What are the main methods of sludge thickening.

     Concentration is a more economical and effective method to reduce sludge volume. The dehydration object of sludge concentration is mainly interstitial water. Sludge concentration includes gravity concentration, air flotation concentration, centrifugal concentration and other forms. Among them, gravity concentration of sludge liquid by natural gravity is a more widely used and simple concentration method.

(1) Gravity concentration

     Usually, gravity concentration tank is used to concentrate sludge. The reasonable design and operation of the thickener depends on the correct understanding of the sludge settling characteristics. Concentration tank can be divided into intermittent type and continuous type. The former is mainly used in the sewage treatment plant of small-scale industrial enterprises. The latter is used in large and medium-sized sewage treatment plants. Generally, centrifugal pump can be used for mud feeding in gravity concentration tank, while piston diaphragm pump, column pump and other mud pumps with high pressure head are needed for mud discharge. The gravity concentration method is simple in operation, low in maintenance management and power cost, but covers a large area.

(2) Air flotation concentration

      It is a method to make the micro air bubble adsorb on the suspended sludge particles and make the particles float together with the small bubbles and separate from the water. It is suitable for the hydrophobic sludge that the particles are easy to float up, or the suspension is difficult to settle and easy to agglomerate. According to the different ways of producing microbubbles, air flotation can be divided into three types: electrolytic air flotation, dispersed air flotation and dissolved air flotation. At present, there are many researches and applications on the former. Activated sludge, aerobic digestion sludge, contact stabilized sludge, delayed aeration sludge without primary sedimentation, and some industrial waste oil and waste oil are suitable for air flotation concentration. Due to its high density and good settling performance, gravity concentration is more economical than air flotation concentration.

(3) Centrifugal concentration

      Using the density and inertia difference of solid and liquid in sludge, they are separated under different centrifugal forces. Centrifugal concentration method occupies a small area and has a low cost, but its operation cost and mechanical maintenance cost are relatively high, so it is less used for sludge concentration. Especially for the sludge that can be effectively concentrated by air floatation or gravity sedimentation without chemicals, centrifugal concentration is not suitable from the economic point of view.

(4) Microporous filter concentration

      The filter screen of microporous filter can be made of metal mesh, polyester fabric or polyester fiber. Sludge is concentrated by filtration.

(5) Diaphragm concentration

      There are two methods, ultrafiltration and reverse osmosis, which separate the molecules of the particles in a certain size range by the separation and filtration of the semi permeable membrane.

(6) Biological flotation concentration

      After the aeration of the activated sludge is stopped, denitrification is generated in the sludge after a certain period of time, and N2 and CO2 are released to float and concentrate the sludge. For the first sludge precipitation, if the water temperature is kept at about 35 ℃ for more than 5 days, it will have the same effect.

(7) Vibration concentration

      By using the moment of inertia of vibration, all kinds of binding water between sludge molecules lose stability and gather, so as to concentrate. It is mainly used in some special occasions, such as small production scale or test needs.

There are many methods of concentration, which should be selected and applied according to the nature of sludge and different conditions.

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