by Bob Dabkowski
Wastewater treatment plants today require optimization and changes that not only make effluent cleaner, but also less expensive to produce. As a result, more facilities are determining that the use of on-line water quality instrumentation can help improve effluent quality and plant performance while significantly reducing power consumption, solids handling costs and other expenditures.
On-line water quality instrumentation at wastewater treatment plants is being used increasingly for process control - not only to provide operators with critical, real-time information to make informed process adjustments, but also for automating critical plant processes to achieve improved effluent quality, optimize plant performance, and significantly reduce costs. As on-line technologies and analytical systems have become more advanced, accurate and reliable, plant operators have become confident in their use of automated process control to achieve what is increasingly being termed as “a better economical effluent.”
Power Reduction
Power costs are by far the largest single consumable expense for conventional wastewater treatment plants and, according to the Water Environment Federation, up to 70 percent of a plant’s budget is allocated to energy just for aeration. Many plants are today successfully reducing these costs while improving microbial efficiency by using advanced on-line sensors to automate, and optimize, the aeration phase of secondary treatment.
The continuous, real-time data from these new sensors installed in a plant’s aeration basins allow for reliable and accurate on-line dissolved oxygen (DO) control. Although dissolved oxygen sensors have been available for decades, it hasn’t been until recent advances in digital sensing technologies that long-term, continuous and reliable DO monitoring could be realistically achieved for many plants.
Many treatment facilities have been designed to provide some level of aeration control, but these systems typically aerate the entire basin uniformly. New digital on-line sensing technologies are now giving plants the ability to perform “tapered aeration,” thereby further reducing power costs and improving effluent quality. With tapered aeration, plants can precisely control the quantities of air into different parts of an aeration basin based on the current dissolved oxygen levels in these areas, and in approximate proportion to the oxygen demand of the mixed liquor under aeration.
Reducing the power costs associated with ultraviolet (UV) disinfection can also now be achieved using new digital on-line sensing technologies. For disinfection, many plants have stopped using chlorine and have adopted UV disinfection, which entails the use of ultraviolet light to destroy target organisms. UV light must hit the organism in order to kill it, and this is dependent upon water turbidity because, if a particle is in the way, the particle and not the organism will intercept the light. In addition to turbidity, if the water contains either suspended or dissolved particles that absorb light at a wavelength of 254 nanometers (the wavelength of UV light), the particles will absorb the UV light and prevent it from hitting the target organisms.
Until recently, controlling UV operation had been based on flow alone, but on-line turbidity sensors combined with new ultraviolet absorbance sensors can now automatically control the power applied to UV systems, thereby optimizing UV disinfection while eliminating unnecessary power consumption. The sensor readings are used to automatically raise and lower the wattage dose that the UV system applies based on the current demand. This not only can potentially reduce power costs significantly, it also potentially allows for the extended life of expensive UV lamps.
Optimizing Solids Handling
After aeration, solids handling typically generates the next highest cost at wastewater treatment plants. Here, advanced on-line instrumentation and analytical systems are being increasingly used in ways that significantly reduce costs, including automating sludge age control for precise wasting. Continuously monitoring suspended solids throughout a plant’s solids handling train can help optimize solids handling operations by providing a continuous feed of solids (controlling load) going into digesters, thus reducing costly upsets and bottlenecks. In addition, today’s more accurate and rugged turbidity and suspended solids sensors are monitoring real-time solids loadings in dissolved air flotation thickeners to more precisely control polymer dosing during sludge thickening operations, thereby reducing polymer consumption by 20 percent and more.
Conclusion
Today, the potential savings in energy, consumables and manpower requirements make the use of proper on-line instrumentation and analytical systems economically worthwhile. In view of the subsequent advances in process measurement technology, the increased costs of energy and solids handling and disposal, and the stricter demands on the operation of wastewater treatment plants, proper on-line instrumentation and analytical systems can help facilities meet the essential demand for producing better, more economical effluent.
About The Author:
Bob Dabkowski is the Wastewater Specialist for Hach Company, and is a Licensed Wastewater Operator in the State of Colorado. Hach is a member company of the Water & Wastewater Equipment Manufacturers Association.
