ROPA company founder Hermann Paintner is also a farmer, who operates a NawaRo biogas facility at the company's headquarters in Sittelsdorf with the sugar beet and pig manure substrates. The planning of the facility began in 2009. The biogas facility is not only meant to cover part of the heat and energy requirements for ROPA's production, but also to gain important insights about the properties of sugar beet as a fermentation substrate.

As a pilot project, the facility was put into operation at the end of 2010 with a 190 kW CHP unit and expanded in 2012 to an electrical output of 550 kW. The biogas system is only fed with substrates suitable for pumping. Due to the automation, no solids are added at all. In the last few years, the experience gained has led to repeated investments in a wide range of optimisations. Among other things, the original fixed-bed fermenters were replaced in 2017 by a stirrer tank fermenter with a self-made central agitator with soil ejection.

Based on the experience, the following process sequence for the liquid storage, storage and removal of the sugar beet mash has been established for the biogas system at ROPA's headquarters.

Harvesting and loading, as for the sugar beet factory
Whole sugar beets with crowns 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 it can be cleaned off with the sugar beet loader. The beets are transported to the biogas system with tractor-trailers or trucks.

Shredding for production of fine and homogeneous sugar beet mash
At the biogas facility, the sugar beets are ground with a self-constructed shredder into a fine, homogeneous mash and fed into the storage silo via a belt conveyor. For cost and labour reasons, extensive washing is deliberately avoided. The viscous mash spreads itself in the silo and becomes more fluid in the course of further ensiling. When it comes to shredding, the rule is the finer, the quicker the implementation; therefore the better.

Repeated filling due to limited storage capacity
Due to the 6,000 ton limited storage capacity of the round tank, the first sugar beets are added in September. For space reasons, the last batch is stored in the pile until February/March. Thanks to the repeated filling, up to 12,000 tons of sugar beets per year are converted into electricity, depending on the harvest.

Storage tank - acid-proof construction
Sugar beets stored as fluid become extremely acidic and are very corrosive. After completion of the ensiling process, the mash has a pH value between 3.2 and 3.5. The acidic beet mash attacks unprotected concrete. That's why an acid-proof storage silo is necessary. The storage tank with a diameter of 30 m and a height of 8 m is cast of in-situ concrete. For acid protection, the inner walls are lined with PU film. The tank floor was constructed with a slope of 15 degrees to the centre. This is not enough for complete emptying, which would require a greater slope.

Ensiling, conversion and segregation
Significant segregation can be observed during the first phase of ensiling. The enclosed oxygen is used up. This produces carbon dioxide, some of which escapes as "bubbles" on the surface.
A certain volume expansion can be observed immediately after ensiling. For this reason, the storage tank should never be filled to the upper edge at one go! A few days after the volume expansion, the substrate settles again and can be topped up. In the conversion stage, often only liquid can be pumped out from the bottom of the storage tank, and the beet pieces float up like breadcrumbs in mineral water. In the course of further ensiling, the sugar beet mash settles, becomes thinner, and more homogeneous.

In the pilot facility, the effect of segregation is particularly evident, as beet mash is also removed during ensiling for facility operation. In addition, the tank is filled up several times with very cold sugar beets in winter; this means that conversion during the ensiling process takes a very long time.  
Digression: if applied to other facilities, it would be advantageous if the sugar beet mash first ferments through for a few weeks after ensiling. 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 loses its benefit. For the supply starting from February to September, mash silage is one of the best and most efficient forms of storage.

Removal of the beet pulp from the top layer
As a consequence of the segregation and continuous refilling of and removal from only one storage silo, the removal at ROPA was modified in such a way that the pumping can be performed from different heights, in particular also close to the surface. Meanwhile, the removal pump hangs on a crane and draws in mash about one metre below the surface. Thus, the more structure-rich and floating material is removed first. Since the layer on the surface is sometimes quite viscous and thick, some fermentation residue/liquid from the fermenter is pumped in via another pump line directly after the removal pump in order to improve the pumping ability.

With the 2,000 m³ fermenter, the 550kW biogas facility 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.

Particular highlight is the central agitator with sand ejection designed by Hermann Paintner. Thanks to the robust construction, it enables complete blending after just a few rotations. A powerful slewing ring with a diameter of 2,300 mm 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 even better blending, a rigid axle with mixing blades is fitted between the two agitator arms. For soil ejection, a third arm is installed near the tank bottom. The diagonally fastened blades push soil and sediment out. The outer carrier pusher pulls the sediment down to a recess 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 moves freely. The soil ejector rotates clockwise. Only the two stirring arms rotate counterclockwise. The duration and number of stirring and cleaning intervals can be set via the system control. Time-consuming tank cleaning or reduced useful volume due to deposits are consistently avoided by the central agitator with sand ejection.