flow measurementCurrently, most countries in the world adopt a combination of pollutant emission concentration control and total control measures. Therefore, while understanding the quality of sewage, hydrological parameters such as water level (m), flow velocity (m/s), and flow rate (m3/s) should also be measured to calculate environmental capacity, control pollution source emissions, and estimate pollution control effectiveness.

surface waterflow measurement
For larger rivers, hydrological departments generally have hydrological monitoring sections and should make use of their measured parameters as much as possible. If there is no hydrological cross-section in the monitored river section, a cross-section with relatively stable hydrological parameters and representative flow rate should be selected for measurement. The main measurement methods include:

1. Flow velocity area method

First, divide the measurement section into several small pieces, measure the area and flow velocity of each small piece, calculate the corresponding flow rate, and then add up the flow rates of each small piece to obtain the water flow rate on the section. The calculation formula is:
（3-1）
In the formula: Q - water flow rate, m³/s；
——Horizontal average flow velocity on each small section, m/s；
F₁， F₂……F_n——Each small cross-sectional area, m².
The flow velocity is mostly measured using a current meter, which includes single point current meters (such as mechanical current meters, electrically connected current meters, etc.), three-dimensional current meters (such as Doppler acoustic current meters, etc.), and multifunctional intelligent current meters. Different settling devices can be used to measure the flow velocity range of 0.020-10m/s.

2. Float method

The float method is a simple method for roughly measuring the water flow velocity in small rivers and canals. When measuring, select a straight river section and measure the cross-sectional areas of the starting point, midpoint, and endpoint within a distance of 2 meters. Based on this, calculate the average cross-sectional area. Put in a buoy upstream, measure the time required for the buoy to flow through a certain river section (L), repeat the measurement several times, and calculate the average time required (t) to calculate the flow velocity (L/t). Then calculate the flow rate according to the following formula:
（3-2）
In the formula: Q - water flow rate, m³/min；
——The average flow velocity of the buoy, m/s， I.e. L/t;
S - cross-sectional area of water flow, m².
K - Buoy coefficient, related to air resistance and the uniformity of flow velocity distribution on the cross-section, generally needs to be calibrated with a flow meter, with a range of 0.84-0.90.

（2） Waste (sewage) water flow measurement

1.

flow measurementFlow meter method
At present, more than ten categories and hundreds of varieties of sewage flow meters have been developed both domestically and internationally, mainly including volumetric, differential pressure, turbine, area, electromagnetic, ultrasonic, and overflow weir flow meters. Suitable flow meters can be selected based on the actual flow range and testing accuracy requirements of the water flow.

2. Volume method

Introduce sewage into a container or sewage tank with a known volume, measure the time it takes to fill the container or sewage tank, and then divide it by the volume of the receiving container or tank to calculate the flow rate. This method is simple and easy to implement, suitable for measuring continuous or intermittent discharge of sewage with low flow rate.

3. Overflow weir plate method

The overflow weir plate method is the main method for measuring water flow rate. This method is suitable for water in irregular sewage ditches and channelsMeasurement of traffic flowThis method involves using triangular, rectangular, or trapezoidal weir plates to intercept water flow and form an overflow weir. The water head and level before and after the weir plate are measured to calculate the flow rate.
Figure 3-3 is a schematic diagram of measuring flow using the triangular weir method, and the flow calculation formula is as follows:
（3-3）
In the formula: Q - water flow rate, m³/s；
H - height of water head over the weir, m
K - flow coefficient;
D - the height from the bottom of the water flow to the edge of the weir, m；
B - upstream water flow width of the weir, m。
Under the following conditions, the error of the above equation is less than ± 1.4%:
0.5m≤ B≤ 1.2m
0.1m≤ D≤ 0.75m
0.07m≤ h≤ 0.26m
H≤ B/3