Bridging the Gap Between Odor and Grit Removal - Saskatoon, Canada
Saskatoon is known throughout Canada as the “Bridge City” due to the seven bridges that cross the South Saskatchewan River in the city’s downtown district. Several kilometers to the north sits The H.M. Weir Wastewater Treatment Plant, located between a residential area and the river’s left bank. Constructed in 1971, the plant has a design daily flow of 120 ML/d (90 ml/d actual), and a peak flow of 300 ML/d.
Since the early 1970s the plant had used two rolling aerated grit chambers, complete with a clamshell crane removal mechanism. Raw sewage was split between the two chambers, while air was used to roll the sewage and promote the settling of the grit. Every six weeks the grit was removed using the clam shell and trucked to an on-site disposal area. Once a year, the tanks required draining and cleaning, which proved to be a labor-intensive and time-consuming task.
A tedious process
“It’s a tedious process,” said Plant Superintendent Joe Zimmer, who recruited eight or nine college students from the local university each summer to help with cleaning the basins and many other “dirty” jobs around the plant.
Not only was the system resource intensive to clean, it was also inefficient. The traditional design had grit removal upstream of the influent screens, which caused rags and other materials to build up in the grit chambers and reduce grit system performance. The resulting increased grit load was filling the plant’s digester and fermenters with sand to the point that it was noticeably reducing valuable process capacity.
“It was putting a lot more stress on primary sedimentation equipment,” said Zimmer.
However, outside of inefficiency and high maintenance, Zimmer believes the biggest problem with his old grit removal system was the odor. Grit was taken by the clam shell, then trucked and buried in a hill behind the plant. Air from the grit removal building was not scrubbed for odors which led to frequent complaints from nearby residents.
“We are in the middle of a residential area…and it stinks,” said Zimmer. “We used to get quite a few odor complaints.”
A better alternative
In 2004, the City began working with Stantec Consulting Ltd. to implement a better alternative. The engineering firm began a system evaluation report, tasked with evaluating the best grit removal system specific to this site. Following evaluation of a number of options, in 2005 the recommendation was made for a system from Hydro International, represented throughout Western Canada by Waste ’n WaterTech. Engineer Ryan Roberts said of the system: “The high efficiency grit removal system was recommended as it provides enhanced protection to downstream treatment processes at an acceptable cost.”
In addition to the high level of grit removal efficiency, Zimmer cited the advantages of the system as being “effective over a wide flow variation, easily expandable by adding trays and energy efficient,” in a later report written about on the project.
In addition to the high percentage of grit removal, Stantec was able to retrofit the new removal systems into the existing aerated grit chambers. This included relocation of influent screening to upstream of the grit handling facility. Through bypass of the plant influent during construction, the upgraded system was modified into the existing inlet/outlet channels, all housed within the existing headworks footprint.
The plant’s new grit removal system consists of four Hydro International HeadCells®, four SlurryCups™ and two Grit Snails®. Following raw influent screening, the raw sewage is split into one of the four HeadCells, where grit is separated. The concentrated grit slurry captured in the bottom of the HeadCell is then pumped to the SlurryCup where the organics are washed away and sent back to the biological treatment process. The concentrated grit is then directed into the Grit Snail, which dewaters and elevates the grit into a solids receptacle.
Noticing the benefits
Shortly after the installation was complete in the spring of 2009, Zimmer began noticing the system’s benefits. From an operator’s perspective, the system has been easier to use, trouble-free and cleaner than before. Additionally, the plant used to use potassium permanganate to chemically treat the plant’s odor, which is no longer needed.
The system has also cut down on maintenance considerably. Since installing the new grit removal system, the plant has not had to clean out the fermenters in over three years, and counting – a significant improvement over the yearly cleaning the old system required.
Zimmer estimates the grit removal system accounted for nearly 75 percent of the plant’s odor reduction. “The last summer since we did the upgrade, we’ve had no odor complaints,” said Zimmer. “They hardly noticed we were even here. We used to get complaints. When we talked to the same people they said [the lack of odor] was a great improvement.”