Flushing Toilets With Seawater Could Protect Marine Life | Chemical & Engineering News
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Web Date: November 10, 2015

Flushing Toilets With Seawater Could Protect Marine Life

Environment: Salty wastewater is less toxic to some marine organisms than freshwater, despite concern about disinfection by-products
Department: Science & Technology
News Channels: Biological SCENE, Environmental SCENE
Keywords: water, sanitation, toilets, wastewater, Hong Kong, disinfection by-products, saltwater, chlorination
Flushing toilets with saltwater could cause less damage to coastal food webs than previously thought.
Credit: Shutterstock
Photo of toilet.
Flushing toilets with saltwater could cause less damage to coastal food webs than previously thought.
Credit: Shutterstock

In Hong Kong, about 80% of residents flush their toilets with seawater, thanks to a separate water distribution system set up in the 1950s. The approach conserves the city’s scarce freshwater resources, and has also been adopted by smaller communities like the Marshall Islands. As coastal populations and water demand rise, this idea may become more attractive elsewhere, though some researchers have worried about the release of potentially toxic by-products to coastal areas from treating seawater with chlorination. To the contrary, a new study suggests that the practice not only helps conserve freshwater but also may protect wildlife in marine ecosystems (Environ. Sci. Technol. 2015, DOI: 10.1021/acs.est.5b03796).

Chlorination during standard wastewater treatment can introduce toxic chlorinated disinfection by-products to coastal zones. Because seawater has higher concentrations of bromide and iodide than freshwater, however, treating it with chlorine can also produce brominated and iodinated by-products that may be more toxic to marine life than chlorinated ones, according to lab studies.

Xiangru Zhang of the Hong Kong University of Science & Technology wanted to understand how the chlorinated effluent, whether fresh or saline, affects the coastal ecosystem. So he and his colleagues, including Susan D. Richardson of the University of South Carolina, designed a study of two organisms at the base of the marine food web, a marine polychaete worm and the alga Tetraselmis marina.

The researchers collected effluent from two wastewater treatment plants in Hong Kong that process saline water and one that processes only freshwater. They chlorinated the effluent, removed residual chlorine, as is typically done in treatment, and added it to seawater at concentrations ranging from 0% to 100%. The researchers exposed worm embryos and algae to these samples for 12 hours and 6 days, respectively, and monitored their development and growth.

At 44% fresh wastewater, none of the worms developed normally, whereas it took higher concentrations of the saline effluents—56% to 63%—to have a similar effect. At 100% freshwater effluent, the embryos died within three minutes. The team found similar trends with the algae experiments.

These results together suggest that the chlorinated saline effluent was generally less acutely toxic to the organisms than its freshwater analog, a result Richardson calls “shocking.” Because of previous toxicity studies of disinfection by-products, “we were expecting the opposite,” she says.

The toxic by-products may be less important in this case than the problem of introducing freshwater into a marine environment. The team tested whether the shock of freshwater exposure on these marine organisms could explain the results. They incubated the worms and algae with pure water at various salinities. Embryos exposed to water with a salinity corresponding to that of both types of wastewater developed abnormally, in a similar fashion to the results of the wastewater experiments.

Seawater flushing might therefore contribute to both water and wildlife conservation by avoiding exposing marine life to high doses of freshwater near outlet areas. However, Zhang notes that it is also important to study the chronic effects of ecosystem exposure to disinfection by-products.

David A. Reckhow, an environmental engineer at the University of Massachusetts, Amherst, says the study is important and breaks new ground in determining how using seawater for toilet flushing could affect ecosystems. “It’s a good indication for the future of the technology,” he says. Despite the challenges of building a separate distribution system, he says, “more coastal communities should consider this.” He notes, however, the study provides a baseline that should be followed up, especially with chemical analysis of the disinfection by-products present.

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Joao Guilherme Rocha Poco (November 18, 2015 2:51 PM)
At first time it seem to be a good idea, but I think that the use of salty water could compromisse the efluente water treatment, specially in the cases where the water is reutilized as potable, because there will be a salt accumulation in the system.
Robert Schwarz (December 7, 2015 2:27 PM)
You don't mix saltwater and freshwater effluent treatment systems. They are and should remain separate. Using saltwater for these types of uses is a great idea. Has anyone thought about how poop whales, porposes, & fish produce? Not to mention all the sea birds. Have you ever seen a boat where seagulls frequent?

Why are seagulls protected? They are a major threat to other birds. Should have some season on seagulls. Crabs need to eat too!
M Todd Coolbaugh (November 18, 2015 4:58 PM)
I wonder how the infrastructure designed with freshwater (sewer lines, etc.) will stand up to salt water?
Gare Henderson, Ph.D. (December 7, 2015 3:47 PM)
Large scale seawater dumping is playing with fire. Potentially a best practice, would be to set up seawater evaporation projects, to return clean water to the local water cycle.

a bio-recharge think tank
Vee (December 7, 2015 10:52 PM)
Hi, this is very interesting kindly give me more information about the technology of this plant and how to design the infrastructure. Like they did in Hong Kong we leave in a coastal area.

Kind Regards
Rowan (December 8, 2015 1:08 AM)
Very interesting findings thank you for sharing this article. As a practitioner the article raises many more questions:
1. Please would you let us know if biological treatment is included in the seawater-based sanitation treatment process train? At what point in the treatment process train is chlorination (disinfection) used?
2. How about inclusion of anaerobic digestion to detoxify halogenated organics?
3. Are Hong Kong plumbers / plumbers associations / sanitation construction standards etc. are these geared for the specifications needed for a separate seawater-based sanitation system - so that easily corroded components are not installed? How common is it for plumbers to fail to meet these standards? How did government take part in establishing sanitation building / plumbing standards?
Rene Javier (December 8, 2015 7:59 AM)
Sorry to say that this research was designed to simply evaluate the impact of flushing toilets with seawater on two simple organisms. The study did not account for the detrimental environmental impact of the BOD, N, P, Total solids loading that direct discharge would do to the receiving waters.
Yabing Nollet (February 4, 2016 4:30 PM)
I also wonder how to treat saline wastewater.
Mario Rivera (April 21, 2017 8:44 PM)
I found this very interesting and wanted to really test if Hong Kong's toilet water is salt water. I did It in this video:


Mario Rivera (August 3, 2017 1:58 PM)
You can see a small experiment here to prof this:


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