Process Optimization Of A Full-scale Orbal Oxidation Ditch System

A full-scale Orbal oxidation ditch(OD)in a municipal wastewater treatment plant(WWTP)beside the Qingyi river shows an unstable performance on nitrogen and phosphorus removal.This caused the effluent of the WWTP can not steadily meet the Class 1 discharge standard of GB 18918-2002,and the residual pollutants can discharge into the Qingyi river.In this study,a set of full-scale experiments have been carried out.The operating process of OD was controlled during typical room temperature and low temperature periods,process control and activated sludge characteristics were also analyzed.The results were as followed:During room temperature period,as the numbers of disc aerators in outer,middle and inner channels were 5,3,3(5-3-3 mode),the effluent chemical oxygen demand(CODCr),NH4+-N,total nitrogen(TN)and total phosphorous(TP)concentrations of this WWTP could meet the Class 1 discharge standard,but the dissolved oxygen(DO)in secondary settling tanks were above 4 mg/L,indicating that there might be excessive aeration in the OD system.Since the aeration mode was adjusted to 4-2-2 mode,the effluent CODcr,NH4+-N,TN and TP concentrations still met the Class 1 discharge standard,the DO in secondary settling tanks decreased to about 2.5 mg/L,and 1248 kW h per day of energy was also saved.During low temperature period,through increasing rotation speed of the disc aerator before inlet,and switching on another group of linked disc in middle and inner channels,as well as the prolonged sludge retention time(SRT)of 20 d and the mixed liquor suspended solids(MLSS)of about 7500 mg/L,the effluent CODCr concentrations of OD system were still met the Class 1 discharge standard of GB 18918-2002.The effluent TN concentrations decreased from 30-35 mg/L before process control to 15 mg/L.Moreover,for the OD running during low temperature period,the CODcr concentrations in outer channel decreased from 45 mg/L before process control to 35 mg/L as well as the NO3--N concentration from 25 mg/L to 10 mg/L.For the sludge,after process control.norunk_f_Saprospiraceae and uncultured_f_Anaeolineaceae became the dominant genus,indicating that there might be endogenous metabolism in OD system after process control in low temperature period,and yielded a part of carbon source for simultaneous nitrification denitrification(SND)process.Organic matters in slime and loosely bound extracelluier polymer substances(LB-EPS)of sludge significantly decreased,such as the combination of tyrosine-like and tryptophan-like substances,while organic matters in tightly bound extracelluier polymer substances(TB-EPS)significantly increased,such as the combination of tyrosine-like and dryptophan-like substances,tryptophan-like substances and soluble microbial by-product-like substances.Polysaccharides.phenols and alcohols at the surface of sludge significantly increased and intermolecular hydrogen bonds also increased,indicating the increased adsorption ability of sludge.This study offers a set of technology about process control of a full-scale Orbal OD System in typical running periods.During room temperature period,the proposed aeration mode is 4-2-2 mode.During low temperature period,the proposed process control methods were increasing rotation speed of the disc aerator before inlet,and switching on another group of linked disc in middle and inner channels,as well as the prolonged SRT of 20 d and the MLSS of about 7500 mg/L.