Date of Award

12-2015

Degree Name

MS in Civil and Environmental Engineering

Department/Program

Civil and Environmental Engineering

Advisor

Trygve Lundquist

Abstract

Microalgae can be grown on municipal wastewater media to both treat the wastewater and produce feedstock for algae biofuel production. However the reliability of treatment must be demonstrated, as well as high areal algae productivity on recycled wastewater media and efficient sedimentation harvesting. This processes was studied at pilot scale in the present research.

A pilot facility was operated with nine CO2-supplemented raceway ponds, each with a 33-m2 surface area and a 0.3-m depth, continuously from March 6, 2013 through September 24, 2014. The ponds were operated as three sets of triplicates with two sets continuously fed primary-clarified municipal wastewater at either a 2-day or 3-day hydraulic residence time (HRT), and one set fed the clarified effluent of the 3-day pond set. This second pond-in-series was operated with a 3-day HRT.

Areal biomass productivity is reported as gross and net, the former based only on biomass in the pond effluents and the latter subtracting the volatile suspended solids in the influent from those in the effluent. An estimate was also made of autotrophic biomass productivity, as differentiated from heterotrophic growth.

Over a year, net productivity averaged 83 metric tons per hectare per year (MT/ha-yr) for the 2-day HRT ponds, 52 MT/ha-yr for the 3-day HRT ponds, and 44 MT/ha-yr for the 3-day HRT ponds receiving clarified effluent of the first set of 3-day HRT ponds (i.e., recycled water). The lower net productivity of the pond receiving water recycling was attributed to two factors. First, the relatively high influent suspended solids concentrations were subtracted from the effluent suspended solids concentrations before net productivity was calculated. Second, the recycled water contained less soluble organic matter than the primary-clarified wastewater leading to less heterotrophic biomass production. The accumulation of inhibitory allelochemicals is a possible third cause of lower productivity

, but no specific information was collected on allelopathy.

Algae were harvested from pond effluent by sedimentation, with harvest efficiency most affected by the extent of natural bioflocculation occurring in the ponds. Some forms of bioflocculation are thought to be mediated by bacteria, which often make-up a substantial fraction of the settled flocs. Pond samples settled in 1-L Imhoff cones averaged/L total suspended solids after 24 hours of settling; but all ponds fell short of meeting an averaged/L total suspended solids after a 2 hour interval which would be ideally achieved for wastewater effluent. No relationship was seen between settling performance and the bacterial content of flocs.

Soluble carbonaceous biochemical oxygen demand (scBOD5) removal by the raceway ponds was sufficient to meet wastewater treatment requirements year around. Influent scBOD5 concentrations averaged 83 mg/L, and the effluent averaged 5.1 mg/L and 4.2 mg/L for the 2-day and 3-day HRT pond sets, respectively.

The variable with the greatest influence on productivity in all pond sets, and settling performance in the recycled water pond set, was season (i.e., co-correlated variables of solar insolation and pond temperature). Neither productivity nor settling appeared to be related to prominent algae genera or prevalence of grazers.

The high net productivity achieved with a growth medium of primary clarifier effluent and the generally high settleability of algal-bacterial flocs indicate a good potential for algae wastewater treatment and biofuel production. However, the settling of algae grown on recycled water needs improvement to achieve the full potential of wastewater-grown algae biofuel production.

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