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Membranfiltrering til rensning af gråvand for vandgenbrug i ejendomme
Summary and conclusions
Background and objective
In connection with reuse of treated grey wastewater, it had been established that several properties had either stopped using the biological treatment plant already installed or had not had the treatment plant
installed yet. In order to test another treatment method already used in connection with treatment of process water within the agri-food industry which requires low chemical and microbiological pollution, it
was arranged to test a mechanical membrane filtration plant at properties with double piping system where it was possible to take samples of grey wastewater.
The general objective of the test was to document the achieved treatment reduction on grey wastewater from properties by means of a pilot test with a membrane filtration plant combined with UV light
treatment with a view to reuse of the treated grey wastewater for toilet flushing etc. in properties. The treatment reduction of the pilot test with the membrane filtration plant is evaluated on the basis of
chemical and microbiological parameters, and the technical possibilities and the treatment price of the plant are described.
The test
Membrane filtration plant
The membrane filtration plant was of the M38 type in a standard unit with filter plates (plate & frame type). Each filter plate has a filter area of 0.15 m2. The capacity (flux) of the pilot plant for purified water
was typically 30-50 l/m2 x hour. In this pilot test, four filter plates were used, corresponding to a filter area of 0.6 m2.
The advantage of using filter plates in the pilot test is that many different membrane types are available and that the system can be dismantled and membranes examined.
Pilot test
The test was conducted at the supplier's factory in Nakskov with grey wastewater from a property located in Copenhagen.
A screening test was conducted to find the membrane type with the highest treatment reduction for suspended matter and turbidity. Nanofiltration was tested in the pressure interval from 5.5 to 6 bar and
ultrafiltration in the interval from 1 to 1.5 bar.
The nanofiltration gave the highest treatment reduction and was then selected for the pilot test in which the treatment reduction was examined for all chemical and microbiological parameters according to
Step 1 of the analytical program for grey wastewater.
The achieved treatment reduction of the nanofiltration was between 95 and 98%. It was therefore decided to conduct a test with removal of the microbiological pollution by means of nanofiltration and
ultrafiltration combined with UV light treatment. The test only covers treatment of grey wastewater by analyses for total viable bacteria grown at temperatures of 22° and 37°C.
Project results
Treatment efficiency
Step 1 - Nanofiltration
The degree of treatment for the chemical parameters in the pilot plant was between 50 and 95% whereas there was no treatment reduction for the amount of organic matter of 20 mg/l (BOD). The treated
grey wastewater had a content of suspended matter of 1.8 mg SS/l, the turbidity was 0.63 FTU and the COD 37 mg/l. The reduction in total viable counts was 89.8 to 99.0%. However, the bacterial counts
found in filtered grey wastewater were higher than expected. The reason may be that the filtered grey wastewater was kept in a non-sterile container from which the samples for analyses were taken.
Step 2 - Nanofiltration and UV light treatment
In step 2 of the test, the reduction in the bacterial counts was determined after combined nanofiltration, ultrafiltration and UV light treatment. Bacteria was not detected in 12 out of 16 samples of UV light
treated grey waste water (<1 cfu per ml). The other samples contained 1-2 cfu per ml. The treated grey wastewater contained few or no bacteria following growth at 22o and 37° and can consequently
comply with the microbiological requirements for bacterial counts in drinking water.
Grey wastewater treated by means of nanofiltration or ultrafiltration without subsequent UV light treatment only contained micro-organismes at 22o and 37° (<100 cfu per ml).
The tests showed a significant reduction in bacterial counts, and consequently both membrane types (nanofiltration and ultrafiltration) must be considered suitable for use in connection with treatment of grey
waterwater for water reuse.
Design parameters
As regards the nanofilter membrane HL 51, a minor increase in the flux and a constant flux/bar ratio in the interval from 2 to 6 bar were measured on grey waste-water. This means that the blockage of the
membrane does not change in this interval. It is expected that the capacity (flux) will be in the interval from 15 to 30 l/m2 x h.
As regards the ultrafilter membrane JW30, an increasing and a decreasing flux/bar ratio in the interval of 1 to 3.5 bar were measured on the grey wastewater. This means that the blockage of the membrane
is increasing within this pressure interval. It is expected that the dimensioning of the capacity (flux) will be in the interval from 30 to 40 l/m2 x bar.
Based on the pilot tests with grey wastewater, it is assessed that the membrane filter surfaces must be cleaned with cleaning materials once every fortnight.
For UV light treatment plants, a light source must be used with a wavelength of 254 nm and a power of 40 mW/cm2.
Treatment cost
The treatment cost for the achieved treatment of grey wastewater in the smallest size standard membrane plant with a capacity (flux) of 1 m3/hour is calculated to be 12 DKK/m3 at 100% utilization. From a
financial point of view, the use of the smallest size membrane plant can be interesting in large housing complexes with 50-100 flats whereas further optimisation of the existing membrane plants will be
necessary to make it financially cost effective, at present, for housing complexes with less than 50 flats.
Recommendation
Based on the significant reductions in bacterial counts, it is recommended that the membrane technology should be used in combination with UV light treatment for treatment of grey water from properties
with a view to reuse.
Depending on the reuse contemplated, risk assessments are recommended. The authorities should also establish quality requirements for treated grey wastewater to be reused.
It would be advisable to establish a pilot plant in order to investigate the membrane plant in continuous service. It would be expedient to install a membrane plant in one or more properties, which have
installed separate piping systems for collection of grey wastewater and have experiences with treatment and reuse of grey wastewater.
If it is possible to achieve similar results with the plant in continuous service as achieved in this test, it is recommended that instructions for dimensioning and operation of membrane plants should be prepared
based on these results.
With the treatment efficiency and the low treatment price of membrane plant compared with current water and discharge prices, the development perspectives for Danish suppliers of starting
developing/optimising membrane plants for housing complexes and single properties will be favourable. The membrane plants will be competitive compared with the expected increases in water prices
because of the limited resources both in Europe and the rest of the world.
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Version 1.0 Juli 2005, © Miljøstyrelsen.
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