The Efficiency And Mechanism Analysis Of Phosphorus Removal By Polyaluminium Chloride-Activated Sludge Co-flocculation Process

With the development of social economy, the water environment of our country isexacerbating, which puts forward higher requirements on wastewater treatment. Theimplement of “The urban sewage treatment plant pollutant dischargestandard”(GB18918-2002) and its revision promulgated in2006makes that most urbansewage treatment plant shoud be upgrated. Engineering practice and research have provedthat the Continuous-flow Intergrated Biological Reactor(CIBR) could reach grade A in theremoval of organic matters, nitrogen and phosphorus, but unstable on phosphorusremoval. The phosphorus removal by chemical and biological flocculation process whendosing chemical coagulants is an economic and simple method to make the phosphorusreach grade A considering the unique structure of CIBR. At present, majority of domesticand foreign researchers only work on the analysis of the experimental results, but themechanism of the phosphorus removal by chemical and biological flocculation process isnot clear and rarely involved. In this paper, the required hydraulic conditions andinfluencing factors on the removal of phosphorus via PAC-activated sludgeco-flocculation in CIBR was studied. The efficiency of phosphorus removal byPAC-activated sludge co-flocculation in CIBR was investigated by analyzing thephosphorus characteristic curve of CIBR. A theoretical and experimental model was putforward to calculate the required dosage of PAC in making the content of phosphorusreach grade A. And the mechanism of phosphorus removal by PAC-activated sludgeco-flocculation was preliminary discussed by investigating the change of polyaluminumspecies and the properties of sludge floc after dosing PAC into the activated sludge. Themain conclusions are as follows:(1) Phosphorus removal by PAC-activated sludge co-flocculation process was afast reaction process，in which activated sludge played an important role. The favorableshearing force in PAC-activated sludge co-flocculation ranged from30to100rpm, andthe optimal hydraulic condition was fast-stirring speed of400rpm for15seconds，thenmid-stirring speed of100rpm for3minutes and slow-stirring speed of40rpm for7minutes. The efficiency of phosphorus removal by PAC-biological co-flooculation inthe secondary treatment process of wastewater mainly depended on the initialconcentration of TP, and little affect was observed by MLSS, pH or temperature. ButPAC attaching to activated sludge first and quickly could improve the efficiency of phosphorus.(2) The flow pattern of the precipitation zone in CIBR was a range of complexforms between the completely plug flow and completely mixed flow, and the threephase separator did not affect the effluent phosphorus concentration. The concentrationof phosphorus in CIBR within one cycle reached grade A was the prerequisite to makethe effluent phosphorus concentration reach grade A. The nitrogen and phosphorusremoval in CIBR could be characterized by the curves of DO/ORP/pH. It waseconomic and effiency that PAC dosing point was chosen at15minutes before or afterthe end of the anaerobic stage, and the optimal dosage could be calculated by themodel summarized in this paper.(3) The removal of PO43--P in PAC-activated sludge co-flocculation process wasclosely related to Alb, but little to Alaand Alcin PAC. Both stable and unstable complexescould formed when PAC reacted with PO43-, and part of the unstable complex could beremoved via adsorption or sweep function of activated sludge. Alband Alcplayed animportant role in biofloccution, where Albreduced the surface charge density of activatedsludge while induced when Alccombined with it.(4) The efficiency of phosphorus removal by PAC-activated sludge co-flocculationwas much higher than chemical flocculation. More polyaluminium could combine withactivated sludge and became a part of the sludge when PAC reacted with activated sludgefirst, which improved the efficiency of phosphorus removal by bioflocculation. Themechanism of phosphorus removal by PAC-activated sludge co-flocculation contained thereaction between the polyaluminium combined with activated sludge with PO43-andactivated sludge with PO43-. The surface charge density and physical fraction of activatedsludge played important roles on the removal of phosphorus by bioflocculation. Thephosphorus removal efficiency increased when the surface charge density reduced，but itdepended on the species of metal combined with activated sludge.