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An Introduction To Wastewater And Sludge Principles


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An Introduction To Wastewater And Sludge Principles

  1. 1. Wastewater and Sewage Sludge A Basic Introduction
  2. 2. What I’m going to talk about Brief introduction to the history of wastewater What a wastewater treatment plant looks like The basic wastewater treatment processes Sewage sludge – a whole new world
  3. 3. Why Treat Sewage?? A Potted History of Sewage Treatment in the UK
  4. 4. 14 th Century - First mention of river pollution in UK in the River Fleet Up till 1800 most rivers clean –last Salmon caught in the River Aire 1820 - Industrial Revolution created large conurbations discharging untreated sewage to rivers and unregulated trade discharges to rivers of organic and toxic pollution – metals and chemicals The Early Days
  5. 5. 1850’s – “ The great stink “ in London that affected even the proceedings in parliament.” Bazalgette built the first large scale public sewers in the world and created the London embankment. But no treatment, he just moved the point of discharge to further downstream Cholera rampaged the land throughout the period 1850 -1870 The Early Days
  6. 6. 1860’s -Dr Snow established that Cholera is waterbourne and spread by contaminated drinking water.(Koch did not isolate the bacillus until 1891) He noticed that a single water pump was causing a large amounts of deaths.. He removed the pump handle stopping the outbreak and preventing the spread of a cholera to a much wider area Dr Snow in Soho
  7. 7. 1865 –Bazalgette’s first pumping stations completed converting London from an open sewer to a city with proper sewerage, transporting sewage to lower down in the Thames. Designed with enough capacity that it is still enough capacity in it to still be in use today The Thames Tideway is the modern equivalent and is being constructed now, almost 150 years later and is going to cost £3.6 billion taking 15 years (the same as Bazalgette) to build 22 miles of sewer (compared to 100 miles for Bazalgette) Bazalgette’s London
  8. 8. Still quite early for wastewater treatment 1850’s – Typically water abstraction downstream of sewage discharge points Water treatment problem resolved by building moorland catchment reservoirs and distribution system to take the “clean” water to people’s taps. 1876 - Rivers Pollution Prevention Act – good idea but ahead of its time because nobody knew how to treat sewage. Get out clause which meant no improvements
  9. 9. 1880’s Sewage farms – First primary settlement tanks (horizontal flow) with Ferrous Sulphate/Lime addition which reduced BOD by 40-50%. Used soil as a media for secondary treatment but not possible to aerate soil and soil went rapidly anaerobic and provided very little treatment 1892 – Stone- First percolating filters at Salford 1914 – Arden and Lockett first activated sludge plant at Davyhulme Early beginnings in Manchester
  10. 10. Sewage gets the Royal Treatment Royal commission set up in 1898 to review methods of sewage treatment Took 17 years and ten reports to complete their findings Trade waste should be treated in admixture with sewage Consent standard 30:SS 20 BOD proposed where a minimum of 1 in 8 dilution in clean water is available Set river water classification based on BOD
  11. 11. Things can only get better……… Local Councils prevented trade discharges to sewer because it “Spoilt” the municipal effluent quality 1936 Public Health Act enabled traders to discharge to sewer but gave them deemed consents – So no control except for new traders who also were charged for discharge. However some councils only applied a temperature limit (43 C) 1951 Rivers Prevention of Pollution Act – required consents for new discharges (trade and municipal 1961 Public Health Act extended charges to deemed consents
  12. 12. …… ..and better 1960’s Mogden Formula introduced for traders as a method of controlling trade loads by charging 1961 Rivers Prevention of Pollution Act – extended this to all existing discharges 1974 Control of Pollution Act – An enabling Act which was brought into law by Statutory Instruments. Act covered discharges to air, water, land and noise. 1977 – Pre 1937 discharges to sewer at last consented From 1977 to present day most environmental law has been through EC Directive although some of it has been included in 1991 Water Act EC produced a black list of chemicals to be banned from discharges Also a red list of Chemicals to be controlled in discharges e.g. toxic metals
  13. 13. …… ..and better 1990 & 1995- Environment Act and Environmental Protection Act 1998 UWWTD required wastewater treatment for all areas above 15,000 PE River Quality Objective standards set to ensure rivers achieve required classification. Standard dependant on available dilution in river and upstream quality. Today standards are set by Stochastic models (statistically based e.g Monte Carlo Models) 2005- UWWTD increased to cover all works >2000 PE 2007 Environmental Permitting Regulations introduced 2009 – First River Basin Asset Management Programmes for the Water Framework Directive. Environmental Permits get tighter
  14. 14. Sewage Treatment What is Sewage and how do we treat it?
  15. 15. What is Sewage? Screenings Grit Faecal matter Urine Water
  16. 16. What is Sewage? Once all the screenings are removed, the 0.5% that isn’t water is: The consent is usually centred around BOD Ammonia Solids and sometimes Phosphorus, organic chemicals and metals
  17. 17. The Wastewater Treatment Process Sludge Handling & Treatment Tertiary Treatment Secondary Treatment Primary Treatment Preliminary Treatment
  18. 18. So what does a Sewage Treatment Works look like?
  19. 21. Esholt WwTW Treats a population of 487k Treats 280 million litres a day Generates 33 tonnes of sludge Processes 51 tonnes of sludge Consumes 60,000 kWh Generates 20,000 kWh
  20. 22. A little more detail…..
  21. 23. The Wastewater Treatment Process Preliminary Treatment
  22. 24. Preliminary Treatment Inlet works Coarse Screens (optional) Grit Removal (optional) Fine Screens Sewer Primary treatment Screenings treatment Screenings treatment Grit treatment
  23. 25. What comes down the sewer?
  24. 26. Getting the big stuff out Coarse Screens Usually around 25mm bars to take the big stuff out
  25. 27. Taking out the fine stuff Fine screens Typically a function of the consent and are normally specified as 6mm in 2 dimensions
  26. 28. What comes off the screens? Screenings Treatment The washer/dryer of the sewage treatment world
  27. 29. Grit – Why’s it a problem? <ul><li>Problems with flow measurement </li></ul><ul><li>Erosion and wear of downstream equipment, pumps etc </li></ul><ul><li>Settles and takes up volume in sludge storage </li></ul><ul><li>Reduces volume in digesters – reduced gas/energy production </li></ul>Better here Than here
  28. 30. How is Grit removed
  29. 31. What happens when preliminary treatment goes wrong
  30. 32. And when it goes right
  31. 33. The Wastewater Treatment Process Preliminary Treatment Primary Treatment
  32. 34. Primary Treatment Settlement Primary Settlement Secondary Treatment Sludge Treatment Preliminary Treatment
  33. 35. Primary Settlement <ul><li>Rely on gravity to settle out lighter SS material </li></ul><ul><li>Circular or Rectangular constructions </li></ul><ul><li>Usual designs : </li></ul><ul><li>minimum 2 hours retention at peak flow </li></ul><ul><li>30-50% removal of organic load and 60% of solids </li></ul><ul><li>High calorific value removed and goes off to digesters to produce energy </li></ul>
  34. 36. The Science <ul><li>Stokes Law </li></ul>In practice this means: A residence time in the primary settlement stage of 2 hours An upflow velocity of 1.2m 3 /m 2 /hr with all tanks in service
  35. 37. What tanks are there? <ul><li>Septic tanks </li></ul><ul><li>Rectangular </li></ul><ul><li>Dortmund </li></ul><ul><li>Upflow (cone) </li></ul><ul><li>Radial Flow </li></ul>
  36. 38. So what happens? <ul><li>Fluid retention time of at least 2 hours </li></ul><ul><li>Scraper bridge scrapes it into the hopper </li></ul><ul><li>Sludge collects in the bottom </li></ul><ul><li>Sludge removed by pump periodically either manually, by timer, solids mode or level control </li></ul>
  37. 39. The Wastewater Treatment Process Preliminary Treatment Primary Treatment Secondary Treatment
  38. 40. Secondary Treatment The engine room of wastewater treatment Secondary Treatment Tertiary Treatment/ Outfall Sludge Treatment Primary Treatment
  39. 41. Secondary Treatment A Biological process <ul><li>Soluble organic fraction removed by biological treatment typically by either trickling filters or activated sludge plant (ASP) but could be something else. </li></ul>
  40. 42. Trickling Filters <ul><li>Fixed film process meaning the bacteria grows on the rocks and settled sewage is applied to it </li></ul><ul><li>The most common type of all treatment work with excess of 50% of all treatment works in the UK ranging from a few hundred people to a hundreds of thousands served </li></ul><ul><li>Settled sewage is dosed onto the filters using mechanical distributors. </li></ul><ul><li>Parts of the slime regularly break away from the media surface and final settlement in humus tanks is required to produce a high quality final effluent. </li></ul>
  41. 43. Trickling Filters <ul><li>Filter Media </li></ul><ul><li>The filter media provides a surface for the biomass to grow upon and is generally made of slag, stone or plastic. </li></ul><ul><li>The biomass requires air to survive and this is achieved through natural ventilation within the filter. This relies on fresh air having a direct path to the base of the filter which is achieved through vent pipes, holes or open centre wells. It is important that these are kept clear. </li></ul><ul><li>Recirculation </li></ul><ul><li>To maintain good wetting of filters most sites recirculate a flow of final effluent. This improves performance by a mixture of dilution and better distribution of flow throughout the filter </li></ul>
  42. 44. Trickling Filters <ul><li>Advantages </li></ul><ul><li>Simple and easy to operate </li></ul><ul><li>Low or no power </li></ul><ul><li>Relatively cheap to build </li></ul><ul><li>Resilient and robust to change and toxic shock </li></ul><ul><li>Very good for small works </li></ul><ul><li>Adaptable </li></ul><ul><li>Disadvantages </li></ul><ul><li>Large land take </li></ul><ul><li>Not as efficient a removal process as suspended growth </li></ul><ul><li>Not resistant to the cold </li></ul><ul><li>Overgrowth and ponding </li></ul>
  43. 45. Activated Sludge (ASP) <ul><li>Activated sludge consists of a mass of micro-organisms which feed on pollutants in the sewage. The bacteria is suspended in liquid and is called “mixed liquor”, it is mixed with sewage and aerated in aeration basins before passing to final settlement tanks where it is settled and the sludge returned. The effluent produced is of a high quality. </li></ul>
  44. 46. Activated Sludge (ASP) <ul><li>Sewage enters the aeration tank and mixes with mixed liquor </li></ul><ul><li>Bacteria eat the sewage and need to be fed with air </li></ul><ul><li>More bacteria are bred and old sludge needs to be wasted to maintain a balance </li></ul><ul><li>Sludge recycled around to ensure the mass of bacteria is maintained. </li></ul>
  45. 47. ASP <ul><li>Surplus Activated Sludge (SAS) </li></ul>Return Activated Sludge (RAS) Anoxic Zone Aeration Lane Grows sludge on dissolved organics FST Selector Aeration adds O 2 Nitrification NH 4  NO 2  NO 3 Denitrification NO 3 NO 2  N 2 50% 50%
  46. 48. Activated Sludge (ASP) <ul><li>Advantages </li></ul><ul><li>Very efficient producing a high quality effluent </li></ul><ul><li>Common technique with a lot of industry knowledge </li></ul><ul><li>Very adaptable to increasing loads </li></ul><ul><li>Can be adapted for Nutrient Removal </li></ul><ul><li>Disadvantages </li></ul><ul><li>Energy intensive </li></ul><ul><li>More complicated to operate than fixed film processes </li></ul><ul><li>Produces more sludge than fixed film </li></ul>
  47. 49. Wait!…there’s more Tertiary Treatment As consents get tighter and tighter or populations increase or things change there is sometimes the need for further treatment. Typically this includes: Sand Filters (RGF & COUF) Drum filters Nitrifying Trickling Filters SAFs HSAFs BAFFs and many others………..
  48. 50. So what happens to all the sludge……..?
  49. 51. Sludge treatment….. a whole world of its own
  50. 52. WHAT IS SLUDGE?
  51. 53. Sludge depending upon its looks, colour, weight and nature depending where its from. It can range from a thin RAS sludge at 0.7% dry solids to dried pellets at 98% dried solids WHAT DOES IT LOOK LIKE
  52. 54. WHAT IS SLUDGE? <ul><li>Mainly water (up to 99%) </li></ul><ul><li>Dissolved solids </li></ul><ul><li>Settled and suspended solids </li></ul><ul><ul><li>Faecal matter </li></ul></ul><ul><ul><li>Bacteria and other micro-organisms </li></ul></ul><ul><ul><li>Nutrients (N, P, K) </li></ul></ul><ul><ul><li>Metals </li></ul></ul><ul><ul><li>Energy </li></ul></ul>
  53. 55. SLUDGE PRODUCTION Primary 50 to 60g ds/person/day Secondary 18 to 29g ds/person/day Typical total sludge per person 70g/day PE x yield x 365 = mass (tds) Mass / concentration = volume mass vol x conc
  54. 56. SLUDGE PRODUCTION Approx 1.2 million tonnes dry solids produced in UK annually this 165kg per person per year YWS production 150,000tds last year
  55. 57. So… what do we do with it all In 2004 62% Agricultural Land 19% Incinerated 11% Land Reclamation 7% Other (including composting) 1% Landfill Source: Water UK
  56. 58. SLUDGE TERMINOLOGY 990L water 10kg solids 1% ds 2% ds 4% ds 490L water 10kg solids 240L water 10kg solids “ Tonnes dry solids” “ Percent dry solids” Volume 1m 3
  58. 60. Primary sludge Bio 1 º 2 º Tertiary Typically 3% ds (up to 5%) Primary cause of site odour SOURCES OF SLUDGE
  59. 61. Primary sludge Bio 1 º 2 º Secondary sludge Tertiary Humus typically 2% ds (1-3% ) SAS typically 0.6% ds (0.25-1.25% ) SOURCES OF SLUDGE
  60. 62. Primary sludge Bio 1 º 2 º Secondary sludge Co-settled sludge Tertiary Typically 2% ds SOURCES OF SLUDGE
  61. 63. Primary sludge Bio 1 º 2 º Secondary sludge Co-settled sludge Tertiary sludge Tertiary Normally returned as backwash liquor SOURCES OF SLUDGE
  62. 64. OBJECTIVES OF SLUDGE MANAGEMENT Reduce volume (removal of water) Reduce/remove odour Stabilise organic material (BOD removal) Remove pathogens Reclaim useful by-products (biogas, soil conditioners) Safe/appropriate disposal & recycling
  63. 65. BIOSOLIDS
  64. 66. >600 Sewage Treatment Works 50 Sludge Treatment Centres Varying degrees of treatment Varying costs of treatment Varying capabilities (sludge intake, sludge output, day and night operability)
  65. 67. <ul><li>Goole </li></ul>Hull Selby Bridlington Staveley Old Whittington Sandall Woodhouse Mill Aldwarke Lundwood Caldervale Mitchell Laithes Leeming Bar Wombwell Northallerton Colburn STC LOCATIONS Naburn Beverley Sutton Blackburn Meadows Calder Valley Incinerator Esholt Knostrop
  67. 69. Main types of treatment Thickening Digestion Advanced Digestion Dewatering Conditioning Phyto-conditioning Incineration
  68. 70. <ul><li>Decreases volume of sludge by removing water to reduce downstream process size </li></ul><ul><ul><li>Gravity settlement (storage tanks, PFT) </li></ul></ul><ul><ul><li>Gravity belt thickener* </li></ul></ul><ul><ul><li>Drum thickener* </li></ul></ul><ul><li>*Polymer added to improve separation </li></ul>THICKENING
  69. 71. To achieve target dry solids with minimal solids loss in the filtrate Target solids is 6% ds – thicker sludge is difficult to pump Filtrate is returned to the works and excessive solids can cause compliance problems THICKENING - aim
  70. 72. <ul><li>1 Sludge is conditioned with polyelectrolyte </li></ul><ul><li>2 Conditioned sludge is fed onto moving belt, water passes through the weave </li></ul><ul><ul><li>Belt continuously washed by a high pressure jets to prevent solids accumulation in the belt weave </li></ul></ul><ul><li>Standard flow rates are between 10 and 50m 3 per m belt per hour </li></ul><ul><li>Be aware of the maximum solids load for the asset (ie the thicker the feed sludge, the lower the allowable flow rate) </li></ul>THICKENING - process
  71. 73. <ul><li>Thin sludge can be due to: </li></ul><ul><ul><li>Sludge application rate too high </li></ul></ul><ul><ul><li>Belt speed too high </li></ul></ul><ul><ul><li>Incorrect polymer dose – excessive dose may blind the belt </li></ul></ul><ul><ul><li>Sludge characteristics eg excessive FOG </li></ul></ul><ul><li>Solids in filtrate can be due to: </li></ul><ul><ul><li>Incorrect polymer dose </li></ul></ul><ul><ul><li>Solids running off the edge of the filter belt </li></ul></ul><ul><li>Problems with belt tracking, lubrication, tension (AMBS) </li></ul><ul><li>Poly suppliers will provide support to determine optimum polymer </li></ul>THICKENING - problems
  72. 74. Gravity belt thickener THICKENING - GBT
  73. 75. THICKENING - PFT
  74. 76. THICKENING - drum
  75. 77. POLYMER Added to sludge prior to mechanical thickening and dewatering Poly adheres to sludge particles, causing the release of surface water, neutralisation of charge and conglomeration of small particles by bridging. Many different types of polymer used eg cationic, ionic, single chain, cross linked Type used depends on characteristics of solids (eg pH, age, source) type of mixing & dewatering device pH, Alkalinity, water hardness, temperature, can affect performance of polymers
  76. 78. Type of sludge is the primary factor affecting the type and quantity of polymer required Raw Primary sludge requires much less poly than SAS Old sludge requires a higher poly dose than fresh sludge Polymer concentration required for a sludge is determined in the lab by jar tests NB sludge feed will vary therefore poly dose will change POLYMER
  77. 79. Mixing of sludge and polymer is essential for effective conditioning Good conditioning depends on polymer addition, retention time and mixing POLYMER - mixing
  78. 80. DIGESTION - requirements Min 12d primary digestion Min 32 º C Min 14d secondary digestion Code of Practice for the Agricultural Use of Sewage Sludge (1989)
  79. 81. DIGESTION - benefits Reduced sludge mass -conversion of solids to gases Reduced odour -conversion of volatile compounds Production of methane -a renewable energy source inert solids volatile sludge
  80. 82. Biogas DIGESTION Heat exchanger Feed Treated sludge Compressor
  81. 83. DIGESTION - biology Complex organics 1. Acidogenesis 2. Methanogenesis Clostridium Bifidobacterium Lactobacillus Methanobaciterium Methanobacillus Methanococcus Volatile acids CH 4 , CO 2
  82. 84. 990L water 10kg solids 490L water 10kg solids 1% ds 2% ds 10kg solids 20% ds DEWATERING 40L water Thickening Dewatering
  83. 85. DEWATERING Sludge feed Polymer Dewatered cake Centrate
  84. 86. DEWATERING Sludge feed Polymer Dewatered cake Filtrate
  85. 87. DEWATERING
  86. 88. Chains of molecules that stick sludge solids together to release water Charge type Charge density Molecular weight POLYMERS
  87. 89. Chains of molecules that stick sludge solids together to release water Charge type Charge density Molecular weight POLYMERS Molecular structure
  89. 91. DEWATERING Centrifuges Feed Bowl Scroll Beach Weir plate
  90. 92. DEWATERING
  92. 94. Provides required secondary retention Allows drainage of free water Forms a stable, friable product Promotes aerobic conditions CONDITIONING - Aim
  94. 96. SPC
  95. 97. Transpiration Predation Competition Root penetration: enzymic degradation & aerobic conversion SPC
  97. 99. INCINERATION Flue gas stack Fluidising air fan Caustic scrubber Ash to disposal 2 field Electrostatic precipitator Heat recovery section Induced draft fan Fluidised bed incinerator Turbo generator Quench scrubber Fixed bed adsorber Sludge feed Filtrate Belt presses MP steam MP steam
  98. 100. Sewage sludge – The opportunities <ul><li>Methane Production from Advanced and Conventional Digestion </li></ul><ul><li>Energy recovery from incineration or pyrolysis </li></ul><ul><li>Nutrient Recovery from sludge liquors </li></ul><ul><li>Utilisation of sludge liquors in BNR Processes </li></ul><ul><li>Biosolids to agricultural land </li></ul><ul><li>Topsoil production from Sludge phytoconditioning </li></ul>
  99. 101. Energy from sludge <ul><li>Maximising energy throughput from sludges </li></ul><ul><li>There are a number of options: </li></ul><ul><li>Digestion and advanced digestion </li></ul><ul><li>Incineration or pyrolysis </li></ul><ul><li>Physical disintegration techniques </li></ul><ul><li>CHP –v- gas turbines –v- biofuel </li></ul>
  100. 102. Sludge as a marketable product Technology exists its how you produce it and how you sell it Sludge as a marketable product Biofert production at 13,200 tDS/annum Sludge phytoconditioning Biogas as a biofuel or supply to the gas grid Ringsend 50% of energy from biogas Sludge liquors as a product Pearl & Crystal Green
  101. 103. Sludge Treatment as a resource Sludge liquors – PEARL Process Process that produces fertilizer by removing nitrogen and phosphorus from sludge liquors Works by adding magnesium and caustic soda to phosphorus rich sludge liquors and passing them through an upflow column The sludge liquors pass through pellets of struvite that are re-circulated around to act as seeds. The pellet size is a function of retention time in the reactor
  102. 104. Sludge Treatment as a resource Sludge liquors – PEARL Process The pellets when ready are removed from the reactor and dried The pellets are then bought, marketed and sold by OSTARA who developed the process as a fertiliser that they term “ Crystal Green” This creates a marketable product from waste
  103. 105. ANY QUESTIONS??