Basically, there are two levels of sewage treatment on the basis of amount of sewage generated by humans: small scale treatment and large scale treatment. Small scale treatment of sewage is done in small homes and rural areas, whereas large scale treatment is done in towns and cities by municipal bodies.
1. Small Scale Sewage Treatment:
There are several methods of sewage treatment on small scale.
A few of them are described below:
(i) Cesspools:
Human waste is thrown in cesspools in many homes. It is constructed in underground part with concrete in such a way that it contains wall of cylindrical rings with pores (Fig. 33.4).
Its opening is near the ground level. Wastewater (sewage) enters the cesspool through the inlet pipe. The bottom of cesspool remains open. Therefore, the suspended solid material falls on the bottom of cesspool and forms sludge after getting deposited in huge amount.
Water passes out through the open bottom of cesspool and through pores into the surrounding soil. The organic materials of the sludge are decomposed by anaerobic bacteria resulting in release and deposition of breakdown products on the ground.
Thus the amount of breakdown products exceeds; it forms thick layers which need to be cleaned by using strong acids. Dried bacterial preparation of Bacillus subtilis or yeast cells should be added at intervals. These accelerate the decomposition of sludge deposited at the bottom of cesspool.
(ii) Septic Tanks:
In rural areas, individual family uses septic tank because of lack of public sewers. Septic tank is a metallic or concrete tank which is kept below the ground level somewhere near the homes. Into septic tanks all the domestic wastes flow through the inlet pipes. A family of four members needs a septic tank of 3 × 5 × 5 feets by accumulating about 750 gallons of sewage.
The suspended organic materials are accumulated at the bottom of tank, whereas the water flows through outlets to a distribution box (Fig. 33.5A) which is connected with perforated pipes that open under the soil surface in the surrounding areas.
Therefore, effluents from the tank are passed to underground surface of soil. Through this process, the pathogenic microbes are not eliminated. Therefore, the drinking water supply must be kept at certain distance of the pipes of septic tanks.
The organic materials accumulated in septic tank is decomposed by anaerobic bacteria releasing into water several by-products such as sugars, alcohols, organic acids, amino acids, fatty acids, glycerols and gases (e.g. H2, H2S, CH4, CO2, etc.).
However, there remain undigested organic materials called sludge. Sludge is removed from the septic tank at certain intervals by pumping process otherwise it will block the tank and pipes. Sludge acts as a source of humus when applied in field.
In addition, in small towns sewage is collected into large ponds which are called oxidation lagoons. The sewage is discharged into oxidation lagoons where organic materials are oxidized first by aerobic organisms and the sediments are decomposed by anaerobic microorganisms.
2. Large Scale Sewage Treatment:
Sewage treatment on a large scale of populations of city is known as large scale sewage treatment. In cities sewage and garbage are generated in massive amount per day which is treated by municipal plants. A schematic view of waste treatment by a municipal plant is shown in Fig. 33.6.
Overall processes of conventional municipal sewage plant can be divided into three steps: the primary treatment, secondary treatment, and tertiary treatment.
The primary treatment is viewed for physical separation of insoluble materials, to lower BOD; the secondary separation is based on microbial decomposition of organic materials in the effluents; the tertiary treatment is the chemical removal of inorganic nutrients and pathogenic microbes (Table 33.1).
Table 33.1 : Major steps in primary, secondary and tertiary treatment of wastes.
(i) Primary Treatment:
Primary treatment is the physical removal of 20-30% of organic materials present in sewage in particulate form. The particulate material is removed by screening, precipitation of small particulate and settling in basin or tanks where the raw sewage is piped into huge and open tanks.
The solid material (sludge) is removed and kept in landfill/composting for anaerobic digestion. The liquid portion is piped into sludge tanks. The accumulated materials in sludge tanks are subjected to aluminium sulfate or the other coagulants so that the suspended particles, organic materials and microorganisms should be trapped as in sedimentation process of water purification.
(ii) Secondary Treatment:
Secondary treatment is also called biological treatment or microbial degradation. By this process about 90-95% of the BOD and many pathogens are removed. There are several means by which BOD can be reduced in secondary treatment.
Reduction of BOD by 90% is achieved through mineralization of small fraction of organic matter and conversion of large proportion to removable solids. The microbial activities may be aerobic or anaerobic.
Secondary treatment is done by several methods as described below:
(a) The oxidation ponds:
The oxidation ponds (also called lagoons or stabilization ponds) permit the growth of algal forms on waste-water effluent (Fig. 33.7). It is used for secondary treatment in rural areas or industrial sectors.
The organic materials are degraded by heterotrophic bacteria into simpler forms that in turn support the growth of algae. Algae use these nutrients to increase their biomass. Air supplies oxygen for biochemical oxidation of organics. Oxygen evolved by algae after photosynthesis maintains the oxygen deficit created by heterotrophs.
The efficiency of oxidation process can be improved by constructing shallow ponds. The algae growing in oxidation ponds are: Chlorella pyrenoidosa, C. ellipsoides, Scenedesmus acutus, S. quadricauda, Spirulina platensis, etc. Secondary treatment through oxidation ponds is the aerobic sewage treatment device.
(b) The trickling filter:
Aerobic secondary treatment also can be carried out with a trickling filter (Fig 33.8). It is a simple sewage treatment device that consists of a bed of a crushed stone, gravel, slag, or synthetic material with drains made at the bottom of the tank.
Thus the trickling filter has a pile of rocks over which sewage or organic wastes slowly trickle. A revolving sprinkler (arm) is suspended over a bed of porous material which distributes the liquid sewage over it, and collects the effluents at the bottom. Due to spraying process, sewage is saturated by oxygen.
The porous filter bed becomes coated with slimy bacterial growth mainly by Zooglea ramigera and other slime producing bacteria. The slime is colonised by the heterotrophic microorganisms e.g. bacteria (Beggiatoa alba, Sphaerotilus natons, Achromobacter spp., species of Pseudomonas and Flavobacterium), fungi, nematodes, protozoa, etc.
These microorganisms form a stationary microbial culture because of continuous supply of nutrients present in sewage and metabolising the organic constituents into the more stable end products. Therefore, BOD of effluent is reduced by these microorganisms. The microorganisms get air through porous bed. A newly constructed bed needs a few weeks to function efficiently unless the zoogleal film is coated over it.
(c) The activated sludge:
It is also one of the widely used aerobic treatment systems, for waste water in which very vigorous aeration of the sewage is done. The sewage is passed into an aeration tank from primary settling tank. Sewage is aerated by mechanical stirring (Fig. 33.9).
Due to vigorous aeration of sewage floc-formation occurs. The colloidal and finely suspended matter of sewage form aggregates which are called floccules. The floes are permitted to settle down in secondary settling tank. The particles of floe i.e. activated sludge contain large amount of metabolising bacteria together with yeasts, fungi and protozoa.
The activated sludge is introduced in primary settling tank and aeration tank just for rapid development of microorganisms and rapid exploitation of organic matter.
This process is repeated i.e. addition of settled sludge to fresh sewage, aeration, sedimentation, addition of settled sludge to fresh sewage, and so on. This repeating process results in complete flocculation of fresh sewage within a few hours. Activated sludge process reduces the BOD of effluent to 10-15% as compared to raw sewage.
The use of activated sludge hastens the efficiency of system. A poor settlement of activated sludge floes adversely affects the efficiency of sewage treatment plant.
The microorganisms found in activated sludge floes are the heterotrophs such as Gram-negative rods (e.g. E. coli, Enterobacter, Pseudomonas, Achromobacter, Flavobacterium, Zooglea, etc.), Arthrobacter, Corynebacterium, Mycobacterium, Sphaerotilus, large filamentous bacteria, some filamentous fungi, yeasts and protozoa. These microbes secrete slime that holds floes.
Thus, the floes are microbial biomass held together by slime. The settled sludge should be removed from settling tank time to time otherwise poor settling will result in bulking of sludge.
The bulking sludge is caused by massive development of filamentous bacteria (e.g. Sphaerotilus, Baggiatoa, Thiothrix), and filamentous fungi (e.g. Cephalosporium, Cladosporium, Geotrichium, etc.). Thus, the settled sludge is permitted to anaerobic treatment and reinoculation of fresh sewage.
Advantage of using activated sludge process are:
(a) Significant reduction in BOD and suspended solids,
(b) Reduction in intestinal pathogens,
(c) Requirement of little land, and
(d) No need of high dilution of final effluent.
(d) Anaerobic digesters:
All the aerobic processes produce excess microbial biomass or sewage sludge which contains many recalcitrant organics. The sludge from aerobic sewage treatment together with the materials settled down in primary treatment is further treated in anaerobic digesters through the process of anaerobic digestion (Fig. 33.10).
These digesters are used only for processing of settled sewage sludge and the treatment of very high BOD industrial effluents. Anaerobic digesters are large fermentation tanks designed to operate anaerobically with continuous supply of untreated sludge and removal of final, stabilized sludge product.
However, in these tanks provisions are made for mechanical mixing, heating, gas collection, sludge addition and removal of final stabilized sludge. High amount of suspended organic materials with high number of bacterial community (109 – 1010 CFU/ml) is found.
The organics are decomposed by a number of anaerobic microorganisms whose population is found to be 2 – 3 times greater than the anaerobes.
Anaerobic digestion involves the following three steps:
Fermentation:
The fermentation of sludge components to form organic acids (including acetate) from organic polymers is done by a number of bacteria such as species of Bacteroides, Clostridium, Peptostreptococcus, Eubacterium, Lactobacillus, etc. The organic acids are butyrate, propionate, lactate, succinate, acetate along with ethanol and H2, CO2, etc.
i. Acetogenic reactions:
The products (e.g. butyrate, propionate, lactate, succinate, ethanol) produced during fermentation are utilized as substrate by several acetogenic bacteria viz., Syntrophomonas, Syntrophobacter and Acetobacterium. The products produced as a result of acetogenic reactions are: acetate, H2 and CO2.
ii. Methanogenesis:
The products produced during acetogenesis are utilized as substrate by methanogenic bacteria. Acetate is used to produce CH4 + CO2 by Methanosarcina and Methanothrix. H2 and HCO3– are used to produce methane by several bacteria e.g. Methanobrevibacter, Methanomicrobium, Methanogenium, Methanobacterium, Methanococcus, and Methanospirillum. A critical balance between oxidants and reductants is maintained during methanogenic processes.
The hydrogen concentration must be maintained at a low level so that it can function most efficiently. Upon accumulation of hydrogen and organic acids, methane production is inhibited. Thus, the final product of anaerobic digestion is a mixture of gases (70% CH4, 30% CO2), microbial biomass and non-biodegradable residues (e.g. heavy metals, polychlorinated biphenyls, etc.).
(iii) Tertiary Treatment:
Tertiary treatment is aimed to remove non-biodegradable organic materials, heavy metals, and minerals. The salts of nitrogen and phosphorus must be removed because they cause eutrophication.
By using activated carbon filters the organic pollutants can be removed, whereas by adding lime the phosphorus is precipitated as calcium phosphate. Nitrogen can be removed by stripping, volatilization as NH3 at high pH values. Ammonia can be converted by chlorination to dichloromine which in turn is converted to N2. Tertiary treatment is expensive, therefore, it is not employed unless very necessary.