Effect of Settle Time onMixed Liquor Volatile Suspended Solids Concentration andBiochemical Oxygen Demand Removal EfficiencyAbstract:Introduction:Procedure:Phase 1: Cell Growth.Phase 2: Cell DepletionResults and Discussion:Sources of Error and Solutions:Conclusions:Suggestions:Effect of Settle Time on Mixed Liquor Volatile Suspended Solids Concentration and Biochemical Oxygen Demand Removal EfficiencyReport Presented by:Melanie TanJohn KimJoe TamilioFinal Report for CEE 473Nutrient Removal ProjectMay 12, 2004Abstract:A model wastewater treatment plant was built and operated with the intent of finding theoptimum Mixed Liquor Volatile Suspended Solids (MLVSS) concentration for removal of bio-chemical oxygen demand through aerobic bacterial assimilation and the system settle timecorresponding to that optimum concentration. The test sequence included decreasing the settlingtime of the model treatment plant thus decreasing the concentration of MLVSS remaining forbiodegradation. This report details the results of the tests as well as an analysis of these results.Further research suggestions and methods for improvement of this experiment are also included. Introduction:The initial design for the wastewater treatment facility model was derived from a simplificationof a Barnard Sequencing Batch Reactor. A basic plant operation outline was designed to simulate the continuous flow sequence of atypical treatment facility. Previous Research:Research to optimize BOD removal via manipulation of individual process durations has beenperformed by varying the hydraulic retention time (HRT or θ) of model wastewater treatmentfacilities.From research done by Nanyang Technological University in Singapore, a correlation betweenthe effects of hydraulic retention time on the stability of aerobically grown microbial granulesrevealed that for a HRT between two and twelve hours, aerobic granules become stabilized withgood MLVSS retention and high volumetric chemical oxygen demand (COD) removal (2). Theapplication of these results will be discussed later in the Procedure portion. Nanyang’s research sets rough guidelines for the set up of the treatment plant based on severalsteps. The first step is design and construction of a small-scale version of the Barnard BatchSequencing Reactor. The second step is to find an appropriate solids retention time. This wouldinclude the selection of an aeration cycle period, the steady decrease of the settle time and thusthe corresponding decrease in MLVSS concentration. Finally data analysis is performed toobtain the peak oxygen uptake rate. The concentration corresponding to this uptake rate shouldbe the optimum MLVSS concentration. Current plants operate at approximately 6 hour HRTsand MLVSS concentrations of 3000 mg/ L. The recycle concentrations are typically on the orderof 10,000 mg/L. Application of Existing Knowledge:To find the optimum MLVSS point via BOD removal a range of settle times were to beattempted. The system was designed to operate at a total system hydraulic residence time ofapproximately six hours, and an optimum MLVSS concentration of 3000 mg/ L. Procedure:Necessary data acquisition included dissolved oxygen concentration, elapsed time, current plantstate and volatile suspended solids (VSS) concentration. With the exception of VSSconcentrations all data was gathered continuously using LabVIEW software. VSS measurementswere obtained manually. The plant was modeled after a batch-sequencing model. Treatment of water was done insequential steps, the first step being the addition of the waste, followed by aeration, then settling,and then emptying of the tank to a level that would allow for a sufficient concentration ofMLVSS to remain in the tank. Settling time was the parameter varied, keeping the effluent wastelevel constant. This allowed for varying of the amount of recycled solids since the MLVSSretained is theoretically a function of the settle time. A 5 L Rubbermaid tank was used as thereservoir, with a pressure sensor inserted at the base outlet of the tank to monitor the water level.A peristaltic pump was used to fill the tank with water and waste. Whatman GF filters were usedto sample the MLVSS and a clamped glass filter connected to a vacuum source was used toprepare the MLVSS sample. A stone diffuser was used to introduce air into the tank for theaeration step and a 1 L pressurized plastic bottle couple with an airflow regulator was used toachieve constant dissolution of oxygen into the tank. Synthetic waste consisted of 3 stocks; 2were added via mixing with the source water, the last was refrigerated and pumped into the tankbefore the start of each cycle. Once the system for the plant was established, steps listed below were used for its operation:StepsPhase 1: Cell Growth.It was attempted to grow a colony of waste activated sludge that had been selectively adapted toefficiently consume the waste used. Waste was to be added at the beginning of each cycle with along settle time and short aeration time to maximize the bacterial growth rate. However, due toerrors in waste addition, a non-uniform waste concentration was added each time. Phase 2: Cell DepletionOnce the maximum concentration from the system setup was attained the settle time wasdecreased to waste a larger portion of the cells. Settle time was incrementally lowered from 7200seconds to 60 seconds and sample measurements were taken to obtain the MLVSS during the testsequence. The MLVSS was measured using the method outlined inhttp://ceeserver.cee.cornell.edu/mw24/cee453/NRP/Suspended%20Solids.htm. Dissolvedoxygen concentration was also measured throughout each test to determine oxygen uptake rate.After settling, 1.2 L of the reservoir water was retained, with the remaining 2.8 L decanted. Thisincludes the wasting of the unsettled solids that remain at a level above the 1.2 L line. After establishing the parameters to be measured, the next major procedure of the lab was thecalibration of the instruments - mainly the calibration and usage of the dissolved oxygen probe. The dissolved oxygen probe uses a permeable membrane and an electrolytic solution to measurethe rate at which oxygen diffuses across the membrane. This rate of diffusion and the differencein the