Date of Award


Degree Name

MS in Civil and Environmental Engineering


Civil and Environmental Engineering


Tryg Lundquist


This thesis examines the effect of solids recycling on nitrification, organic carbon removal, and algal-bacterial productivity in bench-scale photobioreactors (PBRs) simulating winter pond conditions in San Luis Obispo, California and a 6-acre raceway system in Delhi, California. Two sets of duplicate photobioreactors operated with or without solids recycling were fed primary clarifier effluent (1oEff) for the first experiment, and facultative pond effluent (FAC) for the second experiment.

In both experiments mean productivity was lower in solids recycled PBRs (3.67 and 2.3 g/m2-day), than in controls without solids recycling (4.15 and 3.9 g/m2-day). When fed 1oEff which contained a high amount of readily biodegradable COD, solids recycled PBRs had 30% less VSS in supernatant than controls after 1 hour of settling in Imhoff cones. However, when fed facultative pond water with no readily biodegradable COD there was little difference in supernatant VSS after settling.

The type of wastewater influenced COD removal. PBRs had 40-50% soluble COD removal when fed 1oEff, and 10-20% removal when fed FAC. Mean PBR effluent COD was the same in both treatments and controls in when fed 1oEff (42 mg/L). When fed FAC mean effluent COD was marginally lower in solids recycled PBRs (61 mg/L) then in controls (68 mg/L). Most of the COD in 1oEff was readily biodegradable, while most of the COD in FAC was degraded while in facultative ponds leaving mostly recalcitrant and slowly biodegradable COD in FAC.

Nitrification occurred more quickly and more reliably in solids recycled PBRs. The difference in nitrification was seen most notably when using FAC as PBR influent, where solids recycled PBRs had complete nitrification, but controls did not. When fed 1oEff all PBRs had complete nitrification, but solids recycled did so sooner. Overall, PBRs with solids recycling were also more resilient to changes in influent wastewater characteristics and had more consistent effluent water quality when fed wastewater that had occasional spikes in nitrogen and organic carbon. The difference between the two wastewaters indicates a possible inhibitory effect of FAC on nitrification.

Microscopy data from both studies, though mostly qualitative, seems to indicate that solids recycling promotes biodiversity in algal-bacterial cultures, which may be part of the reason why solids recycling promoted more resilient and reliable treatment.

Available for download on Monday, March 21, 2022