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Wastewater treatment consists of a number of steps, such as preliminary and primary treatment , secondary treatment, advanced waste treatment, disinfection, and solids treatment (Hammer et al. 2005).
This paper is offered a design of the plant provided tertiary wastewater treatment through the following processes: coarse screening, aerated grit removal, primary clarification, biological nutrient removal (BNR) followed by secondary clarification and effluent ultra-violet disinfection. The features of the plant design:
The Biological Nutrient Removal (BNR) process for removing phosphorus and nitrogen by using naturally-occurring microorganisms instead of chemicals, eliminating the cost of chemicals and their discharge into natural environment.
The ultraviolet-light disinfection facility for disinfecting the effluent using UV light, eliminating the toxic risks involved with the traditional method of chlorination and dechlorination.
Coarse screening is the first unit process in wastewater treatment which removes large solids, such as rags, cans, rocks, branches, leaves, roots, etc., from the flow before the flow moves on to downstream processes. A bar screen is generally used for wastewater. A bar screen consists of a series of parallel, evenly spaced metal bars, or a perforated screen placed in a channel. Bar screens can be coarse or fine. The wastewater stream passes through the screen, leaving behind trapped on the bars large solids for removal. Solids removal from the screens may be either manual (bars or screens are placed at an angle of 30° for easier solids removal) or mechanical (bars are placed at 45° to 60° angle to improve mechanical cleaner operation), but should occur frequently enough that trash build-up does not block influent flow (Drinan 2001, Spellman 2003).
Aerated grit removal
The purpose of grit removal is to take the heavy inorganic solids (sand, gravel, clay, egg shells, etc) that could cause excessive equipment wear. All of the processes used for grit removal are rely on the fact that grit is heavier than the organic solids, thus organic solids can be kept in suspension to be carried on for treatment . Grit removal may be accomplished in a channel or tank, others in a centrifugal chamber. The processes employ gravity and velocity, aeration, or centrifugal force to separate the solids from the wastewater (Spellman 2003).
In this paper use of aerated grit removal system is suggested. In aerated grit removal systems aeration keeps inorganic suspended solids in suspension allowing heavier grit particles to settle out. The aeration rate is determined by observing mixing and aeration, and sampling fixed suspended solids. The aeration rate is adjusted to produce the desired separation. Too much aeration keeps both the grit and organic materials in suspension. Too little aeration allows both the grit and the organics to settle out. The majority of these systems are mechanically cleaned, however some of them can be cleaned manually (Drinan 2001).
After preliminary treatment the influent still contains suspended organic solids. These settleable organic and floatable solids are readily concentrated and removed by primary clarification. Primary clarification can be expected to remove 90 to 95% of settleable solids, 40 to 60% of total suspended solids, and 25 to 35% of BOD5.
For primary clarification circular tanks or long rectangular tanks can be used. Within these tanks, the heavier primary settled solids, settle to the bottom. The primary settled solids are removed as sludge, and are generally pumped to a sludge-processing area. Those solids which are lighter than water (oil, grease, and other floating materials) float to the top, forming scum. These floating solids are skimmed from the surface and removed. Wastewater flow leaves the sedimentation tank over an effluent weir for further treatment. Process efficiency is controlled by detention time (normally about two hours), temperature, tank design, and equipment condition (Drinan 2001).
The main purpose of secondary treatment is to provide BOD removal beyond what is achievable by primary treatment (Spellman 2003)
As the designed wastewater treatment plant has a focus on the removal of nutrients nitrogen and phosphorus, the advanced method, Biological nutrient removal, occurs at this stage of the treatment process.
Biological nutrient removal
Due to the excess input of nitrogen and phosphorus into the aquatic systems, nutrient contamination is one of the serious water quality problems (Hu 2008). Phosphorus and nitrogen are the most important elements which help algae and water plants to grow in rivers, lakes, etc. Thus, they are referred to eutrophic or life-giving elements. In stagnant surface waters, the sunlight penetration into the water body is prevented by excessive growth of algae and other aquatic vegetation (Vabolien- et al. 2007). As a result, eutrophication symptoms, including low dissolved oxygen (hypoxia) in the water and the occurrence of nuisance and toxic algal blooms, occur in the lower levels (Hu 2008). This inhibits the fish and other natural aquatic life growth, causes undesirable tastes and odours, and thereby decreases the water resource value for domestic, industrial, agricultural and recreational use (Vabolien- et al. 2007).
Municipal and industrial wastewater contains significant quantities of phosphorus and nitrogen and therefore the removal of these nutrients has become an important facet of wastewater treatment (Vabolien- et al. 2007). Traditional chemical and physical methods of the removal of the ammonia nitrogen from wastewater include ammonia-stripping, chemical precipitation with magnesium ammonium phosphate, electrochemical conversion. A stripping tower is used in ammonia stripping and the process consumes much energy. Chemical precipitation by addition of reagents may introduce new pollutants to the water body. Electrochemical method often utilises expensive metal or metal oxide as electrodes, and also consumes large quantity of energy (Lin et al. 2009). Commonly precipitation processes, using calcium, aluminium or iron salts are employed for phosphate removal. However, these methods generate additional solids, increase effluent salinity and add to operational costs of the treatments (Kochany et al. 2009).
Biological processes used for removal of these nutrients are preferred to the chemical removal as they produce a lower waste sludge. The biological processes are also admitted as more environmentally friendly than chemical (Vabolien- et al. 2007). Total nitrogen can be removed from wastewater by biological denitrification. This process is the most common option for the treatment of ammonia nitrogen wastewater. However, the process is only suitable for the removal of relatively low ammonia concentration due to the requirement of appropriate C/N ratio (Lin et al. 2009).
Biological Nutrient Removal (BNR) is becoming more and more popular. The main concept of the treatment is removing both nitrogen and phosphorus in one biomass sequence of anaerobic, anoxic and aerobic processes without chemical addition. The BNR technology has been proven to be very efficient (Kochany et al. 2009). Biological nutrient removal methods have some advantages over physical and chemical methods, including low waste sludge production and low capital and operational costs (Rodgers et al. 2008).
Secondary sedimentation immediately follows biological treatment, and is required to occur before disinfection process. The effluent from the biological treatment systems contains considerable biomass which must be removed by secondary sedimentation to meet acceptable effluent standards. In secondary sedimentation, the effluent is sent to a sedimentation tank called a secondary clarifier which is similar to the primary clarifier. However, differences occur in detention time, overflow rate, and weir loading (Drinan 2001)
Disinfection of wastewater before discharge reduces the number of pathogenic organisms and protects the receiving water, thereby decreasing the risk of waterborne disease outbreaks. This may be accomplished by chemical or physical methods. However, an increased awareness of the many disadvantages of chemical disinfectants, such as chlorine, has resulted in the necessity of application of alternative disinfection methods. Ultraviolet (UV) disinfection has a number of attractive features and benefits. UV light is a physical process used for the disinfection of water and wastewater without the disadvantages associated with chemical disinfection (Trojan Technologies 1999, Do 2004).
There are many advantages of UV disinfection:
Effective disinfectant against bacteria and viruses.
Short contact time.
No residual toxicity.
No chemicals to handle.
Buildings not required.
Minimal space requirement.
However, there are also some disadvantages of UV disinfection:
No immediate measure of effectiveness of disinfection.
No residual effect.
Low doses used to inactivate coliforms may not inactivate some pathogenic viruses, spores and cysts (Trojan Technologies 1999).
Primary sludge is thickened in gravity thickeners. Gravity thickening is most effective on primary sludge. Solids are withdrawn from primary treatment and pumped to the thickener. (Spellman 2003).
Secondary sludge is thickened by dissolved-air flotation prior pumping to digesters for anaerobic digestion. Flotation thickening is used most efficiently for waste sludge from suspended-growth biological treatment process (Spellman 2003)
Mixed primary and secondary sludge is stabilised by anaerobic digestion. This method involves using bacteria that thrive in the absence of oxygen and is slower than aerobic digestion. The advantage of anaerobic digestion is that only a small percentage of the wastes are converted into new bacterial cells. Most of the organics are converted into carbon dioxide and methane gas (Spellman 2003). Digested sludge is regularly pumped to a lagoon for storage and settling.
After the treatment processes the plant effluent is continuously discharged into natural river environment.
The major unit processes that make up the conventional wastewater treatment process are preliminary treatment, primary treatment, secondary treatment, and disinfection. Within modern approach to wastewater treatment some advanced technologies can be used, for example, Biological Nutrient Removal of phosphorus and nitrogen. To ensure environmentally safe disposal, wastewater treatment sludge must undergo appropriate treatment.