“In addition to domestic wastewater we have a relatively large amount of wastewater from industrial areas,” explains plant manager Klaus Jansenn. “That’s why the levels of phosphorus in the incoming water are relatively high, about 15 mg/l on average. Although our permissible limit for phosphorus at the entry point is 2 mg/l, we strive not to exceed the target limit of 0.5 mg/l.
Problems with diaphragm pumps
To achieve this reduction the wastewater plant uses chemical coagulation which converts the phosphate solution in the water into soluble phosphate compounds that can be isolated later. At Riepe a diaphragm pump supplied the coagulant from underground storage containers to the sewage tank, and the actual dosage was managed by two additional diaphragm pumps, one in permanent use and the other in standby mode in case of failure. In fact, breakdowns were an increasingly common result as diaphragms often corroded due to chemical coagulants and had to be replaced.
“Diaphragm pumps also produce an intense pulsating flow which, after a while, caused wear on fittings and valves,” says Jansenn. “Maintenance work started to accumulate hours per month, what’s more, we had to keep a complete set of spare parts including diaphragms and valves in stock”.
Another problem with the diaphragm pumps was suction. Air was getting in and clogging up the valves, preventing the diaphragm pumps from priming and pumping. The result was that the limits of phosphorus discharged into the plant were increased and, in the worst case, the competent authorities had to be informed. To avoid this, the frequency of emergency operations at night or at the weekend was increased.
The problem occurred more frequently when stocks of coagulant were replenished. For this, the chemical dosage pipes had to be manually filled with water in order for the diaphragm pumps to work.
Operation with high phosphate levels
The new Qdos 30 peristaltic pump of this plant doses 150 ml/min, although it offers a maximum flow of 500 ml/min, more than enough for the occasions when there are higher levels of phosphate in the water. The independent Qdos pump is capable of sucking the ferric chloride III directly from the storage tank, which means that it is no longer necessary to have a booster pump, the backup pump and the holding tank, plus all the associated float switches and electronics.
“The accuracy of the diaphragm pumps decreased over time due to corrosion and wear of the diaphragms, whereas the Qdos pump is extremely accurate, so we have been able to reduce the frequency of recalibration” concludes Jansenn.
In total, Janssen estimates that the Qdos pump has generated time savings of between two and five hours per month. “By getting rid of this workload and also saving on spare parts, the investment has quickly paid off.