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DESIGN OF DRINKING WATER AND WASTEWATER TREATMENT PLANTS

Chen Zhou

Instructor:

Dr. Tian C. Zhang

1 PROJECT DESCRIPTION

The town of Waterville is a small city in United States. In recent years, the city has been facing the rapid growth of the population. Therefore, the initial drinking water treatment plant and wastewater treatment plant gradually can’t meet the demand of the city. A new water treatment plant and wastewater treatment plant need to be built in the future. This project basically covers the core process, the calculations, the main design parameters and the floor plans for the two plants.

2 POPULATION ESTIMATE

The Table 2.1 shows the town of Waterville has the following past population data projected beyond 1990.

Table 2.1

We put the data into Excel, and use a log curve to describe the change of the population.

The result is showing in the Figure 2.1,

The Excel automatically calculates the formula,

where y = population, x = year

In 1998,

Because water treatment plant is a kind of structure which is easily to be expanded, we use 15 years as a design periods. The design year should be 2030. The initial year is 2015.

In 2015,

In 2030,

3 WATER DEMANDING

The average water consumption (including every possible sector) is 160 gal/cap/day in 1998. We don’t need to consider about extra water consumption. And as a city grows, the future water consumption increases. For North American cities, the per capita consumption of water has increased by about one-tenth of their population increase.

So in 2015, the percentage of population increase from 1998 =

The per capita consumption of water =

For a maximum day in initial year,

In 2030, the percentage of population increase from 1998 =

The per capita consumption of water =

For a maximum day in design period,

the total water demand in 2030=

Because there is an old drinking water treatment plant with water production capacity of 30 MGD, we sue 25MGD as the design plant capacity for the new water treatment plant.

4 DRINKING WATER TREATMENT TRAIN AND PRELIMINARY DESIGNS

4.1 Water treatment plant process

The water resource is a river near the town. The river has a very big flow rate to allow the town to build a new treatment plant. The surface water quality is as follows: (1) average TSS of 10000 mg/L; (2) 35 mg/L of calcium bicarbonate and 20 mg/L of magnesium bicarbonate; and (3) during the spring runoff season, the river may be contaminated by low concentrations of synthetic organics.

We use a Rapid Sand Filtration Plant. It consists of coarse and fine screens, chemical coagulation, flocculation, sedimentation, granular media filtration, and chlorination. The coarse screens remove large debris, whereas the finer traveling screens remove smaller debris. Chemical coagulation and flocculation produce a precipitate or floc that enmeshes most of the colloidal solids. Most of the floc removed in the settling basins. The granular media filters remove most of the fine non-settling floc, and disinfection kills any pathogenic organisms present. After disinfection, the water is stored in the clear well and is pumped to the distribution system by the high service pumps. The following pictures shows the process of Rapid Sand Filtration Plant and the locations where chemicals are added.

4.2 Rapid mixing tank

We use a square rapid-mixing basin, with a depth of water equal to 1.25 times the width, is to be designed for a flow of 25 MGD. The velocity gradient is to be 790fps/ft, and the detention time is 40sec.

The total volume

We use two mixing tanks, so for each tank,

Use the 8.3ft as width and 10.7ft as height,

The value of the total power for each tank

4.3 Flocculation

We use a cross-flow, horizontal-shaft, paddle-wheel flocculation basin to be designed for 25 MGD, a mean velocity gradient of 26.7sec^-1, and the detention time of 45min. The GT value should be from 50,000 to 100,000. Tapered flocculation is to be provided, and three compartments of equal depth in series are to be used. We use a normal average G value of 26.7 sec^-1. The compartments are to be separated by slotted, redwood baffle fences, and the floor of the basin is level. The basin should be 50ft in width to adjoin the settling basin.

The GT value =

The total volume =

4 basins are designed in flocculation.

Each tank volume =

Use width = depth = 13.2ft, length = 39.6ft,

Figure 4.3.1

Use the paddle-wheel design as shown in Figure 4.3.1, with D₁ = 11.0ft, D₂ = 8.0ft, and D₃ = 5.0ft. Use four paddlewheels per shaft, and the blades are The space between blades is 12in.

4.4 Settling

We use free settling, which is the settling of discrete, non-flocculent particles in a dilute suspension. 4 rectangular sedimentation basin with 50ft width are needed to adjoin the wide of Flocculation basin. The detention time = 4 hr, overflow rate = 700 gpd/ft²/day.

The area of the each basin A =

Use 178.6ft as the length of basin, determine the depth from the detention time,

So use 16ft as a normal depth.

4.5 Granular Filtration

The rapid sand filters are housed in an open concrete basin. We use a sand bed 24in. in depth. The pertinent data are: specific gravity of the sand = 2.65, shape factor () = 0.82, porosity ( 0.45, filtration rate = 2.5 gpm/ft².

Use 2 square filtration tanks,

So use 29.5ft as the length.

From Table 4.5.1, we get that , ,

Table 4.5.1

1ft is remained for the tank, so the total depth

5 CHEMICAL DOSAGES

A jar test was run in the laboratory on a water sample to determine the best chemical dose for clarification. Samples, 2 L in volume, were placed in each jar. After the test, the jar showing that best performance had been dosed with 10 mL of Alum solution containing 5 mg Al³ /ml and 1 ml of polyelectrolyte containing 1 mg/ml.

The raw water volume = 2L,

27 297

The flow rate=25MGD, so

The mass of the Alum:

The mass of the polyelectrolyte:

6 EFFECTS OF WATER QUALITY CHANGES

To deal with the potential water quality changes, we decide to add a membrane filters after the Granular Filtration and before the Chlorine is aerated into the water. In the membrane processes, separation of a substance from a solution containing numerous substances is possible by the use of a selectively permeable. The solution containing the components is separated from the solvent liquid by membrane, which must be differently permeable to the component, which must be differently permeable to the components. Membrane filters are widely used for filtering both drinking water and sewage. For drinking water, membrane filters can remove virtually all particles larger than 0.2 um—including giardia and cryptosporidium. However the membrane filtration cannot remove substances that are actually dissolved in the water such as phosphorus, nitrates and heavy metal ions.

7 WASTEWATER QUANTITY AND QUALITY

7.1 Wastewater quantity

Municipal wastewater consists of liquid wastes from domestic, commercial, and industrial sources. The amount of municipal wastewater will be related to the water demand, since this is the source of the carriage water. An estimate of such wasters is related to water consumption studies, either at the present date or something in the future. We use average percentage of the water demand 76% to calculate the quantity of wastewater. And the design year is 2030.

For the initial year 2015,

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