4 environmental impact factors to consider when treating water for reuse

4 environmental impact factors to consider when treating water for reuse

To help utilities and regulators understand the environmental impact of the water reuse treatment process, Xylem and IVL Swedish Environmental Research Institute conducted in-depth research on several different treatment lines for reuse. Here are four things you should know about sustainable water reuse when developing a treatment plant.

In a previous article, Impeller looked at the research findings regarding the overall life cycle costs of different water reuse treatment lines, including capital and operational costs over 20 years. This article will focus on the results of the research that deal with ten KPIs for environmental impact (see below for the full list).

“We wanted to learn what is the most sustainable and optimal reuse solution,” says Aleksandra Lazic, Senior Process Engineer of R&D Treatment at Xylem. “This is why we looked into the full life cycle cost, the life cycle assessment, which is the environmental impact, and then the social aspect, which is reaching the effluent quality according to regulations for specific regions. This is how we defined sustainability, as an intersection of these three pillars. You need to look at all three of them.”

1. Increasing the effluent quality does not lead to a big increase in environmental impact.

For this study, the researchers looked at eight treatment lines for three water reuse purposes: agriculture use, groundwater recharge and industrial use. These lines ranged from low effluent quality for agricultural use, to high effluent quality with micropollutants removed for groundwater recharge use.

“Our research showed that as you increase the effluent quality, there is an increase in value for most of the environmental impact KPIs, but not all,” says Lazic. “At the same time, we don’t see enormous jumps in the main KPIs, like global warming potential, acidification, and eutrophication, when compared to conventional wastewater treatment. These are not extreme increases, which means that increasing the effluent quality does not lead to a large environmental impact.”

2. The lowest effluent quality has the highest global warming potential.

“For all of the KPIs except one, you do see a small increase in environmental impact as the effluent quality increases, but the global warming potential actually decreases as effluent quality increases,” Lazic says. “Basically the lowest water quality, for agricultural use, has the highest global warming potential. This is something we did not expect. The reason for this is that this line has much higher nitrous oxide (N2O) emissions, 2.1 percent compared to 0.2 percent of the other lines.”

The explanation for the higher levels of N2O is that when producing low-quality effluent for agricultural use, nutrients are kept in the water to act as fertilizer. To achieve this, the secondary treatment step must be repeatedly interrupted, which results in higher levels of N2O.

“Nitrous oxide has 300 times the global warming potential of carbon dioxide, so it is really one of the most problematic gases when we talk about greenhouse gases,” Lazic says. “Though many people are aware of this emission in the treatment process, when they set the value for this gas it is often based on assumptions. Instead we measured N2O emissions for two years.”

3. Energy consumption governs 7 out of 10 environmental impact KPIs.

“We looked at what governs the KPIs, so we could simplify the results as much as we could,” Lazic says. “We found that energy consumption governs most of the KPIs, seven out of ten. This means that if you decrease the energy consumption of your treatment lines, you will decrease the environmental impact. Knowing this, you can also use energy consumption as a surrogate to easily evaluate the environmental impact of your treatment line.”

For the treatment lines for groundwater recharge and industrial use, the N2O emissions were very small, so energy use had the strongest impact on global warming potential. For the agricultural treatment lines, N2O emissions had the strongest impact.

“In our life cycle cost analysis we discovered that the secondary treatment step uses the most energy, and is the largest cost on both the capital and operation side,” Lazic says. “With our life cycle assessment, we can see that decreasing the energy consumption of your secondary treatment step will also decrease the environmental impact. This gives us a clear indication of which step we need to optimize.”

Lazic says that some countries have already gone very far in monitoring how greenhouse gas emissions are correlated to the operation of a plant. To limit energy use and emissions in the UK, water treatment plants there are now subject to a carbon footprint fee. To calculate the annual fee, the plants must report how much energy they consume and their emissions for the entire plant. If a plant’s figures do not fall within the right limits, it has to pay a fee.

4. The location and size of the plant can make a big difference in environmental impact.

The research tested its eight water reuse treatment lines in three full-scale plant sizes: 20,000 PE (person equivalent), 100,000 PE and 500,000 PE. The research concluded that as the size of the plant increased, the C02 emissions per cubic meter of treated water decreased.

“If you have a larger system, with larger equipment like pumps and blowers, they get more efficient the larger they get,” Lazic says. “This means that energy consumption decreases per cubic meter of treated water. This is why people want to have more centralized treatment, having one bigger plant rather than several smaller ones.”

The source of the energy used in the plant is also a significant factor in the environmental impact.

“We based our research on the costs and electricity grid of Spain,” Lazic says. “When we recalculate the impact for a plant in Sweden, which uses much greener energy, the global warming potential drops by 60 percent. If you switch it to the US, it increases by 50 percent, which uses more fossil fuels than Spain. This is why we say that calculating your environmental impact is region specific.”

Xylem’s role in sustainable water reuse

Based on these findings of the environmental impact of water treatment, Xylem decided to take them further to see how emissions could be reduced worldwide.

“We decided to apply our findings on a global scale, to see what the impact would be if we were able to optimize treatment plants using Xylem technology that is available today,” Lazic says. The result was the report Powering the Wastewater Renaissance, which showed that the wastewater industry could cut electricity-related emissions in half.

“Since Xylem can deliver an entire system for wastewater treatment reuse, we are able to optimize the entire treatment line,” Lazic says. “We are not just an equipment supplier, but a solution supplier. We have invested a lot of work in understanding not only the best solution, but the most sustainable solution.”

The ten KPIs for environmental impact

Global warming potential, acidification potential, eutrophication potential, photochemical ozone creation, human toxicity, freshwater ecotoxicity, marine ecotoxicity, terrestric ecotoxicity, abiotic depletion – elements, and abiotic depletion – fossils.

by Chad Henderson