Strategic Management of Non-Point Source

Pollution from Sewage Sludge

L. Bolton1 L. Heathwaite1, P. Whitehead2 and P. Quinn3

1Department of Geography, University of Sheffield2 Aquatic Environments Research Centre,

Reading University3 University of Newcastle upon Tyne

Sewage Sludge DisposalUK, 1991/1992*

26% SEA DISPOSAL

9% OTHER

6%INCINERATION

11%LANDFILL

47% RECYCLING TOFARMLAND

*WaterUK

UK, 2000*

12% OTHER

22% INCINERATION

11% LANDFILL

55%RECYCLING TO FARMLAND

*WaterUK

Sewage Sludge Disposal

Sewage Sludge or “Biosolids”Benefits of recycling to land

– Fertiliser- agronomically useful quantities of nutirents and trace elements

– Increases soil quality– Cheaper than mineral fertilisers (up to

£100/ha)

– Most environmentally sustainable method of disposal

• Best option in most circumstances: EU and UK government

– Supported by environmental groups • Surfers Against Sewage

Drawbacks of recycling to land– Accumulation in soil/transfer to

groundwater/surface water• Heavy metals

– Diffuse nutrient pollution• Fertiliser governed by nitrogen application can lead to

excess phosphorus

Controlled by 1986 EU Directive (86/278/EEC) and ADAS Safe Sludge Matrix

Sewage Sludge or “Biosolids”

Critical Source Areas

CSA

HIGH TRANSPORT

RISK

HIGH SOURCE

POTENTIAL

Soil P Index

• Sludge not applied when risk of P loss is identified• Soil P index ≥3*• UK 56% arable and 30% grassland soil P index ≥3• Current thinking has little understanding of the

vulnerability of sludge P loss to receiving waters • Is it possible to minimise nutrient loss by applying

sludge to land outside CSAs regardless of soil P index status?

*UK Code of Good Agricultural Practice, 1998

Study Area

• Arable farm receiving regular sewage sludge treatments, SE England

• Upper Chalk

• Perched water table

• Groundwater dominated system

• 20m unsaturated zone

Field A

P index

2

3

4

5

Prior to Application

• 30ha field, ephemeral ditch

• Mean soil Olsen’s P 38.8 mgL-1, areas of soil P index 5

• Digested sludge cake and lime stabilised sludge treatment September 2001

• Dominated by subsurface flow

Field A

Post application

P index

2

3

4

5

• Mean ditch total phosphate 0.475 mgL-1

• Mean soil water total phosphate 0.451 mgL-1

Field B

• Field B treated with digested sludge cake, October 2003– Soil Olsen’s P prior to treatment 22.00mgL-1

• Adjacent control field not treated• Both fields are tile drained

• Nutrients concentrations in tile drains from both fields monitored

Field B: tile drains

25/10/2002 06/11/2002 18/11/2002 30/11/200202468

1012

05

101520

rain

fall

(mm

)

02468

1012

tota

l pho

spho

rus

(mgL

-1) control

(untreated)

sewage sludge treated

Field A and Field B

Field A

• P concentration in ditch water relatively low

• P not lost from this field: retained in soil or no connectivity

• High initial soil P is not coincident with transport

• No CSAs for surface water

Field A and Field B

Field B

• Rainfall occurred during application

• Incidental loss of P very important in this situation

• Land drains effectively turned the whole field into a CSA

Modelling

Field Scale Connectivity Modelling• TopManage

– Digital terrain analysis to visualise the effects of land management on hydrology

• TOPCAT – timeseries modelling of flow and nutrients

Catchment Scale Modelling• INCA-N and P models

P leaching: 100% cereal catchment; low connectivity

030 70

low initial Pmedium initial P

high initial P

0

0.4

0.8

1.2

1.6

low initial P

medium initial P

high initial P

To

tal P

lea

chin

g k

g h

a-1 y

-1

Biosolids P input kg ha-1 y-1

030 70

low initial Pmedium initial P

high initial P

0

0.4

0.8

1.2

1.6

2

2.4

2.8

To

tal P

lea

chin

g k

g h

a-1 y

-1

Biosolids P input kg ha-1 y-1

P leaching: 100% cereal catchment; high connectivity

low initial P

medium initial P

high initial P

Nutrient Export Risk Matrix

SOIL TYPE

FLOW CONNECTIVITY

FERTILISER APPLICATION AND

SOIL MANAGEMENT

Low risk

High riskOutput gained from scenario tests with plot and field scale INCA is being used to fill in the nutrient availability axis on the NERM

Phosphorus Export Risk Matrix

FERTILISER APPLICATION AND

SOIL MANAGEMENT

FLOW CONNECTIVITY

Low risk

High risk

Phosphorus Export Risk MatrixSeries of questions are asked relating to:• Flow Connectivity

– Hill slope form– Hedgerows– Remediation options

• Fertiliser Application and Soil Management– How much P do you intend to apply– Current soil P index

• Prototype PERM available on www.sheffield.ac.uk/SEAL and www.ncl.ac.uk/wrgi/TOPCAT/

Conclusions

• Sewage sludge has an environmental and economical use when applied to land

• Export of P occurs when transport and source factors coincide as CSAs

• P export can be controlled by strategic management of applications of sewage sludge

Acknowledgements

• EPSRC (GR/N26074/01) The SEAL Project: Strategic Management of Non-point Source Pollution from Sewage Sludge • Roger Pryor for access to field site• Lister Noble (Farm Systems) for P index data • Thames Water and Terra Ecosystems for sludge data