As the wastewater treatment market for ultrasonic flow-meters continues to grow, it becomes increasingly important to know those applications within a typical treatment plant that are best suited to Transit Time ultrasonic flowmeters and those that are best suited to Doppler Effect flowmeters.
The objective of this application note is to assist specifiers in deciding which type applies best at any given location.
Basic Principles in Flowmeter Selection
The principles for choosing between Transit Time or Doppler flowmeters at any given location are as follows:
1. Specify Transit-Time Ultrasonic flowmeters if all of the following conditions exist:
a. The application calls for verifiable absolute accuracy in the range of 0.5% to 1.0% of reading for flow rates greater than 0.02 m/s
b. Air or other gas is not added to or generated within the waste liquid in the equipment, upstream of the meter.
c. The meter is located as far away as possible downstream from any pump to avoid the adverse effects of pump cavitation. A straight run of at least 10 pipe diameters upstream and three pipe diameters downstream is required between the flowmeter transducer and elbows, valves, tees or pumps.
d. Suspended solids content are not excessive. Solids content of 10 % for small diameter meters and 5 % for large diameter meters can be handled with transit-time meters. The solids content is not the limiting factor in absorbing or scattering sound but invisible gas bubbles that cling to the solids particles can create a problem.
2. Specify Doppler meters if both of these conditions exist:
a. The application calls for repeatability no better than +/- 0.3 m/second.
b. The solids content and/or gas bubble exceed 5-10 %. Doppler meters will work if particle sizes are 40 microns or larger, @ 35ppm, and suspended solids content is 0.2% density difference from the liquid medium.
Ultrasonic Flowmeter Selection Guide
Raw Sewage
The rate of flow of raw sewage into the waste treatment plant is often used as a basis for billing the source of the sewage, perhaps in a municipality or a large industrial facility. Since solids concentration is usually relatively low and absolute accuracy is of paramount importance, Transit-Time meters are normally specified here unless undissolved solids prohibit stable readings. Doppler would then be installed.
Interceptor Storage or Flow Equalization
Not shown in Figure 1, the interceptor / balance storage tanks are typically located on raw sewage lines into the plant and upstream of the bar screens. They are utilized to minimize flow variations within the plant and thus help to achieve consistent performance of the treatment processes. Since repeatability is more important than absolute accuracy, Doppler meters are suggested.
Primary Clarification and Sludge Pumping
Primary clarifiers are tanks used to remove or reduce suspended solids and organic loading from the wastewater before it goes to secondary treatment units. Doppler meters are suggested for the high solids content sludge flow to the thickeners. Since repeatability is more important than absolute accuracy, Doppler meters are usually specified for the primary effluent.
Return Activated Sludge (RAS)
A primary objective is to maintain the stability of the activated sludge process. This is accomplished by recycling an active mass of micro-organisms in sufficient quantity to break down most of the biodegradable organics flowing as effluent from the primary clarifiers. The RAS is usually returned to a point just upstream of the aeration tanks. Because good repeatability is the primary requirement, Doppler flowmeters are suggested.
Waste Activated Sludge (WAS)
The purpose of the secondary clarifiers is to concentrate solids, a portion of which are recycled (RAS) and a portion of which are wasted (WAS). Doppler flowmeters are suggested.
Gravity or Flotation Thickening
The main objective is to concentrate the sludge as much as possible within the constraints of the process to maintain as consistent an underflow quantity and composition as can be achieved. Because solids and/or bubble contents may be rather high, Transit Time meters are usually unsuitable. Doppler flowmeters should be specified.
Digestion
This is the process of breaking down and partially liquefying waste solids by the action of bacteria so that the solids which remain are stable and can be disposed of without posing a health hazard. In primary treatment, the process is anaerobic (i.e., in the absence of air) and methane gas is generated. In secondary treatment, the process is aerobic (i.e. in the presence of air) and air or oxygen is bubbled through the waste system. In either case, the presence of gas bubbles would require use of Doppler meters.
Effluent
The rate of flow of effluent from the waste treatment plant into a nearby lake or river is usually monitored carefully as the basis for reporting to government authorities. Since the effluent is virtually solids-free and absolute accuracy is important, Transit-Time flowmeters are usually suggested. However, effluent cascading from the secondary clarifier or subjected to a final aeration in an open channel (leading to the final closed effluent piping) may be full of bubbles. In such cases, Doppler meters are used.
In general, the above suggests the careful use of Transit-Time meters for raw sewage flowing into the treatment plant and effluent flowing out of the plant. These streams are relatively low in solids and usually call for a high level of accuracy for billing purposes and/or satisfying government regulations. Conversely, the above suggests Doppler meters for use within the waste treatment plant, where repeatability is required for operational control.
Recommended Ultrasonic Flowmeters
1. Flexim F 601 (Transit- Time)
2. Compu-Flow Model C6/CLTV6.2 (Doppler Effect)
Courtesy: James Steves of Teksco USA, Manufacturers of Compuflow Doppler Ultrasonic Flowmeters.
