Sludge Dewatering: Squeezing Out Every Dollar
Intelligent controls could cut costs and improve dewatering performance at U.S. wastewater treatment plants — but they will need to overcome heavy inertia and skepticism.
By William Toffey
Intelligent controls could cut costs and improve dewatering performance at U.S. wastewater treatment plants — but they will need to overcome heavy inertia and skepticism
Bringing approaches common to the Internet of Things (IoT) such as sensors and automatic controls to the handling of municipal wastewater solids has the potential to provide significant cost savings as well as efficiencies in energy, chemical and labor inputs. An estimated cost of solids handling in the U.S., using 7 million dry tons at US$250 per ton, suggests that a reasonable cost savings (i.e., at least 15 percent budget savings) from automation and optimization of solids dewatering could yield $200 million in savings annually to public agency operations.
Solids processing can account for up to 50 percent of an agency’s total operations and maintenance costs. Despite this, until now adoption of automated technologies has been concentrated on the “liquid” side of operations. This has been an overall positive experience, which seems to bode well for the next step of using IoT solutions for solids handling for the approximately 1,500 significant wastewater treatment facilities (those with an average daily flow of over 1 MGD) in the U.S.
Past Failures Loom Large
A vital caveat is that, as it stands today, use of sensors and automatic feedforward/feedback controls in dewatering operations is very nearly absent in the U.S. — current interest among public agencies is stubbornly low. In part, this is because skepticism left over from disappointment in equipment introduced in the early 2000s continues to muffle interest. So far, the very few “wins” in this technology area have not yet moved the needle in the direction of general acceptance of IoT-based solutions.
Success breeds success: published reports of successful advanced control systems will stimulate professional interest. In the U.S., DC Water’s Cambi THP requires a sophisticated advanced control system that could become a reference for other plants with advanced stabilization technologies, and, similarly, NEW Water (Green Bay, Wis.) is nearing completion of a complex solids system with digestion, drying, incineration and energy recovery that will showcase a “mass solids” approach to treatment control.
But the most compelling driver is cost savings. Today’s sensors can detect excess polymer in centrate and variations in sludge feed, thereby enabling feedforward and feedback control on polymer dosage. With a typical cost of polymer at $50 per dry ton of solids dewatered, even a 10 percent reduction in polymer chemical purchases can provide a payback on sensor equipment purchases of shorter than one year in a medium-sized plant.
Opportunities in Solids Treatment Trains
The following tasks present examples where intelligent sensors would provide value.
Metering polymer to the liquid sludge feed: The objective of the control system is to maintain a steady ratio of polymer to the mass flow of solids to the dewatering equipment. A sensor that measures the total solids in the sludge feed provides a feedforward adjustment to the flow rate of mixed polymer. Some examples of this include Hach’s RTC-ST, and Valmet TS, which both offer measurement of feed solids.
Metering centrate: A second control for the polymer dosage can be set by metering the centrate (also referred to as filtrate or pressate), which is the mostly-water fraction separated from the biosolids in the production of cake. Overdosing of polymer can lead to the discharge of excess polymer in the centrate, and underdosing of polymer can lead to a high discharge of unflocculated sludge solids in the centrate. Valmet offers its Valmet LS solution for centrate measurement of solids, for example, while Real Tech’s prototype equipment is at the heart of current research by Professor Banu Örmeci of Carleton University, Canada, into polymer measurements.
Metering cake solids: A third measurement point is the cake itself. Real-time readings of percent total solids in the cake enable verification of the equipment performance against target concentrations, and provides accurate calculations of solids delivery to downstream disposal options. Thermal processes, both incinerators and thermal dryers, are sensitive to even small variations in solids concentrations in cake, as fuel requirements are in direct relationship to percent solids. Also, the cost of solids transport to disposal sites are in direct proportion to cake concentrations — a higher water content decreases cost-effectiveness.
Total System Control?
Placement of sensors on sludge feed, centrate return and cake discharge would give a complete picture of solids flows and provide a workable combination of signals for thorough feedback and feedforward control systems. No full-scale installation of this kind exists; however, Valmet offers a system that may meet these requirements, brought over from the heavily automated paper industry. The company has been engaged by NEW Water in Green Bay to pilot its offering. This could prove to be the catalyst for stimulating future interest among agencies, but it is as likely to be a one-off.
Optical analyzers use high-intensity infrared beams from the tip of light-emitting diodes (LEDs) and measure the light scattered back to fiber optic sensors encircling the LED. The German company BTG was the early developer, and Hach is the current principal manufacturer.
Microwave analyzers use the principle that the travel time of high-power microwaves through material shows a linear relationship to the proportion of water and dry solids in a material. The first sensors for wastewater solids were offered by Kajaani, then by Metso, and finally by Valmet, which has further developed and expanded its offerings.
Ultraviolet (UV) analyzers are based on the characteristic absorption of polymer at a specific UV wavelength, with the concentration of polymer in centrate in a linear relationship to the amount of absorption. A prototype UV analyzer has been developed by Real Tech for use in reference scale operations, with the support of Veolia.
Barriers to Adoption
Several aspects of solids treatment and disposal complicate the marketplace for sensors and automatic controls. The cost of a sale is high; competition is keen among technology companies when a large agency signals interest in exploring control systems, making for an expensive and risky sales cycle. The diffuse decision process involving installation of innovative technologies also makes it difficult to identify the ultimate customer.
There is also resistance to change among public wastewater operations staff, especially if new technologies are seen to threaten the need for staff.
Finally, cost factors that drive operational efficiencies in Europe are weak in the U.S., particularly for transporting and disposing of biosolids cake. Even within the treatment plant, the unit cost of labor, polymer and electricity is lower in the U.S. than European counterparts, making decisions on cost savings harder to support.
Biosolids dewatering is a component of public wastewater treatment that is wide open for optimization and significant money-saving efficiencies. However, control systems vendors face a skeptical market, based on a history of poor performance by other solutions in the past.
Today’s opportunities for companies offering IoT for solids processes will be with service companies and private utilities working under performance contracts to the public sector for dewatering services. For public agencies operating dewatering systems, efficiencies will be gained first with an investment in staff training and capacity building, where operators will learn to effectively tap into information provided by existing meters, scales and lab tests and where they will connect their systems back to wet-side processes and forward to users of the biosolids.
When those efficiencies have been realized, then the tools of IoT can take agencies to the next step of efficiencies and savings. WW
William Toffey is a Technology Assessment Group Expert with BlueTech Research. A new BluePrint report, Opportunities for IOT in Sludge Dewatering, is now available to clients of BlueTech Research. To learn more, visit bluetechresearch.com.
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