The use of microalgae as method for phosphorus removal from a human derived waste stream. From a lab scale to a household scale cultivation system

SUMMARY In this research the possibility of two different microalgae species to remove phosphorus (P) from anaerobically digested human excreta has been investigated. This research was performed on two different levels of scale. First research was performed within a lab scale setting, and then the data derived from these experiments was used to design a household scale cultivation system. The significance of this research is that the phosphate rock reserves, which are used for the production of artificial fertilizers, are going to be depleted within the next 50-100 years. Artificial fertilizers are needed for the production of high yielding crop varieties to feed the ever growing human population. By human activity the natural phosphorus cycle has been disturbed leading to large losses of phosphorus into waterways. This has resulted in eutrophication of waterways, which is a worldwide problem. Since we depend strongly on phosphorus for our food supply and to prevent the eutrophication of waterways, there is a necessity to recover this essential element from sinks and to close the phosphorus cycle again. Especially urban areas are point sources of phosphorus losses caused by the high phosphorus content of humane excreta. With the use of a new sanitation concept, developed by the company of DeSaH, this phosphorus rich stream is being concentrated and used for biogas production through anaerobic digestion. The residue that remains after digestion, called effluent, still contains a significant amount of phosphorus. The cultivation of algae on anaerobically digested animal waste streams to remove phosphorus has proven successful. The produced algal biomass can be used as a biofertilizer to partially close the phosphorus cycle again. Less research has been done on the ability of algae to remove phosphorus from human derived waste streams. Therefore the following main research question has been formulated: What is the possibility of microalgae, cultivated on anaerobically digested human excreta, as method for phosphorus removal? At the experimental level, P. tricornutum and D. tertiolecta were cultured at 10%, 20% and 50% effluent medium. The growth and phosphorus removal of these algae on these three media was compared to the growth and phosphorus removal on a standard growth medium. Both P. tricornutum and D. tertiolecta were able to grow and remove phosphorus from the 10% effluent medium. P. tricornutum was also able to grow and remove phosphorus from the 20% effluent medium. Both species were not able to grow on the 50% effluent medium. The highest phosphorus removal was achieved by D. tertiolecta within 4 days on the 10% effluent medium. When theoretically enlarging this cultivation system for all the households of the Netherlands using outdoor raceway ponds, around 4 million kg op P could be removed from anaerobically digested human excreta. The area needed for such a system would comprise of 2.6 m2 per household. On an experimental scale algae are able to remove phosphorus from anaerobically digested human excreta. However, this concept is technically not feasible within the Netherlands for the next 20-50 years. This is due to the already existing improved sanitation infrastructure and the expected societal resistance to such an outdoor system, because of health and odor concerns. Within developing countries this concept could help improve accessibility to improved sanitations. However, the water demand and area needed by this cultivation system is also seen as a bottleneck within densely populated areas. In order to overcome technical and societal bottlenecks further research is needed on 1) the implementation of large scale algae cultivation using different cultivation technologies and algae, and 2) the ability of algal biomass to be used as biofertilizers in order to close the phosphorus cycle, especially concerning the health aspects and the transferability of pathogens.