Heavy investment in digestion, but most is further treated by conditioning
1 Bifidobacterium 2 Methanobacterium
YWS Water & sewerage services for 1.7M households, 140,000 businesses 130,000 tonnes sludge produced every year Approx half is incinerated, creating 23,000 tonnes of ash Ash currently to YW landfill, but Groundwater legislation etc, plus if it loses ‘inert’ status: extra £0.75M Vast majority of the rest is recycled to agriculture and reclamation
Wastewater and Sewage Sludge A Basic Introduction
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
Why Treat Sewage?? A Potted History of Sewage Treatment in the UK
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
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
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
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
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
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
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
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
…… ..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
…… ..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
Sewage Treatment What is Sewage and how do we treat it?
What is Sewage? Screenings Grit Faecal matter Urine Water
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
The Wastewater Treatment Process Sludge Handling & Treatment Tertiary Treatment Secondary Treatment Primary Treatment Preliminary Treatment
So what does a Sewage Treatment Works look like?
The filter media provides a surface for the biomass to grow upon and is generally made of slag, stone or plastic.
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.
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
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.
More complicated to operate than fixed film processes
Produces more sludge than fixed film
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………..
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
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
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
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
Decreases volume of sludge by removing water to reduce downstream process size
Gravity settlement (storage tanks, PFT)
Gravity belt thickener*
*Polymer added to improve separation
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
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
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
Mixing of sludge and polymer is essential for effective conditioning Good conditioning depends on polymer addition, retention time and mixing POLYMER - mixing
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)
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
Chains of molecules that stick sludge solids together to release water Charge type Charge density Molecular weight POLYMERS
Chains of molecules that stick sludge solids together to release water Charge type Charge density Molecular weight POLYMERS Molecular structure
Jet Wet process WATER SUPPLY LEVEL PROBES STORAGE TANK TRANSFER PUMP MIXER MIXING / AGEING TANK VENTURI EDUCTOR BLOWER AIR SCREWFEEDER POWDER POWDER HOPPER AIR / POWDER JET WET HEAD LEVEL PROBES DRY AREA
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
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
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
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