Sludge and waste treatment solutions
Sludge is an inevitable secondary waste product generated from the treatment of wastewater. Population growth, providing service for sludge treatment and the standard values of the treatment for this kind of waste increases the growing amount of sludge waste.
The sludge contains fecal matter and residual biomass, a variety of natural substances (nutrients, organic matter) and a variety of potentially toxic ones (heavy metals, organic micro-pollutant, pathogen microorganism).
Sludge treatment is required for the following purposes:
- Stabilization to reduce odor and pathogen content
- Reducing the volume and transportation costs
- The requirement of regulations
- Energy generation
- Obtaining the necessary quality for use/storage
The principles underlying sludge management strategies are:
- To avoid the production of waste – in terms of sludge management there are techniques that generate the least amount of sludge.
- Recovery of sludge - refers to the use in agriculture, forestry or soil quality restoration.
- Energy recovery - involving the release of potential energy contained in organic material from the sludge using different techniques: anaerobic digestion (biogas), incineration, co-combustion or other innovative methods such as sludge gasification.
- Final elimination - refers to storage after the pretreatment of sludge in specific locations, with certain features.
nală – se referă la depozitarea nămolului după pre-tratare în amplasamente specifice, având anume caracteristici.
In the European context, final disposal and reuse/recycling of sludge generated in sewage treatment plants is a highly disputed issue. The applying of sludge to the ground can be beneficial as it can help improve the physical, chemical and biological properties of soils, leading to improvement of crops.
On the other hand, the use of sludge in agriculture leads to a potential health risk and the environmental ones by affecting groundwater, surface water and soil. Storing sludge to landfill must be done in compliance with environmental restrictions in effect. Other alternatives such as composting, are much less developed and, in other cases, such as incineration, very expensive.
Many existing wastewater treatment plants ended up in a bad condition, even in certain situation becoming nonfuctional. Few operators can now make a choice in terms of removing sludge - about 90% is stored on site – and sludge treatment, due to cost and complexity, is of little interest and of few practical solutions.
European Directives allow the use of untreated sludge on agricultural land if it is injected or worked into the soil. Otherwise, sludge shall be treated before being used in agriculture, to be more precise, they must undergo biological, chemical or heat treatment, long-term storage or any other appropriate process as to significantly reduce fermentability and the health hazards resulting from its use.
By incinerating sludge, a complete oxidation can be achieved, regarding all organic matter, the final product being water, carbon dioxide, sulfur dioxide and ash (inert material). This process is recommended in cases of sludge resulted from wastewater containing toxic substances, that do not allow exploitation in the agricultural sector, in depositing in landfills or soil to recover useful substances.
For the incineration of sewage sludge, it is mandatory to have a low humidity (prior to application of the reduction of humidity) and the use of an enough calorific power, and the ash quantity to be as reduced as possible.
The calorific value of sludge depends in particular on the amount of organic solids in the composition. From this point of view, fresh sludge contains more dried fresh organic matter in comparison to fermented sludge, which is why it is recommended to avoid anaerobic or aerobic sludge stabilization.
Sludge disposal in landfills is always the last option, being recognized as an unsustainable solution and is subject to legal restrictions in the European Union.
The primary route by which the amount of treated sludge can be minimized is the reduction of the present water volume through thickening and dehydrating the sludge. There is a limit to the amount of water that can be removed mechanically from the sludge and many dehydrated sludge (for example with a belt press, filter press, centrifuge etc.) have a dry matter content in the range of 15% up to 40%.
Any of the alternatives to manage waste, require the stage of drying them firsthand.
Thermal sludge drying will reduce sludge volume by evaporation of water that can not be removed mechanically. Although, for thermally drying sludge, there is a considerable amount of energy and local circumstances that will dictate whether it’s a benefit to the environment by thermal drying rather than transporting large volumes of untreated sludge to be recycled eliminated.
It is vital that these factors to be assessed in the strategic choices. The largest amount of evaporated water is to be recycled to the plant’s wastewater installation after the removal of condensing gas. Storage project must consider the risk of self-ignition.
Disadvantages of thermally drying sludge:
- High capital costs, classical dryers are expensive and take up large spaces
- High operating costs, high fuel consumption due to high energy losses through large volumes of circulating hot air
- Technical complexity
- Risk of fire, explosion due to large amounts of discharged dust
- Requires odor control, in particularly because of large volumes of convection heating that are required which take hot air and water vapor, even odor before being evacuated.
The volume of sludge produced is < 1% of the volume of wastewater treated but can mean > 50% of the operating costs of the wastewater treatment station in relation to the costs of processing, transport and recovery / storage.
It requires the application of efficient energy solutions for advanced sludge drying. Rotary drum dryers with heating radiation realizes, at the same time, drying, sterilizing, deodorizing, disinfecting, deseeding, inactivating enzymes, reducing volume and weight in terms of energy efficiency being superior to any other drying system.
The energy used for the treatment and drying the sludge can be retrieved by:
Use of biogas from anaerobic fermentation
Incineration or co-incineration with other waste, sludge energy value is placed in a wide range depending on the type of sludge, water content and volatile content. The value of treated sludge is high. Because of water content, this value would be low if it would not be dried.
Used as combustion with other fuels in power plants
Used as fuel in industrial processes, such as cement and asphalt production. Conventional fuel can be supplemented by being replaced with sludge.
The thermal energy obtained from the combustion of sludge and biogas always cover the specific consumption for drying the sludge, leaving a surplus which can be used for processes of treating wastewater and sludge.
Electricity from cogeneration can provide over 30% of energy consumption of the entire treatment plant, according to the calorific value of the incinerated sludge and biogas from anaerobic fermentation.
If an energy recovery option is chosen, there is no need for anaerobic fermentation to take place, as this reduces the calorific capacity of the sludge for the receiver. Following the developed feasibility studies, taking in account the specific conditions in each case for large WWTP, fermentation can be applied to some part of the sludge waste in order to generate the energy needed for the treatment/ drying of the sludge, thus improving the possibilities of accepting co-processing and reducing transportation costs and admission.
Operating principle of the rotary dryer with direct heating with radiation:
The granular material is introduced into the rotary drum dryer on the conveyor belt. Here, the continuing drying takes place, though continuous heat supply, predominantly by radiation, transmitted from the radiant burners with metallic fabric from within the drum.
Removing vapor from the rotary drum is achieved by extracting, by case, at the same time with flue gases and by introducing fresh air, heated in this case, by recovering heat from the discharge of flue gases. Some of them are recycled and reintroduced into the drum.
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Description of the rotary drum dryer with direct heating through radiation
Drying aggregate composed out of drying battery with rotary drum through radiation
Sludge structure before mechanical dehydration and drying
- interspacial water and capillary water can be eliminated by mechanical dehydration
- absorbtion water and indoor water can be eliminated only through thermal drying
Mechanical dehydration, followed by warm air drying
drying from outside towards the interior of the grain of sludge
Formation of a waterproof surface on the crust of the sludge’s surface which slows down the water evaporation from the inside
High energy consumption
Mechanical dehydration, followed by radiation drying
Drying from inside out of the grain of sludge
The inside of the grain is dried quickly, the water evaporates smoothly
Low power consumption
The advantages of thermal sludge drying:
Stable for a long time
Elimination of pathogenic microorganisms
Low odor even when using crude sludge
Reducing the volume of dry sludge by 4-5 times compared to the wet sludge
Low transport and storage costs
It’s accepted between farmers as a fertilizer and it’s easy to spread with specific machinery, mainly because of it’s granulated form
Can be incinerated without high fuel consumption
Can be used as a fuel due to higher calorific properties.
Advantages of Rotary drum dryers with radiation heating
In the case of heating with radiation, transmitted energy is independent of the heating sludge temperature, the heating speed is slowed down only if the possible cooling effects of the convection currents run around it.
Heating by radiation can lead to elevated temperatures well above the temperature from within the sludge drum, depending on the source of the radiation and distance from it
Heating sludge by exposing it to radiation is done quickly, radiant energy arises immediately after commissioning of the source of radiation
Heat losses are greatly reduced, radiation of electromagnetic nature heat up the environment and are directed as a beam upon the sludge to be heated up
Heating by radiation with our sources allows a continuous and precise adjustment of radiation intensity and temperature so the sludge during drying.
High share of drying with radiation
Heating / radiation intensity is constant over the entire length of the drum which prevents overheating and is efficient in drying over the entire length of the drum
At the surface there is no crust to slow the evaporation towards the surface of the sludge grain, the drying is done inside out
Lower drying time, up to 5-6 times compared to drying with hot air or flue gas
Fuels consumption is greatly reduced
Superior energy efficiency by significantly reducing losses and circulated hot air recirculation and the possibility of recovering heat from flue gases
Emitted pollutants much lower due to reduced fuel consumption and the use of radiant burners with metallic fabric
The material that needs drying, doesn’t need to be semi-dry and dusting is reduced significantly
The space occupied by the equipment is decreased by 300 %, mainly due to the reduced length of the drum
Commissioning is fast and for very low input regime
Easy maintenance, on the interior wall, due to the lower temperatures, but evenly distributed, material sticking and crusting won’t happen
The possibility of setting the wide variety of drum speed, the intensity of radiation and the temperature inside the rotary drum dryers can cause them to adapt to different humidity, different granular materials and various mass flows, very low regime.
Energy recovery from sludge from municipal sewage plants, must become a technological priority, beneficial to the environment and for reducing the final costs of wastewater treatment and sludge, presenting the following advantages:
It completely eliminates the risk of polluting the environment with organic pollutants, viruses and pathogens, due to high temperatures during the heat treatment process
Sludge from the treatment of municipal wastewater can be effectively exploited as energy, even for a below average loaded influent with organic matter, being able to become a winning situation energetically with over 30% of electricity consumption at the treatment plant, and so a proportional increase of energy efficiency by 30%.
In addition to the energy gain, in the final heat treatment of the sludge, the amount of waste resulting from dehydration is significantly decreased, transforming it into ash, over 15 times reducing the proportion, transport and storage costs.