Save money while saving the planet: New wastewater equipment and control technology
Wastewater treatment plant renewal and replacement projects create fantastic opportunities to improve process performance, reduce nutrients and save energy. With the right approach, cost savings can be a secondary benefit.
Wastewater treatment is a dirty business and mechanical equipment used to convert this unsavory pollutant into a beneficial resource takes a beating 24-7-365. Even the best maintained pumps, blowers, mixers and controllers have a useful life of 15-20 years before wholesale replacement is required. This means that most process equipment being replaced in 2017 was probably installed in the late 1990s.
Wastewater treatment technology has improved more than a little over the past 20 years. Process equipment and control capabilities have been revolutionized and costs have come down for many components. Environmental engineers need to look at R&R projects as cost saving, performance enhancing opportunities rather than expensive, budgetary nightmares.
Most wastewater treatment facilities constructed in the mid to late 90s were designed using technology from the 80s. Fixed speed turbine pumps, centrifugal blowers, coarse bubble diffusers, surface aerators and analog controls were all the rage. There were few requirements for energy conservation, enhanced nutrient removal or public access reclaimed water production.
Fast forward 20 years and the landscape has changed, dramatically. The good news is that most treatment plant structures are designed to last for 50+ years and process basins are often oversized by risk averse design engineers. This leaves a blank canvas for designers to create new works of process art with our favorite medium, raw sewage.
Generally, treatment process design has evolved and the focus has changed from TSS and BOD reduction to advanced nutrient reduction and beneficial utilization of effluent in the form of public access reclaimed water. In some cases this can cause burdensome regulatory mandates, but can also offer opportunities to upgrade equipment and processes that produce a superior product and reduce energy consumption.
As an example, we can look at a 1.0 mgd conventional activated sludge oxidation ditch with flow equalization, secondary clarifiers and traveling bridge sand filters, followed by a chlorine contact chamber. This plant was built in 1997 and has: one manual bar rack, no grit removal, fixed speed centrifugal blowers for the equalization basin and fixed speed surface aerators in the ditch followed by traveling ridge sand filters and chlorine contact.
After 20 years of reliable service, one could elect to replace the blowers, aerators, pumps, motors and filter media with the same equipment and material that it was originally outfitted with, or look at alternatives. These alternatives could dramatically improve plant performance and capabilities while reducing long term energy and maintenance costs.
Some design changes might include installing self-cleaning, fine screens and high efficiency, centrifugal grit removal at the headworks to reduce future grit removal expenses and ragging issues in downline pumps and equipment. Submersible mixers in the EQ basin could replace the mechanical blowers to create an anoxic selector, which will dramatically reduce electrical expenses and provide phosphorous reduction and denitrification. The surface aerators can be replaced, or modified, to include variable frequency drives and lower impellors that sustain channel velocities and mixing at lower speeds. Variable frequency drives, dissolved oxygen and ORP analyzers with programmable logic controllers can be used to adjust aerator speeds create post-anoxic zones within the ditches, further reducing nitrogen and reducing energy costs.
This short list of alternatives does not even begin to address the changes that can be made in the clarifiers, think improved collection mechanisms and weir covers to reduce risk and maintenance. New filter media and backwash technology enables replacement of porous plates and sand to double, or triple filter capacity while improving TSS removal and turbidity reduction. We haven’t even started to talk about alternative disinfection methods.
For us wastewater wonks, new equipment and control technology is awesome. Who would have thought that treating sewage could be so much fun?
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