The Application Of Bioflocculant Combined With Aluminum Salts In Water And Wastewater Treatment

Bioflocculant is a kind of new and envrioment-kind water treatment agent, which is nontoxic and biodegradable. However, so far none of the bioflocculants has been put into practical applications in a large scale because of the high production cost and low cogulation efficiency. In order to promote the application of bioflocculant in water and wastewater treatment, compound bioflocculant (CBF) was selected to combine with aluminum salts (alumminum sulfate and polyalumminum chloride) for the treatment of various water samples, including synthetic kaolin-humic acid solution, surface water and synthetic dyeing waste water. The coagulation performance and mechanism of CBF-alumminum salt dual-coagulant under different coagulation conditions were clearly studied by jar test. The floc growth, floc strength, floc recover capability and floc shape were stusied through on-line floc size monitoring during the coagulation process. The main conclusions are as follows:1. There was a synergistic effect between aluminum sulfate (AS) and CBF in the treatment of kaolin-humic acid solution. The coagulation efficiency increased with the increasing dosage of AS, while it increased first until the maximum was achived at pH6and then decreased as the pH increased. The removal efficiency of the organic matter was enhanced when proper amount of CBF was dosed in combination with AS. Within the dosage range investigated, charge neutralization was the dominant mechanisam for AS. When pH was between4and6, the main mechanism for AS was charge neutralization, and then it turned to enmeshment as the pH further increased. There was an additional adsorption bridge effect for AS-CBF and CBF-AS.2. Increasing the dosage of AS or the pH value could accelerate the floc growth, enlarge the floc size, enhance thenfloc strength but would weaken the floc recover capability during the coagulation of kaolin-humic acid solution. When exposure to high shear force, flocs were easier to get broken and easier to recover at the same time. With the increase of pH the floc strength and recover capability of AS, AS-CBF and CBF-AS decreased first util the minimum was achived at pH6and then increased. Flocs formed by AS-CBF and CBF-AS showed faster growth rate and larger size than that of AS. Floc strength and floc recover ability were borh in the same order as follows:AS-CBF>AS>CBF-AS.3. There was a synergistic effect between polyaluminum chloride (PAC) and CBF in the treatment of surface water in winter. As the dosage of PAC increased the residual turbidity decreased first and then increased, the best turbidity removal efficiency was achived at4.0mg/L. The removal efficiency of organic matter increased with dosage. Organic matter removal was favored at pH6, while the alkaline solution was favored for turbidity removal. The organic matter removal efficiency increased, while the turbidity removal efficiency decreased when CBF was used in combination of PAC. Charge neutralization played an important role in the PAC coagulation process. Both adsorption bridging and charge neutralization performed when PAC-CBF wads used.4. In the coagulation treatment of surface water the floc growth rate increased first until a maximum was achived at4.0mg/L, and then decreased as the dosage further increased. As the dosage increased the floc size decreased gradually. Both the floc growth rate and the floc size first increased and then decreased with the increase of pH. Flocs aggregated more slowly and had smaller sizes in the pH range of7-8. As the PAC dosage increased the floc strength factor increased first and then decreased, while the floc recover factor decreased. At high pH, the recovery factor was larger for all the three coagulants. Flocs formed at high PAC dosage or high pH value were more compact. In addition, flocs after breakage or regrowth became more compact as well. Under the same coagulation conditions, the floc growth rate, the floc size and the floc recover factor were all in the same order as follows: PAC-CBF>CBF-PAC>PAC. However, flocs got weaker when PAC was used in combination witn CBF.5. The color removal efficiencies of AS, PAC, AS-CBF and PAC-CBF all increased with the dosage of Al for the treatment of diperse yellow. AS showed better color removal efficiency than PAC under the same Al dosage. Color removal was favored at pH6for all the coagulants used. The color removal efficencies of AS and AS-CBF were easily influenced by pH at low Al dosage. The color removal efficiencies of PAC and PAC-CBF were less sensitive to pH. The addition of CBF not only enchanced the color removal efficiency but also extended the proper pH range. Adsorption charge neutralization and enmeshment were the main mechanisms for diperse yellow removal.6. Increasing the dosage of alum could accelerate floc growth and sedimentation, enlarge floc size, enhance floec strength but weaken floc recover capability during the coagulation process of diperse yellow. The floc growth rate increased first and then decreased with the increase of pH for all the coagulants used. The floc growth rate was favored at pH6. The floc size and sedimentation rate both increased until the maximum was achieved at pH6and then decreased with the increase of pH. Flocs formed by PAC and PAC-CBF settled faster with larger sizes under acidic conditions. As pH increased the strength factors of AS and AS-CBF increased first until the maximum was achieved at pH6and then decreased, while the recover factors decreased first until the minimum was achived at pH6and then increased. PAC and PAC-CBF gave higher floc strength factors under alkaline conditions, while they gave the minimum recover factors at pH6. The floc showed larger size, faster sedimentation rate, better ability to resist shear force and better recover capability when CBF was used in combination of aluminum salts.