The potential impact of an implementation of microalgae-based wastewater treatment on the energy balance of a municipal wastewater treatment plant in Central europe

An overview on microalgal-bacterial granular consortia for resource recovery and wastewater treatment

Excessive generation of wastewater is a matter of concern around the globe. Wastewater treatment utilizing a microalgae-mediated process is considered an eco-friendly and sustainable method of wastewater treatment. However, low biomass productivity, costly harvesting process, and energy extensive cultivation process are the major bottleneck. The use of the microalgal-bacteria granular consortia (MBGC) process is economic and requires less energy. For efficient utilization of MBGC, knowledge of its structure, composition and interaction are important. Various microscopic, molecular and metabolomics techniques play a significant role in understating consortia structure and interaction between partners. Microalgal-bacteria granular consortia structure is affected by various cultivation parameters like pH, temperature, light intensity, salinity, and the presence of other pollutants in wastewater. In this article, a critical evaluation of recent literature was carried out to develop an understanding related to interaction behavior that can help to engineer consortia having efficient nutrient removal capacity with reduced energy consumption.

Advanced HRT-Controller Aimed at Optimising Nitrogen Recovery by Microalgae: Application in an Outdoor Flat-Panel Membrane Photobioreactor

A fuzzy knowledge-based controller of hydraulic retention time (HRT) was designed and tested in an outdoor membrane photobioreactor (MPBR) to improve nitrogen recovery from a microalgae cultivation system, maintaining the algae as photosynthetically active as possible and limiting their competition with other microorganisms. The hourly flow of the MPBR system was optimised by adjusting the influent flow rate to the outdoor environmental conditions which microalgae were exposed to at any moment and to the nitrogen uptake capacity of the culture. A semi-empirical photosynthetically active radiation (PAR) prediction model was calibrated using total cloud cover (TCC) forecast. Dissolved oxygen, standardised to 25 °C (DO25), was used as an on-line indicator of microalgae photosynthetic activity. Different indexes, based on suspended solids (SS), DO25, and predicted and real PAR, were used as input variables, while the initial HRT of each operating day (HRT0) and the variation of HRT (ΔHRT) served as output variables. The nitrogen recovery efficiency, measured as nitrogen recovery rate (NRR) per nitrogen loading rate (NLR) in pseudo-steady state conditions, was improved by 45% when the HRT-controller was set in comparison to fixed 1.25-d HRT. Consequently, the average effluent total soluble nitrogen (TSN) concentration in the MPBR was reduced by 47%, accomplishing the discharge requirements of the EU Directive 91/271/EEC.

A Review on the Reliability and the Readiness Level of Microalgae-Based Nutrient Recovery Technologies for Secondary Treated Effluent in Municipal Wastewater Treatment Plants

Algae-based wastewater treatment technologies are promising green technologies with huge economical potential and environmental co-benefits. However, despite the immense research, work, and achievement, no publications were found wherein these technologies have been successfully applied in an operational environment for nitrogen and phosphorus removal of secondary treated effluent in municipal wastewater treatment plants. Based on a literature review and targeted comprehensive analysis, the paper seeks to identify the main reasons for this. The reliability (considering inlet wastewater quality variations, operating conditions and process control, algae harvesting method, and produced biomass) as well as the technology readiness level for five types of reactors are discussed. The review shows that the reactors with a higher level of control over the technological parameters are more reliable but algal post-treatment harvesting and additional costs are barriers for their deployment. The least reliable systems continue to be attractive for research due to the non-complex operation and relieved expenditure costs. The rotating biofilm systems are currently undertaking serious development due to their promising features. Among the remaining research gaps and challenges for all the reactor types are the identification of the optimal algal strains, establishment of technological parameters, overcoming seasonal variations in the effluent’s quality, and biomass harvesting.