Process instrumentation and overall solids management are two often overlooked, yet incredibly important components of wastewater treatment operations in small to medium size wastewater treatment facilities. Here are a few examples of the ways in which effective process instrumentation and management of the downstream solids process can greatly improve your facility operations.


Process instrumentation and flow measurement throughout your plant is essential and allows your team to plan for operational changes in the treatment process (this includes the influent flow rate, recycle flow rate, and effluent flow rate). For example, if your facility is experiencing an increase in flow rate, it may be advantageous to put an additional clarifier or aeration basin online to handle the increased flows. 

Process instrumentation can also enhance the activated sludge treatment process. Having real-time total solids concentration and dissolved oxygen levels is critical to making operational adjustments. If the solids concentration is too low, sludge wasting can be reduced; if it is too high, wasting can be increased. 

Solids concentration in the treatment process is closely related to the settleability of sludge, how well the sludge compacts at the bottom of the final clarifiers, and the degree to which nitrification can take place in colder weather. Achieving the target dissolved oxygen concentration in the activated sludge process is critical to the health of microorganisms and necessary for the selection of bugs needed for treatment of different pollutants (carbonaceous biochemical oxygen demand (BOD5), nitrification and denitrification).



Effective process instrumentation also allows for better management of the downstream solids process. Knowing the sludge level in your final clarifiers and holding tanks is essential to making sure denitrification does not take place within the sludge blanket with the associated release of solids. There are several instrument types that can indicate the sludge level in a tank, and your team can use this information to pump more or less sludge to downstream batch thickening or dewatering processes.

An effective sludge management program can reduce labor and final disposal costs, and return valuable nutrients to our environment.

  • The first consideration of a sludge management program is realizing that the upstream liquids treatment process can impact the characteristics of sludge. For example, if biological phosphorus removal is used, sludge can be more difficult to dewater based on the bound water contained in the floc structure of the biomass. Research is being performed based on the Divalent Cation Bridging Theory to evaluate the hypothesis that phosphate release in digesters sequesters divalent cations that would normally be available to bridge negatively charged functional groups and displace water bound within the floc structure, which improves dewaterability.

  • Second, your team must decide whether the end-product will be liquid or solids cake. This decision will depend on the size of your treatment facility, the ability to store and handle cake, capability of plant staff to perform operation and maintenance (O&M) of the dewatering equipment, and the availability and proximity of farmland for final disposal. In general, for larger plants above 4 MGD, it is more economical to have an end-product of cake.


There are various ways to thicken sludge after it is wasted from the activated sludge process. These include:

  • Dissolved air flotation

  • Gravity belt thickeners

  • Gravity thickeners

  • Centrifuges

Centrifuges pc:

Centrifuges pc:

Your equipment needs will change based on your WWTF size, ability to conduct O&M, and building space. For example, centrifuges generally are more economical for larger WWTFs with an average daily flow greater than 4 MGD. For smaller WWTFs with limited maintenance staff and building space, a gravity thickener may be the best choice.

If your desired end product is cake, there are various ways to dewater sludge using a gravity belt press, centrifuge, screw press, or rotary press. Each type of equipment has advantages and disadvantages. For example:

  • A centrifuge uses more energy than other types of equipment but can achieve greater sludge solids content. 

  • A screw press has less set up and clean-up time after the batch operation than the rest of the equipment, but the amount of polymer needed is slightly greater. In addition, the throughput rate of the screw press is less than the other equipment. 


To learn more about the importance of process instrumentation and solids management, don’t miss our free seminar on May 22, 2019. Register here.

About the Author

David W. Arnott

David W. Arnott, P.E.
Team Leader/Senior Project Manager

Dave has extensive experience in the planning, design, and construction project management of wastewater treatment facility renovation and upgrade projects. Areas of expertise include treatment processes, hydraulics, mechanical equipment, and wastewater disinfection. He also has experience in sanitary and water pumping station design and sanitary sewer wet weather facilities.

More Recent Articles