SOLVIA AirECOX is a multi-stage electrochemical system for cleaning ventilation air of odorous and harmful impurities. The air passes a humate pre-scrubber, a catholyte and an anolyte scrubber, a droplet separator and a final sorption stage, while the working solutions are maintained in a remote bypass electrolyser.
The task of cleaning air arises at the same sites where the wastewater is treated: inlet chambers, aeration tanks, sludge handling structures and sewage pumping stations give off several groups of pollutants into the ventilation air at once. Hydrogen sulphide, ammonia, mercaptans, amines, organosulphur compounds, organic odorants, dust, aerosols, entrained droplets and biogenic emissions are present together — substances of different chemical natures, for which there is no single common working medium.
SOLVIA AirECOX gathers into one line a humate pre-scrubber, a catholyte and an anolyte scrubber, a droplet separator and a final sorption stage; the working properties of the solutions are maintained by a remote bypass electrolyser. It is meant for the ventilation air of livestock facilities, wastewater plants, sewage pumping stations, municipal solid waste landfills and composting yards. Each liquid medium works with its own group of impurities, and the loops do not communicate with one another.
The composition of the air at such sites varies. At sewage pumping stations and wastewater plants hydrogen sulphide, mercaptans, humid aggressive air and aerosols prevail, at livestock facilities it is ammonia, amines, organic odours, dust and biological aerosols, and at landfills and composting yards the composition shifts from one day to the next. A carbon bed becomes saturated and loses its service life under high humidity and dust loading, a biofilter is sensitive to temperature, humidity and toxic peaks, and a chemical scrubber rests on a constant supply of reagents.
The reason runs deeper than the list of limitations: hydrogen sulphide and mercaptans are captured in an alkaline medium, ammonia and amines in an acidic one, and in a single liquid these media neutralise each other. Efficiency falls, reagent consumption grows, and the captured products gather into one complex solution. Electrochemical systems with a single loop add to this the fouling of the electrolyser and the risk of by-products.
The development spreads the pollutants over three hydraulically separated liquid loops: the humate loop of the pre-scrubber, the cathode loop of the catholyte scrubber and the anode loop of the anolyte scrubber. The cathode and anode loops have no direct hydraulic connection, and the chambers of the electrolyser are divided by an ion-conducting membrane or a diaphragm. The sulphur-bearing and the nitrogen-bearing capture products are therefore withdrawn separately instead of mixing.
The first stage along the path of the air is the humate pre-scrubber. The humate solution captures dust, fibres, aerosols, entrained droplets and part of the organic odorous substances before the air reaches the electrochemical stages. The humate loop is hydraulically separated from them, so the humates do not enter the electrolyser: the risk of foaming, colour, a rise in COD, organomineral deposits and fouling of the electrodes and the membrane is removed.
The air then passes into the catholyte scrubber, where an alkaline catholyte circulates: hydrogen sulphide, mercaptans and the acidic sulphur components pass into soluble hydrosulphide and sulphide forms, and a blowdown node takes the loaded solution out. In the anolyte scrubber an acidic oxidising anolyte absorbs ammonia and amines; on a nitrate electrolyte the absorbed ammonia passes into an ammonium nitrate solution withdrawn as a liquid stream without drying or crystallisation. A droplet separator and a final sorption or sorption-catalytic block close the line.
Not the whole circulating solution passes through the cathode and anode chambers of the electrolyser, but a share of the flow of the respective loop — from 5 to 50 %, preferably 10–40 % and most preferably 20–30 %. The rest of the liquid goes on working in the scrubbers. The bypass arrangement lowers the load on the electrolyser and the energy consumption, allows pH and the redox potential to be held more precisely, and lets the electrolyser be taken out for service without stopping the scrubbers.
The anode loop runs on a sulphate or a nitrate electrolyte — sodium sulphate, potassium sulphate, sodium nitrate, potassium nitrate or their mixtures. A chloride-free composition lowers the risk of chlorine, hypochlorite forms and chloramines appearing in the treated air, a risk that chloride electrolytes carry in the presence of ammonia and organics. The hydrogen and the oxygen formed in the chambers leave through separate degassers into a safe zone without mixing with the air stream.
The prevailing pollutant at a site changes, and the order of the wet stages is set by gas switching valves and bypass lines. Where hydrogen sulphide prevails, the air is directed first into the catholyte scrubber and then into the anolyte one; where ammonia prevails, first into the anolyte one and then into the catholyte scrubber or straight to the final sorption stage. The scheme is therefore not tied to a single type of pollutant.
Control is exercised by actual readings: sensors of pH, redox potential, conductivity and level stand in the cathode and anode loops, gas sensors of hydrogen sulphide and ammonia at the inlet and the outlet of the scrubbers, and a separate hydrogen sensor in the zone of the degasser and the electrolyser. A controller brings them together and adjusts the electrolyser current, the bypass and circulation flows, the make-up water, the dosing of the electrolyte, the blowdown of the loops and the switching of the stage order.
SOLVIA AirECOX is SOLVIA's own technological development in the cleaning of air from odorous and harmful impurities; the technology holds patent protection in Russia. The solution brings together a humate pre-scrubber, catholyte and anolyte wet scrubbers, a membrane or diaphragm electrolyser, three hydraulically separated liquid loops, chloride-free regeneration of the working solutions, separate withdrawal of the capture products and automatic control by liquid and gas parameters. Structural parameters, the composition of the solutions, the electrolysis regimes, the control settings and the handling of the capture products are selected for the individual site.
A carbon bed becomes saturated, loses efficiency at high humidity, is clogged by dust and aerosols and calls for regular replacement, with the spent sorbent then to be disposed of. Here the main treatment takes place in the wet scrubbing stages, the working solutions are maintained electrochemically, and the sorption block stands as the last stage rather than the only barrier.
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