The 11th IWA Conference on Small Water & Wastewater Systems and Sludge Management Page 1 of 10 Improving the feasibility of on-site wastewater treatment systems in areas of low permeability subsoils by means of water saving technologies Donata Dubber1, Laurence Gill1 1 Department of Environmental Engineering, Trinity College Dublin, Ireland Presenting Author: Donata Dubber

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Improving the feasibility of on-site wastewater treatment systems in
areas of low permeability subsoils by means of water saving
technologies
Donata Dubber1
, Laurence Gill1
1
Department of Environmental Engineering, Trinity College Dublin, Ireland
Presenting Author: Donata Dubber
Abstract
In areas of low subsoil permeability typical on-site wastewater treatment systems, consisting
of a septic tank followed by a percolation area, cannot provide sufficient treatment causing a
risk of nutrient pollution of nearby surface waters as well as microbial risks to human health.
Pressurised effluent dispersal and zero discharge systems such as cesspools and
evapotranspiration systems are currently considered as alternative treatment and disposal
solutions but most of them require large land areas of up to 110 m2
per person. With their size
(and cost) being mainly based upon the hydraulic wastewater load, the aim of this study was
to determine the potential for a reduction in the household’s daily wastewater production by
water saving technologies and the corresponding impact on the design and applicability of
these effluent disposal options. Result show that per capita wastewater production can be
reduced from 150 Lcd down to less than 90 Lcd leading to design size reductions of 42% up
to 49%. Furthermore, construction and operational costs can be reduced for certain systems
by up to 50%. This will improve the applicability of alternative treatment and disposal
systems especially for site remediations where space and resources might be limited.
Keywords: economical feasibility; environmental sustainability; evapotranspiration systems;
pressurised distribution systems; wastewater production; water savings.
Introduction
Over one third of Ireland’s population is living in rural areas with no access to main drainage
and therefore relies on on-site wastewater treatment systems. However, about 25% of the land
surface of the country is covered with subsoils of inadequate permeability (Meehan and Lee,
2012). Under these conditions the typical on-site wastewater treatment systems, consisting of
a septic tank followed by a percolation area, cannot provide sufficient treatment resulting in
surface ponding and direct runoff to surface water. This represents both a risk to human health
and could cause eutrophication in nearby surface waters.
Pressurised on-site effluent dispersal systems such as drip distribution (DD) and low
pressure pipe (LPP) systems may be an alternative in certain low permeability subsoils. They
apply the effluent intermittently and distribute it over a wider area than in traditional gravity
fed percolation trenches (USEPA, 1999; EPRI, 2004). Furthermore, due to their shallow
installation depth in the soil they promote evapotranspiration which helps to reduce the areal
hydraulic load. However, the area required for effluent dispersal increases with decreasing
subsoil permeability (EPRI, 2004). Effluent reduction in conjunction with these technologies
could therefore be considered to keep the required area for those systems to a minimum. In

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areas were subsoil permeability is too low and discharge to ground cannot be considered at all,
zero discharge systems such as storage tanks (cesspools) and closed basin evapotranspiration
systems normally using willow trees (Gregersen and Brix, 2001; Curneen and Gill, 2013), can
be alternative disposal options. For the collection of wastewater in storage tanks with regular
emptying and disposal at an urban wastewater treatment plant (WWTP) a low wastewater
production will be crucial to make this option economical feasible. Willow evapotranspiration
systems are sized according to a water balance between the effluent produced and the climatic
variables of rainfall and evapotranspiration such that there is a zero-discharge of effluent
throughout the year (Gregersen and Brix, 2001). In a maritime temperate climate such as
Ireland’s the systems typically require a very large area of 80 up to 110 m2
per person, with
large basin volumes to hold the wastewater during winter months where there is limited
evapotranspiration from the systems. The design of a willow bed is strongly influenced by the
daily wastewater production and therefore any reductions in effluent production will lead to
the design of smaller systems.
Thus the aim of this study was to determine the potential for a reduction in the daily
wastewater production by water saving technologies and its effect on the design and
applicability of effluent disposal options in areas with low permeability subsoils.
Material and Methods
A review of available water saving devices was carried out in order to determine their
potential water savings compared to those fittings that would be installed in a typical Irish
household. Various water efficient toilet systems were considered including pressure-assist
toilets, different dual flush toilets, as well as vacuum and air assisted toilets. Other water
saving fittings such as water flow restrictors and aerators were also assessed for their water
saving potential. Beside these water saving devices, technologies based upon the principles of
eco-sanitation such as composting toilets, urine separation and greywater recycling were
reviewed with respect to their applicability for rural Irish housing.
Due to the lack of water metering there is little information on the domestic water
consumption in Ireland. The Irish Code of Practice (EPA, 2009) uses a daily hydraulic load of
150 Lcd in order to calculate the design load for on-site wastewater treatment systems. This is
supported by the per capita consumption (PCC) of 147 Lcd that was obtained from water
demand analysis for domestic users in the greater Dublin area (WSP, 2010). Due to the lack of
Irish data in terms of detailed water usage patterns, international data were collated to be able
to estimate potential water savings that can be achieved with the installation of water saving
devices in Irish households.
Based on an average water consumption of 150 Lcd and the estimated usage patterns the
achievable reduction in a household’s wastewater production was calculated. The size as well
as construction and operational costs of on-site wastewater treatment and disposal systems
were then determined and compared to systems designed based on the higher daily hydraulic
load. Additionally energy savings and related reductions in greenhouse gas (GHG) emissions
were estimated.
Results and Discussion
Achievable water savings
The estimated Irish water usage pattern presented in Figure 1 is mainly based on findings of

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the Water Research Centre UK which conducted a large-scale survey to investigate water
consumption trends in different parts of the UK (Liu et al., 2010). Due to the similarity in
housing and living standards the UK data was assumed to provide a good estimate for the
Irish situation but final estimations were also influenced by other international data such as
from Denmark (Revitt et al., 2011) and Germany (BDEW, 2011). These estimated usage
patterns (Fig. 1) together with a PCC of 150 Lcd, have been used for the following water
saving calculations.
Figure 1 Estimation for Irish water usage pattern based on UK and international figures
Table 1 summaries the potential water savings for different toilet systems compared to a 9 L -
single flush toilet, which was estimated as the average flush volume in the UK and in the
greater Dublin area (Liu et al., 2010; WSP, 2010). Together with an estimated water
consumption of 42 Lcd (28% of total PCC of 150 Lcd) used for toilet flushing it can be
concluded that in an average household the toilet is flushed 4.66 times per person per day. The
results show that with the installation of more water efficient toilet systems 14 - 40 Lcd of
flushing water can be saved resulting in a PCC reduction of 9 - 27%.
Table 1 Potential water and cost savings for different toilet systems compared to a reference 9
L - single flush toilet
Single
flush
toilet
Pressure-
assist toilet
Dual flush toilet3
Urine
diverting dual
flush toilet3
Vacuum
toilet
Urine diverting
vacuum toilet3
Flush volume 6 L 4.8 L 3/6 L 3/4.5 L 0.6/4 L 1 L 0.2 / 1 L
Water saving1
14 Lcd 19.6 Lcd 24.5 Lcd 26.3 Lcd 35.23 Lcd 37.33 Lcd 40.13 Lcd
Water cost savings
per person2
3.83 €/y 5.37 €/y 6.71 €/y 7.19 €/y 9.65 €/y 10.22 €/y 10.99 €/y
Reduction in PCC 1
9.33% 13.1% 16.33% 17.5% 23.5% 24.9% 26.8%
1
based on an avg. daily per capita consumption (PCC) of 150 Lcd
2
assuming an average volumetric water charge of 0.75 €/m3
for Irish Group Water Schemes (as of 2011)
3
assuming 3 out of 4 flushes (75% of all flushes) are small flushes

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However, not all of these technologies would be technically feasible for an application in Irish
households. According to existing Building Regulations cold water storage in domestic
dwellings is required. Therefore, pressure-assist toilets fed from a storage tank would
probably not be able to achieve the required pressure, which equals to a hydraulic head of 14 -
21 m and so would not be a functional option for most Irish households. Vacuum toilets are
currently not available in Ireland and would need to be imported. Moreover, such systems
would need qualified contractors for installation and servicing which do not exist at present in
Ireland. It should also be noted that vacuum toilets are an expensive solution and will only be
feasible in the future if major difficulties (e.g. with ordinary pipes, with percolation, with
complicated and expensive transport of collected material) need to be overcome. However, air
assisted flush toilets, such as the Popelair which has been developed in the UK and is
expected to launch the market very soon (www.propelair.com), achieve similar water savings
and could be a reasonable low-tech alternative to vacuum toilets.
Dual flush toilets using 6 L for a full and 3 L for a short flush are a fully accepted option
but concerns have been raised that lower flush volumes might affect an efficient sewerage
network performance through increased blockage (Drinkwater et al., 2008; Schlunke et al.,
2008; WSP, 2010; PERC, 2012). Therefore the installation of 4.5/3 L dual flush and urine
diverting toilets will be only possible in new buildings using new design standards to improve
drainline carriage (pipe diameter, slope, other fixtures such as showers installed upstream of
toilets) or after a satisfactory inspection of the existing drainline condition.
By using a composting toilet no water is used for flushing which reduces the wastewater
production by 28% down to 108 Lcd. Moreover, the black water is completely removed from
the wastewater. With urine and faeces accounting for 91% of nitrogen discharge and 83% of
phosphate (Holtze and Backlund, 2003), a major source for pollution has also been eliminated
and only the lightly polluted grey water needs to be treated and disposed. Cultural acceptance
is still a key issue that restricts the use of composting toilets but they could be a possible
option for holiday homes where homeowners accept the concept and are fully aware of the
operational and maintenance requirements.
Reuse applications of recycled greywater are limited, usually to subsurface irrigation, to
avoid any possible contact with remaining pathogens in the reclaimed water. While this can be
of great use in arid areas it would not be of any significant benefit in the Irish wet climate and
also would not lead to the desired reduction of hydraulic loads onto impermeable clay soils.
The use of treated greywater for toilet flushing could theoretically save up to 28% of potable
water and equally reduce the wastewater production. However, to meet indoor reuse criteria
complex technologies such as MBRs or disinfection systems are needed making it inefficient
especially when other low flush toilet systems are available that already reduce the flush
volume by 60 - 80%.
Tap aerators and low flow shower heads that restrict the flow and aerate the water have
been fairly standard in many countries for several years. Their water saving potential
compared to standard fittings which are still common in Irish households are shown in Table 2
and 3. With an estimated daily water consumption of 43.5 L per person (according to 29% of
PCC in Fig.1) and an average of 5 showers per week the average shower time is about 7 min,
which agrees with other surveys carried out in Ireland. Using a low-flow shower head (5
L/min) could consequently reduce the PCC by 12% (Table 2).

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Table 2 Water and energy cost savings for low-flow shower heads compared to standard ones
Standard shower head Low-flow shower head
Avg. flow rate 8.5 L/min 5 L/min
Avg. no of showers per week 5 5
Avg. shower time 7.16 min 7.16 min
Water usage for showering 43.5 Lcd 25.59 Lcd
Water savings1 n/a 17.91 Lcd
Reduction in total PCC1 n/a 11.94%
Average no. of residents per household 3 3
Total household shower water usage 47,633 L/y 28,019 L/y
Water cost savings per household2 n/a 14.32 €/y
Energy usage (electricity)3
Energy usage (gas)3
1653.43 kWh/y
8083.43 cf/y
972.61 kWh/y
4754.96 cf/y
Energy cost savings:
using electricity
using gas
n/a
n/a
117.71 €/y
51.23 €/y
1
based on an avg. daily per capita consumption (PCC) of 150 Lcd
2
assuming an average volumetric water charge of 0.75 €/m3
for Irish Group Water Schemes (as of 2011)
3
assuming 73% of used shower water is hot and energy requirements of 0.0476 kWh or 0.2325 cf of gas to heat water from
13 to 49°C (US EPA Water 2010)
4
for households heating water with electricity/gas
Assuming that 14% of the total water consumption is used from the bathroom tap, Table 3
shows that 6.18 and 15.32 Lcd can be saved by installing an aerator or a spray aerator onto the
bathroom tap, respectively. This would reduce the total water consumption by 4.12 and
10.21%, respectively
Table 3 Water and energy cost savings for tap aerators compared to a standard tap
Standard tap Aerator Spray aerator
Avg. flow rate 8.5 L/min 6 L/min 2.3 L/min
Proportion of PCC used from bathroom tap1
14% 14% 14%
Water savings1
n/a 6.18 Lcd 15.32 Lcd
Reduction in total PCC1
n/a 4.12% 10.21%
Average no. of residents per household 3 3 3
Total household bathroom tap water usage 22,995 L/y 16,232 L/y 6,222 L/y
Water cost savings per household2
n/a 5.08 €/y 12.58 €/y
Energy usage (electricity)3
Energy usage (gas)3
798 kWh/y
3,902 cf/y
563 kWh/y
2,755 cf/y
216 kWh/y
1,055 cf/y
Energy cost savings:
using electricity
using gas
n/a 40.59 €/y
17.66 €/y
100.67 €/y
43.81 €/y
1
based on an avg. daily per capita consumption (PCC) of 150 Lcd
2
assuming an average volumetric water charge of 0.75 €/m3
for Irish Group Water Schemes (as of 2011)
3
assuming 73% of used shower water is hot and energy requirements of 0.0476 kWh or 0.2325 cf of gas to heat water from
13 to 49°C (US EPA Water Sense, 2010)

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The results of this study show that without major changes to existing standards of living and
with only small changes to people’s behaviour by using dual flush toilets, it is possible to
reduce the water consumption from 150 Lcd down to less than 90 Lcd. In combination with
different water efficient fittings/appliances (shower heads, tap aerators, washing machines)
the use of a 3/4.5 L standard dual flush toilet can reduce the wastewater production to 99.7 -
86.6 Lcd while with a urine diverting dual flush toilet (0.6/4 L) further reductions down to
90.7 - 77.6 Lcd can be achieved.
Consequences for design and application of effluent disposal systems
Low pressure pipe systems or drip irrigation are methods that could prove suitable for
distributing effluent over low permeability subsoils. The NOWRA (2006) guidelines
recommend that drip systems are designed according to the manufacturer recommended
hydraulic loading rates which are expressed as an areal loading rate. For example, Geoflow
Inc. recommends rates of 3.05 L/m2
.d for poor clays. The recommended design loading rate
for LPP systems is 5 L/m2
.d but lower rates might have to be considered for the application in
low permeability subsoils (USEPA, 1999). Based on those loading rates, at low subsoil
permeability equivalent to a percolation rate of 5.2 x 10-6
m/s and including climatic variables
such as rainfall, the plan area required for a LPP or DD system serving a 3 person household
with a wastewater production of 150 Lcd is estimated to be 246 m2
which will be reduced by
42% and 48% down to 142 m2
and 127 m2
with water saving appliances including dual flush
and urine diverting toilet, respectively (Table 4).
Table 4 Design size and operational conditions for effluent disposal options for a 3 person
household in areas of low permeability subsoils. Reductions compared to systems designed
based on the standard PCC are displayed in parentheses.
Standard PCC
(150 Lcd)
Reduced PCC
with dual flush toilet
(86.6 Lcd)
Reduced PCC
with urine diverting toilet
(77.6 Lcd)
Required area for pressurised
distribution systems (T=80)†
246 m2
142 m2
(42%) 127 m2
(48%)
Required area for Willow system 330 m2
189 m2
(43%) 169 m2
(49%)
Emptying frequency for storage
tank of holiday house*
4 months 7 months (43%) 8 months (50%)
* based on an assumed time of occupancy of 17 weeks/y and the use of an 18 000 L storage tank
†
T value is obtained from an on-site falling head test expressed in min/25 mm water head loss and is equal to a percolation
rate of 5.2 x 10-6
m/s
Zero discharge evapotranspiration systems using willow trees can treat on-site wastewater
effluent in any subsoil permeability scenario as they produce no net discharge of effluent to
the ground. The principle is that these lined systems are sized according to a water balance
between the effluent produced and the climatic variables of rainfall and evapotranspiration,
such that there is a zero-discharge of effluent throughout the year (Gregersen and Brix, 2001).
This however, means that the system needs a relatively high capacity to store the water
throughout the winter time where evapotranspiration is at a minimum. With a standard depth

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of 1.8 m willow systems usually require an area of 80-110 m2
per person, depending on
wastewater production and climatic conditions. A willow system for a 3 person household in
Ireland would consequently require a plan area of 330 m2
. By installing water saving
appliances and using a dual flush toilet or urine diverting systems the design size of the
system could be reduced by up to 43% and 49%, down to 189 m2
and 169 m2
, respectively
(Table 4).
A storage tank for wastewater collection and treatment at the local WWTP is considered as
a possible disposal option for holiday houses in areas with very low subsoil permeability.
British Building Regulations (DoCLG, 2000) require a minimum storage tank capacity of
18,000 L for a single domestic dwelling with two residents. This tank volume was used when
estimating the emptying frequencies for a 3 person holiday house with an assumed occupancy
time of 17 weeks/year. The results show that emptying frequencies will decrease from every 4
months (based on standard PCC) to 7 and 8 months when using different water saving
appliances to reduce the daily wastewater production (Table 4).
When water saving devices are installed to reduce the wastewater production, it follows
that the concentration of organics, nutrients and other pollutants thereby increase
proportionally which can increase the risk of shock loading and may have an impact on the
wastewater’s treatability. For storage tank solutions and willow treatment systems, however, a
negative impact due to the higher concentrated wastewater is not expected. Wastewater from
storage tanks will be brought to the nearest urban WWTP for treatment and will be diluted
with other municipal wastewater as well as urban runoff rainwater. In willow treatment
systems the wastewater will also be diluted by rainwater falling onto the willow bed area.
Furthermore, research results have shown that the evapotranspiration rates of willow trees
increase with higher organic content in the receiving wastewater (Curneen and Gill, 2013).
However, where biological wastewater treatment systems are used for secondary treatment
(e.g. before dispersal by DD and LPP systems), care should be taken to ensure that they will
be able to deal with the high influent concentrations. Ideal solutions would be to use treatment
systems based upon fixed film biological treatment (e.g. filter media technology) and/or
incorporate the recirculation of effluent so that the incoming wastewater is diluted with
treated effluent. The occurrence of shock loads in systems can also be avoided by large
primary settlement or buffering tanks which equalise the concentration throughout the day
and ensure a uniform BOD load.
Consequences for economical feasibility
Due to the reduced system size that would apply to households with installed water saving
appliances material and construction costs for a willow system will be up to 43% lower than
for a standard sized system which would cost around €5300 per person without the
consideration of potential wastewater reduction (Table 5). These cost savings are mainly due
to the decreased use of expensive materials (particularly the liner) as well as a significant
reduction in the construction time (for excavations) and hence labour costs. In comparison,
for LPP and DD systems the size does not significantly affect the construction time so that
construction costs are only reduced by up to 5% and 9% respectively (Table 5).
While water saving does not affect the construction costs of a wastewater storage tank it
greatly reduces (40-50%) the annual operation costs related with this disposal method.
However, it should be noted that the use of a cesspool will only be economically feasible for

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holiday houses that are occupied only during parts of the year.
Based on a volumetric water charge of 0.75 €/m3
for rural areas in Ireland the estimated
annual water cost savings are up to €17.36 and €19.82 per person when using water saving
devices including dual flush and urine diverting toilets respectively. Table 1, 2 and 3 include
estimated water cost savings for the individual water saving appliances. At the same time,
appliances such as flow restricted shower heads and tap aerators that reduce the consumption
of warm water will also promote a reduction in energy used to heat water for domestic use.
Based on the estimated PCC reduction of up to 48% it was estimated that up to 73 €/ca.y and
32 €/ca.y can be saved where water is heated using electricity and gas respectively. Potential
energy and cost savings for individual fittings can be found in Table 2 and 3.
Beside the estimated construction costs Table 5 also shows operational costs for the
different disposal options based on a standard PCC and the reduced wastewater productions
due to water saving devices including dual flush and urine diverting toilets. These costs are
expressed as net costs and incorporate annual water and energy cost savings. For DD and LPP
systems annual operational costs can be reduced by 36% up to 68% while the generally low
running costs for the willow system pay back completely by the savings made through water
and energy savings (Table 5).
Table 5 Estimated per capita construction and operational costs (excl. VAT)
Construction costs
Standard PCC
(150 Lcd)
Reduced PCC
with dual flush toilet
(86.6 Lcd)
Reduced PCC
with urine diverting toilet
(77.6 Lcd)
LPP system €1510 €1450 (4%) €1440 (5%)
DD system €2180 €2000 (8%) €1982 (9%)
Willow system €5300 €3400 (36%) €3030 (43%)
Cesspool €1200 €1200 €1200
Annual operational costs*
LPP system €139
€47 (66%)
or €88 (36%)
€45 (68%)
or €86 (38%)
DD system €139
€47 (66%)
or €88 (36%)
€45 (68%)
or €86 (38%)
Willow system €50
€0 (100%)
or €1 (98%)
€0 (100%)
or €0 (100%)
Cesspool €717
€383 (47%)
or €396 (45%)
€339 (53%)
or €353 (51%)
* net costs, incorporating water and energy savings for electricity or gas related to water saving devices
Improving environmental sustainability
Based on Water UK (2007) average company performance values, 0.29 g CO2 is emitted for
every litre of water supplied. Hence, the secondary emission rates for water supply for a
person with a water consumption of 150 Lcd is estimated to be 15.88 kg CO2/ca.y. With the
installation of water saving devices and a reduction of water consumption down to 86.6 or
77.6 Lcd the carbon emission could therefore accordingly be reduced by 6.71 and 7.67 kg
CO2/ca.y respectively. With the average personal CO2 emission in Ireland being estimated at

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5.7 t CO2/y (Kenny and Gray, 2009), then these savings only represents 0.12% and 0.13% of
an individual’s annual primary CO2 footprint.
However, 99% of the energy consumption of the water cycle occurs during the domestic
use (Hackett and Gray, 2009), so that the highest potential in reducing GHG emission is seen
in the reduction of warm water use such as during showering and from bathroom taps. It has
been shown that 305 - 421 kWh/ca.y in electricity can be saved when installing a low flow
shower head together with standard tap or spray aerators. Equally, when gas is used as an
energy source, 1491.8 - 2058.5 cf/ca.y (equal to 480 - 662 kWh/ca.y) can be saved. Applying
conversion factors for electricity and natural gas into carbon emissions of 0.562 and 0.206 kg
CO2/kWh, respectively (Hackett and Gray, 2009), this equates to carbon emission reductions
of 171 - 236.6 kg CO2/ca.y or 99 - 136.4 kg CO2/ca.y, depending on the energy source used to
heat domestic water. With 42.2% of the average personal carbon footprint being related to
home energy use (Kenny and Gray, 2009) these emissions (2.4 t CO2/ca.y) could be reduced
by up to 9.9% and 5.3%, respectively.
Further reductions in GHG emissions can be expected in relation to the construction and
operation of effluent disposal systems when wastewater production is reduced. For DD and
LPP systems for instance 5.6 - 6.4 kg CO2/ca.y can be saved in emissions related to electricity
used to distribute the effluent over the percolation area. The carbon footprint from the
operation of a cesspool depends largely on the distance from the house to the nearest central
WWTP but due to the reduced emptying frequencies (Table 4) similar reductions in CO2
emissions can be expected in relation with the reduced wastewater production.
Conclusions
It has been shown that a significant reduction of the wastewater production in Irish
households can be achieved by installing readily available water saving appliances. This will
reduce the design size as well as construction and operational costs of alternative on-site
effluent disposal systems for areas of low subsoil permeability. Consequently it will improve
their applicability especially for site remediations where space and resources might be limited.
Furthermore, the reduction in water consumption and wastewater production as well as the
reduction in energy used to heat water for domestic use will not only lower the household’s
utility bills but can also help decreasing CO2 emissions.
Acknowledgement
The authors wish to acknowledge the assistance of Arne Backlund (BACKLUND ApS,
Denmark) who provided valuable information from his research and expert knowledge in the
field of ecological sanitation. Acknowledgments are given to the Irish Environmental
Protection Agency for funding this research.
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