Municipal waste treatment, sludge treatment
Urban waste treatment and sludge treatment are key links in modern urban management. Urban waste treatment mainly includes waste collection, classification, transportation and treatment. Common treatment methods include landfill, incineration and composting. Landfill is suitable for treating waste that cannot be directly used, but may cause land pollution and greenhouse gas emissions; incineration can effectively reduce the volume of waste and recover heat energy, but the emission of harmful gases needs to be controlled; composting is suitable for treating organic waste and can convert it into fertilizer.
Sludge treatment involves solid waste generated during sewage treatment. Treatment methods include dehydration, drying, stabilization and resource utilization. Dehydration reduces the volume of sludge, drying further reduces the water content, and stabilization can reduce its corrosiveness and pollution risks. Resource utilization treatment methods such as using dried sludge as soil conditioner or building material aim to achieve resource utilization and reduction of waste. Both need to take into account environmental protection and economic benefits to achieve sustainable management.
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Sludge treatment − properties of drying sludge
Overview of sludge drying properties
When sludge dries, it goes through three different phases as the water evaporates:
- the adaptation/preliminary phase
- the constant drying rate phase, and
- the falling drying rate phase.
During the adaptation/preliminary phase, the rate of water evaporation rapidly increases to a near-constant value. The process then continues at a near-constant drying rate until the most accessible free water has been removed.
Sludges of different origins and characteristics exhibit different behaviours when drying. These are most readily observed by correlating the mass water flux – the mass of water evaporated per unit surface area of solids per unit time (kg m-2 s-1) – against the moisture content of the sludge. This correlation normally reveals the three different drying phases.
Drying curve for typical sewage sludge
Following the removal of most of the free water – the water unassociated with the flocs – the drying rate reduces, corresponding to the falling drying rate phase. This phase may itself have two recognisable zones, respectively corresponding to the removal of:
- a) the interstitial water – the water held within the flocs but not associated with the solids, and
- b) the surface water – the water associated directly with the solid particles and bound to their surfaces by hydrogen bonding.
Evaporation of more than ~95% of the total water is not normally viable or necessary. The last 5% of the moisture relates to the surface water, which is much less easily removed. Thermal drying therefore generally aims to remove most of the interstitial water alongside the free to produce a product of~90% DS or more.
Sludge of ~10% moisture content or less is stabilised, easily conveyed (since it is in granular or powdered form) and generally suitable for either:
- end use, for example, as a soil conditioner, or
- processing by ’dry‘ thermochemical methods such as incineration, pyrolysis or gasification.
The sludge drying curve can be used to determine key dryer design parameters such as temperature and residence time. The rate and extent of drying impacts of the mechanical properties of the sludge, which change significantly in the course of the drying process as the water content decreases:
Sludge drying demands careful control. Dryers are normally designed to avoid the ‘sticky’ phase to prevent handling problems and form a free-flowing granular or powdered product. This is achieved either through backmixing or agitation.
For backmixing, the influent dewatered sludge is blended with some of the pre-dried granular material from the dryer to provide a sludge product where the average water content is below that associated with the sticky phase. This mixed product is then fed into the dryer to reduce the water content further.
Agitation avoids the formation of sticky sludge by efficiently mixing the sludge during the problematic sticky phase in order to maintain its movement throughout. This ensures that the lumps quickly disintegrate before they can become sticky.
Control of the sludge properties during drying is critical. Under certain conditions, and for certain sludge characteristics, the sludge dust particles can explode or the solids self-combust. These risks can be reduced by blanketing with an inert gas.
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Application and effect of dryer in urban garbage treatment and sludge treatment
The treatment of urban garbage and sludge is an important part of modern urban environmental protection. Traditional garbage disposal methods such as landfill and incineration have certain environmental problems. As an advanced drying technology equipment, dryer shows great potential in the treatment of urban garbage and sludge. This paper will conduct an in-depth analysis of the application of dryers in this field, in order to provide theoretical support and practical guidance for future garbage and sludge treatment.
Basic principles and types of dryers
Dryers evaporate the moisture in the material by heating the medium, thereby achieving the purpose of drying. According to their working principles and structures, dryers can be divided into the following types:
- Rotary dryer: The material is fully contacted with the hot air flow by rotating the drum to achieve drying.
- Belt dryer: The material is dried by hot air on the belt conveyor.
- Fluidized bed dryer: The material is fluidized under the action of the air flow and dried by hot air.
- Flash dryer: Drying is achieved by rapid evaporation, suitable for fine powder materials.
Application of dryers in urban garbage treatment
Treatment effect
In the process of urban garbage treatment, dryers are mainly used to reduce the volume and weight of garbage and improve the combustion efficiency of garbage. By drying, the water in the garbage is removed, making it drier, thereby reducing the cost of transportation and landfill. The dried garbage can be incinerated as fuel, reducing the demand for landfills and helping to recover energy.
Environmental impact
Using dryers to treat garbage helps reduce the use of landfills, thereby reducing the risk of landfill pollution to soil and water sources. In addition, the gas emissions during the drying process are relatively low, which can effectively reduce the negative impact of garbage disposal on air quality. However, the use of dryers also needs to pay attention to energy consumption issues, ensuring the use of environmentally friendly energy and efficient equipment to reduce the burden on the environment.
Application of dryers in sludge treatment
Treatment effect
The sludge generated during sewage treatment has a high water content and is difficult to treat. The use of dryers can effectively reduce the water content of the sludge, thereby reducing its volume and weight, facilitating subsequent disposal and resource utilization. The dried sludge can be used as a soil conditioner, building material or further incinerated.
Environmental impact
The process of drying sludge not only reduces its volume, but also reduces the risk of odor and pathogenic microorganisms generated by sludge storage. However, a certain amount of gas and dust may be generated during the drying process, and appropriate control measures need to be taken to ensure environmental protection. In addition, the energy consumption problem of sludge drying also needs attention, and energy-efficient equipment and technology should be adopted.
Comprehensive benefits of dryers in garbage and sludge treatment
Economic benefits
The use of dryers can significantly reduce the transportation and treatment costs of garbage and sludge and improve treatment efficiency. By reducing volume and weight, the cost of landfill and incineration is reduced, while improving resource utilization, which has obvious economic benefits.
Environmental benefits
Drying technology helps to reduce the negative impact of garbage and sludge treatment on the environment, reduce the demand for landfills, reduce waste gas and odor emissions, promote resource recycling, and meet the requirements of sustainable development.
Future development direction
In order to further improve the effect of dryers in garbage and sludge treatment, future research can focus on the following aspects:
- Improve energy efficiency: Develop energy-efficient and energy-saving drying technology to reduce energy consumption.
- Optimize equipment design: Improve the structure and function of the dryer, and improve processing capacity and efficiency.
- Application of environmental protection technology: Strengthen the control of gas and dust to reduce the negative impact on the environment.
- Resource utilization: Explore various resource utilization methods for dried garbage and sludge to achieve waste resource utilization and reduction.
Conclusion
The application of dryers in urban garbage and sludge treatment has shown good treatment effects and economic benefits. By reducing volume, improving combustion efficiency and facilitating resource utilization, dryers provide an effective solution for modern urban garbage and sludge treatment. However, attention should still be paid to its energy consumption and environmental impact. Future research should focus on improving the efficiency and environmental performance of drying technology to achieve a more sustainable urban garbage and sludge treatment goal.
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What is sludge treatment?
Sludge treatment refers to the process of managing, treating and disposing of sludge generated during sewage treatment. Sludge is the solid residue left after sewage is precipitated, filtered and biologically treated, and usually contains a large amount of water, bacteria, organic matter and harmful substances. Since sludge itself is polluting, large in volume and difficult to dispose of directly, it must be treated to reduce its volume, reduce its polluting properties and ultimately safely dispose of or recycle it.
The main goals of sludge treatment include
- Reducing volume: reducing the water content in sludge by dehydration, drying and other methods, reducing the volume of sludge, and facilitating subsequent treatment and transportation.
- Reducing pollution: Sludge contains pathogenic microorganisms, harmful chemicals and heavy metals, and the treatment process should minimize the hazards of these pollutants.
- Resource utilization: Some treated sludge can be used as energy, fertilizer or soil conditioner for resource utilization, thereby converting waste into useful resources.
The main steps of sludge treatment
- Sludge concentration: through physical methods such as gravity sedimentation or flotation, the water content in sludge is reduced and the sludge volume is reduced.
- Sludge dewatering: Mechanical dewatering (such as centrifuges, belt filter presses, etc.) is used to further reduce the water content of sludge and reduce its volume for subsequent treatment or transportation.
- Sludge stabilization: Sludge is treated by physical, chemical or biological methods to reduce its organic matter content and pathogens. Common methods include lime stabilization, composting fermentation, anaerobic digestion, etc.
- Sludge drying: The water in the sludge is further evaporated by heat energy or other means to produce drier sludge, thereby reducing the volume and making it easier to recycle or finally dispose of it.
- Sludge disposal: The treated sludge can be landfilled, incinerated, or used as fertilizer, soil conditioner, etc. for resource utilization. The choice of disposal method depends on the composition of the sludge, the treatment technology, and the local environmental policy.
Main technologies for sludge treatment
Mechanical dewatering: Use filter presses, centrifuges and other equipment to mechanically reduce the water content of sludge and reduce its volume.
- Anaerobic digestion: Through the action of anaerobic microorganisms, organic matter in sludge is decomposed, and biogas (mainly methane) is produced, which can be used for power generation or heating.
- Composting: Sludge is mixed with other organic wastes and converted into organic fertilizer through aerobic fermentation.
- Incineration: Sludge is incinerated at high temperature to reduce its volume and generate electricity through heat recovery.
Challenges of sludge treatment
The treatment cost of sludge is high, and it still needs to be safely disposed after treatment.
The long-term impact of heavy metals, toxic and harmful substances contained in sludge on the environment needs to be strictly controlled.
In the process of resource utilization, the safety of sludge must be ensured to avoid secondary pollution.
In short, sludge treatment is an important part of the urban sewage treatment system, which is of great significance for improving the environment, promoting resource reuse and achieving sustainable development.
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Processing Sludges for Acceptance in the Fertilizer Industry
The following article was written by Lee D. Hoffmann, and presented at the Fertilizer Industry Round Table (FIRT) in October, 1992.
1. Introduction – In 1982 John Naisbitt published a book called “Megatrends”. This book was based on a collection of news stories which were summarized into the (10) major cultural shifts. Below are the trends as described in this book:
a. Industrial Society to an Information Society.
b. Forced Technology to a High Tech/High Touch Society.
c. National Economy to a World Economy.
d. Short Term Oriented to Long Term Oriented.
e. Centralization to Decentralization.
f. Institutional Help to Self Help.
g. Representative Democracy to Participatory Democracy.
h. Hierarchies to Networking.
i. North to South.
j. Either/Or to Multiple Option.
In particular I would like to focus your attention on item “J”. Personal choices for Americans remained rather narrow and limited from the postwar period through much of the 60’s.
I am sure you can all remember all telephones were black, bathroom fixtures were white, checks only came in green, there were only (3) television stations, and soft drinks came in (3) flavors. (Coke, Pepsi, and 7-Up). In contrast, telephones, bathroom fixtures and checks, are now available multi-colors and styles. Cable TV offers over (30) stations and there are at least (8) variations of Coke. Today there is Coke, Diet Coke, Caffeine Free Coke, Caffeine Free Diet Coke, Cherry Coke, Diet Cherry Coke, Caffeine Free Cherry Coke, and
Caffeine Free Diet Cherry Coke. Let alone the multitude of Flavored Mineral Waters.
Well I propose that Sewage Sludge is the Diet Coke of the Fertilizer Industry.
You may wonder why I would open with something so seemingly extraneous to the Fertilizer Industry. Well, believe it or not, the same people that support the above alternatives are the same people that are fertilizing their lawns and nourishing their gardens, and maintaining our golf courses and probably most important of all, the same people that are feeding the world.
A study of our marketing statistics indicates that, never before, have we received more inquiries on equipment needed to process alternative items such as Bat, Chicken, Cattle and Turkey manure, as well as Industrial Sludges and most of all Municipal Wastewater Sludges.
Since we have seen the most activity in Sewage Sludge and it is such an abundant material the balance of this paper will concentrate on its beneficial use.
We are presently working with entrepreneurs who are contracting for the output of these facilities. In most cases they are being paid to take this output and in addition then convert it to fertilizer. Much of the equipment to do so, is very similar to traditional fertilizer manufacturing equipment; however process technology and controls are more refined.
2. Government’s Present and Proposed Position on Agricultural Sewage Sludge Use:
- a. Sewage Sludge Survey – In November of 1990 the EPA published a report called the National Sewage Sludge Survey. This survey consisted of data collection and an informational questionnaire to obtain data on sewage sludge quality and management. The data in this report was based on 1988 disposal methods. The results of the survey have provided the EPA with current data and information essential to establishing numerical pollutant limits in the final part of the 503 rule that will encourage the beneficial reuse of sewage sludge and provide a greater degree of public health and environmental protection.
- b. Because of its origin, wastewater products used in agriculture are subject to regulations which are not imposed on chemical fertilizers. No other material used in agricultural has been subjected to a more rigorous and comprehensive risk analysis.
- c. The biggest negative associated with the agricultural use of sewage sludge presently is the perception around heavy metals. The attached graphs will assist in our discussion of the heavy metals issue:
i. Allowable Metals/Average Metals Graph – Along the bottom of the graph you will notice (10) metals the EPA seems most concerned with.
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The first bar indicates what the EPA has defined as “Cumulative Pollutant Loading Rates” in kilograms per hectare (2.471 acres). These figures were published in February of 1989. In order to contrast these numbers, I needed to assume an application rate. For this I assumed the load would be if the entire EPA allowable, 50 dry metric ton per hectare, sludge load was applied. The metals content is the average derived from the National Sewage Sludge Survey.You will note that in most cases, with the exception of Copper, the metals loading is insignificant. I would also like to point out that a present land application operation, in the Rockford, IL area, is currently applying at a rate that is only 6% of the currently allowable 50 metric ton limit. This would make these insignificant numbers even less significant.
I should also point out that the EPA has several calculations that must be performed to determine the maximum sludge that can be applied per hectare. Generally these calculations will reduce the amount of allowable sludge. However in no event may it exceed the 50 metric ton limit.
ii. Average Metals Mg/Kg Graph – Along the bottom of the graph you will again notice the same list of metals. However, on this graph we compare the average metals contents, again from the National Sewage Sludge Survey, against various products you are familiar with.
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This graph compares the average metals contents, again from the National Sewage Sludge Survey, against various contents you are familiar with.As you can see, according to current EPA publications, metals in sewage sludge do not appear to be that big of an issue. This certainly will need to be re-addressed once the new regulations are published.
3. Present Disposal Statistics – At the time of the National Sewage Sludge Survey (August 1988 – September 1989) there were 11,407 publicly owned sewage treatment facilities in the United States and Puerto Rico. The survey summarized the sludge disposal methods into these following categories:
a. Land Application – The application of liquid, dewatered, dried, or composted sewage sludge to the land by surface spraying, surface spreading, or subsurface injection.
b. Distribution and Marketing – The give-away, transfer, or sale of sewage
sludge or sewage sludge product in either bagged or bulk form (Milorganite, Composting)
c. Sewage Sludge Incineration – The treatment of sewage sludge exclusively in an enclosed device using controlled flame combustion.
d. Monofill – A controlled area of land that contains one or more sewage sludge units. A sewage sludge unit is defined as a controlled area of land where only sewage sludge is placed. The sludge is covered with a cover material at the end of each operating day or at more frequent intervals.
e. Co-Disposal Landfill – An area of land or an excavation that is used for the permanent disposal of soled waste residuals, and sewage sludges.
f. Ocean Disposal – Dumping or controlled release of sewage sludge from a barge or other vessel into marine water.
g. Co-incineration – The combined treatment of sewage sludge and other combustible waste materials in an enclosed device using controlled flame combustion.
h. Surface Disposal – A controlled area of land where only sewage sludge is placed for a period of one year or longer. Sludge placed in this area is not provided with a daily or final cover.
As the chart shows, there were approximately 5.6 million dry metric tons of sewage sludge disposed of in 1988. |
In our opinion the Distribution and Marketing sector holds much potential for growth. Potential uses for dried sludge are as follows:
i. Dry granular sewage sludge is very similar in appearance and handling to Commercial Fertilizers and can be applied with the same equipment.
ii. A component in commercial fertilizer granulation.
iii. A granular ingredient in bulk blends.
iv. It can still be landfilled without the cost of transporting the liquid (80%) component. In addition there is less concern about the liquids leeching.
v. Alternative fuel- the average heat value of dried sludge is approximately 4,000 btu’s/pound. This approaches the values associated with low grade coals.
Other Advantages
• Indefinite shelf life.
• Better odor control.
• Less expensive and more convenient storage.
• Generally more aesthetic.
4. Agricultural Value of Sewage Sludge –
a. Soil Conditioner – The organic components of sewage sludge improves the soils bulk density, porosity, and water retention properties. Many studies have indicated that decreased bulk density and increased soil
aggregate stability, result in better cultivation and less erosion potential. Reduced runoff and sediment losses have also been noted from using sewage sludge. Water retention increases with sludge addition, helping to provide necessary water for plant, particularly during periods of drought or water stress.
b. Nutrient Value – Most programs for beneficial use involve little or no charge for the farmer and, therefore, can represent a significant cost savings to him. The value of 10 dry tons of sludge applied to (1) acre of land, can amount to approximately $90.00 in nutrient value. Sludges vary slightly from facility to facility, but for reference, the Milorganite product manufactured in Milwaukee has an analysis of 6-2-0. Also, Milorganite is currently retailing for around $240.00 per ton, and wholesaling for around $140.00 per ton.
5. Summary – A vast majority of the testing, research, and production, to date.
The vast majority of testing, research, and production are carried out by individuals and organizations outside the fertilizer industry. In my opinion, no one is more capable than you of bringing this product into the hands of farmers.
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