Drinking water standards and therefore water treatment depends on the water source: Three choices: Surface water Groundwater Groundwater under the direct influence of surface water (GWUDI)

The definition of the last source (GWUDI) is groundwater that has physical evidence of surface water contamination (e.g., insect parts, high turbidity), or contains surface water organisms (e.g., cryptospiridium, giardia), or has chemical water quality parameters similar to surface water (e.g., T, conductivity, TDS, pH, color).

Surface water generally requires the most treatment as shown in the following schematics.

For surface waters and GWUDI:

Groundwater requires much less treatment:

At a minimum water treatment will involve disinfection, usually by chlorination. Disinfection: selective killing or inactivation of pathogens as opposed to sterilization (complete elimination of all microoganisms). Chlorine is used because of it’s relative ease of application and low cost.

Chemistry of Chlorination: Chlorination can be accomplished by adding Cl2(gas), NaOCl or Ca(OCl)2 (sodium or calcium hypochlorite). When Cl2 is added to water:

HOCl = hypochlorous acid OCl- = hypochlorite ion The ratio of HOCl/OCl- is a function of pH This is an important concept because HOCl is a better disinfectant than OCl- HOCl and OCl- are called “free residual chlorine”

Free residual chlorine probably works by oxidizing extracellular enzymes of bacterial cells.

Chlorine Demand Because Cl2 or HOCl are strong oxidizers reducing agents will use up some of the chlorine before it can disinfect. These materials exert a chlorine demand.

Some examples of chlorine demand:

All of the above reactions consume the disinfecting power of chlorine. There are some reactions which do not entirely consume this disinfecting power and in some cases the products of these reactions are useful. These reactions involve the reaction of HOCl with NH3 to form chloramines as shown here.

Relative ratio of the chloramine species is a function of the Cl2/NH3 ratio, pH and temperature. All of the chloramines retain the +I oxidation state of HOCl but their oxidizing/disinfection capabilities are reduced. Because the chloramines retain disinfection power They are called “combined available chlorine”

Primary Drinking Water Standards for Disinfectants Chloramines: MCL = 4 mg/L (as Cl2) Chlorine: MCL = 4 mg/L (as Cl2) (MCL = maximum contaminant level, so these numbers represent upper limits of chlorination)

Dosage requirements: Disinfection effectiveness is a function of concentration of disinfectant and contact time. This results in the “Ct” concept. Where k = constant n = constant (usually = 1) t = contact time.

k = a constant for a particular % kill for a particular disinfectant, temperature, pH and microorganism. (to attain a certain % kill the product of C and t must equal this k). The following table gives some values

Required disinfection levels: Goal to reduce microorganism level to the Primary Drinking Water Standards: Total Coliforms: 5% - This means that less than 5% of the samples taken per month can be positive (i.e., show the presence of coliforms) Cryptosporidium, Giardia, HPC, Legionella, Viruses are all regulated by TT standards (resulting from disinfection and filtration)

EPA drinking water standard for disinfection requires water treatment systems to inactivate 99.9% of Giardia cysts and 99.99 % of enteric viruses ( 3 and 4 log reductions respectively). These organisms were chosen as standards because of their resistance to disinfection. “Ct” concept used to determine required retention time and chlorine concentration to achieve these log reductions. See Table 16.2.

Photomicrographs of Cryptospiridium cysts:                                                                                                                                          

Photomicrographs of Giardia cysts:

Note that “C” values are those are the effluent of the chlorine contact tank.

Log Reduction Scale Log Reduction % Removal 1 Log 90 1.5 Log 96.84 99 2.5 Log 99.68 3 Log 99.9 4 Log 99.99

USEPA SWTR (for surface waters and groundwater under the direct influence of surface water): 2 log reduction assumed in conventional treatment (with filtration). Therefore need 1 log reduction from chlorination. Other filters, such as membrane filters, can get up to 2.5 log reductions credit with demonstration of performance.

Regardless of the filtration method used, the water system must achieve a minimum of 0.5-log reduction of Giardia lamblia from disinfection alone after filtration treatment.

Points of chlorination in water treatment plants In many treatment plants chlorine is applied for final disinfection at the storage well (wet well) at the end of the treatment train. There is sufficient contact time here and in the distribution system to provide adequate “Ct”. In some treatment plants chlorine is applied just before filtration.

Typical Chlorine Dosages at Water Treatment Plants Calcium hypochlorite 0.5 – 5 mg/L Sodium hypochlorite 0.2 – 2 mg/L Chlorine gas 1 – 16 mg/L

The backbone of most water treatment plants is: Porous Media Filtration: Definition: Removal of colloidal (usually destabilized) and suspended material from water by passage through layers of porous media ----- turbidity removal

Deep Granular Filters Deep granular filters are made of granular material (sand, anthracite, garnet) arranged in a bed to provide a porous media as shown in the figure below. Filter bed is supported by gravel bed as also shown. Flow is typically in the downflow mode.

Mechanisms of suspended solids removal         Surface removal (straining) Mechanical straining caused by a layer of suspended solids (from the feed water) which builds up on the upper surface of the porous media. This type of removal is to be avoided because of the excessive headloss that results from the suspended solids layer's compressibility.

Flow Suspended solids Top of filter media Filter media

 Depth removal Depth removal refers to SS removal below the surface of the filter bed. There are two types of “depth removal”. Interstitial straining Larger particles become trapped in the void space between granular media particles.

Flow Suspended solid Filter media

Attachment Suspended solids are typically flocculent by design (filter often follows coagulation/flocculation) or by nature (clays, algae, bacteria). Therefore, attachment or adsorption of suspended solids is a good possibility. Attachment can be electrostatic, chemical bridging or specific adsorption. Attachment is enhanced by addition of small amount of coagulant and as the filter bed becomes coated with suspended solids ("ripened" filter). It is easier for suspended solids to attach to other SS that are already attached to the filter media.

Attachment Suspended solids are typically flocculent by design (filter often follows coagulation/flocculation) or by nature (clays, algae, bacteria). Therefore, attachment or adsorption of suspended solids is a good possibility. Attachment can be electrostatic, chemical bridging or specific adsorption. Attachment is enhanced by addition of small amount of coagulant and as the filter bed becomes coated with suspended solids ("ripened" filter). It is easier for suspended solids to attach to other SS that are already attached to the filter media.

Attachment Suspended solids are typically flocculent by design (filter often follows coagulation/flocculation) or by nature (clays, algae, bacteria). Therefore, attachment or adsorption of suspended solids is a good possibility. Attachment can be electrostatic, chemical bridging or specific adsorption. Attachment is enhanced by addition of small amount of coagulant and as the filter bed becomes coated with suspended solids ("ripened" filter). It is easier for suspended solids to attach to other SS that are already attached to the filter media.

Flow Suspended solid Filter media

Filter Cycle As filter run proceeds deposits build up in the upper portion of the filter bed. As a consequence void volume decreases, interstitial flow velocity increases with more hydraulic shear on the trapped and attached SS. This drives some of the filtered SS deeper into the filter bed. Ultimately the SS get washed into the effluent. At this point the filter must be backwashed to clean the filter bed surfaces.

Single media: Sand : 24"-30" depth Effective size = 0.4-1.0 mm. (d10) Uniformity coefficient < 1.65 (d60/d10) Density = 2.65. porosity = 0.43

Dual media: To compensate for the unfavorable gradation that occurs in the single media filters we can use dual media (reverse graded) filters. Place a less dense, larger diameter media on top of sand. This results in a higher porosity (0.55) at top of filter. Sand has porosity of about 0.4. Lower density also allows the less dense media to remain on top after backwashing.

Media Depth (in) Eff size(mm) Uniform Coeff Anthracite 12 –20 0.9 –1 < 1.8 Sand 12- 16 0.5- 0.55 <1.65

Filtration rate 1 - 8 gpm/ft2 = acceptable range. 2-3 gpm/ft2 = average flow loading rates. 4-5 gpm/ft2 = peak flow loading rate

Terminal headloss Commonly 3 - 5 ft for water treatment Filter run = T = f(floc strength, Q and suspended solids concentration in influent).

Backwash sequence Bed expansion is between 15-30 %. This is accomplished by applying a backflow rate of about 15 gpm/ft2 for about 5 - 10 mins. Hydrodynamic shear cleans the media particles (attached, as well as strained). Optimum shearing occurs at about 50 % expansion but this tends to require excessive backwash velocities with the coarser media particles and these high flow backwashs could fluidize the gravel underdrain.

Head applied above sand: 3-5 ft. Depth of sand is also about 3- 5 ft. Loading rates: 0.05 - 0.1 gpm/ft2 T: 1-6 months

Bolton Point Water Treatment Plant (Ithaca):