1 Principles and functions

The function of sludge concentration is to remove part of the water in the sludge by gravity or mechanical means to reduce the volume, and the function of sludge dehydration is to mechanically separate part of the interstitial water in the sludge to further reduce the volume. The moisture content of concentrated sludge can generally reach 94% ~ 96%. The moisture content of dewatered sludge can generally reach about 80%.

2 Application principles 


The sludge concentration and dehydration process shall be comprehensively determined according to the adopted sewage treatment process, sludge characteristics, subsequent treatment and disposal methods, environmental requirements, site area, investment and operating costs and other factors.


3 Conventional concentration and dehydration 


3.1 Main types and characteristics of concentration processes 
 

The methods of sludge concentration are mainly divided into gravity concentration, mechanical concentration and air flotation concentration. At present, gravity concentration and mechanical concentration are often used.


Gravity concentration has low power consumption and strong buffering capacity, but it has a large floor area, is prone to release phosphorus, and has a large odor. It is necessary to increase deodorization facilities. The primary sedimentation tank sludge is concentrated by gravity, and the water content can generally be reduced from 97% to 98% to less than 95%. The remaining sludge should not be gravity concentrated separately. The primary sedimentation sludge is mixed with the remaining activated sludge for gravity concentration. The water content can be reduced from 96% to 98.5% to less than 95%.


The mechanical concentration mainly includes centrifugal concentration, belt concentration, drum concentration and screw pressure concentration. It has the characteristics of land occupation and avoiding phosphorus release. Compared with gravity concentration, it has higher power consumption and requires adding polymer coagulant. Mechanical concentration can generally reduce the moisture content of the remaining sludge from 99.2% to 99.5% to 94% to 96%.


3.2 Main types and characteristics of dewatering process 


Mechanical dewatering mainly includes belt filter press dehydration, centrifugal dewatering, and plate and frame filter press dehydration.


Belt dehydration has low noise and low power consumption, but it takes up a lot of floor space and flushing water, and the workshop environment is poor. The water content of the belt dewatering mud is generally less than 97.5%, and the water content of the mud is generally less than 82%.


Centrifugal dewatering has a small floor area, no flushing water is needed, and the workshop environment is good, but the power consumption is high, the drug dose is high, and the noise is large. The centrifugal dewatering water content is generally 95% to 99.5%, and the water content of the mud is generally 75% to 80%.


The plate and frame filter press cement paste has a low water content, but it takes up a lot of land and washing water, and the workshop environment is poor. The water content of the dewatering mud in the plate and frame filter press is generally below 97%, and the water content of the mud out can reach 65% ~ 75%.

Spiral press dewatering and rolling dewatering have a small footprint, small amount of flushing water, low noise, and a good workshop environment. However, the stand-alone capacity is small, and the supernatant has a high solid content. There are not many examples of large-scale applications. The water content of the screw press for dewatering mud is generally 95% ~ 99.5%, and the water content of the mud is generally 75% ~ 80%.


4 Deep dewatering of sludge 


The so-called deep dehydration means that the moisture content of the sludge after dehydration reaches 55% to 65%, and the moisture content of the sludge can be lower under special conditions. At present, most urban sewage treatment plants in China do not have primary sedimentation tanks and do not undergo anaerobic digestion treatment, so the moisture content of the dewatered sludge is mostly between 78% and 85%. The high water content has brought great difficulties to the subsequent treatment, transportation and disposal of sludge. Therefore, in conditional areas, deep dehydration of sludge can be performed.


The sludge should be effectively conditioned before deep dehydration. The conditioning mechanism is mainly to modify the organic matter on the surface of sludge particles, or to destroy the cell and colloidal structure of sludge, reduce the moisture binding capacity of sludge, and reduce the compressibility of sludge, so that the sludge can meet high requirements. Requirements for dryness dehydration process.


There are three types of conditioning methods: chemical conditioning, physical conditioning, and thermal conditioning:


= 1 \ * GB3 \ * MERGEFORMAT① The chemical agents added by the chemical conditioning institute mainly include inorganic metal salt agents, organic polymer agents, and various sludge modifiers.


= 2 \ * GB3 \ * MERGEFORMAT ② Physical conditioning is the addition of substances that do not produce chemical reactions to the treated sludge, reducing or improving the compressibility of the sludge. Such substances are mainly: flue ash, diatomaceous earth, sludge ash after incineration, and fly ash.


= 3 \ * GB3 \ * MERGEFORMAT③Thermal conditioning includes freezing, medium temperature and high temperature heating conditioning. Commonly used are high temperature thermal conditioning. High-temperature thermal conditioning can be divided into two types: thermal hydrolysis and wet oxidation. High-temperature thermal conditioning can achieve a certain degree of reduction while achieving deep dehydration.


At present, the dehydration effect that can be achieved by combining various conditioning methods with the main mechanical dehydration methods is shown in Table 4-1.

Table 4-1 Dehydration effect of various conditioning methods combined with main mechanical dehydration methods

5 Secondary pollution and control requirements caused by the concentration dehydration unit 


The sludge concentration and dehydration processes produce a large amount of malodorous gases, and the main sources are sludge storage tanks, concentration tanks, sludge dewatering machine rooms, and sludge stacking sheds or silos. The malodorous gas in the dehydration machine room is not easy to emit, and it is the key area for the odor treatment in the sludge concentration dehydration process.


Deodorizing measures shall be taken in accordance with the requirements of the environmental impact assessment. The newly built sewage treatment plant shall adopt closed measures for the concentration tank, mud storage tank, dewatering machine room, and sludge storage and transportation room, and supply air to the deodorization system for deodorization treatment. The structure of the structure shall be capped or closed, and the exhaust duct and deodorization system shall be added. Generally, biological deodorization methods are used, and chemical deodorization methods can also be used when necessary.