The waste water from the three paper mills enters IWE through a sewer pipe. This water contains both solid and dissolved matter. The solids (mainly fibres, chalk and clay) are separated from the water containing dissolved (mainly organic) matter by sedimentation.
Gas en waterleidingstraject vanaf IWE naar DSSThe dissolved substances are removed from the waste water in a three-stage biological purification process. In order to carry out this process, the bacteria need nitrogen and phosphate, as well as the pollutants in the water. These nutrients are supplemented by adding specific waste water from other companies. This makes the use of chemicals unnecessary.
In the first stage of the purification process, bacteria convert around 80% of the dissolved organic matter into biogas. This takes place under anaerobic (oxygen-less) conditions (click here for more information about anaerobic purification). Bacterial growth is regularly removed and sold to new purification plants or plants that have lost their bacteria.
In the oxygen-rich second stage, residues (around 5%) of easily removable organic matter are cleared from the water, which also reduces odour emissions. The component of organic pollution that is more difficult to remove is dealt with in the third stage. Stage three is a robust process that uses aerobic (oxygen-loving) bacteria. By the end of stage three, the water is clean.
Bacteria and water are separated in the final process. At this point, the water can be discharged into the river IJssel.
The bacteria are returned to the start of stage three of the process, to be reused as decontaminators. Bacterial growth is constantly removed and transferred to sludge processing.
Part of the purified water flows to the return water processing facility, where excess dissolved calcium is removed, without the addition of any further chemicals. Following a series of filtration stages and disinfection, the return water is returned to the DS Smith de Hoop paper mill, where it is used as process water, and partly as a replacement for groundwater. IWE also uses a small proportion of this water to replace groundwater. The calcium released during this process is 80% dewatered, and sold as agricultural chalk fertiliser (for more information about the preparation of return water click here).
The biogas released during anaerobic purification has 99% of its sulphur-bearing substances (H2S) removed, using a washing fluid. In a biological reactor, the contaminated washing fluid is regenerated by separating off the sulphur (for more information about the biological biogas desulphurisation process click here). The washing fluid can then be reused. Any remaining moisture and the final H2S residues in the biogas are removed before the gas is supplied back to DS Smith De Hoop, as a replacement for natural gas.
The settled sludge from the paper mills and the bacteria that are removed from the final anaerobic purification process are dried using biogas, following dewatering. As a result, the sludge is no longer biologically active, no longer releases any odours and is suitable as a base product for a range of applications. We are currently researching the best potential application