Approaches for Improving WRRF Treatment Efficiency: All These Acronyms; How Do You Spell Relief?
By David Cohen
This article references recent developments in water testing which allow for rapid, on-site determinations of cATP; cellular ATP and dissolved organic carbon (DOC). These testing protocols can be implemented to provide substantial cost savings and enhanced understanding of the biological processes in traditional municipal and industrial wastewater treatment plants.
Let’s face it, the writing is on the wall. Water reclamation is the new lithium-ion battery. Orange County California has had water reclamation projects online for 20 years. The effluent of these plants discharges into groundwater recharge areas of confined aquifers and, more recently, into the recharge areas of unconfined aquifers. Currently, there are demonstration plants where both direct water reuse (in which water is fed into advanced potable water plants) and indirect water reuse (in which water is fed into aquifers) has proven effective and safe. In fact, astronauts have reused their wastewater since the 1960s. Australia practices resource mining by tapping into conventional sewer lines in the basement of facilities and providing industrial and even potable water for use within the same facility.
The technology required for effective and safe water reuse is available now; essentially off-the-shelf treatment equipment, sensors and monitoring systems are available and ensure that reclaimed water is as pure or purer than tap water. To facilitate the acceptance of water reclamation and reuse by the general population, however, our focus as water treatment professionals should be to understand exactly what is going on in the processes and effluent of our wastewater treatment plants.
The activated sludge process is a principal method used in water resource recovery facilities (WRRFs)—formally known as wastewater treatment plants (WWTPs) or publicly owned treatment works (POTWs)—to concentrate biomass, which naturally degrades the carbon-bearing substances found in most industrial and municipal wastewater treatment systems. Traditional municipal wastewater treatment plants rely on conventional tests, such as TSS, BOD, chemical oxygen demand (COD) and mixed liquor volatile suspended solids (MLVSS), to calculate the food-to-mass ratio, which is also known as the food-to-microorganism ratio (F:M). Traditionally, efficient plants usually remain between 0.2 to 0.6 F:M and an active biomass ratio (ABR) of 40 percent. Recently, the concept of ABR has been used to provide a more useful tool in measuring treatment-plant performance.
Although useful, these traditional metrics describe much more than just the living biomass and include variables like dead biomass and inert material. Moreover, the protocol for taking these measurements does not accurately or quickly reflect process changes that materially alter inventory control and subsequent process optimization.
Solids inventory optimization leveraging ATP monitoring is becoming a key leading indicator in realizing peak process efficiency. Optimization occurs through continuously monitoring ABR, which denotes the percentage of total solids that are biologically active. Traditionally difficult to calculate, ABR can now be calculated in minutes with field monitoring tools.
Why is it important to accurately measure F:M and ABR? Volumetric plant efficiency, energy reduction, equipment maintenance requirements, process cost control and negating the need for expensive plant expansion are primary drivers. Chemical costs, operator intervention costs, sludge drying costs and sludge pumping costs—such as recycling and disposal costs—all materially affect plant operational costs.
Understanding the make-up of the biomass by identifying live biomass from dead biomass and undigested food is critical in determining biomass concentration. Common practice is to err on the side of high biomass. This practice is a generally accepted principal of operation since additional biomass, in theory, equates to additional capacity to degrade incoming wastewater; however, the following performance issues are of significant consequence with this approach:
• Sluggish digestion: Microorganisms can perform at less-than-peak efficiency. This can occur if the plant has accumulated too large of a biomass population or if the plant experiences low flows and/or a drop in available food. More time for digestion means slower throughput and may require additional unnecessary plant resources as influent flows increase.
• Poor settling: High total solids inventory promotes microorganism competition and results in growth advantages for filamentous microorganisms, which are costly to deal with.
• Sub-optimal mass transfer: An increased non-viable solids concentration creates additional resistance to mass transfer during aeration, decreasing the efficiency of blower or other oxygen-generating and transfer equipment and increased energy costs, presenting a case for increased blower capacity.
• Increased pumping: Higher solids recirculation flows (sludge recycle) creates additional wear on pumps, requires more dewatering, more energy and disposal costs.
• Aerobic digestion: High non-viable biomass concentration increases aerobic digestion of dead biomass, resulting in difficult to degrade wastewater constituents. Digestion of old biomass consumes costly process materials, such as oxygen, treatment chemistries, supplemental nutrients and energy.
New technology, new guidance
Recent technological advances have facilitated a new approach for determining F:M and ABR by measuring dissolved organic carbon (DOC) and cATP. MLVSS doesn’t take into account the distinction between active biomass and non-cellular organic matter or unhealthy biomass and dead biomass. Mixed liquid suspended solids (MLSS) doesn’t distinguish between biomass and other particulate matter.
By using cATP, a system’s active biomass can be accurately quantified. Utilizing on-site, bench-top instrumentation, cATP can now be easily measured in minutes to aid in plant optimization. On-line instrumentation can measure DOC as a function of the F:M ratio, generating prediction curves that can have a confidence interval of 95 percent without typical interferences with other measurement techniques. Bioaugmentation of process sectors with genetically selected bacteria strains and/or nutrient supplements can be used and tested to optimize digestive biomes.
Best-in-class plant management goals should include reducing total solids inventory while maintaining the same active biomass population of a specific species. A comprehensive optimization project should include the daily collection and analysis of MLSS, cATP and DOC at all return activated sludge (RAS) and waste activated sludge (WAS) process locations.
Each biological water resource reclamation facility is unique, especially if the plant utilizes bioreactor employing membranes, surface-enhancing media like bio-balls and growth mats and/or any other state-of-the-art treatment systems. It is best to establish a baseline of process measurements with a minimum of 20 samples taken over a three to four week period of typical operations. The optimization process should be initiated when the influent wastewater composition and flow are relatively stable. This ensures that process tweaking and process responses are associated with optimization and not influent changes.
Slow and methodical
Operators can optimize the total solids inventory target over a few days by using small step changes and observing the biological response. Each system is different and adjustments to various process flows, chemistries and nutrient additions will affect system optimization, depending on equipment OEM recommendations. Implementing the adjustments described below to specific treatment processes should guide operators’ efforts, however.
In a traditional plant, the primary method of reducing the total solids inventory is performed by adjusting WAS and RAS flowrates. The inventory level is gradually decreased from the baseline levels by temporarily increasing the WAS rate, while simultaneously decreasing the RAS rate in order to purge the system of solids. Each change is sustained for a few days while monitoring the cATP, MLSS and other measurements as outlined above. Adjustments to the WAS and RAS rates will remove active biomass, dead biomass and non-biological solids; however, only the active biomass will grow back.
It is recommended that step changes of 10 percent of the total bioreactor inventory should be made during each purge. This level is significant enough to promote a reliable change, yet small enough to ensure that treatment capacity is not adversely affected. During this process, it is imperative to balance the RAS and WAS flowrates to prevent degradation of the sludge blanket in the secondary clarifier or the equivalent process sector in an advanced treatment facility. A significant degradation of the sludge blanket could result in flocculant carryover in the final effluent. In addition, as the WAS rate is increased and RAS rate is incrementally decreased during its strategic wasting period, biostimulants can be added to boost the performance of the active biomass population to mitigate drops in biological effectiveness while the active biomass levels rebound. Effective biostimulant addition would decrease the excursions and level out cATP levels, as shown below.
After each purge, the cATP should be closely monitored. It should gradually increase and eventually return to close to the original baseline. If the population does not recover or if process performance degrades, the flows should be adjusted to return to the previous solids inventory step level and readjusted to a new control target. The optimization program is complete when either: a) the cATP does not recover, whereby adjustments are made to return to the previous solids inventory level or b) an acceptable ABR is achieved with a measurable decrease in costs of moving and treating dead biomasses.
The entire process should continue to be monitored and scrutinized for at least two weeks and testing for COD, BOD, phosphorus (P), ammonia (NH3), MLSS and additional parameters to obtain a complete profile of the process change effects. It is important to monitor and maintain the new inventory at a stable level during this time to allow for sufficient adaptation and stabilization to be accurately reflected in cATP measurements.
Operators can further optimize the overall process by adjusting dissolved oxygen (DO) targets using a similar stepped reduction of blower operation and/or reduction in oxidation chemistries. Next, simple microscopic observations of the biome at various points of the system will yield valuable information about the specific bacteria and higher organisms within the treatment plant. Nutrient, enzyme and specific biological enhancement seeding may be considered to further optimize overall plant efficiency. Additional optimization of chemical additions, such as polymers, pH adjustment chemicals, adsorbents like powdered activated carbon (PAC) and other consumables, may be considered.
When adjusting consumable product dose rate(s), a cautious step reduction of one to three percent is advised over a 10-percent step reduction. Aggressive plant optimization efforts at the expense of unplanned plant upsets must be avoided at all costs. When optimization efforts are successful, even a 10 percent reduction in solids and a one- to two-percent reduction in energy and/or consumables will qualify as a major system improvement and result in significant operational cost savings over time.
(1) Jason Curl, et al. Solving Future Water Challenges: Trends in Water Reuse. Journal AWWA. v.111, No.8
(2) Application Note Biomass Inventory Management Using QG21W. Luminultra. June 2018. http://2018.www.luminultra.com/
(3) Case Study: Solids Inventory Optimization. Luminultra. September 2017. http://2017.www.luminultra.com/
(4) Dave Mason. Precise Count New Instrumentation Improves Measurement. WE&T. October2019
About the author
David Cohen is a Senior Water Treatment Engineer for ATS Innova, which provides clean water solutions for municipal, industrial and petrochemical markets. His areas of expertise include membrane system applications, construction management and utility services’ design. Cohen can be reached at (801) 255-5336 and David.Cohen@ATSInnova.com. ATS Innova; Improving LIFE One Drop at a Time. www.ATSSmartSolutions.com
About the company
ATS works with water professionals to help solve some of the toughest clean-water issues in America and beyond. For 40 years, the company has offered a wide variety of proprietary and advanced specialty chemicals that have given public water works and water treatment plants remarkable results. ATS empowers its clients with in-depth knowledge and the most innovative products on the market.