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RAS Special Study

Halton Region

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Georgetown WWTP

  • The Georgetown WWTP is a tertiary treatment plant consisting of screening, raw sewage pumping, grit removal, primary clarification, nitrifying activated sludge, secondary clarification, tertiary filtration, and two stage anaerobic digestion
  • High blankets in the primaries and a high TSS in the secondaries
  • Georgetown WWTP were having solids management issues in the secondary clarifiers
    • High blankets in the primaries and a high TSS in the secondaries
  • Suspected the RAS wasn’t telling the whole story, underestimating with grab sampling

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Georgetown WWTP

  • Hypothesis
    • Grab sampling the RAS vault to determine RAS concentration is underestimating the actual RAS concentration over a 24hr period
  • Approach
    • Grab the daily RAS sample from the B1 and B2 clarifiers and split the samples to make a separate composite
    • For every hour of the remaining shift, collect RAS grab samples from clarifiers B1 and B2, and add to the separate composite
    • At the end of the shift, collect the composite, store in lab fridge and analyse for SS the following work day.
    • Data to be collected will be the two different RAS results, MLSS, Plant Flow, and Plant Recycle Flow

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Georgetown WWTP

  • Data was gathered at the plant over a few weeks
  • Grab and 8 hrs. composite samples were collected for the study to compare the RAS
  • Grabs were underestimating
  • The predictive calculation was used to determine if the grabs were underestimating the RAS concentration
  • R/Q = MLSS/(RAS-MLSS)

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Georgetown WWTP

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Georgetown WWTP

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Georgetown WWTP

Study Summary

  • The 24 hrs. composite samples were on average ~500 - 1000mg/L more than the grab samples
  • Composite samples were closer to the theoretical RAS calculation
  • Georgetown installed composite samplers for their RAS samples
  • Although the RAS wasn’t the primary contributor to the solids issue in the secondary, it did improve solids management at the plant

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Mid-Halton WWTP

  • With the success of the RAS study at the Georgetown WWTP, a study was also conducted at the Mid-Halton WWTP
  • The Mid-Halton WWTP is a Secondary conventional treatment plant located in Oakville, and discharges to Lake Ontario
  • The staff used the same approach as Georgetown WWTP
  • The plant believed the grab sample was representative enough and that there wouldn’t be much of a difference between a composite and grab
  • Operations grabbed a sample every hour for 8 hours
  • Compared the Daily Grab vs Composite

RAS Sample Time

7:45 AM

8:45 AM

9:45 AM

10:45 AM

11:45 AM

12:45 PM

1:45 PM

2:45 PM

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Date

Daily Grab

(mg/L)

Composite (8 hrs)

(mg/L)

16-Feb

3946

5282

17-Feb

4604

4258

18-Feb

4370

4054

19-Feb

4262

4568

23-Feb

5880

5720

24-Feb

6280

5802

25-Feb

6290

6458

26-Feb

6496

5774

1-Mar

5604

6186

2-Mar

5522

6034

4-Mar

5378

5924

5-Mar

5234

5830

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Takeaways for Mid-Halton

  • The grab samples are similar to the 8 hour composite
  • The difference between the grab and composite ranges from 0 to 1000 mg/L in differences from the samples taken in the 2 week period
  • The two week average is 500 mg/L between the 2 samples
  • The highest daily value from the composite study are closer to the theoretical RAS calculation
  • A future study with a 24 hour composite sample is in the works through sampler or solids probe

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Thanks�

  • Thanks to the Jess Dean, Maureen Hart, Wendy Derjugin, at the Georgetown WWTP and Chandra Baker @ Mid-Halton for gathering the samples for the data analysis.

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  • Questions ?