Sludge Drying

What is sludge drying?

Sludge drying is a sludge treatment process in which the water in the sludge is removed as water vapor by heating. It significantly reduces the volume and weight of the sludge, making it easier to recycle and reuse.

Purpose of sludge drying

Dried sludge can be recycled for a variety of uses, such as fertilizer, compost by-products or fuel for incineration plants.

Weight and volume reduction

Removing as much water as possible from the sludge by evaporation reduces the weight and volume of the sludge, making it easier to store and transport.

Stabilization of sludge

Drying stabilizes sludge, thereby reducing olfactory nuisance and improving disinfection, while making it less likely to leak during transport.

Easier recovery

Disposed sludge can be recycled for agricultural fertilizer.

Easier sludge treatment

Drying can reach the minimum dry content levels required by law for use in cement plant ovens. It also increases the calorific value of the sludge used for incineration.

Sludge drying methods

In the sludge drying process, the water is not removed in liquid form by mechanical means (e.g. in thickening or dewatering). Therefore the sludge is usually dried to a solids concentration above 60% to produce granules, or to a powder at a maximum solids concentration of 90%. Granular or powdered sludge is easier to convey than dewatered sludge with higher moisture content, which is present in bulk form.

This sludge drying process includes both direct and indirect methods. Indirect thermal drying is used to reduce the sludge volume by removing the moisture content and achieving a 90% dry solids content. This process reduces the environmental impact and produces a stable dried granular product that can be easily stored and transported and is suitable for agricultural use.

Dryers are usually classified according to the primary method of transferring heat to the wet solid being dried. These methods are conduction (indirect drying), convection (direct drying) and radiation (infrared drying).

A large amount of air is not required in the contact drying process, because warmth is provided by the contact between the moist product and the heated wall. Usually only a minimal gas flow is planned to exhaust the vapors. The advantage of this is that the cost of exhaust gas cleaning is minimal.

Convection drying obtains its effect by treating the sludge with hot air. In addition, ambient air is heated to high temperatures using a burner or steam heat exchanger and brought into contact with the sludge in a drum or belt dryer.

The distinctive feature of radiation drying is that the heat required for the drying process is supplied to the sludge by radiation. General examples of heating by radiation are solar radiation or infrared heating elements.

We offer two different types of sludge drying systems:

  • Hollow paddle dryer — Horizontal agitator dryer with heat transfer. The drying process only needs a small amount of gas to take away the moisture, which greatly reduces the loss of this part of the heat taken away by the gas and improves the utilization rate of heat, it is suitable for the drying of granular and powder materials, and can also dry paste materials smoothly.
  • Low temperature sludge drying system — Combined heat pump drying host, closed drying room, sludge automatic conveyor belt and automatic control system in one. Closed cycle system, zero emission and no bad smell, not affected by external climate: gradient dehumidification, low air volume, low temperature drying, fast drying and no dust; with heat recovery function, significantly improve energy utilization efficiency.


Most drying processes take 20-30 minutes to dry sludge from 20% solids to 90% solids.

The original water content of sludge is high and the volume of wet sludge is relatively large. After solid-liquid separation equipment, the water content of sludge is generally above 80%, and the water content of some sludge may reach 90%. The volume of wet sludge is relatively large, which is not convenient for storage and transportation.

The volume and net weight of the sludge can be reduced to one-third or less of the original volume after drying by the sludge dryer. After drying, the sludge has a significant use value.

1. Composting landfill

Compost is an essential fertilizer component for crop growth and development, due to organic chemical sludge with nitrogen, phosphorus and potassium. Crude protein or hemoprotein of organic chemical composition in specific sludge is a very good soil conditioner. The sludge has smooth characteristics and can be used as fertilizer with agricultural and livestock utilization use value. However, not all sludge can be used for composting, such as printing and dyeing plant sludge.

2. Centralized incineration treatment

The heat content of dried sludge is about 1300-1500 calories. Three tons of dry sludge is equivalent to 4500 calories of coal, and the sludge is mixed with coal and ignited in a heating furnace. One ton of dry sludge can produce one ton of steam.

3. Making bricks from sludge

It can be added to clay to make bricks with a mass ratio of 1:10. Its compressive strength is equal to that of a normal red brick and has a certain amount of heat generation.

4. Production and manufacture of biochemical plywood

Under the standard of partial alkali, a series of physics and chemical reactions are produced after heating, drying and filling pressure. Such transitions are used to make specific sludge epoxy resin (protein glue) to bind its chemical fibers and inhibit the plate.

First, the quality of materials

1. when the material humidity is very big, the use of the sludge drying machine drying out the sludge and humidity less use dryer drying out of the sludge is different, it was difficult because of the humidity larger sludge in dry dry, so the drying time is too long, dry will be uneven, even some of the sludge will be excessive and dry, and humidity is low even when the sludge drying, Dry out the effect is better, the quality is more consistent.

2. When there are more impurities in the material, the sludge dryer will be more laborious and time-consuming to dry the sludge, and there may be damage to the sludge dryer because of impurities. In the case of impurities and more impurities, the drying effect will not be very good, and the quality of the sludge will be poor, or even the need for secondary treatment.

Two, the quality of sludge dryer

When the quality of sludge dryer is not good, there will be many problems in the process of sludge drying, such as: Uneven heating dry, dust and so on, so want to sludge drying quality is good, be about to choose suitable sludge drying machine, sludge drying system quality cannot careless, so the performance of sludge drying machine, working efficiency and power, all must carry on the detailed understanding, sure you can use, under the premise of can ensure the quality of sludge drying and then to use.

Three, sludge drying time

Sludge at the time of drying needs a certain time, drying time is too short, the sludge is not drying, the drying time is too long, can cause excessive drying, dust, and even affect the secondary use, so the sludge drying has a certain degree, when you need to choose the drying effect according to the specific situation, thus determine the time of drying, control of the drying time, Is to control the quality of sludge drying, so as to ensure the effect of secondary utilization.

Four, the experience of the operator

Sludge in dry processing is generally by the operating personnel check before work, the operator can according to the sludge drying conditions determine whether continue to dry, dry or stop, through their experience to make judgments about the quality of the sludge drying, but if inexperienced operators, or did not fulfill his duties, This will lead to errors in judgment, thus affecting the quality of sludge drying.

The main purpose of activated sludge drying is to reduce the amount of water, i.e. the water content before treatment, by technical means, to drain the water from the sludge and prepare it for later treatment. If the sludge is not dried, the material transportation process will cause a lot of transportation costs in addition to pollution. The second problem is that the secondary evaporation of water must consume energy when processing energy. This is why sludge dewatering and drying is an important part of sludge resource utilization. The traditional way is heating, draining and drying by means of hot air ovens and so on. Such a way is relatively simple, but the energy consumption is very large, and at the same time the pollution of the environment and so on is also very large.

Sludge moisture content: The percentage of the weight of moisture contained in the sludge over the total weight of the sludge is called sludge moisture content.

  1. The water in sludge exists in the following forms:
    Void water, about 70% free water between particles, can be separated by gravity precipitation (concentration and compaction).
    Capillary water, which is formed by capillary phenomena around highly dense fine sludge particles, is about 20% passable
    Apply centrifugal force, negative pressure and other external forces, destroy the capillary surface tension and cohesion of the force and separation:
    Particle surface adsorption water and internal binding water, about 10%. Surface adsorbed water is the water attached to the surface of sludge particles
    Strong force, often in colloidal particles, biological sludge and other solid surface, the use of coagulation method, through colloidal particles mutual flocculation,
    Exclude water on the attached surface: internal bound water, which is the water bound inside the sludge particles, such as the water inside the cells in biological sludge, none
    The crystalline water of metal compounds in sludge can be removed by biological separation or thermal method.
    Usually when the moisture content is above 85%, the sludge is in flow state, 65%~85% in plastic state, and less than 60% in solid state.
  2. The relationship between the volume and weight of sludge and the concentration of solids contained in sludge:
    V/V=W/W2= (100-p2)/(100-p) =C/Ci (8-1)
    Where: D, V, W, C — Sludge volume, weight and solid concentration when sludge moisture content is p
    P2, V2, W2, C2 — Sludge volume, weight and solid concentration when sludge moisture content is D

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