Use of anaerobic digestion effluent and secondary treated sewage plant discharge for algal cultivation and prevention of algal pond crash

An anaerobic digestion effluent (ADE) from the filtration facility of an anaerobic digestion process in a sewage treatment plant was used to cultivate the halophilic oil-producing microalga Chlorella sp. strain JPCC 0782. Secondary treated sewage (STS) was used as a dilution medium (ADE/STS) to avoid growth inhibition caused by high ADE ammonium nitrogen content. Chlorella sp. grew and produced lipids in the sterilized ADE/STS with 3.6 mM–14.4 mM of the initial ammonium nitrogen concentration, thereby confirming the usability of ADE/STS as an algal medium. Although the microalga grew in the unsterilized ADE/STS in the raceway pond, ciliates also grew, resulting in consumption of the microalga biomass produced. Because the ciliates in the ADE/STS were inactivated at 10 psu, the salinity of ADE/STS was adjusted from 0.84 psu to 10 psu by adding seawater. We also demonstrated that adjusting salinity by adding seawater is a promising way to inactivate ciliates appearing in ADE/STS to prevent algal pond crash.