ROPA company founder Hermann Paintner is also a farmer, who operates a NawaRo biogas system at the company's headquarters in Sittelsdorf with the substrates sugar beets and pig manure. Planning for the system began in 2009. The biogas system is not only supposed to provide part of the heat and energy requirements for ROPA production, important insights about the properties of sugar beets as fermentation substrate are also supposed to be gained.

The system was put into service at the end of 2010 as a pilot project, with a 190 kW BHKW; in 2012, it was expanded to an electrical output of 550 kW. The biogas system is fed only with substrates suitable for pumping. Due to the automation, solid material input was avoided entirely. In the course of the last few years, the company has invested in a wide variety of optimizations based on the experience gained. Among other things, the original fixed bed fermenter was replaced in 2017 with a stirrer tank fermenter with a central stirrer with soil ejection produced in-house.

Based on experience, the biogas system at ROPA's company headquarters has established the following process flow for fluid silaging, storage, and removal of the sugar beet mash.

Harvest and loading, as for the sugar beet factory
Whole sugar beets with top are harvested, however without leaves - micro-topping is the right approach. The leaf stalks are removed with a minimal top cut in order to reduce unnecessary soil addition and respiration losses during pile storage. Ideally, the sugar beets are stored in the pile for at least one week, so that the soil dries up and most of the soil can be cleaned off with the sugar beet harvester. Transport to the biogas system is done with tractor teams or trucks.

Shredding for production of fine and homogeneous sugar beet mash
At the biogas system, the sugar beets are ground with a shredder produced in-house into a fine, homogeneous mash and put into the storage silo with a belt conveyor. For cost and labor reasons, there is no time-consuming washing. As a viscous mash, the mash distributes itself in the silo and becomes more fluid in the course of additional sileage. For shredding, the rule is the finer, the quicker the implementation; therefore the better.

Multiple filling due to limited storage capacity
Due to the 6,000 ton restricted storage capacity of the round tank, the first sugar beets are added in September. For space reasons, the last batch is put into the pile by February/March. Thanks to the multiple fillings, up to 12,000 tons of sugar beets per year are converted into electricity.

Storage tank - acid-proof construction
Sugar beets stored as fluid become extremely acidic and are very corrosive. After completion of the silage process, there is a pH value between 3.2 and 3.5. Unprotected concrete is attacked by the acidic sugar beet mash. That's why an acid-proof storage silo is necessary. The storage tank, which is 30 m across and 8 m deep, is cast of local concrete. For acid protection, the inner walls are lined with PU film. The tank floor was created with a slope of 15 degrees to the middle. This is not sufficient for complete draining; a steeper slope would be required for that.

Silaging, conversion, and separation
During the first phase of silaging, there is a significant separation. The enclosed oxygen is used up. Carbon dioxide is produced, some of which escapes as "blubber bubbles" on the surface.
Immediately after silage, a certain volumetric expansion can be observed. That's why the storage tank may never be filled to the upper edge all at one time! A few days after the volumetric expansion, the substrate settles and the tank can be topped up. In the stage of conversion, often only fluid can be pumped up from the tank bottom, for the sugar beet pieces float up - like breadcrumbs in carbonated water. In the course of further sileage, the sugar beet mash settles, it becomes thinner, and more homogeneous.

With the pilot system, the effect of the separation is especially clear since sugar beet mash is removed during sileage for system operation. In addition, the tank is filled up several times with very cold sugar beets in the winter; this means that conversion during the sileage process takes a very long time.  
Digression: when transferring to other systems, it would be advantageous if the sugar beet mash would be passed through the silo only a few weeks after storage. In the winter months up to February, fresh sugar beets can easily be fed in as solids. With increasing temperatures in the spring, however, the respiration losses in the sugar beet pile increase, so that feeding fresh sugar beets is no longer interesting. For the whole year's supply starting in February until September, mash sileage is one of the best and most efficient forms of storage.

Removal of the sugar beet mash from the topmost layer
As a consequence of the separation and continuous refilling of and removal from just one storage silo, the removal was modified at ROPA so that pumping is done optionally from various heights, especially close to the surface. Meanwhile, the removal pump hangs on a crane and draws in mash approximately one meter below the surface. The material that is structure-richer and rising is thus removed first. Since the layer on the surface is sometimes quite tough and viscous, a little fermentation residue/fluid from the fermenter is pumped in via another pump line directly below the removal pump in order to improve the pump's capacity.

With the 2,000 m³ fermenter, the 550kW biogas system can be operated flexibly and in a heat-optimized manner since this fermenter "puts a lot away" and also tolerates large quantities of freshly stored sugar beets or daily rations increased a lot for the short term.

Highlight is the central agitator with sand ejection designed by Hermann Paintner. Thanks to the robust construction, it enables complete separation after just a few rotations. A powerful slewing ring that is 2,300 mm across is driven by a hydraulic unit and 4 oil motors. The power is transferred from the slewing ring to the 15.6 m-wide stirring arms of the central agitator.

For still better mixing, there is a rigid axis with mixing wings between the two stirring arms. For soil ejection, a third arm is installed near the tank bottom. The diagonally fastened blades push soil and sediment out. The outer slide gate pusher pushes the sediment along up to a deeper point in the tank bottom. Soil, foreign bodies, and accumulated sediment are pushed out of the fermenter by a hydraulically driven auger. The lowest arm for the soil ejection can move freely. The soil ejector turns along clockwise. Only the two stirring arms turn counter-clockwise. The duration and number of stirring and cleaning intervals can be set with the system control. Time-consuming tank cleaning and reduced volume due to deposits are prevented consistently by the central agitator with sand ejection.