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Summary and conclusions

From a global point of view recycling of grey water has been extended till now primarily in

single households or in delimited groups of buildings where the wastewater is subjected to

a given treatment before being reused. Recycling of grey water is, however, not common in

Denmark.

The problem of decentralized recycling of grey water is that if the treatment plants in question

are not properly maintained they are not able to treat the grey water to a satisfactory

degree and consequently recycling of the wastewater in public institutions and private

households will represent a considerable risk of infection. Decentralized recycling of grey

water therefore seems to be complicated as well as expensive compared to the potential

water saving.

From a technical, economic and hygienic point of view a more centralized and voluminous

recycling of grey water would be more advantageous as it would ensure better possibilities

for supervising the plants.

This project focuses on the possibilities of recycling grey water from large producers

(swimming baths, sports centres, certain industrial enterprises, laundries, etc.). Recycling

will typically be interesting for large consumers having less strict requirements to the supplied

water than what applies to drinking water (industrial enterprises, car washers, etc.).

The purpose of the study has therefore been:

• to identify large primary plants (producers of grey water) and large secondary plants

(consumers of grey water) within a given case area

• to identify the potential of grey water exchange of the enterprises in question

• to characterize the content in the grey water of substances and microbes influencing the

technical and hygienic conditions

• to lay down requirements to the quality of grey water for the secondary plants of relevance

• to make a technological, economic and environmental overall evaluation of selected

scenarios of coupling between primary and secondary plants

• to make general and specific recommendations for future planning of mixed plants

among which grey water can be exchanged

The project is based on the municipality of Herlev as a case area. The water consumption in

the municipality of Herlev is characterized by a relatively high water consumption from

households and institutions (86%), while the water consumption of the trades and industries

is low compared to the national average (10% compared to 18% on a national basis). The

largest water consumer in the municipality of Herlev is definitely the county hospital (Herlev

Amtssygehus) which takes 8% (155.000 m 3 /year) of the total water consumption of the

municipality. The largest industrial and institutional water consumers include production

and manufacturing enterprises (6.000 – 13.000 m 3 /year), offices and service companies

(2.000 – 4.000 m 3 /year), swimming baths (7.000 – 9.000 m 3 /year) and public schools

(2.000 – 4.000 m 3 /year). During recent years the development of the water consumption

among the most important industrial water consumers has been influenced by the trade

conditions and thus unaffected by the prices in the municipality of Herlev, which have been

doubled during the last 6 years. On the other hand, the water consumption of households

and institutions has decreased by 25% within the last 10 years, which is attributable primarily

to the rapidly increasing water prices of the municipality.

The collected business information has been obtained from a questionnaire study among the

20 most water consuming enterprises and institutions in the municipality of Herlev. The

attitude of the enterprises to participate in the study was mainly positive and more than 90%

answered the questionnaire. Based on the questionnaire study information was collected in

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