Simultaneous Removal of Phosphorus and Nitrogen by Sequencing Batch Reactor Activated Sludge Process

Abstract–Phosphorus removal by biological means in continuous-flow aerobic/ anaerobic activated sludge processes is now in a stage of full-scale operations. The similar aerobic/anaerobic treatment is also found in biological processes for nitrogen removal by nitrification followed by denitrification. These processes are successfully applied not only to continuous-flow system but also to sequencing batch reactor (SBR) activated sludge processes, whereas little attempts have been reported on phosphorus removal in SBR activated sludge processes. It is most probable that both phosphorus and nitrogen in addition to organic matter can be removed by the SBR activated sludge processes if aerobic and anaerobic treatments were properly incorporated into a cycle of batch operation. Laboratory scale experiments on aerobic/anaerobic operations of the SBR processes were conducted aiming at simultaneous removal of phosphorus, nitrogen, and organic matter without any addition of chemicals. SBR of 5 1 in working volume was fed with synthetic wastewater in which TOC = 120-200 mg/l, BOD = 200-400 mg/l, total phosphorus = 6-12 mg/1 and total nitrogen = 36-60 mg/1. The following sequence of operations were conducted in a batch cycle; 1) mixing and inflow of wastewater, 2) aeration and mixing, 3) mixing, 4) aeration and mixing, 5) settling and 6) decanting. It was secured from continuous monitoring of dissolved oxygen concentration in the mixed liquor that both anaerobic (stages 1 and 3) and aerobic (stages 2 and 4) treatments were repeated twice in a cycle. In some operations, stages 3 and 4 were omitted for comparison, i.e. anaerobic and aerobic treatments were conducted only once per cycle. The volume of mixed liquor before the inflow of wastewater at the beginning of a cycle (low level) ranged from 33 % to 50 % of that during full volume stages from 2 to 5 (high level). In stage 6, the supernatant was discharged down to the low level and followed by the next cycle of operation. The length of time for a cycle of operation was β h or 9.5 h. Among various types of operations tried, the following sequence was the best in the quality of effluent; 1) 2 h for mixing and inflow, 2) 3 h for aeration and mixing, 3) 3 h for mixing, 4) 20 min for aeration and mixing, 5) 1 h for settling, and 6) 10 min for decanting in a cycle of 9.5 h if influent BOD, total phosphorus and total nitrogen concentrations were 400 mg/1, 12 mg/1 and 60 mg/1, respectively, and BOD loading was 0.68 kg/cu m/d. Total phosphorus and nitrogen concentrations in the effluent were 1.2 mg/1 and 8.0 mg/1, respectively. Similar results were obtained in operations where anaerobic and aerobic treatments were repeated twice in a cycle. In operations where effluent quality was satisfactory, release of phosphorus from the sludge was observed in stage 1. The reactor concentration of filterable total phosphorus (FTP) increased rapidly and its maximum value observed at the end of the stage was ca. 50 mg/1. Phosphorus uptake under aerobic condition (stage 2) decreased FTP to the level of effluent FTP. The luxury uptake of phosphorus by the sludge was noted, i.e. phosphorus content in the sludge ranged from 2.0 % to 4.0 %(w/w). The release of phosphorus from the sludge and subsequent luxury uptake were not significant during stages 3 to 4, hence, further removal of phosphorus was not remarkable. Nitrate nitrogen concentration increased during stage 2 by nitrification. Denitrification was noted both in stages 1 and 3. In stage 1, filterable total organic carbon (FTOC) increased by the inflow of wastewater. It should be, therefore, utilized for denitrification as hydrogen donor. FTOC decreased rapidly after the initiation of aeration in stage 2 and little FTOC remained after the latter half of stage 2. Intracellular organic substances of the sludge, therefore, were regarded to be utilized for denitrification without any addition of chemicals at stage 3. In the best operation, from 50% to 70% out of total nitrogen inflow was removed by denitrification. Effluent BOD was less than 10 mg/l. Although further investigations would be required to determine optimum scheduling in a cycle such as the combination of anaerobic and aerobic periods, the ratio between low and high levels in the reactor, the length of a cycle, and etc. for a given wastewater, the SBR activated sludge process would be a promising wastewater treatment process for simultaneous removal of phosphorus, nitrogen and organiC matter by a single reactor. In spite of complicated operational sequence, full scale automatic operations of SBR activated sludge process would be possible economically even in small-scale plants by using recently advanced microcomputer technology.