Reuse of wastewater saves water and money
A large Danish wastewater treatment plant has, over 10 years, saved more than 5 million DKK by reusing wastewater for cleaning and other processes. The solution is to filter and disinfect the treated wastewater so that it can be used rather than clean drinking water.
The wastewater treatment plant has annually reused between 50,000 and 70,000 m³ of the treated wastewater for, among other things, cleaning the facility, flushing surfaces and mixing. Previously, they would have used clean drinking water. The treated wastewater can safely be used as technical water for these purposes because it is free of harmful bacteria. This is thanks to the disinfection unit from Danish Clean Water (DCW) that the plant installed several years ago.
The engineer at the facility, who takes care of operations and maintenance, says: “The solution is good for both the environment and our accounts. We all have a responsibility to save drinking water where we can, and this is an obvious place where we are able to do that. The disinfection unit means that there are no harmful bacteria in the treated water and also no biofilm in the water pipes.”
Before the plant installed the DCW unit, they tried a UV system to disinfect the water. However, this system did not work properly, which affected operations.
The treated wastewater – ready to be filtered and disinfected for reuse.
Simple ‘disinfect-and-reuse’ process
The solution makes wastewater recycling safe and user-friendly in daily operation. First, the treated wastewater goes through a water filter. Then a small amount of Neuthox disinfectant is injected into the storage tank and the technical water is ready for cleaning. The disinfectant is produced on site by the DCW unit using common salt, water and electricity. It kills potentially dangerous bacteria such as Legionella, Salmonella and E. coli in the wastewater and eliminates biofilm in the pipes and system. The active substance in the disinfectant is hypochlorous acid (HOCl), which works effectively and at the same time is safe for humans to use and gentle on the environment in the low concentrations needed.
“Once the solution is installed and running, it’s almost as easy as turning on the tap. The only thing we have to remember is the weekly refill of salt for the unit and the twice-yearly service. Otherwise, it pretty much takes care of itself,” says the facility engineer.
Due to the solution, the wastewater treatment plant has been granted an exemption to use technical water for spraying tasks in accordance with the Danish Working Environment Authority’s regulations on sewage work, etc.
The wastewater is reused for:
- Rough cleaning
- Rinsing of surfaces and pipes
- Automatic cleaning of machines such as inlet grates and dewaterers
- Mixing of substances, e.g. polymers, in the drainage
- Dilution water/rinse water in pipe systems
- Sludge vacuum trucks in the city
Making great savings
The calculation for the wastewater treatment plant is made as an average per year over the 10 years they have had the DCW disinfection unit. The unit and technical water filtration system have been paid off, leaving only the operating costs, and making the savings even bigger. The price of the drinking water also plays a role, of course. In the municipality with this plant, the price is low compared to other places in Denmark, where the profit from reusing wastewater would be correspondingly greater.
“Today, wastewater and sewage sludge are perceived as an important energy resource. It’s quite natural to also see technical water as an important water resource, and it makes good sense to expand its use – both from an economic and environmental point of view,” says the facility engineer.
The savings on average per year (over 10 years)
| Purchase of drinking water (DKK) (60,000m³) |
DCW unit (DKK) | Filtration system (DKK) | |
|---|---|---|---|
| Saved drinking water | 730,000 | ||
| DCW unit, depreciation over 10 years (incl. interest) | 60,000 | ||
| Filtration system over 10 years | 50,000 | ||
| Salt | 14,000 | ||
| Electricity | 31,000 | 5,000 | |
| Service/maintenance | 17,000 | 5,000 | |
| Total | 730,000 | 122,000 | 60,000 |
| Savings per year | 548,000 | ||
| Savings over 10 years | 5,480,000 | ||
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