Increasing gas production from algae in anaerobic digesters

June 3, 2015
AlgaeIndustryMagazine.com

Thermal pretreatment laboratory-scale system.

ater and wastewater utilities are dealing with separated algae waste from water reservoirs and wastewater treatment plant (WWTP) effluent holding ponds, respectively. This algae waste is typically hauled to landfills or composting facilities for disposal. In some cases, separated algae waste is added to anaerobic digesters that receive primary and waste activated sludge (WAS).

Adding algae decreases the capacity of the digestion and dewatering facilities with little benefits in terms of additional gas production because of limited algae degradation under mesophilic conditions within the typical hydraulic retention time of anaerobic digesters (20 to 30 days).

It has been previously reported that thermal pretreatment increases the anaerobic degradability of algae (Chen, 1998). Direct steam injection is used to heat anaerobic digesters by mixing steam directly with sludge, providing an instantaneous heat transfer from steam to the liquid. The Hydroheater (Hydro-Thermal Corporation) is a direct steam injector (DSI) that combines high temperature (up to 120 °C) and mechanical shear.

For digester heating, these units are typically operated at temperatures of 35 to 55 °C and differential pressures of 0.5 to 2.0 bar. These units are also widely used for starch hydrolysis in slurry from dry grinding or wet milling processes at temperatures of up to 125 °C and differential pressures of 3 to 7 bar. Hydroheater operation at high temperature and high differential pressure may achieve thermal hydrolysis of algae and improve the anaerobic degradability of algae digestion.

A recent study at Georgia Institute of Technology explored the development of a process that combines thermal pretreatment with high shear to increase the degradability of separated algae waste.

The algae waste samples were subjected to different temperatures (170, 230, and 350 ˚F), differential pressures (1 to 4 bar), and high shear using a Hydroheater DSI, maintained at the target temperature for 30 min, and cooled to ambient temperature.

Thermal pretreatment with high shear resulted in 230 and 500% increases in the soluble chemical oxygen demand (COD) levels of the algae samples when treated at 230 and 350 ˚F, respectively. Pretreatment at 170 ˚F did not increase the soluble COD concentration in the algae waste. These results indicate that the level of hydrolysis is proportional to the pretreatment temperature.

Anaerobic biodegradability batch tests showed specific gas production of 6.4 ft3 per lbs feed volatile solids (VS) for the untreated algae and 6.6, 8.4, and 8.0 ft3 per lbs feed VS for the samples pretreated at 170, 230, and 350 ˚F, respectively. These results showed that increasing temperature from 230 to 350 ˚F increased solubilization but did not result in higher gas production. Among the conditions evaluated in this study, the optimum temperature for thermal pretreatment with high shear was 230 ˚F.

Thus, the proposed thermal pretreatment system would receive separated algae waste, heat it at 230 ˚F for 30 min, and blend it with unheated digester feed (primary sludge and/or WAS). Because the thermally pretreated material would be cooled by transferring heat to the digester feed, the heat added in the thermal pretreatment process would off-set digester heating requirements.

Thermal pretreatment with high shear at 230 ˚F is a feasible process that enhances separated algae waste degradability in anaerobic digesters treating primary sludge and WAS. This process adds negligible energy consumption to existing anaerobic digestion facilities because the heat required for thermal pretreatment offsets the digester heating requirements.

The research group consisted of Toshio Shimada, Michael Jupe, Lee Vandixhorn, Spyros G. Pavlostathis and Rodolfo E. Kilian of Carollo Engineers, Dallas, TX;
 with cooperation from Waco Water Utility Services, Waco, TX;
 Hydro-Thermal Corporation, Waukesha, WI; and Georgia Institute of Technology, Atlanta, GA.