Lessons learned after three seasons of intensive follow-up of a moving bed bioreactor (MBBR)
Highly loaded drainage water
The 15 m3 underground MBBR filter, located in Onze-Lieve-Vrouw-Waver (Belgium), processed in the past 3 years more than 6500 m3 of drainage water with a high concentration of nitrate. The drainage water originates from a 1.43 ha tile-drained field of a grower of cauliflower and chrysanthemums. It was demonstrated that the MBBR installation can remove 70 to 74% of the nitrate and this even at low winter temperatures. In Figure 1, the nitrate concentration in the drainage water, in the effluent from the MBBR, in the ditch and at the measuring point of the environmental agency over the last 2 drainage seasons can be derived. During the 2020-2021 season, the reduced removal efficiency that occurred at a given time was resolved by intensifying mixing in the reactor.
Optimized carbon source dosing
In 2021-2022, the primary intention was to minimize carbon source dosing to reduce the operating cost of the MBBR. At the time when the discharge standard could not be met, somewhere around day 400, the dosage of the glycerol was systematically increased. In this way, the optimal carbon to nitrogen ratio (C/N) that must be maintained could be determined. In addition to optimizing carbon source dosing, carbon leaching to the ditch was also addressed during the 2021-2022 season.
A downstream aerated MBBR of 1 m3 was installed to remove residual carbon from the effluent. An additional benefit of the aerated MBBR is that the effluent discharged into the ditch is rich in oxygen, which is important to many forms of aquatic life.
The improvements to the operation of the MBBR (optimization of mixing and carbon dosing and effluent polishing via the downstream aerobic MBBR) ultimately increased the removal efficiency for nitrate by 9% while consuming 34% less carbon source relative to the 2020-2021 season (See Figure 2). This ultimately resulted in a cost savings of 15%.
Overall, it can be concluded that the MBBR technology is becoming mature for implementing in the agricultural sector!
Figure 1: Nitrate measurements for the pilot MBBR plant at Onze-Lieve-Vrouw-Waver in Belgium for the last two growing seasons.
Figure 2: Comparative parameters for the last two growing seasons: 2020-2021 and 2021-2022.