Utilizing algal turf scrubbers for bioremediation and bioenergy production

Abstract

This dissertation researched the conversion of algal biomass that was generated as a byproduct of bioremediation by algal turf scrubbers (ATS) into bioenergy via anaerobic digestion. Anaerobic digestion is a bacterial process that converts organic material into bioenergy in the form of biogas that contains methane (CH4), the primary component of natural gas. Bioenergy yield was quantified as the volume of CH4 generated from digestion of the algae in relation to seasonal changes in algal biomass yield, different digester operational parameters, co-digestion of the biomass with more conventional digestion feedstock, and flocculation pre-treatment for dewatering of algae prior to digestion.

The first study used a pilot-scale mesophilic digester at the Port of Baltimore (Baltimore, MD, USA) to continuously digest algae from a 122 m2 ATS on the Patapsco River over two years. Biomass generation was significantly correlated to maximum daily air temperature, water temperature, and flow rate in Year 1 but only water flow rate in Year 2. Algae of the taxa Ochrophyta dominated the algal turf, especially the filamentous diatom Melosira sp., in both years. In Year 1 of the study, two anaerobic digestion systems with variable hydraulic retention times (HRT), designated D1 (average HRT 45.0 ± 5.8 days) and D2-D3 (average HRT 61.0 ± 8.1 days) were used to digest the algae. The D1 generated 1090 L CH4 from 2416 L of algae over a 39-day HRT (59.1 ± 8.9 L algae/kg VS), and D2-D3 generated 1170 L CH4 from 2337 L of algae over a 53-day HRT (67.9 ± 11.0 L algae/kg VS). The difference in CH4 yield with two different HRTs was not significant. In Year 2, only the D2-D3 was operated and was modified to test the use of active recirculation and heating to improve digestion efficiency and CH4 yield. The D2-D3 system generated 4000 L of CH4 (163 ± 42 L algae/kg VS) from 3310 L of algae in Year 2.

The second study consisted of laboratory-scale biomethane potential tests to test changes in CH4 yield when algae harvested from an Anacostia River (Bladensburg, MD, USA) ATS was co-digested with three wastes (dairy manure, food waste, and poultry litter) at algae:waste loading ratios of at 1:1, 1:2, 1:5, and 1:10 by organic material, or volatile solids (VS), content. The algal biomass was the least efficient substrate at generating CH4 when normalized by both mass VS digested (109 ± 4 mL CH4/g VS) and total mass of substrate digested (0.687 ± 0.025 mL CH4/g substrate). Co-digestion with all three of the wastes at all ratios tested significantly increased CH4 generation efficiency per mass VS compared to only digesting algae. However, the high moisture content of the algae (95.2%) relative to the other co-digestion wastes (29.0-84.6%) significantly decreased CH4 production on a mass basis for the dairy manure, food waste, and poultry litter when algae was added at any loading ratio. A lettuce growth experiment using the effluent of the digestion vessels showed no signs of acute toxicity when any of the diluted (8-fold) digester effluents were applied as fertilizer to the developing plants.

The third and final study consisted of flocculation experiments that tested 500-mL of algae using four experimental treatments (FeCl3, electrocoagulation, chitosan, and Bacillus sp. RP 1137) to dewater algae harvested from the Anacostia River ATS and compared to gravity settling as a control. The experimental flocculants successfully increased the total solids (TS) of the ATS algae by 14-291% depending on the treatment, with electrocoagulation being the least effective and bacterial flocculation being the most effective flocculant. All treatments reduced total suspended solids (TSS) in the drained supernatant by >98%. The raw ATS algae and dewatered solids from the settling experiment were then digested for 35-days, with the algae yielding 49.6 ± 3.6 mL of CH4/g VS. The dewatered solids had reduced digestion efficiency by 29.6-71.0% compared to untreated algae. Dewatering pre-treatment increased CH4 yield from the algae when normalized by total g substrate fed to the reactor (1.65 ± 0.12 mL CH4/g substrate) for all treatments except bacteria 1x, however the effect was only significant for solids dewatered with electrocoagulation.

The results from the three studies show that temperature drives algal growth patterns in temperate climates, which results in seasonally variable biomass yield from ATS, with a corresponding variability in CH4 production due to inconsistent availability of the algal feedstock. Algae can be co-digested with agricultural and food wastes that are generated year-round to reduce variability in feedstock availability. Thickening and dewatering the algae improves CH4 yield on a mass basis, however the digestion efficiency was reduced. In conclusion, the findings suggest that anaerobic digestion is a viable means of managing the algae harvested from ATS systems with and without co-digestion of the algal biomass.