Seawater Flushing Toilets
By Rami Elias Kremesti M.Sc., CSci, CEnv, CWEM
Kremesti Environmental Consulting Ltd
Introduction
Seawater flushing toilets represent an under-developed technology in the West that can reduce up to 30% of the West’s consumption of fresh water supplies. The Population Equivalent or PE of waste water produced per capita in the UK for example is 150 litres. 50 litres of that is used to flush toilets daily! Perfectly clean, most of the time, potable water what a waste. It is an environmental crime which many of us participate in, in the West on a daily basis. It is unthinkable in many arid parts of Africa to waste potable water on flushing poo and pee.
Ways To Eliminate or Reduce the Environmental Impact of Toilet Flushing
A beautiful way to reduce potable water waste for toilet flushing is either to use composting toilets or if not convenient to separate grey water from urine and black water. Grey water from laundry, showers and hand washing can be recycled into toilet flushing water using a more simple process that complete sewage treatment. In open country, you can use it for irrigation without treatment if you take precautions not to get it on the fruits/vegetables due to the risk of E. Coli contamination.
Dry urinals eliminate the consumption of fresh water for flushing. And it can be treated and valuable N and P can be recovered from it using the EAWAG developed process. Note that 50% of the load of N and P on STP’s comes from Urine !!!
The other way is to use seawater flushing.
Seawater Flushing
Hong Kong, Singapore, and some coastal cities in China (like Qingdao) use seawater toilet flushing, with Hong Kong being the most experienced in this area. Other smaller locations, such as the Cayman Islands, Gibraltar, Kiribati, the Marshall Islands, and the US Virgin Islands, also use seawater for domestic purposes like flushing. Avalon City Waste Water Treatment works on Catalina Island in California treats sewage from seawater flushing toilets.
The Challenge
Seawater is saline and full of chlorides/sulphates which makes it corrosive to most metal pipes. Hence the water conveying systems need to be made of PVC or PVDF and the sewage pumps need to be made of Bronze/Brass which is not corroded by seawater. The waste water pipes typically made of PVC or concrete are not an issue. The sulphates present another challenge: formation of unpleasant H2S gases.
The other important challenge is that halophilic or salt loving organisms need to be incubated in the sewage treatment plant so that they can break down N and BOD without being inhibited by the high TDS salt water.
The Hong Kong Experience
Hong Kong treats mixed saline wastewater through conventional methods such as screening, sedimentation, and biological treatment in large sewage treatment works, but it also employs innovative techniques to manage high sulphate levels from seawater flushing. The city uses processes like the A-stage/B-stage (Partial Nitritation-Anammox) and has developed specific methods to control sulphide production, enhance energy efficiency, and improve overall treatment performance for saline sewage.
Nitritation refers to the process of oxidizing Ammonia intro Nitrite not Nitrate.
Micro-Biology of Halophiles
Where it not for seawater loving bacteria, aka halophiles, the bottom of the sea would accumulate large amounts of fish poop and biological debris. Also ammonia concentrations would become toxic.
Seawater halophiles from several genera, including Halomonas, Halobacter, Stappiaceae, and Rhodobacteraceae, are known to break down both biochemical oxygen demand (BOD) components (organic matter) and ammonia. These organisms are used in the biological treatment of saline wastewater where conventional non-halophilic bacteria struggle due to high salt concentrations.
Halobacter halobium: This halophilic archaeon has been shown to improve the efficiency of organic matter (measured as Chemical Oxygen Demand or COD, which correlates with BOD) removal in saline wastewater, achieving over 85% efficiency in studies with salt concentrations greater than 2%.
Halomonas_ species: Members of the Halomonas genus are effective in treating high-salt wastewater and have low contamination levels, making them suitable for use in open bioreactors. They are involved in the general degradation of organic pollutants.
Stappiaceae and Rhodobacteraceae: These families were identified as the dominant genera in halophilic aerobic granular sludge (hAGS) that achieved high (82-99%) total nitrogen removal, which includes the breakdown of ammonia via nitritation-denitritation pathways, even in hypersaline conditions (up to 12% salt). They also remove organic matter effectively.
Ammonia-Oxidizing Archaea (AOA): A new strain of archaea capable of oxidizing ammonia was isolated from a marine aquarium tank, and AOA are now recognized as important actors in the nitrogen cycle in various marine ecosystems, including those with low nutrient levels.
Nitrosomonas marina: This bacterium, or one closely related to it, was found to be a dominant ammonia-oxidizer in artificial seawater systems, highlighting the role of specific bacteria in marine nitrification.
Wider Applications:
These halophiles are particularly valuable for treating industrial effluents (e.g., from textile, tannery, vegetable pickling and fish processing industries) that have high salinity and contain significant organic and nitrogenous pollutants. Utilizing these naturally salt-tolerant organisms in biological treatment systems improves efficiency where non-halophilic bacteria would undergo plasmolysis and fail.
Technologies Used for Saline Sewage Treatment
AB-Process:
This two-stage system is designed for saline sewage and focuses on recovering carbon and removing nitrogen efficiently. The “A-stage” captures carbon, and the “B-stage” uses a specific process for nitrogen removal the SANI®™ Process:
Developed at the Hong Kong University of Science and Technology, this process leverages the high sulphate content from seawater flushing. It uses anaerobic conversion and autotrophic denitrification to achieve significant reductions in sludge, energy, and space requirements compared to conventional methods.
Sulphide control: Saline wastewater is high in sulphates, which can lead to the production of hydrogen sulphide (H2S). Technologies like the Low Energy Electrical Odor control (LEEO®) system have been developed to suppress H2S production and improve sludge dewaterability.
Membrane technology: Advanced techniques like immersed membrane bioreactors (MBRs) are being studied and used. They can achieve high levels of BOD, COD, and total nitrogen removal, though saline wastewater may require more frequent membrane cleaning. The secret is cultivating halophilic or salt loving bacteria that thrive in the high TDS water.
NEREDA:
Nereda technology is capable of treating the high salinity wastewater that results from using seawater for toilet flushing, and has been successfully piloted and implemented for this specific application in Hong Kong.
Proven Application: Hong Kong, where over 80% of toilets use seawater for flushing, has conducted successful pilot programs and is implementing full-scale Nereda wastewater treatment plants (WWTPs). The pilot results confirmed that Nereda technology can efficiently treat the specific characteristics of this high-salinity wastewater.
Salinity Tolerance: The robust aerobic granular sludge used in the Nereda process can withstand high salinity influent, which was previously a challenge for conventional activated sludge (CAS) systems where the salt could be detrimental to treatment performance.
Efficiency: The technology provides high-quality effluent, even with challenging wastewater compositions.
Compact Footprint: Nereda plants are significantly more compact than conventional treatment facilities (up to 75% smaller footprint), making them ideal for densely populated coastal areas like Hong Kong where land availability is limited.
Sustainability: The process is a sustainable and cost-effective solution, requiring less energy and fewer chemicals than traditional methods. Trials in Gibraltar are ongoing to test its efficacy in treating the sewage from seawater flushing toilets.
Research in KAUST
Researchers at King Abdullah University of Science and Technology (KAUST), led by Associate Professor Pascal Saikaly, have conducted significant work into using halophilic (salt-tolerant) bacteria cultured from the Red Sea to effectively treat saline wastewater generated by seawater toilet flushing.
The primary challenge with using seawater for toilet flushing is that the high salt content inhibits the performance of conventional nitrogen-removing bacteria used in standard treatment processes. The KAUST team addressed this by identifying and testing a specific salt-tolerant bacterium: Candidatus Scalindua sp. AMX11.
Key Findings:
High Nitrogen Removal: The bacterium demonstrated high nitrogen removal rates, proving about 90 percent effective at removing nitrogen from a nitrogen-rich seawater solution with a salinity of around 1.2% (typical for seawater sewage).
Real Seawater Testing: The tests were conducted using real seawater, which adds to the practicality and relevance of the findings.
Proof of Concept: The research provided a successful “proof of concept” for using marine anammox (anaerobic ammonium oxidation) bacteria in treating saline wastewater.
Future Steps: The next phase of research involves demonstrating this technology in a full-scale microbial granular system, which would incorporate other necessary bacteria for a complete treatment process.
Risks
Dual water systems in the Netherlands have been associated with significant public health issues, primarily due to cross-connections causing microbial contamination of the drinking water supply. These problems led the Dutch government to largely abandon and discourage these systems.
Key Public Health Issues
Microbial Contamination Outbreaks: The most prominent issue was the risk of drinking water contamination with non-potable “household water” (grey water/rainwater). A notable incident occurred in a new housing estate around November 2001, where a human error following maintenance work resulted in a cross-connection, causing grey water to enter the drinking water pipes. This led to a large gastroenteritis outbreak affecting over 54% of the exposed households.
References
https://vprd.hkust.edu.hk/highlight-recognition/impact-cases/sani
About the Author
Rami Elias Kremesti is a chartered water treatment and environmental manager residing in the UK. He holds a B.Sc. degree from the American University of Beirut and an M.Sc. in chemistry from UNT in Denton Texas. He is passionate about various water and waste water treatment technologies. He grew up swimming and fishing in the Mediterranean in Beirut, Lebanon and as a kid he contracted a fungal infection due to sewage contamination on one of the beautiful beaches in Beirut. His dream is one day to help build sewage treatment projects in his native homeland. Rami also loves to write about hot Sci-Tech topics.
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Rami Elias Kremesti Portrait