Utilisation of wastewater nutrients for microalgae growth for anaerobic co-digestion.

The feasibility of growing microalgae in natural light using wastewater high in nutrients (N & P) for the production of more bioenergy was examined. The main retrofitting unit would be a photobioreactor for wastewater treatment plants (wwtp) having anaerobic digesters in close proximity. Theoretical microalgae production rates from different wastewater sources (municipal wwtp, source separation of human and animal wastewaters) were estimated using mass balance. Mass and energy balances for a conventional wwtp using chemically enhanced primary treatment was investigated for microalgae growth for a situation limited by availability of carbon dioxide (CO2) generated onsite and where additional CO2 was imported from outside source. Reject water from dewatering of anaerobically digested sludge from four wwtp around Oslo region were pretreated for improved light penetration and examined for microalgae growth. Several pre-treatment methods were investigated. Pretreatment using flocculation + settling + anthracite filtration yielded high light transmittance. A maximum microalgae growth rate of 13 g TSS/m(2)-d was achieved using this pretreated reject water. The challenges of integrating photobioreactors with existing units have been highlighted.

A new process for enriching nitrifiers in activated sludge through separate heterotrophic wasting from biofilm carriers.

A new process, the biofilm-activated sludge innovative nitrification (BASIN) process, consisting of a moving-bed biofilm reactor (MBBR) with separate heterotrophic wasting, followed by an activated-sludge process, has been proposed to reduce the volumetric requirements of the activated-sludge process for nitrification. The basic principle is to remove chemical oxygen demand on the biofilm carriers by heterotrophic organisms and then to waste a portion of the heterotrophic biomass before it can be released into the activated-sludge reactor. By this means, the amount of heterotrophic organisms grown in the activated-sludge reactor is reduced, thereby reducing the volume of that tank needed for nitrification. For nitrification applications, the simplest method for stripping biomass was to use an in-tank technique using high shearing rates with aeration. Bench-scale testing showed sludge yields in the BASIN process were one-half of that in a control activated-sludge process and twice that of a process line with intermediate settling between the MBBR and activated-sludge stage. Critical washout solids retention times for nitrifiers were the same for all three lines, so activated-sludge volumes for the BASIN process could be reduced by 50% compared with the control. Originally conceived process concepts for the BASIN process were confirmed by the experimental work.