What Is a Primary Sedimentation Tank?
A primary sedimentation tank can remove settleable and floating matter from wastewater. After primary sedimentation, about 50% of settleable solids, grease, and floating matter and about 20% of BOD can be removed. Calculated per unit mass of BOD or solids removed, the primary sedimentation tank is the most economical purification step, and primary sedimentation pretreatment is easily adopted for both domestic sewage and industrial wastewater with high suspended solids.
(1) Removing settleable and floating matter to reduce the load on subsequent treatment facilities.
(2) Causing fine solids to flocculate into larger particles, enhancing the solid-liquid separation effect.
(3) Having a certain adsorption and removal effect on colloidal substances.
(4) To a certain extent, the primary sedimentation tank can act as a regulating tank, providing a degree of homogenization of water quality and mitigating the impact of water-quality fluctuations on the subsequent biochemical system.
(5) Some wastewater treatment process systems return part of the secondary sedimentation tank sludge to the primary sedimentation tank, leveraging the bio-flocculation effect of the secondary sludge to adsorb more dissolved and colloidal organic matter and improve the removal efficiency of the primary sedimentation tank.
(6) An iron-containing coagulant can also be added ahead of the primary sedimentation tank to enhance phosphorus removal. After the iron-containing primary sludge enters the sludge digestion system, it can also increase the activity of methanogenic bacteria and reduce the sulfide content in the biogas, thereby both increasing biogas output and saving biogas desulfurization costs.
(1) Surface loading An increase in surface loading can affect the effective settling of suspended solids and reduce their removal rate. Sedimentation tanks with different functions have different surface loadings; for example, secondary sedimentation tanks are generally 1-3 m3/m2.h, while a primary sedimentation tank located at the front end of physicochemical treatment can be somewhat larger.
Freshness Fresh sewage has a higher removal rate after settling. The freshness of wastewater depends on the length of the sewage pipeline, the number of pumping-station stages, etc.; in addition, anaerobic high-concentration industrial wastewater is prone to putrefaction and deterioration.
Temperature As the wastewater temperature drops, the viscosity of suspended solids in the water increases; for example, suspended solids settle 50% faster at 27C than at 10C. However, a high water temperature also accelerates the putrefaction and anaerobic fermentation of the sewage, reducing the density difference of the effluent, which is unfavorable for particle settling and thus lowers the settling performance of suspended solids. Therefore, these two factors should be considered together in combination with the specific conditions of the sewage pipe-network system.
Size, shape, and density of solid particles When the solids in wastewater are large in size, regular in shape, and high in relative density, they settle faster.
(3) Operational factors The operating conditions of preceding processes such as the grit chamber or bar-screen well can directly affect the operation of the primary sedimentation tank. Poor operation of the preceding process will increase the load on the primary sedimentation tank and reduce the removal effect.
(4) In systems where secondary sedimentation tank sludge and digested sludge from the sludge digestion tank enter the primary sedimentation tank, special attention should be paid to feeding the sludge evenly and steadily. Intermittent or shock feeding must be avoided; otherwise, the primary sedimentation tank will be overloaded, the amount of putrefied sludge will greatly increase, affecting solids removal and adversely affecting the environment.
