Phosphorus Removal Technology Enhanced By Granular Sludge

Phosphorus is one of the main limiting factors causing eutrophication in water bodies.Controlling total phosphorus emissions is beneficial to alleviate and eradicate eutrophication of water bodies.At the same time,phosphorus is also a hard-to-renew resource with strong demand.It is valuable to recycle phosphorus from wastewater.Therefore,it is of great significance to develop effective technologies for phosphorus removal as well as recycle.Granular sludge is difficult to dispose as waste generated during wastewater treatment,but it has adsorption performance as a medium and filtration performance after being accumulated into a pile.Using granular sludge to enhance the phosphorus removal with calcium salt may achieve the phosphorus removal and recovery efficiently.Thus,it is promising to develop the process.In this research,the adsorption performance of granular sludge for phosphorus is studied.And then granular sludge is used to enhance phosphorus removal with calcium salt.The phosphorus removal technology using granular sludge enhancing calcium salt was developed.The influence of reaction conditions was explored,and the process operating parameters were optimized.The properties of enhanced phosphorus removal products were also investigated.The main conclusions are as follows:1)The phosphorus removal performance of granular sludge/calcium salt was studied.The effects of reaction time,Ca/P and pH on the phosphorus removal using granular sludge-calcium salt were ascertained,and the process characteristics were revealed.Granular sludge has a strong adsorption effect on phosphate/phosphate-calcium.When pH was 7,the phosphorus adsorption capacity was 4.77 mg P g-1,and the equilibrium time was about 4 d.The phosphorus adsorption capacity of granular sludge is no less than that of granular activated carbon.Thus it can be used to phosphorus removal.Its contribution can't be ignored as a mechanism of the phosphorus removal from low-phosphorus wastewater in anaerobic reactor.Ca/P has a significant effect on the performance of phosphorus removal using granular sludge-calcium salt.When the reaction was sufficient(reaction time was 168 h),the phosphorus removal rate increased with the increase of Ca/P,but when the critical value was exceeded,the return rate of calcium decreased,so Ca/P = 2 was appropriate.On the basis of the ratio of ion product to solubility product,it is determined whether the solution can precipitate or not.Due to different stoichiometric coefficients(theoretical Ca/P),increasing the same concentration of different ion can produce different ion product effects.According to the stoichiometric formula,the theoretical Ca/P ratio is 5:3,but the actual Ca/P is always slightly higher than the theoretical Ca/P since the precipitation reaction effect of calcium is stronger than that of phosphorus.The pH has a significant effect on the performance of phosphorus removal using granular sludge-calcium salt.Alkalinity(Hydroxyl)can reduce the solubility of the precipitated phosphorus removal products,and increase the concentration of reactants.Thus,alkaline conditions are more favorable than acidic conditions.Due to the poor acid-solubility of the phosphorus removal product,however,the effect of pH on the phosphorus removal efficiency was not significant when the reaction was sufficient.When pH ? 7.20,the phosphorus removal efficiency was as high as 98%after 168 h,and the effluent P was 2 mg·L-1.Even if pH ? 2.12,the phosphorus removal efficiency was 89.33±0.98%.The pH is preferably controlled at 8-11.The reaction process of phosphorus removal using granular sludge-calcium salt fitted the second-order reaction kinetics,and the calcium ion concentration has a great influence on the reaction rate.When pH = 5 and 11,the phosphorus removal reaction fit the second-order reaction kinetics;at pH = 2.1,7.2,and 10,the phosphorus removal reaction fit the first-order reaction kinetics.Phosphorus removal product can enrich the phosphate in wastewater,but the enrichment was poor,which was far away from the standard of rich phosphate rock.2)The phosphorus removal technology using granular sludge enhancing calcium salt was developed.And the effect of HRT,Ca/P and pH was tested.Suitable conditions,operating performance and product characteristics were ascertained.The self-developed variable flow phosphorus removal reactor was operated.Granular sludge was used to strengthen phosphorus removal with calcium salt.Granular sludge enhanced calcium salt phosphorus removal technology was developed.The phosphorus removal efficiency of this technology was 80-98%,and the volumetric removal rate was 2.0-3.7 kg-P/(m3·d).The replacement period of granular sludge is once every 250-320 h.The optimal conditions for this technology were:HRT = 0.5 h,Ca/P = 2,pH = 11.Both the higher phosphorus removal efficiency and the longer stable operation time were maintained under optimized conditions.Phosphorus removal products under various HRT conditions were mainly hydroxyapatite,which easily leads to the entry of carbonate into the hydroxyapatite and the decrease of purity under long HRT.Phosphorus removal products produced under various Ca/P conditions were also mainly hydroxyapatite,which also contained decacalcium hexakis(phosphate(V))oxide.Phosphorus removal products produced under various pH conditions are different.Almost no phosphorus products are observed at pH 2.Hydroxyapatite is the main phosphorus product at pH 7 and 11.Hydroxyapatite and carbonatehydroxylapatite are the main phosphorus product at pH 12.3)Phosphorus removal technology using granular sludge enhancing calcium salt was optimized and the performance of phosphorus removal and recovery was determined after optimization.The reactor configuration,granular sludge dosage and ventilation frequency have a better optimization effect on phosphorus removal technology using granular sludge enhancing calcium salt.Double-zone(influent zone and reaction zone)split reactor,half-bed,3 times·d-1(that is,5 min every 8 hours)can increase the phosphorus removal rate,extend the stable operation time,and reduce the blocking of reactor.The reactor configuration,granular sludge dosage and ventilation frequency can improve the recovery performance of phosphorus removal product.The phosphorus removal product formed by the dual-zone split-type reactor was more pure,and the quality and value were higher;the less the granular sludge dosage,the higher the purity of phosphorus removal product was,and the higher the phosphorus content was;the ventilation frequency is 3 times d-1(i.e.,5 min ventilation every 8 hours)can increase the crystallization stability and the phosphorus content of phosphorus removal product.The optimized parameters of technology were:dual zone partitioning,Ca/P = 2,pH=10,HRT=0.5 h,csludge=400 g·L-1,ventilation frequency 3 times·d-1.The ventilation intensity was 600 mL·min-1 and the duration of ventilation was 5 min.4)The composition,morphology and structure of phosphorus removal product were observed,and the phosphorus enrichment performance was ascertained.Granular sludge dosage,pre-drying,pH and other conditions significantly affected the composition,morphology,structure and phosphorus recovery performance of phosphorus removal product.The pre-dried granular sludge surface is not conducive to the reaction and crystallization of calcium and phosphorus.The un-dried granular sludge is favorable for the reaction and crystallization.Phosphorus-enriched pre-dried granular sludge,phosphorus-enriched native granular sludge produced by adjusting the pH of the phosphorus solution,phosphorus-enriched native granular sludge produced by adjusting the pH of phosphorus solution and calcium salt solution,and phosphorus-enriched native granular sludge produced by increasing granular sludge dosage formed cubes,twigs,lamellae and smooth flake-shaped crystals,respectively.The phosphorus-enriched pre-dried granular sludge had low crystallinity and phosphorus content.Phosphorus-enriched native granular sludge produced by adjusting the pH of phosphorus solution,phosphorus-enriched native granular sludge produced by adjusting the pH of phosphorus solution and calcium salt solution,and phosphorus-enriched native granular sludge produced by increasing granular sludge dosage had a higher crystallinity and phosphorus content,which were easier and more valuable to recover.Theoretically,the surface phosphorus content of phosphorus-enriched granular sludge can reach 18.53 wt%.Comprehensively,the solution pH 10.5 and the operating conditions without the granular sludge pile were favorable to the enhanced phosphorus removal by the granular sludge.