Waste sludge treatment describes the processes used to manage and dispose of sludge waste generated during wastewater treatment. Sludge is mostly water with less amount of solid material removed from wastewater. Primary sludge includes solids solids removed during primary care in the primary clarifier. Secondary sludge separated in secondary clarification includes sludge treated from secondary treatment bioreactors.
Sludge treatment is focused on reducing the weight of mud and volumes to reduce disposal costs, and reduce the potential health risks of disposal options. Water removal is the primary means to lose weight and volume, while pathogen destruction is often done through heating during thermophilic digestion, composting, or incineration. The choice of sludge treatment methods depends on the volume of sludge produced, and the comparison of maintenance costs required for the available disposal options. Air drying and composting may be of interest to rural communities, while the limited availability of land can make aerobic digestion and mechanical dewatering preferable to municipalities, and economies of scale can encourage alternative energy recovery in metropolitan areas.
Energy can be recovered from sludge through the production of methane gas during anaerobic digestion or through incineration of dry sludge, but energy yields are often insufficient to evaporate moisture content or to blowers, pumps, or centrifuges required for dewatering. Rough primary solids and secondary sludge waste may include toxic chemicals released from wastewater by uptake into solid particles in clarifier sludge. Reducing the volume of sludge can increase the concentration of some of these toxic chemicals in the mud.
Video Sewage sludge treatment
Terminologi
Biosolids
"Biosolids" is a term often used in wastewater engineering publications and community relations efforts by local water authorities when they want to focus on reuse of mud waste, once the mud has undergone the appropriate treatment process. In fact, biosolids are defined as organic wastewater solids that can be reused after stabilization processes such as anaerobic digestion and composting. The term "biosolids" was introduced by the Federation of the Water Environment in the US in 1998. However, some argue that the term is a form of "propaganda" (or at least euphemism) in order to conceal the fact that sewage sludge may also contain substances that could be harmful to the environment when the treated sludge is applied to the soil, eg environmental persistent pharmaceutical pollutants.
Maps Sewage sludge treatment
Treatment process
Thickening
Thickening is often the first step in mud processing. Sludge from primary or secondary clarifiers can be stirred (often after the addition of clarifying agents) to form larger and faster aggregates. Primary sludge can thicken up to about 8 or 10 percent of solids, while secondary sludge can thicken up to about 4 percent solids. Thickers often resemble clarifiers by adding stirring mechanisms. Thick mud with less than ten percent solids may receive additional mud treatments while the overflow thickener is returned to the waste treatment process.
Drying
The water content of the sludge can be reduced by centrifugation, filtration, and/or evaporation to reduce the cost of disposal transport, or to improve suitability for composting. Centrifugation may be the first step to reduce the volume of sludge for subsequent filtration or evaporation. Filtration can occur through underdrains in bed drying sand or as a separate mechanical process in the press of a belt filter. Filtration and centrate are usually returned to the waste treatment process. After dewatering the mud can be handled as a solid containing 50 to 75 percent water. Drained mud with higher moisture content is usually treated as a liquid.
Sidestream treatment technology
The sludge treatment technology used for thickening or dewatering of sludge has two products: thickened or dried sludge, and a liquid fraction called a sludge treatment fluid, mud flow stream, liquor, centrate (if derived from centrifuge), filtrate (if derived from press filter belt) or similar. This fluid requires further treatment because of high nitrogen and phosphorus, especially if sludge has been digested anaerobically. Treatment can be done at the waste treatment plant itself (by recycling the liquid to the beginning of the treatment process) or as a separate process.
Phosphor recovery
One method to treat mud dewatering flow is to use a process that is also used for phosphorus recovery. Another benefit for sewage treatment plant operators processing mud dewatering flow for phosphorus recovery is that it reduces the formation of obstructive struvite scales in pipes, pumps and valves. Such barriers can be a maintenance headache especially for biological pest basting plants where the content of phosphorus in the sewage sludge increases. For example, the Canadian company Ostara Nutrient Recovery Technologies is marketing a process based on deposition of phosphorus chemical precipitation in a fluidized bed reactor that finds struvite in the form of crystalline pellets from the flow of the sludge stream. The resulting crystal products are sold to the agricultural sector, grasses and ornamental plants as fertilizers under the registered trademark "Crystal Green".
Digestion
Much of the mud is treated using a variety of digestive techniques, whose goal is to reduce the amount of organic matter and the number of disease - causing microorganisms present in the solid. The most common treatment options include anaerobic digestion, aerobic digestion, and composting. Sludge digestion offers significant cost advantages by reducing the mud quantity to almost 50% and providing biogas as a valuable energy source.
Anaerobic digestion
Anaerobic digestion is a bacterial process done in the absence of oxygen. This process can be a thermophilic digestion, in which the sludge is fermented in a tank at a temperature of 55 ° C, or mesophilic, at a temperature of about 36 ° C. Although it allows for shorter retention times (and thus smaller tanks), thermophilic digestion is more expensive in terms of energy consumption to heat sludge.
Mesophilic anaerobic digestion (MAD) is also a common method for treating sludge produced in sewage plants. Mud is put into a large tank and stored for a minimum of 12 days to allow the digestive process to perform the four stages required to digest the mud. These are hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In this process proteins and complex sugars are broken down to form simpler compounds such as water, carbon dioxide, and methane.
Anaerobic digestion produces biogas with a high proportion of methane that can be used to heat a tank and run a machine or microturbin to process elsewhere. Methane formation is a major advantage of anaerobic processes. The main drawback is the time required for the process (up to 30 days) and high capital costs. Many larger sites utilize biogas for combined heat and power, using cooling water from the generator to keep the digestive plant temperature at 35 35 Â ± 3 Â ° C as needed. Adequate energy can be generated in this way to generate more electricity than is required by the machine.
Aerobic digestion
Aerobic digestion is a bacterial process that occurs in the presence of oxygen resembling the continuation of the active sludge process. Under aerobic conditions, bacteria quickly consume organic matter and convert it to carbon dioxide. Once there is a shortage of organic matter, the bacteria die and are used as food by other bacteria. This stage of the process is known as endogenous respiration . Reduction of solids occurs in this phase. Since aerobic digestion occurs much faster than anaerobic digestion, the cost of aerobic digestion is lower. However, the operating costs are characteristically much larger for aerobic digestion because of the energy used by the blowers, pumps and motors that are needed to supplement the oxygen to the process. However, recent technological advances include aerated aeration filter systems that use natural air currents for aeration rather than electronically operated machines.
Aerobic digestion can also be achieved by using a diffuser system or a jet aerator to oxidize the sludge. Diffuser smooth bubbles are usually a more cost-effective method of diffusion, but blockage is usually a problem because of sediments that settle in smaller airways. Rough bubble diffusers are more commonly used in active sludge tanks or in flocculation stages. The main component for selecting a diffuser type is to ensure it will produce the required oxygen transfer rate.
Composting
Composting is the aerobic process of mixing sewage sludge with carbon by-products from carbon such as sawdust, straw or wood chips. In the presence of oxygen, bacteria digest mud waste and plant matter to generate heat to kill microorganisms and disease-causing parasites. Maintaining aerobic conditions with 10 to 15 percent oxygen requires a bulking agent that allows air to circulate through a good mud solid. Rigid materials such as corncobs, peanut shells, tree trimming wastes, or bark from wood or paper mills are better at separating mud for ventilation than the softer leaves and grass clippings. Light, biological inverting agents such as shredded tires can be used to provide structures where small, soft crop materials are the main source of carbon.
The uniform distribution of pathogen-killer temperatures can be helped by placing an isolation blanket from the previous compost mud on an animated compost pile. The initial moisture content of the compost mix should be about 50 percent; but the temperature may be inadequate for the reduction of pathogens in which wet or settling muds increase the moisture content of compost above 60 percent. The composting mixture can be stacked on concrete pads with built-in air channels to be covered by an unmixed bulking agent layer. The odor can be minimized by using a vacuum aeration blower pull through the composting pile through the underlying duct and exhausting through the pile of previous composting sludge that will be replaced when the moisture content reaches 70 percent. The liquids accumulated in ducting underdrain can be returned to the sewage treatment plant; and the composting pads may be roofed to provide better moisture control.
After sufficient composting intervals for pathogen reduction, the compost pile can be screened to retrieve undigested bulking agents for reuse; and solid compost that passes through the screen can be used as a land amendment material with the same benefits as peat. The optimum initial carbon-to-nitrogen ratio of the compost mixture is between 26-30: 1; but the composting ratio of agricultural byproducts can be determined by the amount necessary to dilute the concentration of toxic chemicals in the slurry to an acceptable level for the intended use of the compost. Despite low toxicity in most agricultural by-products, suburban grass clippings may have residual herbicide levels that are detrimental to some agricultural uses; and newly composted wood products may contain phytotoxins that inhibit seedling germination until it is detoxified by the soil fungi.
Incineration
Mud incineration is less common because of the problem of air emissions and additional fuel (usually natural gas or fuel oil) needed to burn low-calorie sludge and evaporate waste water. On a dry solid basis, the sludge fuel value varies from about 9,500 British thermal units per pound (980 cal/g) of undigested waste sludge to 2,500 British thermal units per pound (260 calories/g) of digested primary mud. Stepping on some fireplace incinerators with a high residence time and fluidized bed incinerator is the most commonly used system for burning sewage sludge. Co-firing in city waste-to-energy plants is sometimes done, this option becomes cheaper assuming existing facilities for solid waste and no additional fuel needs. Incineration tends to maximize heavy metal concentrations in ash residue requiring disposal; but the option to return the wet waste scrubber to the waste treatment process can reduce air emissions by increasing the concentration of the dissolved salt in the sewage treatment plant waste.
Drying bed
Simple mud drying places are used in many countries, especially in developing countries, because they are a cheap and simple method to dry mud. Water drainage should be captured; The drying bed is sometimes closed but usually left uncovered. Mechanical equipment for reversing sludge in the early stages of the drying process is also available on the market.
The drying bed usually consists of four layers consisting of gravel and sand. The first layer is a rough pebble that is 15 to 20 centimeters thick. Followed by a fine pebble of 10 cm thick. The third layer is sand that can be between 10 to 15 centimeters and serves as a filter between mud and gravel. Sludge dries and water seeps into the first layer collected in the drainage pipe that is under all layers.
New technology
- The recovery of phosphorus from sewage sludge or from the drainage stream of sludge is increasingly noticed especially in Sweden, Germany and Canada, as phosphorus is a limited resource (a concept also known as "peak phosphor") and is needed as a fertilizer to feed the world's growing population. Phosphorus recovery methods of wastewater or sludge can be categorized based on the origin of the materials used (wastewater, slurry, digested or undigested sludge, ash) or by type of recovery process (deposition, extraction and wet chemical deposition, thermal treatment). Research on phosphorus recovery methods from sewage sludge has been done in Sweden and Germany since about 2003, but the technology currently under development has not been cost-effective, given the current phosphorus price in the world market.
- The Omni processor is a process currently under development that handles mud waste and can generate a surplus of electrical energy if the input material has the right dryness level.
- Thermal depolymerization produces light hydrocarbons from sludge heated to 250 Ã, Â ° C and compressed to 40 MPa.
- Thermal hydrolysis is a two-stage process that combines high boiling mud pressures, followed by rapid decompression. This combined action disinfects sludge and makes it more biodegradable, which improves digestion performance. Sterilization destroys pathogens in the mud so that it exceeds the strict requirements for land applications (agriculture). The thermal hydrolysis system operates in sewage plants in Europe, China and North America, and can produce high quality electricity and sludge.
Disposal or use as fertilizer
When the sludge liquid is produced, further treatment may be needed to make it suitable for final disposal. Sludge is usually thickened and/or dried to reduce the volume transported off-site for disposal. The process for reducing water content includes lagooning in a drying bed to produce a cake that can be applied to the ground or burned; pressing, in which the sludge is mechanically filtered, often through a cloth screen to produce a sturdy cake; and centrifugation in which the deposits thicken by centrifugally separating solids and liquids. Sludge can be disposed of by injecting liquid to land or discharged to landfill.
No process completely eliminates the need to dispose of treated sewage sludge.
Much of the mud originating from commercial or industrial areas is contaminated with toxic materials that are released into sewers from industrial processes. The increased concentration of these materials can make sludge unsuitable for agricultural use and may then have to be burned or disposed of in landfills.
Example
Edmonton, Alberta, Canada
The Edmonton Composting Facility, in Edmonton, Alberta, Canada, is the largest mud-waste composting site in North America.
New York City, USA.
Sludge waste can be heated and converted into high pelletized nitrogen granules and other organic materials. In New York City, for example, some sewage plants have dewatering facilities that use large centrifuges along with the addition of chemicals such as polymers to remove liquids from the mud. The remaining product is called "cake," and it is taken by a company that turns it into a pellet fertilizer. This product, also called biosolid, is then sold to local farmers and grass farms as an amendment or soil fertilizer, reducing the amount of space needed to dispose of mud in landfills. Southern California, USA.
In the immense metropolitan area of ​​the southern California inland community, the mud wastes back into the lower-level community sewer systems to be reprocessed at some of the very large maintenance plants on the Pacific coast. This reduces the size of the required interceptor gutter and enables local recycling of treated wastewater while maintaining the economy from a single mud treatment facility and is an example of how waste sludge can help solve the energy crisis.
Controversy
Some campaigners consider mud waste treatment as a problem and a hazard to the environment - largely because systems in most industrialized countries mix industrial wastes with domestic sewerage. This has led some to claim that the term "biosolids" was created by the waste treatment industry to take the focus of material origin to make reuse more acceptable to the public, and some studies have suggested that this is actually a form of propaganda.
See also
- Sludge management
References
Source
Source of the article : Wikipedia
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