Biomass production through the Fitoplan system allows to reach extremely high concentrations, beyond 1 billion cells/ml. A standard system composed of a single six cylinders cluster produces every week about 360 l of high concentration algal solution. When the biomass volume needs to be reduced in order to make the storage at 5°C easier. It is advisable to further concentrate 5/10 times the biomass produced. In the example described above, the super-concentration treatment the final biomass volume obtainable would be of 50 l, reducing the need to stock hundreds liters in a refrigerator.This treatment was traditionally performed with plates centrifuges, that have two important limits:
The plates rotation speed of the centrifuges breaks the cell wall causing the cells death;
The cost of such machinery is extremely high, at least 35,000 EUR.
These two limits, especially the elevated cost, have limited the super-concentrated biomass production above 7 billions cells/ml.Today Separeco introduces a new technology based on microfiltration which maintains the same performances without breaking the cell walls, with a drastic decrease in the machinery cost.The Microplan microfiltration system allows to concentrate high amounts of algal biomass, eliminating the water up to 90%. It is a continuously operating plant, much faster and more efficient than the traditional plates centrifuges. The system, in the single membrane version, is able to concentrate around 150 l per hour, but multi membrane models can increase proportionally the hourly flow rate of the concentrated biomass. The cleaning and regeneration systems is automatized, one night long, allowing the elimination of dead times due to plant cleaning.
Functioning principle
The plant is constituted by a skid in which all the system components are placed. The stocking reservoirs for the biomass to treat, nor the reservoirs for the treated biomass. The biomass microfiltration membrane is placed inside the skid, in an easily reachable position. The membrane works with the transversal flow filtration method. Such method, compared to the direct flow filtration method of the conventional filters, has the benefit to realize a high filtration degree. The transversal flow filtration is a continuous process in which the feeding flow is parallel (tangential) to the filtration surface of the membrane, generating two exiting flows. A small fraction of the feeding, called permeated or filtrated, separates as a purified liquid through the membrane. The remaining fraction, called held or concentrated, contains the particles not filtered by the membrane. The separation is guided by the pressure gap through the membrane.The feeding parallel flow, combined to the turbulence of the boundary generated by the transversal flow speed, sweeps away the particles and the other materials which otherwise would bulk on the membrane surface. The liquid flows inside the membrane at a very high speed parallel to the membrane surface, keeping it clean.
Advantages
Membranes offer several advantages. This separation technology can be defined environmentally friendly compared to the other separation techniques. No additives are required, and they offer a much wider temperature range for the process. Therefore filtration assumes a light character, highly selective without phase modifications. The cost for the system are low.
Hints on the microfiltration technique
Membranes are able to selectively filtrate gases or liquids in solution or mixture, separating the different components. The micropores of the membranes are such to guarantee that some molecules, given their size, are restrained, whereas others are allowed to flow. Each membrane results to be specific thanks to the special molecular structure purposely studied for the components separation. Membranes filtration is considered a BAT in the BREF IPPC (Best Available Techniques Reference Document) for the food industry because of the reduced impact in terms of hydraulic consumption and wastewater production. The applicable techniques are three, and they differentiate on the basis of the micropores size, hence in their retention capacity: microfiltration, ultrafiltration and inverted osmosis.
Microfiltration
Low-pressure membrane process in countercurrent for the suspended colloidal particles, having 0.1-10 μm diameter, separation.It is a strictly physical process, in which particles are kept on the membrane surface. Any particles bigger than the membrane pores diameter cannot pass through it. It is applicable, for example, for fluids sterilization.
Ultrafiltration
In the ultrafiltration the membranes hold the particles having 0.01-0.1 μm diameter under a pressure of 0.5-10 bar. It is a suitable method for the concentration alternative to size exclusion chromatography.UF membranes are interesting for biopharmaceutical applications.The main advantages of ultrafiltration compared to chromatographic techniques are:
- High capacity;
- Easy scale-up;
- Easy cleaning and sanitation of the plants.
Reverse osmosis
Osmosis acts through a semi-permeable membrane, therefore for two saline solutions having different concentrations water passes trough the less concentrated solution to the more concentrated one, until the equilibrium is reached. Acting with an external work (pressure) on the more concentrated solution, water is forced to pass through the membrane, so a higher concentration is obtained (reverse osmosis, RO). The semi-permeable membrane holds any particles having size bigger than water's. It is a high efficiency technique for the concentration/separation of solute molecules having relatively low molecular weight. It is not selective, high energy demanding, and quite costly.