems by run off to nearby water bodies and lands, seepage into the underground aquifers and by affecting the microbial decomposition in the landfill site. Therefore, the sites should be properly constructed with impermeable lining at the bottom to avoid leahates to damage surrounding land and water. The underground aquifer contaminated with leachates remains polluted for many years because of slow movement, minimal self- purification process, low microorganisms, 02 and nutrients of underground water-table. Remedial actions are inadequate and expensive, e.g., pumping out the aquifer followed by its treatment or in situ treatment by oxygenation or nutrient supplementation. Therefore, prevention of groundwater contamination is the most effective remedy. An impermeable clay cap prevents water infiltration and leaching of pollutants. The recovery of CH4 needs investment for installation of collection pipes and pumping systems and for sealing of landfill to prevent the escape of methane. Finally, the leachate collected at the bottom can be pumped out for treatment (Fig. 17.9).

The limited numbers of suitable disposal sites in urban areas are rapidly becoming filled. Premature construction on landfill sites may result in structural damage to the buildings because of subsidence and explosion hazard due to seepage of CH4 into the basements. CH4 seepage may also damage planting on disposal site,

Fig. 17.9 Landfill technology.

(i)                 Composting: It is an attractive alternative to landfills for the decomposition of solid, domestic and agricultural wastes and has environmental advantages (landfill operations may have lower costs but the groundwater contamination due to land filling, composting process is favoured). Only organic wastes are sorted for decomposition by separate collection of garbage. The waste can he ground and mixed with sewage sludge (that may contain heavy metals, like Cd, Cr, etc.) or wood chips and composted. Traditionally at small scale, most of the solid wastes are composted. In urban areas and due to large-scale agriculture the other treatments of wastes are expensive. It involves the aerobic microbial decomposition by bacteria and fungi of solid organic wastes from domestic, agriculture and food industry, which gets converted into sanitary human-like material. The waste is converted into huge piles of is dumped into a pit lined with a straw or wood chips and is covered with dried leaves or a layer of soil to control odour. The waste is watered once or twice a week to
keep it moist (50-60% moisture is optimum as excessive moisture interferes with aeration) and the aeration is provided by regular mechanical turning (after every 14 days) of the waste to facilitate proper oxidation of organic matter into CO2, H2O and organic byproducts. Self-heating raises the temperature to 55-60°C in 2-3 days and then, thermophilic microorganisms are involved in the aerobic decomposition. In well-aerated or oxygenated piles the temperature may rise to 70-80°C. Thereafter, the temperature declines gradually and mesophilic microorganisms now decompose the organic matter. These alternate thermophilic and mesophilic phases are repeated several times. The high temperatures thus attained help in killing of most of the pathogens present in fecal matter and sewage sludge. Aeration or turning may be adjusted to prevent the excessive heating. Periodic water spraying can also help to reduce the temperature. Inside the heap, O2 concentration is about 5 times lesser than the atmospheric concentration. C:N ratio should be greater than 4:1 (lower N restricts microbial decomposition, whereas, excessive N leads to volatilization of NH3 to lower the fertilizer value of compost and also cause odour problem).
Composting is completed in about 3 weeks. Uniform and more stable compost can be obtained more rapidly in the bioreactors in about 2-4 days, which requires curing for
about a month (Fig. 17.10). Compost from toxic wastes can be used in parks and gardens to prevent heavy metal and other types of toxicity in foods.

Fig. 17.10 A heap of compost

Problem of bad odour arises due to the presence of various S and N containing organic compounds in the waste. The escape of various gases produced during decomposition can be inhibited by allowing the decomposition in closed chambers and using biofilters or gas scrubbers. The compost thus, obtained is reduced in bulk and used as manure for soil improvement and cultivation of mushroom (Agaricus bisporus). Presence of high amount of cellulose and lignin (e.g. in wood, straw, etc.), which are relatively resistant to decomposition, slow down the compost formation.

Solid waste can be inoculated with commercially available cultures of earthworm, which help in the breakage of large pieces of organic waste into smaller particles to be easily decomposed by the indigenous bacteria and fungi in about 45 days. The compost thus obtained is rich in nutrients and is called vermicompost (Fig.17.11)

Fig. 17.11 Verrnicompost production.

4. Bioremediation

Bioremediation (or biotreatment or bioreclamation or biorestoration) is a new technology and involves application of biological system (mostly microorganisms) forthe treatment of various hazardous wastes, particularly the non-biodegradable ones (xenobiotic). These microorganisms either degrade and detoxify the pollutant or accumulate them in their body. Two types of bioremedial technologies, which employ generally a group of microorganisms, can be applied:

i.            After prolonged exposure to a particular pollutant, some of the indigenous microbes develop ability to utilize or degrade that pollutant due to spontaneous mutation or acquiring some plasmid DNA from other microorganisms by conjugation. At the affected site, if some of the nutrients are limiting for the growth of such microorganisms, the addition of such nutrients (like urea (N), single super phosphate (P), etc.) may help in the promotion of growth of these microbes. This technology was successfully applied by using fertilizers to clean the oil spillage in the ocean due to accident of the oil tanker Exxon Valdez at the Alaskan beaches in 1989 where the oil was decomposed to less harmful products that became the part of the food chain. Similarly, the soils contaminated with highly toxic industrial recalcitrant chemical, polychlorinated biphenyls, can be dechlorinated by the microbial activity.

ii.            In another technology, if the population of the indigenous microbes involved in cleaning the pollutant is limiting, the microbial samples are isolated from the polluted site and cultivated artificially in bioreactors. The biomass thus, obtained is used for the reinoculation of the polluted site to build up the desired population size. This technology has been found to be successful at some polluted sites.

iii.            Some microbial cultures are commercially available for this purpose, e.g., the white rot fungus, Phanaerochyte chrysosporium, helps in the degradation of lignocellulosic materials, Pseudomonas putida can cure oil spills, Pseudomonas fluorescens biodegrades HCN. In this approach the introduced microbes must be able to survive and compete with the indigenous microbes in the new environment.

iv.            Alternatively, the microorganisms may be genetically engineered to develop the ability to degrade the pollutant molecules and at present, the research is in progress in this field and would need years to be widely used for the environmental applications.

Though, the technology is slow as compared to physical clean up methods and is applicable only in certain environmental conditions only for certain chemicals and sometimes, the added chemicals may contaminate the environment, it is simple and cheaper and leads to the conversion of toxic substances to non-toxic ones.

5. Microbial Mining

By using certain microbes, e.g. Thiobacillus ferrooxidans, many commercially important metals like Cu, Au (gold), U, Fe, etc. can be extracted economically from the poor grade ores containing very small quantity of metal sulphides (ores) (the conventional metallurgical processes would be uneconomical with such ores). Though, this method of metal extraction is slow, it does not affect the environment (no evolution of NO2 and SO2 as in convetional metallurgical operations). The heap of the crushed ore (ex situ extraction), e.g. CuS, or the mining site by drilling the hole (in situ extraction) is inoculated with the desired bacterium, e.g. Thiobacillus ferrooxidans (a chemoautotroph that derives its energy by the oxidation of sulphides to sulphates) along with some nutrients are pumped into the hole. This leads to the oxidation of S2 in CuS ore to SO42- (H2SO4) and water soluble CuSO4following a series of reactions is formed while percolating through the orepile by the bacterium:

The CuSO4 thus, collected from the bottom of the ore heap (or pumped out from mine site) is drained to recovery plant where it is allowed to flow through the surface of Fe metal leading to the production of pure Cu metal and FeSO4 due to displacement reaction:

CuSO4+ Fe            →             FeSO4+ Cu↓

Alternatively, many microbes (e.g. bacteria Bacillus and Streptococcus, algae Chlorella and Anabaena, etc.) can accumulate inside their cells the desired rare metals (e.g. Mn, Au, U, etc) by growing them in very dilute metal solution (e.g. many industrieal effluents) from such microbial biomass, the metal can be extracted economically.

6. Desulphurisation of Coal

Generally the coal contains 0.2-8.0 % S but, many poor quality coals contain higher amount of S, sometimes exceeding 11 %. Burning of such poor quality S-containing coal leads to the release of SOx into the atmosphere. Such coal can be cured by inoculating the mine sites through drilling the hole with the desired microbe (e.g. Thiobacillus ferrooxidans) and water. After microbial growth the S2- (mainly FeS) is be oxidised to SO42- that is leached out.

7. Biocontrol Agents

Discussed in the chapter “Biotechnology in Agriculture and Forestry”.

8. Biofertilizers

Discussed in the chapter “Biotechnology in Agriculture and Forestry”.

QUESTIONS

1. Write short notes on the following:

(i)Sewage treatment

(ii)  Biofertilizer

(iii)Biocontrol agent

(iv) Recovery of metals by biomining

(v) Anaerobic treatment of wastewater and sewage sludge

(vi) Xenobiotic compounds

(vii) Landfill technology

(viii)Activated sludge treatment

(ix)Treatment of solid wastes

2. Discuss the applications of biotechnology in the field of environment giving suitable examples.

3. Define waste and pollutants. Briefly describe the various sources of wastes and discuss the hazards from them.