Wastewater Treatment Methods PDF
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This document discusses preliminary and primary wastewater treatment methods, focusing on the process of separating floating materials and heavy solids. It also details methods like screening, grit chambers, and skimming tanks. Sedimentation processes are further explained, along with coagulation methods for fine suspended particles.
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PRELIMINARY AND PRIMARY WASTEWATER TREATMENT METHODS 1.5 Preliminary Treatment Preliminary treatment consists solely in separating the floating materials (like dead animals, tree branches, papers, pieces of rags, wood, etc.), and also the heavy settle able inorganic solids. It also helps in removing...
PRELIMINARY AND PRIMARY WASTEWATER TREATMENT METHODS 1.5 Preliminary Treatment Preliminary treatment consists solely in separating the floating materials (like dead animals, tree branches, papers, pieces of rags, wood, etc.), and also the heavy settle able inorganic solids. It also helps in removing the oils and greases, etc. from the sewage. This treatment reduces the BOD of the wastewater, by about 15 to 30%. The processes used are: Screening for removing floating papers, rags, clothes, etc. Grit chambers or Detritus tanks for removing grit and sand; and Skimming tanks for removing oils and greases Screening Screening is the very first operation carried out at a sewage treatment plant and consists of passing the sewage through different types of screens, to trap and remove the floating matter, such as pieces of cloth, paper, wood, cork, hair, fiber, kitchen refuse, fecal solids, etc. present in sewage. These floating materials, if not removed, will choke the pipes, or adversely affect the working of the sewage pumps. Thus, the main idea of providing screens is to protect the pumps and other equipment’s from possible damage due to the floating matter of the sewage. Primary Wastewater Treatment Primary treatment consists in removing large suspended organic solids. This is usually accomplished by sedimentation in settling basins. The liquid effluent from primary treatment, often contains a large amount of suspended organic material, and has a high BOD (about 60% of original). Sometimes, the preliminary as well as primary treatments are classified together, under primary treatment. The organic solids which are separated out in the sedimentation tanks (in primary treatment) are often stabilized by anaerobic decomposition in a digestion tank or are incinerated. The residue is used for landfills or soil conditioners. Sedimentation 1. Necessity of Sedimentation in Treatment of Wastewaters As discussed in the previous pages, the screens and the grit chambers do remove most of the floating materials (like paper, rags, cloth, wood, tree branches, etc.) and the heavy inorganic settleable solids from the sewage. However, a part of the suspended organic solids which are too heavy to be removed as floating matters, and too light to be removed by grit chambers (designed to remove only the heavy inorganic solids of size more than 0.2 mm and of sp. gravity 2.65) are generally removed by the sedimentation tanks. The sedimentation tanks are thus designed to remove a part of the organic matter from the sewage effluent coming out from the grit chambers. In a complete sewage treatment, the sedimentation is, in fact, carried out twice; once before the biological treatment (i.e. primary sedimentation) and once after the biological treatment (i.e. Secondary sedimentation). When chemical coagulants are also used for flocculating the organic matter during the process of sedimentation, the process is called chemical precipitation or sedimentation aided with coagulation. Other sewage treatment units which work on the principle of sedimentation are: Septic tanks, Imhoff tanks, etc. Septic tanks and Imhoff tanks combine sludge digestion with sedimentation, whereas the sludge deposited in primary as well as in the secondary settling tanks, is separately digested in the sludge-digestion tanks. 2. Types of Settling Depending on the particles concentration and the interaction between particles, four types of Settling can occur, see also Discrete, Flocculent ,Hindered and Compression 1. Discrete particle settling The particles settle without interaction and occur under low solids concentration. A typical occurrence of this type of settling is the removal of sand particles. 2. Flocculent settling This is defined as a condition where particles initially settle independently, but flocculate in the depth of the clarification unit. The velocity of settling particles is usually increasing as the particles aggregates. The mechanisms of flocculent settling are not well understood. 3. Hindered/zone settling Inter-particle forces are sufficient to hinder the settling of neighboring particles. The particles tend to remain in fixed positions with respect to each other’s. This type of settling is typical in the settler for the activated sludge process (secondary clarifier). 4. Compression settling This occurs when the particle concentration is so high that particles at one level are mechanically influenced by particles on lower levels. The settling velocity then drastically reduce. Discrete Flocculent Hidered Compression 3.2.2 Sedimentation Aided with Coagulation (Type II Sedimentation) 1. Chemical Precipitation and Coagulation Very fine suspend particles, present in wastewaters, which cannot be removed in plain. sedimentation, may sometimes, be settled by increasing their size be changing them into flocculated particles. For this purpose, certain chemical compounds (like ferric chloride, ferric sulphate, alum, chlorinated copperas, etc.) called coagulants are added to the wastewaters, which on thorough mixing form a gelatinous precipitate called floc. The fine mud particles and other colloidal matter present in wastewaters get absorbed in these floes forming the bigger sized flocculated particles. The process of addition and mixing of chemicals is called coagulation. The coagulated sewage is then made to pass through sedimentation tank where the flocculated particles settle down and get removed. B.O.D. SS removed removed pH value Dosage Name of as required.No. as required Remarks coagulant percentage percentage for proper in ppm of total of total functioning present present This coagulant is widely Ferric used for sewage l. 80 - 90 90 - 95 25 - 35 5.5 to 7.0 chloride treatment, wherever, coagulation is adopted. Ferric sulphate has been found to be more effective than chlorinated Ferric copperas, if used in 2. sulphate 60 80 35 - 40 8.0 to 8.5 conjunction with lime. with lime Hence ferric chloride and ferric sulphate are mainly used, as coagulants in sewage. It is generally not used in sewage although used for 3. Alum 60 80 40 - 90 6 to 8.5 treating water supplies on a large scale. 5.5 to 7.0 This coagulant is Chlorinated 70 - 80 and effective for producing 4. 80 - 90 35 - 80 copperas 9.0 to 9.5 sludge for activated sludge process. SECONDARY/BIOLOGICAL AND TERTIARY WASTEWATER TREATMENT The Role of Microorganisms in Wastewater Treatment Micro-organisms, such as bacteria, play an important role in the natural cycling of materials and particularly in the decomposition of organic wastes. The role of micro-organisms is elaborated further here because they are also important in the treatment of wastewater. Waste form humans become a useful food substrate for the micro-organisms. In both natural and engineered treatment systems micro-organisms such as bacteria, fungi, protozoa, and crustaceans play an essential role in the conversion of organic waste to more stable, less polluting substances. They form what is termed a 'food chain'. In a natural water body, e.g. river or lake, the number and type of micro-organisms depends on the degree of pollution. The general effect of pollution appears to be a reduction in species numbers. For example in a badly polluted lake, there are fewer species but in larger numbers, while in a healthy lake there can be many species present but in lower numbers. Micro-organisms are always present in the environment and given the right conditions of food availability, temperature and other environmental factors, they grow and multiply Generalized representation of growth of micro-organisms Micro-organisms require cellular building blocks, such as (carbon) C, (hydrogen) H, (oxygen) O, (nitrogen) N, (phosphorus) P, and minerals for growth. These can be obtained through consuming organic substances containing these elements, or from inorganic materials, such as carbon dioxide, water, nitrate and phosphate. Micro-organisms also require energy. They obtain this through respiration. In this process organic carbon is oxidized to release its energy. Oxygen or other hydrogen acceptors is needed for the respiration process. Algae and photosynthetic bacteria can also utilize energy from sunlight, while certain types of bacteria can utilize energy from chemical reactions not involving respiration. The building blocks and energy are used to synthesize more cells for growth and also for reproduction. In the treatment of wastewater three types of overall processes are distinguished to represent the conversion of organic wastes by micro-organisms. The classification is based on whether the environment where the process takes place is aerobic, anaerobic or photosynthetic. Under aerobic conditions (in the presence of oxygen), micro-organisms utilize oxygen to oxidize organic substances to obtain energy for maintenance, mobility and the synthesis of cellular material. Under anaerobic conditions (in the absence of oxygen) the micro-organisms utilize nitrates, sulphates and other hydrogen acceptors to obtain energy for the synthesis of cellular material from organic substances. Photosynthetic organisms use carbon dioxide as a carbon source, inorganic nutrients as sources of phosphate and nitrogen and utilize light energy to drive the conversion process. Micro-organisms also produce waste products, some of which are desirable and some undesirable. Gases such as carbon dioxide and nitrogen are desirable, since they can be easily separated and do not produce pollution. Gases such as hydrogen sulphide and mercaptans, although easily separated require treatment for odour. Micro-organisms’ cellular materials are organic in nature and can also cause pollution. It would be desirable if the cellular materials have undergone self oxidation (endogenous respiration utilizing own body cells) to produce nonbiodegradable materials that are relatively stable. Self-oxidation is achieved when there is no substrate/food available. The microbiological conversion reactions of organic waste into cellular material can be empirically represented as shown below. (i) Conversion under aerobic conditions (see diagram below): Under aerobic conditions ammonia is further oxidized to nitrate. Phosphorus and sulphur contained in the organic substances are oxidized to phosphate and sulphate. These can be further utilized by the micro-organisms for synthesis. (ii) Conversion under anaerobic conditions (see diagram below): Methane (CH4) is a useful gaseous by-product of anaerobic conversion, because it can be combusted to produce heat/energy. On the other hand if it is released to the atmosphere without being combusted, it contributes to the greenhouse gas effect. (iii) Conversion under photosynthetic conditions: As shown by the conversion reactions (the utilization of organic wastes for food by microorganisms) the product is mainly the cellular material of the micro-organisms i.e. more organisms are produced. The growth yield is the weight of micro-organisms produced per unit weight of organic substances consumed by the micro-organisms. The growth yield depends on the type of substrate and environmental conditions. The smaller the value of the growth yield the better it is for waste treatment, because less sludge is produced which requires disposal. Its value is usually between 0.2 and 0.5 for aerobic conversion, while the corresponding value for anaerobic conversion is smaller. Biological Wastewater Treatment Purpose: The idea behind all biological methods of wastewater treatment is to introduce contact with bacteria (cells) which feed on the organic materials in the wastewater, thereby reducing its BOD content. In other words, the purpose of biological treatment is BOD reduction. Typically, wastewater enters the treatment plant with a BOD higher than 200mg/L, but primary settling has already reduced it to a certain extent (30 – 35% of the original) by the time it enters the biological component of the system. It needs to exit with a BOD content no higher than about 20 - 30mg/L, so that after dilution in the nearby receiving water body (river, lake), the BOD is less than 2 - 3mg/L. Principle: Simple bacteria (cells) eat the organic material present in the wastewater. Through their metabolism, the organic material is transformed into cellular mass, which is no longer in solution but can be precipitated at the bottom of a settling tank or retained as slime on solid surfaces or vegetation in the system. The wastewater exiting the system is then much clearer than it entered. A key factor is the operation of any biological system is an adequate supply of oxygen. Indeed, cells need not only organic material as food but also oxygen to breath, just like humans. Without an adequate supply of oxygen, the biological degradation of the waste is slowed down, thereby requiring a longer residency time of the wastewater in the system. For a given flow rate of wastewater to be treated, this translates into a system with a larger volume and thus taking more space. Types of Biological Process for Wastewater Treatment (reading assignment) The common methods of biological wastewater treatment are: a) Aerobic processes such as trickling filters, rotating biological contactors, activated sludge process, oxidation ponds and lagoons, oxidation ditches, b) Anaerobic processes such as anaerobic digestion, and c) Anoxic processes such as denitrification. Waste Stabilization Pond In which wastewater is biologically treated by natural processes involving pond algae and bacteria. WSP comprise a single series of anaerobic, facultative and maturation ponds or several of such series in parallel. A long hydraulic retention time is necessary because of the slow rate at which the organic waste is oxidized. Typical hydraulic retention times range from 10 days to 100 days depending on the temperature of a particular region. WSP are considered as the most effective and appropriate method of wastewater treatment in warm climates where sufficient land is available and where the temperature is most favorable for their operation. WSP are employed for treatment of a range of wastewaters, from domestic wastewater to complex industrial wastes. The design of WSP depends on the treatment objectives. It may be designed to receive untreated domestic or industrial wastes, to treat primary or secondary treatment plant effluents, excess activated sludge. Anaerobic, facultative and maturation ponds are the three major types of pond in a WSP system. These ponds are normally arranged in series to achieve effective treatment of raw wastewater. Anaerobic and facultative ponds are employed for BOD removal, while maturation ponds remove excreted pathogens. A series of anaerobic and facultative ponds can treat wastewater to a sufficient degree to allow it to be used in a restricted way for irrigating crops. It has been argued that such pond systems remove nematode eggs significantly by sedimentation (WHO, 1989). Maturation ponds are normally used if the treated wastewater is to be used for unrestricted crop irrigation complying with WHO guidelines of less than 1000 faecal coliforms (FC) per 100ml (WHO, 1989). Maturation ponds have also been used when stronger wastewaters with high concentrations of nutrients (nitrogen, phosphorus) are to be treated prior to surface discharge. Advantages and Disadvantages of Waste Stabilization Ponds Waste stabilization ponds (WSP) are shallow man-made basins into which wastewater flows and from which, after a retention time of several days (rather than several hours in conventional treatment processes), a well-treated effluent discharged. The advantages of WSP systems, which can be summarized; as simplicity, low cost and high efficiency, are as follows: High Efficiency BOD removals > 90% readily obtained in a series of well-designed ponds. The removal of suspended solids is less, due To the presence of algae in the final effluent. Total nitrogen removal is 70 – 90%, and total phosphorus removals 30 – 45%. WSP are particularly efficient in removing excreted pathogens, whereas in contrast all other treatment processes are very inefficient in this, and require a tertiary treatment process such as chlorination (with all its inherent operational and environmental problems) to achieve the destruction of fecal bacteria. Disadvantages of Waste Stabilization Ponds The major disadvantage of WSPs is the large area that is required (2 - 5 m /capita), the potentially high algal content of the effluent, evaporation losses, the potential odour and mosquito nuisance and the sensitivity of algae to toxic matter present in raw municipal sewage.