Økologisk byfornyelse og spildevandsrensning, 57 – Genanvendelse af gråt spildevand på campingpladser – Fase 2 og 3

Genanvendelse af gråt spildevand på campingpladser – Fase 2 og 3

Summary and conclusions

To further elaborate on the unit from Phase 1 of the project, the demonstration unit for the cleaning of grey water from douche rooms on camping sites was placed on Gals Klint Camping.

The demonstration unit has the following superior structure:

Settling tank
Inlet buffer tank (pumping station with buffer volume)
Biological sandfilter
Recycling buffer tank (storage tank for biologically treated grey water)
Installation for disinfection (installation for dosing)

Detailed design for a demonstration unit on Gals Klint has been elaborated with basis in the design and experience from Phase 1 of the project. Invitation to tender was sent to selective contractors.

Due to the outcome of the tender and extra costs for groundwater lowering, as a result of a high groundwater level and sodden soil on Gals Klint, it was necessary to reduce the construction works and thus the total construction cost.

It was important that the functionality would not be altered by the adjustment of the project. Therefore it was decided to reduce the size of the biological sandfilter by 50%, thus maintaining the possibility of a later adjustment to treating dimension and capacity.

Construction works began primo April 2002 whilst the start/running-in was initiated ultimo May 2002. Running-in was expected to last 4 – 6 weeks due to the forming of a functional biofilm in the biological sandfilter. Grey water was led to and recycled in the plant throughout the entire running-in period.

After running-in a control analysis of the effect of biological treatment in the sandfilter was carried out. The result of this analysis showed that the sandfilter was fully operational after approx. 7 weeks.

Immediately upon termination of running-in, the test period started.

The purpose of testing the functionality of the grey water unit was to prove the cleaning effect of the biological treatment and the disinfection. Furthermore the purpose of testing was to gather practical experience on the operation of the grey water plant, on amongst others:

Odour and noise
Visual nuisances
Working environment and behaviour
Practical conditions

The DEPA's standard measure program was used as basis. The measure program is split in two groups, one with physical/chemical parameters and one with microbiological parameters.

Samples were taken three places during the functionality test:

In the inlet to the settling tank (untreated grey water)
In the recycling buffer tank (mechanically/biologically treated grey water)
In the cisterns in the douche room building (treated and disinfected grey water)

The sampling places were chosen to prove the effect of the mechanical/biological treatment and the disinfection.

Twelve samples were drawn in all, one in the inlet to the settling tank, three in the recycling buffer tank and eight in the cisterns.

Finally the grey water was disinfected with hydrogen peroxide (35%). The purpose of this was to reduce the number of micro-organisms in the biologically treated grey water to the lowest possible level and in this way determine the necessary dose of hydrogen peroxide, both from a hygienic and economic point of view. Apart from this, the disinfection served as additional oxygen potential for the water, reducing the risk of odour from water settling in the cisterns.

As mentioned above eight samples on the treated and disinfected grey water and one sample from a reference toilet with drinking water were drawn in the functionality test. Two samples were stored at room temperature by the accredited laboratory for three weeks before being analysed. The purpose was to control whether additional growth would occur over time.

Water, which is treated and reused in the households, in this case for toilet flush, must have a degree of purity corresponding to the drinking water quality as stated in Regulation no. 871 of 21/09-2001: ”Regulation on water quality and supervision of water supplies”.

As the grey water is solemnly used for toilet flush, it is primarily the content of microorganisms, which is interesting, but in principle all demands for quality must be observed. A comparison of the results of the analyses with the remediation values shows that the treated disinfected grey water from Gals Klint has a quality, which corresponds to drinking water on the microbiological parameters. However, please note that the content of microorganisms in the biologically treated grey water has been on a very low level, which must be taken into consideration when evaluating the efficiency.

Apart from the hygienic aspects, the reuse of grey water for toilet flush involves a number of aesthetic and operational circumstances, which can cause nuisances for the users and the caretaker of the facilities in the douche rooms. Especially odours have been a problem and have resulted in the closing down of earlier installations.

The following conclusions can be drawn from the present grey water project on Gals Klint:

The construction costs for a grey water unit with a treatment capacity of 12 m³/d amounts to approx. DKK 275,000.- excl. VAT. Yearly expenses are expected to amount to approx. DKK 7,750.- which equals approx. DKK 7.-/m³ cleaned and reused grey water.

The direct depreciation is approx. 13 – 14 years. When considering all Danish Local Authorities on a whole, the depreciation period will be more than 10 years in practically all of them.

The overall conclusion is, that at present there is only a limited economic potential in constructing grey water units in Local Authorities where the water price is less than 40 – 50 DKK/m³. An alternative demand is, that the saving potential when reusing grey water is above approx. 1,500 m³/season.

A future rise in the total price of water may, however, have the effect that it will generally be more attractive to clean and reuse grey water.

Based on the results from the functionality test, the conclusion is that the grey water unit on Gals Klint biologically treats the grey water to a very low level when measured on traditional wastewater parameters. Organic and suspended matter and ammonium is reduced to a very low level, which is below the limit of detection. Ammonium, total phosphorus, sulphide and oil/grease are also reduced to a level close to zero.

The environmentally unacceptable substances, incl. Phthalate, Phenol etc. are reduced equally to a level close to zero or below the limit of detection. However, please note that a part of these parameters have a very low concentration level in the untreated grey water, why the degree of removal is with certain reservations.

A part of the heavy metals is reduced from a relatively high concentration level to a consequently lower level. This applies to Al, Ba, Cu and Zn, and partly to K. The conclusion for the other metals is that As, Ca, Mg, Na and Ni are very likely to remain unchanged in the sandfilter. No unambiguous conclusion can be drawn on changes in the concentration level for other metals in the biological sandfilter.

The sandfilter reduces the microbiological parameters by 97 – 99,9%.

The conclusion of the disinfection analyses is that disinfection with hydrogen peroxide has the effect of reducing all parameters to a level corresponding to the microbial demand for drinking water. A general recommendation for the dosing level of hydrogen peroxide cannot be established with basis in the functionality test, but for the unit on Gals Klint a level of approx. 0.05 – 0.1 ml H₂O₂/l grey water, is estimated to secure a satisfactorily microbial quality of grey water, that is a quality corresponding to the demands in the Regulation on Drinking Water.

The functionality test has not shown any nuisances in connection with the installation including odour and noise. Furthermore in entire period, the recycled water has appeared clear and odourless, with the exception of a shorter period, where the water turned yellowish due to settling in the recycling buffer tank caused by damage on the wastewater pump. If this type of installation shall be used in the future, the conditions stated in section 4.5.5 shall be implemented to operate the installation without annoyances in the working environment.

Based on the result from the functionality test it is concluded that the grey water unit on Gals Klint has been operating very satisfactorily and has cleaned the water to a acceptable level with regards to health factors and wastewater techniques.

Based on the above we estimate that generally no nuisances are connected with the reuse of grey water for toilet flush on Gals Klint. However, please note that the characteristics of the grey water and the design of the plant is essential for the result of the treatment, for which reason the experience drawn from the project cannot be used directly and uncritically in other similar units.