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Microalgae Growth For Nutrient Recovery From Sludge Liquor and Production of Renewable Bioenergy

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Proof-of-concept has been demonstrated for a process that will utilize nutrients from sludge liquor, natural light, and CO2 from biogas to grow microalgae at wastewater treatment plants. The microalgae will remove nitrogen and phosphorus from the sludge liquor, significantly reducing the impact of returning side-streams to the head of the treatment plant. The produced algae will be harvested and fed to existing anaerobic digesters for co-digestion with wastewater sludge for increased production of renewable bioenergy.

Dewatering of anaerobically digested sludge in centrifuges produces reject water that often is black, contains a lot of very fine solids particles, and has extremely low transmittance of light. Pre-treatment of the reject water is necessary to make it a suitable substrate for growth of algae. A procedure was developed that improved light transmittance for reject water from the FREVAR, Norway, wastewater treatment plant from 0.1 % T to 77 % T (670 nm, 1 cm path).

Chlorella sp. microalgae were found to be suitable for growth in pre-treated reject water that was undiluted. Typical nitrogen removal was 80-90 g N/kg TSS of produced microalgae.

The Chlorella sp. microalgae were successfully harvested by chemically assisted flocculation followed by straining through a 33 μm sieve cloth. Up to 99 % recovery of the produced algae was achieved. Harvested algae were anaerobically co-digested with wastewater sludge. The specific methane gas production (mL CH4/g VS fed) for the algae varied from 74 to 90 % of the specific methane gas production for the wastewater sludge, depending on digester temperature and pre-treatment of the algae biomass.
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Keywords: Anaerobic co-digestion; Chlorella sp; methane; microalgae growth; microalgae harvesting; sludge liquor nutrients

Document Type: Research Article

Publication date: 2011-01-01

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