Particle Size Separation Implications on COD Removal before BNR:

A Case Study at Heyburn, Idaho

Co-Authors:Remy Newcombe, Mark LoppTravis Higby, Ralph MartiniBjorn Rusten

• The Heyburn, Idaho WWTP had: screening, grit removal, oxidation ditches, clarifiers, UV, & aerobic digesters

• New requirements: phosphorus discharge to the Snake River, capacity increase from 0.39 to 0.563 MGD

Heyburn WWTP

• Stipulations: no precipitating chemical addition for P removal• Heyburn added: headworks, anaerobic bio-selector tank, new

oxidation ditch, effluent filters• Plant was complete by the end of 2009

Heyburn WWTP

• Rotating belt, primary filter (Salsnes Filter)

• 40–70% TSS reduction• Up to 35% BOD reduction• Produces 25-40% solids cake

Headworks: Rotating Belt Sieve

Salsnes Filter

Dual SF-2000

• TSS and BOD removal creates aerobic treatment capacity

• Flexibility for varying flows, CSO, multiple models available

Fine-Sieve Mesh Belt

• Nylon with Kevlar supports• Patented, self-aligning belt

• Multiple sieve sizes available

• Belt life is 1-5 years

Mat Forms on Screen Cloth

Mat Filters Water

Belt Lifts Solids Away

Solids Drop into Hopper

Air Cleans the Screen Cloth

Direction of Belt

Air Knife Action

Intermittent Hot Water Degreasing

Spray Nozzles

Solids in Auger, Prior to Extruder

40% Solids Exiting Extruder

Filter Mat Principle

40%

200 µm

80%

20 µm

60%

100 µm

50%

150 µm

350 µm mesh

350 µm mesh yields 35-100 µm effective filtration

Belt direction

70%

50 µm

• Recent work (Tas et al., 2009) suggested removal of particulate matter before BNR may impede denitrification potential

Question for BNR Operations?

With 35-100 µm filtration, is too much particulate removed by a RBS?

• COD: suggested as more useful than BOD for determining biological conversion rates in modeling (Orhon and Cokgor, 1997)

• Only 21% of COD is “settleable” (>63 micron), and 27% of COD is “suspended” (5-63 micron) (Nieuwenhuijzen, 2000)

• Significant amount of settleable COD found to be biodegradable (Tas et al., 2009)

• This led to assumption that removal of too much particulate matter could impede BNR

Review of COD

• Removal of particulate matter may be possible while maintaining critical COD/N ratios for efficient pre- denitrification

• Experiment: Sample before and after a RBS install Analyze for COD with filtered and unfiltered samples Confirm nitrogen removal at plant

…but maybe not

• “Settleable COD” might capture more of the readily biodegradable particulate range than simple size exclusion

• Therefore RBS devices might be able to remove a large amount of solids, but allow more biodegradable COD to pass than primary clarifiers

Theory

• RBS (Salsnes Filter) removing solids & BOD as expected

Results: TSS & BOD

(mg/L) RBS Influent RBS Effluent % Removal

TSSst. dev.

155+/- 36, n=3

85+/- 36 , n=3

45%

BODst. dev.

177+/- 25 , n=3

121+/- 55 , n=3

32%

• Unfortunately the COD data quality is poor• Sample 1 appears erroneous – the COD is too high in

relation to BOD (182 mg/L influent)• Removals in Samples 2-3 do not coordinate with the

observed BOD & TSS removals

Results: COD(mg/L) RBS Influent RBS Effluent RemovalCOD-1 1860 1600 14%COD-2 475 520 -9.5%COD-3 280 330 -18%Average 872 817 6%St. dev. 861 685 NA

• Filtered COD data have similar quality issues

Results: COD Fractions

Fractions(microns)

Influent(mg/L)

% of Total

Total COD 872+/- 861

100%

>29 236+/- 139

27%

>0.45 270+/- 26

31%

0.45-29 34+/- 121

4%

<0.45 602+/- 836

69%

<0.45 um

>29 um

• Grab sampling was used in this study, composite sampling may provide better data quality

• Sampling method may not have pulled proper representative samples, particularly for influent

• Laboratory method may not have properly homogenized samples before analysis

• Filtering of samples may have been difficult – filtering primary influent through 0.45 micron is not easy

Sampling & Analysis

• Heyburn is achieving effluent nitrogen <10 mg/L

Results: Nitrogen Removal

(mg/L) RBS Influent

RBS Effluent

Plant Effluent

Removal (whole plant)

TN 29 24 9.2 68%

TKN 28 24 5.6 80%

NO2 <0.2 <0.2 <0.2 NA

NO3 0.6 0.5 3.6 NA

• COD/N ratios are critical to efficient BNR operation

• Influent COD*/N ratio: 377 g COD / 29 g N = 13

• Effluent COD*/N ratio: 425 g COD / 24 g N = 18

• COD/N ratios may be maintained, but better data are needed (*Sample 1 data are excluded as outliers)

• Influent BOD/N ratio: 177 g BOD / 29 g N = 6.1

• Effluent BOD/N ratio: 121 g BOD / 24 g N = 5.1

• BOD data suggest the ratio is only lowered by 16%, since N is also lowered across the RBS

Results: COD or BOD/N Ratios

• Collect more data! Need statistically significant data for COD.

• Collect composite samples and reexamine influent sampling location.

• Use more filter sizes for samples, before and after RBS installs, to better understand COD particle size distribution.

• Examine laboratory methods for sample preparation and analysis of COD.

Future Work

• Rotating belt sieves (Salsnes Filter) remove significant TSS and BOD, increasing treatment capacity in a plant

• Heyburn is achieving <10 mg/L total nitrogen• Data show the BOD/N ratio is only lowered a

small amount across the RBS• The majority of readily biodegradable COD may

pass the RBS, allowing efficient BNR, but more data are needed to understand this in detail

Conclusions