Jeremy Ayre

Anaerobic digestate animal effluent (ADAE) contains high N and P nutrients which need to be treated. In this study, an integrated process was proposed using a microalgae consortium of Chlorella and Scenedesmus. The system was designed for 71 m3/d (medium-sized) and 355 m3/d (large-sized) animals of ADAE. Process simulation estimated to produce 83–417 kg d-1 of microalgae biomass which can be used as further products. As much as 2 kg of animal feed and 36–180 L/d of bio-oil can be produced during the treatment of 1 m3/d of ADAE. The produced biogas can generate 247–1,217 MWh y-1 of electricity. Likewise, the process can reduce greenhouse gas emissions by 2 kg-CO2eq kg−1 of hot standard carcass weight (HSCW). This integrated system offers merits in treating ADAE as well as producing chemicals and energy with low environmental burdens.

Within the biorefinery concept, microalgal cultivation has potential as one component of the wastewater treatment toolkit for anaerobic digestates. Recovering nutrients from digestate such as anaerobic digestate of piggery effluent, has been well demonstrated with Scenedesmus sp. and Chlorella sp. in mixed cultures. Less understood during microalgae cultivation, is the participation of bacterial communities as they play a fundamental role in biological nutrient cycling processes with potential to optimise algal productivity and nutrient recovery. To this end, we batch cultivated microalgae on increasing concentrations of digestate (250, 500 and 890 mg N NH4+ L−1), took samples under time series and quantified culture conditions including water chemistry properties with a focus on nitrogen values during treatment. Chlorophyll and dry weight were measured to provide reasonable estimates of the health of the microalgal culture. We additionally characterised the bacterial community using next generation 16S rRNA sequencing on the ION torrent, followed by an in-silico analysis of functional nitrogen and carbon cycling genes using PICRUSt. Our data suggest the microalgae form symbiotic relationships with a number of bacterial groups including Bacteriodetes, Cyanobacteria, nitrifying and N-fixing bacteria. These microalgae-microbial consortia favour NH4+ and NO2− removal possibly via nitrification and nitrifier denitrification pathways while accumulating NO3− in the inoculated diluted digestate treatment systems. In the absence of inoculation and at high ammonium concentrations in the digestate, almost all NH4+, NO2− and NO3− are driven from the system, largely due to stripping and are unable to be captured for any further use. Thus, a microalgae-microbial consortia-driven digestate treatment system offers the potential to recapture and recover N, enabling production of N fertiliser. These data demonstrate the integral role of syntrophic relationships for microalgae and bacteria in third generation biorefinery concepts.

Microalgal biomass grown in wastewater can be a sustainable source of animal feedstock. We have previously shown the feasibility of mass algal cultivation on undiluted anaerobic digested piggery effluent (ADPE). In this study, we evaluated the nutritional value, pathogen load, in vitro digestibility and potential physiological energy (PPE) of ADPE-grown microalgae as a potential feedstock for pigs. Pathogen load of ADPE-grown microalgae was within regulatory limits. Crude protein of ADPE-grown microalgae was higher than full fat soybeans but was much lower than conventional soybean meals (SBM) currently employed as a source of protein in pig feeds. The essential amino acid content of the microalgae was also lower than SBM. Fatty acid composition of the microalgae was favourable with an omega-3:omega 6 ratio of ~1.9, which may offer potential for value-adding use in some diets. In vitro digestibilities were higher in faeces than at the ileum and were lower for the defatted microalgal biomass. The (theoretical) net energy values of ground and bead-milled algae samples were found to be comparable to that of deshelled sunflower meal used as a feeding ingredient for pigs, but were lower than SBM.

Anaerobic digestate of piggery effluent (ADPE) is extremely high in ammonia toxic to many microorganisms. Bioprospecting and nutrient enrichment of several freshwater and wastewater samples combined and further acclimation resulted in a mixed culture containing at least three microalgae species capable of growing on undiluted ADPE. Outdoor growth of the mixed culture using raceway ponds showed potential for up to 63.7 ± 12.1 mg N-NH4 + L −1 d −1 ammonium removal from the ADPE. The microalgal consortium was dominated by Chlorella sp. and was stable at between 800 and 1600 mg N-NH4 + L −1. Regulation of CO2 addition to the ponds to maintain a pH of 8 increased chlorophyll content of the microalgal consortium. Average microalgal biomass productivity of 800 mg N-NH4 + L −1 culture conditions during five weeks semicontinuous growth was 18.5 mg ash-free dry weight L −1 d −1. Doubling the ammonium concentration from 800 to 1600 mg N-NH4 + L −1 resulted in a 21% reduction of productivity, however the culture grown at 1600 mg N-NH4 + L −1 with the addition of CO2 by keeping pH at pH = 8 led to a 17% increase in biomass productivity.

The overwhelming interest in the use of microalgae to handle associated nutrient surge from anaerobic digestion technologies for the treatment of wastewater, is driven by the need for efficient nutrient recovery, greenhouse gas mitigation, wastewater treatment and biomass reuse. Here, the feasibility of growth and ammonium nitrogen removal rate of semi-continuous mixed microalgae culture in paddle wheel-driven raceway pond and helical tubular closed photobioreactor (Biocoil) for treating sand-filtered, undiluted anaerobic digestion piggery effluent (ADPE) was compared under outdoor climatic conditions between June and September 2015 austral winter season. Two Biocoils, (airlift and submersible centrifugal pump driven) were tested. Despite several attempts in using airlift-driven Biocoil (e.g. modification of the sparger design), no net microalgae growth was observed due to intense foaming and loss of culture. Initial ammonium nitrogen concentration in the Biocoil and pond was 893.03 ± 17.0 mg NH4 +-N L-1. Overall, similar average ammonium nitrogen removal rate in Biocoil (24.6 ± 7.18 mg NH4 +-N L-1 day-1) and raceway pond (25.9 ± 8.6 mg NH4 +-N L-1 day-1) was achieved. The average volumetric biomass productivity of microalgae grown in the Biocoil (25.03 ± 0.24 mg AFDW L-1 day-1) was 2.1 times higher than in raceway pond. While no significant differences were detected between the cultivation systems, the overall carbohydrate, lipid and protein contents of the consortium averaged 29.17 ± 3.22, 32.79 ± 3.26 and 23.29 ± 2.15% AFDW respectively, revealing its suitability as animal feed or potential biofuel feedstock. The consortium could be maintained in semi-continuous culture for more than three months without changes in the algal composition. Results indicated that microalgae consortium is suitable for simultaneous nutrient removal and biomass production from piggery effluent.