PREDICTION OF FILTER HEADLOSS PREDICTION OF FILTER HEADLOSS AND BED EXPANSION AFFECTED AND BED EXPANSION AFFECTED

BY BIOFILM AT WATER BY BIOFILM AT WATER TREATMENT PLANTSTREATMENT PLANTS

Michele Clements, Johannes Haarhoff & Tony Ceronio

Department of Civil and Urban Engineering

University of Johannesburg

BackgroundBackground• Empirical observation showed that the filters were insufficiently cleaned by the backwash system

• Media losses were unexpectedly high

• Specific deposit tests developed by the Water Research Group indicated that the dirtiness correlated with the organic content of the water being treated

Plant After drying & sieving

Background (2)Background (2)

This led to the hypothesis that biofilm is:

- present on the media, - somehow causing both the media loss- and the difficulty to attain efficient backwashing

Background (3)Background (3)Biofilm consists of

a) organisms, b) inorganic particles, all surrounded byc) a sticky, gelatinous polysaccharide matrix (EPS)

EPS makes up the bulk (50-90%) of the biofilm

BackgroundBackground (4)(4)

Two aspects of mechanical behaviour are deemed important in this study:

•Headloss - under prediction in headloss will result in a higher than expected backwash frequency•Bed expansion - under prediction in bed expansion will lead to media washout

The objectives of the study were to determine what effect the biofilm had on headloss and bed expansion and how to predict this effect.

MethodsMethods

• Samples for experimental work were drawn from full scale operating water treatment plants. • The treatment plants were spread over four provinces of South Africa with

- different raw water sources- using approximately the same media

•The sampling was done on three occasions (Winter 2003, Summer 2003 & Winter 2004)

Methods (2)Methods (2)

• Headloss and bed expansion test were done on the samples with biofilm, and after the biofilm was removed. • Parallel to these tests, EPS and volatile fraction quantification tests were done. •Mathematical models were used to reduce the data from multiple headloss and bed expansion experiments. •For the headloss data the Ergun (1952) equation was used and the sphericity (ϕ) was retained as the only unmeasured calibration constant. •For the bed expansion data the Dharmarajah equation (1986) was used and the sphericity was retained as the only unmeasured calibration constant. •Calibration was done with least square fitting, and separating the media bed in ten computational layers

ResultsResultsEPS measurements

0.000.100.20

0.300.400.500.600.70

0.800.901.00

Midvaal Rand Water 3 Rietvlei Sedibeng Vaalkop

mg/

g

ResultsResults (2)(2)Volatile fraction of media deposit

0.00

0.50

1.00

1.50

2.00

2.50

Midvaal Rand Water 3 Rietvlei Sedibeng Vaalkop

mg/

g

Results (2)Results (2)

Now that it has been established that biofilm changes the mechanical behaviour of filter media the reasons for these changes are investigated.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004

V (m/s)

Hea

dlos

s/be

d de

pth

(m/m

Measured - before drying Model - before dryingMeasured - after drying Model - after drying

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004

V (m/s)

Hea

dlos

s/be

d de

pth

(m/m

Measured - before drying Model - before dryingMeasured - after drying Model - after drying

0.500

0.520

0.540

0.560

0.580

0.600

0.620

0.640

0 0.005 0.01 0.015 0.02 0.025

V (m/s)

Exp

ande

d po

rosi

ty (-

)

Measured - before drying Model - before drying

Measured - after drying Model - after drying

0.500

0.520

0.540

0.560

0.580

0.600

0.620

0.640

0.660

0.680

0.700

0 0.005 0.01 0.015 0.02 0.025

V (m/s)

Exp

ande

d po

rosi

ty (-

)

Measured - before drying Model - before drying

Measured - after drying Model -after drying

Headlossresults

Bed expansionresults

Discussion Discussion

Using the Ergun equation, a decrease of 3.4% is calculated, whereas the result attained in this study indicated an increase in headloss of 17%.

TESTING OF THE TESTING OF THE ““SMOOTH FILMSMOOTH FILM”” HYPOTHESISHYPOTHESISHeadlossHeadloss

The most intuitive model one would expect is a single grain with a “smooth film” uniformly distributed around the media particle:

Discussion (2)Discussion (2)

Using the Dharmarajah equation, an increase of 0.92% is calculated, whereas the result attained in this study indicated an increase in expanded porosity of 13%.

It is clear that the “smooth film” model cannot be responsible for the observed effects.

TESTING OF THE TESTING OF THE ““SMOOTH FILMSMOOTH FILM”” HYPOTHESISHYPOTHESISBed ExpansionBed Expansion

Discussion (3)Discussion (3)

The next possible hypothesis is that rather than the grains expandingindividually, the EPS causes grains to stick together and that biofilm growth incorporates filaments that span more than one grain:

TESTING OF THE TESTING OF THE ““PARTICLE CONGLOMERATIONPARTICLE CONGLOMERATION”” HYPOTHESISHYPOTHESISHeadlossHeadloss

Discussion (4)Discussion (4)

This time, a 30% increase in headloss is calculated, a better account for the 40% (summer) increase in headloss measured.

An added theory of biofilm plugging the flow paths may also be possible, but calculating such effects will be very difficult.

TESTING OF THE TESTING OF THE ““PARTICLE CONGLOMERATIONPARTICLE CONGLOMERATION”” HYPOTHESISHYPOTHESISHeadlossHeadloss (2)(2)

Discussion (5)Discussion (5)

•With 5% of the pore space filled with EPS, a 44% increase in expanded porosity is achieved. •Perhaps, as with headloss, it is only necessary for some of the particles to form a conglomerate to achieve the 9% (summer) expanded porosity measured in this study.

When these built-up particles expand during backwashing, they will obviously expand morethan would be expected, as the overall densityof this bigger conglomerate is less than a single particle with the same equivalent diameter.

TESTING OF THE TESTING OF THE ““PARTICLE CONGLOMERATIONPARTICLE CONGLOMERATION”” HYPOTHESISHYPOTHESISBed expansionBed expansion

Why did plants with the same source have different results?

Stimulated by:- eutrophic raw water - the presence of pre-ozonation

Inhibited by- high pH lime processes

Discussion (6)Discussion (6)

Designers can compensate for this increase in headloss and bed expansion in two ways:

They could either apply a correction factor afterapplication of the models to allow for more headloss or bed expansion during eventual plant operation, or they could adjust parameters within the models to account for the larger headloss or bed expansion.

As the surface area sphericity was used as a calibration factor in this study and could account for different sets of complex unknowns, it is suggested that this factor is used for adjustment of the model.

RecommendationsRecommendations

The best route would be to estimate the quantities of biofilm being experienced by:

- a plant with similar processes- or a plant with a similar raw water source

It appears as if the adjustment needs to be highest when- pre-treatment includes ozone- eutrophic water with high nutrient content is treated

while turbid water with low nutrient content will not require adjustment at all.

Recommendations (2)Recommendations (2)

The author acknowledges the help at all the water treatment plants visited.

Acknowledgements