High phosphate concentrations due to VFA production from sieved activated sludge at WWTP Ommen

Abstract

Environmental guidelines (EU WFD) impose a strict effluent phosphate target on wastewater treatment plants (WWTP). An abundance of phosphate (and other nutrients) in a water body is called eutrophication. Eutrophication can lead to algal blooms which result in very low oxygen concentrations in the water. Other species, like fish and aquatic plants, can die because of the lack of oxygen. Most WWTPs in the Netherlands are performing well and reach phosphate effluent concentrations below 1 mg/l. Nevertheless, Dutch water authorities are looking for ways to improve nutrient removal to perform even better then the WDF guidelines. At the same time they are anticipating on the expectation of more strict phosphate effluent targets in the future.

This research is focusing on VFA production from the sieved fraction of activated sludge. Sieves with a mesh size of 0.5 mm were used to sieve cellulose fibers from the sludge. Microbial conversion processes are stimulated in the VFA production reactors. A pilot plant that was performed resulted in very high phosphate concentrations in the reactor. The first phase of this research was devoted to identifying the source of the high phosphate concentration. The second phase was focused on phosphate removal and recovery strategies.

It was found that 20% of the sievings consisted of PAO sludge. The conditions in the VFA production pilot lead to extreme phosphate release by PAO. This resulted in high phosphate levels ranging up to 200 mg/l.

The released phosphate potentiates recovery methods. It was not able to precipitate struvite at the low pH levels at which the VFA production reactor was operated. Calcium carbonate addition could decrease phosphate levels (50%) and can be used to control pH in the reactor. The applied method does not allow for easy recovery because the precipitate ends up in the sievings. Finally vivianite precipitation was achieved by adding FeCl2. 90% phosphate removal was achieved. The para-magnetic property of vivianite allows for easy recovery with a magnetic separator. This results in the production of 2 valuable streams from a waste stream: VFA and vivianite.