constructed farm wetlands (cfws) for remediation of farmyard runoff in scotland: water treatment...
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Constructed Farm Wetlands (CFWs) for
remediation of farmyard runoff in Scotland:
Water treatment efficiency, ecological
value and cost-effectiveness
Fabrice Gouriveau
The University of Edinburgh, School of GeoSciences
Supervisors: Dr. Kate Heal, Dr. Graham Russell, Dr. Andy Vinten
Background
• Agriculture is a major source of
“diffuse” water pollution from
arable fields, grasslands,
farmyards, etc.
However, the Scottish experience is limited and
investigation is needed to understand existing systems and
build more efficient ones…
• Constructed Farm Wetlands
(CFWs) [Surface flow] are
promoted to catch and treat
contaminated farmyard runoff
(e.g. yards, silage pits, roofs,
tracks)
Research objectives
1) Study the link between farm practices, rainfall
characteristics and quality & quantity of farmyard
runoff
2) Assess spatially and temporally the water treatment
performance of two CFWs
3) Assess their ecological value
4) Document the cost-effectiveness of CFWs
5) Contribute to the improvement of design guidance
Work undertaken
Two sites: CFW 1 and CFW 2, in south-east Scotland
• Rainfall, water level and flow monitoring
• Water sampling to assess pollutant loadings and removal:
– Monthly grab samples
– Storm event sampling
– Samples analysed for: NO3-, NH4
+, IP, TP, BOD5, TSS, FIOs
• Ecological surveys
– Aquatic macro-invertebrates (3 times a year)
– Wetland vegetation (once a year)
• Sediment sampling once a year
• Interviews with farmers to assess acceptance, costs,
problems
CFW 1: Overview
Farm 1: beef and arable farm – 125 cows Average annual rainfall ~ 870 mm
Design (Treatment Volume): 2 x Vt = 2 x 1400 = 2800 m3
(Cost ~ £ 5000)
Inlets
Pond 2
Pond 3
Pond 4
Sheep/cattle track
Farmyard + roofs (1.6 ha)Septic tanks
Manhole Pasture (cattle, sheep)
280 m
Field Runoff
Total Area fenced ~ 0.9 ha
Burn
Overflow
Pond 5(2500 m2)
OutletWater Sampling Points (17)
Field drainage (33 ha)Pond 1
Field Runoff
Not to scaleN Inlets
Pond 2
Pond 3
Pond 4
Sheep/cattle track
Farmyard + roofs (1.6 ha)Septic tanks
Manhole Pasture (cattle, sheep)
280 m
Field Runoff
Total Area fenced ~ 0.9 ha
Burn
Overflow
Pond 5(2500 m2)
OutletWater Sampling Points (17)
Field drainage (33 ha)Pond 1
Field Runoff
Inlets
Pond 2
Pond 3
Pond 4
Sheep/cattle track
Farmyard + roofs (1.6 ha)Septic tanks
Manhole Pasture (cattle, sheep)
280 m
Field Runoff
Total Area fenced ~ 0.9 ha
Burn
Overflow
Pond 5(2500 m2)
OutletWater Sampling Points (17)
Field drainage (33 ha)Pond 1
Field Runoff
Not to scaleN
CFW 1: Ponds
Open ponds surrounded by Phragmites australis and Juncus effusus
CFW 1: Wetland areas
Wetland areas: grasses, watercress, rushes, etc.
CFW 1: Final pond
Inlets
Outlet
CFW 2: Overview
Design (Treatment Volume): 5 x Vt = 5 x 340 = 1700
m3 (Cost ~ £ 4000)
Farm 2: dairy farm – 400 cowsAnnual average rainfall ~ 700 mm
Farmyards + roofs + tracks (3.2 ha)Field drainage
Water cooling the milkSeptic tanks effluents
Outlet
Overflow
Inlet 2
Ditch
Pond(2000 m2 / 1700 m3)
Pasture (cattle)
Field Runoff
Swale
Sampling Points (9)
Total Area fenced ~ 0.4 ha
Gate
Field Runoff
Inlet 1
Farmyards + roofs + tracks (3.2 ha)Field drainage
Water cooling the milkSeptic tanks effluents
Outlet
Overflow
Inlet 2
Ditch
Pond(2000 m2 / 1700 m3)
Pasture (cattle)
Field Runoff
Swale
Sampling Points (9)
Total Area fenced ~ 0.4 ha
Gate
Field Runoff
Inlet 1
Not to scaleN
CFW 2: Swale and pond
Swale
Outlet
Inlets
CFW 1: Water quality overview
Mean inlet
Max inlet
Mean outlet
Max outlet
Treatment Efficiency
(concentration)
BOD5 (mg l-
1)1.3 10 2.1 10 < 0 %
NH4+
(mg l-1) 0.6 3 0.3 1.5 50 %
NO3- (mg l-1) 27.7 58 14.5 73 47 %
IP (mg l-1) 0.2 0.9 0.1 0.5 50 %
TSS (mg l-1) 65 703 8 55 88 %
FC (CFU/100 ml)
1800 3500 75000 > 150000 < 0 %
CFW 2: Water quality overview
Mean inlet
Max inlet
Mean outlet
Max outlet
Treatment Efficiency
(concentration)
BOD5 (mg l-
1)157 560 19.5 50 87 %
NH4+ (mg l-1) 17 65.1 9.7 31.7 44 %
NO3- (mg l-1) 17.7 152.7 7.2 45 59 %
IP (mg l-1) 1.7 9.2 1.4 2.5 20 %
TSS (mg l-1) 239 1696 71 160 72 %
FC (CFU/100 ml)
104500>
1500006625 12250 93 %
CFW 1: Spatial & temporal variability
CFW 1: Storm-event sampling
Treatment estimation:
Rainfall ~ 20 mm in 15 h ; Vol. in = Vol. out ~ 500 m3
IP loading in = Ci x Vi = 0.60 x 500 ~ 300 g
IP loading out = Co x Vo = 0.25 x 500 ~ 125 g
Treatment Efficiency for IP: 58 % mass removal
CFW 2: Spatial & temporal variability
CFW 2: Spatial & temporal variability
CFW 2: Storm event sampling
Macroinvertebrates
CFW 1 - Good ecological value (habitat heterogeneity, low
contamination)
CFW 2 - Moderate ecological value (low habitat heterogeneity,
high contamination and rapid pH and DO changes)
Poor
Moderate
Good
Very good
QUALITY
Summary of Findings
CFW 1: Receives lightly polluted runoff & discharges a good
quality effluent meeting UK standards (except for
nitrate and FIOs); good ecological value.
Limitations: Farmyard runoff not fully intercepted;
excess of field drainage & groundwater; preferential
flow & sub-optimal use of the land
CFW 2: Receives highly polluted runoff, achieves some
treatment but discharges a poor quality effluent
which does not meet UK standards; moderate
ecological value.
Limitations: Non-vegetated, too small, single cell,
short HRT. But could be modified and planted to
ensure better treatment.These CFWs are a low-cost option but design & performance could be improved
Conclusions & Prospects
• Lack of studies of performance of CFWs but increasing
knowledge and data (e.g. Irish ICWs, Scottish CFWs)
• Lack of clear water quality targets to be achieved by
CFWs (common, site-specific or depending on receiving
watercourse?)
• Limited design guidance but CFW Design Manual in
progress
• Failure to implement CFWs according to design ; limited
follow-up
• Cost analysis need to be inclusive (construction,
planting, maintenance, loss of land, SFP) and compared
with other alternatives
• Need for stronger financial incentives to support
sustainable CFWs
Design Suggestion
Pond 4
•Deeper pond (sediment retention + spillage contingency) & shallow wetland cells connected by elbow pipes to control water level (<40 cm).
•Wetland area/volume ? f (rainfall on impermeable surfaces, ~ 3 weeks residence time)
Farmyard runoff
Wetland cell 1
Wetland cell 2
Pond 1
2 stage outflow
Wetland cell 3
Thank You for your Attention !
Many thanks to:
David Kinloch Michie Studentship, Torrance Bequest, University of Edinburgh Development Trust, Scottish
Environment Protection Agency (SEPA), Macaulay Institute, Scottish Agricultural College (SAC), Martha Lucía Gouriveau, Alan Frost, Rory Harrington, Marjan Van de Weg, Andrew Gray, John Morman, Rob Briers, Alison Cole, Carole Christian, Andrew Colman, James
Sukias and all the farmers involved.