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Wastewater

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Submitted By mysrebwar
Words 824
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Introduction:
This is a preliminary wastewater treatment plant design for Modeltown City with average daily flow rate of 0.500m3/s. For redundancy, the plant is designed with two trains capable of handling the full plant load individually. Bar Rack:
The bar rack system should consist of at least two bar racks each capable of handling the plant full peak flow. It should be a self-cleaning system to maximize efficiency and reliability.
Grit Chamber:
The grit chamber is necessary for the removal of the larger suspended inorganic solids. A vortex grit chamber is recommended for constant energy efficiency over a wide range of flows, small footprint and minimal headloss.
Flow Measurement:
A parshall flume should be installed after the grit chamber for proper flow measurement of plant influent. For more accuracy a magnetic flow meter can be placed after the parshall flume.
Equalization Basin:
The wastewater treatment plant being designed requires an equalization basin to even out flow and BOD variations. The average daily flow is 0.500 m3 /s. The following flows and BOD5 have been found to be typical of the average variation over a day. This design includes an aeration system in the equalization basin which can reduce the influent BOD by 10-20%. The volume of the basin will be 9545m3. The mass of BOD5 leaving the tank will be 388kg.
Un-equalized BOD Mass Loading Ratios:
• Peak to Average 1.97:1
• Minimum to Average 0.140:1
• Peak to Minimum 14.06:1
Equalized BOD Mass Loading Ratios:
• Peak to Average 1.47:1
• Minimum to Average 0.630:1
• Peak to Minimum 2.34:1
Power required for aeration system is 57.27kW.

Primary Clarifier:
This design was made using an overflow rate of 28.0 m/d and a hydraulic retention time of 2.0 hours. All calculations are attached. Assuming four primary sedimentation tanks with length to width ratio of 4:1 for redundancy and system reliability. The sludge collected is sent for sludge processing along with the sludge from the secondary clarifier. The overflow rate be for the unequalized maximum flow would be 169 m/d. All calculations are attached.
Aeration Tank:
Assuming the following wastewater characteristics.

Assumptions:
1. Temperature of 20C
2. Ratio of mixed-liquor volatile suspended solids (MLVSS) to mixed-liquor suspended solids
(MLSS) = 0.75
3. Return-sludge concentration is 15,000 mg/L of suspended solids (SS)
4. Design MLVSS = 4000 mg/L
5. Design mean cell-residence time (c) or solids retention time (SRT) = 7 d
6. Effluent contains 20 mg/L of biological solids, of which 60 percent is biodegradable
7. BOD5 = 0.7  Ultimate BOD
8. Wastewater contains adequate nutrients for biological growth.
For an effluent with BOD ≤ 15 mg/L. The aeration tank has a volume of 2082m3. A hydraulic retention time of 1 hour, which seems low. The design is based on a MLVSS concentration of 4000mg/L and a SRT of 7 days. All calculations are attached.
Secondary Clarifier:
The secondary clarifier was designed using MLSS settling data from a pilot plant study.

Disinfection:
An ultraviolet (UV) light disinfection system is recommended for this plant. Using UV light for disinfection avoids the addition of chemical and increases plant efficiency.
Sludge Treatment:
For the sludge treatment gravity thickening to increase solids concentration from 1 to 3 % is recommended. Thickening should be followed by anaerobic digestion where methane gas is produced and used to power plant operations. The biosolids are then passed through a dewatering process and dried out on drying beds before being composted for agricultural use.

Questions:
1. Based on the given data can you estimate the number of people served by the new WWTP?
Assuming wastewater generation is about 100 gallons per day per capita and a daily flow of about 12 million gallons per day. The population served is about 120,000 people.
2. What would be the effect on the design process if the temperature were 10C?
The drop in temperature would slow the biodegradation increasing the required retention times.
3. What effect would you expect if the mean cell residence time were reduced to five days?
The time in the aeration tank would have to increase.
4. How does the theoretical yield coefficient compared with the observed value?
The assumed value was greater than the observed value.
5. Are you satisfied that the activated-sludge system that you have designed is optimal for your wastewater? Would you recommend using a conventional plug flow aeration tank or trickling filters or some other attached growth biological treatment for this wastewater?
6. What actions or design/operation modifications would you recommend in the event that sludge bulking becomes a problem at the Modeltown WWTP?
Adding a bulking agent to improve plant performance.
7. What would you suggest in order to accomplish nitrification with an effluent NH4-N concentration of 0.5 mg/L?
Using the A2/OTM process.

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