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Øget genanvendelse af gråt spildevand i fællesanlæg i større bysamfund

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 m³/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 m³/year), offices and service companies (2.000 – 4.000 m³/year), swimming baths (7.000 – 9.000 m³/year) and public schools (2.000 – 4.000 m³/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 relation to water and chemical consumption, water consumption pattern, necessary water quality and wastewater characterization, economic conditions as well as the general situation of the enterprises. Based on the answers and on interviews of the enterprises 7 potential primary plants and 6 potential secondary plants were selected to participate in the project.

The identification of the grey water resources of the potential primary plants indicated that institutions such as swimming baths, public schools and sports centres can contribute considerable grey water resources as it is seen from Table 0.1.

Table 0.1
Survey of potential primary plants and their grey water resources in the municipality of Herlev.

A spot check analysis of the grey water from 4 potential primary plants was performed for determination of pertinent physical-chemical and microbiological parameters.

Grey water from swimming baths and other institutions, where the principal source of the grey water production comes from showering, is estimated to contain a certain potential for microbiological after-growth in situations where the grey water is to be collected and stored in equalizing tanks (COD: 50 – 100 mg/l, SS: 100 – 200 mg/l, total – N: 5 – 10 mg/l, total – P: 1 – 2 mg/l). Due to technical problems it was only possible to analyse the filter rinse water from swimming baths and the quality of shower water from institutions having typically a large consumption of water for showering was consequently estimated based on other studies of similar types of grey water.

At industrial manufacturing enterprises slightly polluted wastewater from production-like conditions was classified as grey water. However, the quality of the grey water varies much dependent on which type of process the water comes from. For example, the quality of grey water from a medical production enterprise was found to almost meet the requirements of the Executive Order on Drinking Water (Danish Ministry of Environment). Another enterprise manufacturing cleaning materials produced less amounts of grey water during their cleaning of the process tanks, but in this case a considerable degree of pollution was seen in relation to COD (> 3.000 mg COD/l).

Rainwater – collected at the Herlev County Hospital – was also included in the study because of an important surplus production. This type of water was estimated to be periodically very unhygienic because of a heavy after-growth of microbes in certain periods. Due to a low hardness, conductivity and salinity, however, the technical water quality was estimated to be high apart from the winter periods, where salt is sprinkled.

Table 0.2
Survey of potential secondary plants in the municipality of Herlev and their need for process water.

The identification of highly water consuming enterprises has indicated that a number of industries would be willing to accept a reduced quality of the supplied recycled water compared to the normal drinking water quality. These enterprises are listed in Table 0.2.

Based on a distance and water volume matrix a number of possible couplings between the selected potential primary and secondary plants were evaluated in order to find the optimum conditions in relation to exchange of heat volume and geographic proximity. The two most obvious scenarios were:

Scenario 1: The Central Pharmacy (P) – Northor A/S (S)
Scenario 2: The swimming bath "Hjortespringbadet" (P) – Papyro-tex A/S (S)

In scenario 1 the need for upgrading of the water quality is insignificant, as the quality of the water exchanged in this case is close to that of drinking water. The water, which consists of surplus distillation water and a dilute concentrate from an Reverse Osmosis plant is supposed to be slightly aggressive but this can be remedied by means of a pH adjustment and by adding calcium oxide, if necessary. The water is primarily meant to be used in cooling water and phosphatizing processes at Northor, which is an iron and steel manufacturing enterprise situated approx. 100 m from the Central Pharmacy. It should potentially be possible to exchange approx. 4.400 m³/year if an equalizing tank at a size of 10 m³ is established near the Central Pharmacy in order to counterbalance the flow variations. Since the water quality is high, no crucial working occupational health problems need to be taken into consideration. Based on a logistic review of indoor and outdoor conditions at the enterprises in question in relation to the establishment of a complete grey water exchange system, the economic situation was analysed. Thus, the total construction investment was calculated to 509.575 DKK not counting the yearly expenses of 15.690 DKK for maintenance and operation of the system. The water savings correspond to 89.900 DKK/year based on a water price of 20,43 DKK (exclusive of VAT and water charge), resulting in a net present value of the investment of 52.600 DKK/year during a depreciation period of 20 years with a real interest rate of 4%.

Scenario 2 deals with exchange of shower water collected from the bathrooms of the swimming bath "Hjortespringbadet". The water is transported to the company Papyro-tex, which is a hot-pressing enterprise (manufacturing PVC foil) using water for cleaning of process air. The distance between the two enterprises is approx. 250 m. It was required by Papyro-tex that no particles, which might cause a blocking of the sprinkler jets, were allowed in the grey water. Furthermore, it was important to ensure that no odour nuisances could arise during recycling of the water. The swimming bath uses approx. 5.500 m³/year of water for showering and approx. 1.200 m³/year for filter rinse while Papyro-tex for the time being only needs approx. 3.500 m³/year, which can be covered by grey water from the "Hjortespringbadet". In order to equalize the difference in flow variations it will be necessary to establish an equalizing tank of 15 m³.

For treatment of the grey water a combination of a pressure sand filter and a UV-system was suggested to remove particles and inhibit the microbial growth in the equalizing tank. In addition, a cartridge filter should be installed at the drainage for further removal of particles in the water flow. Thus the treatment system was not designed for a complete treatment of grey water, since the relatively low concentrations of dissolved nutrients in the water were not considered problematic for the application at Papyro-tex.

As the treated grey water from the "Hjortespringbadet" is to be applied in a closed process at Papyro-tex, no immediate occupational health problems are anticipated, since there is no physical contact in relation to using the grey water.

A review of the logistic situation at the enterprises of scenario 2 showed that there was a need for only minor indoor changes corresponding to expenses of approx. 95.000 DKK for establishment of the necessary facilities to collect and distribute the grey water at both enterprises. The necessary outdoor construction works, however, are extensive, including establishment of wells, drain pipes, pressure pipes, equalizing tanks and treatment system. Working expenses amounts to 32.600 DKK corresponding to a net present (loss of) value of –4.100 DKK concerning the initial expenditures. If the total amount of grey water from the "Hjortespringbadet", corresponding to 7.700 m³/year, could have been recycled the result would have been a net present value of 82.000 DKK per year during a depreciation period of 20 years. It should be noted that the initial expenses will remain almost unchanged and that the working expenses will increase only very little in case the extra amount of water is to be treated, too.

On this background it can be concluded that in case of no particular circumstances, which might raise the installation expenses or the treatment expenses during the establishment of a grey water collecting system, it would be economically profitable to establish and run such grey water exchange systems, provided that they are situated in the neighbourhood (at a distance of up to approx. 500 m). The main condition is however that the price of water of the municipality is sufficiently high (20 DKK or more per m³ exclusive of VAT and water tax) and that a minimum of 4.000 – 5.000 m³/year of grey water can be exchanged. Furthermore, it is important to ensure that no occupational health problems will arise for instance as a result of microbiological after-growth in the recycling systems.

When making future plans for common water consuming systems it would be obvious to prepare the sewer system in a way which makes it possible afterwards to collect the grey water without having to make expensive changes of the existing installations. It is furthermore recommended to locate the grey water producers in areas allowing potential grey water consumers to exploit the available grey water resource.

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