Sludge Conditioning With Fly-ash And Preparation And Application Of Ceramsite

At first, this paper used fly-ash as conditioner to study the dewatering pretreatment of sewage sludge. Then, the preparation and mechanisms of ceramsite were investigated by taking sludge and fly-ash as the raw material and clay as binder. A pilot-scale production was carried out based the process parameters of bench-scale. And the application performance of sludge ceramsite was investigated by the experiments of sewage treatment in biological aerated filter (BAF). In these experiments, the fly-ash influence mechanism to sludge dewatering was explored; the effects of high content of organic and flux in sludge on ceramsite were studied, which revealed the expansion mechanism of sludge ceramsite. The main contents and results are as follows:1. Through the single-factor experiments, contrast to cationic polyacrylamide (PAM),the effects of fly-ash on dewatering of sewage sludge were studied according to specific filtration resistance (SRF), dehydrated speed, and filter cake moisture content (FCMC) as investigation indexes. The results showed that with the rising of fly-ash dosage, SRF kept decreasing. When the dosage of fly-ash achieved 0.02g/mL, SRF fell from 0.92×109 s2/g to 0.32×109s2/g,65.2% was dropped, and so it was easy to filter. The most obvious effect on FCMC is addition of fly-ash only. When the fly-ash dosage is 0.03g/mL, FCMC fell to 66.9%. FCMC could only drop to 80% when adding PAM into sludge. But when adding fly-ash as 0.03g/mL at the same time, FCMC could be reduced to 71.8%. When adding PAM combined with fly-ash, the dehydrated speed will be increased more than 90%.2. By chemical composition analysis, microstructure observation (SEM), and surface electrical behavior (Zeta potential), combined with the dehydration experiments, it was thought that the improvement on SRF by fly-ash was because fly-ash particles were loose and porous, which evenly dispersed in sludge floc structure as skeleton to maintain the permeability of the filter cake during compression dewatering. The surface of fly-ash and sludge are both negatively charged, so there is no electrostatic charge neutralization. Fly-ash cannot promote the sludge size to grow up. Fly-ash won't produce flocculation to sludge.3. The raw materials, sludge, fly-ash and clay, were tested by chemical composition plastic index, and differential scanning calorimetry-thermogravimetric analysis (DSC-TGA). Taking the physical properties of ceramsite, including hardness, water absorption, and density, as the indexes, the effects of mass ratio of raw materials, preheating temperature and time, and sintering temperature and time on the performance were investigated by some single-factor experiments. The preparation parameters of sludge ceramsite were as follows:the mass ratio of sludge should not be more than 50% and clay not be less than 50%; preheating temperature is 400℃and time is 20min; for making sludge ceramsite filter materials, sintering temperature is 1050℃and sintering time is 5min; for making expansion ceramsite, sintering temperature is 1150℃, and time is 10min. The results showed that the sintering temperature is the most key factor in ceramsite preparation. As the temperature increases, more and more materials become melted glass phase, which made the ceramsite shrank, density increased, the surface thicken and dense, and water absorption decreased. When the sintering temperatures is above 1125℃, the reactions of gas producing occurred which increased the internal pressures, made the sludge ceramsite expanded, and the density began to decrease.4. The influences of preheating temperature on density and water absorption revealed the expansion mechanism of sludge ceramsite about the high content of organic composition:i) dehydration and carbonation occurred in organic foaming agent (organic content in sludge) and generated reducing carbon in the preheating process (<500℃); ii) in the sintering process (1150℃), carbonate (CaCO3) and sulfate in inorganic foaming agent firstly decomposed into corresponding oxides and CO2, SO2, and then Fe2O3 decomposed at high temperature into FeO, Fe3O4 and O2, and these gases bloated the body of sludge ceramsite; iii) the O2 produced by ferric oxide decomposition reacted with reducing carbon produced CO and CO2, these secondary gas made the ceramsite expanded again.5. Interaction mechanism about the flux in sludge and sintering temperature was revealed by the affection of mass ratio of raw materials and sintering temperature on ceramsite properties and microstructure observation:at high temperature, many alkali and alkaline metal oxides in sludge made Si-O (bridging oxygen) bond broken, which destroyed the steady structure of [SiO4] tetrahedron skeleton network, made it easier to collapse to generate more glass phases. For this reason, sludge ceramsite was easier to be sintered than fly-ash ceramsite, which meaned a lower sintering temperature and shorter sintering time.6. Expansion mechanism about sludge ceramsite was revealed by comparing the density and expansion rate of sludge-clay ceramsite and fly-ash-clay ceramsite varies with the sintering temperature:when temperature is below the bloating temperature (1150℃), the critical factor is the quantity of molten materials to determine the sludge ceramsite physical properties, which related to flux, the more flux, the lower melting point of framework material, the more glass phase materials melted at the same temperature, which lead to shrink harder and form a denser body. When reached bloating temperature, the gas pressure generated from reaction mentioned above turned to be the critical factor. When the gas pressure could overcome the surface tension, the body of cemamsite would expand. When the temperature is above 1150℃, the reaction of gas producing occurred. This kind of reaction in sludge ceramsite was mainly redox reactions between carbon and ferric oxide, and the reducing carbon was from the pyrolysis product of organic matters in preheating process. Therefore, the content of carbon and ferric oxide and their ratio were the factors determining the gas producing. For their higher organic matters and iron content in sludge ceramsite, the quantity of gas generated was more than fly-ash ceramsite, which lead to a higher porosity inside and a lower density.7. The results of X-ray diffraction (XRD) analysis showed that the main mineral compositions were anorthite, albite, and potassium feldspar, and in sludge ceramsite was mainly quartz. But for these melted and sintered materials between powder particles at high temperature, formed tectosilicate, such as feldspar and quartz, and amorphous substances, which made ceramsite with high strength and higher hardness.8. It was thought that the sintering of sludge ceramsite filter material was an incomplete sintering process by the investigation of preparation and expansion mechanism of sludge ceramsite. On the one hand, certain bonding force between power partical formed by sintering to improve the mechanical strength of sludge ceramsite. On the other hand, the body could be expanded and porous inside by gas production in order to have a lower density. And at the same time, it must to make the ceramsite surface melted not so easy by reducing the content of flux composition so as to improve the surface porosity.9. The heavy metal contents leached from sludge ceramsite (sludge:clay=1:1) were lower than lppm, and the solidified rates were all above 80%, even the curing rate of Pb was 99.1%. Most heavy metals were likely to replace with network forming elements, such as Si4+ and Al3+ as isomorphism state sintering at high temperature, thus were firmly fixed in the new crystal texture. The stability and security of sludge ceramsite were conformed to be satisfactory for wastewater treatment as filter material.10. The preparation procedure of sludge ceramsite was proved to be reasonable, easy to control, having industrialized amplification conditions, and the performance of products was stable by production testing in a bench-scale rotating furnace and pilot-scale rotary kiln respectively. The results of sewage treatment in biological aerated filter (BAF) showed that, the sludge ceramsite filter material was fit for bacteria growth, and its performance was no less than the commercial ceramsite. Moreover, it had advantage on ammonia nitrogen removal over the commercial ceramsite. So sludge ceramsite had good performance in wastewater treatment as filter material.From above researches, the disposal of dewatering sludge and recycling approaches after fly-ash condition were prospected. For small wastewater treatment plant, if only the moisture content of filter cake could be below 60% through pressure filtration dehydration by dosing fly-ash, it could choose sanitary landfill, the most economical way, for sludge disposal. For large municipal sewage treatment plant, adding auxiliary materials to prepare sludge ceramsite after dehydration by PAM joint with fly-ash, could achieve the aim of harmless and recycling of sludge simultaneously.