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Water Services Training Group

Water Services Training Group

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Water Services Training Group

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  1. Water Services Training Group WRc FOG Project Engineering/Operations Issues

  2. WRc FOG Project • Work Package 1: Legal • Work Package 2: Sociological • Work Package 3: Biological/chemical • Work Package 4a: Engineering – designing FOG out (sewers/ pumping stations, grease separators) • Work Package 4b: Operations – cleaning techniques etc (sewers/ pumping stations)

  3. Engineering/Operations Issues • Grease separator selection • Products database • Jetting and cleaning of FOG from sewers • Sewer design • Pumping station design

  4. FOG Work Package 4 - Deliverables Reports • Operational Problems caused by FOG • Guidance document on Sewer Cleaning and Blockage Removal Techniques • Factors that Influence FOG Accumulation on Sewer Pipes • Design to Minimise FOG Deposition and Associated Operational Disruption • Review of Use of Grease Separators • Grease Separator Advice

  5. FOG Work Package 4 - Deliverables FOG Strategy – various advice sheets and leaflets, including: • Grease separator advice • Operational guidance – improved cleaning procedures • Design guidance for sewers and pumping stations • Guidance for improving existing sewer systems and pumping stations

  6. Grease separator selection • Large number of different types of separator are now commercially available • Many are not of the traditional design and, as such, are not covered, by a standard • However, many of the new designs show promise and deserve further investigation

  7. Grease separator selection (2) • When considering new types of separator, look at: • Their practicality - ease to install, where to install, ease to maintain/clean etc • Their overall effectiveness • Guidance given by manufacturers/ suppliers etc – size/installation etc

  8. The standard (EN 1825) Difficult to understand (information spread between Part 1 and Part 2) Only considers conventional ‘large’ underground separators – Not practical for most town centre catering outlets No consideration of more recent separator designs, including skimmers. Grease separator standard

  9. Grease separator standard (2) The standard (EN 1825) Does not consider hybrids – Dosing and separators Most separator suppliers don’t understand EN 1825 Suppliers have a variety of sizing methods Seems to be little standardisation but plenty of confusion Often, sizing is only for the food trap part ofthe separator

  10. EN 1825 separator

  11. Active separator (skimmer)

  12. Grease separator selection • Three main issues to consider: • Location/access • Size • Maintenance • Each property is unique • Selection is an iterative process • may need combination of separator/ biological solution

  13. Grease separator selection • Location • Close to source – connect all sources (dishwashers issue) • Far enough away for effluent cooling • Upstream of other inputs (rainwater and foul water) • Outside units preferable - Consider internal units at properties with no external space • Enable access for cleaning/maintenance

  14. Grease separator selection • Size • To provide adequate retention time for separation to take place - determined by: • Flow rate to the separator • Volume of FOG in discharge • Maintenance frequency v size

  15. Grease separator selection Maintenance • By whom • Staff – small separators, easy access, cleaning record, storage of wastes • Contractor – for larger underground separators, less frequent visits, vactor units remove wastes from site • Frequency – determined by storage capacity of the separator

  16. Inadequate grease separators Examples of two small and poorly maintained separators

  17. Products database • Database including details of: • Grease separators and Biological treatments • Product/company name • Contact details for products • Basic product information • Guidance on appropriate applications • Available to contributors and their agents through password access

  18. Jetting and cleaning • The most frequent symptom of FOG in the sewer system is a blockage • Sewer blockages may occur at any point on the sewer system, from the point of source to the wastewater treatment works • Sewer blockages can be categorised as a primary or secondary blockage

  19. Jetting and cleaning • Primary FOG related blockages are due to natural deposition of FOG at a specific location as a result of the combination of: • a source and appropriate type of FOG, • concentration of FOG in the flow, • the natural processes of separation, and • hydraulic conditions

  20. Jetting and cleaning • Secondary FOG blockages are those that occur as a direct result of a previous blockage or severe flow restriction. • Flow velocity upstream of the original blockage will reduce - FOG that would otherwise remain in the flow will separate out. • FOG will be high on the pipe wall or in the crown of the upstream pipe, due to the increased depth of flow • Sewer debris, (sanitary towels and other sewage debris) then sticks to the FOG on the crown of small pipes, a secondary blockage subsequently develops

  21. Jetting and cleaning practice The following may seem obvious BUT experience often shows otherwise ! • All FOG related blockages must be effectively cleared and the upstream pipe effectively cleaned to prevent repeat incidents • A recommended procedure has been proposed to ensure effective cleansing

  22. Jetting and cleaning practice (2) • General purpose jets are a compromise • Removal of FOG from a sewer should be carried out as a staged and incremental process • General purpose jet to remove soft debris • Plough to remove hard sediments, • Spinning jet to remove FOG • Survey with a camera

  23. Jetting heads

  24. Jetting heads

  25. Jetting and cleaning practice (3) • Analysis shows that the correct cleaning practice will be beneficial in terms of: • Reducing the number of operational disruptions • Improving customer satisfaction • Cost effectiveness

  26. Jetting and cleaning Before and after using recommended procedure

  27. Jetting and cleaning practice (4) • Should the dislodged FOG be: • Removed at the manhole: or • Passed forward ? • No evidence of significant problems if:

  28. Jetting and cleaning practice (5) • Break fragments into small pieces, remove large fragments • Remove large quantities • Remove if FOG is mixed with sediment

  29. Jetting and cleaning practice (6) • Remove if downstream problems may occur • Notify operators of pumping stations and works of the possibility of dislodged FOG being passed downstream

  30. Jetting and cleaning practice

  31. Sewer design • Greater likelihood of FOG deposition/ adherence: • Higher concentration of FOG • Lower the temperatures of the pipe surfaces • Slower the velocity of flow/movement of effluent

  32. Existing sewer improvements (1) • First option – control FOG at source • Good kitchen practice • Install grease separators and/or • Biological treatment systems • Modest improvements to drainage system to reduce areas with low velocity • Other measures: • Improve sewer cleaning/blockage practice • Consider off line, as opposed to on-line tanks

  33. Existing sewer improvements (2) • Modest improvements to drainage system to reduce areas with low velocity: • Removal of Buchan traps • Relay pipes with sags, to enable positive gradient • Removal of tree roots • Make good intruding laterals (use of saddle connections etc) • Improve benching/pipe layout in manholes • Install drop pipes to high level connections

  34. Existing sewer improvements (3) • Where existing systems need to be altered to reduce CSO spills etc, ensure that the ‘improvement’ will not result in areas of low velocity/tranquil flow where serious FOG deposition could occur. • Similarly, where storage needs to be added to reduce sewer flooding (These may seem obvious, but they have happened)

  35. New sewer design to avoid FOG • The following should be done as good practice, to ensure there are no areas with very low velocity: • Pipe velocity, 0.6 m/s or greater • Use minimum number of pipe joints (joints dissipate energy in small flows) • Use easier to clean pipe materials • Good detailing of connections (saddles etc) • Good detailing in manholes

  36. New sewer design to avoid FOG • Also: • Avoid flow control devices • If tanks are necessary, use off-line systems

  37. Pumping station design • Greater likelihood of FOG build up with: • Higher concentration of FOG • Low velocity/movement in the wet well: • More tranquil the effluent in the wet well • If there are dead zones in the wet well

  38. Existing pumping station improvements (1) • Where possible, remove dead zones • Remove redundant/non essential equipment • Remove baffles from incoming flows • Consider re-profiling wet well floor • Ensure effluent is mixed at start of pump down

  39. Existing pumping station improvements (2) • Refit with easy clean materials • Replace fouled float switches with ultrasonics (located clear of the effluent) • Use varying pump on/off levels , to avoid FOG shelves at ‘pump on’ level • Improve access to wet wells, for cleaning purposes • Review pump regimes to reduce FOG deposition in rising mains

  40. Problem pumping station

  41. Problem pumping station

  42. Problem pumping station

  43. Problem pumping station

  44. New pumping station design (1) • Greater likelihood of FOG build up: • Higher concentration of FOG • Low velocity/movement in the wet well • Ensure wet well does not have dead zones

  45. New pumping station design (2) • Avoid dead zones: • Use circular wet wells • Use deep/small diameter wet wells – better mixing • Avoid excessive use of equipment in the wet well • Minimise use of dividing walls • Make use of energy/turbulence caused by incoming flow • Use of non stick materials

  46. FOG Work Package 4 - Deliverables Reports • Operational problems caused by FOG • Guidance document on Sewer Cleaning and Blockage Removal Techniques • Factors that Influence FOG Accumulation on Sewer Pipes • Design to Minimise FOG Deposition and Associated Operational disruption • Review of Use of Grease Separators • Grease Separator Advice

  47. FOG Work Package 4 - Deliverables FOG Strategy – various advice sheets and leaflets, including: • Grease separator advice • Operational guidance – improved cleaning procedures • Design guidance for sewers and pumping stations • Guidance for improving existing sewer systems and pumping stations