Ph.D. researcher Nadine Boelee looked into different scenarios to use microalgae biofilms to clean wastewater. Algal biofilms are especially efficient when algae grow together with bacteria. Boelee concluded that these symbiotic biofilms are very promising for year round wastewater cleaning in countries at lower latitudes, where temperature and irradiation are sufficiently high.
Conventional wastewater plants use microbial processes, sometimes in combination with suspended microalgae and/or chemicals to remove nitrogen and phosphorous from wastewater. In The Netherlands, the maximum concentrations of these nutrients in discharge water are by law 10 mg/ml for nitrogen and 1 mg/l for phosphorous. For discharge in ‘sensitive’ waters, guidelines are even more stringent: 2.2 mg/l and 0.15 mg/l for nitrogen and phosphorous, respectively. In 2015 EU regulations require strict purity guidelines for all water bodies. To reach these targets, post treatment of wastewater effluent can be needed. Microalgae could be a sustainable alternative to existing (post) treatment methods since they use nitrogen and phosphorous for growth. Microalgae could be a sustainable alternative to existing (post) treatment methods since they use nitrogen and phosphorous for growth.
Microalgae may grow in suspension, but also in biofilms. A biofilm looks as a slimy, green layer and consists of large numbers of microalgae entrapped in a gel-like matrix (fig. 1). They may grow on almost any surface. Removing nutrients from wastewater by using algae biofilms is potentially more sustainable than traditional methods. Less energy and chemicals are needed, while microalgae utilize CO2 as a carbon source and produce valuable biomass. Microalgal biofilms have several advantages over suspended algae cultures. ‘Biofilms grow on a surface separate from waste water, and can easily be harvested by simply scraping off the algae layer’, Boelee explains. ‘Biomass density of a biofilm is hundreds of times higher than that of algae in suspension. Consequently, harvesting requires no energy-consuming centrifugation step.’ (fig. 2).
Lab scale reactor
Boelee investigated the possibility to use microalgae biofilms to reduce nutrients in wastewater to reach 2015 emission targets. In collaboration with Wetsus, Leeuwarden, she tested their performance using both lab scale and pilot scale phototrophic biofilm reactors of different designs.
First she built a lab scale reactor aimed at investigating the capacity of microalgal biofilms as a post treatment step for the effluent of a wastewater plant. The maximum uptake of nitrogen and phosphorous by the algae was 1.0 and 0.13 g/m2/day respectively, while target values of the new EU regulations were still reached. She estimated that a full scale post-treatment reactor of this design would need about 10 ha per 100.000 people, producing 2000 kg of dry biomass per day.
Pilot scale reactor: lower productivity
An up-scaled pilot version of the laboratory reactor was built to test its capabilities ‘in real life’ at the municipal wastewater plant in Leeuwarden (fig. 3) from June to October. The algae biofilm was growing on a vertically placed carrier material, where a continuous thin layer of wastewater was flowing over.
This vertical system had substantially lower productivity than the original lab version and consequently also lower nitrogen and phosphorous removal rates: 0.13 g/m2/day and 0.023 g/m2/day, respectively. This amount varied due to light availability and temperature fluctuations during the day. Target values for 2015 were not reached. ‘Unfortunately, this type of reactor is unsuitable for the Dutch climatic conditions, possibly because temperatures and irradiance are limiting’, Boelee concludes. ‘As a result, nutrient removal is too low, while the surface area needed is too large.’ The reactor can be made more energy-efficient, by placing the carrier material horizontally, instead of vertically, resulting in reduced pumping. In addition, automatic sampling, pH control and limiting heat loss are important tools to increase reactor efficiency.
Further experiments with a symbiotic microalgal biofilm, where algae and bacteria grow in the same biofilm, showed promising applications for this design. Boelee studied this biofilm in the lab (fig. 4), with ammonium, phosphate and acetate mimicking a degradable organic pollutant. Microalgae remove nitrogen and phosphorous, use carbon dioxide (CO2), while producing oxygen.
The bacteria use oxygen (O2) for aerobic degradation of organic pollutants, while producing CO2. Once the system was running, there was no need to supply extra O2 or CO2. The symbiotic biofilm showed a large capacity to clean waste water with relatively high nutrient concentrations, removing 3.2 g nitrogen and 0.41 g phosphorous/m2/day. ‘If this symbiotic biofilm is used as a single method to remove all nitrogen and phosphorous from wastewater, nutrient concentrations should be adapted to available light intensity, otherwise a second cleaning step is needed’, Boelee concludes. ‘In southern locations the symbiotic biofilm is a very promising method for year-round wastewater cleaning.’