Open channel flow is constrained liquid flowing with a free surface. Rivers and uncovered conduits are obviously open channels, but enclosed channels such as sewers, when flowing partially full and not under pressure, are also classified as open channels.

A restrictive hydraulic structure (primary device) in the channel, such as a weir or flume, controls the flow's shape and velocity. A secondary device used in conjunction with the primary device measures the liquid level and converts this into an appropriate flow rate, based upon the known level to flow relationship of the primary device.

In some situations, the flow conduit itself serves as the primary device. A resistance equation, such as the Manning formula, is used to estimate flow rate based on the channel shape, dimensions, slope and a roughness coefficient that represents the friction the conduit material exerts on flowing water. Generally speaking, this information is configured into the monitoring instrument and used with fluid depth measurements to calculate flow rate.

The main advancement in open channel flow measurement technology has been the evolution of instruments that use the area velocity method. An area velocity flow meter is configured for channel shape and dimensions but there is no need for variables such as slope or roughness coefficients. The depth of flowing water is measured at a specific point and then the instrument calculates the cross-sectional area of the flow stream at that point. The average velocity of the flow is measured at the same point, and the meter multiplies the area (A) of the flow by the average velocity (V) to calculate flow rate (Q). This is represented by the continuity equation, Q = A x V.

Sampling

Sampling is an extremely important consideration in properly characterizing wastewater for biological phosphorus removal. Flow rate and wastewater quality change continuously, and these changes may affect the ability of a wastewater treatment plant to achieve consistent biological phosphorus removal. Obtaining samples that will actually represent the wastewater flow throughout the months and years to come is difficult at best. Diurnal fluctuations occur in concentration and flow volume; seasonal fluctuations occur in concentration, flow volume, and temperature; and industrial contributions to the collection system may cause wastewater characteristics to change on a short- or long- term basis. Given the variable nature of wastewater and the necessity of attaining consistent phosphorus removal, it may be necessary to collect samples that will represent "average" characteristics and approximate characteristics under more extreme conditions.

A desirable sampling method is to collect a 3-4 hour composite sample. This will provide data that may be considered representative of average wastewater characteristics throughout the day while minimizing the sample holding time. A careful review of flow monitoring records and reports generated by a facility over the past couple of years will also be helpful in assessing the seasonal characteristics of the wastewater throughout the year. If records reveal a wastewater that is highly variable in flow volume and concentration, further analysis may be required. It is not unusual to find that a particular facility may remove an adequate amount of phosphorus biologically during certain times of the year, with chemical precipitation being required during times when the wastewater characteristics are not as conducive to biological removal.

Preservation

Once a sample is taken, the constituents of the sample should be maintained in the same condition as when collected. When it is not possible to analyze collected samples immediately, samples should be preserved properly. Biological activity such as microbial respiration, chemical activity such as precipitation or pH change, and physical activity such as aeration or high temperature must be kept to a minimum. Methods of preservation include cooling, pH control, and chemical addition. Freezing is usually not recommended. The length of time that a constituent in wastewater will remain stable is related to the character of the constituent and the preservation method used.