Microalgae-bacteria biomass was digested through anaerobic digestion (AD) to produce biogas. Since microalgae typically have a more resistant cell wall than activated sludge (AS), its anaerobic biodegradability is limited. Therefore, three different pre-treatment techniques were evaluated for their effect on anaerobic biodegradability, especially of the extracellular polymeric substances (EPS). Firstly, heat at 70 °C for 1.5 h. Secondly, exposure to a mixture of commercially available enzymes (cellulase, α-amylase and protease), and thirdly, adding crude hydrolytic enzymes extracted by ultrasonication from a highly loaded anaerobic digestion cascade reactor. The first and third treatments had better effects than enzymatic pre-treatment with commercial enzymes, improving anaerobic biodegradability, the solubilisation of organic matter and increasing the methane production rate by 78 and 21 %, respectively. The EPS content of microalgae-bacteria biomass was considerably lower than reported for WAS, and about 40–50 % of the EPS consisted of proteins and polysaccharides. The hydrolysis of proteins and polysaccharides was quantified, and its effect on AD was discussed. A COD balance showed that the increase of soluble COD is due to the conversion of tightly bound EPS into loosely bound and soluble EPS but also due to the release of organic matter from cellular material. Although all pre-treatments increased the soluble organic fractions, especially those corresponding to EPS, none significantly improved the overall methane yield. Nevertheless, the methane production rate increased after thermal and pre-treatment with hydrolytic enzymes, which could result in smaller and more efficient anaerobic reactors.@en

Removal of nitrogen from wastewater without using electricity consuming aerators was previously observed in photo-bioreactors with a mixed algal-bacterial biomass. Algammox is the particular process based on algae, ammonium oxidizing organisms and anammox bacteria. In this research the activity of anammox bacteria in such an oxygen-producing environment was tested, as well as the effect of short-duration increase in dissolved oxygen (DO) to values potentially inhibiting anammox activity. Sequencing batch photo-bioreactors were fed with settled domestic wastewater enriched with ammonium (200 mg NH4 +-N/L) and exposed to light within the photosynthetic active range with intensity of about 500 μmol/m2·s. Each cycle consisted of 12 h illumination and 12 h darkness. A well-settling biomass (10 days solids retention time) developed that carried out nitritation, nitrification and anammox. Ammonium removal rate during the light period was 4.5 mg N-NH4 +/L·h, equal to 858 mg N-NH4 +/m2·h or 477 mg N-NH4 +/(mol photons). When the reactors were aerated for 3 h to temporarily increase the DO, anammox was inhibited at bulk DO values larger than 0.4–1.0 mg/L. For almost oxygen saturated conditions, recovery time was about 9 days. Algammox photo-bioreactors are therefore able to overcome short periods of oxygen stress, provided they occur only occasionally.@en

This study demonstrates the feasibility of combining microalgae, ammonia-oxidizing bacteria (AOB), and Anammox in a photosequencing batch reactor. Alternating light and dark periods were applied to achieve biological nitrogen removal without mechanical aeration or external electron donor addition. This process is termed ALGAMMOX (algal anaerobic ammonium oxidation) and differs from the SHARON-Anammox process in that oxygen is generated during light periods through microalgal photosynthesis, replacing mechanical aeration. Results from bench-scale ALGAMMOX experiments with high-ammonia strength wastewater (COD/TN from 1 to 3) showed that influent ammonia was converted to nitrite during light periods at a rate of 7.0 mg of NH4 +-N L-1 h-1. Nitrite was subsequently reduced by an average of 82% during the dark (anoxic) periods due to Anammox activity. Further studies are needed to optimize the system to maximize nitrogen removal rates and to assess long-term process stability.@en