am-pro-wwt-wwt 3.1 primary treatment-sec3 issue 1.1

THAMES WATER Asset ManagementInternal Company and Partners Asset Standards

Technical Lead: Bryan Bleeker Issue 1.1

AM-PRO-WWT-WWT 3.1 Primary Treatment-SEC3 Page 1 of 21

UNCONTROLLED WHEN PRINTED

Asset Management Asset Standard Section 3 of 5 Operating Standard Wastewater Non-Infrastructure WWT 3.1 Primary Treatment

Document Reference AM-PRO-WWT-WWT 3.1 Primary Treatment-SEC3

Version Issue 1.1

Data Owner Sarah Shipsey

Data Steward Kevin Kelleher

Technical Lead Bryan Bleeker

Document Author Tim Wheatley

Authorised by Asset Standards

Document Location AM Lotus Notes Best Practice Database on LNAPP01

Date of Issue December 2012

Reason for Issue Conversion and validation of old BOP into new Standard format

Next Review December 2014





Contents:- 3.0 Operating Standard..................................................................................................................... 3

3.1 Primary Treatment Performance Measures ............................................................................ 3 3.2 Operator Activities ................................................................................................................... 5

3.2.1 Process Checklist Circular Primary Settlement Tanks.................................................. 5 3.2.2 Process Checklist Horizontal Scraped Primary Settlement Tanks ............................... 6 3.2.3 Process Checklist Pyramidal Autodesludged Primary Settlement Tanks..................... 7 3.2.4 Process Checklist Pyramidal No Power Primary Settlement Tanks ............................. 8 3.2.5 Process Checklist Horizontal Unscraped Primary Settlement Tanks ........................... 9 3.2.6 Process Checklist Lamella Primary Settlement Tanks ............................................... 10 3.2.7 Process Checklist Notes ................................................................................................ 11

3.3 Troubleshooting Guide .......................................................................................................... 12 3.3.1 Dos and Donts ............................................................................................................... 14

Appendices .......................................................................................................................................... 15 3.4 Operation ............................................................................................................................... 15

3.4.1 Primary Settlement Efficiency ........................................................................................ 15 3.4.2 Impact on Secondary Treatment Processes .................................................................. 16 3.4.3 Primary Settlement Tank Optimisation........................................................................... 18 3.4.4 Contingency Planning .................................................................................................... 19 3.4.5 Plant Release / Taking out of and Bringing into service ................................................ 20





3.0 Operating Standard

3.1 Primary Treatment Performance Measures

Measure Why Frequency Method of Collection

Screened Crude Sewage

Concentration of BOD to calculate organic load and TSS to calculate suspended solids load. This is taken downstream of site liquor returns.

Operational Sampling Program

Sampling / Laboratory Analysis

Site Return Stream Flow

Site Return stream that may be required for process calculations. This could be SAS from Activated sludge works, humus sludge from biological filtration works, backwash sludge from tertiary plants and liquor return from sludge works.

Daily or continuous

SCADA, Onsite flow reading, Daily log

Settled Sewage

Concentration of BOD / NH3 to calculate organic load and TSS to calculate suspended solids load.

Works PE< 5000

5,000 - 25,00025,000-100,000

100,000-250,000250,000-1,000,000

>1,000,000

Quarterly Quarterly Monthly Monthly Weekly Weekly


Settled Sewage Flow

Settled Sewage Flow required for Process Calculations. Continuous SCADA, Onsite flow reading, Daily log

Sludge Blanket Level

To ensure Sludge Blanket levels are controlled so that gross solids are not lost or discharged in the settled sewage

Daily or Continuous

SCADA, Fixed or Portable Blanket Detectors.

Hydraulic Retention Time

The hydraulic retention time is the length of time that the screened sewage actually spends in the primary settlement tank(s). Typically it should be above 2 hours but the ideal figure varies for different tank configurations. A reduction in the retention period occurs for example during periods of peak flow. Measure within process design ensures robust plant performance and compliance the withdrawal of fresh sludge.

Quarterly Process Calculation

Sludge Flow Control of Primary Sludge withdrawal Rates (on large works): The amount of solids withdrawn from the system will determine the amount of solids settled in the tanks and therefore the efficiency and performance of this stage of treatment.

Daily or Continuous

Automatic De-sludge Pump Control. SCADA





Measure Why Frequency Method of Collection

Percentage dry solids of the sludge thickening Plant

Concentration of suspended solids to calculate organic load to calculate suspended solids load withdrawn from the primary treatment stage.

Operational Sampling Program


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3.2 Operator Activities 3.2.1 Process Checklist Circular Primary Settlement Tanks Any of the following checks that result in faulty operation needs to be logged and reported for maintenance / repair

Task Low PE (10-1000)

Low-Med (1000-5000)

Med PE (5000-20,000)

High PE (20,000-350,000)

Manned (350,000+)

PST - CIRCULAR - CHECK BRIDGE/SCRAPER OPERATION OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D PST - CIRCULAR - CHECK SAFETY GUARDS OP/1W OP/1W OP/1W OP/1W OP/1W PST - CIRCULAR - CHECK SCUM TRAP FOR NORMAL OPERATION AND CLEAN/HOSE OUT OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D PST - CIRCULAR - CHECK DE-SLUDGE PUMP AND TIMER FOR NORMAL OPERATION OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D PST - CIRCULAR - CHECK AND LOG SLUDGE LEVEL BY DIPPING TANKS OP/1W OP/1W/3D OP/1W/3D OP/1W/3D OP/1W/3D PST - CIRCULAR - CHECK AND REMOVE ANY BUILD-UP OF SCUM OR RAG ON TANK SURFACE, WEIRS OR LAUNDER CHANNELS

OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D

PST - CIRCULAR - CHECK SCUM BOARDS FOR BREAKS OR CARRY UNDER OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D PST - CIRCULAR - CHECK SETTLED SEWAGE QUALITY (VISUAL CHECK ONLY) OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D PST - CIRCULAR - CHECK STILLING CHAMBER FOR RAG, CLEAR AS NECESSARY OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D PST - CIRCULAR - PERIODICALLY EMPTY AND CHECK INTEGRITY T1/48M T1/48M T1/48M T1/48M T1/48M PST - CIRCULAR - DISTRIBUTION CHAMBER - CHECK ALL VALVES, PENSTOCKS AND VARIABLE WEIR POSITIONS/OPERATION

OP/1W OP/1W OP/1W OP/1W OP/1W

PST - CIRCULAR - DISTRIBUTION CHAMBER - CHECK COPASACS AND CHANGE AS NECESSARY


PST - CIRCULAR - DISTRIBUTION CHAMBER - CHECK FLOW SPLIT IS CORRECT OP/1W OP/1W OP/1W OP/1W OP/1W

PST - CIRCULAR - DISTRIBUTION CHAMBER - CHECK WEIRS AND CLEAN OP/1W OP/1W OP/1W OP/1W OP/1W

PST - CIRCULAR - DISTRIBUTION CHAMBER - REMOVE ANY SCUM AND DEBRIS, INCLUDING GRIT


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3.2.2




Process Checklist Horizontal Scraped Primary Settlement Tanks

Any of the following checks that result in faulty operation needs to be logged and reported for maintenance / repair


Low-Med (1000-5000)

Med PE (5000-20,000)

High PE (20,000-350,000)

Manned (350,000+)

PST - HORIZONTAL SCRAPED - CHECK BRIDGE/SCRAPER OPERATION OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D PST - HORIZONTAL SCRAPED - CHECK SAFETY GUARDS OP/1W OP/1W OP/1W OP/1W OP/1W PST - HORIZONTAL SCRAPED - CHECK SCUM TRAP FOR NORMAL OPERATION AND CLEAN/HOSE OUT


PST - HORIZONTAL SCRAPED - CHECK DE-SLUDGE PUMP AND TIMER FOR NORMAL OPERATION


PST - HORIZONTAL SCRAPED - CHECK AND LOG SLUDGE LEVEL BY DIPPING TANKS OP/1W OP/1W/3D OP/1W/3D OP/1W/3D OP/1W/3D PST - HORIZONTAL SCRAPED - CHECK AND REMOVE ANY BUILD-UP OF SCUM OR RAG ON TANK SURFACE, WEIRS OR LAUNDER CHANNELS


PST - HORIZONTAL SCRAPED - CHECK SCUM BOARDS FOR BREAKS OR CARRY UNDER OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D PST - HORIZONTAL SCRAPED - CHECK SETTLED SEWAGE QUALITY (VISUAL CHECK ONLY)


PST - HORIZONTAL SCRAPED - PERIODICALLY EMPTY AND CHECK INTEGRITY T1/48M T1/48M T1/48M T1/48M T1/48M PST - HORIZONTAL SCRAPED - DISTRIBUTION CHAMBER - CHECK ALL VALVES, PENSTOCKS AND VARIABLE WEIR POSITIONS/OPERATION


PST - HORIZONTAL SCRAPED - DISTRIBUTION CHAMBER - CHECK COPASACS AND CHANGE AS NECESSARY


PST - HORIZONTAL SCRAPED - DISTRIBUTION CHAMBER - CHECK FLOW SPLIT IS CORRECT


PST - HORIZONTAL SCRAPED - DISTRIBUTION CHAMBER - CHECK WEIRS AND CLEAN OP/1W OP/1W OP/1W OP/1W OP/1W PST - HORIZONTAL SCRAPED - DISTRIBUTION CHAMBER - REMOVE ANY SCUM AND DEBRIS, INCLUDING GRIT



3.2.3




Process Checklist Pyramidal Autodesludged Primary Settlement Tanks



Low-Med (1000-5000)

Med PE (5000-20,000)

High PE (20,000-350,000)

Manned (350,000+)

PST - PYRAMIDAL AUTODESLUDGED - CHECK DE-SLUDGE PUMP AND TIMER FOR NORMAL OPERATION

OP/1W OP/1W/3D OP/1W/5D N/A N/A

PST - PYRAMIDAL AUTODESLUDGED - CHECK AND LOG SLUDGE LEVEL BY DIPPING TANKS


PST - PYRAMIDAL AUTODESLUDGED - CHECK AND REMOVE ANY BUILD-UP OF SCUM OR RAG ON TANK SURFACE, WEIRS OR LAUNDER CHANNELS


PST - PYRAMIDAL AUTODESLUDGED - CHECK SCUM BOARDS FOR BREAKS OR CARRY UNDER


PST - PYRAMIDAL AUTODESLUDGED - CHECK SETTLED SEWAGE QUALITY (VISUAL CHECK ONLY)


PST - PYRAMIDAL AUTODESLUDGED - CHECK STILLING CHAMBER FOR RAG, CLEAR AS NECESSARY


PST - PYRAMIDAL AUTODESLUDGED - MANUALLY DE-SLUDGE TO WATER OP/1W OP/1W OP/1W N/A N/A PST - PYRAMIDAL AUTODESLUDGED - PERIODICALLY EMPTY AND CHECK INTEGRITY T1/48M T1/48M T1/48M N/A N/A PST - PYRAMIDAL AUTODESLUDGED - DISTRIBUTION CHAMBER - CHECK ALL VALVES, PENSTOCKS AND VARIABLE WEIR POSITIONS/OPERATION

OP/1W OP/1W OP/1W N/A N/A

PST - PYRAMIDAL AUTODESLUDGED - DISTRIBUTION CHAMBER - CHECK COPASACS AND CHANGE AS NECESSARY


PST - PYRAMIDAL AUTODESLUDGED - DISTRIBUTION CHAMBER - CHECK FLOW SPLIT IS CORRECT


PST - PYRAMIDAL AUTODESLUDGED - DISTRIBUTION CHAMBER - CHECK WEIRS AND CLEAN


PST - PYRAMIDAL AUTODESLUDGED - DISTRIBUTION CHAMBER - REMOVE ANY SCUM AND DEBRIS, INCLUDING GRIT



3.2.4




Process Checklist Pyramidal No Power Primary Settlement Tanks



Low-Med (1000-5000)

Med PE (5000-20,000)

High PE (20,000-350,000)

Manned (350,000+)

PST - PYRAMIDAL NO POWER - CHECK AND LOG SLUDGE LEVEL BY DIPPING TANKS OP/1W OP/1W/3D OP/1W/3D N/A N/A PST - PYRAMIDAL NO POWER - CHECK AND REMOVE ANY BUILD-UP OF SCUM OR RAG ON TANK SURFACE, WEIRS OR LAUNDER CHANNELS


PST - PYRAMIDAL NO POWER - CHECK SCUM BOARDS FOR BREAKS OR CARRY UNDER OP/1W OP/1W/3D OP/1W/5D N/A N/A PST - PYRAMIDAL NO POWER - CHECK SETTLED SEWAGE QUALITY (VISUAL CHECK ONLY) OP/1W OP/1W/3D OP/1W/5D N/A N/A PST - PYRAMIDAL NO POWER - CHECK STILLING CHAMBER FOR RAG, CLEAR AS NECESSARY


PST - PYRAMIDAL NO POWER - DESLUDGE ACCORDING TO AGREED PROCEDURES. ON MOST PLANTS WHERE TWO TANKS EXIST, ALTERNATE THE TANKS; RETAINING ONE IN USE WHILE THE OTHER IS DESLUDGED MANUALLY.FOR MULTIPLE TANKS THE DESLUDGING OPERATION WILL BE STAGGERED.


PST - PYRAMIDAL NO POWER - MANUALLY DE-SLUDGE TO WATER OP/1W OP/1W OP/1W N/A N/A PST - PYRAMIDAL NO POWER - RECORD OPERATION AND VOLUME OF SLUDGE REMOVED IN LOG


PST - PYRAMIDAL NO POWER - PERIODICALLY EMPTY AND CHECK INTEGRITY T1/48M T1/48M T1/48M N/A N/A PST - PYRAMIDAL NO POWER - DISTRIBUTION CHAMBER - CHECK ALL VALVES, PENSTOCKS AND VARIABLE WEIR POSITIONS/OPERATION


PST - PYRAMIDAL NO POWER - DISTRIBUTION CHAMBER - CHECK COPASACS AND CHANGE AS NECESSARY


PST - PYRAMIDAL NO POWER - DISTRIBUTION CHAMBER - CHECK FLOW SPLIT IS CORRECT


PST - PYRAMIDAL NO POWER - DISTRIBUTION CHAMBER - CHECK WEIRS AND CLEAN OP/1W OP/1W OP/1W N/A N/A PST - PYRAMIDAL NO POWER - DISTRIBUTION CHAMBER - REMOVE ANY SCUM AND DEBRIS, INCLUDING GRIT



3.2.5




Process Checklist Horizontal Unscraped Primary Settlement Tanks


Task Low PE (10-

1000)

Low-Med (1000-5000)

Med PE (5000-20,000)

High PE (20,000-350,000)

Manned (350,000+)

PST - HORIZONTAL UNSCRAPED - CHECK AND LOG SLUDGE LEVEL BY DIPPING TANKS OP/1W OP/1W/3D OP/1W/3D N/A N/A PST - HORIZONTAL UNSCRAPED - CHECK AND REMOVE ANY BUILD-UP OF SCUM OR RAG ON TANK SURFACE, WEIRS OR LAUNDER CHANNELS


PST - HORIZONTAL UNSCRAPED - CHECK SETTLED SEWAGE QUALITY (VISUAL CHECK ONLY) OP/1W OP/1W/3D OP/1W/5D N/A N/A PST - HORIZONTAL UNSCRAPED - PERIODICALLY EMPTY AND CHECK INTEGRITY T1/48M T1/48M T1/48M N/A N/A PST - HORIZONTAL UNSCRAPED - ON SMALL SITES WITH ONE TANK AND NO SLUDGE HOLDING FACILITY: - CHECK CONDITION OF TANK.

OP/3M OP/3M OP/3M N/A N/A

PST - HORIZONTAL UNSCRAPED - SITES WITH TWO OR MORE TANKS: - CAUTIOUSLY EMPTY TANK. OP/3M OP/3M OP/3M N/A N/A PST - HORIZONTAL UNSCRAPED - SITES WITH TWO OR MORE TANKS: - CHECK CONDITION OF TANK. OP/3M OP/3M OP/3M N/A N/A PST - HORIZONTAL UNSCRAPED - SITES WITH TWO OR MORE TANKS: - DE-SLUDGE ONE TANK UNTIL WATER IS DETECTED.

OP/3M OP/3M OP/3M N/A N/A

PST - HORIZONTAL UNSCRAPED - SITES WITH TWO OR MORE TANKS: - RETURN IT ON-LINE. OP/3M OP/3M OP/3M N/A N/A PST - HORIZONTAL UNSCRAPED - SITES WITH TWO OR MORE TANKS: - START ON THE NEXT TANK. OP/3M OP/3M OP/3M N/A N/A PST - HORIZONTAL UNSCRAPED - RECORD OPERATION AND VOLUME OF SLUDGE REMOVED IN LOG OP/1W OP/1W OP/1W N/A N/A PST - HORIZONTAL UNSCRAPED - DISTRIBUTION CHAMBER - CHECK ALL VALVES, PENSTOCKS AND VARIABLE WEIR POSITIONS/OPERATION


PST - HORIZONTAL UNSCRAPED - DISTRIBUTION CHAMBER - CHECK COPASACS AND CHANGE AS NECESSARY


PST - HORIZONTAL UNSCRAPED - DISTRIBUTION CHAMBER - CHECK FLOW SPLIT IS CORRECT OP/1W OP/1W OP/1W OP/1W OP/1W PST - HORIZONTAL UNSCRAPED - DISTRIBUTION CHAMBER - CHECK WEIRS AND CLEAN OP/1W OP/1W OP/1W OP/1W OP/1W PST - HORIZONTAL UNSCRAPED - DISTRIBUTION CHAMBER - REMOVE ANY SCUM AND DEBRIS, INCLUDING GRIT



3.2.6




Process Checklist Lamella Primary Settlement Tanks



Low-Med (1000-5000)

Med PE (5000-20,000)

High PE (20,000-350,000)

Manned (350,000+)

PST - LAMELLA - CHECK BRIDGE/SCRAPER OPERATION OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D

PST - LAMELLA - CHECK SAFETY GUARDS OP/1W OP/1W OP/1W OP/1W OP/1W

PST - LAMELLA - CHECK SCUM TRAP FOR NORMAL OPERATION AND CLEAN/HOSE OUT OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D

PST - LAMELLA - CHECK DE-SLUDGE PUMP AND TIMER FOR NORMAL OPERATION OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D

PST - LAMELLA - CHECK AND LOG SLUDGE LEVEL BY DIPPING TANKS OP/1W OP/1W/3D OP/1W/3D OP/1W/3D OP/1W/3D

PST - LAMELLA - CHECK AND REMOVE ANY BUILD-UP OF SCUM OR RAG ON TANK SURFACE, WEIRS OR LAUNDER CHANNELS


PST - LAMELLA - CHECK SCUM BOARDS FOR BREAKS OR CARRY UNDER OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D

PST - LAMELLA - CHECK SETTLED SEWAGE QUALITY (VISUAL CHECK ONLY) OP/1W OP/1W/3D OP/1W/5D OP/1W/5D OP/1W/7D

PST - LAMELLA - PERIODICALLY EMPTY AND CHECK INTEGRITY T1/48M T1/48M T1/48M T1/48M T1/48M

PST - LAMELLA - DISTRIBUTION CHAMBER - CHECK ALL VALVES, PENSTOCKS AND VARIABLE WEIR POSITIONS/OPERATION


PST - LAMELLA - DISTRIBUTION CHAMBER - CHECK COPASACS AND CHANGE AS NECESSARY


PST - LAMELLA - DISTRIBUTION CHAMBER - CHECK FLOW SPLIT IS CORRECT OP/1W OP/1W OP/1W OP/1W OP/1W

PST - LAMELLA - DISTRIBUTION CHAMBER - CHECK WEIRS AND CLEAN OP/1W OP/1W OP/1W OP/1W OP/1W

PST - LAMELLA - DISTRIBUTION CHAMBER - REMOVE ANY SCUM AND DEBRIS, INCLUDING GRIT






3.2.7 Process Checklist Notes

What to check Action / What to check for on deviation Sludge blanket level Check to compare level against the target level for optimum performance. Dip

the PSTs at the same time of day and at the same location for accurate readings. The selected location(s) must be assessed for health and Safety and be clearly marked. Check that no more than half a metre of sludge is held within a tank.

Screened Sewage Quality

Check colour of the sewage. Greyish/green colour shows fresh sewage. Dark grey or black indicates the sewage may be septic. The colour of incoming sewage may be affected by trade effluent. The colour of incoming sewage may also be affected by works returns i.e. sludge liquors which can turn the sewage black, or from septic / cess tanker discharges, or SPS wet well level control.

Operate with the correct de-sludge regime

This determines that the de-sludge timers are set correctly. Where this is not possible the last de-sludge time must be recorded. In cases of manual de-sludging the correct de-sludge valve needs to be used. Some control systems will inhibit de-sludging from a high level Sludge Holding Tank or Sludge Blending Tank. For multiple lamella or conventional PSTs the de-sludging times will normally be staggered. De-sludging times to be as detailed in the Site Operational Manual. If changes are required they must be logged in the Log Book at the Site Information Point.

Tank Bridge/Scraper operation.

Check for smooth rotation / travel with no jerky movements. Check for excessive noise from drive wheels, motors or gear boxes. Check the integrity of the concrete surface of the PST walls. Periodically check condition of PST scraper blades by emptying tank. Bridge rotation on circular PSTs may be prone to stopping due to the concrete top of the tank breaking up and causing a hole that the wheel gets stuck in. On rectangular PSTs with scraper bridges there is the need to ensure that the bridge has moved throughout the day, as it may appear that bridge is not moving. On Lamella Settlement Tanks with scraper systems there is the need to ensure that the scraper mechanism has moved throughout the day, as it may appear that it is not moving. In icy conditions consider using salt or sand on walls to prevent drive wheel slipping.

Scum trap blockage Check the traps for normal operation. This avoids excessive build up on tank surface. Remove scum manually where possible. In some cases the scum trap(s) may be blocked and kept blocked on purpose to avoid excess water going to Sludge Holding Tanks (SHTs).

Tank structural integrity

Check frequency of tank emptying for inspection.

Scum board integrity

Check scum boards for cracks or breaks. Check for scum breakthrough and carryover into settled sewage channel.

Cockpit Data Collection

Sludge level, de-sludge timer operation, power and flow readings should be logged and captured on the Site Cockpit / Dashboard.





3.3 Troubleshooting Guide WWT 3.1 Troubleshooting Guide - Primary Settlement Tanks Symptom Possible Cause Action Risk / Consequence Excessive gassing and/or rising sludge Sewage colour and odour

Inadequate sludge removal. De-sludge pump inhibited from

downstream processes Strong influent. Tank overloaded. Scraper not working or damaged. Blocked desludge line Scraper operation and de-sludging

out of sequence. Poor flow distribution to tanks. Sludge has floated with excess

gassing Sludge build up on walls

(Pyramidal Tanks) Timer clock faulty / incorrect

settings / no settings Seasonal increase in sludge

Manually de-sludge Review Process Control

Chart and action lines Review dip frequency. Review de-sludging

regime. Review flow distribution. Check control

philosophy. Check flow split Check scraper Consider cleaning /

emptying tank

Poor sludge condition.

Higher load to secondary treatment.

Poor Picket Fence Thickener / Sludge Holding Tank performance.

Possible Effluent failure

High sludge blanket which may be a result of blockages on sludge line

Sludge too thick. Reduced de-sludge pumping rate. Blockages on sludge line De-sludge pump inhibited by

downstream processes Valve damaged / jammed Build-up of rag or grit. Impact of storm washout. Seasonal Issues producing extra

sludge Rag

If possible de-sludge by hand.

Review action lines on Process Control Chart.

Review de-sludge pump timers.

Review dip frequency. Check flow split Review grit removal

frequency / Inspect grit removal system.

Flush lines Repair Valve Review SAS rate. Check screens

Storage of sludge in PST with increased sludge blanket.

PST Performance Impact on Picket

Fence Thickener/ Sludge Holding Tank performance.

Impact on Secondary treatment


Excessive solids carryover

Inadequate sludge removal. Strong influent. Tank overloaded. Scraper operation and de-sludging

out of sequence. Valve damaged / jammed. Poor flow distribution. Incorrect SAS rate Sludge build up on walls

(Pyramidal Tanks) Stilling chamber and scum board

condition Timer clock faulty / incorrect

settings / no settings

Check de-sludge pump timer.

Review dip frequency. Check valve operation Review de-sludging

regime Check flow split If applicable, is SAS

return rate correct (for co-settling)

Check stilling chamber and scum boards.

Higher load to secondary treatment.

Increased aeration costs

Filter ponding. High / increased

SAS production. Possible Effluent

failure

Scraper not operating

Fault Drive wheel slipping on wall. Alarm Fault

Check for build-up of sludge in tank.

Repair fault. Salt/sand wall if due to

ice.

Build-up of sludge in tank.

Impact on all downstream processes.





Alarm Call Out Troubleshooting Guide WWT 3.1 Alarm Troubleshooting Guide - Primary Settlement Tanks Symptom Possible Cause Action Risk / Consequence PST Scraper Fail Alarm No De-sludge in 24 hours Alarm

Alarm fault Power Failure Gear box Failure Drive Wheel

Failure / slipping Obstruction on

tank wall Ice on surface of

wall Scraper not

working or damaged.

Faulty limit switches

Restore power Arrange for Defective

Maintenance Check sludge blanket levels Salt/sand wall if due to ice Review Process Control

Chart and action lines Review dip frequency. Review de-sludging regime. Review flow distribution. Check control philosophy Check flow split Check scraper Consider cleaning / emptying

tank

Poor sludge condition. Higher load to secondary

treatment. Possible Effluent failure Inadequate sludge removal Scraper operation and de-

sludging out of sequence Excessive gassing and/or

rising sludge Sewage colour and odour

PST De-sludge Pump Fail Alarm No De-sludge in 24 hours Alarm

De-sludge pump failure

De-sludge pump inhibited from downstream processes

Build up of rag or grit

Sludge too thick.

If possible de-sludge by hand.

Repair / reset pump Arrange for Defective

Maintenance Check sludge blanket levels Review Process Control

Chart and action lines Review dip frequency Check flow split Review grit removal

frequency Inspect grit / rag removal

system Flush lines

Storage of sludge in PST with increased sludge blanket

Reduced de-sludge pumping rate

High sludge blanket which may be a result of blockages on sludge line

Impact on Picket Fence Thickener / Sludge Holding Tank performance



PST De-sludge Valve Fail Alarm

Valve damaged / jammed

Build-up of rag or grit.

Check valve operation Repair Valve Arrange for Defective

Maintenance Check sludge blanket levels

Reduced de-sludge pumping rate


Possible Effluent failure PST De-sludge Inhibit No De-sludge in 24 hours Alarm

De-sludge pump inhibited from downstream processes not working, i.e. SHT full, Lime Plant not working

Check sludge blanket levels Review dip frequency Determine how long

downstream plant will be out of action

Review desludge pump timer Tanker out sludge off site if

downstream plant remains unavailable

Storage of sludge in PST with increased sludge blanket

Excessive gassing and/or rising sludge


Possible Effluent failure PST Distribution Chamber High

Impact of storm wash-out

Poor flow distribution

Check for downstream blockages

Check flow split

Poor PST performance PST flooded





3.3.1 Dos and Donts

Dos Donts

Regularly check de-sludge lines & plant operability in line with SOM.

Leave de-sludge lines or scum removal systems blocked

Control sludge blanket levels to within target values

Allow sludge blanket levels & hence sludge stocks to build unchecked

Sample the settled sewage quality of each Primary Settlement Tank if problems arise to see if it is one or multiple tanks, which are causing the issues

Presume once optimisation / commissioning is complete that everything will be great it needs regular checks & adjustments

Ask for additional support if problems persist & inform your Line Manager

Ignore issues hoping they will resolve themselves.





Appendices 3.4 Operation The operation of Primary Settlement Tanks must be in accordance with the design. Effective operation will ensure a reduced and optimal biological loading onto the secondary treatment processes and will reduce the whole life cost of assets.

For sites with SCADA (Supervisory Control And Data Acquisition) the key areas for primary settlement tank operation are controlled and monitored. These areas of control are bridge rotation / motion and de-sludge pump / valve operation.

The most important activity for Primary Settlement Tank (PST) operation, sludge management and optimisation is the use of manual sludge dips with a sludge blanket detector and altering the de-sludging (duration, frequency or flow rate) whilst also monitoring the Mixed Liquor Suspended Solids (MLSS) in the case of Activated Sludge Plants.

This will also provide an early indication of blockages, scraper failure or de-sludge pump issues. Taking proactive measures will minimise risk of sludge blanket inversion, i.e. the sludge blanket rising to the surface of the tank. Every time an Operator visits site the Primary Settlement Tank (PST) sludge blanket dips must be carried out, recorded and trended. There must also be an action line for when the sludge blanket level rises too far on the run chart and Cockpit / Dashboard.

3.4.1 Primary Settlement Efficiency

For Activated Sludge Plants, if there is an increased air demand and the Mixed Liquor Suspended Solids (MLSS) have not increased, check the Primary Settlement Tank (PST) sludge blanket levels first.

Using Primary Settlement Tank (PST) sludge blanket level monitoring, the aim is to maintain good Biochemical Oxygen Demand (BOD) and suspended solids removal and thus minimise the loading to aeration plants, prevent high Mixed Liquor Suspended Solids (MLSS) or loading to biological filters downstream.

Good operation of the Primary Settlement Tank and thus reduced loading will minimise aeration power costs and Surplus Activated Sludge production on aeration plants and thus improve sludge thickening and Final Effluent quality. Good operation will also help to maintain robust nitrification on biological filter plants.

The difference in monitoring Primary Settlement Tank (PST) blanket levels and adjusting the de-sludging accordingly is the key to controlling the load to the secondary biological treatment process. For Activated Sludge Plants reducing the loading to the aeration plant consequently reduces the amount of aeration required, hence reducing the aerator running costs.

In the event of the secondary treatment processes struggling, the sludge blankets must be checked to ensure that the Primary Settlement Tank performance is as good as it could be.

High sludge blankets will also result in the production of dissolved hydrogen sulphide. This will be stripped out when the settled sewage flows over the outlet weirs and into the collection channel, this turbulence in the presence of air will strip the hydrogen sulphide out of the water, giving rise to odour and corrosion problems.

Allowing sludge blanket levels to rise will detrimentally affect the Primary Settlement Tank performance and ultimately affect downstream processes and Final Effluent compliance.

High sludge blankets will lead to increased aeration costs and SAS production on activated sludge plants and may jeopardise final effluent compliance on trickling filter plants and activated sludge plants





3.4.2 Impact on Secondary Treatment Processes

The wastewater secondary treatment process is essentially a biological process in which aerobic microorganisms consume the organic contaminants in the incoming wastewater. Organic carbon in the waste stream is the food on which these organisms live; hence the amount of food or Biochemical Oxygen Demand (BOD) & solids within the settled sewage coming from the Primary Settlement tanks directly affects the biomass within the biological process.

The amount of solids within sewage is usually related to the amount of Biochemical Oxygen Demand (BOD), with Biochemical Oxygen Demand (BOD) being the food source for the microorganisms, which we rely upon to treat the sewage.

The following table details the potential impacts on secondary treatment processes as result of poor PST performance.

Symptom Consequence

Poor quality settled sewage with high suspended solids.

For Activated Sludge works the solids within the sewage add directly to the Mixed Liquor Suspended Solids (MLSS) within the aeration plant. If the Mixed Liquor Suspended Solids (MLSS) are higher than target levels then we start to have trouble getting enough air into the aeration plant, consequentially the ammonia within the final effluent starts to rise.

Increase in Mixed Liquor Suspended Solids (MLSS) due to poor quality settled sewage with high suspended solids.

If the Mixed Liquor Suspended Solids (MLSS) rise even further, depending upon the specific plant, then we have problems removing enough Surplus Activated Sludge (SAS) from the Final Settlement Tanks (FSTs), which can lead to increased solids within the final effluent as well, resulting from poor settleability & even sludge blanket loss.

Excessively F:M levels The relationship between the Biochemical Oxygen Demand (BOD) loading from the Primary Settlement Tanks and the amount of biomass or Mixed Liquor Suspended Solids (MLSS) within aeration plant is quantified as the food to mass ratio (F:M).

Excessively low F: M levels will reduce the food available to the organisms to metabolise, which will result in reduced growth which in extreme cases can lead to the mortality rate exceeding the reproductive rate, resulting in a loss of treatment. However, in most cases, there is a high F: M consequential from the over loading of solids from the Primary Settlement Tank (PST).

This is resultant from either / or both the primary sludge blankets being too high resulting in solids being carried over, or insufficient solids removal or settlement from high flows which also results in solids being carried over. Furthermore, if the removal of primary (RAW) sludge is too low, i.e. insufficient desludging, then the sludge within the Primary Settlement Tanks can age & go septic.





Symptom Consequence

Septic sludge in Primary Settlement Tank

If sludge is left in the primary tank for too long it will begin to undergo anaerobic digestion. The first stage of this is that some of the BOD in the settled solids becomes solublised and therefore adds to the Biochemical Oxygen Demand concentration in the settled sewage. The next phase of this process is gas production. The gas produced will be trapped in the sludge and will cause it to float to the surface. This rising sludge that carries over into the Activated Sludge Plant will have the further detrimental effect of some toxic inhibition of the Mixed Liquor Suspended Solids (MLSS) biomass resultant from the septicity.

High or low F:M ratio Further problems with either high or low F: M ratio is the increased potential to cause a filamentous bloom, which can further detrimentally affect the treatment process.

Increased Primary Sludge production as a result of chemical dosing for Phosphorus removal

In some cases where sites are required to remove phosphorus the Primary Settlement Tanks are chemical dosed, usually with Ferric Chloride, into the Primary Settlement Tank distribution chamber or inlet channel. In these cases it is found that increased primary sludge is produced as more solids in the sewage are removed as a consequence and settle as raw sludge.

Desludge rates need to be set up to remove this extra raw sludge. It is also important to manage downstream sludge handling processes effectively so that desludge pumps are not inhibited and sludge does not build up in the Primary Settlement Tanks.

Weaker settled sewage as a result of chemical dosing for phosphorus removal

As a result of chemical dosing into Primary Settlement Tanks for Phosphorus removal the result is generally a weaker settled sewage with reduced organic load.

This can have an impact on downstream secondary biological processes. Activated Sludge Plants can operate with lower Mixed Liquor Suspended Solids (MLSS) or less aeration lanes in operation. The reduced load passed onto percolating Filters will produce better quality effluents but can also lead to increased to weed and moss growth on the filter surface.

Odour Emissions of odour from our sites are more likely to occur when we are working on Primary Settlement Tanks. On these tanks and on some sites this is more of an issue and must be mentioned in the works Site Operating Manual in more detail. Effective operation of Primary Settlement Tanks will minimise odour issues, however if the site does have issues with odour then refer to the Asset Management Odour Strategy. Odour can be minimised when a tank is out of service for repairs / maintenance by covering the sludge on the tank floor with water.


Issue 1.1

Page 18 of 21

3.4.3 Primary Settlement Tank Optimisation

Adjusting de-sludging: It is important to remember that all changes must be made gradually; some sludge blanket being present will aid settlement within the Primary Settlement Tank (PST), too large a blanket will cause carry over. A medium / optimum depth target range must be established.

Where downstream Sludge Thickening processes are in operation, the aim ideally is to have no sludge blanket in the Primary Settlement Tanks with Raw Sludge Dry Solids of about 1.0 to 1.5% being produced. It is better to pump water or thin sludge to Sludge Thickeners than to have sludge ageing on the Primary Settlement Tank floors. Some sludge blanket is possibly required if thick sludge from the Primary Settlement Tanks is required to be produced or if site is an Activated Sludge plant running in Biological Nutrient Removal (BNR).

For sites that use Sludge Cages in the Sludge Holding Tanks, as the dewatering is so effective the Primary Settlement Tanks can be run to water with no sludge blanket. Care needs to taken with approach as decanted liquors being returned to the inlet from the Sludge Holding Tanks (s) are much stronger than sewage and where possible be returned continually and managed to avoid shock loading and effluent failure.

Ensure that there are no blockages or maintenance issues that will detrimentally effect / inhibit raw sludge removal. Monitor the sludge blanket depth with a sludge blanket meter, record and trend all readings on a run chart or performance pack/cockpit, for each Primary Settlement Tank.

Check that there are no bottlenecks in the raw sludge-dewatering stream, if applicable. If the sludge blanket levels start to rise, increase the de-sludging slightly and monitor, if they continue to rise adjust accordingly. When the sludge blanket levels start to reach the lower end of their target range, reduce the de-sludging in a stepped manner.

On small sites its recommended to manually operate the de-sludge pumps to ensure they are working effectively and the Primary Settlement Tank has no sludge in it and runs to water. Also on small rural sites its recommended that the Primary Settlement Tank(s) are emptied and cleaned out periodically.

Technical Lead: Bryan Bleeker

AM-PRO-WWT-WWT 3.1 Primary Treatment-SEC3






Some of the Costs associated with poor Primary Settlement Tank performance (downstream aeration and SAS polymer costs)

Impact of Primary Settlement Tank operation on OPEX(200K pe, power 0.08/Kwh)

0

10

20

30

40

50

60

70

1520253035404550

BOD removal across PSTs %

OPE

X K

/a

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2

4

6

8

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12

SAS

mak

e t/d

Increased aeration costs K/a Increased OPEX (exc sludge) K/d SAS make t/d

3.4.4 Contingency Planning If the Primary Settlement Tank (PST) sludge blanket levels get too high and are in danger of weiring over or going septic, the sludge blanket levels can be brought down quickly by over pumping into a disused tank if available or removal by tanker off site. For Activated Sludge Plants additional aeration can also be utilised in anticipation of this additional load. For sites with percolating filters is it essential that sludge is not allowed to be discharged on to the filters. If there is a danger that raw sludge will spill over from the Primary Settlement Tank (PST) on to the percolating filters arrange for the sludge to be removed and taken by tanker off site before the sludge spills onto the filters. Where sludge has built up to excessive levels recirculation flows can be increased to thin the Settled Sewage down until the sludge can be removed from the Primary Settlement Tank (PST). DO NOT ALLOW SLUDGE TO GO ONTO PERCOLATING FILTERS





3.4.5 Plant Release / Taking out of and Bringing into service Taking out of service - Planned When Primary Settlement Tank(s) are being taken out of service the Plant Release procedure needs to be followed together with site process risk assessment and plant availability. If a Primary Settlement Tank cannot be taken out of service due to process risk then this needs to be highlighted in the Risk Register for potential investment / capital delivery purposes. The frequency for removing Primary Settlement Tanks from service must be set at a level that ensures the performance of the tank is maintained and inspection of the scraper can be made prior to any damage occurring. As a guide, this should be initially set at four years. Draining of Primary Settlement Tanks for inspection is normally carried out in summer months or during periods of low inlet flow to the works when plenty of time needs to be allowed for draining. Primary Settlement Tanks can also be taken out of service in the summer months to improve settled sewage quality due to septicity and reduced flows. This should be linked to any Odour Management Plans. The rapid return of sewage when draining Primary Settlement Tanks may overload the other Primary Settlement Tanks in service and flush out sludge. The alternative is to pump off the top water to a balancing or other tank for slow return to main flow. In small works where there is only one Primary Settlement Tank the top water can be taken off by a tanker. Increase the de-sludging run time and / or frequency on the other Primary Settlement Tanks, as the higher loading on these tanks will result in an increase in sludge production. Taking out of service - Unplanned The Primary Settlement Tank may well have a lot of sludge in it that will need removing. This must be removed in a controlled way either to the Sludge Holding Tank, directly into the sludge stream or if unavoidable to the inlet works. The following points must be taken into account when taking a Primary Settlement Tank out of service to maintain site operation, maintenance and compliance. Always protect effluent quality. Review operation of upstream processes, i.e. screens which may be causing problems Control the Primary Settlement Tank pump down to protect the treatment process. Be aware of Primary Settlement Tank flotation risk. Plan flow diversion needs i.e. Tanker, pumping station etc. Isolation of Primary Settlement Tank inflow. When Primary Settlement Tank(s) are taken out of service ensure flow split and de-sludging is

redistributed among the other Primary Settlement Tanks. De-sludge to water prior to emptying the Primary Settlement Tank. Sludge Holding Tank capacity. Primary Settlement Tank cleaning. Health & Safety Inspections required Monitor downstream processes Alarms that could lead to needless Call Outs, i.e. not temporarily disabled during outage. Odour





Bringing into service Check Primary Settlement Tank (PST) alarms before leaving site. Check that all Primary Settlement Tanks have equal flow distribution to ensure optimal operation. Tanks should be brought back into service slowly or there is a risk of solids carry over. More than one tank on site Fill Primary Settlement Tank by passing a proportion of the incoming flow through it. It is important that the other Primary Settlement Tanks still receive flow. One tank only on site Fill with Humus Tank effluent / final effluent before restoring flow to the Primary Settlement Tank. General It is advisable that an inspection of the Primary Settlement Tank and its de-sludging system is carried out within a few days of being put back in service. Lessons Learnt In the case example of Esher STW, due to a capital project, there was a requirement to carry out works shutdowns during dry weather; this resulted in having to return strong (undiluted) sewage from the storm tanks. When this was being done, there was no account made for the increase in solids loading into the Primary Settlement Tanks, hence no increase in de-sludging times were made. This resulted in solids being carried over into the aeration plant. Initially this meant that the back-up aerators had to run extra hours to maintain Dissolved Oxygen (DO) levels and ammonia treatment. However, after a short period of time, this resulted in the Mixed Liquor Suspended Solids (MLSS) increasing from 2,500mg/l (as target) to 8,000mg/l. This resulted in a breach in consent of ammonia & solids within the final effluent. There was also the further impact of requiring additional sludge to be removed from site, incurring additional tanker cost, not to mention the extra power consumed by both the aerators & the sludge thickening plant. This scenario could also occur as a result of heavy rainfall after prolonged dry periods whereby the network may be cleaned / flushed out resulting in additional solids being washed to the Sewage Treatment Works. It is important to note that these solids maybe old and septic causing loading and settlement difficulties within the Primary Settlement Tanks (PSTs). Regular monitoring of sludge dips and pump timer adjustments can identify and combat this changing situation.

---- End of section 3 ----

3.0 Operating Standard3.1 Primary Treatment Performance Measures3.2 Operator Activities3.2.1 Process Checklist Circular Primary Settlement Tanks3.2.2 Process Checklist Horizontal Scraped Primary Settlement Tanks3.2.3 Process Checklist Pyramidal Autodesludged Primary Settlement Tanks3.2.4 Process Checklist Pyramidal No Power Primary Settlement Tanks3.2.5 Process Checklist Horizontal Unscraped Primary Settlement Tanks3.2.6 Process Checklist Lamella Primary Settlement Tanks3.2.7 Process Checklist Notes

3.3 Troubleshooting Guide3.3.1 Dos and Donts

Appendices3.4 Operation3.4.1 Primary Settlement Efficiency3.4.2 Impact on Secondary Treatment Processes3.4.3 Primary Settlement Tank Optimisation3.4.4 Contingency Planning3.4.5 Plant Release / Taking out of and Bringing into service

am-pro-wwt-wwt 3.1 primary treatment-sec3 issue 1.1

Documents

primary settlement efficiency

power primary settlement

primary treatmentsec3

process checklist notes

bryan bleeker issue

issue conversion

secondary treatment

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