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
