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Vacuum Toilets and Treatment of Collected Material in Biogas Plant or
Wet Composting Plant
In the municipality of Nysted on Lolland vacuum toilets of a new design were installed
two detached houses in 2000. The toilets were installed as part of a project to llustrate
the possibilities of recycling concentrations of "black" wastewater (wastewater
from water closets) for agricultural purposes. The project report describes experience
gained from the new toilet systems for detached houses and from earlier common vacuum
systems in summer cottage areas on Lolland and Falster.
The new vacuum toilet systems for detached houses, which are continuously developed,
have very low water and electricity consumption. Many of the difficulties connected to the
operation of the earer vacuum systems have been solved. Studies indicate that vacuum
toilets allow collection of a product applicable for co-treatment at a biogas and wet
composting plant. The report also includes experience from our neighbouring countries.
Storstrøm County has initiated the project in co-operation with A & B Barklund
ApS, and the project is supported by the Danish Environmental Protection Agency through
the "Sustainable Urban Renewal and Wastewater Treatment" programme.
Background and objective
By the end of the 1960s and at the beginning of the 1970s, Storstrøm County
established vacuum toilet systems in a number of summer cottage areas, where - due to the
conditions of the soil or elevated groundwater level - wastewater cannot move by means of
gravitation. Instead of treating this wastewater at sewage plants, the content of energy
and nutrients is exploited through treatment at biogas or wet composting plants. The
material may be utilized for energy purposes (biogas plant) or for the generation of heat
(wet composting plant). The nutrients contained in the residual product may be used for
agricultural purposes.
During the project experience was collected from the installation and operation of two
new types of vacuum toilets for detached houses, including a source segregation toilet
with separate collection of urine and faeces, as well as the operation of large, common
vacuum toilet systems in the county. Based on test results and theoretical calculations,
concentrations of organic material/dry matter and nutrients have been tested with a view
to identifying the applicability of the material for biogas or wet composting treatment.
At the same time it has been examined whether it is possible to treat the collected
"black" wastewater at the municipal biogas plant as well. In comparison to this,
experience has been gathered from the establishment and/or the running of vacuum toilet
systems and wet composting works in our neighbouring countries.
The study
The two new vacuum toilet systems were established in 2000. The toilets are equipped
with direct/temporary instead of the permanent vacuum system, which is more commonly used.
The collected material is transported to storage tanks by means of vacuum. In the source
segregation system urine plus 0.1-0.2 liter flush water run to a separate tank by means of
gravitation. The users' and the plumbers' experience from the installations, the operation
and use of the toilets has been gathered during the years 2000 and 2001.
Experience was gathered from the running of large vacuum toilet systems in four
municipalities as well as from the running of two minor public systems in Storstrøm
County. We have interviewed both the technical and the administrative staff of the
municipalities as well as the local plumbing service, which repaired the systems and
replaced the toilets. Test results from vacuum toilet systems in Sweden and Germany as
well as the treatment of human residual products in a Swedish wet composting works have
been consulted throughout the entire project period.
Thestudy and evaluation of the suitability collected product for treatment in a wet
composting or biogas plant were made on the basis of theoretic calculations of the content
of dry matter, organic material and nitrogen, as well as of individual analyses of the
material collected in two vacuum systems in the municipality of Rudbjerg. Moreover, test
results from Swedish works were applied. Due to operating troubles it proved impossible to
take out representative samples of the collected wastewater from the new installations.
The costs of establishment and running of vacuum toilets, including disposal of the
collected material, are illustrated on the basis of experience from detached houses in
Nysted. As the economy will depend on specific local conditions, no theoretical
calculations were made concerning the profitability of establishing toilet systems in a
larger number of houses in combination with treatment of the material at a biogas or wet
composting works. Economic aspects in connection with plamts in Norway and Sweden are
presented.
Main conclusions
- Vacuum toilet systems can collect material with a high concentration of organic material
and nutrients and with very few xenobiotic substances.
- In detached houses with storage tanks the quantity and frequency of emptying will be
considerably lower than is the case with an ordinary water-flushing toilet. With the pilot
installations, water savings of 40-50% have been obtained. The largest reduction is seen
in connection with source segregation toilets.
- With the new toilets the electricity consumption is negligible and considerably lower
per person for the toilet equipped with a temporary vacuum, compared to the consumption by
permanent vacuum systems.
- Normally the plumbers do not know about the system and its components, since the systems
are not commonly used. It is therefore very important that detailed assembling
instructions are available to the plumbers.
- To obtain the optimal function of the toilet systems, sufficient user information is
crucial. The way the users' use the toilets is important – both to the quantities and
concentrations supplied to the storage tank.
- The toilets must be cleaned more frequently than ordinary toilets. The residents of the
two houses, who have chosen the system themselves, do not find the extra work a problem in
their everyday lives, because they compare it with the water savings achieved.
- Experience from Bälinge, Sweden, and Storstrøm County concerning vacuum toilet systems
for many connected households shows that it is possible to run such systems with a minimum
of operating troubles, provided the system is in good repair and the users are well
informed. The systems require more maintenance than the gravitation systems
- The concentrations of organic material tested in large systems with permanent vacuums in
Storstrøm County and in Sweden indicate that the material is treatable in a biogas or wet
composting plants. The concentrations are not high enough, however, for the works to
operate exclusively on the basis of this material. Large quantities of organic material
with considerably higher contents of dry matter and organic matter must be added. The
application of source segregation vacuum toilets can increase the concentration
significantly.
- The costs of establishing a vacuum toilet system combined with one or two storage tanks
are 70-150% higher than for ordinary toilet system with a storage tank. On the other hand,
the annual expenses related to a traditional toilet system are much higher than for a
vacuum toilet system.
- At the biogas plant in the municipality of Nysted, the co-treatment price for organic
residual products is DKK 65-180 per cubic meter, depending on the content of e.g. dry
matter. The profitability of the treatment depends on e.g. the pricing of the protection
of the aquatic environment and the utility value.
Project results
Experience from newly established vacuum toilet systems in two houses in the
municipality of Nysted
Both installations had problems in the initial phases, i.e. for about six months, but
most of them were solved. It will thus be possible to make them function perfectly in all
houses. However, some of the system components are too fragile and need improvement.
The installation of the toilet, including piping, burial of the tank etc. Does not
require more work than for traditional systems, provided efficient documentation and
instruction are available. The electricity work is more extensive, however, since,
normally, electric control systems are not used. Only very few people are familiar with
the technical aspects, detailed information is crucial, both to reduce time consumption,
and to prevent a number of subsequent operating troubles.
Control
The plumbers found it difficult to adjust the electric control of the vacuum system.
The guide plate and the adjusting screws are produced from fragile material, and not
strong enough for frequent adjustment. Even during the running-in period the guide plate
had to be changed. The adjustment of the correct sucking-out intervals was made according
to the instructions of the firm. The adjustment depends on a certain expert feeling and
several tests, before it functions optimally. The required three meter between the toilet
and the flush pipe valve is too short a distance to ensure stable operation. Due to the
unstable operation of one of the installations with a short distance, the vacuum toilet
system was replaced after six months by a source segregation double-flush toilet, DS.
Flush pipe valve
The flush pipe valves first delivered were too fragile. After the valves were equipped
with heavier flaps and stronger fixing, no problems were seen.
Vacuum pumps, filter and piping
The installation itself is unproblematic. The filter is indispensable to the motor and
to avoid leaving a repulsive toilet bowl. It is easy for the users to clean the system.
Water consumption and emptying frequency
In the household with the non-segregating vacuum toilet, water consumption was reduced
by approx. 45% during the project period. In the future, the production of
"black" wastewater is estimated to be about 5 m3 per year. This means
emptying one or two times a year, compared to earlier approx. six times a year. The water
consumption of the source segregation vacuum toilet was reduced by approx. 40%. The
emptying frequency could be expected to be reduced from approx. 12 times a year (approx.
28 m3 in one tank) to about two times (11 m3 in two tanks), had the
vacuum toilet been retained. Because of the replacement of the toilet and the varying
number of users, it has not been possible to estimate the production of "black"
wastewater.
Power consumption
In the household without source segregation, the power consumption is assessed to total
7 kWh/year somewhat more than 1 kWh per cubic meter, corresponding to annual expenses of
DKK 12 at a price of DKK 1.7/kWh. Using source segregation as an alternative, the annual
consumption would roughly be reduced to approx. 4 kWh in total, i.e. half the consumption
of the non-segregating system. This corresponds to annual expenses of DKK 7 at a price of
DKK 1.7 per kWh.
Users' experience
Using the toilets has caused no problems to the visitors, among whom many were
children. Because of the source segregation, all users, irrespective of sex, must sit down
when using the two-part toilet. The family with the segregation toilet did not miss a
child's seat, and the children using the toilet did not cause faeces in the urine bowl or
on the partition between the two toilet bowls. Apparently it has been no problem, either,
to use a sanitary pail for toilet paper after a "short visit". The toilets
require somewhat more cleaning than usually, which, however, is acceptable because of the
considerable water savings. It is fairly easy to explain to guests how to use the toilet.
The motor may make a lot of noise, and therefore is very important to locate it
properly. To avoid noise nuisances, the pump (motor) can be placed at the entrance to the
tank.
Treatment of the collected "black" wastewater
It has not been possible to treat the collected "black" wastewater at the
biogas plant in Kettinge in the municipality of Nysted, however not because of
environmental requirements, but because of attitudes. The biogas plant is authorized to
treat sludge from sedimentation tanks (septic tank sludge). The residual products of the
plant are spread on farmland of farmers who, due to agreement made with Danisco, are not
allowed to use residual sludge. Therefore the biogas plant in question accepts no residual
sludge and in this case ranks the "human residual products" alongside with
sludge instead of with semi-liquid manure. The collected wastewater was supplied to a
municipal wastewater treatment plant.
Storage tanks and vacuum toilet systems in Storstrøm County
In the Storstrøm County only a few houses have storage tanks. In 2001 26,600 houses
were not connected to the sewage disposal system (Storstrøm County 2002), and of these
only 320 houses had storage tanks. In some cases the "gray" wastewater is
percolated, in other cases the "black" and the "gray" wastewater are
both supplied to the storage tanks.
Vacuum toilet systems
About 30 years ago several major vacuum toilet systems with permanent vacuums were
established in the summer cottage areas of the county. Experience from the operation of
these systems in the municipalities of Holeby, Ravnsborg, Rudbjerg and Sydfalster are
presented in the report. The "black" wastewater is transported by vacuum to a
common wastewater treatment plant for purification, whereas the "gray"
wastewater is percolated at the individual property. Experience shows that these
30-year-old transportation systems are worn down and requires extensive maintenance work,
even though the municipalities have reduced the operating problems considerably by
information and various other initiatives. Generally, plans to dismantle the systems have
not been made. However, in some cases there are problems percolating the "gray"
wastewater, especially during the winter, and therefore other disposal methods would be
better.
Operating experience
The systems require more frequent inspection than the gravitation systems. Vacuum
valves are often leaky, which means that an entire pipe (the passage) may be out of order.
In many cases the valves are not accessible to the personnel; this is e.g. the case if the
problem occurs in an unoccupied summer cottage area. The valves turn leaky for many
reasons, e.g. with age or if the system is left unused during the winter with a constant
vacuum maintained. If the users try to flush out napkins, cloths etc., which often occurs,
the pipes will choke. These problems are caused by a large number of different tenants
during a season, who are not familiar with the system or just do not care about the
special care required. Apart from that, the pipes may choke because of calcareous
sediments.
How to remedy operating troubles
In recent years the municipalities have taken steps to remedy the operating troubles in
many respects. They have established clean-out holes at the roads or even on the service
pipes to the houses and done much to inform the users. They have reduced the number of
operating stops considerably. Experience from Sweden also shows that the installations can
be run satisfactorily after the running-in period.
Description of collected "black" wastewater
Test samples from two installations in the municipality of Rudbjerg show concentrations
of organic material of 5.4 mg/l and 5.8 mg/l respectively. These figures make the material
suitable for treatment at a biogas or wet composting plant. The concentrations measured
correspond to the ones found in Sweden.
Power consumption
The power consumption of the two installations in the municipality of Rudbjerg has been
estimated at somewhat above 60 kWh/m3, which is considerably larger than in two
residential areas in Sweden and Germany. This is due to the transport distance being much
longer in Rudbjerg.
Wet composting and wet composting systems
Wet composting is a thermophilic, biological treatment process, which develops under
the supply of oxygen. In a composting drum the energy content in a biological residual
product is used to produce high treatment temperature. The surplus heat may be used for
external heating. The treated material (the residual product) is used as a fertilizer. The
report states power consumption figures during operation to be 19-35 kWh/m3
treated raw material and possibilities of using 50-60 kWh/m3 raw sludge for
external heating purposes. Moreover, reference is made to Swedish heat balance studies.
Biodegassing and biogas works
Biodegasification/digestion is an anaerobic, microbial decomposition of organic
material under the generation of biogas (methane). The treated material (the residual
product) is used as an agricultural fertilizer. Farm biogas installations, biogas
installations at purifying plants or biogas joint installations have gas production based
on domestic animal manure, source segregated household waste, residual sludge and organic
industrial waste. Main focus is normally on livestock manure. Residence time and process
temperatures may vary. A distinction is made between mesophilic digestion (20-52° C) and
thermophilic digestion (52-60° C). The biogas potential per year from a person is
assessed at approx. 4 m3 biogas from faeces, and 5.4 m3 from urine,
i.e. a total of 9.4 m3 biogas, estimated from the content of organic matter.
The high content of nitrogen in urine can have an inhibitory effect on the thermophilic
processes.
Theoretic calculation of the concentration of collected material from vacuum toilets
The report presents calculations of the concentrations of dry matter and organic matter
that can be obtained through vacuum toilet systems. These results may be used to estimate
the dimensions of tanks, emptying frequencies, costs of emptying and treatment. They may
also be used to assess whether the material (the "black" wastewater) is suitable
for treatment at existing biogas plants or at newly established wet composting plants at
an acceptable price. The report also presents the test results of components of material
collected by other techniques, e.g. sedimentation tanks etc.
Expenses for the establishment of vacuum toilet systems in detached houses
The costs of establishment of the vacuum systems depend on whether one or two storage
tanks are used. Costs are estimated to be 70-150% higher that those required for the
establishment of a conventional system with an ordinary toilet (3/6 litres).
The operation of vacuum toilet systems
The operating costs include emptying the tanks, power expenses and maintenance of the
components. The annual costs of operating conventional systems with storage tanks are
normally several thousand DKK higher than the costs related to vacuum toilet systems. The
higher costs of establishing vacuum toilet systems may be recovered within a few years,
provided no repair work is required. Another question is if it pays to co-treat the
"black" wastewater at a biogas plant. The average costs at county level for
treatment of wastewater at a wastewater treatment plant are approx. DKK 30 per cubic
meter. The treatment price at the biogas plant at Kettinge is DKK 180 per cubic meter
– an additional DKK 150 per cubic meter. This amount should be compared to the
benefits of using the material for power generation or as a fertilizer instead of
discharging the wastewater to the aquatic environment.
Future prospects
About 15 toilet systems have been established, including the latest changes in Sweden
and Finland. Here it is possible to monitor operation and to take samples. Vacuum systems
consisting of a single toilet with a temporary vacuum system give rise to great interest,
because both establishment and operating costs for systems with permanent vacuums are much
higher. The latest changes seem to have solved the problems experienced in the
municipality of Nysted.
The present knowledge is not sufficient to determine the actual possibilities of
supplying biogas common installations with "black" wastewater collected from
vacuum toilets.
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