Processes & Water Filters
The experimental objective of this lab is to design and assemble a low-cost and efficient water filter. The contaminated water supply must first have its acidity neutralized and then filtered. The filter will be evaluated on the cost and the amount of contaminants it filters out. A process flow diagram will be designed based on the water treatment system guidelines and the physical filter design.
Access to safe drinking water is a privilege that is often taken for granted. According to the United Nations, water scarcity affects more than 40% of the global population. It is estimated that by 2050, 2.3 billion people are expected to live in areas that experience severe water stress. One of the United Nations' 17 goals to sustainably transform the world includes available and sustainable management of water and sanitation for all.
Engineers have experimented with different ways of making drinking water more accessible. Currently, water is sourced from freshwater sources, such as surface water, which includes lakes, rivers, and streams. Non-conventional water sources, such as desalinated ocean water, treated wastewater, and agricultural drainage water, are also used. Regardless of how it is sourced, all water must go through a treatment process to be safe for human consumption (Figure 1).
Water Treatment Methods
Regardless of its natural source, water for everyday us undergoes treatment to remove contaminants (Figure 2). There are five steps in the water treatment process, but this lab will use just the first three steps: coagulation and flocculation, sedimentation, and filtration.
The first step in the water treatment process is coagulation and flocculation. This is a process where positively-charged chemicals called flocculants are added to the water to neutralize the negative charge of the contaminants in the water. The contaminants combine with the flocculants to form larger particles that are easier to filter out, known as floc. In this lab, aluminum sulfate, also known as alum, is used as the flocculant.
The second step is sedimentation. The difference in densities between the floc and the water cause the particles to eventually settle to the bottom of the water supply.
The third step is filtration where floc is removed from the water. During filtration, the water on top of the floc goes through a series of filters that are made up of sandbeds, gravel, or other materials. The materials typically have different pore sizes to contain contaminants of different sizes. In this lab, sand, gravel, and activated carbon will be used to filter the water. At some water treatment plants, water also filters through a granulated carbon filter that removes any organic material and other particles.
The fourth step is disinfection. For human consumption, a disinfectant may be added to kill any remaining bacteria, viruses, or parasites during disinfection. Chlorine is commonly used to treat water at this stage and rid it of any chemical pollutants. It is important that all the sediments are removed from the water as chlorine can react with them and produce trihalomethanes, which can cause cancer. Chlorine also produces a residue that will ensure water is disinfected and safe for consumption.
The final step is storage. The treated water is stored in tanks for future use.
Water Acidity & Alkalinity
Before filtration, water must first be neutralized to alter its acidity or alkalinity. If it is not neutralized, the water may be unsafe to drink and cause corrosion of pipes in the water treatment system. Water and water-based solutions are measured for acidity or alkalinity by their power of hydrogen or potential for hydrogen (pH). The pH scale is logarithmic based on the concentration of hydrogen ions (1).
The pH of a solution indicates the concentration of hydrogen ions (H) and hydroxide ions (OH). pH is measured on a scale from 0 to 14. A pH of 7 indicates a neutral solution, which signifies that the concentration of H and OH ions are equal. If the concentration of hydrogen ions increases in a solution, it becomes more acidic. A pH of 0 to 6 indicates an acidic solution. If the concentration of hydroxide ions increases, the solution becomes more alkaline or basic. A pH of 8 to 14 indicates an alkaline solution. The pH scale and examples of solutions of varying pH values can be seen in Figure 3.
The pH of a solution can be measured using a pH strip, which is a piece of filter paper previously soaked in various acid/base indicators. Once placed in a solution, the pH strip will change color based on the acidic or alkaline nature of the solution. The pH of the solution can be approximated based on the color change of the pH strip (Figure 4).
Turbidity, which is measured in nephelometric turbidity units (NTU), is the measure of a liquid’s clarity. It is an optical characteristic of water and is found by measuring the amount of scattered light caused by materials in water. A turbidity sensor works by shining light into the water and reading how much light is reflected back to it. The higher the level of turbidity, the higher the intensity of scattered light (Figure 5).
In a study on water resources conducted by Dr. Edward D. Schroeder at UC Davis, he concluded that the measure of turbidity is important regarding drinking water because microorganisms attached to contaminants are more likely to survive the disinfection process of water treatment systems.
Qualifications for Clean Water
To protect the public, the Environmental Protection Agency (EPA) sets legal limits of no more than 90 contaminants in drinking water. Secondary drinking water regulations that focus on the pH and turbidity are also set. Safe drinking water should have a pH between 6.5 and 8.5, and a standard turbidity of 1.0 NTU. More information about the EPA standards for clean drinking water can be found here.
To estimate the amount of water retained after filtration, engineers measure mass balance of a system. Mass balance is a measure of the total mass flow in and out of a system during a chemical or physical process. Mass balance is also known as material balance. The law of conservation of mass states that in a closed system, the mass of the system must remain constant over time. A closed system is a type of thermodynamic system where the exchange of energy can take place with the surroundings, but the mass is conserved within the boundaries of the system. An example is a thermos that may lose heat from a hot liquid it contains, but when properly capped does not allow the liquid to escape. Energy conservation is beyond the scope of this lab and will not be considered.
Applying the law of conservation of mass to a flow chart, the total mass that enters a system must equal the total mass exiting the same system. In Figure 6, the box represents the system, and the arrows into and out of the box represent the movement of mass into and out of the system. In a continuous process, the mass can be expressed as the mass flow rate. Mass flow rate is the pound mass of a fluid that passes through a cross-sectional area of the system per unit of time. In Figure 6, 6 lb/s of material A and 12 lb/s of the material B enter the system per second. The sum of the material entering, 18 lb/s, must leave the system per second to satisfy mass conservation.
Mass balance can also be described with an equation (2).
In (2), all the materials that are fed into the process unit are the input and all the product streams that flow out of the process unit are the output. When chemical reactions occur, generation and consumption also contribute to the mass balance. Generation is the amount of materials formed in a process unit and consumption is the amount of materials broken down in a process unit. As these reactions require energy input, they will not be part of this lab.
Process Flow Diagram
In many engineering fields, process flow diagrams (PFD) are used to indicate the sequential movement and processing of a material through multiple processing units and equipment. A PFD tracks the movement and composition of water in a water filtration system. Drawing PFDs is an essential technique to properly and clearly relay key information about a process design. When drawing PFDs, several elements must be included in the diagram. These elements include:
- Process vessels and equipment
- Process and utility flow lines
- Full energy and material balance
- Composition of every stream
- Bypass and recycle streams
All processes in a PFD must abide by the law of conservation of mass and the law of conservation of energy. Energy conservation is beyond the scope of this lab and will not be considered in the PFDs.
Three streams of fruit material enter a mixer: a stream of pineapple juice extract at a rate of 500 kg/s, a stream of frozen strawberries at a rate of 100 kg/s, and a stream of fresh peaches at a rate of 75 kg/s. The fruit mixture product stream then leaves the first mixer and enters a strainer where 135 kg/s of solids exit and are thrown into the trash. The remaining 540 kg/s of liquid exits in one stream. The liquid stream enters a final mixer alongside a stream of club soda that enters at a rate of 700 kg/s. The resulting product stream enters a storage unit.
The complete PFD in Figure 7 shows the elements of the system.
Process vessels and equipment are the units that the working stream passes through to change some of its physical properties, such as temperature, mass flow, or pressure. Some examples of process vessels and equipment are mixers, steamers, turbines, and reactors. In Figure 7, the process vessels are Mixer 1, Mixer 2, Strainer, Trash, and Storage. They can be indicated by different closed form shapes.
The process and utility flow lines include the one-sided arrows in Figure 7, although a process can involve two-sided arrows. The flow lines indicate the direction the materials flow and the composition of each stream.
Full energy and material balance in a process involves carrying out the conservation of mass and energy in a process. At Mixer 1 in Figure 7, 675 kg/s of total material enters the mixer (500 kg/s of pineapple juice, 100 kg/s of frozen strawberries, and 75 kg/s of fresh peaches), and 675 kg/s leaves the mixer. The mass and energy flow rates are usually listed above the process flow lines.
The composition of the streams is usually listed under the flow lines and indicates the percentage of the streams that each material comprises. For Mixer 1, the composition of the exit stream is indicated as 74% pineapple juice (P.J.), 15% frozen strawberries (F.S.), and 11% fresh peaches (F.P.).
Bypass and recycle streams are not considered in the PFDs in this lab, but they are also indicated by process flow lines. Bypass streams consist of streams of undesirable products, such as waste products, that usually flow to a disposal unit. Recycle streams consist of unprocessed materials that are fed back into the initial feed stream to be processed again, as most real process units are not 100% effective. In Figure 7, the flow line of 135 kg/s of solid waste that flows into the trash unit is a bypass stream.
The design of a PFD based on a problem statement will be walked through in detail.
Consider the first part of the example. Three streams enter the mixer and one stream leaves. There are three inputs and one output, as shown in Figure 8.
Each stream should be labeled with the flow rate above the flow line and the composition of the stream below the flow line. Due to mass balance, the flow rate of the product stream is equal to the total flow rate of the input streams. The composition of the product stream can be calculated using (3). To find the percentage of the stream, multiply the result in (3) by 100.
In the second part of the example, the combined product stream from Mixer 1 enters a strainer and 20% of the stream is separated into a solid stream and 80% of the stream is separated into a liquid stream. The solid stream is directed into the trash. The flow rate of the solid and liquid streams can be calculated by multiplying the percentages, 20% and 80%, by the input stream from Mixer 1, as shown in Figure 9. Due to the lack of information, the specific composition of each stream of fruit is unknown in the output and can be omitted in the PFD.
In the final part, the liquid stream from the strainer enters a mixer as an input with a 700 kg/s stream of club soda. These are the input streams for the final part of the example. The product stream then enters a storage unit. Similar to the first part, the flow rate of the product stream is the sum of the input streams, as shown in Figure 10.
- Which aspects of the competition components are most advantageous?
- Which materials will filter large sediments out of the contaminated water?
- Which materials will improve the water quality?
- What order of materials would create a winning design?
Materials and Equipment
- A computer with access to the internet
- pH strips
- Contaminated water
- Erlenmeyer flask
- A scale
- Sodium bicarbonate
- Glass stirring rod
- Activated carbon
- Turbidity Sensor
|Material||Unit||Cost Per Unit|
|Activated Carbon||1 tsp||$0.50|
Note: Water filtered in this experiment SHOULD NOT be consumed
1. Water Filter Design Analysis: Warm-Up Exercise
In this exercise, the relationship between % mass lost and the quantity of each material will be analyzed when the quantity of each material for the filter is varied at a time while keeping the quantity of other two materials constant. The models are merely approximations and they are not accurate representations of the filter process. The following steps outline the procedure to create a scatter plot for the sand-varied filter dataset.
- Download the Lab 8 Data Sheet containing data for this exercise.
- On the Excel ribbon, click on the Insert tab. Navigate to the Insert Scatter option in the Charts groups and click Scatter. This will create a blank scatter plot, as shown in Figure 11.
- Right-click on the empty graph and click Select Data. The Select Data Source dialog box will appear.
- Click Add to add a new dataset to the graph. Enter the series name % Mass Lost vs Teaspoons of Sand.
- For Series X values, highlight the cells containing the number of teaspoons of sand from the sand-varied filter dataset (A9:A12). For Series Y values, highlight the % mass lost data for the sand-varied filter (D9:D12).
- Rename the axes and chart titles by clicking the Chart Design tab on the Excel ribbon, then navigating to Add Chart Element and selecting the Title option.
- To insert a trendline, select Add Chart Element, then navigate to the Trendline option and select Linear.
- Double click on the newly created trendline. A Format Trendline dialog box will appear on the right side of the Excel window.
- Click on the Trendline options category with the icon that looks like a column chart, as shown in Figure 12. Check off Display Equation on the chart.
The finished scatter plot should look like Figure 13.
Repeat the steps to create a scatter plot for the gravel-varied and activated carbon-varied filters.
Pay attention to the linear relationship illustrated by the trendline equation in all three scatter plots. It can be deduced that a filter in which the quantity of all three materials is varied would be represented by a linear model of the form (4).
In (4), d is the total percent mass lost, a is the teaspoons of sand, b is the teaspoons of gravel, c is the teaspoons of activated carbon, and the constants, X, Y, and Z, correspond to the slope of the trendline for the sand-varied, gravel-varied, and activated-carbon varied filters, respectively.
When data is not provided, as in the tables in this exercise, experimental data can be used to derive the relationship given by (4).
2. Initial Data
- Record all data observed in the lab in the Excel datasheet found here. Only cells highlighted in green should be edited.
- Weigh the empty 500 mL beaker in grams using the scale. Make sure to zero the scale by hitting the tare button before weighing the beaker.
- After recording the weight of the beaker, the TA will add ~200 g of the contaminated water to the beaker. Take note of the color, odor, and any visual observations.
- The TA will use a turbidity sensor and measure the initial turbidity of the water sample. Be sure to record the value in the data sheet. Note that turbidity is being measured in volts.
- Reweigh the beaker and the contaminated water sample and calculate the weight of just the contaminated water. Record the weight in the datasheet. It should be around 200 g.
- Immerse one end of the pH strip into the contaminated water for a few seconds.
- Compare the color on the end of the strip to the pH color chart in Figure 4 of this manual page and record the pH.
3. pH Neutralization
Measure the pH of the water before constructing the water filter. The EPA standard for drinking water is a pH from 6.5 to 8.5. Although the human body can occasionally consume liquids with higher and lower pH with no health concerns, excessively drinking acidic water can lead to weakened teeth and gastrointestinal problems.
- From the pH obtained in the initial data, determine if the contaminated water is acidic or alkaline.
- For acidic water, one method of increasing the pH is to add sodium bicarbonate, or baking soda, which has a pH of approximately 9. If the solution is acidic, add sodium bicarbonate.
- Place your beaker of contaminated water onto the scale. Tare the scale.
- Add in a 1/8 tsp or 0.3 g of baking soda to the contaminated water and mix well. From the mass displayed on the scale, record the amount of the baking soda added.
- Stir the water to make sure the baking soda dissolves and test the pH of the contaminated water again with a pH strip.
- Compare the color on the end of the strip to the pH color chart and record the final pH. If the pH is not between the range of 6.5 to 8.5, repeat the process until the pH meets the EPA standards.
4. Flocculation and Coagulation
- Place the beaker of neutralized water onto the scale and tare the scale. Add around 0.5 g or half a spoonful of alum to the contaminated water. Record the mass of alum added in the datasheet.
- Mix the contaminated water well for 10 s
- Let the contaminated water rest for 10-15 min to allow flocculation to occur. Note any changes observed in the mixture. Your final coagulated and flocculated water sample should look similar to Figure 14.
5. Water Filter Design and Assembly
- Using the available materials and a funnel, design a water filter in the lab notes that will filter the physical sediments with the following limitations:
- Use a minimum of three scoops and a maximum of six scoops.
- Each material can only be used a maximum of three times.
- Record the following information in your lab notes:
- Materials used to design the filter.
- Order of adding each material.
- Quantity of each used material.
- Total cost.
- Have a TA approve the design.
- Record the materials used in the datasheet.
- Obtain the required materials and assemble the water filter. A gauze pad must be placed on the inside of the funnel to contain the filter materials.
- Place an empty Erlenmeyer flask on the scale.
- Place the assembled water filter in the funnel on top of the Erlenmeyer flask and tare the scale.
- After waiting 10-15 min for the flocculation process to complete, slowly pour the contaminated water through the filter. But before pouring, please read the following instructions:
- Measure the flow rate. Start a timer the second the water is poured into the filter. Stop the timer when the water in the Erlenmeyer flask reaches 100 mL. Record the time to calculate the flow rate in Step 18. Do not stop filtering once 100 mL is reached. All of the contaminated water must be filtered.
- Do not overfill the filter. The water must be poured slowly to first filter out all of the clear water. Exclude the floc at the bottom.
- Once that filters through, pour in the rest of the contaminated water sample including the floc.
- Once the contaminated water has been completely filtered, wait a few minutes before removing the filter. Once no water is observed to be coming out of the filter, remove the assembled water filter from the Erlenmeyer flask and rest the filter on the now empty beaker.
- Record the filtration time in the datasheet.
- Have a TA measure the turbidity of the filtered water. Record the new turbidity reading in the datasheet.
- The competition judges the teams using a competition ratio (CR) that measures 4 factors (Mass of Filtered Water, Change in Water Turbidity, Cost, and Flow Rate). These values should appear at the bottom of the datasheet. The team with the highest competition ratio wins.
- Calculate the percent voltage change using (6) and record it in the datasheet. The percent voltage change measures the decrease in reflected light. Less light being reflected indicates a lower turbidity.
- Calculate the flow rate (7).
(7)Note: Note that 1 cm equals 1 mL and 100 cm represents the 100 mL of water measured earlier in the procedure.
- The competition judges the filter in four categories (% mass lost, cost, % voltage change, flow rate). The categories will be ranked separately with each place receiving points (1st place = 1 point, 2nd place = 2 points, etc.). The winner of the competition is the filter with the fewest points.
- Share your results with a TA. The TA will record the results in the competition spreadsheet.
4. Process Flow Diagram
- Design a simplified industrial water treatment system PFD based on the experiments done in this lab. EG1003 recommends Diagrams.net for creating the PFD. Include:
- pH Neutralization
- Coagulation and flocculation
- The design will assume that the initial flow of contaminated water is 1000 kg/s. The contaminated water will flow through the following process units:
- pH Neutralizer
- Based on the numerical data and the given initial mass flow rate, be sure to add the following:
- Amount of contaminated water entering the system.
- Mass flow rate of alum added to the system.
- Divide the mass of the alum by the mass of the contaminated water to find the flow rate of alum needed for the PFD
- Calculate the mass flow rate of physical sediment removed from the system.
- Calculate the mass flow rate of filtered water leaving the system.
Keep in mind the order of filtration and any other process units needed to logically complete the process flow (for example a mixer after adding the alum). See the PFD example in the Overview section for reference. Be mindful of the mass balance (8).
The lab work is now complete. Refer to the Assignment section for the instructions to prepare the lab report.
Team Lab Report
Follow the lab report guidelines laid out in the EG1004 Writing Style Guide in the Technical Writing section of the manual. The following points should be addressed in the appropriate section of the lab report.
- Describe the water treatment process.
- What is pH? Why is it important to maintain the pH of drinking water?
- What potential effects would an increase in turbidity have on a lake?
- What was the final pH of the contaminated water? Is this within the EPA standards for drinking water?
- At what step in the water treatment system was the alum added? What did alum do to the contaminated water? Discuss any observations.
- Discuss minimal design. Describe the importance of minimal design and explain how it was employed in the design. Did the design minimize cost?
- Explain the water filter design, including why certain materials were selected and the order in which they were added.
- Compare your design with the other water filters designed in class. Which was the most cost-effective? Which was the cheapest? Which filtered out the most material?
- Overall, which filter media was the most effective at filtering the contaminated water?
- Explain the thought process behind the PFD designed in Part 5 of the procedure.
- How might the water filter design be improved (both individual and large scale)?
- Include pictures of the water filter, the polluted water, the filtered water, the pH strips, and the complete PFD.
- Discuss what part of the lab each member completed for the group and how it was important to the overall experiment.
Remember: Lab notes must be taken. Experimental details are easily forgotten unless written down. EG1004 Lab Notes paper can be downloaded and printed from the EG1004 Website. Use the lab notes to write the Procedure section of the lab report. At the end of each lab, a TA will scan the lab notes and upload them to the Lab Documents section of the EG1004 Website. One point of extra credit is awarded if the lab notes are attached at the end of the lab report. Keeping careful notes is an essential component of all scientific practice.
Follow the presentation guidelines laid out in the EG1004 Lab Presentation Format in the Technical Presentations section of the manual. When preparing the presentation, consider the following points.
- Since one term in the competition ratio is cost, present the cost of the water filter. Use the page How to Show Cost Data in Presentations for instructions on how to do this.
- How might the water filter design be improved?
- Include pictures of the water filters, the coagulation and flocculation results, the unfiltered water sample, and the filtered water samples.
- Which design was the most cost effective and why?
- Include picture/screenshots of the complete PFD.
2018 Edition of the Drinking Water Standards and Health Advisories Tables (2018, March). Retrieved from https://www.epa.gov/sites/production/files/2018-03/documents/dwtable2018.pdf
FAO's Global Water Information System. Prepared by AQUASTAT, Food and Agriculture Organization of the United Nations, www.fao.org/3/I9253EN/i9253en.pdf.
Guppy, L., Anderson, K., 2017. Water Crisis Report. United Nations University Institute for Water, Environment and Health, Hamilton, Canada.
Healthy Water. (2020, March 23). Retrieved July 02, 2020, from https://www.cdc.gov/healthywater
Schroeder , Edward D. “Water Resources.” Encyclopedia of Physical Science and Technology, by Robert Allen Meyers, 3rd ed., Academic Press, 2002, pp. 721–751. https://reader.elsevier.com/reader/sd/pii/B0122274105008218?token=709604225F511CB5F3C8E980E1DB5766BBFF9ABB22B7502BA791BC763A910230D5E1BA5444EE32BC25FD8DF7B67DD75F
Water Treatment. (2015, January 20). Retrieved July 02, 2020, from https://www.cdc.gov/healthywater/drinking/public/water_treatment.html