Sludge disposal is an important part of sewage treatment systems, and its cost accounts for more than half of the total cost of sewage treatment. Sludge disposal and management are thus major challenges in the global water industry.
Sludge dewatering is an essential process in sludge disposal, and it is important for the effective reduction of the final processing cost. Flocculation/coagulation, a relatively mature, cost-effective, user-friendly sludge dewatering technology, is one of the most commonly-used sludge conditioning approaches. The effects have been influenced by various environmental factors; moreover, the combination with other pretreatments are significant to improve the dewatering properties of coagulation/flocculation.
What is sludge and sludge dewatering?
Sludge is the product of sewage treatment, and the total solid content in it is generally below 8% before treatment. The components of sludge are highly complicated, and they could be divided into water and various types of solids. With regard to the existing water in sludge, a clear classification, which generally includes free water, interstitial water, surface water, and hydration (or internal) water according to the degree of removal difficulty, is essential for the study of dewatering.
Different sludge sources contain dissimilar solid components, and waste activated sludge produced in sewage treatment using the biological method could be divided into five fractions: supernatant, slime, loosely-bound extracellular polymeric substances (EPS), tightly-bound EPS, and pellet. The existence of EPS in waste activated sludge is regarded as the main cause of the difficulty in efficient sludge dewatering. As for both water and solids in sludge, their certification criteria need to be unified and standardized.
The process of sludge dewatering is closely related to the formation of sludge cake formed by the aggregation of flocs with different size, compactness, and surface characteristics. Sludge dewatering by draining and repelling water includes two procedures: formation of sludge cake and subsequent compression of the sludge cake. Therefore, permeability and compressibility are important for dewatering performance, and many studies have characterized the compressibility and expression characteristics via experimental evaluations.
Coagulation/flocculation and their application in sludge dewatering
Coagulation/flocculation is one of the most commonly-applied techniques to achieve efficient solid-liquid separation in water treatment. In the coagulation process, small suspended colloids in water are destabilized after diminishing their surface charges by the addition of coagulants with an opposite charge; then, the destabilized particles aggregate and settle down. To accelerate particle aggregation further and improve settlement efficiency, polymeric flocculants with flexible long-chain conformation are sometimes fed after coagulation as coagulant aids. These polymeric flocculants act as bridges that adsorb and connect various colloidal particles in water to form large flocs that can be effectively removed by sedimentation. The latter process is called flocculation. Coagulation/flocculation technology is widely employed in many industrial fields, including mining, oil extraction, and paper production, aside from water treatment.
Coagulants/flocculants are the key in coagulation/flocculation, which include inorganic salt coagulants, organic synthetic polymeric flocculants, natural polymeric flocculants, and bioflocculants. Inorganic coagulants and synthetic polymeric flocculants are the most frequently used dewatering conditioners, and they mainly include aluminum and ferric salts and polyacrylamide (PAM) derivatives, respectively. However, their applications are restricted by the drawbacks: inorganic coagulants are usually in high dose, sensitive to system pH, and have residual metal ions while synthetic polymeric flocculants, mainly PAM derivatives, are costly and may lead to secondary pollution and health risks. Bioflocculants require a relatively long retention time. Lastly, natural polymers have attracted increasing attention due to their many advantages, such as environmental friendliness, widespread availability, biodegradability, and prominent structural features, but its practical applications still lack and need further investigation.
Other pretreatments with coagulation/flocculation in sludge dewatering
Other commonly used pretreatments, such as adding skeleton builders, microwave treatment, ultrasonic treatment, acid/alkali treatment, oxidation treatment, and enzymatic treatment, are divided into physical and chemical conditioning methods. After pretreatment with these methods, the release of bound water and degradation of the EPS structure simultaneously occur. Then, coagulants/flocculants can make the best use of their functionality through charge neutralization and the bridging effect. Thus these pretreatments combined with coagulation/flocculation could efficiently enhance the dewatering properties.
However, practical operation indicates that the current dewatering performance for waste activated sludge is still relatively poor. The large amount of EPS, the high energy demand for bound water drainage, and the strong compressibility of sludge cake are considered difficulties in the dewatering process. Other reasons include insufficient coincident standards for testing these above parameters. However, the major reason is the lack of a well-established structure-activity relationship due to the highly complicated and multilevel structural features of sludge. To understand sludge better, the characteristics of sludge cake should be studied thoroughly in addition to the water content, solid components, and floc properties.
To further improve the sludge dewatering performance, novel coagulants/flocculants should be developed urgently and should have the characteristics of high dewatering efficiency, low cost, and environmental friendliness. Dewatering mechanisms should thus be studied in detail based on not only the structural characteristics of sludge but also on those of the coagulants/flocculants. In addition, the combination of coagulants and flocculants and the combination of coagulation/flocculation with other pretreatments could be investigated.
These findings are described in the article entitled Coagulation/flocculation in dewatering of sludge: A review, recently published in the journal Water Research. This work was conducted by Hua Wei, Boqiang Gao, Jie Ren, Aimin Li, and Hu Yang from Nanjing University.