1.4 CONVENTIONAL WASTEWATER TREATMENT PROCESS
Conventional wastewater treatment consists of a combination of physical, chemical and biological operations to remove solids, organic matter and nutrients from wastewater. The general terms used to describe different degrees of treatment, in order of increasing treatment levels are preliminary, primary, secondary and tertiary or advanced wastewater treatment.
Preliminary Treatment
The objective of preliminary treatment is removal of coarse solids and large size materials often found in raw wastewaters, to enhance the operation and maintenance of subsequent treatment units. Preliminary treatment typically include coarse screening, grit removal and in some case combination of large objects.
Primary Treatment
Primary treatment involves removal of settleable organic and inorganic solids by sedimentation, and removal of materials that float (scum) by skimming. Approximately 25-50 percent of the incoming BOD, 50-70 percent of the total suspended solids and 65 percent of the oil and grease are removed during primary treatment. Some organic nitrogen, organic phosphorous, and heavy metals associated with solids are also removed during primary sedimentation but colloidal and dissolved constituents is not affected. Primary sedimentation tanks or clarifiers may be round or rectangular basins, 3-5 meter deep, with hydraulic retention time between 2-3 hours. Settled solids (primary sludge) are normally removed from the bottom of tanks by sludge rakes to central well from where it is pumped to sludge processing units in small sewage plants. Sludge is processed in a variety of ways including aerobic digestion, storage in sludge lagoons, direct application to sludge drying beds and land application. Secondary Treatment Secondary treatment is the further treatment of the effluent from primary treatment to remove residual organic and suspended solids. In most cases, it follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matters using aerobic biological treatment processes by aerobic microorganisms (principally bacteria). These microorganisms metabolise the organic matter in the wastewater, thereby producing more microorganisms and inorganic end products (CO2, NH 3, and H20). Several aerobic biologicalnprocesses are used for secondary treatment differing primarily in the manner in which oxygen is supplied and rate at which organisms metabolise the organic matter. High rate biological processes are characterised by relatively small reactor volumes and high concentrations of microorganisms compared with low rate processes. High-rate processes include activated sludge processes, trickling filters or biofilters, oxidation ditches, and rotating biological contractors. Microorganisms must be separated from the treated wastewater by sedimentation to produce clarified secondary emuent. Secondary clarifiers (sedimentation tanks) remove the secondary sludge, which are contained with primary sludge for processing. Tertiary Treatment Tertiary or advanced wastewater treatment is employed when specific wastewater constituents that cannot be removed by secondary treatment must be removed. Individual treatment processes are necessary to remove nitrogen, phosphorus, additional suspended solids, refractory organics, heavy metals and dissolved solids. 1.5 NATURAL WASTEWATER TREATMENT SYSTEMS As energy and construction costs have increased, new methods of wastewater treatment with potentially lower energy and capital costs have become increasingly attractive. …show more content…
Relatively simple wastewater technologies can be designed to provide low cost sanitation and environmental protection while providing additional benefits from the reuse of water. Effluents from conventional treatment facility not only contain some unstable organic matter but also nutrients, which represent potential energy. It is therefore illogical to utilise external energy sources to dissipate energy stored within the wastewater. A natural treatment system makes use of aquatic animals and plants to improve he water quality and reclaim nutrients. In addition, persistent toxic environmental contaminants such as pesticides, aromatic hydrocarbons and metals can be removed from wastewater through biological treatment process. Natural wastewater treatment systems include algal mass culture, macrophyte treatment systems (including constructed wetlands) and aquaculture. Natural systems may be utilised to treat agricultural, domestic and industrial wastewater. Climate, soil conditions, space, waste characteristics and other factors relate to its applicability. 1.5.1 Algal Mass Culture Algae are a diverse group of microorganisms that can perform photosynthesis. Of these, unicellular algae (e.g. green algae, blue green algae) are of interest in wastewater treatment and recycling process. They are tolerant to changes in environmental conditions. The biological reactions occurring in algal ponds reduce the organic content and nutrients of wastewater by bacterial decomposition
Conventional wastewater treatment consists of a combination of physical, chemical and biological operations to remove solids, organic matter and nutrients from wastewater. The general terms used to describe different degrees of treatment, in order of increasing treatment levels are preliminary, primary, secondary and tertiary or advanced wastewater treatment.
Preliminary Treatment
The objective of preliminary treatment is removal of coarse solids and large size materials often found in raw wastewaters, to enhance the operation and maintenance of subsequent treatment units. Preliminary treatment typically include coarse screening, grit removal and in some case combination of large objects.
Primary Treatment
Primary treatment involves removal of settleable organic and inorganic solids by sedimentation, and removal of materials that float (scum) by skimming. Approximately 25-50 percent of the incoming BOD, 50-70 percent of the total suspended solids and 65 percent of the oil and grease are removed during primary treatment. Some organic nitrogen, organic phosphorous, and heavy metals associated with solids are also removed during primary sedimentation but colloidal and dissolved constituents is not affected. Primary sedimentation tanks or clarifiers may be round or rectangular basins, 3-5 meter deep, with hydraulic retention time between 2-3 hours. Settled solids (primary sludge) are normally removed from the bottom of tanks by sludge rakes to central well from where it is pumped to sludge processing units in small sewage plants. Sludge is processed in a variety of ways including aerobic digestion, storage in sludge lagoons, direct application to sludge drying beds and land application. Secondary Treatment Secondary treatment is the further treatment of the effluent from primary treatment to remove residual organic and suspended solids. In most cases, it follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matters using aerobic biological treatment processes by aerobic microorganisms (principally bacteria). These microorganisms metabolise the organic matter in the wastewater, thereby producing more microorganisms and inorganic end products (CO2, NH 3, and H20). Several aerobic biologicalnprocesses are used for secondary treatment differing primarily in the manner in which oxygen is supplied and rate at which organisms metabolise the organic matter. High rate biological processes are characterised by relatively small reactor volumes and high concentrations of microorganisms compared with low rate processes. High-rate processes include activated sludge processes, trickling filters or biofilters, oxidation ditches, and rotating biological contractors. Microorganisms must be separated from the treated wastewater by sedimentation to produce clarified secondary emuent. Secondary clarifiers (sedimentation tanks) remove the secondary sludge, which are contained with primary sludge for processing. Tertiary Treatment Tertiary or advanced wastewater treatment is employed when specific wastewater constituents that cannot be removed by secondary treatment must be removed. Individual treatment processes are necessary to remove nitrogen, phosphorus, additional suspended solids, refractory organics, heavy metals and dissolved solids. 1.5 NATURAL WASTEWATER TREATMENT SYSTEMS As energy and construction costs have increased, new methods of wastewater treatment with potentially lower energy and capital costs have become increasingly attractive. …show more content…
Relatively simple wastewater technologies can be designed to provide low cost sanitation and environmental protection while providing additional benefits from the reuse of water. Effluents from conventional treatment facility not only contain some unstable organic matter but also nutrients, which represent potential energy. It is therefore illogical to utilise external energy sources to dissipate energy stored within the wastewater. A natural treatment system makes use of aquatic animals and plants to improve he water quality and reclaim nutrients. In addition, persistent toxic environmental contaminants such as pesticides, aromatic hydrocarbons and metals can be removed from wastewater through biological treatment process. Natural wastewater treatment systems include algal mass culture, macrophyte treatment systems (including constructed wetlands) and aquaculture. Natural systems may be utilised to treat agricultural, domestic and industrial wastewater. Climate, soil conditions, space, waste characteristics and other factors relate to its applicability. 1.5.1 Algal Mass Culture Algae are a diverse group of microorganisms that can perform photosynthesis. Of these, unicellular algae (e.g. green algae, blue green algae) are of interest in wastewater treatment and recycling process. They are tolerant to changes in environmental conditions. The biological reactions occurring in algal ponds reduce the organic content and nutrients of wastewater by bacterial decomposition