A multi-stage municipal wastewater treatment system is proposed to comply with Mexican standards for discharge into receiving water bodies. The system is located in Santa Fe de la Laguna, Mexico, an area with a temperate climate. It was designed for 2,700 people equivalent (259.2 m³/d) and consists of a preliminary treatment, a septic tank as well as two modules operating in parallel, each consisting of a horizontal subsurface-flow wetland, a maturation pond and a vertical flow polishing wetland. After two years of operation, on-site research was performed. An efficient biochemical oxygen demand (BOD₅) (94–98%), chemical oxygen demand (91–93%), total suspended solids (93–97%), total Kjeldahl nitrogen (56–88%) and fecal coliform (4–5 logs) removal was obtained. Significant phosphorus removal was not accomplished in this study (25–52%). Evapotranspiration was measured in different treatment units. This study demonstrates that during the dry season wastewater treatment by this multi-stage system cannot comply with the limits established by Mexican standards for receiving water bodies type ‘C’. However, it has demonstrated the system's potential for less restrictive uses such as agricultural irrigation, recreation and provides the opportunity for wastewater treatment in rural areas without electric energy.

This work evaluates the use of native microalgae consortia for a dual role: polishing treatment of municipal wastewater effluents and microalgae biomass feedstock potential for biodiesel or biofertilizer production. An initial screening was undertaken to test N and P removal from secondary effluents and biomass production by 12 consortia. A subsequent treatment was performed by selected consortia (01 and 12) under three operational conditions: stirring (S), S + 12 h of daily aeration (S + A) and S + A enriched with CO₂ (S + AC). All treatments resulted in compliance with environmental regulations (e.g. Directive 91/271/EEC) and high removal efficiency of nutrients: 64–79% and 80–94% of total N and PO₄³⁻-P respectively. During the experiments it was shown that pH alkalinization due to microalgae growth benefits the chemical removal of ammonia and phosphorus. Moreover, advantages of pH increase could be accomplished by intermittent CO₂ addition which in this research (treatment S + AC) promoted higher yield and lipid concentration. The resulting dry biomass analysis showed a low lipid content (0.5–4.3%) not ideal for biodiesel production. Moreover, the high rate of ash (29.3–53.0%) suggests that biomass could be readily recycled as a biofertilizer due to mineral supply and organic constituents formed by C, N and P (e.g. carbohydrate, protein, and lipids).

Microalgae hold great potential for producing purified high-value products (e.g. pigments and polyunsaturated fatty acids) and represent a source of bioavailable nutrients in aquaculture feeds; however, its production is limited by the high costs of harvesting. Recently, microalgae autoflocculation has been considered a useful solution due to the easy recovery of cell aggregates and natural increase in pH that prevents cell lysis. This work evaluates some cultivation conditions that could contribute to autoflocculation, such as natural pH increase and precipitate formation over the productivity and flocculation of 16 microalgae consortia. Results showed a biomass production of 1.4 to 4.4 dry weight (g L-1) and high flocculation of 81 to ~100%, probably due to Ca3(PO4)2 formation. Moreover, the alkaline pH could have facilitated the assimilation of CO2 and explained the increase of inorganic carbon in the solution obtained. Likewise, results showed a marked descent in electrical conductivity (EC) in solution and high mineral content in the biomass (21.4-35.9%). Finally, this study suggests that the studied culture conditions facilitated microalgae harvest, and the resulting biomass could be a source of bioavailable dietary minerals. Therefore, some of these results might be addressed in future studies with individual microalgae species.