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In this study two wet microalgae cultures and one dried microalgae culture were co-digested in different proportions with sewage sludge in mesophilic and thermophilic conditions. The aim was to evaluate if the co-digestion could lead to an increased efficiency of methane production compared to digestion of sewage sludge alone. The results showed that co-digestion with both wet and dried microalgae, in certain proportions, increased the biochemical methane potential (BMP) compared with digestion of sewage sludge alone in mesophilic conditions. The BMP was significantly higher than the calculated BMP in many of the mixtures. This synergetic effect was statistically significant in a mixture containing 63% (w/w VS based) undigested sewage sludge and 37% (w/w VS based) wet algae slurry, which produced 23% more methane than observed with undigested sewage sludge alone. The trend was that thermophilic co-digestion of microalgae and undigested sewage sludge did not give the same synergy.

In municipal wastewater treatment plants (WWTPs), algae could be utilised for cleaning the water and, at thesame time, produce a biomass that can be used for energy. Through anaerobic digestion, microalgae can contributeto biogas production when co-digested with sewage sludge. In this paper, previous published results onthe co-digestion of sewage sludge and microalgae are summarised and reviewed, and any remaining knowledgegaps are identified. The batch tests currently documented in literature mostly concern digestion under mesophilicconditions, and studies investigating thermophilic conditions are less common. The average biochemicalmethane potential (BMP) for 29 different mixtures co-digested under mesophilic conditions is 317 ± 101 N cm3CH4 gVS−1 while the result for 12 different mixtures investigated under thermophilic conditions is a BMP of318 ± 60 N cm3 CH4 gVS−1. An evaluation of the heat required for increasing the temperature from mesophilicto thermophilic conditions shows that increased methane production under thermophilic conditions can beenough to create a positive energy balance. For a full-scale WWTP, using thermophilic digestion on sludge, or acombination of sludge and microalgae could therefore be of interest. This is dependent on the demands onsanitation of the sludge and the possibilities for heat recovery.Most of the mesophilic investigations indicate a synergetic effect for co-digestion, with enhancements of up toalmost 70%. However, the results are uncertain since the standard deviations for some of the BMP tests are in thesame order of magnitude as the identified enhancement. Neither of the presented publications provide an understandingof the basic mechanisms that led to higher or lower BMP when microalgae were mixed with wastewatersludge. We, therefore, call for care to be taken when assuming any effects related to the specification ofsubstrates. Microalgae and wastewater sludge have several similarities, and the specific results of BMP in themixtures relate more to the specifics of the respective materials than the materials themselves.Investigations into semi-continuous processes of co-digestion of microalgae and sludge are scarce. The yieldsfor three co-digestion studies show high variation, with an average of 293 ± 112 N cm3 gVSin−1. The availableresults show strong potential for co-digestion of sewage sludge and microalgae. Further investigations are requiredto identify optimal conditions for biogas production, and analysis of microalgae implementation onwastewater treatment at a system level is also needed to identify the total mass balance of substrate and nutrientrecovery.

In this study a natural mix of microalgae grown in wastewater of municipal character was co-digested with sewage sludge in mesophilic conditions, in both batch and semi-continuous modes. The semicontinuous experiment was divided into two periods with OLR 1 (Organic Loading Rate) of 2.4 kg VS m³ d^-1 and HRT1 (Hydraulic Retention Time) of 15 days, and OLR2 of 3.5 kg VS m³ d^-1 and HRT2 of 10 days respectively. Results showed stable conditions during both periods. The methane yield was reduced when adding microalgae (from 200 ± 25 NmL CH₄ g VSin^-1 , to 168±22 NmL CH₄ g VSin^-1). VS reduction was also decreased by 51%. This low digestability was confirmed in the anaerobic batch test. However, adding microalgae improved the dewaterability of the digested sludge. The high heavy metals content in the microalgae resulted in a high heavy metals content in the digestate, making it more difficult to reuse the digestate as fertilizer on arable land. The heavy metals are thought to originate from the flue gas used as a CO₂ source during the microalgae cultivation. Therefore the implementation of CO₂ mitigation via algal cultivation requires careful consideration regarding thesource of the CO₂-rich gas.

This study explores the possibility of using a microalgae based activated sludge – process (MAAS-process) to increase the reduction of pharmaceutical residues in outgoing wastewater, compared to a conventional wastewater treatment plant with activated sludge process. In an on-site study, residual sludge from four pilot scale digesters fed with primary sludge and waste activated sludge or microalgae were sampled and analysed for pharmaceutical residues. The aim of the study was to compare the reduction efficiencies of a microalgae based process with a conventional biological treatment and also to explore the reduction of the residues in the different process steps including the sewage sludge thickening before the anaerobic digestion, the digestion and the secondary treatment with the sludge dewatering process. The results show that the total reduction of pharmaceutical residues in the water phase appears to be significantly higher in the MAAS-process. The substance diclofenac was not degraded in any of the biological processes in the study. The reduction of pharmaceutical residues in digested sludge seems to be higher in mesophilic conditions compared with thermophilic conditions.