Leachate is a complex effluent that is hard to treat. It is characterized by, amongst other things, a highly variable composition, a high conductivity (high salt level), high nitrogen load and a limited carbon source (low BOD level).
This complex effluent needs to be purified in a high-quality way so that it can be discharged without causing any damage to the environment. Hooge Maey has its own biological water purification unit that is recognized as one of the best performing units in Western Europe.
As the old leachate in the landfill is processed and the landfill is being capped, the water purification unit will no longer operate at full capacity. This now provides additional space for the high-quality treatment of third-party effluent.
From the landill the leachate is pumped into the water purification unit through a buffer tank.
Through a pre-denitrification area the water ends up in the circuit reactor. This reactor consists of an aerated area (nitrification) and non-aerated areas (denitrification). A propeller drives the sludge-water mixture at high speed through the circuit reactor. In this way ammonium is converted into nitrate (in the aerobic area) and nitrate into nitrogen gas (in the non-aerated areas).
From the circuit reactor water will flow to the membranes through the post-nitrification area. By means of these membranes the sludge is then separated from the water. The membranes are of the ultra-filtration type.
This ultra-filtration unit consists of two parallel sets each with six modules of cross-flow membranes. Each module consists of a large number of membrane tubes.
In these membrane modules the sludge-water mixture flows at a high long-flow rate of 4.5 m/s (so that contamination has no chance to settle).
The water that is flowing through the membrane wall, the so-called permeate, is almost 100% free of suspended matter. The residual flow (the concentrate) goes back to the biological unit.
The second phase comprises the removal of non-biodegradable organic compounds. As a matter of fact, after the biological purification of the leachate, a number of persistent contaminations remain in the effluent, due to the presence of humus acids. The water is further purified by active carbon filters. The active carbon filters are positioned in two parallel lanes. Each lane has three serial filters. Each filter contains 10,000 kg of active carbon. The purified water is discharged into the 'Verlegde Schijn' (re-routed Schijn river).
The effluent quality is subject to permanent monitoring. If the quality does not comply, the effluent is automatically sent back to the installation.
Probably the most eye-catching feature of the water treatment unit is its enormous capacity for nitrogen removal .
In general, the total load of nitrogen of the incoming leachate is 400 mg N/l. The discharge conditions for ammoniacal nitrogen and nitrates must be below the 10 mg N/l standard. The total nitrogen load of the discharged water cannot exceed 40 mg N/l. The fact that the water treatment unit of Hooge Maey complies with these discharge standards, shows the high performance level of nitrogen removal. Therefore, it cannot be denied that both the nitrification and the denitrification processes are performing excellently.
The two most important parameters determining the capacity of a water purification unit are the following:
- hydraulic load: 20 m3/h
- biological load: 700 kg N/d
When compared to household waste water, these values correspond to the equivalent of:
- hydraulic: 3.200 inhabitants
- biological: +/- 100.000 inhabitants