Grantee Research Project Results

Final Report: Electrolytic Reactor for N Removal from Existing Septic Tanks

EPA Contract Number: EPD13037
Title: Electrolytic Reactor for N Removal from Existing Septic Tanks
Investigators: Monnett, Gregory T.
Small Business: Enviro Utilities Inc.
EPA Contact: Richards, April
Phase: I
Project Period: May 15, 2013 through November 14, 2013
Project Amount: $79,749
RFA: Small Business Innovation Research (SBIR) - Phase I (2013) RFA Text | Recipients Lists
Research Category: SBIR - Water , Small Business Innovation Research (SBIR)

Description:

Enviro Utilities (EU) has developed an innovative device that is small enough to be inserted into existing septic tanks and remove nitrogen (N) from wastewater without the need for organic compound digestion. Almost all current treatment technologies on the market rely on organic compound digestion as a precursor to N removal. N removal in the EU system is conducted by isolating ammonium (NH₄⁺) from the waste stream into an electrolysis reactor through a cation selective membrane and electrode assembly (MEA). The N is ultimately converted to dinitrogen gas (N₂) and released into the atmosphere.

The two main objectives of the Phase 1 award were to test the device configuration and to measure the N removal efficiency. Multiple experiments were conducted to test the configuration and components of the MEA. The MEA was tested for durability, electrical requirements, and electrode materials. The N removal rates were determined using an ion selective electrode (ISE). Bench-top scale units were tested using synthetic wastewater.

Summary/Accomplishments (Outputs/Outcomes):

Configuration evaluations found that the anode material was critical to the device. Several anode materials were tested, including nickel wire, gold plated metal, stainless steel, graphite powder, graphite rods and graphite felt. The graphite felt was found to be the most durable in the high oxidation conditions of an electro-chemical cell. The felt material also demonstrated the best N removal rates. The cation selective membrane used in the MEA was proven to be durable and have a good chemical resistance. The most promising MEA configuration was used in subsequent tests to measure N removal rates. The electrical requirements of the system was low with sufficient ion transfer occurring at less than 1 watt.

Nitrogen removal from the synthetic wastewater by the MEA was above 70 percent in most of the evaluations. The graphite felt configuration had the highest average removal rate of 87 percent with 62 percent occurring in the first 3 hours. The MEA was also shown to continuously remove NH₄ in an experiment set up to mimic a septic tank system with daily additions of waste.

The pH of the catholyte fluid collected in the MEA was surprisingly high. Values were consistently above 11.0. The pH was attributed to water hydrolysis in the MEA in which hydrogen gas was generated, thus removing H⁺ ions from solution. The high pH caused a design change in the device in which the nitrification chamber was replaced with an ammonia electrolysis component.

Ammonia electrolysis will convert NH₃ into H₂ and N₂ gases. Since the MEA concentrates the ammonium from septic tank effluent into a small volume catholyte with a high pH, ammonia electrolysis is a viable treatment to transform ammonia into N₂ gas. Several experiments evaluated ammonia electrolysis of the catholyte using different electrode materials. A manufactured electrode material made of graphite felt was proven to be the best material. Nitrogen removal by electrolysis was 62 percent within 3 hours.

Conclusions:

The Phase 1 results demonstrated proof of concept for the device. The membrane electrode assembly effectively removed and isolated NH₄ from a synthetic wastewater in less than a few hours. The electrical requirements to transfer the ions was small and would equate to less than 1.7 cents per day for a typical household. The final materials used for the MEA were found to be durable in the harsh chemical environment. A design change in the device was demonstrated to remove N using ammonia electrolysis instead of relying on microbial population to transform the ammonium to N₂ gas. Currently, the National Sanitation Federation (NSF) 245 testing deems 50 percent removal of N as a suitable N removal technology. Our preliminary results indicate that the innovative device could exceed that standard. The device could be an alternative to current technologies that rely on organic compound digestion and microbial populations for effective N treatment.

Supplemental Keywords:

nitrogen, water quality, wastewater, septic tank, electrode

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