Two sets of triplicate pilot algal raceway ponds (1000-L, 0.30-m deep, paddle wheel mixed) were operated for 14 months at a California wastewater treatment plant to treat wastewater and generate algal-bacterial biomass as biofuel feedstock. Two experiments were run to determine the effect on biomass productivity of (1) hydraulic residence time (HRT: 2, 3, 4, or 4.5 days) and (2) feeding schedule (18 small pulses during 8 AM-4 PM [diurnal] versus 20 large pulses during 4 AM-12 AM [diel]). The target productivity was at least 20 g volatile suspended solids per m2 of pond per day. Additional output variables were followed during the experiments: treatment performance and the effectiveness of biomass harvesting though bioflocculation. Productivity was consistently higher in ponds with a 2-d HRT versus longer HRTs. Average productivity for the 2-d HRT ponds and the variable-HRT ponds (3.6-d average HRT) were 30.1 and 23.4 g/m2-d, respectively. Productivity data collected during the feed regime experiment were highly variable, and average productivities were the same at 26 g/m2-d. During both experiments, both pond sets exceeded the target of 20 g/m2-d on an annual basis. During the hydraulic residence time experiment, the average pond productivity throughout the HRT experiment for the 2-d HRT and 3-d HRT ponds were 30.1 and 23.4 g/m2-d, respectively. Settling efficiency was high for both 2- and 3-d HRT ponds with average turbidity removal of 87-89%. However, total ammonia nitrogen (TAN) concentrations in the 2-day HRT pond effluent were 50-94% higher than in the 3-d HRT pond effluents, although effluent TAN concentrations in both ponds were approximately the same during mid-summer. Furthermore, effluent biochemical oxygen demand (BOD5) concentrations were similar, with the supernatant of Imhoff cones settled for 24 hours containing 24-27 mg/L BOD5 (81-92% removal). In general, the 3-d HRT ponds provided better treatment than the 2-d HRT ponds. During the feeding regime experiment, no productivity or BOD5 removal differences were evident. However, the 3-d HRT ponds had consistently 8 mg/L more effluent TAN than the 2-d HRT ponds.

Raceway ponds can be used to cultivate microalgae for purposes such as wastewater treatment and feedstock for animal feeds and biofuels. Besides nitrogen assimilation in biomass, nitrification can be a wastewater treatment mechanism but one that is hindered by the low nighttime dissolved oxygen concentrations (DO) common in wastewater treatment raceways. The large diel difference in pond conditions and changing influent wastewater quality also raises the question of the optimal frequency and timing of grab sample collection, to obtain representative water quality data. Three sets of triplicate pilot raceways (1,000-L, 0.30-m deep, fed primary clarifier effluent) were operated at an inland central California wastewater treatment plant. Two pilot pond experiments were conducted to determine the effect of (1) night aeration (1800 to 0600) on nitrification, biomass productivity, DO, and BOD5 treatment and (2) sampling frequency (diel, consecutive daily, and once-per-week; 1x/week) on the resulting mean biomass concentration (ash-free dry weight; AFDW) values. The two pond sets with night aeration both displayed a higher nitrification rate than control ponds. Night aeration in well-mixed, 3.7- to 5.4-day hydraulic residence time (HRT) pilot-ponds receiving influent from facultative pond-fed head tank (HT) effluent 16 to 20 hours per day lead to 8.8% more nitrification than night aeration in 2-day HRT ponds fed primary effluent wastewater (PEFF) eight hours per day during late-spring and summer. The cBOD5 concentration in HT was 61% less than PEFF, indicating that low cBOD5 concentrations in influent waters likely promote nitrification when night aeration is provided. Ponds increased in DO by 15% over a 24-hour period on average and by 57% at nighttime hours from 0600 to 1800, suggesting that night aeration significantly increased DO in ponds over a 24-hour period and at nighttime hours when DO may be limited for nitrifying bacteria. However, night aeration is likely to have a negative effect on productivity; the pond set with the highest nitrification experienced a 16% decline in productivity, and the pond set with the lowest nitrification displayed a 6% decline in productivity with night aeration installed. In the sampling frequency study, productivity from samples collected 1x/week at 0900 PST± 2 hour was compared to productivity determined from samples collected at the consecutive daily frequency for four to six consecutive days at 0900 PST± 2 hour and to samples collected at equal time intervals over 24 hours (diel) to determine the optimal sampling frequency. Based on AFDW concentrations, gross productivities were compared to evaluate the effect of sampling frequency. Average diel productivity from three diel experiments, conducted June 2015, December 2015, and July 2016, indicate 1x/week samples might overestimate productivity; however, at most, the difference was 11%. Average consecutive daily productivity from the five experiments, conducted February, March, June, and July of 2016, indicate 1x/week samples may underestimate productivity. The difference was also, at most, 11%. Because the 1x/week samples led to higher productivities than the diel samples and lower productivities than the consecutive daily samples, it seems likely that over many months, the 1x/week sampling frequency represents actual productivity with reasonable accuracy.