HYDROTECH MBR is a membrane bioreactor in which the treated water is separated from the mixed liquor by an ultrafiltration module rather than by a secondary clarifier. Anaerobic, anoxic and aerobic zones, a membrane separation zone and fixed biofilm media are gathered into one scheme with a controlled nitrate recycle.
The development brings together in one structure a biological scheme of anaerobic, anoxic and aerobic zones, membrane separation and attached biofilm. The ultrafiltration module stands in a separate membrane bioreactor zone and holds the activated sludge inside the system; the fixed biofilm media sit in the aerobic zone, fine-bubble aeration works there as well, coarse-bubble aeration under the membrane module, and the nitrate recycle runs off the same pump that feeds the mixed liquor to the membranes.
It is meant for compact treatment where the discharge has to meet raised requirements and where there is not enough room for a classical scheme with a clarifier. Holding the biomass with a membrane allows work at a raised concentration of activated sludge and yields a flow of steady quality after the biological stage, fit for polishing by sorption, ultraviolet disinfection or reverse osmosis and for the reuse of water for technical purposes.
Membrane separation closes the task of parting the treated water from the sludge, but it does not set the behaviour of the biological part. In operation that shows up as unstable phosphorus removal, as nitrates carried into the anaerobic zone, as an uneven distribution of the mixed liquor across the membrane zone and as rapid fouling of the membranes, and after them grow the load on the module, the number of pumps, the energy consumption and the labour of maintenance.
In HYDROTECH MBR the membrane module is not a separate filter at the end of the scheme but one of the zones of a single system, where the biology, the hydraulics, the recirculation and the membrane separation are set up together. Hence the reliance on the biological removal of phosphorus: plants that take phosphorus out with a reagent produce chemical sludge and depend on continuous dosing, while AO-MBR schemes without a separate anaerobic zone close the biological route altogether.
Biological phosphorus removal proceeds only where the anaerobic zone holds neither oxygen nor nitrates: phosphorus-accumulating organisms release phosphorus there and store internal carbon compounds, which are then spent at the following stages. The nitrate recycle, without which there is no denitrification, is what disturbs that very regime — even a small carry-over of nitrates worsens the conditions for those organisms and lowers the stability of phosphorus removal.
The separation here is structural rather than a matter of regime. The partition between the anaerobic and the anoxic zones is made with a bottom opening along its whole length, and the mixed liquor passes into the anoxic zone from below; the nitrate recycle, on the contrary, is fed into the upper part of that zone. The flows stratify by height, the nitrate-bearing medium is held above, and the carry-over of nitrates back into the anaerobic zone is impeded. It is by this detail that the plant differs from A²O-MBR schemes.
Fouling of the membranes is the main operating limit of membrane bioreactors. As the deposits grow the transmembrane pressure rises, the throughput falls, cleanings become more frequent and the running costs go up, and so the design of the membrane zone answers not so much for the quality of the filtrate in the design regime as for how long that quality holds without intervention.
Two solutions work against fouling. Coarse-bubble aeration under the membrane module creates an intense movement of liquid at the surface of the fibres and cleans them mechanically. The distribution device at the inlet of the membrane bioreactor zone delivers the mixed liquor not as a single jet but through a manifold with evenly spaced outlet openings, so that the flow spreads across the section of the zone and the local overloads of the membrane block are reduced.
In A²O-MBR schemes the feed of the mixed liquor into the membrane zone and the nitrate recycle are usually served by separate pumps. Here one pump works: it delivers the liquor from the aerobic zone into the membrane bioreactor zone, and an adjustable branch is taken off its pressure pipeline, directing part of the flow back into the anoxic zone. The share of the recycle is set at that branch.
The consequences of the solution belong to operation rather than to the depth of treatment. The installed power and the number of pumps go down, the pipework is simplified, the structure becomes more compact. For local treatment works this is a marked part of the solution: there reliability, ease of maintenance and energy consumption weigh no less than the design figures for nitrogen and phosphorus.
The media are held in the aerobic zone permanently, and in this the scheme differs from MBBR-MBR with its mobile carriers. A mobile fill calls for constant mixing and for a retention screen, and in a plant with membranes it adds the risk of mechanical action on the module as well. Fixed carriers do not touch the membranes, no screen is needed to hold them back, and the biofilm grows on a motionless surface.
The outer layers of the biofilm work under aerobic conditions, in the inner ones the oxygen content is reduced, and simultaneous nitrification-denitrification is possible there: part of the nitrogen is removed inside the film itself and not in the anoxic zone alone. The oxidation of organics and the nitrification of ammonium nitrogen proceed in that same zone on the same carriers, while the attached biomass develops on a stable surface, and the steadiness of the biological process rises.
HYDROTECH MBR is SOLVIA's own technological development in the deep biological treatment of wastewater. The solution joins anaerobic, anoxic and aerobic treatment with a membrane bioreactor, fixed biofilm media, a controlled nitrate recycle and protection of the membranes from fouling. The technology is being developed as a product platform for sites where compactness, the quality of the treated water, a lower dependence on chemical reagents and steady work under variable loads matter. Structural parameters, the tuning regimes and the layout decisions are chosen individually for the particular site.
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