At the wastewater treatment plant
The finescreen installation that was built at WWTP Beemster, sieves unsuspended solids from the sewage water. The screens have a mesh size of 0,35 mm and are placed directly behind the rough screen (which sieves out material > 6 mm).
The effect of the finescreen installation on the wastewater treatment process was monitored during 2017. This is what we found:
- 37% of the unsuspended solids in waste water are filtered out by the finescreens;
- the harvested screening material consists for about 43% out of toilet paper (cellulose);
- the waste collected at the finescreens does not reach the aeration tank, as a consequence the energy consumption for aeration decreases 9%;
- 23% less sewage sludge is formed.
So less energy is needed for aeration and for dewatering of sludge. On the other hand the finescreen installation also consumes energy, so overall only 3% less energy is used at the WWTP. Because less sludge is formed, the number of transports decreases and less sludge has to be incinerated. This has a positive effect on the emission of greenhouse gasses. Overall can be concluded that the finescreen installation has a positive effect on the wastewater treatment process.
The harvested screening material from WWTP Beemster is processed by Attero for PLA production. Therefore Attero has built a glucose plant with the following parts:
- hygienization of the screening material at 130°C;
- adding enzymes that convert cellulose into glucose;
- filtering out unsuspended solids from the glucose solution;
- transform the glucose into PLA through fermentation/polymerization.
Unfortunately Cellu2PLA did not succeed in making PLA out of used toilet paper. A glucose solution could be formed, but due to a bacterial infection the quality was not good enough to produce any PLA from it.
Cellu2PLA proved that it is possible to form glucose from cellulose out of sewage. Unfortunately no stable process could be reached, but it demonstrated that this technique has potential. With the knowledge gained Cellu2PLA contributes to the biobased economy. At this moment however, PLA from cellulose out of sewage can't compete with PLA from resources like sugarcane or mais.