The ASU Tempe Campus Water Treatment Plant
Posted: August 27th, 2021
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The ASU Tempe Campus Water Treatment Plant
Executive Summary
With
the recent population projection in the ASU Tempe campus, there are estimations
that the size of both the students and staff might increase in the future. In
particular, the population size is estimated to reach 85,000 people in a design
period of 25 years. Therefore, the ASU Tempe campus needs to start planning for
the design of its water treatment plant, which can meet the demands of the
projected population increase. The intention of building the water treatment
plant is for the campus to attain its independence of water demands without
having to compete with other residents who obtain water from the city Tempe
reservoir. In this regard, the design process will entail correct estimations
of power mixing coagulants, flocculation, and sedimentation systems. Apart from
that, the design process of the water treatment plant will feature in the
annual costs for treating and supplying water coupled with the provision of
water quality requirements. Categorically, the design unit of treatment plant
processes composed of preliminary treatment and sedimentation, rapid mix,
flocculation, media filtration, disinfection, and Clearwell. Therefore, water
flow rates and population of the place is 141,176-meter cubic and 85,000 people
respectively
Table of Contents
Power Requirements for Mixing. 4
Dimensions for the Coagulation System.. 5
Power Requirements for Mixing. 7
Dimensions for the Flocculation System.. 8
Dimensions for the Sedimentation System.. 10
Particles Removal Efficiency. 12
Typical Dimensions and Layout of Filtration System.. 13
Introduction
The recent extended shutdown of buildings at the ASU Tempe campus due to the prevalence of Covid-19 has impacted water quality negatively. With the closing of gyms, dine-restaurants, bars, and classes, the left water sitting in pipes could alter in quality. There is a possibility that the water left for long periods during the Covid-19 shutdown would experience a concentration of pathogens and heavy metals. Undoubtedly, the stagnant water inside the pipes could encourage the growth of disease-causing microorganisms in buildings, resulting from almost non-existent residual disinfectants as well as poor temperature control. Therefore, this paper seeks to design a water treatment plant for the ASU Tempe campus that would meet the water demand of more than 85,000 staff and students in the next 25 years. The approach for undertaking a water treatment plant is to remove contaminants and odor, thus restoring the taste and quality for human consumption.
Water Demand Calculation
The universally UN water demand is 12,000,000,000 cubic meter (“water and wastewater treatment trains,” 1). For a population of 85,000 staff and students; therefore, the average day water demand (ADD) is calculated as follows:
ADD =
= 141,176 m3
| Maximum Day Water Demand: | MDD = | 1.6 x ADD = 1.6 x 141,176 = 225,882 |
| Peak Hour Water Demand: | PHD = | 2.5 x ADD = 2.5 x 141,176 = 352,941 |
Coagulation
Coagulation is a process of removing suspended particles from both surface and underground water. These particles which are categorized as colloids vary in sizes from 1 nm to 1 µm. While being absorbed by contaminants, colloids settle slowly due to their charged electrostatic surfaces, which prevent aggregation. Therefore, coagulation entails the addition of chemicals to contaminant surface water and thereafter followed by rapid mixing to destabilize the particles (“Coagulation and flocculation,” 1). Indeed, the whole process undertakes less than a minute. Apart from that, precipitated aluminum or ferrous hydroxide complexes are useful in enmeshing the natural colloids in surface water due to their highly positive charges. Consequently, this choice of coagulants is important in facilitating the removals of colloids.
Power Requirements for Mixing
The power requirement in the rapid mixing tank is described by the following equation that signifies the amount of power being imparted towards the liquid in a baffled tank by a specific impeller. This equation for power requirements sums up the process of coagulation in a fully turbulent flow.
P = KT (n)3(Di)5ρ
Where P = power, W
KT = impeller constant
N = rotational speed, revolutions/s
Di = impeller diameter, m
ρ= density of the liquid, kg/m3
For unbaffled tanks, however, the imparted power might be as low as one-sixth of the value obtained from this equation. Therefore, the velocity gradient is always associated with the power imparted to the water by a mechanical agitator, as demonstrated by the following graph and equation respectively.
G = =
Where G = velocity gradient, fps/ft or sec-1 mps/m or s-1
W = power imparted to the water per unit volume, N-m/s-m3
P = power imparted to the water, N-m/s
V = basin volume, ft3, m3
µ = absolute viscosity of water, 0.00131 N-s/m2 at 500F/100C
γ = specific weight of water, 1 g/cm3
To make power imparted to the water the subject of the equation, therefore, one has to square velocity gradient as follows:
G2 =( )2
Therefore, G2 =
Whereas the imparted power, P = G2 x
With a gradient velocity of 1,000 sec-1 and absolute viscosity of water at 20 N-s/m2, the imparted power of the rapid mixing can be approximated as follows.
P = G2 x
= (1,000)2 x
Power requirements = 2.0 x 1010 N-m/s-m3
Dimensions for the Coagulation System
Mechanical agitation is the most common method for rapid mixing inside the coagulants since it entails vertical shaft rotary mixing devices. For example, an impeller rapid mixing initiates a variable speed drive over the coagulants. Besides, the rapid mixing has varied basin design considerations, which are useful in determining the effectiveness of removing colloids. For instance, the bafflers are held at a gradient velocity of 20 to 60 seconds, either in a circular or rectangular basin with a fluid depth of 1.25x the basin diameter (“Coagulation and flocculation”, 1). In this way, baffles seem desirable in reducing vortex and rotational flow. Otherwise, the size of the baffles is demonstrated by 0.1 tank diameter.
Figure 1. Display of two and 6 bladed impeller
| Design | Dimensions |
| Turbine impeller | The diameter of the impeller is 30-50% of the coagulant tank. |
| The impeller is half the diameter of the bottom tank. | |
| The most effective speed range is from10 to 150 rpm. | |
| Paddle impeller | 2-6 blade sets. |
| The diameter of the impeller is 50-80% of the coagulant tank diameter. | |
| The width of the paddle ranges from one-sixth to one-tenth diameter. | |
| The impeller is one-half of the bottom tank diameters. | |
| Speed ranges from 20-150 rpm. | |
| Propeller impeller | 2-3 blades fixed on axial flow. |
| Speed ranges from 400-1,750 rpm. | |
| The maximum propeller diameter is 18″. |
Table 1. Design dimensions of a coagulant system
Annual Coagulants Cost
For an industrial purpose, a coagulant mixer will cost an annual cost of approximately $1.5 million, especially for a water treatment plant with a capacity of more than 150,000 GPD. Therefore, the following table shows the total annual costs of coagulants up to the startup stage.
| Coagulation Equipment | Costs |
| Design | $500,000 |
| Engineering | $300,000 |
| Equipment | $600,000 |
| Installation | $30,000 |
| Startup | $70,000 |
Table 2. Annual coagulants cost
Flocculation
Figure 2. Example of a common flocculation basin with a mechanical agitator
Power Requirements for Mixing
Power requirements for flocculation mixing range greatly in respect to the number of Reynolds a mechanical rapid mixer possesses. Therefore, it implies that a mechanical mixer having a greater number of Reynolds turbulent flows than 1,000 will be defined by the following equation.
P = KT n3 ρ
However, for a laminar flow whose Reynolds number does not exceed 20, there is a completely different equation for determining the power requirements for mixing as illustrated below.
P = KLn2
Where;
P = Power imparted to water, ft-lb/s, N-m/s
KT = Impeller constant for turbulent flow
KL = Impeller constant for laminar flow
n = rotational speed, rps
Di = Impeller diameter
ρ= density of the liquid
µ= absolute viscosity of water, 0.00121 N-s/m2 at 500F/100C
Dimensions for the Flocculation System
| Flocculation Tanks | Dimensions |
| Shape | Circular, square, rectangular |
| Circular tanks | 15-300 ft diameter, 6-16 ft deep. Our choice of tank is 150 ft diameter, 14 ft deep. Steps of 5’ diameter. |
| Square tanks | 35-200 ft wide, 6-19 ft deep. Steps of 5’. |
Sedimentation
Sedimentation is a process involving the removal of solid particles by use of gravity. The design of a sedimentation tank would entail deriving the flow rate in the WTP while determining the design of various power requirement expressions.The objective of sedimentation design in a water treatment plant entails determining a settling velocity of a particle in a bid to expound further on the association between overflow rate and removal of the particle (“Sedimentation,” 1). Therefore, the whole assumption of particle removal is pegged on an ideal basin theory. In particular, the process of separation happens between solid-liquid through the aid of gravity. In this perspective of treating water for use in the ASU Tempe campus, various processes occur systematically. First, there is the settling of surface water. Second, it involves the settling of coagulated or flocculated water. Lastly, the process involves the settling of insoluble compounds, which are fashioned during the removal of iron. Moreover, the settling processes are categorized into four types. Free settling does not involve the interaction of particles like silt and sand because these particles are non-flocculent. For flocculent settling, however, there is a broad interaction among particles, causing the particle size to increases as it flocs. Next, there is a high concentration of particles in hindered settling in that sand and silt interact closely, thus bringing about a similar settling velocity. At compressed settling, there is some accumulation of particles at the bottom which tends to deter the settling of newer particles. Thus, it implies the existence of lower depths of clarifiers.
Dimensions for the Sedimentation System
The criteria for designing a sedimentation basin are based on the settling velocity of a particle that needs to be removed from the water treatment plant (WTP). Therefore, removing 100% of particles of sedimentation at a rate of VS will equally require the design overflow rate to be equated to VS.
Overflow Rate (OR) = Sedimentation Rate
OR = = ==VS
| Sedimentation Slant Plate Clarifier (Models) | SPC-80 | SPC-150 | SPC-200 | SPC-300 | SPC-400 |
| Design flow maximum (GPM) | 80 | 150 | 200 | 300 | 400 |
| Flash tank-mix volume (gal) | 79 | 110 | 110 | 186 | 186 |
| Tank volume (gal) | 189 | 316 | 316 | 742 | 742 |
| Total pre-treatment volume (gal) | 268 | 426 | 426 | 928 | 928 |
| Solids discharge connections | 4″ | 4″ | 4″ | 6″ | 6″ |
| Plate area (ft2) | 345 | 728 | 1,153 | 1,339 | 1,633 |
| Projected plate area (ft2) | 189 | 400 | 633 | 747 | 911 |
| Empty shipping weight (lb) | 4,800 | 7,400 | 11,000 | 11,800 | 16,500 |
| Full operating weight (lb) | 15,480 | 22,900 | 32,760 | 56,550 | 60,320 |
| Overall length | 104″ | 135″ | 163″ | 179″ | 210″ |
| Overall width | 73″ | 77″ | 77″ | 90″ | 100″ |
| Overall height | 129″ | 147″ | 147″ | 147″ | 143″ |
Table 4. The required dimensions for the sedimentation system
FiltrationProcess
The filtration process entails removing the deferred solids within the sedimentation unit by defining the relationship between single or multimedia filters, removal of depth or surface water, and heat loss. Indeed, the process of filtration is significant in calculating the head loss in a filtration bed as it seeks to describe the hydraulics of a backwash cycle. Sometimes the removal of suspended particles might take a long time, especially if they are placed outside the basin. In this regard, the use of media filtration will help facilitate rapid sand depth filtration through porous media. Therefore, the use of this media filtration will remove fine suspended solids.
Figure 3. Detail of media filtration system
The needed size coupled with the uniformity coefficient of the media filter tanks is approximated at 0.5 and 1.6 respectively. Nonetheless, the recommended depth of the media filter is 50 cm, signifying that varied supporting layers of approximately 20 and 30 cm. Moreover, the head of the water above the filter media is regarded 2 meters high with the backwash process recommended for an average of 15 minutes (CEE 462/598. “5). Consequently, the total number of minutes for backwashing is 30, while the media filter run time is presumed at 24 hrs. Apart from that, the removal of particles via media filtration is classified under various scenarios. First, there is an inertia impaction or impingement that seems helpful in removing too dense particles to move along the streamline. Also, there is diffusion which compels the deviated small particles to be drawn towards the streamline by Brownian motion. Furthermore, some particles having a larger radius undergo interception while they follow the streamlined media filtration flow. Consequently, the gravitational settling can be considered efficient in undertaking media filtration for the reason that dense particles have a settling rate faster as compared to stream velocity.
Particles Removal Efficiency
Regarding particle removal efficiency, there is a significantly large rate of removing particles, which are highly associated with media filtration processes like interception, impaction, straining, as well as gravity. Nonetheless, there is an experienced large rate of removing small particles being occasioned by higher diffusion rates, as illustrated by figure 4.
Figure 4. A graph displaying the efficiency of removing particles
On the other hand, the media filtration signifies the possibility of an increased percentage efficiency of particle removal against particle size, especially when various particles are removed via different methods. For example, when a new filter tends to lose its charge mechanism due to electrostatic, the efficiency might drop down meaningfully, as illustrated by dashed lines in figure 5. Thus, the graph implies the integration of all types of media filtration intending to bolster the total particle removal efficiency.
Figure 5.A graph displaying the percentage efficiency of particle removal against particle size
Typical Dimensions and Layout of Filtration System
During the process of filtration, some valves seem either open or closed. Particularly, both the influent and effluent valves are open, whereas those valves for wash-water troughs are normally closed. With the design of two filter as minimum, four might be considered standard because the sand bed is about 14-30″ deep. However, the underlying gravel is set approximately between 6-24″ deep together with its arrangement in 5 layers. This ranges from the top to the bottom at the depth of 2″ (fig.6). Water has a height of 4″ over the filter bed, thus signifying a standard filtration rate of 2 gals per min-ft2. Likewise, the most effective size of particles that will be sieved through the filtration bed is nearly 10% of the sand (“Filtration,” 1). Precisely, the normal effective size varies from 0.35 to 0.5 mm of sand. Consequently, the uniformity coefficient associated with the passing of sieve size particles is approximately 60% of the sand.
Figure 6. A typical layout of filtration system
| Parameter | Units | Value |
| Filter type | ____ | Conventional, deep-bed mono-media |
| Flow control | ____ | Influent weir split, constant level |
| Number | ____ | 8 |
| Inside dimensions | M .M | 10 x 4.55 x 2 cells |
| Media surface are (each filter) | m2 | 91 |
| Media surface is (total filter) | m2 | 728 |
| Maximum available head | m2 | 2.5 |
| Filtration rate (at plant design flow rate) One-filter offline All filters in service | ||
| m/h | 15 | |
| m/h | 13.2 | |
| Filter media Type Depth Effective size | ||
| ____ | Anthracite | |
| M | 1.8 | |
| Mm | 1.0 | |
| Backwash criteria Maximum rate Normal rate Duration | ||
| m/h | 48.2 | |
| m/h | 38.6 | |
| Min | 15 |
Table 5. Summary of design criteria for rapid filter design
Disinfection System
Since there might be bacteria or disease-causing micro-organisms in the filtered water, it is important to disinfect the filter units. Carrying out disinfection would ensure the possibility of removing all the pathogens that might cause water-borne disease (Aziz and Mustafa 9). Therefore, disinfection encompasses a series of procedural methods. In this case, the use of chlorine is reliable in disinfecting the water. The reason is that the applied method is inexpensive as it is easy for a person to handle it based on the required safety measures. With the demand for water estimated at 141,176m3per day, therefore the needed chlorine and residual chlorine are estimated at 0.36 mg/L and 0.2 mg/L levels respectively.
Chlorine demand = 0.36 mg/L – 0.2 mg/L = 0.16 mg/L
Consumed chlorine = 0.36 mg/L × (0.0106) × 141,176 × 1000 = 79.6 kg/day
The time required to complete the disinfection performed in a storage tank is 0.5 h.
Q = Volume/time
Volume = Q × time
Volume = 141,176 m3 per day × (1/24) × 0.5 h = 2,941 m3
Clearwell
With the completion of the final stages of the WTP process, water will bedisseminated to the consumers or storage tanks through the use of high lift pumps. Subsequently, the treated water is used for drinking among the households on the ASU Tempe campus.
Figure 7. Overview of the Clearwell storage and distribution of water at ASU Tempe campus
Conclusion
With the prevalence of
Covid-19 hitting hard globally, schools have also been left behind as a
majority of students at universities have left for home. With schools’ rooms
unoccupied during the lockdown and curfew, piped waters have been rendered
untreated for long periods, leading to disease-causing pathogens and
micro-organisms. Therefore, the untreated water cannot be sufficient for the
growing number of ASU Tempe campus once schools resume duties post-covid-19.
Designing a water treatment plant will help alleviate the challenge of water
scarcity at school in a bid to meet the average water demand of 141,176 cubic
meters for a population size of 85,000. In summary, the whole process of
designing involves estimations of power mixing coagulants, flocculation, and
sedimentation systems coupled with the design costs.
Works Cited
CEE 462/598. “5 – Coagulation and flocculation.” Unit Operations in Environmental Engineering, Arizona State University (ASU), n.d.
CEE 462/598. “6 – Sedimentation.” Unit Operations in Environmental Engineering, Arizona State University (ASU), n.d.
CEE 462/598. “7 – Filtration.” Unit Operations in Environmental Engineering, Arizona State University (ASU), n.d.
CEE
462/598. “1 – Water and wastewater treatment trains.” Unit Operations in Environmental Engineering, Arizona State
University (ASU), n.d.
Annexes
Annex 1. General treatment train for surface water
Annex 2. A rapid sand filtration plant
Annex 3. Coagulation
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