An Introduction To Wastewater And Sludge Principles

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?

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

The Wastewater Treatment Process Preliminary Treatment

Preliminary Treatment Inlet works Coarse Screens (optional) Grit Removal (optional) Fine Screens Sewer Primary treatment Screenings treatment Screenings treatment Grit treatment

Getting the big stuff out Coarse Screens Usually around 25mm bars to take the big stuff out

Taking out the fine stuff Fine screens Typically a function of the consent and are normally specified as 6mm in 2 dimensions

What comes off the screens? Screenings Treatment The washer/dryer of the sewage treatment world

Grit – Why’s it a problem? Problems with flow measurement Erosion and wear of downstream equipment, pumps etc Settles and takes up volume in sludge storage Reduces volume in digesters – reduced gas/energy production Better here Than here

What happens when preliminary treatment goes wrong

The Wastewater Treatment Process Preliminary Treatment Primary Treatment

Primary Treatment Settlement Primary Settlement Secondary Treatment Sludge Treatment Preliminary Treatment

Primary Settlement Rely on gravity to settle out lighter SS material Circular or Rectangular constructions Usual designs : minimum 2 hours retention at peak flow 30-50% removal of organic load and 60% of solids High calorific value removed and goes off to digesters to produce energy

The Science Stokes Law 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

What tanks are there? Septic tanks Rectangular Dortmund Upflow (cone) Radial Flow

So what happens? Fluid retention time of at least 2 hours Scraper bridge scrapes it into the hopper Sludge collects in the bottom Sludge removed by pump periodically either manually, by timer, solids mode or level control

The Wastewater Treatment Process Preliminary Treatment Primary Treatment Secondary Treatment

Secondary Treatment The engine room of wastewater treatment Secondary Treatment Tertiary Treatment/ Outfall Sludge Treatment Primary Treatment

Secondary Treatment A Biological process Soluble organic fraction removed by biological treatment typically by either trickling filters or activated sludge plant (ASP) but could be something else.

Trickling Filters Fixed film process meaning the bacteria grows on the rocks and settled sewage is applied to it 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 Settled sewage is dosed onto the filters using mechanical distributors. 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.

Trickling Filters Filter Media 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. Recirculation 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

Trickling Filters Advantages Simple and easy to operate Low or no power Relatively cheap to build Resilient and robust to change and toxic shock Very good for small works Adaptable Disadvantages Large land take Not as efficient a removal process as suspended growth Not resistant to the cold Overgrowth and ponding

Activated Sludge (ASP) 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.

Activated Sludge (ASP) Sewage enters the aeration tank and mixes with mixed liquor Bacteria eat the sewage and need to be fed with air More bacteria are bred and old sludge needs to be wasted to maintain a balance Sludge recycled around to ensure the mass of bacteria is maintained.

ASP Surplus Activated Sludge (SAS) 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%

Activated Sludge (ASP) Advantages Very efficient producing a high quality effluent Common technique with a lot of industry knowledge Very adaptable to increasing loads Can be adapted for Nutrient Removal Disadvantages Energy intensive 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………..

So what happens to all the sludge……..?

Sludge treatment….. a whole world of its own

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

WHAT IS SLUDGE? Mainly water (up to 99%) Dissolved solids Settled and suspended solids Faecal matter Bacteria and other micro-organisms Nutrients (N, P, K) Metals Energy

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

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

Primary sludge Bio 1 º 2 º Tertiary Typically 3% ds (up to 5%) Primary cause of site odour SOURCES OF SLUDGE

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

Primary sludge Bio 1 º 2 º Secondary sludge Co-settled sludge Tertiary Typically 2% ds SOURCES OF SLUDGE

Primary sludge Bio 1 º 2 º Secondary sludge Co-settled sludge Tertiary sludge Tertiary Normally returned as backwash liquor SOURCES OF SLUDGE

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

>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)

Goole 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

Main types of treatment Thickening Digestion Advanced Digestion Dewatering Conditioning Phyto-conditioning Incineration

Decreases volume of sludge by removing water to reduce downstream process size Gravity settlement (storage tanks, PFT) Gravity belt thickener* Drum thickener* *Polymer added to improve separation THICKENING

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

1 Sludge is conditioned with polyelectrolyte 2 Conditioned sludge is fed onto moving belt, water passes through the weave Belt continuously washed by a high pressure jets to prevent solids accumulation in the belt weave Standard flow rates are between 10 and 50m 3 per m belt per hour Be aware of the maximum solids load for the asset (ie the thicker the feed sludge, the lower the allowable flow rate) THICKENING - process

Thin sludge can be due to: Sludge application rate too high Belt speed too high Incorrect polymer dose – excessive dose may blind the belt Sludge characteristics eg excessive FOG Solids in filtrate can be due to: Incorrect polymer dose Solids running off the edge of the filter belt Problems with belt tracking, lubrication, tension (AMBS) Poly suppliers will provide support to determine optimum polymer THICKENING - problems

Gravity belt thickener THICKENING - GBT

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

Biogas DIGESTION Heat exchanger Feed Treated sludge Compressor

DIGESTION - biology Complex organics 1. Acidogenesis 2. Methanogenesis Clostridium Bifidobacterium Lactobacillus Methanobaciterium Methanobacillus Methanococcus Volatile acids CH 4 , CO 2

990L water 10kg solids 490L water 10kg solids 1% ds 2% ds 10kg solids 20% ds DEWATERING 40L water Thickening Dewatering

DEWATERING Sludge feed Polymer Dewatered cake Centrate

DEWATERING Sludge feed Polymer Dewatered cake Filtrate

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

DEWATERING Centrifuges Feed Bowl Scroll Beach Weir plate

Provides required secondary retention Allows drainage of free water Forms a stable, friable product Promotes aerobic conditions CONDITIONING - Aim

Transpiration Predation Competition Root penetration: enzymic degradation & aerobic conversion SPC

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

Sewage sludge – The opportunities Methane Production from Advanced and Conventional Digestion Energy recovery from incineration or pyrolysis Nutrient Recovery from sludge liquors Utilisation of sludge liquors in BNR Processes Biosolids to agricultural land Topsoil production from Sludge phytoconditioning

Energy from sludge Maximising energy throughput from sludges There are a number of options: Digestion and advanced digestion Incineration or pyrolysis Physical disintegration techniques CHP –v- gas turbines –v- biofuel

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

An Introduction To Wastewater And Sludge Principles

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