The utilization of ozone vent gas for aeration at a new biological nutrient removal (BNR) wastewater plant expansion in China

摘要

An expansion of a municipal wastewater treatment plant (WWTP) in China was designed tornutilize vent gas from an ozone contactor, instead of compressed air, for aeration in the secondaryrnprocess. The capacity of the existing oxidation ditch plant was increased from 120,000 m~3/d torn150,000 m~3/d with the construction of additional secondary trains and clarifiers based on arnreverse A2O process (Anoxic, Anaerobic, Oxic) for biological nutrient removal. The oxygenrnrequirement for the incremental flow (30,000 m~3/day) is met completely with vent gas from arntertiary ozonation system.rnOzonation of the filtered secondary effluent from the whole plant uses 16.5 mtpd pure oxygenrnfor generating about 1.6 mtpd of ozone (at ～10% w/w) at the current flow rate of 150,000 m~3/d.rnThe balance of the ozonation gas (14.9 mtpd) is oxygen, which can be largely recovered fromrnthe contactor vent gas. Optimal oxygen recovery requires minimization of oxygen losses to thernatmosphere and effluent. Following ozone contacting, there is a net value of about 14 mtpdrnoxygen in the vent gas (see Figure 1). The oxygen-rich vent gas (75-85% pure oxygen) is appliedrnto the aeration zone of the A2O process in the plant expansion (30,000 m3/d).rnThe use of the vent gas stream has enabled a significant reduction in the capital (aerationrnequipment) and operating (electrical) cost for aeration compared to conventional air-basedrnaeration. The low-pressure vent gas stream is dissolved using Praxair’s I-SO? oxygenatingrnsystems which are in-situ, floating mechanically agitated contacting systems that are able torninduce gas flows using a high strength vortex generated by the rotational action of a helicalrnimpeller (see Figure 2). The I-SO? system’s capacity for gas induction eliminates the need forrnthe compression of the vent gas stream, with attendant savings in costs.rnAs the WWTP must meet ammonia and total nitrogen limits, there were concerns about therninhibition of nitrification at reduced pH levels common to closed basin pure oxygen wastewaterrnplants. As a result, the design included a provision for an additional surface aerator to strip CO_2rnand raise the Ph. Over six months after start-up however, nitrification was found to occurrnwithout evidence of inhibition in the open basin pure oxygen-based aeration within thernsecondary process at the WWTP, suggesting that some of the concerns associated with Phrncontrol and nitrification in closed basin pure oxygen systems are avoided, or significantlyrnmitigated in open-basin pure-oxygen systems.rnOur findings suggest that the cost of ozone treatment can be potentially offset by consideringrnreuse options that enable the oxygen content in the vent stream for aeration in the biologicalrnbasin. Despite concerns that any oxygen-based aeration can inhibit nitrification, the wwtp in thisrncase study has successfully met BNR limits without any adjustments to the aeration process.