Wastewater Treatment Efficiency under Different Salinity Levels
Authors: Lyuqin Liu, Yong Sang Kim, Myeong-Ho Yeo & Yuming Wen | Published in Springer Journal, Water, Air & Soil Pollution (2024)
Water security and sustainable water resource management have become urgent global issues due to increasing water shortages and frequent droughts. To address these challenges, various alternative methods for utilizing existing water resources have been implemented, including wastewater recycling, rainwater catchment, and the use of seawater. In islands, seawater can be utilized for various purposes, such as desalination and flushing toilets.
With funding support from the Guam Waterworks Authority and the Guam Hydrogeologic Survey (Guam Public Law 24-247), WERI conducted research on water security and sustainability, focusing on the use of seawater for flushing toilets—a promising technique to conserve freshwater resources. Literature reviews indicate that toilet flushing accounts for a significant proportion of household water consumption, ranging from 29% to 47%.
Currently, Guam’s wastewater treatment plants handle 59% of household wastewater, while the remaining 41% of households use septic tanks, which are expected to be eventually connected to sewage lines. The main pollutants in septic tanks are ammonium, typically ranging from 20 to 55 mg/L. Consequently, ammonium and other ions, such as chloride, would significantly increase in wastewater if septic tank connections and seawater-flushing toilet systems were implemented. Some reports indicate that Guam’s wastewater treatment plants occasionally experience ammonium levels in effluent as high as 24.9 mg/L. Thus, controlling ammonium levels in wastewater treatment plants is a major concern in Guam.
Nitrification, a biological oxidation process that converts ammonia to nitrite (nitritation) and then nitrite to nitrate (nitratation), is the most critical process for determining wastewater treatment efficiency. The metabolisms of nitrifying bacteria, such as Nitrosomonas and Nitrobacter, are largely responsible for nitrification.
Salinity is known to impact the activity of nitrifying bacteria. If seawater is used for flushing toilets, the salinity levels of wastewater will gradually increase as the number of seawater-flushing toilet systems rises. To assess the feasibility of seawater-flushing toilets, it is important to evaluate the efficiency of ammonium removal in the biological treatment process under varying salt concentrations. Additionally, examining the effect of stepwise increases in salt levels on the biological treatment process is essential. The gradual increase in salinity levels due to seawater-flushing toilets may cause nitrifying bacteria to respond differently at each stage.
Based on this concept, we designed a laboratory scale fully aerated bioreactors with added polyvinyl alcohol (PVA) gel as the biocarrier, simulating Guam’s wastewater conditions with 30 mg/L of ammonium-nitrogen. The reactors underwent seven salinity increment steps: 0%, 20%, 30%, 50%, 70%, 80%, and 100%. After reaching 100% salinity, an additional step at 0% salinity was included to observe any potential changes or recovery processes.
The results revealed that between 0% and 30% salinity, the ammonium-nitrogen removal efficiency averaged 95.0%, indicating no inhibition of nitrification. However, at 50% salinity, ammonium-nitrogen removal began to decrease, continuing up to 80% salinity, where the removal efficiency dropped to over 60.0%. At 100% salinity, the removal efficiency rapidly fell from 59.2% to 15.4% within ten days. When the bioreactor was returned to 0% salinity, the removal efficiency quickly recovered to 93.7% within five days. This experiment identified 50% salinity as a critical threshold for nitrification efficiency, while 100% salinity proved to be detrimental.
Figure 1. Fully aerated PVA gel bioreactor system.
Figure 2. A photo of plexiglass bench-scale bioreactors.