- Many smaller wastewater systems do not have to meet stringent nitrate or phosphate permits at their discharge. Instead, they mainly treat for BOD/COD, pathogen, and ammonia removal. Often this system is based on extended aeration activated sludge concepts designed to combine aerobic treatment and some sludge stabilization/digestion in the main wastewater treatment unit. Often the wasted sludge is treated in a small aerobic digester on-site.
With budgetary costs being a concern, I want to point out a potential way to solve some of the secondary floating sludge problems while also lowering utility costs associated with aeration.
When removing ammonia in aerobic activated sludge treatment systems, the final nitrogen form is nitrate (NO3). Many wastewater bacteria can utilize nitrate to remove BOD/COD when dissolved oxygen falls below measurable levels - this is also ORP or Redox below 0 mV.
Often we find this BOD (food) + nitrate with no dissolved oxygen - if we allow the secondary clarifier blanket to build (solids anoxic for over 2hours) with relatively warm temperatures. This results in floating sludge carried by very fine nitrogen gas bubbles. While more of a nuisance than critical problem, in summer conditions this may cause effluent TSS permit issues.
Cycling back the nitrate rich water with the return sludge line and mix with BOD rich influent water, allows existing biomass to degrade BOD while removing nitrate even without a true denitrification reactor. As most conventional municipal ASU facilities are plug flow in design, you can denitrify by reducing aeration in the first portion of the basin. This does not mean fully cut off aeration - as you need enough for mixing if there is not other mechanical mixing present. The reduced dissolved oxygen will drive flocculated bacteria to use nitrate as an alternative electron acceptor while treating soluble influent BOD. The result is a lower demand for added oxygen/aeration while concurrently removing nitrate/nitrite responsible for floating secondary clarifier solids.
Additionally floc forming bacteria are preferentially selected by influent anoxic zones over most filamentous (bulking) microbes. Therefore, the use of denitrification at the head of the aeration basin may also contribute to better SVI in the secondary clarifiers.
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
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