Engineering Design Cases of Multi-Point Inflow in Multi-Stage AO Biochemical Treatment Process

Engineering Design Cases for Multi-Point Inflow in Multi-Stage AO Biochemical Treatment Process

Introduction

Multi-point inflow has been widely applied in biochemical treatment processes such as inverted A²O, pre-anoxic A²O, and multi-stage AO in wastewater Treatment Plants. Zhu Yichun et al. proposed a total sewage distribution coefficient for multi-point inflow of α=0.43 (anoxic tank) and β=0.57 (anaerobic tank). Lv Liping et al. conducted research on water distribution for multi-point inflow, showing that when influent TN was 29.3–38.8 mg/L, ammonia nitrogen was 14.9–24.7 mg/L, TP was 3.16–4.46 mg/L, and the C/N ratio was 2.81–5.32, the optimal inflow distribution ratio for the pre-anoxic tank, anaerobic tank, and anoxic tank was 6:2:2. Under these conditions, the average effluent ammonia nitrogen concentration was 0.26 mg/L, TN concentration was 4.88–6.69 mg/L, and TP concentration was below 0.15 mg/L, with effluent quality significantly exceeding the national Class A discharge standard. Li Hang et al. adopted a multi-point inflow method for the pre-anoxic tank and anoxic tank in the biological treatment stage, maximizing the use of carbon sources in the raw water for denitrification and anaerobic phosphorus release, achieving stable nitrogen and phosphorus removal relying solely on influent carbon sources. This approach saved costs associated with external carbon sources and labor while effectively ensuring consistent effluent nitrogen and phosphorus levels, although the inflow ratio and water distribution method were not specified. Since multi-point inflow achieves reasonable distribution of influent, TN removal efficiency is better than single-point inflow, and the higher the water temperature, the more pronounced the denitrification advantage of multi-point inflow. When water temperature is low, there is a risk of effluent ammonia nitrogen exceeding standards with multi-point inflow, requiring reasonable adjustment of the inflow ratio based on actual conditions.

The main challenge in engineering applications of multi-point inflow is accurately controlling the flow at each inflow point. There are few reports on the specific design methods for applying this approach to biological nitrogen and phosphorus removal processes, how to control the inflow ratio at each point, and how to measure it. This paper shares three different methods for controlling and measuring multi-point inflow flow rates and ratios based on three engineering design cases, providing reference for wastewater treatment plant designers.

1 Engineering Design Case for Multi-Point Inflow Using Multiple Orifice Weirs

The raw water for this project is domestic sewage, and the biochemical treatment process consists of "anaerobic → first-stage anoxic → first-stage aerobic → second-stage anoxic → second-stage aerobic → third-stage anoxic → third-stage aerobic → secondary sedimentation tank."

Influent enters each inflow point through an inlet channel. There are four inflow points: the anaerobic zone inlet, the first-stage anoxic zone inlet (referred to as Anoxic 1 inlet), the second-stage anoxic zone inlet (referred to as Anoxic 2 inlet), and the third-stage anoxic zone inlet (referred to as Anoxic 3 inlet). The inlets for each anoxic zone are located in the middle section of that zone to maximize the utilization of carbon sources from the raw water. The positions of the inflow points are shown in Figure 1. Each inflow point consists of 2–4 openings (with adjustable slide gates) of identical size and elevation, also known as multiple orifice weirs. Each slide gate has only two states: fully open or fully closed, both manual. When fully open, the bottom of the slide gate opening functions as a horizontal weir. The total weir length, determined by the number of fully open slide gates at each inflow point, dictates the inflow volume for that point. This inflow method is simple, with the flow ratio between inflow points equal to the ratio of the total number of open slide gate openings at each point. The water distribution ratio is relatively accurate during operation, and measurement is convenient. By measuring the water level at the opening, the flow rate through the opening can be calculated based on the opening dimensions. Figure 2 shows a photograph of the Anoxic 3 inlet. When the left slide gate is open, the bottom of the opening functions as a weir for inflow; the right side is closed and does not allow inflow. In this state, the flow rate is calculated using the width of a single opening as the weir length. If both slide gates are open, the flow rate is calculated using the sum of the widths of both openings as the weir length.

Figure 1 Schematic Diagram of Multi-Point Inflow Engineering Case Using Slide Gates

Figure 2 Photograph of Anoxic Tank Inlet (Two Slide Gates)

This design was applied in an engineering project with influent water quality: COD 150–200 mg/L, BOD 88–200 mg/L, TN 31–40 mg/L, BOD/TN 2.5–4. The effluent achieved COD 30 mg/L, BOD 6 mg/L, TN 12 mg/L, NH₃-N 1.5 mg/L. The multi-point inflow design eliminated the need for external carbon sources.

2 Engineering Design Case for Multi-Point Inflow Using Adjustable Weir Gates

To more precisely control the flow at each multi-point inflow point, adjustable weir gates are also a commonly used method. In this engineering case, the inlet channel extends to the inflow points requiring water distribution. The inflow points are reserved as flow-through openings, with adjustable weirs installed at the openings.

There are two types of adjustable weir gates. Type A, shown in Figure 3, adjusts the water depth in front of the weir by changing the angle of the weir plate. Type B, shown in Figure 4, adjusts the height of the weir vertically using a hoist. Both types calculate the flow rate based on the weir length and the water depth in front of the weir. The inflow point can also be closed as needed, offering flexible adjustment. When selecting the weir gate model, the water depth in front of the weir, adjustment range, flow-through opening dimensions, and weir length are back-calculated based on the required flow rate.

Figure 3 Adjustable Weir Type A

Figure 4 Adjustable Weir Type B

3 Engineering Design Case for Multi-Point Inflow Using Slide Gates

An engineering application case using slide gates is shown in Figure 5. First, wastewater enters the inlet channel and flows over the inlet weir into the water distribution channel. The length of the inlet weir is determined based on the flow rate, water depth in front of the weir, tank dimensions, distribution ratio requirements, and tank layout. As shown in Figure 5, the water from the inlet channel is to be distributed to the anaerobic zone and the anoxic zone. After flowing through the inlet weir into the distribution channel, the water can be divided into 10 equal portions. How it is distributed depends on the position of the slide gate. The slide gate can be placed at positions a, b, c, d, e, f, g, h, and i, totaling 9 positions, dividing the flow into 10 equal portions. The slide gate can only be placed in one position, while the other positions allow water to pass through.

Figure 5 Multi-Point Inflow Engineering Using Slide Gates

If the slide gate is placed at position d in the figure, the weir length corresponding to the water entering the anaerobic zone is A1, and the weir length corresponding to the water entering the anoxic zone is B1. The inflow ratio is anaerobic:anoxic = A1:B1, which is 4:6. If the slide gate is placed at position g, the weir length corresponding to the water entering the anaerobic zone is A2, and the weir length corresponding to the water entering the anoxic zone is B2. The inflow ratio is anaerobic:anoxic = A2:B2, which is 7:3. By analogy, as the position of the slide gate changes, the inflow volume to the anaerobic zone and anoxic zone can be flexibly adjusted. This method is similar to the multi-point water distribution method summarized by Zheng Mei.

This design was applied in an engineering project with an actual scale of 80,000 tons/day. Influent water quality was COD 280 mg/L, BOD 120 mg/L, SS 200 mg/L, TN 40 mg/L, NH₃-N 35 mg/L. Effluent achieved COD 30 mg/L, BOD 6 mg/L, TN 10 mg/L, NH₃-N 1.5 mg/L, saving approximately 15 million RMB annually in carbon source costs.

4 Multi-Point Inflow Flow Measurement

As described above, through several different multi-point inflow methods, the flow rate at each inflow point can be flexibly adjusted. All three methods can calculate the flow rate based on weir length and water depth in front of the weir. The calculation method follows the formulas in the water supply and drainage design manual and will not be repeated here. Generally, instruments are not required for measurement, and it is sufficient to know the flow ratio at each inflow point during operation. However, for projects with higher requirements, operators may still wish to read the flow rate at each inflow point directly. In such cases, instruments such as open channel flow meters, Parshall flumes, or ultrasonic level meters can be installed to calculate the flow rate.

Alternatively, a self-made flow meter using a triangular weir can be placed at the inflow point. A perspective view of this device is shown in Figure 6. Water flows through a 90° triangular weir, and the flow rate over the weir can be calculated based on the water level and the triangular weir dimensions using formulas from the water supply and drainage design manual. Once the triangular weir dimensions are set, different water levels correspond to different flow rates. A water level stabilization box is installed upstream of the triangular weir. The water level inside the box is consistent with that in the inlet channel and is free from disturbances. A float with a guide rod is installed inside the box, limiting the float's displacement within an appropriate range. A pointer is attached to the end of the float guide rod. Based on calculation results, the flow rates corresponding to different water levels are marked on the flow scale, allowing direct reading of the flow rate. This method is simple, low-cost, and although not highly accurate, it meets basic daily measurement requirements.

Figure 6 Perspective View of Simple Channel Flow Meter

5 Conclusion

Multi-point inflow plays a crucial role in saving carbon sources and optimizing nitrification and denitrification. There are various design methods for multi-point inflow. The three basic methods summarized in this paper all have successful engineering application cases and can be selected based on the specific circumstances of the project. It is important to note that multi-point inflow requires high construction precision. The dimensions and elevations of multiple inflow points, as well as the bottom elevation of the inlet channel, must be strictly consistent. Slight differences in water depth in front of the weir can lead to significant variations in inflow volume. Therefore, construction errors should be minimized as much as possible.