By Dallas Weaver*
We then noted that if we want higher production per unit volume of water used we will either be storing that N in the form of protein in living biomass as bacteria in bioflocs or living plant cells in algae ponds or aquaponics, or use bacteria in biofilters to convert it to the less toxic NO3 (Nitrate). If we now want to further increase production without more water usage, we will get to the point where we have to deal with the buildup of Nitrate in the recycled water.
Nitrate can be removed from water using bacteria to reduce the NO3 to N2 gas as the bacteria use nitrate instead of oxygen as an electron acceptor. Keeping in mind that bacteria can usually get more energy using oxygen than they can with NO3, these denitrification bacteria usually operate best under strongly hypoxic and fully anaerobic conditions without free oxygen being present for competitors. For energetic reasons bacteria prefer O2 over NO3 over SO4 as electron acceptors.
The other half of the denitrification reaction requires some reduced electron donor material that could range from carbohydrates, alcohols, volatile fatty acids (acetate), to elemental sulfur and H2S to ammonia. It is all about energy, and nature has figured out how to use almost any chemical energy combination one can imagine.
In the 1990’s, Philip Lee at the University of Texas Medical Brach faced a unique problem for most of aquaculture at that time. His squid didn’t like NO3, and he couldn’t let the levels build up, but he didn’t have a flow through water supply. His solution was a packed bed anaerobic bioreactor being fed methanol and system water to denitrify the water. He was using ORP based control systems to maintain proper anaerobic condition by controlling the methanol feed to the reactor. Being a salt water system, letting the ORP become too negative (feeding too much methanol) would allow the bacteria to use sulfate and produce H2S.
Also in the 1990’s, I set up an anaerobic fluidized bed biofilter using sugar as the carbon source (no hazardous material nonsense with the city bureaucrats) using ORP and pH controllers on one of my production systems. The added sugar to the inlet removed all the oxygen; then the bacteria started denitrifying.
In the 2000’s, a lot of research was done using the solid waste from the fish as the carbon source for denitrification, eliminating both nitrate and some of the solid waste at the same time with the goal being zero discharge systems. The fecal material from fish/shrimp contains a mixture of compounds ranging from simple very biodegradable molecules to highly refractory complex organic polymers. If 3% of the initial feed becomes ammonia which is converted to nitrate N in the main RAS, you need 12% of the initial feed as biochemical oxygen demand (BOD) to use all the oxygen in the nitrate. As the amount of solid COD from the feed can be around 20% of the feed amount, and the fraction that is not rapidly biodegradable (COD – BOD5) can be > 50% (lots of insoluble fiber and ash), getting the suspended solids from the fish culture system to provide all the carbon demand for the denitrification depends upon the details of how the suspended solids are removed, stored and utilized.
System designs with mature sludge (incline plate, etc.) will require letting the anaerobic sludge digest longer (longer SRT – sludge retention time) to convert more of that refractory “fiber” and other refractory compounds into better food sources for denitrifying bacteria.
A lot of research has been done on just letting the solids fill a large anaerobic basin where anaerobic fermentation and mineralization produces volatile fatty acids, H2S and other compounds that are then used by denitrifying bacteria to reduce the nitrate to nitrogen gas. When “fresh” solids such as from a micro-screen are used, the system seems fairly balanced between the conversion of the accumulating sludge into suitable energy sources for denitrifying bacteria and the input of fresh solids.
These manure solids from the fish waste also contain some nitrogen compounds in the form of nitrogen-containing organic materials that become mineralized during this anaerobic decomposition and end up adding more ammonia. Under some conditions, it is possible to have anammox bacteria that obtain their energy from ammonia + nitrate to nitrogen gas eliminating both problems at the same time.
When the nitrate is destroyed, the alkalinity that was lost when the nitrate was formed is regained. As a byproduct, it seems that many of these denitrification reaction conditions also result in phosphate concentrations being elevated in the remaining sludge.
With denitrification, we are now approaching zero discharge RAS, which is truly sustainable on water and environmental issues but not on energy and feed issues.
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.