Wastewater and Sewage SludgeA 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

14th 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 pointsWater 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 sewageConsent 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 better1960’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 consentedFrom 1977 to present day most environmental law has been through EC Directive although some of it has been included in 1991 Water ActEC produced a black list of chemicals to be banned from dischargesAlso 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

Preliminary Treatment

Primary Treatment

Secondary Treatment

Tertiary Treatment

Sludge Handling & Treatment

So what does a

Sewage Treatment Works

look like?

Esholt WwTWTreats 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

A little more detail…..

The Wastewater Treatment Process

Preliminary Treatment

Preliminary TreatmentInlet works

Coarse Screens (optional)

Grit Removal (optional)

Fine Screens

Sewer Primary treatment

Screenings treatment

Screenings treatment

Grit treatment

What comes down the sewer?

Getting the big stuff out

Coarse ScreensUsually around 25mm bars to take the big stuff out

Taking out the fine stuffFine screensTypically a function of the consent and are normally specified as 6mm in 2 dimensions

What comes off the screens?

Screenings TreatmentThe 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

How is Grit removed

What happens when preliminary treatment goes wrong

And when it goes right

The Wastewater Treatment Process

Preliminary Treatment

Primary Treatment

Primary TreatmentSettlement

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 1.2m3/m2/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 TreatmentThe engine room of wastewater treatment

Secondary Treatment

Tertiary Treatment/ Outfall

Sludge Treatment

Primary Treatment

Secondary TreatmentA 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 FiltersAdvantages• 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.

ASPSurplus Activated Sludge (SAS)

Return Activated Sludge (RAS)

Anoxic Zone

Aeration Lane Grows sludge on dissolved organics

FSTSelector

Aeration adds O2

Nitrification NH4 NO2 NO3

Denitrification NO3 NO2 N2

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 moreTertiary 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 filtersNitrifying Trickling FiltersSAFsHSAFsBAFFs

and many others………..

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

Sludge treatment…..a whole world of its own

WHAT IS SLUDGE?

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 solidsSettled and suspended solids

Faecal matterBacteria and other micro-organismsNutrients (N, P, K)MetalsEnergy

SLUDGE PRODUCTION

Primary 50 to 60g ds/person/daySecondary 18 to 29g ds/person/dayTypical total sludge per person 70g/day

PE x yield x 365 = mass (tds)

Mass / concentration = volumemass

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 Land19% Incinerated11% Land Reclamation7% Other (including composting)1% Landfill

Source: Water UK

SLUDGE TERMINOLOGY

990L water

10kg solids

490L water

10kg solids

1% ds 2% ds

240L water

10kg solids

4% ds

“Tonnes dry solids”

“Percent dry solids”

Volu

me 1

m3

SOURCES OF SLUDGE

Primary sludge

Bio

1º

2º

Tertiary

Typically 3% ds (up to 5%)Primary cause of site odour

SOURCES OF SLUDGE

Primary sludge

Bio

1º

2º

Secondary sludge

Tertiary

Humus typically 2% ds (1-3% )SAS typically 0.6% ds (0.25-1.25% )

SOURCES OF SLUDGE

Primary sludge

Bio

1º

2º

Secondary sludge

Co-settled sludge

Tertiary

Typically 2% ds

SOURCES OF SLUDGE

Primary sludge

Bio

1º

2º

Secondary sludge

Co-settled sludge

Tertiary sludge

TertiaryNormally returned as backwash liquor

SOURCES OF SLUDGE

OBJECTIVES OF SLUDGE MANAGEMENT

Reduce volume (removal of water)Reduce/remove odourStabilise organic material (BOD removal)Remove pathogensReclaim useful by-products (biogas, soil conditioners)Safe/appropriate disposal & recycling

BIOSOLIDS

>600 Sewage Treatment Works50 Sludge Treatment CentresVarying degrees of treatmentVarying costs of treatmentVarying capabilities (sludge intake, sludge output, day and night operability)

Naburn

Beverley

Hull

Sutton Selby

Blackburn Meadows

Calder Valley Incinerator

Esholt

Knostrop

Bridlington

Staveley

Old Whittington

Sandall

Woodhouse Mill

AldwarkeLundwood

Caldervale

Mitchell Laithes

Leeming Bar

Goole

Wombwell

Northallerton

Colburn

STC LOCATIONS

TYPICAL SITE LAYOUTS

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 filtrateTarget solids is 6% ds – thicker sludge is difficult to pumpFiltrate is returned to the works and excessive solids can cause compliance problems

THICKENING - aim

1 Sludge is conditioned with polyelectrolyte2 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 50m3 per m belt per hourBe 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 beltSludge characteristics eg excessive FOG

Solids in filtrate can be due to: Incorrect polymer doseSolids 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

THICKENING - PFT

THICKENING - drum

POLYMER

Added to sludge prior to mechanical thickening and dewateringPoly 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 devicepH, Alkalinity, water hardness, temperature, can affect performance of polymers

Type of sludge is the primary factor affecting the type and quantity of polymer requiredRaw Primary sludge requires much less poly than SASOld sludge requires a higher poly dose than fresh sludgePolymer 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 conditioningGood conditioning depends on polymer addition, retention time and mixing

POLYMER - mixing

DIGESTION - requirements

Min 12d primary digestionMin 32ºCMin 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

slu

dg

e

inert so

lidsvola

tile

BiogasDIGESTION

Heatexchanger

Feed

Treatedsludge

Compressor

DIGESTION - biology

Complexorganics

Volatileacids

CH4, CO2

1. Acidogenesis

2. Methanogenesis

ClostridiumBifidobacteriumLactobacillus

MethanobaciteriumMethanobacillusMethanococcus

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 feedPolymer

Dewatered cakeFiltrate

DEWATERING

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 TANKTRANSFER PUMP

MIXER

MIXING / AGEING TANK

VENTURIEDUCTOR

BLOWER

AIR

SCREWFEEDER

POWDER

POWDERHOPPER AIR / POWDER

JET WET HEAD

LEVELPROBES

DRY AREA

DEWATERING Centrifuges

Feed

Bowl

Scroll

Beach

Weir plate

DEWATERING

CONDITIONING

Provides required secondary retentionAllows drainage of free waterForms a stable, friable productPromotes aerobic conditions

CONDITIONING - Aim

CONDITIONING

SPC

Transpiration

Predation

Competition

Root penetration:enzymic degradation& aerobic conversion

SPC

INCINERATION

Flue gas stack

Fluidising air fan

Caustic scrubber

Ash to disposal

2 fieldElectrostatic 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

INCINERATION

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 resourceSludge 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 resourceSludge 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

ANY QUESTIONS??