Mechanical type sludge thickening equipment uses physical process to concentrate the sludge by removing the water portion thus leading to increased amount of the solid percentage. There are different methods to achieve this and generally taking out the average of all the available options, usually the sludge content can be increased by 4 – 5 folds depending on how well the equipment is operated. Let us study some of the options available which are commonly used in the industries’ wastewater treatment systems.
Method relying on gravity principle can be applied to both untreated primary and even waste activated sludge. It is normally done in a circular tank similar in design compared with a typical plant sedimentation tank. Flow of sludge coming from the aeration system is directed to the center well and the design is such that there is sufficient detention time enough for good settling to take place. Waste that is collected at the bottom of the tank is allowed to settle, become compact and then pumped out from the bottom effluent outlet pipe going to either a digester or secondary dewatering equipment. Usually there is a weir and channel for the clarified water to overflow out and the rotating raking arm with blade will turn in circular motion in order to create a slow stirring effect. What results is that by doing this, it will ensure that compactness will take place and get the sludge to travel to the bottom. Occasionally process can be improved by slowing down the feed rate while the design must be properly planned out to allow sufficient detention time.
Flotation thickening is considered an improved process of the gravity method and basically it uses similar principle as mentioned before like the DAF system. To recap back, what it does is that compressed air is introduced to the incoming feed channel and then these are diverted to a pressure vessel in order to force air to dissolve in the water. After that, the flow will be slowly released so that when exposed to atmospheric pressure, dissolved air will become finely dispersed bubbles carrying together with it the sludge floating to the top. Flotation method is normally applied to waste activated sludge and the efficiency of the system normally takes into account the air to solid ratio and sometimes addition of coagulant or polymer is used to aid in the separation process. Other parameters that need to be taken into consideration is mainly the loading rate on how the incoming flow can be adjusted or lowered down so that air to solid ratio is improved. Apart from that, settling characteristics of the solid waste measured in SVI value is also another factor that determines the efficiency of the unit. Obviously the lower the SVI (below 50), the better it gets and this is the most ideal condition to get a good separation.
This method is often applied to waste solids coming from suspended growth biological treatment process. Generally it can be used in tandem along with the gravity thickener to further concentrate and increase the solid content. As mentioned earlier, not only does it play the role and function to thicken the sludge but it helps in dewatering process as well. For more information, refer to the solid bowl centrifuge and the imperforated basket design which are two of the main and most commonly used technology in most industrial effluent treatment plants.
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