Fundamental Research On The Anaerobic Biological Phosphorus Removal From Wastewater

Eutrophication has been one of the main water pollution problems in China, andit is important to control phosphorus discharge into waters to relieve the severity asphosphorus is the key factor to cause eutrophication. In recent years, great advanceshave been achieved in anaerobic biotechnology for wastewater treatment. The UASB(Upflow Anaerobic Sludge Blanket) process has become one of the most popularanaerobic treatment processes and has been applied in various wastewaters. Thefunction of UASB reactor for removal of organic pollutants has drawn a lot ofattention, but its function for removal of phosphorus has so far been ignored.A laboratory scale UASB reactor was operated to investigate its performance onphosphorus removal, and the feasibility of its applications in treating phosphoruscontaining wastewater was assessed as well. The main results of the research weresummarized as follows:High performance on phosphorus removal was shown in the laboratory scaleUASB reactor. The volumetric phosphorus removal rate of 21.4 mgP/L·d, the removalrate efficiency above 99 % and the effluent PO₄^3--P concentration below 0.5 mg/L(the requirement in the general discharge standards of wastewater )could be achieved during the operation, when the influent COD and PO₄^3--Pconcentrations were 5000～10000 mg/L and 5～30 mg/L, respectively.The microbial assimilation was supposed to be the most important pathway forphosphorus removal in UASB reactor. The microorganisms in the sludge uptake thephosphate as nutrient during their growth (the phosphorus was transferred from theliquid phase to the solid phase), then phosphorus was removed by separating thesludge from the effluent.The influent concentration of COD and phosphate and the COD/P ratio were themain factors affecting phosphorus removal efficiency. Higher volumetric phosphorusremoval rate was achieved at higher influent COD and phosphate concentrations. Theincrease in influent phosphorus concentration could result in lower phosphorusremoval efficiency but higher volumetric phosphorus removal rate. Larger COD/Pratio ensured higher phosphorus removal efficiency, but limited the furtherimprovement in volumetric phosphorus removal rate. The volumetric removal rates of COD and phosphorus were highly correlated.The volumetric phosphorus removal rate increased as the COD removal rate elevated,while the volumetric COD removal rate was hardly affected by volumetricphosphorus removal rate. With the influent COD/P ratio of 1000:1, a volumetric CODremoval rate of 10 gCOD/L·d was achieved, meanwhile, the ratio of the consumedCOD to phosphorus reached a relatively high value of 1182:1. The ratio of consumedCOD to phosphorus was about 655:1 when the influent COD/P ratio was between1000:2 and 1000:3, and it decreased to about 500:1 as the volumetric COD removalrate increased.The removal rate of ammonia nitrogen was proportional to that of phosphorus,but it was limited by the phosphorus concentration. At the high influent COD/P ratio(1000:1), the ratio of the consumed nitrogen to phosphorus was increased to 18.8:1,otherwise the ratio was relatively steady at 8.0:1 (with influent COD/P ratio of1000:2～1000:3), which was not affected by influent N/P ratio or volumetric nitrogenremoval rate under most conditions.The performance of the phosphorus removal in UASB reactor had relationshipwith the microbial community in its granular sludge. The results of PCR-DGGE(Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis) and SEM(Scanning Electronic Microscope) showed that the microbial community in thegranular sludge shifted during the operation of the UASB reactor. The dominantmicroorganisms in the granules were rod-shaped and coccus in the early stage of theoperation, one of which was identified as Methanobacterium formicicum, while thefilaments became the predominant microorganisms in the latter stage, one of whichwas identified as Methanosaeta concilii. Lower effluent phosphorus concentrationcould be achieved when the granules was dominated by filaments in the UASBreactor.The structure of the granular sludge was affected by the operational condition ofthe UASB reactor. Higher COD/P ratio (1000:1～1000:2) in influent and lower sludgeorganic loading rate (0.36～0.86 gCOD/gVSS·d) were prone to cause the granules todisaggregate and to be washed out.The performance of the UASB reactor in phosphorus removal decreased with theaddition of sulfate. The volumetric phosphorus removal rate decreased from 9.6mgP/L·d to 2.0 mgP/L·d when the COD/sulfate ratio in influent decreased from 7.0 to2.3. The volumetric phosphorus removal rate was in the range of 2.0～4.3 mgP/L, and its phosphorus removal efficiency was 12.3 %～26.2 %, with the influent containing3500 mg/L COD, 1500 mg/L SO₄^2- and 15～25 mg/L PO₄^3-.