Drinking Water Treatment – Chapter 25 Class Objectives

• Be able to define the possible components of a water treatment train and their functions

• Be able to differentiate between rapid and slow filtration

• Identify the components of a water treatment train that are best for a virus. A protozoa.

• List the possible detrimental effects of microbial biofilms in water distribution systems

• Differentiate between dissolved organic carbon and assimable organic carbon

• Describe the AOC test

Where does drinking water come from?

• Rivers• Streams• Lakes• Aquifers

Drinking water treatment processes

• Water treatment processes provide barriers between the consumer and waterborne disease

• One or more of these treatment processes is called a treatment process train

Typical Water Treatment Process Trains

• Chlorination

• Filtration (sand or coal)

• In-Line Filtration

involves a coagulation step (additive

that allows aggregation of

suspended solids, e.g., alum, ferric

sulfate, and ferric chloride,

polyelectrolytes)

• Direct Filtration

involves a flocculation step where

the water is gently stirred to

increase particle collision thereby

forming larger particles

• Conventional Treatment

involves a sedimentation step which

is the gravitational settling of

suspended particles

Filtration Processes Used

• Rapid filtration– used in United States– fast filtration rates through media (sand or anthracite)– backwashing needed

• Slow sand filtration– common in United Kingdom and Europe– slow filtration rates through media (sand and gravel)– removal of biological layer needed– higher removal rates for all microorganisms

Coagulation, Sedimentation, Filtration: Typical Microbial Removal Efficiencies and Effluent Quality

Organisms

Coagulation and sedimentation

(% removal)Rapid filtration (% removal)

Slow sand filtration

(% removal)

Total coliforms 74–97 50–98 >99.999

Fecal coliforms 76–83 50–98 >99.999

Enteric viruses 88–95 10–99 >99.999

Giardia 58–99 97–99.9 >99

Cryptosporidium 90 99–99.9 99

• Giardia and Cryptosporidium– filtration is best

• large size• resistant cyst and oocyst

• Enteric viruses– disinfection is ultimate barrier– filtration and coagulation also help via adsorption to particles

• dependent on surface charge of virus

Removal efficiency is dependent on microbial type:

Water Distribution Systems

Treated drinking water may go through miles of pipe to reach a consumer. The quality of the water is impacted by several things:

Dissolved organic compounds in finished drinking water is

responsible for: • enhanced chlorine demand• trihalomethane production• bacterial colonization of water distribution systems

Increases resistance to disinfection, e.g., E. coli is 2400 X more resistant to chlorine when attached to surfaces

Increases frictional resistance of fluids Increases taste and odor problems, e.g., H2S production Can result in colored water (iron and manganese oxidizing bacteria) Can cause regrowth of coliform bacteria Can cause growth of pathogenic bacteria, e.g., Legionella

Bacterial growth in distribution systems is influenced by:

• Concentration of biodegradable organic matter

• Water temperature

• Nature of the pipes

• Disinfectant residual

• Detention time within distribution system

One way is to determine Assimilable Organic Carbon (AOC)

• This test is used to determine amount of organic carbon capable of being oxidized by microbes

• Measurements of bacterial activity in the test sample are determined over time by plate counts, ATP, turbidity, or direct cell counts

How do you determine biodegradable organic carbon in a water distribution system?

• Performed with a single bacterial species, Spirillum NOX or Pseudomonas fluorescens P-17

• A water sample is pasteurized by heat to kill the indigenous microflora and then inoculated with the test bacterium in stationary phase

• Growth is monitored (7 to 9 days) until stationary phase is reached• Growth is determined and compared to standard growth on acetate (AOC

concentrations are then reported as acetate-carbon equivalents)

AOC can be calculated as follows:AOC (μg carbon/liter) = (Nmax x 1000)/Y where:

Nmax = CFU/mlY = yield coefficient in CFU/μg carbon

When using P. fluorescens strain P-17, Y = 4.1 x 106 CFU/μg acetate-carbon

Thus, if the final yield of the test organism is 5 x 106 CFU/ml after 9 days of incubation:

AOC = 5 x 106 CFU/ml x 1000 ml/L = 1.22 μg acetate-carbon equivalents/liter 4.1 x 106 CFU/μg acetate carbon

AOC Test

Comparison of Concentrations of DOC and AOC in Various Water Samples

Source of water

Dissolved organic carbon

(mg carbon/L)Assimilable organic

carbon (mg carbon/L)

River Lek 6.8 0.062–0.085

River Meuse 4.7 0.118–0.128

Brabantse Diesbosch 4.0 0.08–0.103

Lake Yssel, after open storage 5.6 0.48–0.53

River Lek, after bank filtration 1.6 0.7–1.2

Aerobic groundwater 0.3 <0.15