Application of Coagulation/Flocculation followed by Sedimentation for Treating Urban Combined Sewer Overflows

  • Author / Creator
    Shaikh, Mohammed Z
  • Combined Sewer Systems are commonplace in the United States and the major cities in Canada. These systems consist of both sanitary and stormwater lines which, when combined, create the potential for large surges of wastewater during storm conditions known as Combined Sewer Overflows (CSOs). CSOs have been identified by the USEPA and the European Parliament as damaging to the receiving environment by introducing protozoans such as Cryptosporidium and Giardia into fish as well as extra nutrients that may result in algal blooms in lakes downstream. As a result, the treatment of CSOs is an area of concern in the field of water treatment. CSOs can be treated via physical-chemical processes such as coagulation/flocculation alongside rapid/slow mixing. In Edmonton, the local wastewater treatment plant uses Aluminium Sulfate (Alum) along with coagulant aid to treat CSO flows. This process is known as Enhanced Primary Treatment. The current study’s objectives were to determine the optimum mixing conditions using a jar test apparatus for removal of common wastewater parameters such as chemical oxygen demand, pH, turbidity, alkalinity, total suspended solids, total volatile suspended solids and phosphorus. The coagulants used for this study were Alum, ferric chloride and polyaluminum chloride (PACl). Each coagulant was given a low dose and a high dose while the mixing speed and times were varied on a high/low basis as well. The mixing speeds and times were 1 minute/150 RPM and 3 minutes/300 RPM for rapid mixing and 10 minutes/15 RPM and 20 minutes/30 RPM for slow mixing. Six jars were chosen for the two-level, three-factor factorial design with two levels of dosages for each of the three coagulants with equivalent Al levels ranging from 4.05 mg Al/L to 10.14 mg Al/L. Four sampling campaigns took place between May 2016 and March 2017. The results of the final sampling campaign were analysed in detail. Through factorial design and ANOVA analysis, it was determined that the two most statistically significant factors in this study were slow mixing speed/time and coagulant dosage. The contaminant removal ratios and percent removal indicated that both Alum and PACl were the most effective in the removal of contaminants. The low dose, 4.05 mg Al/L, of both were more effective per mg of coagulant at removal with both removing an average of 23 mg contaminant per mg Al. The high level of slow mixing, 20 minutes/30 RPM, was determined to be the most effective mixing level with an average 9% improvement in contaminant levels over the low level of slow mixing. Alum was determined to be the most cost effective coagulant due to the cost per metric tonne for Alum being $400 as opposed to $800 to $950 for PACl. The benefits of removal between PACl and Alum were not significant enough to warrant being the coagulant of choice. Ferric chloride did not perform well in contaminant removal, possibly due to the high relative molecular weight of iron compared to aluminium as well as the higher amount of iron required in the form of ferric chloride in order to maintain equivalence with the active aluminium in the other coagulants, resulting in less efficient contaminant removal.

  • Subjects / Keywords
  • Graduation date
    Fall 2017
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.