SOLVIA AnammoX is an outboard two-loop electrochemical module for the stabilisation of the anammox process. The cathodic loop works with the anammox reactor and holds reducing conditions in it; the anodic loop works with the nitritation zone, where nitrite-oxidising bacteria are suppressed by pulsed micro-dosing.
Anammox is the anaerobic oxidation of ammonium, in which ammonium and nitrite are converted into molecular nitrogen; it is one of the routes of biological nitrogen removal, applicable above all to streams high in ammonium. The development does not treat the water itself and does not replace the biology: the electrochemical module works on two side streams drawn from the nitritation and the anammox reactors and returns them, having altered the oxidation-reduction potential of each.
It is meant for the control of the anammox process in one-stage and two-stage systems of partial nitritation–anammox, and in CANON, OLAND, SHARON–ANAMMOX and other deammonification reactors as well. The module stands outside the biological reactors, and so it suits new sites and existing ones alike. The configuration of the loops, the share taken by the side streams and the target ranges of potential are chosen for the particular system.
The classical chain of nitrification and denitrification calls for a considerable quantity of oxygen, for organic carbon, for large reactor volumes, for recirculation and for exact control of the regime. The anammox route is shorter: ammonium reacts with nitrite directly. What is paid for the shorter route is a narrow corridor of conditions. Anammox bacteria grow extremely slowly, the doubling time stated in the development materials being 11–30 days, and a system regains lost activity only over a long time.
There are two vulnerable points, and both lie outside the biochemistry itself. Oxygen that reaches the anammox reactor by accident raises the potential of the medium and inhibits the consortium. Nitrite-oxidising bacteria that have grown up in the nitritation zone turn nitrite into nitrate and take from the anammox bacteria one of their two substrates. Ordinary PN/A systems answer that through dissolved oxygen, the reaction of the medium, free ammonia, temperature, aeration regimes and sludge age.
The electrochemical module is carried outside both reactors and is divided within into a cathode and an anode chamber by a membrane, a diaphragm or a porous partition. The cathodic side stream is drawn from the anammox reactor: in the cathode chamber a catholyte of a potential from −300 to −100 mV is formed, and the stream goes back where it was taken from, holding reducing conditions in the reactor.
The anodic side stream is drawn from the nitritation reactor. In the anode chamber an anolyte of a potential from +200 to +500 mV is formed, which is dosed in pulses back into the nitritation zone or is switched over into the polishing zone. The volumes are small: the anodic side stream amounts to 0.1–3 % of the flow of the nitritation reactor. The cathodic and the anodic streams are not mixed hydraulically, and the separate control of the parts of the nitrogen cycle rests on exactly that.
Hydrogen is formed at the cathode. Dissolved in the catholyte, it returns to the anammox reactor as an electron donor for the hydrogenotrophic denitrifying bacteria coupled with the anammox consortium: they reduce nitrate and nitrite to molecular nitrogen and thereby hold the nitrite balance through swings of load. The anammox bacteria themselves go on working on ammonium and nitrite as their principal stoichiometric substrates.
The system runs on no single programme but on three regimes switched automatically. In the normal regime the potential in the anammox reactor is held in the range from −200 to −50 mV, and the ratio of ammonium to nitrite is watched at the inlet. The emergency regime comes in by dissolved oxygen: as soon as it rises above 0.3 mg/l, the current of the cathodic loop is raised, the potential of the catholyte falls and the oxidising action is compensated without any obligatory change of the aeration.
The third regime answers a nitrite imbalance: when the ratio of nitrate to nitrite exceeds a set threshold, pulsed micro-dosing of the anolyte into the nitritation reactor begins. The activity of the ammonium-oxidising bacteria is watched at the same time, and watched not indirectly but by the rate at which ammonium falls and nitrite rises together with it. If the rate of nitrite formation drops below 90 % of the initial one, or the concentration of residual oxidants grows, the dosing is reduced or stopped.
The speed of the response is described by calculation. In a calculated forecast example, at a dissolved oxygen of 0.8 mg/l the current of the cathodic loop is raised over 3–5 minutes, and the time to restore anammox activity is estimated at 6–18 hours instead of the 2–5 days it would take without intervention. That is a calculated forecast and not a measurement on a working site, and for a given installation it stays a reference point.
The electrodes here do not stand in the biological zone — in this the scheme differs from electro-bioreactors, where the electrode assembly is immersed in the biomass. The module carried outside treats side streams only, so that the electrodes are not permanently in the biomass, the risk of their fouling is lower, and service of the electrode assembly asks for no opening of the bioreactor. Connection to structures already built runs along the same side lines.
The installation provides a bypass line, a washing loop and differential pressure sensors for watching the fouling of the membrane, and the module is served without stopping the anammox reactor. All three regimes are controlled from sensors of oxidation-reduction potential, of the reaction of the medium and of dissolved oxygen, from sensors of ammonium, nitrite and nitrate, from the current of the loops and the flows of the side streams.
SOLVIA AnammoX is SOLVIA's own technological development in the field of biological nitrogen removal and the electrochemical stabilisation of the anammox process; the technology has patent protection in Russia. The solution brings together a nitritation reactor and an anammox reactor, an outboard two-loop electrochemical module, a cathodic and an anodic side stream, control of potential, compensation of oxygen surges, pulsed suppression of NOB, protection of AOB and automatic switching of regimes on sensor data. The structural parameters, the electrode materials, the current regimes, the control algorithms, the switching thresholds and the dosing settings are chosen for the particular site and are not fully disclosed in open materials.
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