Conventional treatment of nitrogenous waste in aquaculture effluents involves the nitrification of ammonia by aerobic bacteria. Systems range from highly efficient bioreactors to wetlands that do not provide an artificial substrate for the bacteria. More complete systems also de-nitrify nitrate to nitrogen gas, returning it to the atmosphere, although de-nitrification involves anaerobic processes that may be difficult to control. What all such systems have in common is dependence on bacteria and the enzyme kinetics of metabolic pathways. Is there an alternative?
A few bacteria-free technologies have been developed to treat nitrogenous waste, at least partially, however some can leave unacceptable chemical residues, upset water quality, or be prohibitively expensive. Other techniques, such as reverse osmosis, can isolate ammonia but not convert it.
Electrochemical oxidation is a process that uses electrical pulses to create nitrogen gas directly from aqueous ammonia. The use of this technology to treat aquaculture waste dates back to at least 1996, but it has not previously been demonstrated at scales that would be practical for aquaculture production facilities.
At the Aquaculture America meeting held earlier this year in Seattle, Washington, Dave L. Anderson from OriginOil, Inc of Los Angeles, California, described experiments in which he an his colleagues evaluated a commercial-scale system for the electrochemical oxidation of ammonia directly to nitrogen gas. They manipulated a number of parameters to determine the optimal conditions for the complete conversion of ammonia: pH, conductivity, salinity, initial ammonia concentrations, and current input. Electrochemical oxidation bypasses the bacterial steps of the nitrogen cycle entirely. The technique has been used in wastewater treatment for many years, but it has never been adapted for aquaculture systems before now.
The key to making electrochemical processes suitable for aquaculture is a reaction cell design that allows for continuous treatment (as opposed to batch treatment) of effluent. A unique system, known as Electro-Water Separation (EWS) incorporates multiple reactor cells that can be mounted in a treatment loop along with protein skimmers, heaters, and other conventional components, but replacing the biofilter. There is an added advantage to the EWS system: water that is treated for ammonia removal is also disinfected by the process.
Early iterations of the EWS system were directed at removing relatively large amounts of ammonia in a single pass. In trials with synthetic seawater and fresh water effluents containing 5 mg/l ammonia, a mobile six cell system (72”L x 30”W x 34”H – 1.8m x 0.75m x 0.85m) was sufficient to remove the ammonia in a single pass when operating at a flow rate of 2 usgpm (7.57 lpm).
The delivered voltage is low, around 4 Volts DC, and is related to the conductivity of the water, hence seawater is cheaper to treat than freshwater. Even at higher voltages, the entire system is perfectly safe. The amperage required (see graph) is related to the concentration of ammonia to be removed. Thus the amperage needed to remove 5 ppm ammonia in a freshwater system is over 100A (at a given flow rate), but in a well run system the concentration of ammonia would not likely be allowed to reach levels as high as 3-5 mg/l.
A shrimp system with 100 kg biomass, fed at 5% body weight per day, would produce roughly 150 g of TAN per day and require less than 30A to maintain a steady state of 0.49mg/l of TAN. Removing this much TAN would cost $1.04 per day to treat (at Los Angeles electricity rates of 21.5 cents per kWh, assuming 50% efficiency), or $0.0069 per gram of TAN. This does not take into account costs of moving water, but the system can be installed so that an additional pump is not necessary.
According to Anderson, a certain amount of chlorine will be generated from any salt in the treatment stream. This is central to the ammonia removal process, and a small amount of residual chlorine is expected. Carbon filtration is employed to remove any free chlorine and any possible disinfection by-products.
The principle advantage of an EWS treatment system is that it will allow complete control over the removal of nitrogenous waste, without the need for “pre-conditioning” biofilters, etc. With that comes the ability to site aquaculture operations in hitherto impossible locations: inland areas, urban locations, even within seafood restaurants.
I am grateful to David Anderson, for explaining some of the details of the EWS system, and for furnishing the illustrations. For more information go to: www.originoil.com
– DJ Scarratt