October 2010 NEWS www.cenews.com cefor the business of civil engineering PROGRESSIVE ENGINEERING Cutting-edge wastewater treatment Optimizing clarifier operation with computational fluid dynamics modeling By Alonso Griborio, Ph.D., P.E., and Paul Pitt, Ph.D., P.E. Full-plant simulators are limited in their ability to simulate accurately the performance of clarifiers, which play a significant role in the wastewater treatment process. PROGRESSIVE ENGINEERING s the population contin- ties meet these standards and increase separation of solid materials from the ues to grow, so too does capacity without unnecessary capital liquid stream through particle settling. the waste stream. At the investment. Clarifiers are commonly used in two A same time, government Full-plant simulation models are places in most municipal wastewater regulations regarding the removal of powerful tools for evaluating, sizing, treatment plants: nutrients (typically nitrogen and phos- and optimizing wastewater treatment 1) Wastewater first enters primary phorus) from treated wastewater that facilities. These simulators incorpo- clarification, where influent solids is discharged back into the environ- rate the latest advances in technical settle to the bottom of the tank and ment grow ever more stringent. This knowledge and can reliably predict are typically pumped to the solids- confluence leaves many treatment performance if properly calibrated. handling process. Meanwhile, fats, facilities facing significant capital Full-plant simulators do have limita- oils, and greases rise to the top and costs as they look to expand capacity tions, however, most notably in their are skimmed off and removed. and upgrade treatment processes. Op- ability to simulate accurately the 2) The wastewater then undergoes timization of the treatment process — performance of clarifiers, which play secondary treatment, during which making relatively small modifications a significant role in the wastewater micro-organisms consume much of to equipment and operating strategies treatment process. the soluble and particulate organic rather than expanding the physical Clarifiers, also known as settling pollutant material such as sugars, re- structure of a plant — can help facili- and sedimentation tanks, provide maining fats, and organic shortchain CFD modeling protects public health and saves money In a recent project, operators of a 15 million-gallon-per-day feed of wastewater. The proposed third clarifier could meet (mgd) activated sludge plant planned to upgrade to an enhanced effluent standards, but only with additional chemical treatment nutrient removal facility for phosphorus and nitrogen removal, (polymer addition). while at the same time expanding from a peak flow capacity The CFD software was also used to determine the impacts of 18 mgd to 40 mgd. Among the planned enhancements was of minor modifications to the existing two clarifiers. The the addition of step feed to the aerobic cell just upstream of the simulations predicted greatly enhanced clarifier performance post-anoxic zone to treat wet-weather flows. with modifications to the existing clarifier center wells Prior to design of the upgrade/expansion, the 2Dc CFD model and baffles. Figure 1 illustrates the dramatic difference in was used to simulate the effects of increased hydraulic flows performance of a 130-foot clarifier given the addition of step and solids loading rate on the two existing secondary clarifiers feed and some modifications to the clarifiers. and a third proposed clarifier. A simulation of the largest wet- In this instance, the 2Dc model identified enhanced weather event for which records were available was developed clarifier performance with step feed, chemical addition, using BioWin (a full-plant simulator) to determine the loading to and modifications to the existing clarifier center wells and the secondary clarifiers. baffles. These comparatively minor improvements enabled The first set of simulations demonstrated that the existing the wastewater treatment plant to defer construction of clarifiers would not meet current or future effluent standards additional clarifier capacity, saving approximately $8 million in during wet-weather conditions without the proposed step construction costs. Figure 1: Slight modifications to operating procedure and the clarifier’s center wells and baffles can achieve a dramatic difference in water quality. In the modified tank (right), the higher concentrations of solids (mg/L) settle better, producing much higher-quality water — effluent suspended solids concentration decreases from greater than 150 mg/L to less than 15 mg/L during peak wet weather flows. carbon molecules. These or- drodynamics, turbulence, floccula- accurate simulation. ganisms then bind together, or tion (ability of suspended solids to CFD is an advanced technique flocculate, becoming heavier coagulate), the settling properties of that can be used in clarifier design, and separating from the treated the floc, and solids rheology (solids troubleshooting, and optimization. wastewater by gravity sedimenta- flow characteristics) all have an im- It uses mathematical methods — tion in secondary clarifiers. pact, as do atmospheric conditions, and millions of calculations — to Secondary clarifier performance tank geometry, internal features, analyze systems that involve fluid is vital for efficient wastewater and loading conditions. Current motion, mass transfer, heat transfer, treatment because it is often the full-plant simulators normally use and their associated phenomena. final barrier to organic and solid one-dimensional clarifier models The primary limitation of 2D and material removal. Poor performance that cannot account for all these 3D CFD modeling is that its com- will lead to effluent quality viola- processes and factors and, therefore, putational time is very demanding. tions and a gradual decrease in do not offer an accurate simulation Additionally, the biological reac- process capacity. of clarifier performance. However, tions within — and outside — the Clarifier performance depends 2D and 3D computational fluid dy- clarifiers are normally not included on several interrelated factors. Hy- namics (CFD) can and do provide in the model. This is an important In primary clarification, solids settle to the bottom of the tank while fats, oils, and greases rise to the top. Prior to this step, the wastewater has only passed through a screen to remove larger solids and floating debris. facet of treatment process optimiza- The 2Dc model capitalizes on the combinations of physical geometries tion that must be performed using a exponential increase in the speed and (retrofits with baffles, modification to full-plant model. memory of computers and the ad- inlet conditions, et cetera) and hydro- vancement in our understanding of the dynamics within a clarifier, simulating 2Dc CFD clarifier model physical processes in clarifiers to offer a full spectrum of possible loading and One of the state-of-the-art tools users unprecedented insight. We can atmospheric conditions and comparing in CFD clarifier modeling is the 2Dc now accurately model crucial perfor- the results against current and potential model, developed at the University of mance measures such as discrete, zone, effluent standards. These simulations New Orleans under the supervision and compression settling; flocculation; readily identify needed improvements of Professor J. Alex McCorquodale, non-Newtonian flow; and floatable to existing clarifier infrastructure and Ph.D. He is the North American particles; in addition to variable inter- operating strategies, ensuring im- pioneer in CFD modeling of clarifiers, nal tank options including skirts and proved reliability in meeting stringent and has more than 35 years of experi- baffles. effluent requirements and reducing ence in development, testing, and ap- Users can quickly and cost-effec- operational and capital costs through plication of these types of models. tively model any number of different optimization of existing facilities. By secondary clarification, the wastewater has been cleaned of most solids, and micro-organisms have consumed much of the organic matter. Steps to remove nutrients such as nitrogen and phosphorus have also been performed. This cleaner “effluent” again goes through a separation process, allowing remaining solids to settle to the bottom of the tank. PROGRESSIVE ENGINEERING Calibrating a CFD model monitored until performance reaches to increase, regulations will con- As with any model, CFD must be a determined “failure” point, such as tinue to tighten, and more challeng- used cautiously and with a good un- high effluent total suspended solids ing contaminants will be identified derstanding of the processes and fac- (TSS) or high sludge blanket level, at in the waste stream. Fortunately, our tors that affect clarifier performance. which point the stress testing is ended. understanding of how to be most ef- Model calibration and verification A properly calibrated CFD model ficient and effective in the treatment is crucial to model accuracy and the provides valuable results in just a few of wastewater will also continue to credibility of the model output. days, rather than the weeks it often advance. The data needed for calibration takes to construct physical models. Treatment plant owners and op- and validation of a secondary clarifier The 2Dc model produces clear visual erators already have powerful tools CFD model includes the following: representations of fluid flow,