Abstract: Disclosed herein is a method for dewatering sewage sludge by using sludge-coal-oil co-agglomeration (“SOCA”) which comprises the steps of physically, chemically or biologically conditioning sludge to impart hydrophobicity and lipophilicity to the sludge (conditioning step), supplying oil and coal to the conditioned sludge with stirring to form sludge-coal-oil agglomerates (agglomerating step), enlarging the particle diameter of sludge-coal-oil agglomerates (size enlargement step), and remaining the enlarged sludge-coal-oil agglomerates over a sieve to selectively separate them from hydrophilic materials dispersed in tailing water(screening step). According to the method, since sludge can be rapidly, easily and effectively dewatered and purified when compared to conventional sludge treatment methods. In addition, there is no risk of involving malodor and air pollution. Furthermore, the dewatered sludge can be utilized as a high-quality fuel.
Claims:
1. A method for dewatering sewage sludge by using sludge-coal-oil co-agglomeration (“SOCA”), comprising the steps of: physically, chemically or biologically conditioning sludge to impart hydrophobicity and lipophilicity to the sludge (conditioning step); supplying oil and coal to the conditioned sludge with stirring to form sludge-coal-oil agglomerates comprising separated organic materials having a diameter of 500 ?m or higher (agglomerating step); enlarging the particle diameter of sludge-coal-oil agglomerates to the range from 0.5 mm to 3.0 by increasing the agglomeration force of the sludge-coal-oil agglomerates by stirring the agglomerates at a low speed using an agitator, pan-disk or tumbler, after the agglomerating step (size enlargement step); passing water with the enlarged sludge-coal-oil agglomerates and unagglomerated inorganic materials through a sieve having 48 mesh or less to selectively separate the enlarged sludge-coal-oil agglomerates (screening step); and spray-washing and dewatering the enlarged sludged-coal-oil agglomerates after they are selectively separated from the unagglomerated inorganic materials.
2. The method according to claim 1, wherein the agglomerating step is carried out by separately agglomerating the conditioned sludge and coal with oil (a first agglomeration), mixing the first agglomerates, and agglomerating the mixture (a second agglomeration).
3. The method according to claim 1, wherein the agglomerating step is carried out by agglomerating the conditioned sludge with oil (a first agglomeration), adding coal and oil to the first agglomerates, and agglomerating the mixture (a second agglomeration).
4. The method according to claim 1, wherein the agglomerating step is carried out by agglomerating coal with oil (a first agglomeration), adding the conditioned sludge to the first agglomerates, and agglomerating the mixture (a second agglomeration).
5. The method according to claim 1, wherein the oil used in the agglomerating step is selected from heavy oil, light oil, kerosene, cooking oil, waste cooking oil, castor oil, soybean oil, hempseed oil, waste lubricant and mixtures thereof.
Description:
1. Field of the Invention
The present invention relates to a method for dewatering sewage sludge by a process wherein only organic materials are selectively purified and recovered from a suspension of hydrophilic inorganic materials and organic materials present in sewage sludge based on the hydrophobicity of carbohydrates. More particularly, the present invention relates to a method for dewatering sewage sludge by sludge-coal-oil co-agglomeration (hereinafter, abbreviated as “SOCA”) which comprises lipophilically conditioning the surface of sludge and mixing the surface-conditioned sludge with fine oil particles to form sludge-oil agglomerates in the form of small spheres. The sludge-oil agglomerates are formed when the fine oil particles cover the surface of the lipophilic and hydrophobic sludge. According to the dewatering method of the present invention, sludge can be effectively dewatered.
2. Description of the Related Art
In order to better understand the background of the present invention, conventional techniques and current technological situation concerning the treat of sewage sludge are provided below. Unit processes currently used to treat and dispose of sewage sludge are classified into the following categories according to their purpose and functions.
As explained above, the sludge treatments are divided into a pretreatment process, an intermediate treatment process and a final treatment process. The pretreatment process for the weight reduction of sludge includes concentration, digestion and mechanical dewatering. The intermediate treatment process for the weight reduction and stabilization of dewatered cakes includes composting, incineration, melting, solidification and the like. Most final waste products obtained after the final treatment process are commonly buried without drying, or reused. Leachate generated from the buried waste products becomes a serious problem. In addition, since environmental regulations have recently become more stringent for ground-landfill/coastal-landfill and ocean dumping, special attention has been paid to incineration processing.
Incineration is a process wherein combustible materials are burned using oxygen present in the air. In order to solve the problem of insufficient available landfill sites, weight reduction of waste materials and stabilization for treating septic matters by transforming organic materials into inorganic materials, are widely used as processes for treating waste materials. Since incineration processing is advantageous in that the waste heat released during incineration of waste materials can be reused for electricity generation and district heating, it has been used to treat municipal waste materials for a long time. In the case of hazardous waste materials, weight reduction of sludge is preferential over stabilization and safety effects because of limited landfill sites, particularly, in metropolitan areas. In most big cities of industrialized countries, conventional landfill processing is increasingly changed toward incineration processing.
The major benefits of incineration processing are sanitary disposal, no septicity, less disgust than dewatered cakes, and volume reduction to 10?20% of the initial volume of sludge.
Since concentrated sludge generated from a sewage terminal treatment plant contains a water content of 95% or more, it needs to be dewatered for subsequent processing. The use of a general method for dewatering sludge by mechanical dewatering enables the water content to be reduced to 75?85% (see, FIG. 1).
Meanwhile, in order to better improve the dewatering properties during dewatering of sludge or solid microorganisms, a chemical conditioning or stabilization treatment of sludge is performed. In addition to the chemical conditioning treatment, a physical heating treatment and a freeze-thaw treatment are used. However, since these treatments are economically disadvantageous in terms of their operation, they are limited to small-scale industrial processes.
The chemical conditioning treatment is evaluated to be the most economical sludge dewatering treatment, in terms of high recovery rate and compatibility with other treatments. Depending on the type of sludge to be treated, the chemical conditioning treatment can lower the water content of sludge from 90?99% to 65?85%. According to the chemical conditioning treatment, simple coagulation between sludge and sludge solids takes place, and as a result, absorbed water is discharged (dewatering). The chemical conditioning treatment is commonly used in processes, e.g., centrifugal dewatering, belt-filter press or pressure filter treatment, where a more elaborate dewatering is required. As chemicals used for the chemical conditioning treatment, iron chlorides, limestone, alum, polymeric materials, etc., are used. In the case that a polymeric material is used as a chemical for the chemical conditioning treatment, additional non-combustible inorganic sludge is not generated. However, in the case that an iron chloride or limestone is used as a chemical for the chemical conditioning treatment, 20?30% of sludge based on dried sludge is further generated. The chemical for the chemical conditioning treatment is properly selected according to the concentration of entering sludge and reaction conditions of a reaction solution, for example, pH, alkalinity, reaction time. For instance, a large amount of limestone is required to dewater sludge to a high extent and to increase pH and alkalinity. At this time, a large amount of ammonia gas is evolved during sludge dewatering and additional sludge is generated. The choice of a chemical also depends on the dewatering processes employed, e.g., the polymeric material is mainly used in the centrifugal dewatering process and belt-filter press treatment, but is not suitable in the pressure filter treatment. The amount of entering sludge generally varies according to the characteristics of the entering sludge. As the dewatering of sludge becomes difficult, the amount of the chemical addition increases, the formation of dry cakes is difficult and filtering efficiency is poor. Generally, dewatering of untreated primary sludge is easiest. Mixed sludge containing the untreated primary sludge, anaerobically treated sludge and aerobic sludge follow the untreated primary sludge in terms of ease of dewatering.
As other conditioning treatments, heat treatment, preheating treatment, freeze-thaw treatment and the like can be mentioned. According to the heat treatment for sludge conditioning and stabilization, sludge is heated under pressure for a short time to destroy its gel-structure and to reduce its hydrophilicity. As a result of these heat-treating effects, even moisture present within microorganisms can be removed and the water content of dry cakes can be reduced to 30?50%. In addition, since no additional conditioning treatment is required, the formation of sludge having a heating value of about 30 kJ/g is possible. However, the drying process has disadvantages that a large quantity of noxious gases, including ammonia, is evolved during heat-treating and a supernatant having a high BOD is formed. Accordingly, an additional secondary treatment of wastewater and air is indispensable to remove the noxious gases and supernatant. Furthermore, since equipment used for the heat treatment is very expensive, the heat treatment is partially used in a small-scale process.
The preheating treatment is based on the fact that the preheating of sludge to about 60° C. can increase the dewatering effects of sludge by about 6%. However, since the preheating treatment requires recirculation of filter-treated water having a high BOD, it can be effectively utilized in areas using waste heat.
According to the freeze-thaw treatment, repeated freezing and thawing changes the jelly-structured sludge into fine granules so that the filtering resistance is reduced and thus the sludge is effectively dewatered. The freeze-thaw treatment is ineffective to dewater bound water, but is effective for relatively hard-to-dewater materials. The dewatering operation of the freeze-thaw treatment is relatively easy, and the water content in cake residues can be reduced to 25?40%.
SUMMARY OF THE INVENTION
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an improved method for dewatering sewage sludge by replacing water bound to the surface of sludge with oil, and separating the sludge-oil agglomerates by screening, thereby improving dewatering and purifying effects. According to the method of the present invention, since sludge can be rapidly, easily and effectively dewatered and purified in comparison with conventional sludge dewatering methods, costs taken to dewater sludge can be considerably reduced and facilities associated with sludge treatment can be greatly simplified. In addition, there is no risk of involving malodor and air pollution. Furthermore, the dewatering method of the present invention has an advantage that the dewatered sludge can be utilized as a high-quality fuel.