The latest sonar technology improves accuracy in levels measurement, helping clean-water plants sustain compliance and enhance efficiency.
Wastewater treatment plants are major energy users, and efficient operation requires a fine balance between biological and hydraulic parameters.
Maintaining that balance can be daunting. It depends on reliable and meaningful data in critical parts of the process, such as the primary and secondary clarifiers and thickeners.
Accurate measurement of floc and high-density sludge levels is challenging in murky, turbid settling tanks.
Today, advanced sonar measurement devices can provide such accuracy, helping to improve process control, enhance permit compliance consistency, save energy and extend equipment life.
Even with extensive sample extraction and lab analysis, it is difficult to obtain a clear picture of the sludge density profile: solids densities can range from 200 mg/L at the tank top to 3,000-6,000 mg/L or more at the bottom. Generally, operators are interested in quality return activated sludge with density greater than 2,500 mg/L. However, when problems occur, operators need to know the dynamics of the different layers to effectively control the treatment process.
Sonar measurement systems do not always provide comprehensive and reliable information because they lack the power or the correct frequency to penetrate the suspended solids. Other tools for gaining a full tank profile include manual dipping devices such as tube samplers or gap sensors, but these provide no continuous output for trending and control.
Because sonar systems typically cannot penetrate densities of 1,200 to 1,500 mg/L, they can measure only the upper floc layer. Users can then make the incorrect assumption that the denser RAS layer corresponds with the floc layer’s movement.
As the floc layer rises due to an upset, the assumption may be that the denser RAS layer is also rising. This tells operators either to increase the RAS pumping rate or drop the bellmouth to bring the rising blanket down. These actions have little effect on the lighter floc layer, but will quickly remove the good-quality biomass and then begin to pump back a lower-density, poor-quality biomass.
All this negatively affects the food-to-microorganism ratio, mixed liquor suspended solids and dissolved oxygen. At some sites, it could take weeks to fully rectify the situation, by which time increased aeration may have been required, increasing energy consumption and costs.
The reality is that the denser, good-quality biomass has remained at the bottom of the tank and only the lighter floc layer has lifted. Thus, it is essential to monitor the biomass at 3,000 to 6,000 mg/L in order to properly control the RAS pumps and bellmouth and send only good-quality’ biomass back to aeration or to the thickener.
Now, advanced sonar systems have been developed that submerge a high-power transducer to send sonar pulses through the liquid. These are reflected back by different density layers, including those in excess of 6,000 mg/L and even the tank floor. Signals are processed to provide outputs that relate to both the floc and RAS levels — vital information to help optimize energy usage and site operations. Alarm levels can be set to help operators make process changes in time to avert a permit violation.
Measuring through a liquid is straightforward, and almost any 700 kHz transducer will give reliable and repeatable results. But suspended solids particles attenuate and reflect the high-frequency short wavelength, causing unreliable and unrepeatable measurements.
However, with a lower-frequency (150 to 300 kHz) transducer, the longer wavelength can “ignore” the suspended particles more easily. This is why foghorns use a low frequency/long wavelength to project the sound through moisture particles in the air.
In line with this, the correct transducer frequency needs to be selected for primary sedimentation, primary and secondary clarifiers, sludge thickeners, lamella clarifiers, and sequencing batch reactors. The key to the success of the new systems is their wide range of transducers, from 150 to 700 kHz.
For example, SBRs are typically installed where space or cost is at a premium. They combine primary sedimentation, aeration and secondary settlement in one tank. The liquid levels change within the tanks, and a fixed transducer cannot cater to these variations. The new technology overcomes this with a floating transducer that tracks the settling blanket layer far more accurately, improving batch times up to 20 percent.
Clean and accurate
To provide regular cleaning of the submerged or floating ultrasonic transducers, the new measurement devices use an actuator lever arm system with an automatic cleaning cycle, triggered on a time basis or by a predetermined signal level. The actuator pushes the transducer through the water and returns it to the original position. The sharp shearing action removes debris and scum from the front face, sustaining optimum performance without any operator involvement.
There are several design issues to consider when evaluating a sludge monitoring system, whether electronic or mechanical. Hazardous area versions, used for enclosed settling tanks, are built to minimize odor release or to capture methane gas. These include transducers and cleaning mechanisms suitable for use in the hazardous area.
Communication protocols including Fieldbus, Profibus, HART and DeviceNet deserve consideration for ensuring seamless integration with plant instrumentation and distributed control systems (DCS).
The transducer can be remotely located from the control unit (typically up to 2,000 feet), providing a robust wireless link for instant access to all parameters for servicing, technical support and commissioning. Multiple outputs and relays should be available for alarm and control functions and for cleaner arm actuation.
When using the new measuring devices with two analog outputs, two 4-20mA outputs are used for monitoring the different densities within the tank. For a primary tank, both the blanket level and the suspended solids can be monitored. The second floc output can be used to control the dosing mechanism, thus reducing flocculant/coagulant usage.
For a secondary tank, the two analog outputs can be used to monitor both the RAS and floc layers. Control of the RAS pumps and bellmouth optimizes the RAS density being returned for aeration and ensures that a consistent density is sent to the thickener. The floc output provides an indication of process problems and gives warning of possible permit exceedances.
For a thickener, the two analog outputs can be used to monitor both high-density sludge level and settling rate. Monitoring the high-density sludge level ensures that the filter presses and digester receive sludge of consistent density, and reduces foaming and mechanical wear and tear. Monitoring the floc level provides control for dosing based on settling rate, optimizing dosing and minimizing waste.
Using BUS communications options (HART, Fieldbus, Profibus or Modbus), the PLC or DCS can receive four outputs with any combination of RAS, floc level, high-density sludge level, clarity and temperature.
Whether using an instrument with two analog outputs or the four communications outputs, the information provided improves control, quickly indicates process problems, prevents permit violations, controls dosing in primary tanks and thickeners, and reduces wear and tear on filter presses. The net results are lower energy, maintenance and chemical costs.
About the author
Jack Evans is president of Hawk Measurement, a provider of level, positioning and flow measurement technology based in Lawrence, Massachusetts. He can be reached at firstname.lastname@example.org.