Surahammar – a case study of the impacts of installing food waste disposers in
fifty percent of households.
Tim Evans, FCIWEM1, Per Andersson2, Åsa Wievegg2 and Inge Carlsson2
TIM EVANS ENVIRONMENT, Stonecroft, Park Lane, Ashtead, KT21 1EU, UK
email: tim@timevansenvironment.com
2
Surahammars KommunalTeknik AB, Box 10, 735 21 Surahammar, Sweden
1

Abstract
This paper reviews 15 years of sewage works’ monitoring data to assess the effect of
installing food waste disposers (FWD) and how these effects compare with the
published scientific literature. Within a period of 10 years, 50% of households in the
town of Surahammar in Sweden chosen to have FWD installed as their means ofmanaging their kitchen food waste. The drainage from the households feeds a single
wastewater treatment works (WwTW) that comprises primary settlement, activatedsludge followed by chemical phosphate precipitation and mesophilic anaerobic
digestion. The diameters and gradients of the sewers are not unusual. This paper
reviews the influent, effluent and biogas monitoring data for the 3 years before
installation started and the 10 years after the first peak of installations when they were
installed in 30% of households. This provides a unique opportunity to “ground truth”
the published research on FWD. The paper reviews the monitoring data and confirms
the published research that there has been an absence of impacts on water use, sewer
blockages, vermin and wastewater treatment. The data also provide an indication thatin-sewer biological process may have acclimated to the change in wastewatercomposition and “treat” the dissolved and fine particulate load before it reaches theWwTW. The digesters produce more than 40% more biogas than before FWD were
installed and the tonnage of waste to landfill from the municipality has decreased
from 3600 tonnes/year in 1996 to 1400 tonnes/year in 2007.
Keywords
Ammonium, biogas, BOD, COD, cost, food waste, in-sewer process, landfill directive,
water resources, wastewater treatment
Introduction
The laws of ecology enunciated by Barry Commoner (1971) are timeless andimmutable:
1. Everything is connected to everything else.
2. Everything must go somewhere.
3. Nature knows best.
4. There is no such thing as a free lunch.
Within the European Union it has been decided that in order to reduce the emission of
methane (a climate change gas with a global warming potential 25 times that ofcarbon dioxide, IPPC, 2007) from landfill, Member States (MS) shall send lessbiodegradable municipal waste (BMW) to landfill (CEC, 1999). However, it has togo somewhere. The Landfill Directive set targets for the weight of BMW that can belandfilled compared with the amount that had been landfilled in 1995, the reference
year. These reductions (to 75%, 50% and 35% of the reference year weight) are to beachieved by specific target dates, which are respectively 2006, 2009 and 2016 formost MS though the UK has a 4-year derogation to 2010, 2013 and 2020.
To achieve the diversion targets, Surahammar in Sweden chose (in 1997) to offer its
citizens differential charges for waste collection plus a bring system for cardboard,glass, metal and plastic. The policy has been effective in that the tonnage of waste tolandfill from the municipality has decreased from 3600 tonnes/year in 1996 to 1400
tonnes/year in 2007.
The lowest annual charge was for home composters; householders are required topurchase and maintain their own authorised compost bins. The highest charge was forhouseholds that chose kerbside collection of source-segregated biodegradablemunicipal waste, householders were provided with an additional wheeled bin which
was collected weekly, or twice a week in hot weather. The third option was an 8-year
contract to lease a food waste disposer (FWD) from Surahammars KommunalTeknik
AB (SKT). SKT operates the solid waste, water supply, wastewater, wood-fired
electricity generation and district heating in Surahammar Kommune, it is a company
wholly owned by the municipality. To qualify for the leasing contract SKT inspected
the sewer lateral connecting the property to the main sewer using closed circuit
television (CCTV). Sometimes this revealed problems. When the pipework was
suitable the householder was eligible to have a FWD installed by the municipality as
part of an 8-year leasing agreement during which the municipality repairs any faults.
After 8 years the FWD becomes the property of the householder, whose waste
collection charge reverts to that of a home composter; alternatively the householder
can have a new FWD and start another 8-year contract. The approximate annual costs
to householders are leasing £27 and kerbside collection £209. Unsurprisingly, withthis cost differential, the uptake of FWD was rapid, and it is this that makes
Surahammar an interesting case study.
The municipality of Surahammar comprises Surahammar, Ramnäs and Virsbo about
110 km west of Stockholm in gently rolling countryside. Industry was founded in the
16th and 17th centuries based on iron. Virsbo is about 13km north of Ramnäs which isabout 7km north of Surahammar. The drainage from Surahammar and Ramnäs istreated at Haga wastewater treatment works (WwTW) which is situated at thesouthern end of the catchment about 2km from Surahammar. A rising main connects
the Ramnäs and Surahammar catchments. Virsbo has its own WwTW whose sludge istankered to Haga for digestion.
Sewers are laid at 4-5‰ gradient. Haga WwTW comprises 3 mm screens, grit/sandsettlement, primary clarifiers, diffused-air activated sludge, chemical precipitation ofphosphorus and mesophilic anaerobic digestion. Aeration of the activated sludge iscontrolled by dissolved oxygen (DO) probes. The discharge consent is 15 mgBOD7/L
and 0.5 mgP/L; there is no nitrogen limit. The digested sludge is thickened in former
drying beds and composted/dried with miscanthus grass. The resultant soil-likebiosolids are trucked to Västerås to a topsoil manufacturer.
Even though there has been a substantial amount of research into their effects since
they were invented in USA in 1928, some wastewater operators with no experience of
FWD are apprehensive about their introduction. They fear increased sewer blockage
and extra wastewater treatment costs and operational problems. None of the research,which has been conducted in many countries, bears out these anxieties. The
monitoring data from Surahammar from the years before and through this period of
rapid installation of FWD (1995 to the present) provide a unique opportunity to“ground truth” the existing research at the scale of a wastewater treatment works
catchment.
Background Research
Before launching the new options for managing household kitchen food waste, SKT
commissioned VA-FORSK to research the likely impacts. This pre-projectassessment was published together with post-project appraisal in 1999 (Karlberg andNorin). VA-FORSK was created in 1990 by the Association of Local Authorities andSwedish Water Works Association. It is financed by annual subscriptions frommember municipalities; almost all municipalities are members. VA-FORSK is thewaste and sewage technology research and development programme for themunicipalities in Sweden in the fields of municipal waste and sewage.
In 1993 SKT installed FWD in 32 out of 39 apartments in a housing cooperative; the7 without FWD were in a building with its own drainage and were the control. The
sewers were flushed before installation and video-filmed. They were filmed again
after 1 year and 2 years, on both occasions there was no difference. At the end of thesecond year the sewers were flushed again, there was no discernable difference inparticles, sludge or grease. 96% of residents were satisfied with their FWDs. Refuse
collection decreased from emptying six 400 litre containers twice a week to emptying
three once a week. Water consumption in the cooperative appeared to decrease by
25%, which SKT did not attribute to FWD but the cause remained unexplained. It
was on the basis of this trial that the municipality decided to offer FWD as one of thewaste management options.
Installations started in May 1997. Karlberg and Norin (1999) undertook intensive
monitoring at Haga WwTW in 1998. They concluded that they were not able tomeasure any change in flow, BOD7, P or the electricity consumed by the motors forthe turbo-blowers for the activated sludge plant. The incoming N decreased and thebiogas production increased. Karlberg and Norin concluded that even though theymeasured no change on BOD7, the amount of substrate for biogas production had
increased as a result of FWD installation and that there had been no adverse effect on
the treatment works or on the sewerage.
Water use
Each time they are used, FWD are flushed with cold water, this cools the motor andconveys the food waste out of the grinding chamber. Water resources in south east
England are already under pressure, it has the highest population in the UK and has
low rainfall, however the Chartered Institution of Water and Environmental
Management has concluded (CIWEM, 2003) “The change in water usage associatedwith operation of FWD has been measured to be trivial or not significant.”
A detailed stratified survey in the USA (Ketzenberger, 1995) showed that FWD wereused for about 15 seconds per start irrespective of the number of people in thehousehold; subjectively this seems sensible (because FWD use is linked to foodpreparation events) and accounts for the range of reported water-use when expressedas litres per capita. A study in Sweden fitted FWDs in a community of 100
apartments (155 adults and 56 children); the duration of use per start was 38 seconds(Nilsson et al. 1990). The per capita water use was 13 L/day less during the 11
months after the FWDs had been installed than the 6 months prior to installation but
like Karlberg and Norin (1999) Nilsson et al. concluded it would not be appropriate to
attribute this directly to the fact that FWD had been installed. Jones (1990) in Canada
was unable to detect any greater per-capita volume of water used where FWD had
been installed and concluded the influence on water use was not significant within theoverall “noise” in measured water use. Whilst this inability to measure an increase inwater use when FWDs are installed seems counter intuitive initially, it is perhaps
understandable when one thinks about the routine of food preparation, etc. Afterusing the sink it is normal to wash it down to clean it, if there were a FWD this would
also flush the FWD.
The studies that have been able to measure water use associated with FWD operation
found data ranged from 0.29 L/person*day (large families) to 6.4 L/person*day. The
extremes of the range are probably anomalous. There has only been one study ofwater use in the UK that has included FWD, however the methodology used was
fundamentally flawed. Even when the paper was presented, the statistical analysis
used was criticised as having been demonstrated to be inappropriate for this type ofwork (Thackray et al., 1978).
The study by the New York City Department of Environmental Protection (NYDEP,
1999), which was undertaken to inform its decision whether to change the regulations
regarding FWD installation, is probably the largest field study ever undertaken. It
involved 514 apartments with FWD compared with 535 apartments without FWD;
they were divided into 4 localities to reflect some of the city’s diversity. The survey
comprised 2014 people in total, i.e. 1.92 people per apartment. The report concluded
the average water use attributable to FWD was 3.6 L/person*day. If uses/day
averaged 2.2 as in Ketzenberger’s study, this would equate to 3.1 L/use, i.e. the sameas Ketzenberger. The overall result of the NYDEP study was that the 18-year
restriction on FWD installation in New York City was removed.
Electricity
Domestic FWDs typically have a 350 to 500 W motor (0.5 to 0.75 horsepower), if
usage averages 2.4 times per day for 16 seconds per use the annual electricity
consumption is about 2 to 3 kWh/household*year. The surveys cited under ‘wateruse’ found that usage (starts/day) was largely independent of the number of people ina household because it was determined by food preparation events.
SewersSewer systems are designed to remove wastewater to prevent urban flooding anddisease; the pipe diameters and gradients are designed such that the flow velocity
keeps the typically encountered solids in suspension. During periods when the flow
velocity is low, solids might settle but they should be re-suspended when velocities
increase. Design standards for “self-cleansing velocity” range from 0.48 m/s to 0.9
m/s (Ashley et al., 2004). The field studies already cited in this paper have checked
the effect of FWDs on the conditions in sewers and found no significant

accumulations. The times of day when FWDs are used correspond with times of highflow (Nilsson et al., 1990). In an experimental rig using different types of kitchenfood waste (KFW), sediment-free transport of the output from FWD was observed at
0.1 m/s, i.e. well within the normal design standards (Kegebein et al., 2001).
Kegebein et al. used two mixtures of foods and the waste from the university’scafeteria, they found that 40-50% of the output of a FWD was <0.5 mm and 98% was<2 mm by sieve analysis. All of the output passed a 5 mm sieve. The largest particles
were fragments of lettuce leaves. For the inputs used, between 15 and 36% of theoutput of the FWD was dissolved. The output of the FWD was very finely divided
and very biodegradable.
FOG (fat, oil and grease) is a significant problem in sewerage operations, it can
reduce the capacity of sewers and even block them; FOG can also accumulate inside
the cooling jackets of pumps and cause them to overheat if it is not removed. It
appears that FOG undergoes chemical transformations (possibly involving proteins)that increase its hardness. Field studies have found that domestic FWDs do notincrease FOG; it is supposed that the constituents of FOG coalesce onto food waste
particles in the cold water flush and that they are therefore not “free” to attach/solidify
onto sewer surfaces. De Koning and Graff (1996) concluded that even in Holland
where the gradients of sewers are shallower than elsewhere (and as a consequence
sedimentation would be more likely) ground KFW from FWD would not result insewer obstructions from sedimentation or FOG deposition. Ducoste, et al. (2008)
analysed 27 samples of FOG from 23 sewer utilities in all the regions of the UnitedStates except the north east. They measured the yield strength, chemical composition
and fatty acid profile etc. and also examined samples by microscopy. They concluded
that the mechanisms by which FOG deposits form remained unknown but, thephysical properties and visual characteristics suggest that a majority of deposits (84%)
were metallic salts of fatty acids formed by saponification. They hypothesised thatsaturated oil constituents are preferentially selected and accumulated into a hardenedmass with a porous structure and that the high pH (>10) alkaline detergents,
degreasers, and sanitizers commonly used in food service establishments would
promote saponification. A secondary cause of FOG deposits may be oil accumulations
without saponification, possibly from highly concentrated oil discharges resulting
from illegal dumping or improper grease interceptor management. Keener said there
was no evidence of FWD output being involved in FOG deposits (priv. comm., 2009).
An important question is whether putting more food into the sewers will increase thenumber of rats. A spokesperson for the British Pest Control Association [Adrian
Meyer, Rodent Control Consultant, priv. comm. 2005] advised that installing FWDs
would probably be detrimental to rats and certainly not advantageous because finely
ground food waste would be less attractive to sewer rats than un-ground waste.
Meyer said nobody really knows how rats find their food in sewers, which are dark,
but rats have been seen scooping grains etc. out of the flow. There is invariably
identifiable food such as sweet corn grains in the grit and screenings skips at
WwTWs; these would have been large enough to be identifiable by rats. However, if
they had been through a FWD they would have been liquidised and hence not
identifiable by rats; food residues <2mm would be non-identifiable by rats. Meyer
said rats might not feed in sewers at all but merely use them as refuges and feed on
the surface from waste bins, etc.

Energy
Kegebein et al. (2001) estimated that where the WwTW receiving the KFW treated its
sludge by AD, the biogas from KFW would amount to approximately 300
MJ/resident*year, which they said corresponds to a heating value of 8 litres of diesel
fuel or 183 kWh/household*year (2.2 people per household). At 40% electricity
generation efficiency, this is 73 kWhe/household*year electricity generation, which atthe EU average for electricity generation is a Global Warming Potential of -33
kgCO2e/household*year (i.e. a saving compared with the 6 kg CO2e /t KFW to run the
FWD).
Battistonia et al. (2007) reported a field study in central Italy where KFW from 67%of the population equivalent of a village that drained to its own WwTW installed
FWD. The area is rural with scattered communities mostly <10,000 population.
Gagliole, the village chosen for the study is 38km from the composting plant; 35
families (95 people) decided to participate also a FWD was installed in the canteen of
the local school (population equivalent 60 people). This gave a total ‘‘market
penetration factor’’ of about 67%. The WwTW design was extended aeration forcontrol of BOD and ammonia; it was modified to remove N and P by alternate oxic
and anoxic operation under the control of DO and redox meters. Battistonia et al.
monitored the WwTW for 275 days: 96 before and 179 after FWD were installed.
The chemical–physical characterization of the WWTP influent, effluent and activatedsludge was determined twice a week on daily averaged samples analysed for: COD,
soluble COD, NH4-N, TKN, total phosphorus, total suspended solids, mixed liquor
suspended solids and mixed liquor volatile suspended solids, pH and total alkalinity.
High-performance liquid chromatography was used to determine concentrations ofanions (NO2-, NO3-, PO43-, Cl-, SO42-, F-) and cations (Na+, K+, Mg2+, Ca2+). The
specific rates for nitrogen removal, nitrate utilisation rate, and oxidation, ammonium
utilisation rate, were determined by means of batch tests in order to study the impactof the FWDs on the biological kinetics and, in particular, on the denitrifying
capability of the activated sludge.
Video inspection found no sedimentation even in a length of sewer where the gradient
was only 1‰. The daily flow rate (hydraulic loading / water use) did not change.
Battistonia et al. found it was hard to distinguish the impact of FWD on the WwTW’s
loading. FWDs had no significant impacts on the energy consumption but they foundthat the better availability of biodegradable carbon can optimize the use of thenitrogen-bound oxygen and thus save energy for air blowing. They concluded that fora rural town of 10,000 inhabitants, FWDs would have a payback of only 4 years
compared with kerbside collection of BMW.
Methods
Between 11th January 1995 and 1st April 2009, 180 samples of influent were collected
at Haga WwTW, i.e. one sample every 4 weeks over 742 weeks. Influent samples
were 24-hour composites collected after the 3mm inlet screens and before gritsettlement. Samples were not filtered before analysis. Generally samples werecollected on Wednesdays. At the time of sampling the daily flow was recorded(m3/day). Effluent samples were 24-hour composites collected every 2 weeks.

Samples were analysed for BOD7 using SE EN 1899-1 Water quality. Determinationof biochemical oxygen demand after n days (BODn). Dilution and seeding methodwith allylthiourea addition, COD Chemical oxygen demand by oxidation withdichromate, Total phosphorus using ISO 6878 Water quality - Spectrometricdetermination of phosphorus using ammonium molybdate, Total-N using SS13395(modified) and SS028131(modified) and ammoniacal-N using SS-EN ISO 11732(modified) Water quality - Determination of ammonium nitrogen - Method by flow
analysis (CFA and FIA) and spectrometric detection.
Karlberg and Norin (1999) reported that food waste disposer installation started in
May 1997 and that by October 1998, 1100 households had had one installed (30% of
the total number of households in the municipality). By June 2008 the proportion ofhouseholds with FWD installed had increased to 50%.
Results and Discussion
The sewer maintenance team has found there has been no change in septicity or incorrosion to, or deterioration of, the fabric of the sewers (including the Ramnäs risingmain). The pest control contractor for Surahammar observed that there have been rat
problems associated with some [poor] home composting but none associated with
FWD installation.
Figure 1 shows that although there has been considerable variation in daily flow (asone would expect with rainfall, snow-melt and drought, etc.), there has been
negligible change in the overall trend between January 1995 and April 2009, which
has remained at about 4500 m3/day, indeed the fitted trend line shows a slight decline.
The median for January 1995 to April 1997 was 4020 m3/d whereas the media for
January 1999 to April 2009 was 3575 m3/d. The scatter of results shows that despite
maintenance work, some of it triggered by the CCTV surveys) there has still been asignificant amount of surface water and infiltration. Inevitably, some old appliances
will have been replaced by more water efficient ones during this 14 year monitoring
period.
Figure 1 Daily flow of influent after 3mm screens and grit settlement (m3/day) at the time of 4weekly sample collection and the 13-sample moving average (trend line for all data)

16000
12000
8000
4000
0

FWD installation

Incoming flow m 3/day

20000
y = -0.078x + 7491.9
R² = 0.0022

Figure 1 verifies, against the background of these confounding effects, the conclusionfrom earlier field studies that the net effect on water usage in households with FWD isnegligible and that the hydraulic loading on sewerage and wastewater treatment is
negligible or immeasurable. Comparing the influent data for 11/01/95 to 30/-4/97(before FWD were installed) with 12/01/99 to 01/04/09 (when the majority of FWD
had been installed) using Student’s t-test confirmed that the daily flows were notdifferent (P=0.34).
One of the largely unknown factors from earlier research is the proportion of theoutput from FWDs that actually arrives at a WwTW. We know that sewers are linear
bioreactors but our understanding of in-sewer processes is imperfect. In gravity
sewers with free headspace the bulk of the wastewater is aerobic but biologicalactivity soon strips out the dissolved oxygen when the sewer becomes surcharged; thebiochemistry is further complicated because biofilms on sewer walls can be anaerobic
closer to the walls even if they are aerobic at the interface with the wastewater.
Tendaj, et al. (2008) cited Cedergren (2007) as showing, with respect to the output of
FWD, that it is mostly the organic material that is already in dissolved form that
decomposes during transportation in the sewerage system whereas the particulate
portion does not decompose. As a result of Tendaj et al., Stockholm Water reversed
its antipathy to FWD and now encourages them as a means of generating more biogas
to fuel the city’s busses etc.
The data in Figure 2 have been calculated from the analyses of the composite samples
taken 4-weekly and the daily flows on the days that samples were collected. Theyshow that even when 50% of households use FWD as their means of managing
kitchen food waste, the effect on the BOD7 loading on the WwTW has not increased(samples collected after 3mm inlet screens and before grit settlement). Figure 3
shows that the COD loading did not increase either.
Figure 2 BOD7 loading (kgBOD7/day) - 4 weekly samples and 13-sample moving average(trend line for all data)

Influent BOD7 kg/day

1600
1400
1200
1000
800
600
400
200
0

y = -0.0323x + 1600.7
R² = 0.0624

Figure 3 COD loading (kgCOD/day) - 4 weekly samples and 13-sample moving average (trend line forall data)
4000
3500
y = -0.0579x + 3225.1
R² = 0.0283

Influent kgCOD/day

3000
2500
2000
1500
1000
500
0

Figure 4 shows that despite the BOD7 data, there has been a progressive increase in
the amount of biogas produced by mesophilic anaerobic digestion as more households
have installed FWD. Biogas production was continuously measured by gas flow
meter. The first four data points (1995, 1996, 1997 and 1998) are from Karlberg andNorin (1999) who actually quoted the averages for the 4 months September toDecember inclusive because these were the months that consistently had no
operational irregularities (e.g. maintenance, valves, etc.). This indicates that thecontent of biodegradable material entering the WwTW has increased substantially.
Since the BOD7 was not changed significantly, presumably the additional substrate
for biogas production was composed of particles that settled in the primary clarifiers.
Figure 4 Average annual biogas (m3/day) [1995, 1996, 1997 and 1998 are from Karlberg & Norin, 1999]

600

500
450

y = 12.456x - 24519
R² = 0.6996

FWD installation

Average annual biogas m 3 /day

550

400
350
300
1995

1997

1999

2001

2003

2005

2007

2009

Figure 5 and Figure 6 show that installing FWD in 50% of households has had
negligible impact on the total nitrogen load or the ammonium-nitrogen load on the
WwTW. This is consistent with Karlberg and Norin’s observation over a much
shorter time-scale that electricity use for the air blowers for secondary treatment had
not changed. Presumably part of the biochemistry in the sewerage linear bioreactor
system has been nitrification and denitrification fuelled by BOD.

Figure 5 Total nitrogen loading (kgNtot/day) - 4-weekly samples and 13-sample moving average (trendline for all data)
300

Influent kgN tot/day

250

y = -0.0014x + 159.19
R² = 0.004

200
150
100
50
0

Figure 6 Ammonium loading (kgNH4-N/day) - 4-weekly samples and 13-sample moving
average (trend line for all data)

Incoming ammonia load kgNH4/day

160.0
140.0
120.0
100.0
80.0
60.0
40.0
20.0

y = 0.0007x + 42.238
R² = 0.0023

0.0

Rather surprisingly the total phosphorus loading has decreased over the period (Figure7). The P content of food waste is less than toilet waste, but that does not explain thedecrease, though Battistonia et al. observed a similar effect. We have no explanation
for this other than a contemporaneous change to phosphate-free and low-P detergent
products. Sweden introduced a voluntary limit of 7.5%P in laundry detergents in
1970. The switch to P-free laundry detergents was so effective that the government
was able to introduce a ban of P in laundry detergents from 1st September 2008, aspart of concerted action by Baltic countries, and to propose a ban of P in domestic
dishwasher detergents from 1st July 2011.

Figure 7 Phosphate loading (kgPtot/day) - 4-weekly samples and 13-sample moving average (trend line
for all data)
50
45

Influent kgPtot/day

40
35

y = -0.001x + 51.738
R² = 0.0691

30
25
20
15
10
5
0

Figure 1through Figure 7 give a very clear picture of the effects of installing FWD as
one of the options for householders to manage their food waste. As a further test, theinfluent data for the pre-installation period were compared with the post installation
data using one-tail Student’s t-tests. The results are shown in Table 1. The preinstallation period was 11th January 1995 to 30th April 1997 which was 120 weeks.
Installation started in May 1997 and by October 1998 1100 (30%) of the 3700
households had had FWD installed (Karlberg and Norin, 1999). Three post
installation periods have been compared, the 120 weeks from 12th January 1999 to 2nd
May 2001, the whole data from 12th January 1999 to 1st April 2009, which was 533
weeks and the latest 120 weeks from 13th December 2006 to 1st April 2009. Table 1
also includes the annual average daily biogas production for 1995 to 1997 comparedwith 2000 to 2002, 2000 to 2009 and 2007 to 2009. Thus the major installation period
was excluded from Table 1.
Table 1 shows that, comparing the 120 weeks pre installation with 120 weeks after the
first surge of installations, when 30% of households had FWD, there were significant
increases in BOD7, COD and NH4-N loadings, the BOD7:N ratio and the biogas
production (95% confidence level). The increase in the BOD7:N ratio was beneficial
to nutrient removal, as also observed by Battistonia et al. (2007). However, when the
post-installation period was extended to 533 weeks, the picture changed. Only the P
loading (-14%, P=0.002) and the biogas production (+35%, P=0.04) weresignificantly different pre- and post- FWD installation at the 95% confidence level.
The decrease in ammonium-N was only just outside the 95% confidence test.
Comparing the pre-installation 120 weeks and the most recent 120 weeks, these werestill the only significant changes (Ptot -26%, P=0.002 and biogas +46%, P=0.01).
There was no major change in trade effluent (non-domestic) discharge that could haveinfluenced the data to produce these results. As confirmation that residents continued
to use their FWD (they had no other means of legally disposing of their kitchen foodwaste), the biogas production continued to increase (see also Figure 4).

Table 1 Student's t-test comparing influent and biogas pre and post FWD installation
Flow
kgBOD7/d kgCOD/d
m3/d

kgN/d

kgNH4/dkgP/d

BOD7:Nm3
biogas/d

4706

408

1084

113.6

74.0

18.0

3.50

331

303412346620

394192

979

405

49.9

1.695

1036

5194

520

1420

113.8

62.4

17.5

4.60

410

13156275

69225

425475

1507

391

22.4

2.341

6.937

+10.3%+27.4%+31.0%+0.16%-15.7%-2.6%+31.5%+23.9%

P (1-tail T-test)

0.25

0.04

0.03

0.49

0.02

0.39

0.002

0.03

Mean post FWD
533 weeks

4538

381

1062

108

67

15.4

3.55

447

717153738370

262063

1084

490

26.6

1.902

3005

Difference(post533-pre)

-3.7%

-7.1%

-2.0%

-5.3%

-9.5%

-14%

+1.63%+35%

P (1-tail T-test)

0.34

0.27

0.43

0.19

0.06

0.04

0.42

0.002

Mean post FWD
120 weeks to 01/04/09

4678

331

892

107

71

13.3

3.11

484

5675190

17138

167426

548

282

12.7

1.191

3147

Difference(post120-pre)-0.59%-19.0%-17.7%

-6.1%

-3.9%

-26.1%-11.1%+46%

P (1-tail T-test)

0.50

0.06

0.09

0.18

0.28

0.002

0.11

0.01

Mean pre FWD
120 weeks
Variance

Mean post FWD
120 weeks
Variance

Difference(post120-pre)Variance

Variance

Evans (1997) showed that the global warming potential of delivering source
segregated food waste to anaerobic digestion (AD) via FWD and the sewers was
equivalent to kerbside collection and transport to AD by road and that these were
better than composting, incinerating of landfilling food waste. However theunanswered question was how much the output of FWD affected the costs ofwastewater treatment because of the uncertainty about how much load is “treated” inthe sewers and how much arrives at the WwTW. The evidence from extended (533
week) monitoring at Haga WwTW shows that it has not increased costs, indeed,
where value can be gained from the biogas, it is a positive financial contribution. As
stated in the introduction, sewers are linear bioreactors, the processes are complex but
our understanding of them is incomplete. Perhaps the difference between the 120
week and the 533 week post-installation data reflect acclimation of sewer processes(biofilms etc.) to the additional dissolved and fine particulate load. It also indicatesthe care that is needed when interpreting field studies.

Not surprisingly, since the influent loads did not change, the installation of FWD has
not reduced Haga’s ability to meet its effluent discharge consents. Figure 8 showsthat Haga met its 15 mgBOD7/L and 0.5 mgPtot/L discharge limits.
Figure 8 Effluent monitoring results, trendlines and consent standards (dashed lines, 15 mgBOD7/L and
0.5 mgP/Ll)
40

5

35

4.5
4

20
15
10
5
0

y = -8E-11x3 + 9E-06x2 - 0.3247x + 3989.4
R² = 0.0379

3.5

mgP/l

25

FWD

mg BOD7 /l

30

3

y = 3E-06x + 0.18
R² = 0.0002

2.5
2
1.5
1
0.5
0

Conclusions
This examination of wastewater monitoring data has shown the benefits of dataarchives maintained over extended periods of time and of using standardised methodsof analysis. From the archived data we have been able to examine the effects on thewastewater system of offering citizens food waste disposers (FWD) as one of theoptions for separating food waste at source.
The data from analysing 24-hour composite samples of influent and effluent 4-weekly
and 2-weekly respectively between 11th January 1995 and 1st April 2009 showedsignificant dispersion as can be expected from the real world circumstance but they
did not reveal much effect from installing food waste disposers in 50% of households,
except that the biogas production has increased by more than 40% between the preinstallation period and 2009 by which date 50% of households have FWD. There isno indication that water use has increased. The sewerage team has reported that there
has been no increase in sewer blockages, accumulation of solids, fat oil and grease or
corrosion. After an initial increase following the main surge of installations in 1997/8,
the treatment load (BOD7, COD, Ntot, and NH4-N) returned to the pre-installation
values. Ptot had decreased significantly over this period but this could be partly
because of contemporaneous reductions in the P content of detergent products. An
explanation for the decrease in treatment load, which is consistent with the data
analysis, is that in-sewer biological processes acclimated to the new wastewatercomposition and removed the treatment load before it got to the WwTW. If
acclimation is the mechanism, it is a slow process that can only be revealed byextended periods of monitoring. The fact that biogas production increased over thewhole period shows that biogas substrate (which presumably settling in the primary
clarifiers) was getting through to Haga WwTW. There was no impediment to the
WwTW achieving its effluent quality requirements. By the time that 50% ofhouseholds had installed FWD, the biogas production had increased by more than40%.

This appraisal of operational monitoring data has verified the information from fieldstudies and laboratory investigations reported in the literature. It has shown that evenwhen 50% of households have installed FWD as their means of segregating foodwaste at source and managing it, and the system has equilibrated to the new load, thecost effect on wastewater treatment is neutral and that if value is obtained from theadditional biogas, FWD make a positive financial contribution.
Acknowledgements

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