Primary Settlement Tanks

Why buy when you can modify?

Presented by Dan Course

IChemE Technical Seminar – 17th October 2018

Dan Course

AMIChemE

MEng Environmental Engineering - Swansea

MMB Process Engineer

5 years Industrial and Municipal Wastewater Treatment

Project background

Dortmund Type Primary Settlement Tanks

Existing Works Performance

Proposed Solutions

Chosen Solution

Results

Conclusions

Contents

Works Description

• Built 1970s

• Conventional Trickling Filter Works

• Not permanently manned

• Average Flow = 320 m3/day

• Quality Scheme

• BOD Consent = 22 mg/l (no change)

• TSS Consent = 39 mg/l (no change)

• Ammonia Consent = 7 mg/l (new)

Collection

Chamber

Primary

Settlement

Tanks

Trickling Filters

Humus Tanks

Reed Beds

DischargeH

um

us D

eslu

dg

e Sludge

Tank

Primary

Sludge

Project Background

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5.00

10.00

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25.00

30.00

35.00

40.00

45.00

Concentr

ation (

mg/l)

Final Effluent Quality

BOD

TSS

AmmN

BOD Consent

TSS Consent

AmmN Consent

Project Background

0.00

5.00

10.00

15.00

20.00

25.00

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35.00

40.00

45.00

Concentr

ation (

mg/l)

Final Effluent Quality (Spot Samples)

BOD

TSS

Ammonia

BOD Consent

TSS Consent

AmmN Consent

Project Background

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Tricklin

g F

ilter

Rem

oval P

erf

orm

ance

Trickling Filter Removal Performance (Composites)

BOD Removal

TSS Removal

AmmN Removal

Project Background

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Rem

oval P

erf

orm

ance (

%)

Trickling Filter Removal Performance (Spot Samples)

BOD Removal

TSS Removal

AmmN Removal

Dortmund Type Primary Settlement Tanks

• Separate Solids from Liquids

• Reduces BOD and TSS Load

• Thick sludge concentration

• Beginning of a poorly operating plant

Dortmund Type Primary Settlement Tanks

Existing Performance

Parameter Specification Actual

Hydraulic Loading Rate (all PSTs in Service) at

PFF (m3/m2.hr)1.20 1.08

Hydraulic Loading Rate (One PST Out of

Service) at PFF (m3/m2.hr)3.00 2.15

Hydraulic Retention Time at DWF (hrs) 2.00 9.63

Existing Performance

-20%

-10%

0%

10%

20%

30%

40%

50%

60%

70%

80%

Perc

enta

ge R

em

oval

Primary Settlement Tank Removal Performance (Daily Composites)

BOD Removal

TSS Removal

BOD Target Removal

TSS Target Removal

Existing Performance

-200%

-150%

-100%

-50%

0%

50%

100%

150%

Perc

enta

ge R

em

oval

Primary Settlement Tank Removal Performance (Spot Samples)

BOD Removal

TSS Removal

BOD Target Removal

TSS Removal Target

Existing Primary Settlement Performance

Bad Χ

Rising Sludge

Good ✓

Existing Primary Settlement Performance

What is rising sludge?

• Occurs under anaerobic conditions

• NO3- + 6H+ + 5e-

½N2 + 3H2O + Energy

• NO2- + 4H+ + 3e-

½N2 + 2H2O + Energy

• Nitrogen becomes trapped in sludge mass

• Sludge rises to surface

Existing Primary Settlement Performance

Why does it occur?

• Long sludge detention time

• Rat holing

• High incoming solids concentration

• Sludge removal too short

• Sludge not collecting at bottom of tank

Consequence

Conventional Solution – New PSTs

Item Cost

Divert Incoming Rising Mains £4,200

Primary Tanks £172,800

Desludge pumps + Pipework £26,400

Pipework to Trickling Filters £4,200

Modified Humus Desludge £44,900

Decommission Existing £2,400

New footpaths and road £10,900

TOTAL £265,800

Innovative Solution Reline PST

Reline PSTs Cost

Empty & Clean Tank £1,300

Reline Tank £21,000

TOTAL £22,300

92%

Cost

Saving

Innovative Solution Reline PST

Epoxy Resin HDPE Liner Silicate Coating

Advs Disadvs Advs Disadvs Advs Disadvs

Widely used

product

Confined

Space

requirement

SimpleMaybe susceptible

to cold weatherReputable

Unknown suitability

for application

ReputableNot used for

this applicationWell known

Risk of intrusion

behind sheet

Silicate material

technically soundGrit blast required

Smooth Prefab offsite

Difficult to install

one sheet over

pipework

Spray CoatingConfined space

requirement

Trained

applicatorsTough

Does not include

installation cost

Used in sewer

networks

Easy

installation

Easily secured

with batons

Does not include

cleaning

Resistant to water

after 24 hrs

£21,000 £29,500 £25,000

Material Information – Sikagard 63N

• Solvent Free Two Part Epoxy Resin Coating

• Rendering, protective lining, anti-corrosion coating

• Very good chemical resistance, easy application, solvent free

• No influence by sun radiation

• Concrete to be primed before application

• Application between 10˚C and 30˚C

• Mixture of chemicals, to be applied within 30 mins

• Laminated with special profiled roller

• Waiting time/ Overcoating = Minimum 5 hours, Maximum 3 days

• Curing time minimum = 24 hours

Installation Method

Isolate PST

Empty CleanApply

CoatingCure

Bring Back

Online

Total Time – 1 Week

Not desludged as regularly as it should

Sludge rose to the surface

Results

Rest of works working well

Operational demands on other sites

Before

After

0

10

20

30

40

50

60

70

80

90

100

09/06/18 11/06/18 13/06/18 15/06/18 17/06/18 19/06/18 21/06/18 23/06/18

TS

S %

age R

em

ovals

Comparison of TSS Removals

Unlined PST Removals

Lined PST Removals

TSS Removal Target

Results

Avg.

15%

Conclusions

✓ Improvement in TSS Removal

✓ Less sludge sticking to side of the tank

✓ Cost saving of 92%

✓ Installed within 1 week

Χ Infrequent sludge removal still caused rising sludge

Recommendations

Include Scum Removal

Increase sludge removal frequency

➢ Increase manual operation (increased labour requirements)

➢ Automatic desludge pumps (Extra £18,500, 85% cost saving vs building new)

➢ Use on site operated daily

Thank you