Inagro pilot plots offer good test case for constructed wetlands, woodchips filter and MBBR
The drains at Godelieve fields were diverted to a pond acting as buffer. From this pond, the MBBR and constructed wetland are fed. Because of height differences, a part of the plot could not be diverted to the pond. The drainage water from this part of the plot is also buffered and then flows through a woodchip filter. The operation of the MBBR and constructed wetland was monitored for three consecutive winters. The operation of the woodchip filter was monitored during the last winter.
Installation of different filter systems at the Godelieve fields of Inagro
MBBR doesn't meet the expectations
A MBBR or Moving Bed BioReactor was introduced to the agricultural sector by PCS Ornamental Plant Research about 10 years ago. PCS uses this technique to remove nitrates from own wastewaters from their research site. Thus, this process still occurs under fairly controlled conditions. In collaboration with KU Leuven, the translation to the application on drainage water took place.
MBBR at Godelieve fields, field test site of Inagro in Roeselare
The heart of an MBBR is a 'reactor tank' containing carrier material. Denitrifying bacteria growing on the carrier material (in our case AnoxKaldness®) convert nitrate into nitrogen gas. Carbon is required for this conversion and therefore glycerol (CarboST) is dosed into the reactor tank.
For the past three winter years, the MBBR processed drainage water (collected in a pond) from the Godelieve fields. As long as the MBBR works for 100% perfect, the denitrification is very well. But in the second and third winter year, we had several technical problems or the water temperature was too low (less than 5°C) which hampered the denitrification.
Taking into account the experiences of other agricultural research centers, we can conclude that an MBBR is a good option for the treatment of effluent (ornamental horticulture, greenhouse horticulture, ...) and when drainage water can be collected underground and "treated" underground. When the drainage water ends up in a pond and the MBBR is above ground, proper operation is less evident due to cold temperatures in wintertime.
Constructed wetland meets expectations
A constructed wetland (CWL) is a system in which drainage water flows alternately through sand banks and into a basin. The system constructed at Inagro is divided into three sections by two sandbanks. The collected drainage water (from the same pond as the water flowing through the MBBR) flows into the first compartment of the CWL. It then passes a first time through a sandbank, followed by a second compartment, a second sandbank and finally a third compartment. A drainage system at the end of the third compartment allows the low-nitrate water to leave the CWL. Straw was mixed into the sandbanks acting as carbon source and reed was planted on the sandbanks. The dead reed roots also serve as a carbon source and with time will also be able to replace the straw as a carbon source.
Constructed Wetland at Inagro (Godelieve fields in Roeselare)
A CWL takes up a lot of space. The size of the CWL at Inagro's trial fields is about 2% of the drained area. But once the CWL is in place, there is almost no need to worry about it.
The past three winter years nitrate levels decreased well below 50 mg ( or 11.3 mg Nitrate-N) nitrate/L, when flowing through the CWL, even during periods of frost.
Wood chip filter: compact and efficient
A wood chip filter is a system in which drainage water flows through a basin filled with wood chips. Such a system was also installed at Inagro and treats drainage water (from another part of the trial fields). The drainage water is first buffered after which it flows through the woodchips bassin. In this system, woodchips are the carbon source.
Compared to a CWL, an woodchips filter takes up less space. Over time the wood chips will need to be replaced, but otherwise this system requires no work either.
The wood chips filter only worked last winter. After passage through the wood chips filter, the nitrate level was below detection limit which means a 95% nitrate reduction.
