Clarification basin

Wastewater treatment systems can be classified, in addition to pretreatment, as preliminary, primary, secondary, and tertiary (advanced) treatments. Pretreatment of industrial wastewater is required to prevent adverse effects on the municipal wastewater treatment plants. Preliminary treatment is considered as any physical or chemical process that precedes primary treatment. The preliminary treatment processes may consist of influent screening and grit removal. Its function is mainly to protect subsequent treatment units and to minimize operational problems. Primary treatment is defined as the physical or chemical treatment for the removal of settleable and floatable materials. The screened, degritted raw wastewater from preliminary treatment flows to the primary clarification tanks, which are part of the primary treatment facilities. Secondary wastewater treatment is the process that uses biological and chemical treatment to accomplish substantial removal of dissolved organics and colloidal materials. The secondary treatment facilities may be comprised of biological reactor and secondary clarification basins. Tertiary (advanced) wastewater treatment is used to achieve pollutant reductions by methods other than those used in primary and secondary treatments. The objective of tertiary wastewater treatment is to improve the overall removal of suspended solids, organic matter, dissolved solids, toxic substances, and nutrients. 

Another example for a possible aeration cell occurs with suction scrapers made of unalloyed steel in the clarification basin. In this case, the difference in aeration arises from the oxygen deficiency in the bottom deposit and the relatively high oxygen content in the waste water [17]. 

Continuous physicochemical PAC process systems involving the use of gravity settling basins for clarification (Figure 14). 

A typical metal precipitation system is shown in Figure 5. Some systems have equalization basins to equalize flow into the treatment system, along with a prereaction tank for pH adjustment. The central part of the treatment plant is the rapid mixing tank with a detention time of between 1 and 5 minutes, where coagulating chemicals are added and a slow mix tank or flocculation tank with a detention time of 20 to 30 minutes for the agglomeration of metal precipitates or floes. Organic polymers may be added to aid in the flocculation. The precipitates are settled out in a settling tank or clarification tank with typical overflow rates... 

The refinery clarification equipment has the capability of adding any of the chemicals mentioned above. However, lime clarification was chosen as the method to be used. The combined secondary effluent and the plant return streams (5.64 MGD) are pumped from the surge ponds to the rapid mix basin in the clarifier. The rapid mix basin has two compartments in series and each compartment has a high speed mixer. Lime and sodium hydroxide are added to the first compartment and these are vigorously mixed with the secondary effluent in both compartments. As a result, the pH of the effluent from the rapid mix basin is raised to between 10.8 to 11.0. At this pH, much of the bicarbonate in the water reacts with the lime and forms an insoluble calcium carbonate and the magnesium in the water reacts with hydroxyl ions to form insoluble magnesium hydroxide. 

Sedimentation Either Decanting (2-liquid phases), clarification, inclined plane (IP) clarification, thickener basin, dissolved air flotation tank, oil separator... 

The river waters of the Paris basin exhibit consumptions ranging from 1 to 3 mg-l" of O2. Of this, 30 to 45% can be eliminated during clarification by flocculation with the traces of organic micropollutants that the waters may contain. 

Chemo-mechanical clarification of effluents is carried out almost exclusively in sedimentation plants (round and rectangular basin with bottom sludge removal) as shown in Fig. 10.1. Only in a few cases it is necessary to neutralize the effluents. Rakes for the separation of coarse material and sand traps are seldom used. The clarifying efficiency of sedimentation plants is increased considerably by the use of flocculants. Undissolved substances are removed with an efficiency exceeding 90%. 

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