SPN7. 7th International Conference on
Sewer Processes & Networks
SEWER MISCONNECTIONS IN
ENGLAND AND WALES: ARE THEY
A SERIOUS PROBLEM?
J Bryan Ellis
Urban Pollution Research Centre,
Middlesex University, UK.
Sheffield, August 2013
Surface Water Sewers in England and Wales
Percentage Surface Water Sewers
30
Percentage Surface Water Sewers
Number of Households Served
(Millions)
6
5
25
4
20
3
15
2
10
5
0
1
0
Number of Households Served (Millions)
35
Downstream changes in NH3-N and PO4 in an urban catchment, River
Colne, Oxhey, Herts.
Downstream Increase in NH3-N (mg l-1)
10
9
20%
River Classification
Grade C/D
8
7
6
5
10
4
Average N:P ratio for
sewage (DEFRA, 2008).
3
5%
2
Percentage given is % sewage in
river water
1
1%
0
0
0.5
1
Downstream Increase in PO4 (mg l-1)
1.5
2
MISCONNECTION SOURCES
-household/commercial premise misconnections (greywater
and blackwater)
- connection of industrial/commercial floor drains to surface
water drainage system
- abuse of surface water drainage system e.g illegal disposal
of solvents, paints etc; vehicle jet-washing; wash-down of
cafe/restaurant frontages/courtyards etc; illegal dumping
- failing septic tanks.
- dual/shared manhole chamber overflows—cross-connection
rather than misconnection (??)
Procedural Flow for Misconnection Enforcement Notices
Misconnection Established by EA or WC
Ground Inspection
(EA or WC pre-survey and site survey)
LA Section 59 Enforcement Notice
(Notice and Recommendation of measures)
EA or LA Preliminary Visit
LA Takes No Action
LA Takes Action
Householder Complies
(Cost split 40% to LA)
Repair Carried Out
LA Closure Visit
(Site Inspection and Sign-off)
Pollution
Problem
Remains
MISCONNECTION DATA AND TRENDS
NATIONAL DATA
- CES (1999); >1M; ~29% of all connections)
- Defra (2007); 1.35M (~7M properties; £235M/year; >7% misconnection rate)
- Defra (2009); 300,000 – 500,000 (3% - 5% misconnection rate)
- UKWIR (2013); 130,000 – 140,000 (<1% misconnection rate; £190M)
REGIONAL DATA
- Thames Water (2010); 1.2M or 3% - 5%. (Based on 1 :10/20 misconnection rate;
£78 - £104M reconnection cost + survey + enforcement notice)
SITE SURVEY DATA
- Moston Brook, Manchester (2013); 1.5km2 ; 1700 population; ~24%
misconnections)
- Brent, N London (2008); 30km2 ; 236,464 population; >7% misconnection rate;
£10.4M
Actual Pollution Potential of Domestic Properties in Thames Water Region
600
8000
7800
7600
500
NUMBER OF HOUSES WITH
POLLUTION POTENTIAL
<0.01%
misconnection
rate
HOUSES CAUSING POLLUTION AT
7% MISCONNECTION RATE
HOUSES CAUSING POLLUTION AT
2% MISCONNECTION RATE
400
300
7400
200
7200
100
7000
6800
2007
2008
2009
2010
2011
YEAR
2012
2013
2014
0
2015
Houses Causing Pollution
Number of Houses with Pollution Potential
8200
SECTION 59 ENFORCEMENT NOTICES 2008
Company
Apportioned
Household Pollution
Potential
(x 1000)
%
Population
served
Enforcement
Notices
Served
Thames
2034
24%
60
Severn Trent
1343
16%
86
Yorkshire
Water
599
13%
180
Section 59 actual
326
Estimated total Section 59 requested
609
Total if all LAs fully cooperated with Section 59
869
Domestic Misconnections by Type
Washing Machine
Kitchin sink
Handbasins
Dishwasher
Bath
Shower
Toilet
Bidet
Other
50
45
Percentage Contribution (%)
40
35
30
Kitchen sink
Washing machine
Dishwasher
Wash basin
Shower
bath
Domestic
Misconnections
by Type
25
20
15
10
5
0
Volume
BOD
PO4-P
Household Appliance Misconnection Pollution Yield
Washing
Machine
Sink
Dishwasher
Bath
Shower
Toilet
Total
11
(280-470)
5
(331400)
11
(390-699)
7
(50173)
7
(80-424)
20
62
(110350)
255
175
200
101
884
10060
11675
Volume (litres)
13.7
(17-60)
10.0
(8-14)
1.6
(2-6)
25.0
(16)
33.2
(12-20)
28.1
121.6
(68-134)
Volume assuming 2.36
persons per house
(litres)
32
(41.5)
24
(21-31)
4
(14 – 47)
59
(37.8)
78
(29)
90
287
Probability that
appliance is
misconnected
78%
96%
26%
51%
32%
22%
5%
9
6
3
4
2
4
3
31
Phosphate (mg/l-1)
199
168
52
51
280
2248
384
3582
Volume (litres)
25
23
1
30
25
20
14
138
BOD (mg l-1)
Phosphate (mg/l-1)
BOD (mg l-1)
Average
Totals
Estimated Misconnection Wastewater Volumes and
Pollution Loads Entering Thames Region Surface Waters
National Data House
Potential Misconnections
(No x 103)
Wastewater Company
reported Houses Actually
Causing Pollution
Untreated Wastewater
Volume Based on National
Data (litres x 106)
BOD Loading Based on
National Misconnection Data
(kg)
BOD Loading Based on
Wastewater Company Data
(kg)
2007
2008
2009
2010
2011
2012
2013
2014
2015
274
294
314
335
358
378
400
423
447
480
490
500
509
519
529
539
549
558
37.8
40.6
43.3 46.2 49.4 52.2 55.2 58.4 61.7
2468 2646 2826 3019 3222 3402 3600 3802 4023
4.32
4.4
4.5
4.6
4.7
4.8
4.9
4.94
5.0
EXTRAPOLATING MISCONNECTION DATA
BOD = [(POPt/Hn) x (Ma/Hn) x La x Hn]
where:
BOD = B OD in kg/day
POPt = Total Population of catchment
Hn = Number of occupants per household/property
Ma = Number of each type of misconnected appliance (as
determined from site survey)
Hn = Total number of households/properties in survey
La = BOD loading for each appliance (kg/person/day)
OR
BOD = [POPt x (Ma/Hn) x L
i.e estimate is independent of the number of occupants per property.
CONCLUSIONS
- Site surveys required to give accurate estimation of misconnection
numbers and severity of impact. National estimations deflate likely potential
scale and costs of the misconnection problem.
- Misconnection rates likely to vary between average minimum of 1% to
mean of 3% and average maximum of 7%. Hotspots greater than 12% - 24%
can occur with observable receiving water impacts where SWOs discharge
in groups or in series along an urban reach as demonstrated by N:P ratios.
Need for robust and tested catchment scale extrapolation techniques to
adequately quantify WFD receiving water risks.
- Source tracking and compliance procedures need to be more stringent and
without need to resort to third party engagement.
- Need for twin-track approach with misconnection remediation (“clean-up”)
in conjunction with long term “preventative” community-based
programmes/campaigns of both public and professional trade knowledge
and awareness.
- Surface water misconnections likely to be a continued issue for urban
drainage and urban receiving water quality in the UK into the foreseeable
future.