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Results
Several tests have been run with the uniform baffle configuration. The initial results were inaccurate due to a clog in the inlet valve of the raw water sampling line. The following graph is the turbidity profile obtained on 10/20/08.
The team had difficulty finding the correct alum dose at low temperatures and incoming turbidity this semester. The team determined a relationship between the WFP's PAC (polyaluminum chloride) dosage and the alum dose at the plant. The composition of PAC is a trade secret and its chemical formula is unknown. The relationship was determined between the percent active ingredients in PAC and alum. At the plant 48.8% of 11.1 lb/gal alum is active and 33% of 10.84 lb/gal of PAC is active. Therefore 1 ppm of PAC should have the same effects on flocculation as 0.182 mg/L alum.
Once this relationship was determined, it was used to set the flocculation tank dosage. The resulting dosage was much lower than expected. At average incoming turbidity of 2.3 NTU it was determined that the optimal alum dose would be around 4 mg/L. As can be seen in the figure below this alum dose gave the best effluent quality, although the differences in NTU between 4 mg/L and 9 mg/L are very slight.
The configuration of the uniform and tapered flocculator was changed to agree with the energy dissipation model of flocculation. Previously the end of the tapered flocculator the baffle spacing was 15 cm, this resulted in a energy dissipation lower than the recommended value of 0.4 mW/kg. In order to achieve the recommended value of energy dissipation the spacing in the flocculator was changed to 13.2 cm. This value was calculated using the equation:
The spacing of the uniform flocculator was altered to match the last section of the tapered flocculator so that floc break up would not occur due to shear in the uniform flocculator.
A turbidity profile for the uniform and tapered flocculator was determined using this alum dose. The results are inconclusive- neither configuration produced an effluent with a lower turbidity than the raw water entering the flocculator. The flocculator was clearly working as can be seen by the increase in turbidity along the profile. Flocs that were too small to settle out but large enough increase the turbidity were formed. The tapered flocculator showed slightly better effluent quality than uniform.
Conclusions
The alum dose set based on plant PAC dosage gave the best results for an incoming turbidity of 2.3 NTU. If this relationship is confirmed to work at a broad range of incoming turbidities, it can hopefully be used by future semesters. The plant operators obviously have much more experience determining alum dosing and this relationship can help us take advantage of their expertise.The accuracy of this result is questionable. The tank had to be cleaned before the measurement could be taken. Cleaning may have increased turbidity in the last section of the flocculator. We observed an incoming turbidity of 1 NTU, which is comparable to the raw water that enters the CUWFP. Residual sludge iin the end of the tank could be contributing to the final turbidity creating some error. However a decrease in turbidity can been seen as water travels through the flocculator indicating more flocs are settling later in the flocculator.
The team will develop a more comprehensive profile for the uniform baffle configuration, sampling turbidity at more locations in the flocculator. A comprehensive turbidity profile must also be created for the tapered baffle configuration.Fall 2008Fall 2008