System Tests Performance of Dewatering Automation

The Vallejo Sanitation and Flood Control District (District) retained CH2M Hill to upgrade its solids handling facility, with a requirement to generate Class B biosolids. In addition to new dewatering equipment, the district asked for an evaluation of dewatering automation technology to determine whether a system was available to keep the operating system at maximum efficiency while minimizing staffing requirements.

The Vallejo Sanitation and Flood Control District (District) retained CH2M Hill to upgrade its solids handling facility, with a requirement to generate Class B biosolids. In addition to new dewatering equipment, the district asked for an evaluation of dewatering automation technology to determine whether a system was available to keep the operating system at maximum efficiency while minimizing staffing requirements.

Based on analysis of test data, the district decided to select belt presses as the dewatering technology, and to add slaked lime to the sludge prior to dewatering. The project went out to bid in mid-1994, and the presses were started up in 1997, with the dewatering automation system started late the same year.

The engineer had field experience with dewatering automation technology, initially from designing a 24 belt press installation at Jones Island, Milwaukee, and later with a project at the Mohawk and A.B. Jewell WTPs in Tulsa, OK.

The Jones Island project had installed streaming current technology (sensors that use the electrical charge in polymers and sludge to determine the dosage needed), but the system was eventually withdrawn from service because of the extensive maintenance required. In early 1993 the engineer audited a Sludge Expert (SLX) system from Alpine Technology which was installed at the Knollwood, Il. WWTP. The system used fuzzy logic to control non-linear variables and an infrared meter sensor. A meeting with the plant operators indicated that the system was efficient and reliable. This led the engineer to specify the system for two potable water installations for alum dewatering at the city of Tulsa in 1995.

Twelve-hour unattended performance tests on the SLX were conducted at the two Tulsa plants: Mohawk on August 31, 1995, and at A.B. Jewell on September 1, 1995.

At both plants the SLX successfully controlled the presses for 12 hours, and achieved performance comparable to trials conducted previously by the belt press supplier in terms of solids loading, cake, and feed solids. This performance was met during a period when there was a fluctuation in feed solids from 5 percent down to 2.5 percent at the A.B. Jewell WTP.

Based on the experience in Tulsa and elsewhere, the Vallejo District decided to install the SLX dewatering automation system as part of the upgrade. Start-up of this system was deferred until after the new dewatering equipment was commissioned and operating reliably.

How Automated Dewatering Control Works

Historically, automated sludge dewatering control systems have experienced problems. One has been with instrumentation, where drift caused by contamination resulted in repeatability difficulty. Also, environmental variables which were independent of the dewatering process could affect the instrument output. Another problem experienced with some systems stemmed from the difficulty in relating the measurement being taken to what was happening in the system as a whole, except for a quite narrowly defined operating range and sludge conditions.

How Automated Dewatering Control Works

The SLX system uses a filtrate meter to monitor the combined filtrate streams from the belt press. A major departure from previous systems that increased polymer when the filtrate deteriorated is that the SLX looks only for changes in the filtrate probe reading. The absolute value of the reading is not required for accurate control.

How Automated Dewatering Control Works

This strategy overcomes the problems posed by changes in sludge feed or type, or changes in polymer type or batch that might occur during operation. These changes, which can be expected to take place from time to time, would require a new setpoint to be input for a controller to function. For the Vallejo project, the SLX does not use a fixed setpoint; it dynamically seeks the setpoint as it operates by making process changes and monitoring the reaction from the press in the form of a change in the filtrate turbidity.

How Automated Dewatering Control Works

When the polymer dose rate is decreased, the base level filtrate remains relatively unchanged as the dose rate drops until it reaches the point where there is barely sufficient polymer to flocculate the sludge. At this point, the filtrate solids increase rapidly due to extrusion and passage of solids through the belt fabric in the gravity zone, and the SLX recognizes that it has found the dose rate "knee".

How Automated Dewatering Control Works

The detection of the maximum loading point is found in a similar manner. As the solids loading is increased, the filtrate base level again remains fairly static until the maximum loading is reached. At this time, the press cannot take any more sludge, and it begins to extrude sideways between the belts in the pressure zone and wedge zone. This produces a rapid increase in filtrate solids, which is detected by the filtrate monitor in the same way as the minimum polymer reaction.

How Automated Dewatering Control Works

The system is not affected by progressive probe fouling, as the rate of change is the important parameter, rather than the absolute numerical value sent back by the probe.

Dewatering Automation Results

The main reason for specifying dewatering automation was to improve labor efficiency, by minimizing time spent in attendance at the presses, and to minimize exposure to gas emissions. The SLX has proven reliable at operating the presses, with operator attendance limited to start-up, shutdown, menu priority switching, and maintenance work.

Dewatering Automation Results

In addition, Vallejo has seen significant savings in polymer: when the belt presses were first put under automatic control, the SLX achieved an immediate reduction in polymer consumption from 7.5 lbs./DT to 4.6 lbs./DT. This was achieved because, up to the time of SLX start-up, the flocculated sludge appeared wetter than it actually was. From observing the SLX in operation, the operators gained confidence in using the lower level of polymer consumption, even when in manual mode. This new level of performance has become standard.

Dewatering Automation Results

Automatic dewatering using the SLX was able to achieve similar results to the manual operation. The main benefit of automation is that optimum performance is maintained even when there are variations in the sludge feed characteristics or polymer concentration. This can occur at different times of the year, especially since the district has three different sludge streams.

Conclusions

The district decided that the benefits of installing new generation belt presses, with automation controls, justified the challenge of being one of the first facilities with this technology.

Conclusions

The operational staff was particularly interested in using the new automation system because it added a stimulus to their jobs and made them more efficient as a commercial operation. The owner is already realizing substantial savings in chemical and labor cost, with additional benefits accruing from high solids capture and high cake solids.

Conclusions

The Sludge Expert has now been operating routinely for nearly 12 months. This year, the district plans to install a system upgrade which will provide a seamless interface with the plant DCS system.

Acknowledgements

The authors wish to acknowledge the CH2M Hill personnel who were involved with the project, and who assembled the data for the tables. The authors thanks are due to WEF for permission to use an extract from a paper published at the WEFTEC 98 Residuals Conference, "Vallejo Experience with Dewatering Automation." The authors also want to express their appreciation of the support they received from the operational staff at Vallejo SFCD, and Tulsa Metropolitan Authority.

Acknowledgements

About the Authors: R. Matheson is with the Vallejo SFCD, Vallejo, California. P. Brady, B.E., M.I.E.I., is with Alpine Technology Inc., Austin, Texas.

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