By Dallas Weaver, Ph.D., P.E.
These ammonia + nitrite feeding bacteria were given the name anammox. My own experience and discussions with others (T. Schuur, 2003) indicated that similar reactions were occurring in some aquaculture systems and throwing off mass balance calculations on O2 and nitrogen species.
Meanwhile, some sewerage plants started using fluidized bed bioreactors specifically for anammox bacteria by feeding the system both ammonia and nitrite. This requires a two stage process, where you first do a secondary treatment or similar biotreatment with adequate oxygen to get carbon oxidation, with only some nitrification to nitrite using a controlled sludge. You then pass the water containing little carbon and ammonia + nitrite into a bioreactor system with the ability to operate at high SRT’s (sludge retention times) of greater than two weeks, allowing enough time for the slow growing anammox bacteria to thrive.
These can be bioreactors like packed bed, fluidized bed, granulated bed or membrane type reactors (Strous et al., 1997, Water Research 31, 1955-1962) where the ammonia and nitrite is converted to nitrogen gas and water by the anammox bacteria under slightly anaerobic conditions. Relative to conventional nitrification oxidizing the ammonia all the way to nitrate, like we do in aquaculture, then using a carbon source to reduce the nitrate to nitrogen gas, the anammox process uses 66% less oxygen to convert the ammonia to nitrogen gas and doesn’t require any carbon.
As adding oxygen to water in aquaculture systems is a significant fraction of the total RAS energy, the lower oxygen demand is attractive for zero exchange aquaculture; but not oxidizing and stabilizing the solid waste from an aquaculture facility will add to the solid waste disposal issue. However, in a truly minimal energy system in fresh water (no high sulfate levels), it is possible to visualize a RAS culturing fish in a high loop strength (LS – feed/biotreatment flow – mg/l) system with pH control for ammonia toxicity and oxygen/aeration for tank oxygen requirements, using microscreens or equivalent for solids removal producing a high concentration waste stream containing 10 mg/l of ammonia or more. Something like 300 mg/l of feed would have been added to the culture water so the BOD would also be high, but the flow per kg of feed is low (low pumping energy per kg of feed).
This waste stream could be treated in a variety of biofilters with fairly low SRT and high bacterial growth rates but with long enough SRT to get some nitrification to nitrite. Visualize something like a highly loaded moving bed reactor, bead filter, etc. with a biomass control system shearing extra biomass off the carriers and into a separate discharge stream. This system would be operated near zero DO, so the cost of oxygen would be minimized. With a short SRT and high biomass grow rates and high biomass yields the total amount of oxygen required per kg of feed input is minimized. By controlling the biomass, oxygen and SRT, the ratio of ammonia to nitrite could be controlled with the discharge water at the correct ratios for anammox reactions. Using two stages in series could allow even better biomass yields in the first stage eliminating most of the BOD with very short SRT and the second stage for converting some of the ammonia to nitrite.
This water with ammonia and nitrite could then go into an anaerobic reactor where the ammonia and nitrite are converted by anammox bacteria to N2 and water. As the water from this reactor would have zero oxygen, using a fluidized bed bioreactor with a packed column for re-aeration would be a good choice.
The solids from the fish culture tanks and the extra biomass from the fast growing stages of the bioreactor sub-system could both flow, with suitable concentration step, to an anaerobic digester and be used to produce methane for energy recovery. This energy recovery step eliminates a lot of the biodegradable solid waste, just like using the solid waste for denitrification accomplishes. However, this game is limited to fresh water as seawater contains too much sulfate and H2S is formed at the ORP’s necessary for methane bacteria survival. The effluent from this bio-gas production unit would be high in ammonia, phosphate and residual solids and would require some solids and phosphate removal by mechanisms such as lime addition, filtration or sedimentation. After solids and phosphate removal, the high ammonia water could go back to the ammonia/anammox system.
The above design approach could be very energy efficient, but highly complex with lots of ways to make mistakes. The overall risks of errors and complexity would be too high for all but the largest systems. It would also require a high level of automated chemistry and computer control, more like major modern waste water treatment systems. However, the oxygen and energy cost would be significantly reduced per kg of feed fed.
Dallas Weaver, PhD, started designing and building closed aquaculture systems in 1973 and worked for several engineering/consulting companies in the fields of air pollution, liquid wastes, and solid wastes until 1980. Today, he’s the Owner/President of Scientific Hatcheries.