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

An expansion of a municipal wastewater treatment plant (WWTP) in China was designed to utilize vent gas from an ozone contactor, instead of compressed air, for aeration in the secondary process. The capacity of the existing oxidation ditch plant was increased from 120,000 m3/d to 150,000 m3/d with the construction of additional secondary trains and clarifiers based on a reverse A2O process (Anoxic, Anaerobic, Oxic) for biological nutrient removal. The oxygen requirement for the incremental flow (30,000 m3/day) is met completely with vent gas from a tertiary ozonation system.Ozonation of the filtered secondary effluent from the whole plant uses 16.5 mtpd pure oxygen for generating about 1.6 mtpd of ozone (at ˜10% w/w) at the current flow rate of 150,000 m3/d. The balance of the ozonation gas (14.9 mtpd) is oxygen, which can be largely recovered from the contactor vent gas. Optimal oxygen recovery requires minimization of oxygen losses to the atmosphere and effluent. Following ozone contacting, there is a net value of about 14 mtpd oxygen in the vent gas (see Figure 1). The oxygen-rich vent gas (75-85% pure oxygen) is applied to the aeration zone of the A2O process in the plant expansion (30,000 m3/d).The use of the vent gas stream has enabled a significant reduction in the capital (aeration equipment) and operating (electrical) cost for aeration compared to conventional air-based aeration. The low-pressure vent gas stream is dissolved using Praxair's I-SOTM oxygenating systems which are in-situ, floating mechanically agitated contacting systems that are able to induce gas flows using a high strength vortex generated by the rotational action of a helical impeller (see Figure 2). The I-SOTM system's capacity for gas induction eliminates the need for the compression of the vent gas stream, with attendant savings in costs.As the WWTP must meet ammonia and total nitrogen limits, there were concerns about the inhibition of nitrification at reduced pH levels common to closed basin pure oxygen wastewater plants. As a result, the design included a provision for an additional surface aerator to strip CO2 and raise the pH. Over six months after start-up however, nitrification was found to occur without evidence of inhibition in the open basin pure oxygen-based aeration within the secondary process at the WWTP, suggesting that some of the concerns associated with pH control and nitrification in closed basin pure oxygen systems are avoided, or significantly mitigated in open-basin pure-oxygen systems.Our findings suggest that the cost of ozone treatment can be potentially offset by considering reuse options that enable the oxygen content in the vent stream for aeration in the biological basin. Despite concerns that any oxygen-based aeration can inhibit nitrification, the wwtp in this case study has successfully met BNR limits without any adjustments to the aeration process.