Benopolis Wastewater Treatment Plant Plan
- Ben Quattlebaum
- Dec 15, 2018
- 33 min read
Table of Contents Page
Table of Contents…....…………………………………………………………………………….1
List of Figures…………..………………………………………………………………………….2
1.0 Introduction…………………………………………………………………………………...3
1.1 Benopolis History………………………………………………………………………..…….3
1.2 Our Goal………………………………………………………………………………..……...3
1.3 Objective…………………………………………………………………………………..…..4
1.4 Public Outreach……………………………………………………………………………......4
2.0 Background…………………………………………………………………………………....5
2.1 Regulations for Wastewater Treatment……………………………………………………....5
2.2 Wastewater Treatment Plant Capacity….…………………………………………………....5
2.3 Proposed Location of Wastewater Treatment Facility..……….……………………………...7
3.0 Design………………………………………………………………………………………….8
3.1 Wastewater Treatment Process Overview………….…...……………………..……………..8
3.2 Wastewater Retrieval...……………………………………………………………………….9
3.3 Plant Construction……..……………………………………………………...........................9
3.4 Preliminary Treatment……………………………………………………………………….10
3.4.1 Screening……………………………..…………………………………………………....10
3.4.2 Aerated Grit Chamber..…………………………………………………………………….10
3.5 Primary Clarifiers…....…………………………………………………………………….....11
3.6 Secondary Treatment………………………………………………………………………..12
3.6.1 Aeration Basin………………....…………………………………………………………...12
3.6.2 Secondary Clarifiers…………………..……………………………………………………13
3.6.3 Disinfection...……………………………………………………………………………....14
3.6.4 Aeration..…………………………………………………………………………………..14
3.7 Tertiary Treatment.…………………………………………………………………..............15
3.7.1 Denitrification..…………………………………………………………………………….15
3.7.2 Phosphorus Removal…...………………………………………………………………….16
3.8 Solid Waste Management…………………………………………………………………....16
3.8.1 Digestion………..................................................................................................................16
3.8.2 Dewatering...……………………………………………………………………………….16
4.0 Summary……..……………………………………………………………………………....17
4.1 Overview………….................................................................................................................17
4.2 Public Meeting……………………………………………………………………………….17
References….…………………………………………………………………………………....17
Images…………………...............................................................................................................18
Appendix I (Calculations)..............................................................................................................19
List of Figures
Page
Figure 1: Current and Projected Population of Benopolis……………………………………….. 6
Figure 2: Map of Benopolis with Proposed Locations of Drinking Water Treatment Plant and Wastewater Treatment Plant………...…………..……………………………………………….7
Figure 3: Brief Overview of the Wastewater Treatment Process...….…………………………....8
Figure 4: Hydraulic profile of wastewater treatment plant processes…………….……....……...9
Figure 5: Influent screen for wastewater treatment…………………………………………….10
Figure 6: Motion of particles in aerated grit chamber..………………………………………....11
Figure 7: Interior of a primary clarifier………..………………………………………………....12
Figure 8: Food web of microorganisms in an aeration basin……………………………………13
Figure 9: Fine-pore diffuser aeration system…………………………………………………....15
Figure 10: Diagram of the modified Ludzak-Ettinger (MLE) process…………………………….15
1.0 Introduction
1.1 Benopolis History
Benopolis, CA once had a steadily increasing population of over 28,000 people, with its economy fueled by the Exxon Mobile oil refinery within its limits. The city was located along the Carquinez Strait, which flows into the Pacific Ocean after passing San Francisco. It offered public hiking and biking trails and public access to the bay for recreational activities such as sailing and swimming.
The Exxon oil refinery turned out to be the drastic end of Benopolis, however, as heavy flooding during the spring of 2015 caused enough damage to some of Exxon’s oil containers to create a massive spill. Thousands of gallons of crude oil leaked from the containers and mixed with the floodwater, contaminating a majority of the city’s surface water, including a large portion of the bay around it. While Exxon was legally obligated to fund efforts to clean the contaminated land and water, it would be nearly three years until the city was back to the habitable state it once was. Because the drinking water treatment plant was unable to handle the new levels of toxicity in the city’s water supply, and due to other obvious public health concerns, everyone deserted the town. The only visitors to the city besides the Exxon funded clean up crews were occasional vandals and thieves, who over the clean-up process continued to damage public facilities to the point that they needed to be rebuilt from the ground up.
1.2 Our Goal
The Quattlebaum Engineering Company was started in 1997 and places its core values in the ideas of closed-loop systems, resilience, and design for disassembly. These principles of green engineering represent limiting waste while maximizing efficiency in terms of materials, energy, and space overall. The catastrophe that Benopolis has faced gives us an opportunity to start essentially from scratch, and implementing a system that will not only last, but leave little to no footprint behind once the system is no longer needed.
Of our goals, designing closed-loop systems is by far our most ambitious, especially in terms of gaining public support. While this is merely the design plan for the wastewater treatment plant, the Quattlebaum Engineering Co. has been contracted to design the drinking water treatment plant as well as the solid waste management system in Benopolis. That gives us opportunities to responsibly deal with the waste that each of these facilities will produce. For example, while this is generally unpopular regarding public opinion, reclaiming wastewater to be used as drinking water has been proven to be equally as clean for consumption and wastes less of the earth’s freshwater reserves. This, while it may be more expensive to implement, will end up reducing a lot of the waste involved in disposing of wastewater and gathering freshwater.
Resilience is another important concept that the Quattlebaum Engineering Co. focuses on. It would create an absurd amount of unneeded waste for this plant to be shut down in 25 years or so and replaced with a new, larger one. Therefore, we are planning with space in mind for expansions for each and every step of the drinking water treatment process. The plot of land we have recommended was once a massive parking lot, so it is very level and easily buildable on, making expansion and adapting extremely easy.
Design for disassembly is my favorite aspect of the Quattlebaum Engineering Company’s philosophy. Many materials of our plant are connected via interlocking, temporary connections. For example, our influent screens can be easily slid out of place for cleaning or replacement, and our walkways above the system are connected in a system of “puzzle pieces”, or interlocking steel mesh-like panels. This allows us to disassemble areas of the plant that are no longer in service and also makes maintenance and cleaning much easier, wasting less resources to do the same jobs.
1.3 Objective
This report’s purpose is to explain the reasoning behind every decision the Quattlebaum Engineering Co. made on behalf of Benopolis’ new drinking water treatment plant. These explanations will be supported by graphs and calculations that were used to estimate quantities such as population, sizes of facilities, concentrations of chemicals added, and distances from other industrial facilities in Benopolis.
1.4 Public Outreach
Our number one goal throughout this process is to satisfy the citizens of Benopolis, and in order to ensure that we are sufficiently doing so, we welcome all feedback. On Friday, November 2nd at 1:25 p.m. there will be a town hall gathering to give citizens an opportunity to hear about our process as well as voice any opinions they may have regarding said process. We also encourage feedback via our website, https://benquatt.wixsite.com/qbaumengineerco.
2.0 Background
2.1 Regulations for Drinking Water Treatment
Wastewater treatment plants must follow strict regulations in order to uphold high standards of public and environmental health and safety. The major federal regulations that governs this process are found in the U.S. Code of Federal Regulations Title 40, or 40 C.F.R., Part 1361 and the Clean Water Act, or 33 U.S.C. § 1251 et seq.2 Both of these laws give an overview of the permitted levels of pollutant discharges and regulate the quality of surface water in designated areas.
The state of California also has its own regulations governing wastewater plants. These can be found in the California Code of Regulations Title 23, Division 3.3 These regulations outline the qualification and registration requirements for a wastewater treatment plant. Secondary wastewater that has been treated must have an average monthly effluent BOD5 level of 30 mgL and a maximum average weekly effluent BOD5 level of 45mgL. It also must have an average monthly effluent TSS level of 30mgL with a maximum average weekly effluent BOD5 level of 45mgL.4
2.2 Waste Water Treatment Plant Capacity
One of the most important aspects of sustainable infrastructure design is determining the size of a plant in order to fit the wastewater management needs of a community. A plant that is too large will require resources in the form of materials and energy that will go underutilized most of the time, but a plant that is too small will be unable to process the required amount of effluent wastewater. Therefore, population growth and average wastewater production quantities must be taken into account.
After the recent oil spill, all of the previous residents of Benopolis have decided to return to their homes. The population is low, but it is growing at a constant growth rate. In 2010, there was a population of 24,000, and with a growth rate of 1.56%, Benopolis now has a population of just over 26,000 and, assuming a steady growth rate, is expected to reach a population of 30,000 in 2039 and 32,000 in 2048 (see Appendix A).
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It has been determined that, on an average day, the average hourly wastewater flow rate is 0.59m3s and the peak hourly wastewater flow is 1.65m3s. Using these values, we have determined that it is appropriate to design a plant with the volume of 85 gallons per person per day. This means that, in 2018, the volume of the wastewater treatment plant in Benopolis needs to be able to handle approximately 2.21 million gallons per day. This, however, is not taking into account the population growth that will likely occur over the next several years. In order to be considered sustainable, the plant needs to be built to handle the city’s population for the next 25 years. In 2043, the population of Benopolis has been estimated to be 31,095 people. Therefore, the plant design volume needs to handle 2.64 million gallons per day. By building for the max demand in 2043, this plant will be prepared for all factors that increase wastewater production within the next 25 years. It also leaves room for growth without building in excess for current years, and thus saves materials and is more sustainable overall.
2.3 Proposed Location of Wastewater Treatment Plant
When choosing a wastewater treatment facility’s location, it is necessary to take into account several factors. Because we will be implementing a closed loop treatment process, pairing the wastewater treatment plant with the drinking water treatment plant, proximity to the drinking water plant is one of the most important factors, as the distance can affect the cost of transport of effluent water to the drinking water plant. Proximity to industrial facilities is also important, as factories may be a source of pollutants and extra contaminants that may influence the treatment process. A plot of land in the eastern region of Benopolis provides a suitable location for the wastewater treatment plant. It is land plot of about two acres that is over half a mile from the nearest industrial facility and over two miles away from the nearest residential neighborhood. Before the oil spill, it was a massive parking lot, so the land is already very level and easy to build on, and it is within a mile of the drinking water treatment plant that would be used, so transport of the water would be affordable and easy. While less than one and a half acres are required to meet Benopolis’ daily wastewater treatment demands, it is recommended that the entire two-acre plot be bought in case of any necessary expansion in the future. 📷
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3.0 Design
As stated previously, the wastewater and drinking water treatment plants will be on a closed-loop system, meaning that some effluent wastewater will be sent to the drinking water treatment plant to be recycled and used as drinking water. Other treated wastewater that is not needed by the drinking water plant will be released into the Carquinez Strait about 2 kilometers upstream of the drinking water intake pipe. Because the wastewater has been treated, it is already safe to be treated for drinking purposes, but the river will further reduce any BOD levels found in the wastewater.
3.1 Wastewater Treatment Process Overview
The process for treating wastewater is different from drinking water in that the focus of wastewater treatment is on removing biological pollutants that could pose extreme environmental and health risks if released. Therefore, there are steps that focus on removal of toxic compounds through advanced oxidation, pathogens through sedimentation and disinfection, and solid wastes.
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While the facility will be designed for the maximum load, there will be times that maintenance is required. Thus, backups for every step in the process have been implemented and will be used when needed in order to ensure that the system never backs up.
3.2 Wastewater Retrieval
Wastewater can be retrieved in two different ways: centralized systems and decentralized systems. Centralized systems are what most people have, as they are the public sewer systems that are used for areas with a highly concentrated population. The wastewater treatment plant is located on an elevation lower than most of the population, so minimal electricity will be required to pump Benopolis’ wastewater to the facility. Decentralized systems are rarer, but in the areas to the north where a few agricultural properties are located, septic tanks are used. These must be emptied every three to five years, and private companies offer services to pump these and bring the waste to our treatment plant.
3.3 Plant Construction
In order to be as energy efficient as possible, the site for the wastewater treatment plant has been chosen to have a slightly declining elevation. This allows for the water to flow from one step to the next while minimizing the power required to pump it. There is one step that electricity input is required for pumping, and that is moving the water from the influent screens to the grit chamber, as shown in the hydraulic profile. Pumping this influent water up ensures that large solids are not transported to the rest of the treatment processes.
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As shown above, every other step is on a steady decline, with approximately half a meter of elevation decline between each step. While this does not completely eradicate the need for a pump, it greatly reduces electricity needs of the plant.
3.4 Preliminary Treatment
3.4.1 Screening
In order to prevent damage to pipes and mechanical equipment, as well as to prepare for the settling phase of wastewater treatment, coarse solids must be removed from the influent water flow. This can be done by using a combination of barracks, parallel rods that are about 20mm-150mm in diameter, and bar screens, which have perforations of approximate diameters of less than 10mm.7
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These screens are very effective at removing large objects such as bottles, insoluble hygiene products, and other large objects that are transported through piping. An alternative that is slightly more costly is a comminutor, which grinds up solids and allows them to pass through for sedimentation. Our plant will only be using the combination of barracks and screens, as we have determined that it is cost effective enough for our city’s needs.
3.4.2 Aerated Grit Chamber
Grit is considered solids slightly smaller than those removed through screening. This can include sand, gravel, eggshells, bone fragments, coffee grounds, and fruit and vegetable seeds.7 Grit is removed in order to avoid destruction of pipes from abrasion, as well as to remove some unwanted organic material. Our plant will be using an aerated grit chamber, meaning air will be introduced along the edge of the chamber. This creates a current within the water that settles the grit while leaving the smaller particles suspended in the water for later treatment.7
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Our facility will use two aerated grit chambers, each with a volume of 148.5 m3. The dimensions of each chamber will be a length of 11 m, a width of 4.5 m, and a depth of 3 m. The detention time required in order to fully settle the grit will be approximately 8.4 minutes. The amount of air required to fully settle the grit will be 7.7m3min. These parameters will remove approximately 2.14m3day of grit at peak hours.
3.5 Primary Clarifiers (Primary Settling Basins)
Primary clarifiers are analogous to sedimentation basins in the drinking water treatment process. They remain extremely still to allow particles to settle to the bottom by gravity, where they are collected as sludge by a large rotating arm. Approximately 60% of the total suspended solids will be removed, as well as 30% of the BOD and 20% of the phosphorus, allowing the rest to pass on to the secondary clarifier. Our facility will use alum as a coagulant in order to maximize the amount of particulate matter that settles and reduce the energy requirements in the secondary settling step.
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Our treatment facility will have two circular clarifiers with a depth of 3 m and an overflow rate of 47m3m2-day. The diameters have been rounded up to 30 m in order to ensure that they can process the required volume of wastewater for Benopolis’ waste requirements. This means that the volume of each clarifier will be approximately 2,120 m3. The detention time will be approximately 2 hours and the flow rate of the clarifiers will be about 36m3m2-day. This will result in about 1450kgday of primary solids being produced, which will be stored and transported to a landfill.
3.6 Secondary Treatment
Secondary treatment uses microorganisms to remove the rest of the dissolved particles that were too small for primary settling to remove. Our facility will be using a suspended-growth system, where the microorganisms are directly mixed into the wastewater for removal of small particles and organic matter.
3.6.1 Aeration Basin
The aeration basin is the primary component of the secondary treatment process. There are microorganisms such as bacteria, rotifers, fungi, and protozoa, which, when mixed with the organic matter and particulate matter within the wastewater, are called mixed liquor.7
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The bacteria is added to the water in order to digest the organic matter within the water, and then the excess bacteria are consumed by the protozoa and rotifers, who will be passed along to the secondary clarifier along with excess sludge.
The volume of the aeration basin was determined based on the BOD levels of samples of Benopolis’ wastewater. Because Benopolis has 83% of its wastewater coming from municipal waste and 17% coming from industrial waste, the BOD5 levels of the wastewater have been determined to be approximately 269mgL. This allowed us to calculate the needed volume of the aeration basin to be approximately 9.9 million liters with an aeration period of 2.37 hours.
3.6.2 Secondary Clarifier
The secondary clarifier is similar in design to the primary clarifier, but it contains both microorganisms as well as settling sludge. The microorganisms remain at the bottom of the tank without a food source, as the aeration basin removed all of their food source. These starved microorganisms will become activated, and 20-30% of the sludge-microorganism combination will be returned to the aeration basin to be used again as returned activated sludge (RAS). The rest will be processed as waste activated sludge (WAS), which is shipped to a landfill. The solids retention time (SRT) before the activated sludge is either returned or wasted is only five days, so the food-to-microorganism (F/M) ratio will be relatively low. This means that the microorganisms will be starved for a period of time, making them more efficient at removing BOD from the system. Therefore, the power requirements for aeration will be less, as the microorganisms will be saturated with food.
The secondary waste produced has been calculated using the volume of the aeration basin. It is estimated that about 647kgday of secondary dry solids will be produced by the secondary settling basin, and of that approximately 20%, or about 129kgday, will be returned activated sludge. The rest will be transported to a landfill. The F/M ratio is estimated to be about 0.583, which is extremely close to the recommended ratio of 0.6.12
3.6.3 Disinfection
Before either reclamation or ultimate release of the treated wastewater, possible pathogens must be thoroughly removed from the water as a final secondary step. In Benopolis, this will be done through UV light pulsing. UV light sterilizes any bacteria that were not killed by the other processes, so they will not be able to reproduce through the reclamation or release process. This means that most bacteria and all harmful pathogens will be completely gone by the time the water hits any intake pipes or the Carquinez Strait. The intensity used for the UV sterilization process will be 100 microWatts per cm3. This has been previously determined to be sufficient in sterilizing 99.9% of bacteria.
3.6.4 Aeration
The last step of the secondary wastewater treatment process is aeration, or adding dissolved oxygen back into the water. This helps to dissolve other gases such as carbon dioxide, as well as helps to oxidize dissolved metals and organic compounds that may be left within the water after the other treatment steps. In our facility, we will be incorporating two fine-pore diffusers, which produce small oxygen bubbles that are very efficient in the dissolution process. This requires less energy and less air to achieve the same result as surface aeration or coarse-pore diffuser aeration. Because the pores are extremely small (less than 5mm), they can be easily clogged. They will need to be cleaned periodically using hydrochloric acid, which is why we have incorporated a pairing so that one can be shut down for cleaning without backing the entire system up.
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3.7 Tertiary Treatment
Secondary treatment removes most of the pathogens and pollutants from the effluent wastewater, but sometimes it leaves behind some polluting nutrients such as nitrogen and phosphorus, which are both toxic and caused by human urine.
3.7.1 Denitrification
During secondary treatment, ammonia is converted to nitrate. This raises the levels of nitrogen within the water, and because nitrogen requires much more time, bacteria, and oxygen to dissolve than other nutrients such as carbon dioxide, it requires a separate process for its removal. In order to completely denitrify the effluent water, a process called the modified Ludzak-Ettinger (MLE) process.
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In the MLE process, mixed liquor suspended solids (MLSS) are recycled to an anoxic chamber. This, along with the following aerobic chamber, help remove any COD, or carbon waste. Then, the aerobic chamber converts nitrates from the nitrification process in secondary treatment. It is then recycled back to the anoxic zone where the nitrates are broken down into nitrogen and easily removed.
3.7.2 Phosphorus Removal
Phosphorus is another nutrient present in wastewater that is toxic in large quantities. This can be somewhat removed through the addition of alum, but it is also very effectively dealt with by phosphate-accumulating organisms (PAOs). They, when exposed to an anaerobic zone, convert the phosphorus that is difficult to treat into easily removable phosphorus particles that settle out with sludge.
3.8 Solid Waste Management
3.8.1 Digestion
Once all of the solids have settled out of the primary and secondary clarifiers, they must be stabilized in order to be recycled or taken to a landfill. One method of stabilizing the solids is aerobic digestion, where the waste-activated sludge is pumped back into the aeration chamber for a long period of time. This results in overall reduced organic matter in the sludge.
Anaerobic digestion is the method we will be using, because it requires less energy, as there is no aeration required. It begins with hydrolysis, followed by acidogenesis and acetogenesis, ending with methanogenesis. These steps use specialized bacteria to break down organic matter further to get the same result as aerobic digestion.
3.8.2 Dewatering
Once the sludge has its organic matter reduced, the volume is to be reduced for easier transport. This is done through a process called dewatering, which increases the percent of solids in the total volume from 0.5% to anywhere from 15-50%. Dewatering occurs in a mechanism called a drying bed, which allows water to seep from the sludge into layers of gravel and sand over a time period of about three months. From there, the solids can be scraped up and transported to a landfill.
4.0 Summary
4.1 Overview
This report outlines the plans the Quattlebaum Engineering Company has made for the design of a new wastewater treatment plant in Benopolis, CA. It includes detailed step-by-step summaries and calculations of required dosages of chemicals and volumes of containment units. It also maps out the location of the facility with regard to industry, residential areas, and water source. Laws governing the monitoring the level of contaminants and methods of release were outlined and taken into account. This report’s goal was to show the citizens and leaders of Benopolis that everything was decided based on logic. Public health and safety, fiscal management, and sustainable design played major roles in designing this wastewater treatment facility.
4.2 Public Meeting
This plan will not reach the level of success that is desired if citizens do not accept it. That is why there will be a public town hall meeting on Friday, November 9th, 2018 at 1:25 p.m. to present the plans as well as answer any questions that have not been answered by the presentation. The citizens of Benopolis are the Quattlebaum Engineering Company’s top priority, and to ensure that all of the needs of the citizens are met, it is important that they know the facts and are on board with everything.
References
1U.S. Code of Federal Regulations. National Emission Standards for Organic Hazardous Air Pollutants From the Synthetic Organic Chemical Manufacturing Industry for Process Vents, Storage Vessels, Transfer Operations, and Wastewater. Title 40 Part 63. 4-9-2004 Edition.
2United States Environmental Protection Agency. Summary of the Clean Water Act. Web. 2018.
3California Code of Regulations. State Water Resources Control Board and Regional Water Quality Control Boards Chapter 26. Classification of Wastewater Treatment Plants and Operator Certification. Title 23 Division 3. 6-12-2015 Edition.
4California Waterboards. General Waste Discharge Requirements for Discharges to Land by Small Domestic Systems. Web. 2018.
7Mihelcic and Zimmerman, Environmental Engineering: Fundamentals, Sustainability, Design, John Wiley & Sons, Incorporated, Edition: 2nd, ISBN: 9781118741498.
12Water and Wastes Digest. Headworks: Removing Inorganics and Preventing Wear. Web. 2018.
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Appendix I (Calculations)
Plant Design Volume:
2018:
V = (85gallonsperson*day)(26,038 persons) = 2,213,230gallonsday
2028:
V = (85 gallonsperson*day)(27,954 persons) = 2,376,090gallonsday
2043:
V = (85gallonsperson*day)(31,095 persons) = 2,643,075gallonsday
Aerated Grit Chamber Design:
Volume:
Total Volume = (1.65m3s)(3 min)(60smin)= 297 m3
Chamber Volume = 297m32 chambers= 148.5 m3
Dimensions: (assuming width-to-depth ratio of 1.5:1 and depth = 3)
Grit chamber width = (1.5)(3m) = 4.5m
Grit chamber length = volumewidth*depth= 148.5m3(4.5m)(3m)= 11m
L*W*D = (11m)(4.5m)(3m)
Average Hydraulic Retention Time
V=Q*Θ → Θ=VQ= (148.5m3(.59m3s))(2)(1min60s)= 8.39min
Air Requirements (assuming 0.35 m3m-min)
Total air = (2 tanks)(11m length)(0.35m3m-min) = 7.7 m3min
Quantity Grit Removed @ Peak (assuming 0.015 m3103m3grit in untreated WW)
Grit Removed = (1.65m3s)(0.015m3103m3)(86400sday) = 2.14m3day
Primary Settling Tank Design
Assuming 2 circular tanks with a depth of 3m and a surface overflow rate of 47m3m2-day
Total clarifier area
A = QOR= (0.59m3s)(86400sday)47m3m2-day= (1084.6 m2)(12)= 542.3m2 per clarifier
Tank diameter
D = A4 =542.3m24= 26.28m → rounded up to 30m
New area
A = 4*R = (4)(30m)2 = 706.86m2
Vclarifier = A*d = (706.86m2)(3m) = 2,120.6m3
Detention Time
Θ = VQ= (2120.6m3(0.59m3s)(86400s)2)(24hrday) = 2 hr
Observed Flow Rate
OR = QA= (0.59m3s)(86400sday)2706.86m3= 36.058 m3m2-day
Other Calculations:
Incoming BOD
Municipal flow → 83%; industrial flow → 17%
Municipal BOD5 = 253 mgL; CODlab = 465 mgL
Industrial BOD5 = (0.75)(465mgL) = 348.75 mgL
Total BOD5 = (0.83)(253mgL)+(0.17)(348.75mgL) = 269.28mgL
Aeration Basin
Solids Retention Time (SRT) = 5 days; Decay Constant (kd) = 0.05day
Incoming Flow (Qo) = (2,643,075galday)(m3264.172 gal)(1000Lm3) = 10005129.23Lday
Yield Coefficient (Y) = 0.48 g biomassg BOD5; X = 3,270 mgL
Incoming Substrate (So) = (0.70)(269.28mgL) = 188.49mgL; Substrate (S) = 20mgL
1SRT= [QoYVX(So-S)] - kd → 15 days= [(10*106Lday)(0.48g biomassg BOD5)V(3270mgL)(188.49mgL- 20mgL)] - 0.05day
Basin Volume (V) = 9.898*105 L
Aeration Period
Θ = VQ= 9.898*105L10*106Lday= 0.0989 days (24 hrday) = 2.37 hr
Primary dry solids produced
Influent = 265mgL; Effluent = 120mgL
TSSprimary = (Q)(influent - effluent)(kg106mg) = (10*106 Lday)(265mgL- 120mgL)(kg106mg)
TSSprimary = 1450.74kg primary SSday
Secondary dry solids produced
SRT = VXQwXw= 5 days = (9.898*105 L)(3270mgL)QwXw→ 6.47*108mgday(kg106mg) = 647.3292 kgday
F/M Ratio
F/M = QSoXV= (10*106Lday)(188.49mgL)(3270mgL)(9.898*105L)= 0.583lb BOD5lb MLSS-day
The solids retention time (SRT) is low (5 days), which means that the F/M ratio is low. The power requirements for aeration will be less. The microorganisms will be saturated with food. The mean cell retention time is low. The sludge age is low. The sludge wastage rate may have been recently increased. The MLSS may have been increased.
Table of Contents Page
Table of Contents…....…………………………………………………………………………….1
List of Figures…………..………………………………………………………………………….2
1.0 Introduction…………………………………………………………………………………...3
1.1 Benopolis History………………………………………………………………………..…….3
1.2 Our Goal………………………………………………………………………………..……...3
1.3 Objective…………………………………………………………………………………..…..4
1.4 Public Outreach……………………………………………………………………………......4
2.0 Background…………………………………………………………………………………....5
2.1 Regulations for Wastewater Treatment……………………………………………………....5
2.2 Wastewater Treatment Plant Capacity….…………………………………………………....5
2.3 Proposed Location of Wastewater Treatment Facility..……….……………………………...7
3.0 Design………………………………………………………………………………………….8
3.1 Wastewater Treatment Process Overview………….…...……………………..……………..8
3.2 Wastewater Retrieval...……………………………………………………………………….9
3.3 Plant Construction……..……………………………………………………...........................9
3.4 Preliminary Treatment……………………………………………………………………….10
3.4.1 Screening……………………………..…………………………………………………....10
3.4.2 Aerated Grit Chamber..…………………………………………………………………….10
3.5 Primary Clarifiers…....…………………………………………………………………….....11
3.6 Secondary Treatment………………………………………………………………………..12
3.6.1 Aeration Basin………………....…………………………………………………………...12
3.6.2 Secondary Clarifiers…………………..……………………………………………………13
3.6.3 Disinfection...……………………………………………………………………………....14
3.6.4 Aeration..…………………………………………………………………………………..14
3.7 Tertiary Treatment.…………………………………………………………………..............15
3.7.1 Denitrification..…………………………………………………………………………….15
3.7.2 Phosphorus Removal…...………………………………………………………………….16
3.8 Solid Waste Management…………………………………………………………………....16
3.8.1 Digestion………..................................................................................................................16
3.8.2 Dewatering...……………………………………………………………………………….16
4.0 Summary……..……………………………………………………………………………....17
4.1 Overview………….................................................................................................................17
4.2 Public Meeting……………………………………………………………………………….17
References….…………………………………………………………………………………....17
Images…………………...............................................................................................................18
Appendix I (Calculations)..............................................................................................................19
List of Figures
Page
Figure 1: Current and Projected Population of Benopolis……………………………………….. 6
Figure 2: Map of Benopolis with Proposed Locations of Drinking Water Treatment Plant and Wastewater Treatment Plant………...…………..……………………………………………….7
Figure 3: Brief Overview of the Wastewater Treatment Process...….…………………………....8
Figure 4: Hydraulic profile of wastewater treatment plant processes…………….……....……...9
Figure 5: Influent screen for wastewater treatment…………………………………………….10
Figure 6: Motion of particles in aerated grit chamber..………………………………………....11
Figure 7: Interior of a primary clarifier………..………………………………………………....12
Figure 8: Food web of microorganisms in an aeration basin……………………………………13
Figure 9: Fine-pore diffuser aeration system…………………………………………………....15
Figure 10: Diagram of the modified Ludzak-Ettinger (MLE) process…………………………….15
1.0 Introduction
1.1 Benopolis History
Benopolis, CA once had a steadily increasing population of over 28,000 people, with its economy fueled by the Exxon Mobile oil refinery within its limits. The city was located along the Carquinez Strait, which flows into the Pacific Ocean after passing San Francisco. It offered public hiking and biking trails and public access to the bay for recreational activities such as sailing and swimming.
The Exxon oil refinery turned out to be the drastic end of Benopolis, however, as heavy flooding during the spring of 2015 caused enough damage to some of Exxon’s oil containers to create a massive spill. Thousands of gallons of crude oil leaked from the containers and mixed with the floodwater, contaminating a majority of the city’s surface water, including a large portion of the bay around it. While Exxon was legally obligated to fund efforts to clean the contaminated land and water, it would be nearly three years until the city was back to the habitable state it once was. Because the drinking water treatment plant was unable to handle the new levels of toxicity in the city’s water supply, and due to other obvious public health concerns, everyone deserted the town. The only visitors to the city besides the Exxon funded clean up crews were occasional vandals and thieves, who over the clean-up process continued to damage public facilities to the point that they needed to be rebuilt from the ground up.
1.2 Our Goal
The Quattlebaum Engineering Company was started in 1997 and places its core values in the ideas of closed-loop systems, resilience, and design for disassembly. These principles of green engineering represent limiting waste while maximizing efficiency in terms of materials, energy, and space overall. The catastrophe that Benopolis has faced gives us an opportunity to start essentially from scratch, and implementing a system that will not only last, but leave little to no footprint behind once the system is no longer needed.
Of our goals, designing closed-loop systems is by far our most ambitious, especially in terms of gaining public support. While this is merely the design plan for the wastewater treatment plant, the Quattlebaum Engineering Co. has been contracted to design the drinking water treatment plant as well as the solid waste management system in Benopolis. That gives us opportunities to responsibly deal with the waste that each of these facilities will produce. For example, while this is generally unpopular regarding public opinion, reclaiming wastewater to be used as drinking water has been proven to be equally as clean for consumption and wastes less of the earth’s freshwater reserves. This, while it may be more expensive to implement, will end up reducing a lot of the waste involved in disposing of wastewater and gathering freshwater.
Resilience is another important concept that the Quattlebaum Engineering Co. focuses on. It would create an absurd amount of unneeded waste for this plant to be shut down in 25 years or so and replaced with a new, larger one. Therefore, we are planning with space in mind for expansions for each and every step of the drinking water treatment process. The plot of land we have recommended was once a massive parking lot, so it is very level and easily buildable on, making expansion and adapting extremely easy.
Design for disassembly is my favorite aspect of the Quattlebaum Engineering Company’s philosophy. Many materials of our plant are connected via interlocking, temporary connections. For example, our influent screens can be easily slid out of place for cleaning or replacement, and our walkways above the system are connected in a system of “puzzle pieces”, or interlocking steel mesh-like panels. This allows us to disassemble areas of the plant that are no longer in service and also makes maintenance and cleaning much easier, wasting less resources to do the same jobs.
1.3 Objective
This report’s purpose is to explain the reasoning behind every decision the Quattlebaum Engineering Co. made on behalf of Benopolis’ new drinking water treatment plant. These explanations will be supported by graphs and calculations that were used to estimate quantities such as population, sizes of facilities, concentrations of chemicals added, and distances from other industrial facilities in Benopolis.
1.4 Public Outreach
Our number one goal throughout this process is to satisfy the citizens of Benopolis, and in order to ensure that we are sufficiently doing so, we welcome all feedback. On Friday, November 2nd at 1:25 p.m. there will be a town hall gathering to give citizens an opportunity to hear about our process as well as voice any opinions they may have regarding said process. We also encourage feedback via our website, https://benquatt.wixsite.com/qbaumengineerco.
2.0 Background
2.1 Regulations for Drinking Water Treatment
Wastewater treatment plants must follow strict regulations in order to uphold high standards of public and environmental health and safety. The major federal regulations that governs this process are found in the U.S. Code of Federal Regulations Title 40, or 40 C.F.R., Part 1361 and the Clean Water Act, or 33 U.S.C. § 1251 et seq.2 Both of these laws give an overview of the permitted levels of pollutant discharges and regulate the quality of surface water in designated areas.
The state of California also has its own regulations governing wastewater plants. These can be found in the California Code of Regulations Title 23, Division 3.3 These regulations outline the qualification and registration requirements for a wastewater treatment plant. Secondary wastewater that has been treated must have an average monthly effluent BOD5 level of 30 mgL and a maximum average weekly effluent BOD5 level of 45mgL. It also must have an average monthly effluent TSS level of 30mgL with a maximum average weekly effluent BOD5 level of 45mgL.4
2.2 Waste Water Treatment Plant Capacity
One of the most important aspects of sustainable infrastructure design is determining the size of a plant in order to fit the wastewater management needs of a community. A plant that is too large will require resources in the form of materials and energy that will go underutilized most of the time, but a plant that is too small will be unable to process the required amount of effluent wastewater. Therefore, population growth and average wastewater production quantities must be taken into account.
After the recent oil spill, all of the previous residents of Benopolis have decided to return to their homes. The population is low, but it is growing at a constant growth rate. In 2010, there was a population of 24,000, and with a growth rate of 1.56%, Benopolis now has a population of just over 26,000 and, assuming a steady growth rate, is expected to reach a population of 30,000 in 2039 and 32,000 in 2048 (see Appendix A).
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It has been determined that, on an average day, the average hourly wastewater flow rate is 0.59m3s and the peak hourly wastewater flow is 1.65m3s. Using these values, we have determined that it is appropriate to design a plant with the volume of 85 gallons per person per day. This means that, in 2018, the volume of the wastewater treatment plant in Benopolis needs to be able to handle approximately 2.21 million gallons per day. This, however, is not taking into account the population growth that will likely occur over the next several years. In order to be considered sustainable, the plant needs to be built to handle the city’s population for the next 25 years. In 2043, the population of Benopolis has been estimated to be 31,095 people. Therefore, the plant design volume needs to handle 2.64 million gallons per day. By building for the max demand in 2043, this plant will be prepared for all factors that increase wastewater production within the next 25 years. It also leaves room for growth without building in excess for current years, and thus saves materials and is more sustainable overall.
2.3 Proposed Location of Wastewater Treatment Plant
When choosing a wastewater treatment facility’s location, it is necessary to take into account several factors. Because we will be implementing a closed loop treatment process, pairing the wastewater treatment plant with the drinking water treatment plant, proximity to the drinking water plant is one of the most important factors, as the distance can affect the cost of transport of effluent water to the drinking water plant. Proximity to industrial facilities is also important, as factories may be a source of pollutants and extra contaminants that may influence the treatment process. A plot of land in the eastern region of Benopolis provides a suitable location for the wastewater treatment plant. It is land plot of about two acres that is over half a mile from the nearest industrial facility and over two miles away from the nearest residential neighborhood. Before the oil spill, it was a massive parking lot, so the land is already very level and easy to build on, and it is within a mile of the drinking water treatment plant that would be used, so transport of the water would be affordable and easy. While less than one and a half acres are required to meet Benopolis’ daily wastewater treatment demands, it is recommended that the entire two-acre plot be bought in case of any necessary expansion in the future. 📷
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3.0 Design
As stated previously, the wastewater and drinking water treatment plants will be on a closed-loop system, meaning that some effluent wastewater will be sent to the drinking water treatment plant to be recycled and used as drinking water. Other treated wastewater that is not needed by the drinking water plant will be released into the Carquinez Strait about 2 kilometers upstream of the drinking water intake pipe. Because the wastewater has been treated, it is already safe to be treated for drinking purposes, but the river will further reduce any BOD levels found in the wastewater.
3.1 Wastewater Treatment Process Overview
The process for treating wastewater is different from drinking water in that the focus of wastewater treatment is on removing biological pollutants that could pose extreme environmental and health risks if released. Therefore, there are steps that focus on removal of toxic compounds through advanced oxidation, pathogens through sedimentation and disinfection, and solid wastes.
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While the facility will be designed for the maximum load, there will be times that maintenance is required. Thus, backups for every step in the process have been implemented and will be used when needed in order to ensure that the system never backs up.
3.2 Wastewater Retrieval
Wastewater can be retrieved in two different ways: centralized systems and decentralized systems. Centralized systems are what most people have, as they are the public sewer systems that are used for areas with a highly concentrated population. The wastewater treatment plant is located on an elevation lower than most of the population, so minimal electricity will be required to pump Benopolis’ wastewater to the facility. Decentralized systems are rarer, but in the areas to the north where a few agricultural properties are located, septic tanks are used. These must be emptied every three to five years, and private companies offer services to pump these and bring the waste to our treatment plant.
3.3 Plant Construction
In order to be as energy efficient as possible, the site for the wastewater treatment plant has been chosen to have a slightly declining elevation. This allows for the water to flow from one step to the next while minimizing the power required to pump it. There is one step that electricity input is required for pumping, and that is moving the water from the influent screens to the grit chamber, as shown in the hydraulic profile. Pumping this influent water up ensures that large solids are not transported to the rest of the treatment processes.
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As shown above, every other step is on a steady decline, with approximately half a meter of elevation decline between each step. While this does not completely eradicate the need for a pump, it greatly reduces electricity needs of the plant.
3.4 Preliminary Treatment
3.4.1 Screening
In order to prevent damage to pipes and mechanical equipment, as well as to prepare for the settling phase of wastewater treatment, coarse solids must be removed from the influent water flow. This can be done by using a combination of barracks, parallel rods that are about 20mm-150mm in diameter, and bar screens, which have perforations of approximate diameters of less than 10mm.7
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These screens are very effective at removing large objects such as bottles, insoluble hygiene products, and other large objects that are transported through piping. An alternative that is slightly more costly is a comminutor, which grinds up solids and allows them to pass through for sedimentation. Our plant will only be using the combination of barracks and screens, as we have determined that it is cost effective enough for our city’s needs.
3.4.2 Aerated Grit Chamber
Grit is considered solids slightly smaller than those removed through screening. This can include sand, gravel, eggshells, bone fragments, coffee grounds, and fruit and vegetable seeds.7 Grit is removed in order to avoid destruction of pipes from abrasion, as well as to remove some unwanted organic material. Our plant will be using an aerated grit chamber, meaning air will be introduced along the edge of the chamber. This creates a current within the water that settles the grit while leaving the smaller particles suspended in the water for later treatment.7
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Our facility will use two aerated grit chambers, each with a volume of 148.5 m3. The dimensions of each chamber will be a length of 11 m, a width of 4.5 m, and a depth of 3 m. The detention time required in order to fully settle the grit will be approximately 8.4 minutes. The amount of air required to fully settle the grit will be 7.7m3min. These parameters will remove approximately 2.14m3day of grit at peak hours.
3.5 Primary Clarifiers (Primary Settling Basins)
Primary clarifiers are analogous to sedimentation basins in the drinking water treatment process. They remain extremely still to allow particles to settle to the bottom by gravity, where they are collected as sludge by a large rotating arm. Approximately 60% of the total suspended solids will be removed, as well as 30% of the BOD and 20% of the phosphorus, allowing the rest to pass on to the secondary clarifier. Our facility will use alum as a coagulant in order to maximize the amount of particulate matter that settles and reduce the energy requirements in the secondary settling step.
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Our treatment facility will have two circular clarifiers with a depth of 3 m and an overflow rate of 47m3m2-day. The diameters have been rounded up to 30 m in order to ensure that they can process the required volume of wastewater for Benopolis’ waste requirements. This means that the volume of each clarifier will be approximately 2,120 m3. The detention time will be approximately 2 hours and the flow rate of the clarifiers will be about 36m3m2-day. This will result in about 1450kgday of primary solids being produced, which will be stored and transported to a landfill.
3.6 Secondary Treatment
Secondary treatment uses microorganisms to remove the rest of the dissolved particles that were too small for primary settling to remove. Our facility will be using a suspended-growth system, where the microorganisms are directly mixed into the wastewater for removal of small particles and organic matter.
3.6.1 Aeration Basin
The aeration basin is the primary component of the secondary treatment process. There are microorganisms such as bacteria, rotifers, fungi, and protozoa, which, when mixed with the organic matter and particulate matter within the wastewater, are called mixed liquor.7
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The bacteria is added to the water in order to digest the organic matter within the water, and then the excess bacteria are consumed by the protozoa and rotifers, who will be passed along to the secondary clarifier along with excess sludge.
The volume of the aeration basin was determined based on the BOD levels of samples of Benopolis’ wastewater. Because Benopolis has 83% of its wastewater coming from municipal waste and 17% coming from industrial waste, the BOD5 levels of the wastewater have been determined to be approximately 269mgL. This allowed us to calculate the needed volume of the aeration basin to be approximately 9.9 million liters with an aeration period of 2.37 hours.
3.6.2 Secondary Clarifier
The secondary clarifier is similar in design to the primary clarifier, but it contains both microorganisms as well as settling sludge. The microorganisms remain at the bottom of the tank without a food source, as the aeration basin removed all of their food source. These starved microorganisms will become activated, and 20-30% of the sludge-microorganism combination will be returned to the aeration basin to be used again as returned activated sludge (RAS). The rest will be processed as waste activated sludge (WAS), which is shipped to a landfill. The solids retention time (SRT) before the activated sludge is either returned or wasted is only five days, so the food-to-microorganism (F/M) ratio will be relatively low. This means that the microorganisms will be starved for a period of time, making them more efficient at removing BOD from the system. Therefore, the power requirements for aeration will be less, as the microorganisms will be saturated with food.
The secondary waste produced has been calculated using the volume of the aeration basin. It is estimated that about 647kgday of secondary dry solids will be produced by the secondary settling basin, and of that approximately 20%, or about 129kgday, will be returned activated sludge. The rest will be transported to a landfill. The F/M ratio is estimated to be about 0.583, which is extremely close to the recommended ratio of 0.6.12
3.6.3 Disinfection
Before either reclamation or ultimate release of the treated wastewater, possible pathogens must be thoroughly removed from the water as a final secondary step. In Benopolis, this will be done through UV light pulsing. UV light sterilizes any bacteria that were not killed by the other processes, so they will not be able to reproduce through the reclamation or release process. This means that most bacteria and all harmful pathogens will be completely gone by the time the water hits any intake pipes or the Carquinez Strait. The intensity used for the UV sterilization process will be 100 microWatts per cm3. This has been previously determined to be sufficient in sterilizing 99.9% of bacteria.
3.6.4 Aeration
The last step of the secondary wastewater treatment process is aeration, or adding dissolved oxygen back into the water. This helps to dissolve other gases such as carbon dioxide, as well as helps to oxidize dissolved metals and organic compounds that may be left within the water after the other treatment steps. In our facility, we will be incorporating two fine-pore diffusers, which produce small oxygen bubbles that are very efficient in the dissolution process. This requires less energy and less air to achieve the same result as surface aeration or coarse-pore diffuser aeration. Because the pores are extremely small (less than 5mm), they can be easily clogged. They will need to be cleaned periodically using hydrochloric acid, which is why we have incorporated a pairing so that one can be shut down for cleaning without backing the entire system up.
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3.7 Tertiary Treatment
Secondary treatment removes most of the pathogens and pollutants from the effluent wastewater, but sometimes it leaves behind some polluting nutrients such as nitrogen and phosphorus, which are both toxic and caused by human urine.
3.7.1 Denitrification
During secondary treatment, ammonia is converted to nitrate. This raises the levels of nitrogen within the water, and because nitrogen requires much more time, bacteria, and oxygen to dissolve than other nutrients such as carbon dioxide, it requires a separate process for its removal. In order to completely denitrify the effluent water, a process called the modified Ludzak-Ettinger (MLE) process.
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In the MLE process, mixed liquor suspended solids (MLSS) are recycled to an anoxic chamber. This, along with the following aerobic chamber, help remove any COD, or carbon waste. Then, the aerobic chamber converts nitrates from the nitrification process in secondary treatment. It is then recycled back to the anoxic zone where the nitrates are broken down into nitrogen and easily removed.
3.7.2 Phosphorus Removal
Phosphorus is another nutrient present in wastewater that is toxic in large quantities. This can be somewhat removed through the addition of alum, but it is also very effectively dealt with by phosphate-accumulating organisms (PAOs). They, when exposed to an anaerobic zone, convert the phosphorus that is difficult to treat into easily removable phosphorus particles that settle out with sludge.
3.8 Solid Waste Management
3.8.1 Digestion
Once all of the solids have settled out of the primary and secondary clarifiers, they must be stabilized in order to be recycled or taken to a landfill. One method of stabilizing the solids is aerobic digestion, where the waste-activated sludge is pumped back into the aeration chamber for a long period of time. This results in overall reduced organic matter in the sludge.
Anaerobic digestion is the method we will be using, because it requires less energy, as there is no aeration required. It begins with hydrolysis, followed by acidogenesis and acetogenesis, ending with methanogenesis. These steps use specialized bacteria to break down organic matter further to get the same result as aerobic digestion.
3.8.2 Dewatering
Once the sludge has its organic matter reduced, the volume is to be reduced for easier transport. This is done through a process called dewatering, which increases the percent of solids in the total volume from 0.5% to anywhere from 15-50%. Dewatering occurs in a mechanism called a drying bed, which allows water to seep from the sludge into layers of gravel and sand over a time period of about three months. From there, the solids can be scraped up and transported to a landfill.
4.0 Summary
4.1 Overview
This report outlines the plans the Quattlebaum Engineering Company has made for the design of a new wastewater treatment plant in Benopolis, CA. It includes detailed step-by-step summaries and calculations of required dosages of chemicals and volumes of containment units. It also maps out the location of the facility with regard to industry, residential areas, and water source. Laws governing the monitoring the level of contaminants and methods of release were outlined and taken into account. This report’s goal was to show the citizens and leaders of Benopolis that everything was decided based on logic. Public health and safety, fiscal management, and sustainable design played major roles in designing this wastewater treatment facility.
4.2 Public Meeting
This plan will not reach the level of success that is desired if citizens do not accept it. That is why there will be a public town hall meeting on Friday, November 9th, 2018 at 1:25 p.m. to present the plans as well as answer any questions that have not been answered by the presentation. The citizens of Benopolis are the Quattlebaum Engineering Company’s top priority, and to ensure that all of the needs of the citizens are met, it is important that they know the facts and are on board with everything.
References
1U.S. Code of Federal Regulations. National Emission Standards for Organic Hazardous Air Pollutants From the Synthetic Organic Chemical Manufacturing Industry for Process Vents, Storage Vessels, Transfer Operations, and Wastewater. Title 40 Part 63. 4-9-2004 Edition.
2United States Environmental Protection Agency. Summary of the Clean Water Act. Web. 2018.
3California Code of Regulations. State Water Resources Control Board and Regional Water Quality Control Boards Chapter 26. Classification of Wastewater Treatment Plants and Operator Certification. Title 23 Division 3. 6-12-2015 Edition.
4California Waterboards. General Waste Discharge Requirements for Discharges to Land by Small Domestic Systems. Web. 2018.
7Mihelcic and Zimmerman, Environmental Engineering: Fundamentals, Sustainability, Design, John Wiley & Sons, Incorporated, Edition: 2nd, ISBN: 9781118741498.
12Water and Wastes Digest. Headworks: Removing Inorganics and Preventing Wear. Web. 2018.
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Appendix I (Calculations)
Plant Design Volume:
2018:
V = (85gallonsperson*day)(26,038 persons) = 2,213,230gallonsday
2028:
V = (85 gallonsperson*day)(27,954 persons) = 2,376,090gallonsday
2043:
V = (85gallonsperson*day)(31,095 persons) = 2,643,075gallonsday
Aerated Grit Chamber Design:
Volume:
Total Volume = (1.65m3s)(3 min)(60smin)= 297 m3
Chamber Volume = 297m32 chambers= 148.5 m3
Dimensions: (assuming width-to-depth ratio of 1.5:1 and depth = 3)
Grit chamber width = (1.5)(3m) = 4.5m
Grit chamber length = volumewidth*depth= 148.5m3(4.5m)(3m)= 11m
L*W*D = (11m)(4.5m)(3m)
Average Hydraulic Retention Time
V=Q*Θ → Θ=VQ= (148.5m3(.59m3s))(2)(1min60s)= 8.39min
Air Requirements (assuming 0.35 m3m-min)
Total air = (2 tanks)(11m length)(0.35m3m-min) = 7.7 m3min
Quantity Grit Removed @ Peak (assuming 0.015 m3103m3grit in untreated WW)
Grit Removed = (1.65m3s)(0.015m3103m3)(86400sday) = 2.14m3day
Primary Settling Tank Design
Assuming 2 circular tanks with a depth of 3m and a surface overflow rate of 47m3m2-day
Total clarifier area
A = QOR= (0.59m3s)(86400sday)47m3m2-day= (1084.6 m2)(12)= 542.3m2 per clarifier
Tank diameter
D = A4 =542.3m24= 26.28m → rounded up to 30m
New area
A = 4*R = (4)(30m)2 = 706.86m2
Vclarifier = A*d = (706.86m2)(3m) = 2,120.6m3
Detention Time
Θ = VQ= (2120.6m3(0.59m3s)(86400s)2)(24hrday) = 2 hr
Observed Flow Rate
OR = QA= (0.59m3s)(86400sday)2706.86m3= 36.058 m3m2-day
Other Calculations:
Incoming BOD
Municipal flow → 83%; industrial flow → 17%
Municipal BOD5 = 253 mgL; CODlab = 465 mgL
Industrial BOD5 = (0.75)(465mgL) = 348.75 mgL
Total BOD5 = (0.83)(253mgL)+(0.17)(348.75mgL) = 269.28mgL
Aeration Basin
Solids Retention Time (SRT) = 5 days; Decay Constant (kd) = 0.05day
Incoming Flow (Qo) = (2,643,075galday)(m3264.172 gal)(1000Lm3) = 10005129.23Lday
Yield Coefficient (Y) = 0.48 g biomassg BOD5; X = 3,270 mgL
Incoming Substrate (So) = (0.70)(269.28mgL) = 188.49mgL; Substrate (S) = 20mgL
1SRT= [QoYVX(So-S)] - kd → 15 days= [(10*106Lday)(0.48g biomassg BOD5)V(3270mgL)(188.49mgL- 20mgL)] - 0.05day
Basin Volume (V) = 9.898*105 L
Aeration Period
Θ = VQ= 9.898*105L10*106Lday= 0.0989 days (24 hrday) = 2.37 hr
Primary dry solids produced
Influent = 265mgL; Effluent = 120mgL
TSSprimary = (Q)(influent - effluent)(kg106mg) = (10*106 Lday)(265mgL- 120mgL)(kg106mg)
TSSprimary = 1450.74kg primary SSday
Secondary dry solids produced
SRT = VXQwXw= 5 days = (9.898*105 L)(3270mgL)QwXw→ 6.47*108mgday(kg106mg) = 647.3292 kgday
F/M Ratio
F/M = QSoXV= (10*106Lday)(188.49mgL)(3270mgL)(9.898*105L)= 0.583lb BOD5lb MLSS-day
The solids retention time (SRT) is low (5 days), which means that the F/M ratio is low. The power requirements for aeration will be less. The microorganisms will be saturated with food. The mean cell retention time is low. The sludge age is low. The sludge wastage rate may have been recently increased. The MLSS may have been increased.
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