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CORNELL CEE 453 - Study Notes

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MethodsPlant Setup – ConstructionPlant Setup — Process ControllerSet PointResults and DiscussionSuggestions and CommentsBibliographyDesign and Operation of a Bench ScaleSemi-Continuous Waste Water TreatmentPlantCEE 453: Laboratory Experience in EnvironmentalEngineeringGroup 4Dan Fronhofer, Aliza Cohen, Dave Pomeranz5/11/04AbstractA bench-scale semi-continuous flow wastewater treatment plant was constructed in the lab using materials available in the Environmental Teaching Laboratory. Solids retention was achieved with 90% Volatile Suspended Solids (VSS) removal. This is comparable and competitive to other bench-scale treatment plants operating in a batch process. A separate clarifying tank and sludge recycle pump were added to make this process semi-continuous. The dimensions of the conical clarifier limited clarifier efficiency while software and mechanical limitations prohibited this process from achieving truly continuous flow. With a few modifications, bench-scale continuous flow reactors are entirely possible within the Environmental Teaching Laboratory. IntroductionMuch of this class was devoted to the construction of a small-scale wastewater treatment plant using a batch reactor process. The process was easy to apply to the available laboratory equipment and hardware and, on first sight, produced a reasonably clean effluent. However, on a larger scale, these batch reactors have many limitations that may deem them impractical. When large volumes of continuous flow must be treated (for example: the city of Ithaca’s wastewater), the batch reactor will not be able to process thewaste fast enough to be practical. Instead, a continuous flow reactor is ideal for this situation. In a continuous flow reactor, the wastewater input is pumped continuously in and out of the system. The flow of the waste through system and reactor tank volumes provide the required residence time for treatment, unlike a batch reactor where all processes occur in one reactor tank in sequence.Previous groups that attempted to create a continuous flow treatment plant in this class found difficulties in sludge movement and solids retention. A continuous flow wastewatertreatment plant also requires a number of additional reactors, pumps, and valves, all of which must be configured to work within the technical restraints of the Environmental Teaching Lab. The number of inputs and outputs required for a continuous flow treatmentplant exceeds the number of available data ports in the black control box. A conical plastic tank found in the next-door lab was used for the additional reactor (the clarifying tank) required for our process. The limitations of this tank imposed by its size, shape, andthe modifications made for its previous use(s), were ultimately critical in the performanceof our treatment system. Lastly, due to the limitations of the process controller (LabView software) and hardware, our system could never truly achieve “continuous flow.” Instead we did our best to approximate continuous flow by minimizing the amount of time allotted for each individual process as it cycled through a continuous loop.Our goal for this lab project was to create a functioning automated continuous flow wastewater treatment system to process the synthetic waste prepared for the class. Specifically we hoped to achieve solids retention in our reactor while producing a cleaneffluent (low in organic matter) that could compete with the ranks of the other batch reactors already running in the lab. VSS analysis of effluent and mixed reactor contents of our group along with two others from our lab provided us with a quantitative verification of our results. Methods Plant Setup – Construction The construction of the bench-scale semi-continuous flow reactor was modeled after the basic principles of a full-scale wastewater treatment plant (WWTP). Final plant layout is displayed in Figure 1. - Synthetic waste (at 20x concentration) is stored in a refrigerator and is pumped via a peristaltic pump into a 6 L Rubbermaid container, which serves as the activated sludge reactor. A single solenoid valve controls waste flow into the input line.- Water is also pumped via the peristaltic pump into the activated sludge reactor. The water is pumped through the same line as the synthetic waste so as to prevent microbial growth within the lines. The quantity of water added is such that it dilutes the synthetic waste to the desired 1x concentration. A single solenoid valve controls water flow into the input line.- The activated sludge reactor is continuously stirred by a stir bar and is also continuously aerated. Two automated solenoid valves on either side of an air accumulator (1 L plastic bottle) and a single manual needle valve control the aeration. - Mixed activated sludge is pumped from the Rubbermaid container to a 7-L Pyrex conical settling tank via a second line through the peristaltic pump. Volume is regulated in the activated sludge reactor by utilizing an elbow pipe in the reactor and different size tubing through the peristaltic pump. The elbow pipe is set with the open end at the desired liquid level in the reactor. Because these two lines runoff the same pump, the tubing size of the activated sludge to clarifier line is larger (size 18) than the synthetic waste and water feed line (size 16), which ensures thatthe liquid level in the reactor does not rise significantly above the elbow pipe.- The conical settling (clarifier) tank has three effluent ports, two at the bottom and one approximately 2 inches from the top. One bottom port is to the sludge recycle line, the other is to a manual valve sludge removal line, and the top port isto a clarified effluent drain. The clarified effluent flow is controlled by another solenoid valve. The outflow of the line which transports activated sludge from the reactor to the clarifier is placed at the very bottom of the clarifier, immersed insettled solids. This is done so that when activated sludge is added, solids are mixed throughout the conical clarifier via turbulence to promote flocculation and settling.- The sludge recycle is pumped from the clarifier back into the activated sludge reactor via a centrifugal pump controlled by a power switch and a single solenoid valve.- The solenoid valves, the peristaltic pump, the pressure sensor for the aeration accumulator, and the centrifugal pump power switch are all connected to ports on the black control box.


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