英文文献

摘要

A novel oxidation ditch system using anaerobic tanks and innovative dual dissolved oxygen (DO) control technology is proposed for biological nitrogen and phosphorus removal from domestic sewage. A continuous bench-scale experiment running for more than 300 days was performed to evaluate the system. Monitoring and controlling the airflow and recirculation flow rate independently using DO values at two points along the ditch permitted maintenance of aerobic and anoxic zone ratios of around 0.30 and 0.50, respectively. The ability to optimize aerobic and anoxic zone ratios using the dual DO control technology meant that a total nitrogen removal efficiency of 83.2–92.9% could be maintained. This remarkable nitrogen removal performance minimized the nitrate recycle to anaerobic tanks inhibiting the phosphorus release. Hence, the total phosphorus removal efficiency was also improved and ranged within

72.6–88.0%. These results demonstrated that stabilization of the aerobic and anoxic zone ratio by dual DO control technology not only resulted in a marked improvement of nitrogen removal, but it also enhanced phosphorus removal.

关键词

Biological nitrogen and phosphorus removal, dual DO control technology,

oxidation ditch

介绍

Removal of biological nitrogen and phosphorus from domestic sewage is an important consideration for avoiding serious eutrophication problems in closed water ecosystem (Metcalf & Eddy 2003). Oxidation ditch (OD) technology is a modified activated sludge system that has been extensively applied for the removal of pollutants given the advantages associated with the process characteristics and ditch configurations. Typical oxidation ditches are completely mixed systems, and horizontally or vertically mounted aerators provide circulation and oxygen supply in the ditch simultaneously. The conventional oxidation ditch systems have been commonly operated with influent BOD loading (0.24 kg BOD/m3d) and hydraulic retention time (HRT) as long as 6–30 h (USEPA 2000).

Since nitrogen removal potential is dependent upon the aerobic phase for nitrification and the anoxic phase for denitrification, the dissolved oxygen (DO) distribution in the ditch is an important control parameter for optimal nitrogen removal (Brouwer et al. 1998; Furukawa et al.

1998;Ekman et al.2006). Surface aerators mentioned above hardly establish a stable anoxic zone under the daily variations of influent quality and water temperature due to the simultaneous change in circulation speed and oxygen supply. Hence, separation of aeration and mixing is effective for stabilizing the anoxic zone (Haoet al.1997). It was also reported that the efficiency of removing nitrogen could be increased by an oxidation ditch with the separation of the two functions of aeration (by flat flexible membranes) and mixing (by horizontal mixer) (Roustan et al.1993).

Moreover, the combination of the aeration and mixing will cause high energy consumption and a perturbation of biological system without consideration the wastewater characteristics. The DO control system at one point maintains DO concentrations in the aerobic phase without consideration of the remaining DO subsequently entering the anoxic phase (Kiet al. 2008), limiting the rate of denitrification in anoxic phase and increasing the energy costs of municipal facilities (Ekmanet al.2006; Holenda et al.2008).

In this study, the innovative dual DO control technology which was capable of controlling the aeration flow and recirculation rate independently was proposed for oxidation ditches with fine-bubble diffusers and submerged impellers.This novel system could effectively control the DO concentration at two points at the optimal level for the biological sewage treatment. In this research, the typical oxidation ditch was replicated by eight completely mixed tanks in series with huge inner recirculation rate similar to the previous studies (Furukawaet al. 1998; Henzeet al.2000). The objective of this research was to evaluate the novel oxidation ditch process employing dual DO control technology and anaerobic tanks prior to the ditch for the biological nitrogen and phosphorus removal.

材料和方法

反应器的安装和操作