Tapered vs. Uniform Baffle Configuration
Abstract
The vertical flow hydraulic flocculator has been used to test tapered and uniform baffle configurations during previous semesters, but the two configurations have never been compared with similar raw water conditions. For this reason the Fall 2008 Pilot Plant Flocculation team focused on determining which flocculation configuration is optimal. The tapered flocculation configuration model is based on the G-theta Model, while uniform configuration is based on the energy dissipation model. In our final results, the tapered baffle configuration produced marginally better turbidity than the uniform baffle configuration. However, due to low temperature conditions, the incoming turbidity was too low in order to produce conclusive results.
Introduction and Objectives
Methods
Results
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.