Økologisk byfornyelse og spildevandsrensning, 46 – Demonstrationsprojekt med genanvendelse af gråt spildevand fra en større gråvandsproducent

Demonstration Project: Recycling of Greywater from Large Producer of Greywater

Summary

The objective of the project was to carry out a demonstration project for treatment of grey water from a large producer of grey water, in this case Vestbadet I/S, a large public swimming bath. The purpose of the project was to establish a system for collection, treatment and storage of grey water to be reused for toilet flushing and machine-wash of the clothes used by the staff.

The system was installed at the public swimming bath of Vestbadet I/S, Nykær 26, 2605 Brøndby, which has an annual visit rate of approximately 300,000 people. Vestbadet's annual water consumption is approximately 17,000-18,000 m³ - equalling 45-50 m³ daily. It is estimated that the annual water consumption for showers and washbasins is between 12,000 and 15,000 m³.

In the course of the project various treatment methods were investigated and assessed. A flotation system called "the mussel", which is a Norwegian concept, was the originally proposed technology. Further investigations showed, however, that the proposed technology would result in considerable operational problems. In stead, a sand filter technique was used.

The treatment system consisted of the following treatment modules:

Collection tank
Double sand filter, where decomposition and transformation of organic substances took place, as well as retention of particles
System for oxygen supply to ensure the transformation of the organic substances under aerobe conditions
UV-light treatment
Reservoir

The main objective of the project was to assess the efficiency of the treatment system to reduce the number of selected microbiological measuring parameters in grey water, in order, thus, to determine the microbiological quality of treated and stored grey water. A further objective was an overall assessment of the system's operational stability and technology, as well as a recording of any unacceptable smell. The latter has been a problem in connection with the operation of several previous systems for treatment of grey water. Finally, the analyses was to contribute to generally increased knowledge of the microbiological and chemical composition of grey water.

An overall and summarising evaluation was carried out of the health and hygienic risks to the people who use treated grey water for toilet flushing and machine-washing of clothes, whereas no risk assessment was made of the failures and errors with the technical system. It should be mentioned, however, that a number of procedures have been implemented to reduce the risk of failures and operational breakdowns. No significant evaluation was made of the health and working environment of the people, who did the maintenance of the system.

The Danish Environmental Protection Agency decided that the measuring programme for projects with grey water should be carried out in two steps:

A general characterisation of grey water when transferred to the grey water system, before the water was treated. This step included a series of chemical parameters and microbiological standard parameters.
In step two, water samples should be taken from the inlet as well as the outlet to the treatment section of the system. The number of measuring parameters in step two was reduced in comparison to step 1. On the basis of the measuring programme step 1, the most essential parameters should be identified, i.e. the most sensitive or the most describing parameters (parameters as indicators for problematic polluting components), so that a less extensive but more specific measuring programme could be constructed in step two.

In step 1, it was assessed that in the collected grey water at Vestbadet, the content of potentially dangerous or toxic substances, such as metals and other xenobiotic substances, was very low. However, potentially infectious matter in both treated and untreated grey water may occur.

In step 2, it was assessed that the untreated grey water contained relatively high numbers of all microbiological measuring parameters, although the number of faecal indicator bacteria (thermo-tolerant coliform bacteria and enterococci) was low. Furthermore, in general there was considerable variation in the total bacterial counts.

The treatment of grey water in the sand filters had only a limited reducing effect on the total bacterial counts. The high bacterial counts found in the UV-light treated grey water, which was discharged from the system, compared to germ counts from samples taken immediately after the UV-light treatment, indicated that not all the treated water discharged from the tank had been treated with UV-light. Furthermore, the content of BOD found in the treated grey water caused an increased risk of microbial growth.

On the basis of the experience gained from this project on sand filter systems, and experience from a similar system at BO90 /1/, it is evaluated that the technology with sand filter systems presented in this report is not suitable for cleaning grey water. Due to the project results, no further evaluation was carried out of possible areas for use of grey water.