First, the raw sludge is pumped from the secondary settling tank to the other tank through the sludge pump to be separated from the supernatant. Since the moisture content of the raw sludge can usually reach 99.5%, the sludge must be concentrated and there are many feasible methods for reducing the volume of the sludge. Methods of mechanical treatment such as vacuum filtration and centrifugation are typically used before the sludge is disposed of in a semi-solid form. Usually these methods are preparations for sludge incineration. If biological treatment is planned, most will be concentrated by gravity sedimentation or air flotation. The sludge corresponding to these two conditions is still in a fluid state.
The gravity concentrate tank is designed and operated similar to the secondary settling tank in wastewater treatment. The concentrating function is the main design parameter. In order to meet the greater concentration capacity, the concentrating tank is basically deeper than the secondary settling tank. A properly designed, well-functioning gravity concentrating tank can at least double the sludge content. That is to say, the moisture content of the sludge can be reduced from 99.5% to 98%, or less. It is worth mentioning here that the design of the gravity concentration tank should be based on the analysis of the Chinese results as much as possible, because the appropriate sludge loading rate has a lot to do with the properties of the sludge.
If dissolved air flotation is used, a small portion of water is required, usually the secondary settling tank, which is inflated at a pressure of 400 kPa. This supersaturated liquid passes to the bottom of the tank and the sludge passes under atmospheric pressure. The gas adheres to the solid particles in the sludge in the form of small bubbles, or is surrounded, thereby causing the solid particles to float to the surface. The upper portion of the concentrated sludge is removed, and the liquid flows back from the bottom to the dissolved gas tank.
After the volume is reduced, the sludge contains a large amount of harmful components, which need to be converted into inert components before disposal. The most common method is stable biodegradation. Because this process aims to convert the material into a final sterile product, the method of digestion is often applied. Sludge digestion can further reduce the volume of sludge and also convert the solids contained into inert materials and generally no pathogens. Sludge digestion can be achieved by anaerobic digestion or good digestion.
Sludge contains a variety of organic matter and therefore requires a variety of microorganisms to decompose. The relevant data divides the microorganisms in anaerobic digestion into two categories: acid-producing bacteria and methanogens. Therefore, we can also divide anaerobic digestion into two steps.
In the first step, an acid-producing bacterium consisting of facultative anaerobic bacteria and anaerobic bacteria dissolves the organic solid by hydrolysis. The solute is then converted to alcohol and low molecular weight molecules by fermentation.
In the second step, methanogens consisting of strict anaerobic bacteria convert acetic acid, alcohol, water and carbon dioxide into methane. Because the two flora can only survive in an anaerobic environment, the anaerobic digestion reactor must be sealed. There are other factors to consider when designing the container, such as temperature, pH, and mixture agitation.
Sludge can also be stabilized by aerobic digestion. This digestion can basically only be used for biochemical sludge and not for primary sedimentation tanks. With the reduction of sludge volume in the secondary sedimentation tank and sludge concentration tank, this process requires constant aeration. Aerobic digestion is mostly used in deep aeration systems. Furthermore, aerobic digestion is not sensitive to environmental conditions and is not limited to epidemic changes.
After the sludge is digested, the organic matter in the sludge can be removed and the sludge volume can be further reduced. Next, the sludge needs to be disposed of. A variety of methods can be used to effectively dispose of the sludge. These include incineration, sanitary landfills and use as fertilizers and soil amendments. The raw sludge can be used for incineration, which can effectively reduce the water content. Adding fuel can be used to cause and sustain combustion, and municipal waste can also be used to achieve this goal.
Raw sludge and digested sludge can also be disposed of in sanitary landfills. The land application of sludge has been practiced for several years and is now limited to the treatment of digested sludge. The nutrient content of the sludge is conducive to plant growth, and its particle characteristics can be used for land improvement. These applications are limited to forage crops and non-human consumption, and the possibility of supporting edible plants is under investigation. The main limiting factors for sludge land application are plant enrichment metal toxicity and water eutrophication. The application of sludge can be carried out by sprays from sprinklers, diversion of ditches or direct injection into the soil. Dewatered sludge can be laid on the land and cultivated by traditional agricultural machinery.
The above text refers to the treatment of general sludge. Because sludge can cause environmental pollution, we need to do our best to make it harmless. Nowadays, many sludges with different characteristics that lead to type pollution are under study. In this article, I will describe a sludge from the human oil and petroleum industry. This representative sludge is called oily sludge.
A large amount of sludge is produced, and this sludge contains a considerable amount of oil and must be removed before final disposal. Sludge from refineries cannot be disposed of safely unless the oil content is removed to a certain extent.
In addition, in the oil-water separation system and the oil storage tank of the refinery, the sludge generated due to the accumulation of the oil-containing raw materials is expensive to handle, and causes serious pollution to the environment. Petroleum is a hydrophobic mixture such as alkanes, aromatic hydrocarbons, resins and asphalt. Many compounds are toxic, mutagenic and carcinogenic. Their emissions are tightly controlled because of their negative impact on human health and the environment, and they are classified by the US Environmental Protection Agency as a priority for environmental pollutants.
There are many ways to treat oily sludge. Chemical and physical methods such as incineration, chlorine oxidation, ozone oxidation and combustion, biological treatment methods such as bioremediation, traditional composting, and the like. Nowadays, with the development of technology, low temperature cold treatment and bioremediation of oily sludge have become two effective treatment methods.
As a physical treatment method, low temperature cold treatment technology can effectively increase the dewatering properties of sludge, change the structure of flocculant and reduce the water content around the sludge. Comparing the “initial settling”, the cold treatment removes impurities from the solution and thus achieves better concentration. This is the benefit of cold treatment. To the best of our knowledge, there is no discussion of the feasibility of cold treatment techniques to separate oil from sludge in today’s data. However, in many countries where natural conditions permit, cold treatment technology provides an effective means of handling the treatment and disposal of oily sludge.
By comparing the conventional method with the sludge after the cold treatment, we can find that a layer of oil floated on the sample after the cold treatment. Finally, we can find that the test tube is divided into three layers: the uppermost layer is a clear oil slick, the bottom layer is a layer of dark sediment, and the middle layer is clear water. After the original sludge settled for 24 hours, the supernatant and bottom sediment were visible, but there was no visible oil phase. The phenomenon described above reveals that a simple cold treatment can effectively separate the oil in the sludge.
The physicochemical method can be used to treat sludge, but the cost is very high. Composting and bioremediation by degrading oil strains or activating existing organisms by inoculation is seen as two economical methods to deal with oil pollution. Some of the compelling advantages of composting are: low capital and maintenance costs, simple design and operation, and the ability to remove some of the oil. However, composting is basically unable to meet the standards of today’s environment.
