The implementation of a fine filtration step on the influent of the WWTP, after the removal of bulky waste, is a good basis for achieving a robust and efficient wastewater treatment process in which the sieved material (screenings) has great reuse potential. The screenings consist to a large extent of cellulose. There was insufficient knowledge about the impact of the removal from the influent of this fibrous stream on the downstream treatment process. This was the focus of the Screencap research at the Aarle-Rixtel WWTP.
The situation at the Aarle-Rixtel WWTP is unique in that the plant has two identical treatment trains. The finescreen installation was incorporated into one of the trains, so that the impact of the finescreens on the operational performance could be closely monitored. The screened material removed by the finescreen installation at the beginning of the treatment process consists primarily of cellulose fibres from toilet paper. This incorporated treatment step increases the processing capacity of the treatment and generates a cellulose-rich residual stream (screenings). These screenings can be used for the production of energy: they are easily digested and produce 2 to 3 times more biogas than does the sludge generated by the downstream treatment process. Screenings can also be used as a fibre source, for instance, for the optimisation of the dewatering installation, or as reprocessed clean cellulose for (road) construction or chemical industry applications. Reprocessing of cellulose is already carried out in the Cellvation installation at the Geestmerambacht WWTP.
The impact of the finescreen technology on the downstream treatment process applied at the Aarle-Rixtel WWTP can be summarised as follows:
- The finescreen technology has no negative or positive impact on the ultimate effluent quality: both treatment trains produced a comparable effluent quality.
- The finescreen technology has no impact on the sludge settling. The SVI was comparable for both trains.
- The excess sludge production in the finescreen train decreased by about 10%.
- Besides the decreased amount of sludge, there was no noticeable impact on the sludge dewatering; the dry matter percentage of the dewatered material did not change nor did the chemical requirements.
- The train with the finescreen technology required about 15% less aeration energy.
- The capacity of the WWTP increased by about 10% through the implementation of the finescreen technology.