Wastewater Treatment Plant - Wastewater Engineering - Old Exam Paper, Exams of Business Management and Analysis

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Cork Institute of Technology

CORK INSTITUTE OF TECHNOLOGY INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ Semester 2 Examinations 2008/

Module Title: Wastewater Engineering

Module Code: CIVL

School: Building and Civil Engineering

Programme Title: Bachelor of Engineering (Honours) in Structural Engineering

Programme Code: CSTRU_8_Y

External Examiner(s): Mr P Anthony; Prof P O’Donoghue Internal Examiner(s): Mr TL O’Driscoll

Instructions: Answer Question 1 and 2 other questions.

Duration: 2 hours

Sitting: Summer 2009

Requirements for this examination:

Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination paper. If in doubt please contact an Invigilator.

Q1 A treatment plant is proposed for a town with a population equivalent (PE) of 50,000. Assume for this plant that 1PE equates to 60g BOD/day and 220L/PE/day. Wastewater is to be treated to the 25/35 standard. Suspended solids concentration at the inlet to the works is 230 mg SS/L. Design a conventional activated sludge system wastewater treatment plant, using the design parameters given in Table 1. The treatment plant will also include a primary settlement stage. (a) Calculate the plan area and depth of the primary sedimentation tanks and the primary sludge production rate, given the following design parameters:

• Average overflow rate in the range 1-2 m^3 /m^2 /hr
• Peak hourly overflow rate = 4m^3 /m^2 /hr
• Average hydraulic retention time, 2.5 hours
• BOD removal rate = 25%
• Suspended solids removal rate 50%
• Waste sludge concentration 2% solids (6 marks) (b) Calculate daily primary sludge production (tonnes/day) (6 marks) (c) Calculate the plan area of the aeration tanks, given the following design parameters:
• Sludge depth 4m
• MLSS 3000mg/L (6 marks) (d) Establish the peak oxygen demand for the activated sludge plant, given:
• OD = 0.75 Q (BOD (^) i – BODe) + 2V (^) a.MLSS (4 marks) Where: OD is oxygen demand (g/h) Q is flow through plant (m^3 /h) BOD (^) i and BODe are the influent and effluent BOD levels in mg/L V (^) a is the volume of the basin in m^3 MLSS is the concentration of mixed liquor suspended solids in g/L (e) Calculate the average volume (m^3 /day) of sludge to be wasted from the activated sludge system each day, given:
• Qw = MLSS(VA + VC) / Sludge Age * SW Where: VA = Aeration tank volume (m^3 ); VC = clarifier volume (m^3 ); S (^) W = WAS suspended solids (mg/L)
• Average sludge age: 10 days
• Waste sludge concentration: 1.5% solids (4 marks)

(f) What are the advantages of the activated sludge secondary treatment system, when compared with the trickling filter treatment system? (7 marks) (g) Describe a number of tertiary treatment technologies which could be used, if the outfall from this treatment plant discharged to sensitive waters. (7 marks)

Table 1

Q2 A new storm-water sewer is to be designed as shown in Figure 1. The lengths of the pipes and the catchments are shown in Table 2.

Design the sewer system in Figure 1, for a return period of 5 years. Indicate the pipe diameters & pipe slopes using the attached Table 2.

Design Assumptions

1. Assume the ground falls uniformly from manhole to manhole.
2. A minimum cover of 1.0m is allowed between cover level, (CL), and top of pipe. Invert Level, (IL) = CL – 1.0m – diameter of pipe.
3. Assume Time of Entry of 4 minutes.
4. For each pipe, round the time of concentration to the nearest ½ minute.
5. Use the Dillon equation to establish rain intensity, I = 152.4 T (^) p 0.2^ / t 0. I is rain intensity in mm/hr T (^) p is the return period t is the storm duration in minutes.
6. Use the Modified Rational method to establish flow, Q = 2.78 C (^) v.Cr .I. A Where: Q is the flow in L/s, I is rain intensity in mm/hr and A is the impermeable area in Ha. PR = 70%; Cr = 1.

7. Minimum and maximum velocities are 0.75m/s and 3 m/s respectively.
8. For pipeline design, see Attachments 1: Colebrook White Chart with Ks = 0.6 and pipe design sheet.

(30 marks)

Figure 1

MH IL 4.731mOD Area 0 ha

Pipe 2.

Pipe 2.

Pipe 1.

Pipe 1.

Pipe 1. Pipe 3.

MH A GL 7.87mOD Area 1.2 ha

MH B GL 7.57mOD Area 1.8 ha MH C GL 6.98mOD Area 1.3 ha

MH D GL 6.87mOD Area 1.8 ha

MH E GL 7.43mOD Area 0.8 ha

MH F GL 6.93mOD Area 1.2 ha

Cork Institute of Technology Table 2PipeNr

Pipelength(m)

Fall(m)

Slope(m/m)

Diameter(mm)

Velocity(m/s)

Timeof Entry(mins)

Timeof Flow(mins)

Timeof Conc(mins)

ImpArea(ha)

TotalImpArea(ha)

Rain(mm/hr)

Q (L/s)

Capacity(L/s)

Upstream IL

(mOD)

D/stream IL

(mOD)

63

74

58

67

84

81

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Q3 A storm-water pumping station is to be designed for an urban location in Cork. Peak flow

from the pumping station is to be designed for 5 year return period storm, with a 30 minute

duration. The total impermeable area upstream of the pumping station is 136,958m^2. The

static lift is 25m and the length of rising main is 4100m.

Use the Dillon equation to establish rain intensity, I = 152.4 T p 0.2^ / t 0.

I is rain intensity in mm/hr

T p is the return period

t is the storm duration in minutes.

Use the Modified Rational method to establish flow, Q = 2.78 Cv.Cr .I. A

Where:

Q is the flow in L/s, I is rain intensity in mm/hr and A is the impermeable area in Ha.

PR = 62%; Cr = 1.

(a) Calculate the peak flow required from the pumps (litres/sec). (4 marks)

(b) Calculate the total friction losses in the rising mains for the pipeline for each of the

rising main options listed. (5 marks)

(c) Size the pump motor for each of the rising main options listed. (6 marks)

(d) Calculate the annualised cost of each option. Establish which is the most economic

pump and rising main option. (9 marks)

(e) Describe the typical layout and arrangement of duty and standby pumps in a

submersible pumping station to cater for varying storm-water flow rates, using sketches to

illustrate your answer. (6 marks)

Available rising main diameter: 750mm, 825mm or 900mm.

Assumptions:

ks = 1.

Pump Power required = {Q(m^3 /hr) x H (m)}/125 kW

Ignore standby pumps in economic analysis.

Allow for 0.5m of station loss.

Capital Costs:

Rising main: 750 mm dia. = €400/m; 825 mm dia. = €450/m; 900 dia. = €500/m

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Pumps: 1000kW - €750,000; 100kW - €80,000; 50 kW - €50,000; 10 kW - €15,000;

Pump Station: 80 % of capital cost of pumps

Running Costs:

Cost of Capital: R = P{(1+r) Nr}/{(1+r) N^ - 1}

Where:

P is the capitalised amount of annual payments R with return on investment r over N years.

Use 5% return on investment over 10 years

Annual maintenance costs = 7% of capital cost of pumps.

Cost of electricity = €0.1/kWh

Attachments: Colebrook White Chart with ks = 1.

Q4(a) Describe how Sustainable Urban Drainage (SUDS) systems should be used within a new

urban housing development design? Use sketches to illustrate your answer.

(10 marks)

Q4(b) The septic tank system has been used for the treatment of domestic wastewater for single

rural houses in Ireland for a number of years. Describe how this system works.

(5 marks)

What are the advantages and disadvantages of these treatment systems?

(5 marks)

Describe the site suitability tests that must be completed prior to recommendation of an

appropriate small-scale wastewater treatment system for a single domestic dwelling.

(5 marks)

Q4(c) Describe the mechanism of the automatic tipping-bucket rain-gauge. Use sketches where

necessary. (5 marks)

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