| Sewage sludge has the characteristics of high organic matter content, high compressibility and strong hydrophily. Thus, how to improve sludge dewatering performance and enhance dewatering efficiency becomes a challenge in wastewater and sludge treatments. Activated persulfate (S2O82-) oxidation technology can generate powerful SO4·-with stable oxidation ability, which can effectively destroy the hydrophilic structure of the sludge, and is less affected by the pH of the solution. Industrial solid waste phosphogypsum (PG) can form skeleton builder in the sludge to solve the problem of sludge compressibility. Accordingly, this paper proposes a composite conditioner (S-P composite conditioner for short), combining Fe2+/S2O82-with PG for sludge conditioning and deep dewatering. The main contents are as follows:1. Optimization of S-P compositions and parametersThe results of conditioning sludge with PG at different calcination temperatures showed that:when PG calcined at150～300℃for2h was used alone for sludge conditioning, a lot of hemihydrate gypsum (CaSO4·0.5H2O) still existed in the phase composition. However, ions by-product of Fe2+/SPS can rapid promote the plate-like crystal form of hemihydrate rapid hydration reaction occurs, generating columnar or club-shaped dihydrate gypsum (CaSO4·2H2O) in the S-P condition system. When adding PG calcined at150℃as skeleton builder, the most content of columnar crystals could be obtained. When the PG calcination pretreatment temperature was higher than400℃, the columnar crystals gradually disappear, but long and thin needle structure of anhydrous gypsum (CaSO4) began to appear. Ions promote the rapid formation and growth of CaSO4·2H2O in the S-P condition system. PG calcined at150℃was chosen as the skeleton builder combined with Fe2+/SPS in the S-P composite conditioner. Based on response surface methodology, the optimum formula of S-P composite conditioner was:Fe2+=23.5mg/g DS (Dry solid), SPS=100.0mg/g DS, PG=300.0mg/g DS, at which condition, the filter cake moisture content and dewatering efficiency could achieve45.7%and91.7%, respectively.2. Mechanism of the S-P composite conditioningThree sectional dosing of Fe2+was beneficial to maintain a higher oxidation potential of Fe2+/SPS system. Radical quenching tests confirmed that a large amount of SO4·-radicals exist. The destructive effect of SO4·-radicals on floc structure was mainly reflected in the following aspects:(1) Changed the content and distribution of extractable proteins and polysaccharides in LB-EPS (Loosely Bound-Extracellular Polymeric Substances) and TB-EPS (Tightly Bound-Extracellular Polymeric Substances). The proteins and polysaccharides in LB-EPS and TB-EPS decreased under Fe2+/SPS conditioning. By the S-P conditioning, the proteins in LB-EPS increased by53.6%, the polysaccharides decreased by21.2%, while the proteins and polysaccharides in TB-EPS by21.2%and20.5%, respectively.(2) Promoted the transformation of bound water into free water. The bound water content in raw sludge reduced from2.60g/g DS to0.81g/g DS after S-P conditioning.(3) Changed the particle size distribution and transferred to smaller particle size, making small particle size range of3～40μm particle volume fraction frequency increased by7.3%, large particle size range of40～130μm particle volume fraction frequency decreased by7.4%, and (4) specific resistance to filtration (SRF) and capillary suction time (CST) were reduced by91.6%and88.4%, respectively.After adding calcined PG as skeleton builder, Fe2+/SPS conditioning promotes hemihydrates quickly generating gypsum with a rod-like structure, which "fill" in sludge flocs and play a role of skeleton builder, causing the pore distribution of SP conditioned sludge is0.4～6.0μm, maintaining the water filtering channel, thus contributing to improve dewaterability.3. Adaptability of the S-P composite conditioner to sludge from different sourcesThe characteristics of sludge from five different sewage treatment plants in Wuhan in four seasons were analyzed and the S-P composite conditioner was used to condition the sludge before the diaphragm filter press dewatering process. The results showed that solid content of filter cake was positively correlated with initial solid content of sludge, while negatively correlated with the organic matter content. It was easier to get a better dewatering effect and to control the filter cake moisture content lower than60%when the initial content of organic matter was less than50%, as well as the solid content was more than3%. The results showed that S-P composite conditioner was adaptable for conditioning and dewatering of any sludge from different sewage treatment technologies and different seasons. 4. Transformation tendency of nitrogen, phosphorus and heavy metals in deep dewatering with S-P conditioningA sludge deep dewatering pilot-scale demonstration including sludge thickening, conditioning, automatic control dewatering process was established. The feed process with "four step-ups" holding pressure (0.2MPa pressure holding20min,0.4MPa pressure holding20min,0.6MPa pressure holding20min,0.8MPa pressure holding30min) will help to improve the dewatering performance. In the pilot scale, moisture content of dewatered cake could be reduced to49～65%, dewatering efficiency enhanced to93～98%, and the dewatering system met the mass balance as well. In the process of S-P conditioning and dewatering, around95.4～97.1%of the nitrogen in sludge and99.3～99.9%of the phosphorus in sludge were retained in solid cake. The transformation tendency of heavy metals to filtrate was Cd> Zn/Pb> Cr> Cu. The filtrate could be recycled in the wastewater treatment plant and would not affect the normal operation of the plant.The S-P composite conditioning can directly realized sludge deep dewatering in wastewater treatment plant, conducive to realizing the combination of deep dewatering and subsequent disposal, and providing a theoretical reference for promoting the study of integrated technology of sludge dewatering and disposal. |
PDF Full Text Request